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

Comparison of anterior column reconstruction techniques after en bloc spondylectomy: a finite element study

Total en bloc spondylectomy (TES) effectively treats spinal tumors. The surgery requires a vertebral body replacement (VBR), for which several solutions were developed, whereas the biomechanical differences between these devices still need to be completely understood. This study aimed to compare a femur graft, a polyetheretherketone implant (PEEK-IMP-C), a titan mesh cage (MESH-C), and a polymethylmethacrylate replacement (PMMA-C) using a finite element model of the lumbar spine after a TES of L3. Several biomechanical parameters (rotational stiffness, segmental range of motion (ROM), and von Mises stress) were assessed to compare the VBRs. All models provided adequate initial stability by increasing the rotational stiffness and decreasing the ROM between L2 and L4. The PMMA-C had the highest stiffness for flexion-extension, lateral bending, and axial rotation (215%, 216%, and 170% of intact model), and it had the lowest segmental ROM in the instrumented segment (0.2°, 0.5°, and 0.7°, respectively). Maximum endplate stress was similar for PMMA-C and PEEK-IMP-C but lower for both compared to MESH-C across all loading directions. These results suggest that PMMA-C had similar or better primary spinal stability than other VBRs, which may be related to the larger contact surface and the potential to adapt to the patient's anatomy.

Efficacy of Titanium Mesh Cages for Anterior Column Reconstruction after Thoracolumbar Corpectomy

STUDY DESIGN

This retrospective study was conducted to determine the safety and efficacy of titanium cage reconstruction and anterior plating after thoracolumbar corpectomy.

PURPOSE

To study the clinical and radiological outcome of anterior column reconstruction after thoracolumbar corpectomy.

OVERVIEW OF LITERATURE

Anterior column reconstruction aims to optimize neural decompression with adequate stabilization.

METHODS

A series of 16 patients underwent reconstruction after thoracolumbar corpectomy to treat injury due to trauma (n=10), tuberculosis (n=3), and tumor (n=3). The average duration of follow-up was 18 months (range, 8-58 months). The degree of kyphosis, construct height, and the subsidence of the cage in relation to the vertebral endplates were measured. The approach was thoracoabdominal in 10 cases and retroperitoneal in 6 cases.

RESULTS

Four patients were neurologically intact with Frankel grade E on admission, and all remained intact postoperatively. Of the 6 patients with Frankel grade D, all fully recovered full motor and sensory functions. Of the 6 patients with Frankel grade C, three improved one grade and the other three improved two grades. The mean height of the vertebra before surgery was 41 mm and the mean construct height immediately after surgery and at follow-up was 47 mm and 44 mm, respectively. Solid fusion was observed in all patients. The sagittal alignment of the fractured segment was restored immediately after surgery as a significant decrease in the local kyphotic angle.

CONCLUSIONS

Anterior instrumentation is an effective and safe treatment for thoracolumbar instability with satisfactory clinical and radiological outcomes.

Relationship between autologous bone graft osteointegration and correction loss after antero-posterior spondylodesis of traumatic vertebral body fracture

BACKGROUND

A common method to restore the sagittal alignment and stabilize the spinal column is a dorso-ventral spondylodesis. It is assumed that correction loss after posttraumatic spondylodesis results from inadequate incorporation of the autologous iliac crest graft.

MATERIALS AND METHODS

Retrospective documentation of patients with unstable vertebral body fractures of the thoracic or lumbar spine with concomitant rupture of at least one adjacent intervertebral disk who received surgical treatment at our institution from 2000 to 2006. Followed by analysis of the computer tomography documentation of a total of 142 patients with unstable vertebral body fracture stabilized by posterior internal fixator and anterior iliac crest spondylodesis.

RESULTS

The following mean angle changes were derived from the second series of CT scans performed on average 283 days after anterior spondylodesis: vertebral wedge angle (VWA): 2.1°; segmental kyphotic angle: 4.9°; adjusted-SKA: 4.8°; sagittal index (SI): -0.04; segmental-scoliotic-angle (SSA): 0°; adjusted-SSA: 0°. Changes in VWA, both SKAs and SI postoperatively and prior to ME, were statistically significant (P<0.05). The McAfee fusion assessment of the graft showed: full fusion: cranial 64%, caudal 47%; partial fusion: cranial 20.5%, caudal 29%; lysis: cranial 8.5%, caudal 17%; graft resorption: 7%. No correlation was found between the above-mentioned angle changes and fusions grade.

DISCUSSION

The importance of radiological evidence of fusion deficiency is questionable, because the extent of fusion only has a minimal effect on correction loss.

LEVEL OF EVIDENCE

Level IV.

Design and biomechanical properties of a new reconstruction device for treating thoracolumbar burst fractures

Implants currently used for reconstruction of a burst vertebral body are associated with complications, including subsidence, nonunion, and substantial intraoperative blood loss. A new reconstruction device, the U-Cage (Double Engine Medical Material Ltd, Xiamen, Fujian, China), was designed to minimize complications.Six intact adult cadaver thoracolumbar (T11-L3) spines were collected and scanned by dual-energy X-ray absorptiometry (DEXA). The stiffness of the burst spine was subsequently compared with its previous intact state during flexion/extension, lateral bending, and rotation, and then subjected to a cyclic test to predict cage subsidence and device loosening. Axial load was applied continuously until failure to test the peak load that the specimen could withstand during the cyclic test. The correlation of bone mineral density and peak load was also analyzed. The instrumented specimens were found to be equivalent to intact bone in all directions (P>.05), with the exception of left rotation (P<.05). All specimens could withstand the cyclic test, and no subsidence or loosening of the device was detected. Average peak load for the instrumented specimens was 4137.5 N, which correlated with the average bone mineral density (r=0.915; P=.011).Thoracolumbar burst fractures instrumented with a U-Cage and anterolateral D-rod fixation achieved a stiffness similar to that of intact spines. This procedure may avoid the subsidence of the cage in vivo and serve as a better option for treating thoracolumbar burst fractures.

The posterior transpedicular approach for circumferential decompression and instrumented stabilization with titanium cage vertebrectomy reconstruction for spinal tumors: consecutive case series of 50 patients

STUDY DESIGN

A retrospective case series.

OBJECTIVE

To demonstrate the feasibility, safety, and results of the posterior transpedicular approach for circumferential decompression and instrumented reconstruction of thoracolumbar spinal tumors.

SUMMARY OF BACKGROUND DATA

Patients presenting with spinal tumor disease requiring 3-column instrumented stabilization are typically treated with a combined anterior and posterior surgical approach. However, circumferential decompression and instrumented stabilization may also be achieved through a single-stage, midline posterior transpedicular approach.

METHODS

Fifty consecutive patients (27 women and 23 men) underwent surgery between 2003 and 2010 at a single institution by the senior author. Mean age was 55.9 years (range, 25-79 yr).Single or multilevel, contiguous subtotal vertebrectomy was performed ranging from T1 to L4 (38 thoracic and 12 lumbar). Three-column spinal stabilization was achieved using posterior pedicle screw fixation and vertebral body reconstruction, with a titanium cage introduced through the posterior transpedicular route. The mean follow-up period was 17 months (range, 1-54 mo).

RESULTS

The mean operating time was 4.2 hours. The mean estimated blood loss for a subgroup of 9 patients with hypervascular tumor pathology was 3933 mL (range, 2700-5800 mL). The mean blood loss in the remaining 41 patients was 1262 mL (range, 250-2500 mL).Postoperative neurological status was maintained or improved in all patients. Mean postoperative stay was 7.7 days (range, 3-12 d). At last review, 14 patients were alive, with a mean survival of 36 months (range, 13-71 mo). The mean survival for the 36 patients who died was 19 months (range, 2 weeks to 54 mo).

CONCLUSION

This is the largest reported series of patients with spinal tumor disease undergoing circumferential decompression and 3-column instrumented stabilization through the posterior transpedicular approach.This surgical approach provides sufficient access for safe and effective circumferential decompression and stabilization, with reduced complications compared with costotransversectomy or combined anterior transcavitary and posterior approaches.

Anterior Decompression and Shortening Reconstruction with a Titanium Mesh Cage through a Posterior Approach Alone for the Treatment of Lumbar Burst Fractures

STUDY DESIGN

A retrospective study.

PURPOSE

To examine the efficacy and safety for a posterior-approach circumferential decompression and shortening reconstruction with a titanium mesh cage for lumbar burst fractures.

OVERVIEW OF LITERATURE

Surgical decompression and reconstruction for severely unstable lumbar burst fractures requires an anterior or combined anteroposterior approach. Furthermore, anterior instrumentation for the lower lumbar is restricted through the presence of major vessels.

METHODS

Three patients with an L1 burst fracture, one with an L3 and three with an L4 (5 men, 2 women; mean age, 65.0 years) who underwent circumferential decompression and shortening reconstruction with a titanium mesh cage through a posterior approach alone and a 4-year follow-up were evaluated regarding the clinical and radiological course.

RESULTS

Mean operative time was 277 minutes. Mean blood loss was 471 ml. In 6 patients, the Frankel score improved more than one grade after surgery, and the remaining patient was at Frankel E both before and after surgery. Mean preoperative visual analogue scale was 7.0, improving to 0.7 postoperatively. Local kyphosis improved from 15.7° before surgery to -11.0° after surgery. In 3 cases regarding the mid to lower lumbar patients, local kyphosis increased more than 10° by 3 months following surgery, due to subsidence of the cages. One patient developed severe tilting and subsidence of the cage, requiring additional surgery.

CONCLUSIONS

The results concerning this small series suggest the feasibility, efficacy, and safety of this treatment for unstable lumbar burst fractures. This technique from a posterior approach alone offers several advantages over traditional anterior or combined anteroposterior approaches.

Single-stage posterior spondylectomy, circumferential decompression and reconstruction using mesh cage for spinal tumors

OBJECTIVE

To explore the feasibility and clinical results of circumferential decompression and three-column reconstruction through single-stage posterior transpedicular approach for spinal tumor treatment.

METHODS

Totally, 24 patients with spinal tumor underwent tumor resection and spinal reconstruction through single-stage posterior transpedicular approach. Preoperatively, according to the Frankel classification, 12 patients were grade E, 9 grade D, and 3 grade C. Anterior column was reconstructed with non-expandable titanium cages. Posterior segmental instrumentation was used to maintain the stability of spine in all cases. Anterior and posterolateral fusion was performed with autograft and allogenic bone. The following data were followed up in these patients: deformity angle, local recurrence, neurological function, and spinal bony fusion.

RESULTS

The average operating time and blood loss was 5.6 hours and 3,400 ml respectively. No intraoperative and postoperative complications were observed in this group. Postoperatively, 21 patients were Frankel grade E, 2 grade D, and 1 grade C. Four patients reported significant functional restoration and twenty patients reported complete resolution of pain. At follow-up (range, 6-42 months), implant failure or recurrent neurological symptoms was not found.

CONCLUSIONS

The tumor resection and spinal reconstruction through single-stage posterior transpedicular approach is a safe and effective technique for the treatment of spinal tumor. It can fully decompress the neurological structures, correct the kyphosis, and achieve early weight-bearing. This technique can improve life quality for the patients with spinal tumor.

Five-year follow-up examination after purely minimally invasive posterior stabilization of thoracolumbar fractures: a comparison of minimally invasive percutaneously and conventionally open treated patients

INTRODUCTION

It is well known that during posterior stabilization of the spinal column conventionally open methods are predominantly used. However, in this study a minimally invasive method was chosen to decrease the morbidity of the operative access and to protect the paravertebral musculature, which serves as an important spine-stabilizing factor during posterior stabilization. The aims of this retrospective non-randomized case-control study were to compare the clinical and radiological results of minimally invasive on the one hand and conventionally open posterior surgery on the other with each other and to measure the loss of correction after purely posterior stabilization.

METHODS

Twenty-one consecutive non-randomized patients with thoracolumbar vertebral body fractures, which had been stabilized posteriorly without any intervertebral body fusion between 1996 and 1997, and without any neurological symptoms, were examined retrospectively more than 5 years after trauma. Eleven patients had been treated conventionally open and 10 patients minimally invasive. As methods of evaluation, the intra- and postoperative amount of blood loss, the X-ray time, the Hannover-Spine-Score, the SF-36 Health Questionnaire and radiological assessment of the bisegmental wedge and vertebral body angle were made use of.

RESULTS

The blood loss was significantly lower among those patients who had been operated in a minimally invasive way. The operating time, the time of X-ray exposure and the loss of correction were identical in both groups. The first year after implant removal, the loss of correction was the highest with 2.1 degrees for the body angle and 6.86 degrees for the bisegmental wedge angle. Neither in the Hannover-Spine-Score nor in the SF-36 Health Questionnaire did both groups show a difference. A correlation between the loss of correction and the clinical results could not be demonstrated.

CONCLUSION

The minimally invasive posterior stabilization leads to lower blood loss in comparison to the conventionally open method and can be carried out without any special effort limited to A-fractures without any neurological symptoms.

Harms titanium mesh cage fracture

Interbody fusion has become a mainstay of surgical management for lumbar fractures, tumors, spondylosis, spondylolisthesis and deformities. Over the years, it has undergone a number of metamorphoses, as novel instrumentation and approaches have arisen to reduce complications and enhance outcomes. Interbody fusion procedures are common and successful, complications are rare and most often do not involve the interbody device itself. We present here a patient who underwent an anterior L4 corpectomy with Harms cage placement and who later developed a fracture of the lumbar titanium mesh cage (TMC). This report details the presentation and management of this rare complication, as well as discusses the biomechanics underlying this rare instrumentation failure.

Two in vivo surgical approaches for lumbar corpectomy using allograft and a metallic implant: a controlled clinical and biomechanical study

BACKGROUND CONTEXT

Both bone graft and metallic implants have been used in combination with the necessary anterior rod or plate instrumentation to fill the voids left by vertebral body removal, with the ultimate goal of restoring stability. One type of device that has recently been introduced is an expandable titanium telescoping cage that is designed to be used as a strut implant to fill corpectomy defects. The use of these devices has met varying success. Acceptance by surgeons and spine biomechanicians has been limited by clinical failure with subsequent loss of reduction and increase in kyphosis. In order to further improve patient care, it is critical to evaluate the use of these implants through biomechanical as well as other modes of testing.

PURPOSE

To compare and contrast the spinal fusion outcome of using allograft bone versus the expandable vertebral body replacement titanium implant in a lumbar corpectomy procedure.

STUDY DESIGN

Controlled biomechanical study of lumbar spine fusion using bone graft and the expandable cage in an in vivo bovine model after a 4-month postoperative healing period (n=6). ANIMAL MODEL: Twelve Holstein calves aged 4-6 months with L3 and adjacent discs removed to create a simulated lumbar corpectomy defect.

OUTCOME MEASURES

Lumbar spine stability after corpectomy repair was quantified by biomechanical parameters. Strength of fusion was assessed by stiffness of ex vivo spine specimens in flexion-extension, lateral bending, and torsion obtained from biomechanical testing. Uniaxial strain at various positions on the surface of the anterior plate was measured during loading as an additional stability parameter. Loading tests were repeated after removal of the anterior instrumentation (plate and the screws).

METHODS

The calves were randomly allocated to groups for corpectomy defect repair with 1) Allograft metatarsal bone and thoracolumbar spine locking plate, n=6; or 2) Expandable vertebral body replacement device, and thoracolumbar spine locking plate, n=6. After a 4-month postoperative period, anterior-posterior and lateral radiographs were taken of the spine, followed by animal sacrifice and harvesting of the lumbar spine for biomechanical and histological testing. For biomechanical testing, uniaxial strain gauges were applied to the thoracolumbar spine locking plate to measure plate deformation during loading in a custom built fixture for application of flexion-extension, lateral bending, and torsion moments in an Instron materials testing machine. These loading tests were repeated with the thoracolumbar spine locking plate removed, thereby loading solely the fused segment.

RESULTS

At 4 months postoperative, the stiffness of the calf spines repaired by the metatarsal allograft and thoracolumbar spine locking plate was significantly greater than that of the spines repaired by the expandable cage and thoracolumbar spine locking plate. This finding was true for all three directions of loading (flexion-extension, left-right lateral bending, and torsion). Concordantly, the neutral zone, elastic zone, and range of motion of the spines repaired with the allograft bone were less than that of the spines repaired with the expandable cage. Greater strain values were observed from the gauges on the thoracolumbar spine locking plate of the spines using the expandable cage than the spines using allograft bone. This finding held for all gauge positions (anterior edge, anterior face, posterior edge, and posterior face at the longitudinal midpoint of the plate). After thoracolumbar spine locking plate removal and a repeat of the loading tests, a decrease in stiffness of the construct and a rise in the motion parameters were observed for both the allograft and cage groups.

CONCLUSIONS

The use of allograft bone for corpectomy defect repair in the lumbar spine appears to contribute to a stiffer and perhaps more stable spine segment compared with using the expandable cage device for such a repair after a 4-month healing period in this in vivo calf model. These findings thus far are based upon the biomechanical data gathered.

Anterior thoracic spine reconstruction using a titanium mesh cage and pedicled rib flap

STUDY DESIGN

A retrospective clinical study.

OBJECTIVES

To evaluate the safety and efficacy of using an integrated titanium mesh cage and pedicled rib flap for thoracic spine reconstruction in patients at high risk of pseudarthrosis.

SUMMARY OF BACKGROUND DATA

A variety of materials are available for interbody thoracic reconstruction; however, the optimal treatment of patients at high risk of pseudarthrosis remains a challenging problem. Free or pedicled bone flaps have been shown to be highly effective in terms of promoting fusion and titanium mesh cages provide excellent structural support.

METHODS

Eleven patients who underwent anterior thoracic corpectomy and spinal reconstruction using an integrated titanium mesh cage and pedicled rib flap were analyzed with a mean follow-up of 37 months (range, 25-55 months). The etiology of spinal disease was infection in 7 (64%) patients and tumor in the remaining 4 (36%) patients. Seven (64%) patients were treated with only an anterior approach while the remaining 4 (36%) patients underwent circumferential spinal reconstruction.

RESULTS

All patients demonstrated clinical and radiographic evidence of spinal fusion at the time of follow-up. All patients had stable or improved Frankel grades after surgery. There was a mean kyphosis correction of 7 degrees for both the focal and regional thoracic kyphosis. There were three significant postoperative complications: bilateral pleural effusion, gram-negative bacteremia, and transient right lower extremity weakness requiring reoperation and pedicle screw revision. Two patients died after surgery: one from aneurysmal subarachnoid hemorrhage and the other from complications of breast cancer.

CONCLUSIONS

The use of an integrated rib flap and titanium mesh cage construct appears to be a safe and effective means of providing immediate and substantial anterior column support as well as achieving arthrodesis in challenging fusion candidates.

Anterior stabilization of three-column thoracolumbar spinal trauma

Object

The purpose of this study was to evaluate the results obtained in patients who underwent anterior stabilization for three-column thoracolumbar fractures.

Methods

The authors retrospectively reviewed available clinical and radiographic data (1997–2006) to classify three-column thoracolumbar fractures according to the Association for the Study of Internal Fixation (AO) system, neurological status, spinal canal compromise, pre- and postoperative segmental angulation, and arthrodesis rate.

The mean computed tomography–measured preoperative spinal canal compromise was 48.3% (range 8–92%), and the mean vertebral body height loss was 39.4%. The mean preoperative kyphotic deformity of 14.9° improved to 4.6° at the final follow-up examination. Although this angulation had increased a mean of 1.8° during the follow-up period, the extent of correction was still significant compared with the preoperative angulation (p < 0.01). There were no cases of vascular complication or neurological deterioration.

Conclusions

Contemporary anterior spinal reconstruction techniques can allow certain types of unstable three-column thoracolumbar fractures to be treated via an anterior approach alone. Compared with traditional posterior approaches, the anterior route spares lumbar motion segments and obviates the need for harvesting of the iliac crest.

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.

Endoskopische Knochentransplantation an der Wirbels�ule

The application of autogenous bone grafts represents the golden standard for reconstruction of the load-bearing anterior column in the thoracolumbar spine. However, the osseous integration of the implanted grafts is demanding and delayed union or pseudarthrosis may occur. There are no standardized data available yet indicating the further course in such cases. The aim of this study was to evaluate the incorporation of endoscopically applied grafts and to develop therapeutic strategies for delayed or non-fusions. Twenty patients suffering from unstable injuries of the thoracolumbar spine were studied in a prospective clinical trial. After primary dorsal stabilization, the anterior column was thoracoscopically reconstructed with an autogenous iliac crest graft and a fixed-angle implant (MACS). The osseous integration of the bone grafts was detected by MSCT 1 year postoperatively. Complete integration of the transplanted bone grafts was observed in only 65% of the cases. In 25% partial integration was detected and in two cases a fracture of the transplanted iliac crest graft occurred. Despite the incomplete integration of the bone grafts, the further course without surgical intervention revealed no clinical or radiological evidence of a concomitant implant loosening or a relevant secondary loss of correction. Similar to the open technique, endoscopic reconstruction of the anterior column with autogenous bone grafts may lead to disadvantageous results concerning the integration and healing of the applied bone grafts. Decision making in such cases depends on the individual clinical and radiological findings (i.e., evidence of implant loosening and concomitant loss of correction).

[Vertebral body replacement in spine surgery]

Autografts and allogeneous bone grafts as well as cages are used for the reconstruction of the anterior column after corpectomy. Recently, expandable cages for vertebral body replacement have been developed. Based on our own experience, the purpose of this study was to summarize the available biomechanical and clinical data of expandable corpectomy cages and to compare it with established fixation techniques. If used correctly, expandable cages offer several surgical advantages in comparison to non-expandable cages. However there were no significant differences between the biomechanical properties of expandable and non-expandable cages. Additionally, design variations of expandable corpectomy cages did not show any significant impact on the biomechanical stability. Currently available mid-term clinical and radiological data on the treatment of fractures, metastasis and infection of the cervical, thoracic and lumbar spine demonstrated no significant difference between expandable and non-expandable cages. However, the increased stress-shielding effect of expandable cages compared to non-expandable cages might result in a deterioration of the long-term clinical outcome.

Radiologic Stability of Titanium Mesh Cages for Anterior Spinal Reconstruction Following Thoracolumbar Corpectomy

BACKGROUND

This work evaluated the radiologic stability of titanium mesh cages (TMCs) when used for single-level corpectomy reconstruction of thoracic and thoracolumbar spine.

METHODS

Thirty-one patients underwent reconstruction for acute fractures (n = 15), posttraumatic deformity reconstruction (n = 10), neoplastic disorders (n = 4), and infection (n = 2). The cages were placed after corpectomy and excision of the adjacent intervertebral discs. Additional stabilization devices included anterior plates alone (n = 18), anterior double screw and rod constructs alone (n = 9), a single anterior rod system (n = 1), posterior stabilization alone (n = 6), and additional posterior stabilization (n = 2).

RESULTS

Mean kyphosis correction was from 16 degrees to 5 degrees with 3 degrees of recurrence at 1-year follow-up (P < 0.0001 for both postoperative and final follow-up). In patients with greater initial kyphosis (>20 degrees ), mean correction was from 33 degrees to 10 degrees without recurrence (P = 0.004). Distance between adjacent vertebral bodies improved by 13 mm after cage placement, with a mean of 2mm of settling at final follow-up. There was one asymptomatic cage fracture without evidence of other problems. Two patients had construct failure after complex three-dimensional deformities were inadequately corrected and the cages had been placed in an angulated position.

CONCLUSIONS

This report suggests that TMCs are a sound reconstruction alternative after thoracic and thoracolumbar corpectomy at a single level and may prevent complications associated with the harvest and use of large structural autografts for these reconstructions. Failure to correctly align the spine so the cage can be vertically placed is a contraindication to the use of TMCs.

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.

Influence of cage design on interbody fusion in a sheep cervical spine model

OBJECT

The purpose of this study was to compare the characteristics of interbody fusion achieved using an autologous tricortical iliac crest bone graft with those of a cylinder- and a box-design cage in a sheep cervical spine model. This study was designed to determine whether there are differences between three interbody fusion procedures in: 1) ability to preserve postoperative distraction; 2) biomechanical stability; and 3) histological characteristics of intervertebral bone matrix formation.

METHODS

Twenty-four sheep underwent C3-4 discectomy and fusion in which the following were used: Group 1, autologous tricortical iliac crest bone graft (eight sheep); Group 2, titanium cylinder-design cage filled with autologous iliac crest bone graft (eight sheep); and Group 3, titanium box-design cage filled with autologous iliac crest graft (eight sheep). Radiography was performed pre- and postoperatively and after 1, 2, 4, 8, and 12 weeks. At the same time points, disc space height, intervertebral angle, and lordosis angle were measured. After 12 weeks, the sheep were killed, and fusion sites were evaluated by obtaining functional radiographs in flexion and extension. Quantitative computerized tomography scans were acquired to assess bone mineral density, bone mineral content, and bone callus volume. Biomechanical testing was performed in flexion, extension, axial rotation, and lateral bending. Stiffness, range of motion, neutral zone, and elastic zone were determined. Histomorphological and histomorphometric analyses were performed, and polychrome sequential labeling was used to determine the time frame of new bone formation. Over a 12-week period significantly higher values for disc space height and intervertebral angle were shown in cage-treated sheep than in those that received bone graft. Functional radiographic assessment revealed significantly lower residual flexion-extension movement in sheep with the cylinder cage-fixed spines than in those that received bone graft group. The cylinder-design cages showed significantly higher values for bone mineral content, bone callus content, and stiffness in axial rotation and lateral bending than the other cages or grafts. Histomorphometric evaluation and polychrome sequential labeling showed a more progressed bone matrix formation in the cylindrical cage group than in both other groups.

CONCLUSIONS

Compared with the tricortical bone graft, both cages showed significantly better distractive properties. The cylindrical cage demonstrated a significantly higher biomechanical stiffness and an accelerated interbody fusion compared with the box-design cage and the tricortical bone graft. The differences in bone matrix formation within both cages were the result of the significantly lower stress shielding on the bone graft by the cylinder-design cage.

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.

Use of folded vascularized rib graft in anterior fusion after treatment of thoracic and upper lumbar lesions. Technical note

For lesions involving the anterior and/or middle column of the spine, an anterior approach is adequate for curetting the lesion and restoring spinal stability. Materials such as autogenous bone grafts, cages with bone chips, some artificial materials, or allografts are used as strut materials. Rib material is usually removed when the anterior approach is conducted for thoracic or thoracolumbar lesions. A rib itself is not rigid enough to support the load, and a bone union is not easily obtained. The purpose of this paper is to describe a method of grafting vascularized rib in folded form to fill the defects left after removal of a spinal lesion. The rib, with the artery and vein at two levels cranial to the involved vertebral body, was isolated from surrounding tissues such as the intercostal nerve, muscles, and pleura. After curetting the lesion, the rib was folded into three or four pieces to a length adequate to fill the defect and inserted as a pedicled vascularized graft. A total of 23 cases, including 14 men and nine women, underwent surgery in which this grafting technique was used. The pathological conditions requiring anterior decompression and fusion were spinal trauma in nine cases, spinal infection in six cases, osteoporotic fracture in seven cases, and spinal metastasis in one case. In all cases a solid bone union was obtained and all infections resolved. With vascularized rib graft folded into three to four pieces, solid bone union can be obtained without use of any other grafted materials even in cases of infection and osteoporosis.

A biomechanical comparison of modern anterior and posterior plate fixation of the cervical spine

STUDY DESIGN

A biomechanical study was designed to assess relative rigidity provided by anterior, posterior, or combined cervical fixation using cadaveric cervical spine models for flexion-distraction injury and burst fracture.

OBJECTIVES

To compare the construct stability provided by anterior plating with locked fixation screws, posterior plating with lateral mass screws, and combined anterior-posterior fixation in clinically simulated 3-column injury or corpectomy models.

SUMMARY OF BACKGROUND DATA

Anterior plating with locked fixation screws is the most recent design and is found to provide better stability than the conventional unlocked anterior plating. However, there are few data on the direct comparison of biomechanical stability provided by anterior plating with locked fixation screws versus posterior plating with lateral mass screws. Biomechanical advantages of using combined anterior-posterior fixation compared with that of using either anterior or posterior fixation alone also have not been well investigated yet.

METHODS

Biomechanical flexibility tests were performed using cervical spines (C2-T1) obtained from 10 fresh human cadavers. In group I (5 specimens), one-level, 3-column injury was created at C4-C5 by removing the ligamentum flavum and bilateral facet capsules, the posterior longitudinal ligament, and the posterior half of the intervertebral disc. In group II (5 specimens), complete corpectomy of C5 was performed to simulate burst injury. In each specimen, the intact spine underwent flexibility tests, and the following constructs were tested: (1) posterior lateral mass screw fixation (Axis plate) after injury; (2) polymethylmethacrylate anterior fusion block plus posterior fixation; (3) polymethylmethacrylate block plus anterior (Orion plate) and posterior plate fixation; and (4) polymethylmethacrylate block plus anterior fixation. Rotational angles of the C4-C5 (or C4-C6) segment were measured and normalized by the corresponding angles of the intact specimen to study the overall stabilizing effects.

RESULTS

Posterior plating with an interbody graft showed effective stabilization of the unstable cervical segments in all loading modes in all cases. There was no significant stability improvement by the use of combined fixation compared with the posterior fixation with interbody grafting, although combined anterior-posterior fixation tended to provide greater stability than both anterior and posterior fixation alone. Anterior fixation alone was found to fail in stabilizing the cervical spine, particularly in the flexion-distraction injury model in which no contribution of posterior ligaments is available. Anterior plating fixation provided much greater fixation in the corpectomy model than in the flexion-distraction injury model. This finding suggests that preservation of the posterior ligaments may be an important factor in anterior plating fixation.

CONCLUSIONS

This study showed that the posterior plating with interbody grafting is biomechanically superior to anterior plating with locked fixation screws for stabilizing the one-level flexion-distraction injury or burst injury. More rigid postoperative external orthoses should be considered if the anterior plating is used alone for the treatment of unstable cervical injuries. It was also found that combined anterior and posterior fixation may not improve the stability significantly as compared with posterior grafting with lateral mass screws and interbody grafting.