Biomechanical evaluation of anterior thoracolumbar spinal instrumentation

Surgical outcomes of anterior column reconstruction for spinal fractures caused by minor trauma-preoperative examination of the number of intervertebral bone bridges is key to obtaining good bone fusion

BACKGROUND

To achieve good bone fusion in anterior column reconstruction for vertebral fractures, not only bone mineral density (BMD) and bone metabolism markers but also lever arms due to bone bridging between vertebral bodies should be evaluated. However, until now, no lever arm index has been devised. Therefore, we believe that the maximum number of vertebral bodies that are bony and cross-linked with the contiguous adjacent vertebrae (maxVB) can be used as a measure for lever arms. The purpose of this study is to investigate the surgical outcomes of anterior column reconstruction for spinal fractures and to determine the effect of bone bridging between vertebral bodies on the rate of bone fusion using the maxVB as an indicator of the length of the lever arm.

METHODS

The clinical data of 81 patients who underwent anterior column reconstruction for spinal fracture between 2014 and 2022 were evaluated. The bone fusion rate, back pain score, between the maxVB = 0 and the maxVB ≥ 2 patients were adjusted for confounding factors (age, smoking history, diabetes mellitus history, BMD, osteoporosis drugs, surgical technique, number of fixed vertebrae, materials used for the anterior props, etc.) and analysed with multivariate or multiple regression analyses. The bone healing rate and incidence of postoperative back pain were compared among the three groups (maxVB = 0, 2≦maxVB≦8, maxVB ≧ 9) and divided by the maxVB after adjusting for confounding factors.

RESULTS

Patients with a maxVB ≥ 2 had a significantly higher bone fusion rate (p < 0.01) and postoperative back pain score (p < 0.01) than those with a maxVB = 0. Among the three groups, the bone fusion rate and back pain score were significantly higher in the 2≦maxVB≦8 group (p = 0.01, p < 0.01).

CONCLUSIONS

Examination of the maxVB as an indicator of the use of a lever arm is beneficial for anterior column reconstruction for vertebral fractures. Patients with no intervertebral bone bridging or a high number of bone bridges are in more need of measures to promote bone fusion than patients with a moderate number of bone bridges are.

Mechanical Analysis of Posterior Pedicle Screw System Placement and Internal Fixation in the Treatment of Lumbar Fractures

Objective

Image segmentation technology is applied to separate a single vertebra from the three-dimensional model of the spine, so as to separate a single vertebra image with smaller error, higher degree of automation, and better results. The objectives are to study the biomechanical characteristics of posterior short-segment pedicle screw fixation by three-dimensional finite element method, analyze the mechanical characteristics of posterior pedicle screw rod fixation system under different factors, and demonstrate the feasibility of its application in the treatment of lumbar fracture.

Methods

The authors searched the database for articles about the treatment of lumbar spine fracture, screw rod internal fixation system, and its mechanical parameters. The threshold segmentation method based on region segmentation method was used to segment the image, and the three-dimensional finite element model was used to analyze the biomechanical characteristics of different posterior internal fixation for lumbar spine fracture.

Results

The posterior pedicle internal fixation system for the treatment of multilevel spinal fractures is a mature surgical technique and has fewer postoperative complications. Transpedicle fixation is effective and reliable. It can effectively restore the coronal and sagittal curvature of the vertebral body and restore the stability of the spine better. But the choice of internal fixation method should be individualized based on fracture type, identification of critical and secondary injury sites, and stability assessment. Only after mastering the biomechanical characteristics of the posterior screw rod system for the treatment of lumbar fracture, selecting the appropriate method, and fixing the appropriate movement unit can the best fixation be achieved.

Conclusion

Threshold method is the most direct and simple image segmentation method. The core technology of thresholding is the selection of threshold, which will affect the final segmentation effect. The most common segmentation method is to calculate the segmentation threshold by histogram. The threshold method has less computation and good segmentation effect for the image with large contrast between background and target. Posterior pedicle screw rod system internal fixation has the advantages of less trauma, good reduction, reliable fixation, and less complications. The design, placement angle and depth of various internal fixation systems, and the number of fixed segments all show different mechanical characteristics. As long as we master the above characteristics, choose the appropriate method and fix the appropriate motor unit, and we can get the best fixation; it can be used as an effective treatment for lumbar fracture.

Lumbar muscle atrophy caused by harness replacement in a chronic calf model of total artificial heart implantation

The postoperative care of animals implanted with mechanical circulatory support devices is complex. The standard of care requires continuous monitoring of hemodynamic parameters post implant, wound care, and maintenance of the animal's well-being, but also includes controlling the animal's biomechanics under conditions of continuous restraint and harnessing. In such studies, a harness provides secure fixation of the exteriorized device driveline and pressure lines and aids animal handling (lifting, position adjustment, and assistance with standing up). Harnessing is a key element in large-animal surgery. It affects the animal's conditions, safety, and post-procedure troubleshooting and thus may drastically worsen postoperative outcomes if improperly handled. Here we report a case associated with an unplanned harness replacement in a chronic animal model implanted with the Cleveland Clinic continuous-flow total artificial heart. Inadvertent changes to the harness resulted in posture change caused by muscular atrophy of the calf's spine that had been under long-term harness support.

A history of spine biomechanics. Focus on 20th century progress

The application of mechanical principles to problems of the spine dates to antiquity. Significant developments related to spinal anatomy and biomechanical behaviour made by Renaissance and post-Renaissance scholars through the end of the 19th century laid a strong foundation for the developments since that time. The objective of this article is to provide a historical overview of spine biomechanics with a focus on the developments in the 20th century. The topics of spine loading, spinal posture and stability, spinal kinematics, spinal injury, and surgical strategies were reviewed.

Range of motion after thoracolumbar corpectomy: evaluation of analogous constructs with a novel low-profile anterior dual-rod system and a traditional dual-rod system

STUDY DESIGN

An in vitro biomechanical study.

OBJECTIVES

To compare the biomechanical stability of traditional and low-profile thorocolumbar anterior instrumentation after a corpectomy with cross-connectors. Dual-rod anterior thoracolumbar lateral plates (ATLP) have been used clinically to stabilize the thorocolumbar spine.

METHODS

The stability of a low-profile dual-rod system (LP DRS) and a traditional dual-rod system (DRS) was compared using a calf spine model. Two groups of seven specimens were tested intact and then in the following order: (1) ATLP with two cross-connectors and spacer; (2) ATLP with one cross-connector and spacer; (3) ATLP with spacer. Data were normalized to intact (100 %) and statistical analysis was used to determine between-group significances.

RESULTS

Both constructs reduced motion compared to intact in flexion-extension and lateral bending. Axial rotation motion became unstable after the corpectomy and motion was greater than intact, even with two cross-connectors with both systems. Relative to their respective intact groups, LP DRS significantly reduced motion compared to analogous DRS in flexion-extension. The addition of cross-connectors reduced motion in all loading modes.

CONCLUSIONS

The LP DRS provides 7.5 mm of reduced height with similar biomechanical performance. The reduced height may be beneficiary by reduced irritation and impingement on adjacent structures.

Outcome after thoracoscopic ventral stabilisation of thoracic and lumbar spine fractures

BACKGROUND AND PURPOSE

Thoracoscopic-assisted ventral stabilisation for thoracolumbar fractures has been shown to be associated with decreased recovery time and less morbidity when compared with open procedures. However, there are a limited number of studies evaluating late clinical and radiological results after thoracoscopic spinal surgery.

METHODS

We performed an analysis of the late outcomes of thoracolumbar fractures after minimally invasive thoracoscopic ventral instrumentation. Between August 2003 and December 2008, 70 patients with thoracolumbar fractures (T5-L2) underwent ventral thoracoscopic stabilisation. Tricortical bone grafts, anterior plating systems (MACS-System), and cage implants were used for stabilisation. Outcomes measured include radiologic images (superior inferior endplate angle), Visual Analogue Scale (VAS), VAS Spine Score, quality of life scores SF-36 and Oswestry Disability Index (ODI).

RESULTS

Forty seven patients (67%, 47 out of 70) were recruited for the follow up evaluation (2.2 ± 1.5 years). Lower VAS Spine scores were calculated in patients with intra- or postoperative complications (44.7 (± 16.7) vs. 65.8 (± 24.5), p=0.0447). There was no difference in outcome between patients treated with bone graft vs. cage implants. Loss of correction was observed in both bone graft and titanium cage groups.

INTERPRETATION

The present study demonstrates diminished long-term quality of life in patients treated with thoracoscopic ventral spine when compared with the outcome of german reference population. In contrast to the other patients, those patients without intra-operative or post-operative complications were associated with improved outcome. The stabilisation method (bone graft versus spinal cage) did not affect the long-term clinical or radiographic results in this series.

Comparison of allograft bone and titanium cages for vertebral body replacement in the thoracolumbar spine: a biomechanical study

BACKGROUND

When an anterior approach to repair a burst fracture is indicated, several devices can be used to restore spinal stability (eg, bone graft, free-standing titanium cage, and expandable titanium cage).

OBJECTIVE

We compare the biomechanical stability and prices of each of these systems.

MATERIALS AND METHODS

Eight fresh human cadaver T11 through L3 vertebral specimens were harvested and cleaned of soft tissues. T11-T12 and L2-L3 were fixed by screws. The fixed ends were then set in automotive body filler (Bondo). The prepared specimens were tested in the Biaxial Instron tester (8874, Norwood, MA) after a sequence of the following: intact, after the creation of an anterior corpectomy at L1, and after insertion of both of the 2 different titanium cages and the fibular graft. A titanium screw-and-plate anterolateral system was used to secure the construct (VANTAGE, Medtronic Sofamor Danek, Memphis, TN). The conditions of displacement testing were as follows: rotation (+/- 3.5 degrees ), flexion and extension, and left and right bending (+/- 3.5 mm). For each mode of testing, the stiffness was calculated.

RESULTS

The stiffness data, when statistically analyzed by repeated-measures analysis of variance (at P = .05 and power > 0.9), indicated no significant differences among these devices.

CONCLUSION

On the basis of this biomechanical study, the stiffness of the fibular graft was similar to that of the other metallic devices in this cadaver model.

Anterior approach to thoracic and lumbar spine lesions: results in 145 consecutive cases

Object

The authors report on a series of 145 consecutive patients with different types of spine lesions surgically treated via an anterior approach (AA) at the thoracic and lumbar levels during the past 10 years. Indications, techniques, and surgical results are described.

Methods

This series included 92 patients with fractures, 30 with neoplasms, 13 with thoracic disc hernias, and 10 with spinal infections. Based on the lesion to be addressed, the AA was used for lesion excision, corpectomy, vertebral body reconstruction with cages, realignment, and/or plating or screwing. The approach was extracavitary in 55 patients and intracavitary in 90. In 126 patients (86.8%), neural decompression and spine stabilization were achieved via a stand-alone AA (SA-AA), whereas 19 patients (13.1%) were treated using a 2-stage anteroposterior approach. This circumferential approach was reserved for select cases of severe traumatic dislocation, particular types of tumors, or specific anatomical locations. The authors developed a simple neuronavigation-based method of identifying the severely injured patients who were eligible for the SA-AA by evaluating the angle of lateral dislocation.

Results

There were no deaths and no instances of major surgery-related morbidity. Minor morbidity was almost always transitory and was reported in 13 patients (8.9%). Neurological improvement was reported in 20% of injured patients with a preoperative incomplete lesion. Postoperatively, all patients were able to stand or at least sit without load pain. During the follow-up (mean ± standard deviation 3.8 ± 2.4 years), there were no cases of failure, fracture, dislocation, or bending of the anterior instrumentation, and the rate of pseudarthrosis was 0%.

Conclusion

The anterior route provides direct access to most spine diseases and allows optimal neural decompression and the possibility of adequate realignment and strong reconstruction/fixation. Stability of the vertebral column is achieved, resolution of clinical pain is rapid and almost complete, and the rate of surgical complications is very low. The authors assert that the SA-AA offers so many advantages and has such good results that the 2-stage anteroposterior approach can be reserved for a minority of select cases and that the time for using the posterior approach alone is over.

Two column lesions in the thoracolumbar junction: anterior, posterior or combined approach? A comparative biomechanical in vitro investigation

There are various surgical techniques for the treatment of spinal fractures in the thoracolumbar region. Several implants have been developed for anterior or posterior instrumentation. Optimal treatment of unstable thoracolumbar osseous and ligamentous injuries remains controversial. To compare the stabilizing effects of an antero-lateral, thoracoscopically implantable plate system (macsTL, Aesculap, Germany) with the stability provided by a fixateur interne (SOCON, Aesculap, Germany), this in vitro investigation examined six human bisegmental (T12-L2) spinal units. Specimens were tested intact, and with simulation of osseous lesions in the anterior and ligamentous lesions in the posterior column (combined A/B-fracture). While loaded in the main anatomical planes such as flexion/extension, left and right lateral bending and left and right axial rotation with a bending moment of 7.5 Nm in a special testing jigs, motion analysis was performed. Quantitative interpretation of the stabilizing effect was achieved using a contactless three-dimensional motion analysis system. Each specimen was tested in four different scenarios: the first step measured movements of intact spinal segments. For the second step, specimens underwent simulation of combined A/B-fracture provided with bisegmental (T12/L2) antero-lateral fixation and bone strut graft from the iliac crest. For the third step, segments were additionally stabilized by the fixateur interne. The last measurement (fourth step) was performed after removing the anterior instrumentation. Range of motion (ROM) values were compared and statistically evaluated. Compared to the intact specimens the anterior instrumentation of the combined lesion, simulated A/B-fracture, leads to a stabilizing effect in flexion/extension and lateral bending. In contrast to these findings the torsional instability increased for the upper segment and bisegmentally. A maximum rigidity, beyond intact values, was registered for each anatomical plane with the combined instrumentation: antero-lateral and fixateur interne. After removing the anterior screw plate system maximum movements, in all segments for flexion/extension and lateral bending, bisegmentally and for the upper segment in axial rotation, were less than ROM values measured with the anterior system only. With respect to these findings a combined ventro-dorsal stabilization procedure should be considered for ligamentous disruptions of the posterior column in combination with A-fractures in the thoracolumbar junction.

Unstable Thoracolumbar Burst Fractures

Operative management of a thoracolumbar burst fracture varies according to many factors. Fracture morphology, neurologic status, and surgeon preference play major roles in deciding upon anterior, posterior, or combined approaches. Optimizing neural decompression while providing stable internal fixation over the least number of spinal segments is the goal. Short-segment constructs via a single-stage approach (anterior versus posterior) have become viable options with advances in instrumentation and techniques. This study compares anterior-only fixation utilizing a corpectomy strut graft and a modern thoracolumbar plating system with a posterior-only construct using pedicle screws and load sharing hooks for the treatment of unstable burst fractures. Functional outcome and sagittal plane restoration and maintenance of sagittal plane alignment were evaluated. Fifty-three patients with unstable burst fractures were assessed with 40 undergoing an anterior-only construct and 13 having a short-segment posterior-only construct. The posterior-only group had no hardware failures; however, the loss of sagittal plane correction averaged 8.1 degrees, whereas the anterior-only group averaged only a 1.8-degree increase in sagittal plane kyphosis. Both techniques resulted in statistically significant initial improvement in sagittal alignment; however, the posterior short-segment group lost this statistical significance at follow-up whereas the anterior-only group continued to demonstrate statistically significant improvement in sagittal alignment at follow-up compared to preoperative measurements.

Influence of screw positioning in a new anterior spine fixator on implant loosening in osteoporotic vertebrae

STUDY DESIGN

A biomechanical study was designed to assess implant cut-out of three different angular stable anterior spinal implants. Subsidence of the implant relative to the vertebral body was measured during an in vitro cyclic loading test.

OBJECTIVES

The objective of the study was to evaluate two prototypes (Synthes) of a new anterior spine fixator with different screw angulations in comparison to the established MACSTL(R) Twin Screw Concept (Aesculap). The influence of factors like load-bearing cross-sectional area, screw angulation and bone mineral density upon implant stability should be investigated.

SUMMARY OF BACKGROUND DATA

Epidemiologic data predict a growing demand for appropriate anterior spinal fixation devices especially in patients with inferior structural and mechanical bone properties. Although different concepts for anterior spinal instrumentation systems have been tried out, implant stability is still a problem.

METHODS

Three angular stable, anterior spinal implants were tested using 24 human lumbar osteoporotic vertebrae (L1-L5; age 84 (73-92)): MASC TL system (Aesculap); prototype 1 (MP1) with 18 degrees and prototype 2 (MP2) with 40 degrees screw angulation (both Synthes). All implants consisted of two screws with different outer screw diameters: 7-mm polyaxial screw with 6.5-mm stabilization screw (MASC TL), two 5-mm locking-head screws each (MP1 and MP2). Bone mineral density (BMD) and vertebral body width of the three specimen groups were evenly distributed. The specimens were loaded in craniocaudal direction (1Hz) for 1000 cycles each at three consecutive load steps; 10-100 N, 10-200 N and 10-400 N. During cyclic loading subsidence of the implant relative to the vertebral body was measured in the unloaded condition. Cycle number at failure (defined as a subsidence of 2 mm) was determined for each specimen. A survival analysis (Cox Regression) was performed to detect differences between implant groups at a probability level of 95%.

RESULTS

High correlations were found between BMD and number of cycles until failure (MP1; r = 0.905, P = 0.013; MP2: r = 0.640, P = 0.121; MACS TL: r = 0.904, P = 0.013) and between load bearing cross sectional area and number of cycles until failure (MP1: r = 0.849, P = 0.032;MP2: r = 0.692, P = 0.085; MACS TL: r = 0.902, P = 0.014). Both Prototypes survived significantly longer than the MACS TL implant (MP1: P = 0.012, MP2: P = 0.014). The survival behaviour of MP1 and MP2 was not significantly different (P = 0.354).

CONCLUSIONS

Implant stability within each implant group was influenced by BMD and load bearing cross-sectional area. The angulation of the two screws did not have a significant influence on cut-out. As conclusion from this study, promising approaches for further implant development are: 1) increase of load-bearing cross-sectional area (e.g., larger outer diameter of the anchorage device), 2) screw positioning in areas of higher BMD (e.g., opposite cortex, proximity to pedicles or the endplates).

Primary stability of anterior lumbar stabilization: interdependence of implant type and endplate retention or removal

This is a comparative in vitro biomechanical study of the primary stability of an anterior lumbar interbody stabilization. The objective was to compare the stability of a interbody stabilizing titanium cage with and without the retention of the bordering vertebral endplates, as well as to compare the titanium cage with a tricalcium phosphate block when the endplates are removed. An adequate blood supply is critical for interbody fusion, which suggests surgical treatment of the bordering endplates. On the other hand, primary stability is improved by the retention of the endplates. Furthermore, bone substitute materials are finding more frequent use due to complications associated with autologous bone grafts. Ten bovine lumbar spine motion segments (average age 6 months) were investigated. Pure bending loadings as well as eccentric axial compression loadings were applied. A titanium cage and tricalcium phosphate block, were tested in conjunction with an anterior augmentation (MACS). Range of motion, neutral zone (NZ) and bending stiffness were measured under pure bending to 10 Nm, and bending stiffness under axial loads of up to 1,500 N. Range of motion of both implants in flexion-extension was significantly smaller than physiologic (cage without endplates 4.3 degrees , cage with 2.8 degrees , block without 3.4 degrees , and physiologic 6.6 degrees , all p<0.001). The cage with endplates and the block without endplates were both significantly stiffer than physiologic in all directions except left lateral bending. The block without endplates and the cage with endplates were both stiffer than the cage without endplates. The results suggest that the use of the bone substitute block provides better stability than the cage when the endplates are removed.

Comparative biomechanical testing of anterior and posterior stabilization procedures

STUDY DESIGN

This is a comparative in vitro biomechanical study in a calf lumbar spine model.

OBJECTIVES

The objective was to compare the primary stability of an anterior instrumentation, an intercorporal cage in combination with an anterior instrumentation, and a posterior instrumentation for monosegmental spondylodesis.

SUMMARY OF BACKGROUND DATA

Spondylodesis can be achieved through a posterior lumbar fusion, posterior lumbar intercorporal fusion, or an anterior lumbar intercorporal fusion. The posterior lumbar fusion is the gold standard, although the anterior approach offers some potential advantages to the transpedicular posterior techniques.

METHODS

Stability testing was performed on 30 calf lumbar spine motion segments in a physiologic state (n = 30), with either an isolated anterior (MACS) or posterior instrumentation (SOCON), and with an anterior instrumentation augmented with an intercorporal cage (MACS-Cage, n = 10, respectively). Range of motion, neutral zone, and bending stiffness were measured under pure bending to 10 Nm, and bending stiffness under axial loads of up to 1500 N.

RESULTS

The isolated posterior instrumentation was found to be more stable than the isolated or augmented anterior instrumentation in flexion/extension, although no significant differences were observed in lateral bending or axial rotation. The results of this biomechanical study suggest that an augmented anterior instrumentation provides similar stability for bony fusion as does the golden standard posterior instrumentation, with the exception of flexion/extension.

CONCLUSION

An augmented anterior instrumentation may provide similar stability for bony fusion as does the posterior instrumentation.

Range of motion in reconstruction situations following corpectomy in the lumbar spine: a question of bone mineral density?

STUDY DESIGN

In vitro biomechanical study to evaluate the stability of different types of instrumentation in the lumbar spine following corpectomy in relation to bone mineral density (BMD).

OBJECTIVES

To investigate the relation between the stability of a spinal instrumentation and BMD. To determine a threshold value of BMD allowing a single ventral instrumentation following corpectomy in the lumbar spine.

SUMMARY OF BACKGROUND DATA

Some in vitro studies determined the biomechanical properties of different spinal instrumentations in various spinal injury models. To the authors' knowledge, there are no published data available concerning stabilization in relation to BMD. A guideline for the treatment of a corpectomy depending on BMD would be helpful in order to choose the appropriate surgical method.

METHODS

Twenty-four fresh frozen human lumbar cadaveric spine specimens L1-L3 were used for testing of biomechanical properties. Plain radiographs were taken. BMD was determined using quantitative computed tomography (QCT). Testing in a 6 df loading device included native specimens and specimens after corpectomy of L2, restoration of the defect with a titanium cage, and two reconstruction situations: single ventral and additional dorsal instrumentation. Load-displacement curves and range of motion parameters were recorded and correlated with BMD.

RESULTS

A significant (P < 0.05) influence of BMD on range of motion was found. Single ventral instrumentation was critical concerning axial rotation. Combined dorsoventral instrumentation offered sufficient stability. The threshold value for use of single ventral instrumentation is a BMD > or = 0.22 g/cm.

CONCLUSIONS

Single ventral instrumentation can provide sufficient stability following corpectomy in the lumbar spine under the condition of a high BMD. Determination of BMD and the use of this guideline provides a valid tool for surgical planning.

Is a single anterolateral screw-plate fixation sufficient for the treatment of spinal fractures in the thoracolumbar junction? A biomechanical in vitro investigation

Controversy exists about the indications, advantages and disadvantages of various surgical techniques used for anterior interbody fusion of spinal fractures in the thoracolumbar junction. The purpose of this study was to evaluate the stabilizing effect of an anterolateral and thoracoscopically implantable screw-plate system. Six human bisegmental spinal units (T12-L2) were used for the biomechanical in vitro testing procedure. Each specimen was tested in three different scenarios: (1) intact spinal segments vs (2) monosegmental (T12/L1) anterolateral fixation (macsTL, Aesculap, Germany) with an interbody bone strut graft from the iliac crest after both partial corpectomy (L1) and discectomy (T12/L1) vs (3) bisegmental anterolateral instrumentation after extended partial corpectomy (L1), and bisegmental discectomy (T12/L1 and L1/L2). Specimens were loaded with an alternating, nondestructive maximum bending moment of +/-7.5 Nm in six directions: flexion/extension, right and left lateral bending, and right and left axial rotation. Motion analysis was performed by a contact-less three-dimensional optical measuring system. Segmental stiffness of the three different scenarios was evaluated by the relative alteration of the intervertebral angles in the three main anatomical planes. With each stabilization technique, the specimens were more rigid, compared with the intact spine, for flexion/extension (sagittal plane) as well as in left and right lateral bending (frontal plane). In these planes the bisegmental instrumentation compared to the monosegmental case had an even larger stiffening effect on the specimens. In contrast to these findings, axial rotation showed a modest increase of motion after bisegmental instrumentation. To conclude, the immobilization of monosegmental fractures in the thoracolumbar junction can be secured by means of bone grafting and the implant used in this study for all three anatomical planes. After bisegmental anterolateral stabilization a sufficient reduction of the movements was registered for flexion/extension and lateral bending. However, the observed slight increase of the range of motion in the transversal plane may lead to loosening of the implant before union. Therefore, the use of an additional dorsal fixation device should be considered.

Biomechanical in vitro comparison of different mono- and bisegmental anterior procedures with regard to the strategy for fracture stabilisation using minimally invasive techniques

Endoscopic minimally invasive techniques have become an established method of fracture stabilisation in the spine. In view of this fact, anterior stabilisation strategies must be reconsidered, as monosegmental A 3.1 compression fractures are increasingly being stabilised endoscopically from the anterior aspect using minimally invasive techniques. This study investigated the biomechanical necessity of anterior two-point or four-point stabilisation in the instrumentation of mono- and bisegmental fractures. In three biomechanical in vitro studies, burst fracture stabilisation was simulated, and anterior short fixation devices were tested under load with pure moments up to 3.75 Nm to evaluate the biomechanical stabilising characteristics of different kinds of instrumentations in flexion/extension, lateral bending, and axial rotation. Only anterior four-point stabilisation resulted in sufficient primary stability both in mono- and bisegmental instrumentation and therefore represents the standard procedure in open as well as in minimally invasive spinal surgery.

Anterior-Only Stabilization of Three-Column Thoracolumbar Injuries

OBJECTIVE

The optimal treatment of "unstable" thoracolumbar injuries remains controversial. Studies have shown the advantages of direct anterior decompression of thoracolumbar injuries along with supplemental posterior instrumentation as a combined or staged procedure. Others have also shown success in decompression as a single-stage anterior procedure, largely limited to two-column (anterior and middle) injuries. A retrospective review of all available clinical and radiographic data was used to classify unstable three-column thoracolumbar fractures according to the Association for the Study of Internal Fixation (AO) classification system. This was conducted to evaluate the efficacy of stand-alone anterior decompression and reconstruction of unstable three-column thoracolumbar injuries, utilizing current-generation anterior spinal instrumentation.

METHODS

Between 1992 and 1998, 40 patients underwent anterior decompression and two-segment anteriorly instrumented reconstruction for three-column thoracolumbar fractures. Retrospective review of all available clinical and radiographic data was used to classify these unstable injuries according to the AO classification system, evaluating for neurologic changes, spinal canal compromise, preoperative and postoperative segmental angulation, and arthrodesis rate.

RESULTS

According to the AO classification system, there were 24 (60%) type B1.2, 10 (25%) type B2.3, 5 (12.5%) type C1.3, and 1 (2.5%) type C2.1 three-column injuries. Preoperative canal compromise averaged 68.5% and vertebral height loss averaged 44.5%. There were no cases of neurologic deterioration, and 30 (91%) patients with incomplete neurologic deficits improved by at least one modified Frankel grade. Mean preoperative segmental kyphosis of 22.7 degrees was improved to an early mean of 7.4 degrees (P < 0.0001). At latest follow-up, angulation had increased by an average 2.1 degrees but maintained significant improvement from preoperative measurements (P < 0.0001). There was one early construct failure due to technical error. Thirty-seven of the remaining patients (95%) went on to apparently stable arthrodesis.

CONCLUSIONS

Current types of anterior spinal instrumentation and reconstruction techniques can allow some types of unstable three-column thoracolumbar injuries to be treated in an anterior stand-alone fashion. This allows direct anterior decompression of neural elements, improvement in segmental angulation, and acceptable rates of arthrodesis without the need for supplemental posterior instrumentation.

Influence of screw-cement enhancement on the stability of anterior thoracolumbar fracture stabilization with circumferential instability

The influence of additional dorsal structure damage on anterior stabilization of a thoracolumbar fracture is still unknown. Screw-cement enhancement can be used to reinforce the stability of anterior instrumentation. We have developed a new anchorage system for fixation of anterior stabilization devices, adapted through geometric optimization and the additional option of cementation after screw insertion. This study examines the question of whether this enhancement is strong enough to enable a single anterior procedure and still compensate for dorsal instability. Various spinal reconstruction procedures were evaluated biomechanically in an increasing ventrodorsal instability model for thoracolumbar fracture stabilization. A biomechanical in vitro study, simulating stabilized defect situations (corporectomy/vertebrectomy) with strut grafting and overbridging instrumentation, was performed on six human T10-L2 cadaveric specimens. The primary stability parameters, range of motion and neutral zone, were evaluated with or without anterior screw-cement enhancement. This was compared with a single conventional anterior stabilization without a dorsal defect (corporectomy). It was also compared with a single anterior, posterior or combined procedure in the presence of additional dorsal structure damage (vertebrectomy). The use of an additional cementable screw dowel enhanced the primary stability of the anterior instrumentation, compensating for dorsal instability. These results are warranted for the clinical use of minimally open or endoscopic techniques, creating the highest possible primary stability while performing a single anterior enhanced instrumentation with a tissue-preserving approach.

The management of acute thoracolumbar burst fractures with anterior corpectomy and Z-plate fixation

STUDY DESIGN

A retrospective review of a consecutive series of patients with acute thoracolumbar burst fractures who were surgically treated with an anterior corpectomy and fusion with anterolateral Z-plate fixation.

OBJECTIVES

To evaluate the clinical and radiographic success of the management of acute thoracolumbar burst fractures by corpectomy, structural grafting, and anterolateral internal fixation.

SUMMARY OF BACKGROUND DATA

Burst fractures are frequently associated with instability or neurologic deficit. Modern surgical procedures for these fractures have been performed via both anterior and posterior approaches. Anterior surgical treatment allows direct decompression of the neural elements and correction of deformity. Newer anterior instrumentation devices, combined with a structural graft, allow a stable construct that may obviate a posterior procedure. An anterior procedure generally requires fusion of only two levels compared to posterior fusion, which generally requires more.

METHODS

A retrospective review of a consecutive series of patients with thoracolumbar burst fractures treated with anterior surgery, strut graft, and fixation with a Z-plate was carried out. Fractures were considered acute if surgically treated within 30 days. Clinical and radiographic evaluation was performed on all 35 patients with acute thoracolumbar burst fractures. Surgical indications were incomplete neurologic deficit, segmental kyphotic deformity, or significant comminution. All patients with acute thoracolumbar burst fractures with spinal cord injury were treated with an intravenous steroid protocol and were operated on within 24 hours of admission unless medically precluded. Forty-six percent (16 of 35) of patients with acute thoracolumbar burst fractures presented with a neurologic deficit.

RESULTS

All 16 patients with neurologic deficit demonstrated at least one Frankel grade improvement on final observation, with 11 (69%) patients demonstrating complete neurologic recovery. Thirty-three patients were treated with anterolateral instrumentation only. Twenty-nine of thirty patients demonstrated radiographic healing. Five were lost to follow-up observation. One patient required subsequent posterior fusion for increasing kyphotic deformity. There were no instances of hardware failure. Sagittal alignment was improved from a mean preoperative kyphosis of 18 degrees to 6 degrees at final follow-up observation.

CONCLUSIONS

Anterior corpectomy, strut graft, and Z-plate fixation is an effective treatment for thoracolumbar burst fractures. It allows direct decompression of the spinal cord in the acute setting and was associated with a high rate of neurologic improvement, no instances of neurologic worsening in any case, and a low complication rate.

Solvent-preserved, bovine cancellous bone blocks used for reconstruction of thoracolumbar fractures in minimally invasive spinal surgery-first clinical results

We investigated the osseointegration of solvent-preserved, xenogenous cancellous bone blocks in the treatment of unstable fractures of the thoracolumbar junction. In 22 patients, the anterior repair procedure was performed by thoracoscopy or minimally invasive retroperitoneal surgery. Twenty-two patients had undergone monosegmental anterior fusion and were surveyed prospectively. Solvent-preserved, bovine cancellous bone blocks were used in 11 patients; iliac crest bone graft was used in the others. Follow-up after 12 months included CT scans, which revealed successful osseointegration in eight out of 11 patients who had received autogenous iliac crest bone grafts, while three patients showed a partial integration. There were no graft fragmentations. In patients who had received solvent-preserved, xenogenous cancellous bone blocks, complete osseointegration was achieved at the graft-bone interface in only two out of 11 cases, after 1 year. Partial integration was found in three patients. In view of these results, autogenous iliac crest bone grafts are still the unrivalled standard for defect repair in spinal surgery.

A biomechanical comparison of calf versus cadaver lumbar spine models

STUDY DESIGN

Biomechanical flexibility tests were performed using calf and human cadaveric lumbar spine models to investigate the effect of anatomic differences.

OBJECTIVES

The purpose is to determine if differences exist in biomechanical flexibility testing results between calf and human cadaveric spines when using identical methods and instrumentation.

SUMMARY OF BACKGROUND DATA

Calf spines are commonly used in biomechanical research as a substitute for human cadaveric spines in an attempt to reduce expense and specimen variability. Despite widespread use, the validity of this model has not been thoroughly investigated.

METHODS

Five fresh calf spines and five human cadaveric spines (L2-L5) were used for nondestructive biomechanical flexibility testing. Maximum moments of 6.4 Nm were achieved in five increments of 1.6 Nm. The rotations of L3 with respect to L4 were measured in 5 cases: 1) intact; 2) following partial discectomy, including partial laminectomy and partial facetectomy; 3) partial discectomy with pedicle screw instrumentation; 4) total discectomy with pedicle screw instrumentation; and 5) pedicle screw instrumentation with interbody graft. Rotational angles were normalized to the intact case to determine the stabilizing effect during each testing case. Data were analyzed using analysis of variance to determine if significant differences existed between the calf spine results and the human cadaveric spine results.

RESULTS

In both models, motion increased following discectomy, decreased with instrumentation, and increased with total discectomy. Placement of the interbody graft decreased motion during axial rotation, flexion, and extension but increased lateral bending motion. A two-way analysis of variance revealed no significant differences in the two models during flexion or extension (P > 0.05), but significant differences were discovered in axial rotation and lateral bending (P < 0.05).

CONCLUSIONS

Significant differences were identified in flexibility testing between calf and human cadaveric specimens. The calf spine model overestimated the stabilizing effect of instrumentation during lateral bending and underestimated stability during axial rotation. The extrapolation of calf spine data to the in vivo case, especially during axial rotation and lateral bending, should carefully consider the variation between these two models.

Thoracolumbar fracture stabilization: comparative biomechanical evaluation of a new video-assisted implantable system

Minimally invasive techniques for spinal surgery are becoming more widespread as improved technologies are developed. Stabilization plays an important role in fracture treatment, but appropriate instrumentation systems for endoscopic circumstances are lacking. Therefore a new thoracoscopically implantable stabilization system for thoracolumbar fracture treatment was developed and its biomechanical in vitro properties were compared. In a biomechanical in vitro study, burst fracture stabilization was simulated and anterior short fixation devices were tested under load with pure moments to evaluate the biomechanical stabilizing characteristics of the new system in comparison with a currently available system. With interbody graft and fixation the new system demonstrated higher stabilizing effects in flexion/extension and lateral bending and restored axial stability beyond the intact spine, as well as having comparable or improved effects compared with the current system. Because of this biomechanical characterization a clinical trial is warranted; the usefulness of the new system has already been demonstrated in 45 patients in our department and more than 300 cases in a multicenter study which is currently under way.

Biomechanical evaluation of contemporary posterior spinal internal fixation configurations in an unstable burst-fracture calf spine model: special references of hook configurations and pedicle screws

STUDY DESIGN

This study attempts to determine the most biomechanically rigid posterior spinal instrumentation configuration in a burst-fracture calf spine model.

OBJECTIVES

To compare the biomechanical stability of contemporary posterior spinal instrumentation in various hook and screw configurations in an unstable calf spine model.

SUMMARY OF BACKGROUND DATA

Burst-fractures are relatively common injuries seen in the setting of spinal trauma. The use of posterior-only configurations in the treatment of this deformity has become a much more popular approach because of the relative ease of applying the instrumentation.

METHODS

Fresh frozen in vitro study using 10 calf spines involving the T11-S1 vertebral segments. Pure moment forces including flexion, extension, axial rotation, and lateral bending were applied to the top of the spinal column at T11. Testing was first performed on all intact specimens. A corpectomy was then performed at L2. Testing was then repeated on each of the ten specimens after internal fixation with different posterior spinal configurations using ISOLA instrumentation (DePuy AcroMed Inc., Raynham, MA).

RESULTS

With regards to flexion-extension and lateral bending, all configurations except for distraction hook-rod construct provided stability greater than the intact spine. The distraction hook-rod configuration failed to control extension (P > 0.05) above the intact specimen. All pedicle screw constructs were more rigid than the hook-rod constructs in axial rotation at the level of injury (P < 0.001).

CONCLUSIONS

The motion segment at the corpectomy site is adequately stabilized by contemporary spinal internal fixation configurations tested except for the distraction-hook stabilization. Axial rotation is generally poorly controlled by posterior-only internal fixation. Pedicle screw instrumentation was the most rigid compared with other forms of stabilization in stabilizing a burst-corpectomy defect. Based on this study, pedicle screw configurations are preferred over hook-rod strategies in the posterior stabilization of a burst-corpectomy anterior defect. Among hook-rod configurations, the distraction hook-rod strategy provided the least stability.

Thoracolumbar fracture management: anterior approach

The surgeon who treats patients with spine trauma must be able to apply a variety of management techniques to achieve optimal care of the patient. The anterior surgical approach is appropriate for some thoracolumbar burst fractures in patients with neurologic deficit and without posterior ligamentous injury. Surgery is most often indicated for patients with incomplete deficit, especially those with a large retropulsed fragment, marked canal compromise, severe anterior comminution, or kyphosis <30 degrees. This approach provides excellent visualization of the anterior aspect of the dura mater for decompression. Reconstruction of the anterior body defect can be done with autograft, allograft, or a cage. Supplementation of the graft with anterior internal fixation helps prevent kyphosis. Clinical results demonstrate improved neurologic function in most patients as well as low pseudarthrosis rates. In patients with incomplete deficit, improvement in neurologic function usually can be expected with few complications.

Posterior thoracic extrapedicular fixation: a biomechanical study

STUDY DESIGN

In vitro biomechanical testing of thoracic spine specimens using a standardized three-dimensional spine flexibility protocol.

OBJECTIVES

To compare the mechanical stability of the intrapedicular and extrapedicular technique for pedicle screw placement. The hypothesis was that extrapedicular screw placement provides an equally rigid construct.

SUMMARY OF BACKGROUND DATA

Pedicle screws provide rigid fixation of instabilities in the lumbar and lumbosacral spine. Anatomic considerations and the potential risk of neurologic complications are the main reasons to hesitate using pedicle screws in the thoracic spine. Extrapedicular fixation would allow safer insertion due to an increased distance to the spinal canal.

METHODS

Twelve human cadaveric thoracic spines (six intra-, six extrapedicular) were instrumented with the USS system, using computed tomography-based computer navigation to ensure accurate placement. The specimens were tested in flexion-extension, torsion, and lateral bending. The ROM was measured using an optoelectronic system, and the two methods were compared before and after implantation of the USS construct and before and after fatigue testing of the construct.

RESULTS

The ROM of the instrumented spine was reduced to less than 50% that of its original ROM. There were no statistically significant differences in the ROM reduction between the intra- and the extrapedicular technique. Cyclic fatiguing of the construct did not significantly increase the ROM.

CONCLUSIONS

The extrapedicular technique provides a construct for stabilization of the thoracic spine that is as rigid as the conventional intrapedicular technique, but has the advantage of a safer surgical screw insertion.

Axial compression force measurement acting across the strut graft in thoracolumbar instrumentation testing

OBJECTIVE

Current recommendations for spinal implant testing do not consider the determination of axial compression forces of the overbridging implant on the strut graft. No direct data exist on the influence of load transfer through the strut graft and of the kind of instrumentation, especially in thoracolumbar corpectomy models.

DESIGN

Therefore in this biomechanical in vitro study a method for measurement of the axial compression force acting across the strut graft in different thoracolumbar instrumentations was developed.

METHODS

In this in vitro study, a corpectomy model was simulated and anterior, posterior and combined short fixation devices currently available were tested under pure moments to evaluate their biomechanical stabilizing characteristics. Range of motion, neutral zone and the axial compressive force acting on the strut graft were measured continuously in the three primary directions.

RESULTS

Without loads, the combined stabilization and followed by anterior instrumentation created a higher axial compression force than the dorsal instrumentation on the strut graft. Especially during maximal extension there was no axial compression of the dorsal instrumentation on the strut graft, which resulted in an increase of the range of motion.

CONCLUSION

The feasibility of the new method was demonstrated in this study. For the purpose of standardization and comparison it should be considered in spinal implant testing.

Enhanced primary stability through additional cementable cannulated rescue screw for anterior thoracolumbar plate application

OBJECT

The authors conducted a study to investigate the biomechanical in vitro influence of a new anchorage system for fixation of anterior stabilization devices and the possibility of using additional cement after screw insertion to compensate for poor bone quality. The incidence of osteoporosis-related fractures has increased nearly twofold in the last decade. Because of problems associated with anterior screw fixation such as loosening, mechanical failure, and the weakness of osteoporotic bone, current surgical treatments of vertebral body (VB) fractures are problematic. This is due to poor fixation strength of anterior screws in the adjacent segments. The aim of this study was to determine whether a new cemented and uncemented VB screw provides improved primary stability following placement of anterior instrumentation in cases of fracture.

METHODS

The primary stability-related parameters of a new uncemented/cemented screw were compared with those of conventional monocortical screw fixation in a burst fracture model in which strut graft and anterior overbridging instrumentation were used. The use of the new uncemented screw improved the range of motion (ROM) of the stabilized spine in flexion-extension by approximately 22%, in rotation by 20%, and in lateral bending by 15%. Additional cementation improved the ROM by approximately 41% in flexion-extension, 32% in rotation, and 30% in lateral bending compared with conventional monocortical screw fixation.

CONCLUSIONS

The new cannulated screw improves fixation strength and primary stability parameters. It is useful in the initial treatment of fractures in cases of poor bone quality and as a rescue device if previously inserted screws do not remain securely in place.

In vitro biomechanical studies of an anterior thoracolumbar implant

After L1 corpectomy in T11-L3 human cadaveric spine, anterior thoracolumbar instrumentation with strut grafting restores spinal stability. T12-L2 angular rotation was measured in response to moments of 0.0, 1.5, 3.0, 4.5, and 6.0 Nm in flexion, extension, lateral bending, and axial rotation, respectively. The spines were tested: 1) intact; 2) after partial L1 corpectomy, grafting, and instrumentation (Profile plate, DePuy-AcroMed, Raynham, MA), with the wooden dowel graft screwed to the plate; 3) without graft screw fixation; and 4) after flexion-extension cyclic fatiguing for 5000 cycles at a load of +/-3.0 Nm. Before and after fatiguing, the instrumented spine was significantly (p <or= 0.05) stiffer than the intact spine in flexion, extension, and right and left lateral bending but not in axial rotation. There were no significant differences between the constructs with or without graft-to-plate fixation before or after fatigue. The instrumented spines were more rigid in bending away from the implant than bending toward the implant. Anterior spinal instrumentation with the Profile implant augments stiffness in the sagittal and coronal planes but not in the axial plane. Although graft-to-plate fixation may prevent graft migration into the canal, it does not contribute to spinal rigidity.

Current concepts in anterior surgery for thoracolumbar trauma

Our goals in writing this article are to facilitate understanding of issues related to (1) why anterior fixation for thoracolumbar fractures are an important tool for managing these injuries, (2) when to perform these as a single procedure, or in combination with other procedures such as vertebrectomy and/or posterior stabilization and fusion, (3) to appreciate the biomechanical and design-related issues of available systems, and (4) what the clinical outcomes are following these procedures.

The use of allograft bone and cages in fractures of the cervical, thoracic, and lumbar spine

The advantages of allograft tissues and cage devices in anterior spinal reconstruction for trauma is the absence or minimization of donor site morbidity and unlimited choices of graft shapes and sizes. Osteoinductive matrices often are added to these grafting alternatives to improve healing rate and success. Allografts and cages seem to be the most frequent grafting materials used in the thoracolumbar region. Currently, autologous iliac crest is the most popular grafting material in the cervical region although cylindrical cages are gaining popularity. As the material properties of cage devices improve to better match the modulus of elasticity of host vertebral bone, their frequency of use undoubtedly will increase in spinal trauma and other spinal disorders.

Biomechanical evaluation of diagonal fixation in pedicle screw instrumentation

STUDY DESIGN

Flexibility tests and finite element analyses were performed for the biomechanical evaluation of diagonal transfixation in pedicle screw instrumentation.

OBJECTIVE

To assess the biomechanical advantages of diagonal transfixation compared with conventional horizontal transfixation.

SUMMARY AND BACKGROUND DATA

A few pedicle screw instrumentation systems allow the use of cross-links in the diagonal direction. Such a diagonal transfixation is anticipated to improve the surgical construct stability, but its biomechanical qualities have not been completely evaluated.

METHODS

Flexibility tests were performed on 10 calf lumbar spines (L2-L5). Specimens were subjected to pure moments up to 8.2 Nm in flexion, extension, lateral bending, and extension while the resulting movements of L3 and L4 were measured by a three-dimensional motion analysis system. The tested cases included (1) intact, (2) pedicle screw fixation without transfixation after total removal of the L3-L4 disc, (3) pedicle screw fixation with diagonal transfixation, and (4) pedicle screw fixation with horizontal transfixation. Three-dimensional finite element models of the tested surgical constructs were also developed by use of three-dimensional beam elements to investigate the effect of diagonal transfixation and horizontal transfixation on the construct stability and the corresponding stress changes in the screws.

RESULTS

When compared with no transfixation, horizontal transfixation significantly improved the lateral bending and axial rotation stability by 15.7% and 13.9%, respectively, but there was no improvement of stability in flexion and extension. By contrast, diagonal transfixation significantly improved the flexion and extension stability by 12% and 10.7%, respectively, but not the lateral bending and axial rotation stability in comparison with no transfixation. Comparison between horizontal transfixation and diagonal transfixation showed that the stabilizing effect of diagonal transfixation was greater in flexion and extension (13% and 11%, P < 0.01) than that of horizontal transfixation but smaller in lateral bending (11%, P < 0.05) and axial rotation (6.6%, P > 0.1). Finite element model predictions of the motion changes were similar to the changes observed in flexibility tests. In horizontal transfixation, the load changes, compared with no transfixion, were a 0.02% increase in flexion-extension, a 27.5% increase in lateral bending, and a 58% decrease in axial rotation, and the magnitudes of the moments applied on both the right and left pedicle screws were identical. However, when diagonal transfixation was achieved by connecting the left superior screw and the right inferior screw, the loads in the left screw were increased by 11.5% in flexion-extension, 43.6% in lateral bending, and 7.9% in axial rotation, whereas the loads in the right screw were decreased by 10.9% in flexion-extension, increased by 0.06% in lateral bending, and decreased by 18.1% in axial rotation.

CONCLUSIONS

The results of this study showed that diagonal transfixation provides more rigid fixation in flexion and extension but less in lateral bending and axial rotation in comparison with horizontal transfixation. Furthermore, greater stresses in the pedicle screws were predicted in the diagonal transfixation model. These limitations of diagonal transfixation should be considered carefully for clinical application.

Pedicle screw adjustments affect stability of thoracolumbar burst fracture

STUDY DESIGN

An in vitro biomechanical study of the stabilizing effect of pedicle screw instrumentation on experimental thoracolumbar burst fractures.

OBJECTIVES

To evaluate the effects of different adjustments applied by the pedicle screw fixation device on the stability of the spine-device construct.

SUMMARY OF BACKGROUND DATA

Pedicle screw devices are widely used to accomplish spinal reduction and provide stability to an injured spine. In previous biomechanical studies the stability of the spine-device constructs has been examined for many devices. However, no study has quantitatively assessed the associations between the device adjustments and the stability of the construct.

METHODS

Five-vertebrae human cadaveric specimens with burst fracture at L1 vertebra were studied. Pedicle screw fixation device was attached to the T12 and L2 vertebrae. Five device adjustments (pure compression, pure distraction, pure extension, a combination distraction-extension, and neutral posture) were studied. Multidirectional flexibility test was performed when intact, after burst fracture, and after each device adjustment to document spinal stability.

RESULTS

The construct stability had a complex association to the device adjustment. For example, the maximum flexion and extension stabilities were achieved by pure compression and distraction-extension combination adjustments, respectively. Pure distraction and pure extension adjustments decreased the construct stability.

CONCLUSIONS

The device adjustments affected the spinal construct stability differently in different directions. Although pure compression provided the most stability in most directions, the combined distraction-extension adjustment may be more suitable considering the neural decompression also.

In vitro biomechanical analysis of three anterior thoracolumbar implants

OBJECT

The goal of this study was to evaluate the comparative efficacy of three commonly used anterior thoracolumbar implants: the anterior thoracolumbar locking plate (ATLP), the smooth-rod Kaneda (SRK), and the Z-plate.

METHODS

In vitro testing was performed using the T9-L3 segments of human cadaver spines. An L-1 corpectomy was performed, and stabilization was achieved using one of three anterior devices: the ATLP in nine spines, the SRK in 10, and the Z-plate in 10. Specimens were load tested with 1.5-, 3-, 4.5-, and 6-Nm in flexion and extension, right and left lateral bending, and right and left axial rotation. Angular motion was monitored using two video cameras that tracked light-emitting diodes attached to the vertebral bodies. Testing was performed in the intact state in spines stabilized with one of the three aforementioned devices after the devices had been fatigued to 5000 cycles at +/- 3 Nm and after bilateral facetectomy. There was no difference in the stability of the intact spines with use of the three devices. There were no differences between the SRK- and Z-plate-instrumented spines in any state. In extension testing, the mean angular rotation (+/- standard deviation) of spines instrumented with the SRK (4.7 +/- 3.2 degrees) and Z-plate devices (3.3 +/- 2.3 degrees) was more rigid than that observed in the ATLP-stabilized spines (9 +/- 4.8 degrees). In flexion testing after induction of fatigue, however, only the SRK (4.2 +/- 3.2 degrees) was stiffer than the ATLP (8.9 +/- 4.9 degrees). Also, in extension postfatigue, only the SRK (2.4 +/- 3.4 degrees) provided more rigid fixation than the ATLP (6.4 +/- 2.9 degrees). All three devices were equally unstable after bilateral facetectomy. The SRK and Z-plate anterior thoracolumbar implants were both more rigid than the ATLP, and of the former two the SRK was stiffer.

CONCLUSIONS

The authors' results suggest that in cases in which profile and ease of application are not of paramount importance, the SRK has an advantage over the other two tested implants in achieving rigid fixation immediately postoperatively.

Biomechanical properties of anterior thoracolumbar multisegmental fixation: an analysis of construct stiffness and screw-rod strain

STUDY DESIGN

Three types of anterior thoracolumbar multisegmental fixation were biomechanically compared in construct stiffness and rod-screw strain.

OBJECTIVES

To investigate the effects of rod diameter and rod number on construct stiffness and rod-screw strain in anterior thoracolumbar multisegmental instrumentation.

SUMMARY OF BACKGROUND DATA

No studies have been undertaken to investigate the biomechanical effects of rod diameter and rod number in thoracolumbar anterior instrumentation.

METHODS

Ten fresh-frozen calf spines (T13-L5) were used. After intact analysis, a total discectomy and transection of the ALL and PLL were performed at L1-L2, L2-L3, and L3-L4 with intervertebral reconstruction using carbon fiber cages. Three types of anterior fixation were then performed at L1-L4: 1) 4.75-mm diameter single-rod, 2) 4.75-mm dual-rod, and 3) 6.35-mm single-rod systems. Single screws at each vertebra were used for single-rod and two screws for dual-rod fixation. These systems share the same basic design except rod diameter. Nondestructive biomechanical testing was performed and included compression, torsion, flexion-extension, and lateral bending. Construct stiffness and rod-screw strain of the three reconstructions were compared.

RESULTS

The 6.35-mm single-rod fixation significantly improved construct stiffness compared with the 4.75-mm single rod fixation only under torsion (P < 0.05). The 4. 75-mm dual rod construct resulted in significantly higher stiffness than did both single-rod fixations (P < 0.05), except under compression. No statistical differences were observed in rod-screw strain between the two types of single rods, whereas dual-rod reconstruction exhibited less rod-screw strain (P < 0.05).

CONCLUSIONS

For single-rod fixation, increased rod diameter neither markedly improved construct stiffness nor affected rod-screw strain, indicating the limitations of a single-rod system. In thoracolumbar anterior multisegmental instrumentation, the dual-rod fixation provides higher construct stiffness and less rod-screw strain compared with single-rod fixation.

Biomechanical effect of anterior grafting devices on the rotational stability of spinal constructs

In the thoracolumbar spine, frequently strut grafting is used to restore the anterior and middle column defects. Biomechanical stability of the surgical construct may be altered significantly depending on the type of anterior grafting devices. In this study, a biomechanical flexibility test was conducted to compare the stabilizing role of various types of anterior grafting devices, such as a polymethylmethacrylate block, tricortical iliac crest bone graft, one large Harms cage, and two small Harms cages using a calf lumbar corpectomy model. The Harms cage, especially one large cage, improved the axial rotational stability significantly in both anterior and posterior fixation groups as compared with the iliac bone or polymethylmethacrylate. No significant difference in the stabilizing role was found among different grafting devices in lateral bending, flexion, and extension. These results suggest that a more rigid spinal construct can be obtained by using a metal cage with improved friction at the cage-bone interface.

Biomechanical studies of a dynamized anterior thoracolumbar implant

STUDY DESIGN

An in vitro investigation into the biomechanical properties of a dynamized anterolateral compression implant that allows controlled subsidence.

OBJECTIVES

To determine the extent to which both modes of the anterolateral compression implant (controlled collapsing and rigid) are able to reestablish the stability of the lumbar spine after L4 corpectomy.

SUMMARY OF BACKGROUND DATA

Over time, anterior and posterior spinal implants have been associated with progressive angulation, and occasionally implant failure and breakage. To circumvent this occurrence and provide better graft loading, dynamized or collapsing devices for clinical use have been developed.

METHODS

Eight fresh calf spines (L1-L6) were placed in a biomechanical testing frame. Pure moments of 6 Nm were loaded onto the intact spine in six directions: flexion, extension, right and left lateral bending, and right and left axial rotation. A total L4 corpectomy then was performed, and the defect grafted with a wooden dowel. Loading was repeated after the specimens were stabilized using the two modes of the anterolateral compression implant in succession.

RESULTS

The results showed that both modes of the implant (the rigid mode in particular) restore the stiffness of the unstable spine to normal levels of flexion, extension, and right and left lateral bending, even to levels exceeding normal. These devices, however, fall short of achieving normal stability in right and left axial rotation.

CONCLUSION

In the cadaveric calf spine after L4 corpectomy, restoration of stability with a dynamized anterior spinal implant is possible in flexion, extension, and right and left lateral bending, but not in axial rotation.

The stability of reconstruction methods after thoracolumbar total spondylectomy. An in vitro investigation

STUDY DESIGN

After total spondylectomy, five types of spinal reconstruction techniques were compared biomechanically.

OBJECTIVES

To evaluate the stability provided by five reconstruction methods after total spondylectomy.

SUMMARY OF BACKGROUND DATA

Total spondylectomy presents a worst-case scenario for spinal reconstruction. However, few investigators have biomechanically investigated spinal reconstruction stability after total spondylectomy.

METHODS

Eight human cadaveric spines (T11-L5) were used. After intact analysis, a total spondylectomy was performed at L2 and reconstructed using Harms titanium mesh (Depuy-Motech, Warsaw, IN) as an anterior strut. Anterior, posterior, or circumferential instrumentation techniques were then performed using the Kaneda SR and ISOLA pedicle screw systems (AcroMed Corp., Cleveland, OH) as follows: 1) anterior instrumentation at L1-L3 with multisegmental posterior instrumentation at T12-L4 (AMP), 2) anterior instrumentation at L1-L3 with short posterior instrumentation at L1-L3 (ASP), 3) anterior instrumentation at L1-L3 (A), 4) multilevel posterior instrumentation at T12-L4 (MP), and 5) short posterior instrumentation at L1-L3 (SP). Nondestructive biomechanical testing was performed under axial compression, flexion-extension, and lateral bending loading modes.

RESULTS

Only circumferential instrumentation techniques (AMP, ASP) exhibited higher stiffness than the intact spine in all loading modes (P < 0.05). Short circumferential fixation provided more stability than did multilevel posterior instrumentation (P < 0.05). Multilevel posterior fixation provided more stiffness than did short posterior and anterior instrumentation alone (P < 0.05).

CONCLUSIONS

Only circumferential fixation techniques provide more stability than the intact spine in all testing modes. Short circumferential instrumentation provides more stability than multilevel posterior instrumentation alone and requires fewer levels of spinal fusion.

Static and fatigue biomechanical properties of anterior thoracolumbar instrumentation systems. A synthetic testing model

STUDY DESIGN

A mechanical testing standard for anterior thoracolumbar instrumentation systems was introduced, using a synthetic model. Twelve recent instrumentation systems were tested in static and fatigue modes.

OBJECTIVES

To establish the testing standard for anterior thoracolumbar instrumentation systems using a synthetic model and to evaluate the static and fatigue biomechanical properties of 12 anterior thoracolumbar instrumentation systems.

SUMMARY OF BACKGROUND DATA

Although numerous studies have been performed to evaluate the biomechanics of anterior spinal instrumentation using a cadaveric or animal tissue, problems of specimen variation, lack of reproducibility, and inability to perform fatigue testing have been pointed out. In no studies has a precise synthetic testing standard for anterior thoracolumbar instrumentation systems been described.

METHODS

An ultra-high-molecular-weight polyethylene cylinder was designed according to the anatomic dimensions of the vertebral body. Two cylinders spanned by spinal instrumentation simulated a total corpectomy defect, and a compressive lateral bending load was applied. The instrumentation assembly was precisely standardized. The static destructive and fatigue tests up to 2 million cycles at three load levels were conducted, followed by the failure mode analysis. Twelve anterior instrumentation systems, consisting of five plate and seven rod systems were compared in stiffness, bending strength, and cycles to failure.

RESULTS

Static and fatigue test parameters both demonstrated highly significant differences between devices. The stiffness ranged from 280.5 kN/m in the Synthes plate (Synthes, Paoli, PA) to 67.9 kN/m in the Z-plate ATL (SofamorDanek, Memphis, TN). The Synthes plate and Kaneda SR titanium (AcroMed, Cleveland, OH) formed the highest subset in bending strength of 1516.1 N and 1209.9 N, respectively, whereas the Z-plate showed the lowest value of 407.3 N. There were no substantial differences between plate and rod devices. In fatigue, only three systems: Synthes plate, Kaneda SR titanium, and Olerud plate (Nord Opedic AB, Sweden) withstood 2 million cycles at 600 N. The failure mode analysis demonstrated plate or bolt fractures in plate systems and rod fractures in rod systems.

CONCLUSIONS

The biomechanical testing standard for anterior thoracolumbar instrumentation systems was successfully designed. It provided a repeatable and consistent experimental condition and controlling dimensional and surgical factors. The comparison of 12 instrumentation systems highlights the importance of mechanically balanced device design without a weak link in the development of instrumentation.

Biomechanical studies on two anterior thoracolumbar implants in cadaveric spines

STUDY DESIGN

A biomechanical comparison of two commonly used anterior spinal devices: the Smooth Rod Kaneda and the Synthes Anterior Thoracolumbar Spinal Plate.

OBJECTIVES

To compare the stability imparted to the human cadaveric spine by the Smooth Rod Kaneda and Synthes Anterior Spinal Plate, and to assess how well these devices withstand fatigue and uni- and bilateral facetectomy.

SUMMARY OF BACKGROUND DATA

Biomechanical studies on the aforementioned and similar devices have been performed using synthetic, porcine, calf, or dog spines. As of the time of this writing, studies comparing anterior spinal implants using human cadaveric spines are scarce.

METHODS

An L1 corpectomy was performed on 19 spines. Stabilization was accomplished by an interbody wooden graft and the application of the Smooth Rod Kaneda in 10 spines and the Synthes Anterior Spinal Plate in the remaining 9. Biomechanical testing of the spines was performed in six degrees of freedom before and after stabilization, and after fatiguing to 5000 cycles of +/- 3 Nm of flexion and extension. Testing was repeated after uni- and bilateral facetectomy.

RESULTS

After stabilization, the Smooth Rod Kaneda was significantly more rigid than the anterior thoracolumbar bar spinal plate in extension. After fatigue, the Smooth Rod Kaneda was significantly stiffer than the anterior thoracolumbar spinal plate in flexion, extension, right lateral bending, left lateral bending, and right axial rotation. A significant decrease in stiffness was noted with the Synthes device in flexion after bilateral facetectomy compared with the stabilized spine.

CONCLUSIONS

The smooth Rod Kaneda device tends to be stiffer than the anterior thoracolumbar spinal plate, particularly in extension, exceeding the anterior thoracolumbar spinal plate in fatigue tolerance. The spine stabilized with the anterior thoracolumbar spinal plate is more susceptible to the destabilizing effect of bilateral facetectomy than than that stabilized with the Smooth Rod Kaneda. The additional rigidity encountered with the Smooth Rod Kaneda must be weighed against the simplicity of anterior thoracolumbar spinal plate application.

Changes in cadaveric cancellous vertebral bone strength in relation to time. A biomechanical investigation

STUDY DESIGN

A biomechanical study was conducted during a 3.5-day period to test for changes occurring in pullout strengths of cancellous screws inserted into human cadaveric vertebral bodies.

OBJECTIVES

To quantify, within the testing time of 3.5 days, the possible changes to the mechanical properties of cadaveric vertebral bodies, resulting from structural degradation caused by postmortem, time-dependent, autolytic processes during mechanical testing of implant-bone biomechanics.

SUMMARY OF BACKGROUND DATA

Biomechanical testing of whole spinal implants and analysis of the screw-bone interface of spinal implants is an area of clinical interest that frequently requires the use of cadaveric spine specimens. Changes in vertebral bone properties during the testing period may invalidate experimental results, but no data are available on degradation of bone during the testing period.

METHODS

Anterior oblique cancellous screws were inserted into human vertebral bodies from which the ventral cortex had been removed. The pullout strength was measured at 0, 24, 60, and 84 hours after insertion. The tests were performed on 48 human vertebral bodies, which were stored by freezing to -23 C, thawed for testing, and kept at room temperature during the testing time for as long as 84 hours.

RESULTS

The axial pullout strength showed no statistically significant change during 84 hours (P = 0.15). There were no significant differences attributable to vertebral level from T4 to L4, probably because the ventral cortices had been removed (P = 0.7).

CONCLUSIONS

During 3.5 days, there were no changes in pullout strength of vertebral cancellous bone. In biomechanical studies during a maximum period of 3 days with a small number of cadaveric spines (e.g., four spine specimen) the time-dependent changes in pullout strength play a less significant role than do the interspine differences. Interspine differences should be regarded as an important factor to be considered in the design of biomechanical tests.

Stability potential of spinal instrumentations in tumor vertebral body replacement surgery

STUDY DESIGN

The multidirectional stability potential of anterior, posterior, and combined instrumentations applied at L1-L3 was studied after L2 corpectomy and replacement with a carbon-fiber implant.

OBJECTIVES

To evaluate the biomechanical characteristics of short-segment anterior, posterior, and combined instrumentations in lumbar spine tumor vertebral body replacement surgery.

SUMMARY OF BACKGROUND DATA

The biomechanical properties of many different spinal instrumentations have been studied in various spinal injury models. Only a few studies, however, investigate the stabilization methods in spinal tumor vertebral body replacement surgery.

METHODS

Eight fresh frozen human cadaveric thoracolumbar spine specimens (T12-L4) were prepared for biomechanical testing. Pure moments (2.5 Nm, 5 Nm, and 7.5 Nm) of flexion-extension, left-right axial torsion, and left-right lateral bending were applied to the top vertebra in a flexibility machine, and the motions of the L1 vertebra with respect to L3 were recorded with an optoelectronic motion measurement system after reconditioning. The L2 vertebral body was resected and replaced by a carbon-fiber cage. Different fixation methods were applied to the L1 and L3 vertebrae. One anterior, two posterior, and two combined instrumentations were tested. Load-displacement curves were recorded and neutral zone and range of motion parameters were determined.

RESULTS

The anterior instrumentation provided less potential stability than the posterior and combined instrumentations in all motion directions. The anterior instrumentation, after vertebral body replacement, showed greater motion than the intact spine, especially in axial torsion (range of motion, 10.3 degrees vs 5.5 degrees; neutral zone, 2.9 degrees vs. 0.7 degrees; P < 0.05). Posterior instrumentation provided greater rigidity than the anterior instrumentation, especially in flexion-extension (range of motion, 2.1 degrees vs. 12.6 degrees; neutral zone, 0.6 degrees vs. 6.1 degrees; P < 0.05). The combined instrumentation provided superior rigidity in all directions compared with all other instrumentations.

CONCLUSIONS

Posterior and combined instrumentations provided greater rigidity than anterior instrumentation. Anterior instrumentation should not be used alone in vertebral body replacement.

Biomechanical evaluation of anterior and posterior fixations in an unstable calf spine model

STUDY DESIGN

Fresh calf lumbar spines were used to perform flexibility tests in multiple loading directions to compare the stabilizing effects of anterior and posterior rigid instrumentations.

OBJECTIVE

To compare the biomechanical flexibility of anterior and posterior instrumentation constructs using an unstable calf spine model.

SUMMARY OF BACKGROUND DATA

Unstable burst fractures of the thoracolumbar spine can be managed anteriorly or posteriorly. Controversy persists, however, on the merit of anterior fixation versus that of posterior fixation in terms of how much stability can be achieved.

METHODS

Fifteen fresh calf spines (L2-L5) were loaded with pure unconstrained moments in flexion, extension, axial rotation, and lateral bending directions. After removal of L3-L4 disc and endplates to create an 1.5-cm anterior and middle column defect, testing was performed on five specimens after anterior Kaneda rod fixation, anterior University Plate fixation, or posterior ISOLA pedicle screw fixation (AcroMed, Cleveland, OH). Testing was repeated after inserting a polymethylmethacrylate block to stimulate an interbody anterior graft with instrumentation.

RESULTS

All fixation devices provided a significant stabilizing effect in flexion and lateral bending. In extension, all constructs except ISOLA (AcroMed) without graft were stiffer than the intact specimen. In axial rotation with no graft, only the Kaneda device significantly reduced the flexibility from that of the intact specimen. The interbody graft provided additional rigidity to the ISOLA (AcroMed) instrumentation construct in flexion and extension and to the Kaneda construct in lateral bending. There was no significant effect of grafting in axial rotation.

CONCLUSIONS

A short, transpedicular instrumentation, such as ISOLA (AcroMed), provided less rigid fixation in flexion and extension without the anterior structural graft. The Kaneda rod and University plate with grafting provided a significant stabilizing effect in all directions compared with the intact specimen. When no graft was inserted, the Kaneda device was more effective in preventing axial rotation than the other devices. In lateral bending, the University plate provided more rigid fixation than the Kaneda device without grafting.

Anterior decompression and stabilization with the Kaneda device for thoracolumbar burst fractures associated with neurological deficits

One hundred and fifty consecutive patients who had a burst fracture of the thoracolumbar spine and associated neurological deficits were managed with a single-stage anterior spinal decompression, strut-grafting, and Kaneda spinal instrumentation. At a mean of eight years (range, five years to twelve years and eleven months) after the operation, radiographs showed successful fusion of the injured spinal segment in 140 patients (93 per cent). Ten patients had a pseudarthrosis, and all were managed successfully with posterior spinal instrumentation and a posterolateral arthrodesis. The percentage of the canal that was obstructed, as measured on computed tomography, improved from a preoperative mean of 47 per cent (range, 24 to 92 per cent) to a postoperative mean of 2 per cent (range, 0 to 8 per cent). Despite breakage of the Kaneda device in nine patients, removal of the implant was not necessary in any patient. None of the patients had iatrogenic neurological deficits. After the anterior decompression, the neurological function of 142 (95 per cent) of the 150 patients improved by at least one grade, as measured with a modification of the grading scale of Frankel et al. Fifty-six (72 per cent) of the seventy-eight patients who had preoperative paralysis or dysfunction of the bladder recovered completely. One hundred and twenty-five (96 per cent) of the 130 patients who were employed before the injury returned to work after the operation, and 112 (86 per cent) of them returned to their previous job without restrictions. We concluded that anterior decompression, strut-grafting, and fixation with the Kaneda device in patients who had a burst fracture of the thoracolumbar spine and associated neurological deficits yielded good radiographic and functional results.