Biomechanical evaluation of posterior cervical stabilization after a wide laminectomy

Concepts and Techniques to Prevent Cervical Spine Deformity After Spine Surgery: A Narrative Review

Adult cervical spine deformity is associated with decreased health-related quality of life, disability, and myelopathy. A number of radiographic parameters help to characterize cervical deformity and aid in the diagnosis and treatment. There are several etiologies for cervical spine deformity, the most common being iatrogenic. Additionally, spine surgery can accelerate adjacent segment degeneration which may lead to deformity. It is therefore important for all spine surgeons to be aware of the potential to cause iatrogenic cervical deformity. The aim of this review is to highlight concepts and techniques to prevent cervical deformity after spine surgery.

Effects of rod diameter on kinematics of posterior cervical spine instrumented constructs: an ex vivo study

OBJECTIVE

Posterior cervical spine fixation is a robust strategy for stabilizing the spine for a wide range of spinal disorders. With the evolution of spinal implant technology, posterior fixation with lateral mass screws in the subaxial spine is now common. Despite interest in variable rod diameters to meet a wide range of clinical needs such as trauma, revision, and deformity surgery, indications for use of posterior cervical spine fixation are not clear. This laboratory investigation evaluates the mechanical stability and kinematic properties of lateral mass fixation with various commercially available rod diameters.

METHODS

The authors conducted an ex vivo experiment using 13 fresh-frozen human cervical spine specimens, instrumented from C3 to C6 with lateral mass screws, to evaluate the effects of titanium rod diameter on kinematic stability. Each intact spine was tested using a kinematic profiling machine with an optoelectrical camera and infrared sensors applying 1.5-Nm bending moments to the cranial vertebra (C2) simulating flexion-extension, lateral bending, and axial rotation anatomical motions. A compressive follower preload of 150 N was applied in flexion-extension prior to application of a bending moment. Instrumented spines were then tested with rod diameters of 3.5, 4.0, and 4.5 mm. The kinematic data between intact and surgical cases were studied using a nonparametric Wilcoxon signed-rank test. A multivariable, multilevel linear regression model was built to identify the relationship between segmental motion and rod diameter.

RESULTS

Instrumentation resulted in significant reduction in range of motion in all three rod constructs versus intact specimens in flexion-extension, lateral bending, and axial rotation (p < 0.05). The maximum reductions in segmental ROM versus intact spines in 3.5-, 4.0-, and 4.5-mm rod constructs were 61%, 71%, and 81% in flexion-extension; 70%, 76%, and 81% in lateral bending; and 50%, 60%, and 75% in axial rotation, respectively. Segmental motion at the adjacent segments (C2-3 and C6-7) increased significantly (p < 0.05) with increasing rod diameter. The 4.5-mm rod construct had the greatest increase in motion compared to the intact spine.

CONCLUSIONS

With increasing rod diameters from 3.5 to 4.0 mm, flexion-extension, lateral bending, and axial rotation across C3-6 were significantly reduced (p < 0.05). Similar trends were observed with a statistically significant reduction in motion in all anatomical planes when the rod diameter was increased to 4.5 mm. Although the increase in rod diameter resulted in a more rigid construct, it also created an increase (p < 0.05) in the kinematics of the adjacent segments (C2-3 and C6-7). Whether this increase translates into adverse long-term clinical effects in vivo requires further investigation and clinical assessment.

Biomechanical Evaluation of Cervicothoracic Junction Fusion Constructs

OBJECTIVE

We studied the effect of different cervicothoracic construct design variables on biomechanical stability in vitro.

METHODS

Six fresh-frozen human cadaveric spines (C5-T4) were used. After intact analysis, each specimen was destabilized and reconstructed, with all groups having 4.0-mm pedicle screws placed at T1-T3. The 2 hook-rod constructs included interlaminar hooks at C6 and C7, with either 3.5-mm or 4.0-mm rods (C6-T3). The 2 screw-rod constructs tested included lateral mass screws at C6 and C7, with either 3.5-mm or 4.0-mm rods (C6-T3). The 2 screw-connector-rod constructs tested included lateral mass screws at C6 and C7, with either 3.5-mm or 4.0-mm rods; 1 rod spanned C6-C7 with a connector to a second rod of the same size spanning T1-T3. Global (C6-T3) and intervertebral (C6-C7, C7-T1, T1-T2, and T2-T3) ranges of motion were compared for each construct.

RESULTS

In terms of global (C6-T3) stability, 3.5-mm versus 4.0-mm rod constructs were not significantly different, regardless of whether the construct was hook-rod, screw-rod, or screw-connector-rod. The hook-rod constructs provided less stability compared with the screw-rod and screw-connector-rod constructs in lateral bending (P < 0.04) and axial rotation (P < 0.001). The screw-rod constructs demonstrated a similar range of motion to that of the screw-connector-rod constructs, except for significantly less axial rotation at the C6-C7 level with 3.5-mm rods (P = 0.04).

CONCLUSIONS

We found that the rod diameter of a construct does not appear to significantly influence the biomechanical stability of subaxial constructs. The screw-rod construct resulted in certain biomechanical advantages compared with the screw-connector-rod construct, and both were significantly superior to the hook-rod construct.

Impact of Connector Placement and Design on Bending Stiffness of Spinal Constructs

OBJECTIVE

To evaluate the stability of multiple rod-connector construct designs using a mechanical 4-point bending testing frame.

METHODS

A mechanical study was used to evaluate the bending stiffness of 3 connectors across 12 different configurations of rod-connector-rod constructs. Stability was evaluated in flexion-extension and lateral bending. Combinations of rods having 1 of 3 diameters (4.0 mm, 5.5 mm, and 6.0 mm) connected by 1 of 3 connector types (parallel open, snap-on, and hinged) were compared. Configurations with single connectors and with double connectors with variable spacing were also compared to simulate revision surgery conditions.

RESULTS

Constructs consisting of 4.0-mm rods connected to 4.0-mm rods were significantly less stiff as the total number of connectors used in a series exceeded 2. When single-connector configurations were compared, parallel open rod connectors demonstrated greater stiffness in flexion-extension than hinged open connectors, whereas hinged open connectors demonstrated greater stiffness in lateral bending. Using double connectors increased stiffness of 4.0- to 4.0-mm rod configurations in flexion-extension and lateral bending, 4.0- to 6.0-mm rod configurations in flexion-extension, and 5.5- to 6.0-mm rod configurations in lateral bending. Spacing the double connectors significantly improved lateral bending stiffness of 4.0- to 4.0-mm and 5.5- to 6.0-mm rod configurations.

CONCLUSIONS

Our data indicate that the design, number, and placement of rod connectors have a significant impact on the bending stiffness of a surgical construct. Such mechanical data may influence construct design in primary and revision surgeries of the cervical spine and cervicothoracic junction.

Posterior cervical spine crisscross fixation: Biomechanical evaluation

BACKGROUND

Biomechanical/anatomic limitations may limit the successful implantation, maintenance, and risk acceptance of posterior cervical plate/rod fixation for one stage decompression-fusion. A method of posterior fixation (crisscross) that resolves biomechanical deficiencies of previous facet wiring techniques and not reliant upon screw implantation has been devised. The biomechanical performance of the new method of facet fixation was compared to the traditional lateral mass plate/screw fixation method.

METHODS

Thirteen human cadaver spine segments (C2-T1) were tested under flexion-compression loading and four were evaluated additionally under pure-moment load. Preparations were evaluated in a sequence of surgical alterations with intact, laminectomy, lateral mass plate/screw fixation, and crisscross facet fixation using forces, displacements and kinematics.

FINDINGS

Combined loading demonstrated significantly lower bending stiffness (p < 0.05) between laminectomy compared to crisscross and lateral mass plate/screw preparations. Crisscross fixation showed a comparative tendency for increased stiffness. The increased overall motion induced by laminectomy was resolved by both fixation techniques, with crisscross fixation demonstrating a comparatively more uniform change in segmental motions.

INTERPRETATION

The crisscross technique of facet fixation offers immediate mechanical stability with resolution of increased flexural rotations induced by multi-level laminectomy. Many of the anatomic limitations and potentially deleterious variables that may be associated with multi-level screw fixation are not associated with facet wire passage, and the subsequent fixation using a pattern of wire connection crossing each facet joint exhibits a comparatively more uniform load distribution. Crisscross wire fixation is a valuable addition to the surgical armamentarium for extensive posterior cervical single-stage decompression-fixation.

Posterior Vertebral Column Resection With Intraoperative Manual Retraction for the Treatment of Posttubercular Kyphosis in Upper Thoracic Spine or Cervicothoracic Junction

STUDY DESIGN

This is a case-series.

OBJECTIVE

To evaluate the clinical and radiologic outcomes of posterior vertebral column resection (PVCR) for treatment of posttubercular kyphosis in upper thoracic spine and cervicothoracic junction (CTJ).

SUMMARY OF BACKGROUND DATA

Surgical management of severe posttubercular kyphosis in upper thoracic spine or CTJ is challenging. A new technique that combines PVCR and intraoperative manual traction in a single procedure, was developed to maximize the correction rate and minimize the risk of dural buckling and spinal cord injury. However, the safety and effectiveness of the procedure is yet to be established.

MATERIALS AND METHODS

Ten patients with upper thoracic or CTJ posttubercular kyphosis, who underwent PVCR with intraoperative manual traction between December 2008 and 2013, were studied retrospectively. Data pertaining to operation time, blood loss, and perioperative complications were collected. Outcomes were evaluated using the visual analog scale; the 12-Item Short Form Health Survey (SF-12), Mental Component Summary, and SF-12 Physical Component Summary. Neurological status was assessed using American Spinal Injury Association (ASIA) impairment scale. Radiologic outcomes including fusion and deformity correction were assessed using plain radiography and computed tomography.

RESULTS

The average duration of follow-up was 53.6±7.2 months. Average operating time and blood loss were 263±35.3 minutes and 2490±569 mL, respectively. Visual analog scale score for back pain, SF-12 Mental Component Summary, and Physical Component Summary showed significant improvement postoperatively. Kyphosis improved from a preoperative average of 73.6±13.1 degrees to an average of 37.9±8.7 degrees at final follow-up (correction rate: 48.5%). No postoperative neurological deterioration was observed based on the ASIA score. As of the last follow-up, solid fusion was not achieved only in 1 patient.

CONCLUSIONS

PVCR with intraoperative manual traction is a safe and effective procedure for treatment of upper thoracic or CTJ posttubercular kyphosis.

LEVEL OF EVIDENCE

Level 4.

Posterior Cervicothoracic Instrumentation: Testing the Clinical Efficacy of Tapered Rods (Dual-Diameter Rods)

STUDY DESIGN

Prospective study.

OBJECTIVE

To study the clinical efficacy of tapered rods in posterior cervicothoracic instrumentation.

SUMMARY OF BACKGROUND DATA

The cervicothoracic spine is a junctional area with complex biomechanics. A variety of disorders affect this region, rendering it unstable. Numerous posterior constructs have been evaluated by in vitro biomechanical studies. There are no data available on the clinical efficacy of a screw-rod system utilizing tapered (dual-diameter) rods. This is the first study analyzing the efficacy of this system in clinical scenarios in the short term.

MATERIALS AND METHODS

All consecutive patients with cervicothoracic junctional pathologies undergoing surgical treatment by posterior instrumentation utilizing tapered rods, between April 2007 and April 2012 were included in the study. The tapered rod tapers from a diameter of 5.5-3.5 mm to accommodate thoracic pedicle screws and lateral mass screws/pedicle screws of cervical spine, respectively. The cases were periodically followed up. Postoperative radiographs and computed tomography scans were analyzed.

RESULTS

There were 14 cases, 11 males and 3 females. The etiology was tuberculosis in 7 cases, neoplasm in 5, and 1 each of trauma and deformity. Three patients required combined anterior and posterior surgeries and remaining 10 were managed by posterior-only procedures. The average follow-up was 28 months. Three patients died (surgically unrelated reasons) and 1 was lost to follow-up. Excluding 2 patients with neoplastic and traumatic etiologies, patients with neurological deficit had significant improvement. One patient had postoperative cerebrospinal fluid leak which was recognized and corrected subsequently. No biomechanical failure occurred in any of the patients. No intraoperative complications were noted.

CONCLUSIONS

This study demonstrates that tapered rods are an excellent and a viable option to connect screws to stabilize cervicothoracic junction in the short term. This study complements the biomechanical studies previously reported.

Comparison of structural allograft and traditional autograft technique in occipitocervical fusion: radiological and clinical outcomes from a single institution

OBJECT The authors' objectives were to compare the rate of fusion after occipitoatlantoaxial arthrodesis using structural allograft with the fusion rate from using autograft, to evaluate correction of radiographic parameters, and to describe symptom relief with each graft technique. METHODS The authors assessed radiological fusion at 6 and 12 months after surgery and obtained radiographic measurements of C1-2 and C2-7 lordotic angles, C2-7 sagittal vertical alignments, and posterior occipitocervical angles at preoperative, postoperative, and final follow-up examinations. Demographic data, intraoperative details, adverse events, and functional outcomes were collected from hospitalization records. Radiological fusion was defined as the presence of bone trabeculation and no movement between the graft and the occiput or C-2 on routine flexion-extension cervical radiographs. Radiographic measurements were obtained from lateral standing radiographs with patients in the neutral position. RESULTS At the University of Utah, 28 adult patients underwent occipitoatlantoaxial arthrodesis between 2003 and 2010 using bicortical allograft, and 11 patients were treated using iliac crest autograft. Mean follow-up for all patients was 20 months (range 1-108 months). Of the 27 patients with a minimum of 12 months of follow-up, 18 (95%) of 19 in the allograft group and 8 (100%) of 8 in the autograft group demonstrated evidence of bony fusion shown by imaging. Patients in both groups demonstrated minimal deterioration of sagittal vertical alignment at final follow-up. Operative times were comparable, but patients undergoing occipitocervical fusion with autograft demonstrated greater blood loss (316 ml vs 195 ml). One (9%) of 11 patients suffered a significant complication related to autograft harvesting. CONCLUSIONS The use of allograft in occipitocervical fusion allows a high rate of successful arthrodesis yet avoids the potentially significant morbidity and pain associated with autograft harvesting. The safety and effectiveness profile is comparable with previously published rates for posterior C1-2 fusion using allograft.

Adjacent-level range of motion and intradiscal pressure after posterior cervical decompression and fixation: an in vitro human cadaveric model

STUDY DESIGN

This in vitro human cadaveric study measured adjacent-level kinematics after posterior cervical decompression and fixation.

OBJECTIVE

Quantify adjacent-level changes in range of motion (ROM) and intradiscal pressure after posterior cervical decompression and fixation.

SUMMARY OF BACKGROUND DATA

Optimal length of instrumentation after posterior decompression is unclear. Longer posterior cervical fixation constructs may increase the risk of adjacent-segment degeneration.

METHODS

Eight cervicothoracic spines were evaluated intact, with C3-C6 laminectomy, C3-C6 laminectomy + C3-C6 fixation, C3-C6 laminectomy + C3-C7 fixation, C3-C7 laminectomy, C3-C7 laminectomy + C3-C7 fixation, C3-C7 laminectomy + C2-C7 fixation, C3-C7 laminectomy + C3-T2 fixation, and C3-C7 laminectomy + C2-T2 fixation. Testing included intact moments (± 2.0 N·m) in flexion/extension, axial rotation, and lateral bending, with quantification of ROM at C2-C3, C6-C7, and C7-T1 normalized to the intact spine. Intradiscal pressures were also measured at each level.

RESULTS

For the C3-C6 laminectomy group, there were no differences in adjacent-level flexion/extension ROM or intradiscal pressure based on construct length, except at C6-C7, where ROM was significantly decreased when fixation was extended to C7 (P < 0.05). After C3-C7 laminectomy and reconstruction, the greatest increase in C2-C3 flexion/extension ROM and intradiscal pressure occurred in the C3-T2 fixation subgroup (ROM: 348% [P < 0.05]; intradiscal pressure: 319 ± 243 psi [pounds per square inch] vs. 65 ± 41 psi intact [P < 0.05]). At C7-T1, the greatest increase in flexion/extension ROM and intradiscal pressure occurred after C2-C7 fixation (ROM: 531% [P < 0.05]; intradiscal pressure: 152 ± 83 psi vs. 21 ± 14 psi intact [P < 0.05]).

CONCLUSION

For C3-C6 laminectomy, instrumentation to C7 significantly decreased flexion/extension ROM and intradiscal pressure at C6-C7 without significantly increasing either measure at C2-C3 or C7-T1 relative to C3-C6 fixation. In the setting of a C3-C7 laminectomy, when instrumenting to either C2 or T2, consideration should be given to including both levels within these constructs.

A computed tomography-based anatomic comparison of three different types of c7 posterior fixation techniques : pedicle, intralaminar, and lateral mass screws

OBJECTIVE

The intralaminar screw (ILS) fixation technique offers an alternative to pedicle screw (PS) and lateral mass screw (LMS) fixation in the C7 spine. Although cadaveric studies have described the anatomy of the pedicles, laminae, and lateral masses at C7, 3-dimensional computed tomography (CT) imaging is the modality of choice for pre-surgical planning. In this study, the goal was to determine the anatomical parameter and optimal screw trajectory for ILS placement at C7, and to compare this information to PS and LMS placement in the C7 spine as determined by CT evaluation.

METHODS

A total of 120 patients (60 men and 60 women) with an average age of 51.7±13.6 years were selected by retrospective review of a trauma registry database over a 2-year period. Patients were included in the study if they were older than 15 years of age, had standardized axial bone-window CT imaging at C7, and had no evidence of spinal trauma. For each lamina and pedicle, width (outer cortical and inner cancellous), maximal screw length, and optimal screw trajectory were measured, and the maximal screw length of the lateral mass were measured using m-view 5.4 software. Statistical analysis was performed using Student's t-test.

RESULTS

At C7, the maximal PS length was significantly greater than the ILS and LMS length (PS, 33.9±3.1 mm; ILS, 30.8±3.1 mm; LMS, 10.6±1.3; p<0.01). When the outer cortical and inner cancellous width was compared between the pedicle and lamina, the mean pedicle outer cortical width at C7 was wider than the lamina by an average of 0.6 mm (pedicle, 6.8±1.2 mm; lamina, 6.2±1.2 mm; p<0.01). At C7, 95.8% of the laminae measured accepted a 4.0-mm screw with a 1.0 mm of clearance, compared with 99.2% of pedicle. Of the laminae measured, 99.2% accepted a 3.5-mm screw with a 1.0 mm clearance, compared with 100% of the pedicle. When the outer cortical and inner cancellous height was compared between pedicle and lamina, the mean lamina outer cortical height at C7 was wider than the pedicle by an average of 9.9 mm (lamina, 18.6±2.0 mm; pedicle, 8.7±1.3 mm; p<0.01). The ideal screw trajectory at C7 was also measured (47.8±4.8° for ILS and 35.1±8.1° for PS).

CONCLUSION

Although pedicle screw fixation is the most ideal instrumentation method for C7 fixation with respect to length and cortical diameter, anatomical aspect of C7 lamina is affordable to place screw. Therefore, the C7 intralaminar screw could be an alternative fixation technique with few anatomic limitations in the cases when C7 pedicle screw fixation is not favorable. However, anatomical variations in the length and width must be considered when placing an intralaminar or pedicle screw at C7.

Transverse connectors providing increased stability to the cervical spine rod-screw construct: an in vitro human cadaveric study

Object

The object of this study was to determine if the addition of transverse connectors (TCs) to a rod-screw construct leads to increased stabilization of the cervical spine.

Methods

Eleven human cadaveric cervical spines (C2–T1) were used to examine the effect of adding connectors to a C3–7 rod-screw construct in 3 models of instability: 1) C3–6 wide laminectomy, 2) wide laminectomy and 50% foraminotomy at C4–5 and C5–6, and 3) wide laminectomy with full medial to lateral foraminotomy. Following each destabilization procedure, specimens were tested with no TC, 1 TC between the C-5 screws, and 2 TCs between the C-4 and C-6 screws. Testing of the connectors was conducted in random order. Specimens were subjected to ± 2 Nm of torque in flexion and extension, lateral bending, and axial rotation. Range of motion was determined for each experimental condition. Statistical comparisons were made between the destabilized and intact conditions, and between the addition of TCs and the absence of TCs.

Results

The progressive destabilization procedures significantly increased motion. The addition of TCs did not significantly change motion in flexion and extension. Lateral bending was significantly decreased with 2 connectors, but not with 1 connector. The greatest effect was on axial rotation. In general, 2 TCs were more restrictive than 1 TC, and decreased motion 10% more than fixation alone.

Conclusions

Regardless of the degree of cervical destabilization, 1 or 2 TCs decreased motion compared with rods and screws alone. Axial rotation was most affected. Transverse connectors effectively increase the rigidity of rod-screw constructs in the cervical spine. Severe cervical instability can be overcome with the use of 2 TCs, but in cases in which 2 cannot be used, 1 should be adequate and superior to none.

Cervicothoracic junction kyphosis: surgical reconstruction with pedicle subtraction osteotomy and Smith-Petersen osteotomy

Object

Sagittal plane deformities can be subdivided into kyphotic and lordotic forms and further characterized according to their global or regional (focal) presentation. Regional deformities of a significant magnitude constitute a gibbous deformity. Pedicle subtraction osteotomy (PSO) and interlaminar Smith-Petersen osteotomies have been used to correct sagittal plane deformities in the cervical, thoracic, and lumbar spine. By resecting a portion of the vertebral body and closing in the gap of this vertebra, the spine is placed in local lordosis and kyphosis is corrected. These osteotomies have generally been carried out in the lumbar or less frequently in the thoracic area. While PSO has been performed in the mid and lower thoracic spine, there have been no case series of patients undergoing PSO at the CTJ. Specifically, a PSO approach that addresses the challenges of the CTJ is needed. Here, the authors review their case series of PSOs performed in the CTJ. Their goal in the treatment of these patients was to correct the regional CTJ kyphosis, restore forward gaze, and reduce the pain associated with the deformity.

Methods

Eight patients (5 males and 3 females, mean age 63 years) underwent PSO for the correction of CTJ kyphosis. Pedicle subtraction osteotomy was performed at C-7 or the upper thoracic vertebrae and was facilitated by a computer-guided intraoperative monitoring system. Surgical indications included postlaminectomy kyphosis, spinal cord tumor resection, posttraumatic kyphosis, and degenerative cervical spondylosis.

Results

The mean follow-up was 15.3 months (range 12–20 months), and the mean preoperative CTJ kyphosis was 38.67° (range 25°–60°). Clinically satisfactory correction of the regional deformity was accomplished in all patients, achieving a mean correction of 35.63° (range 15°–66°) at the CTJ, with restoration of forward gaze and significant reduction in pain.

Conclusions

A CTJ deformity is a distinctive form of kyphosis that presents as a variable local deformity and requires complex spinal reconstructive techniques to restore sagittal balance and forward gaze. Pedicle subtraction osteotomy allows for significant correction through one spinal segment, and it can be used safely to correct the regional sagittal alignment of the cervical spine and head in relation to the pelvis. Pedicle subtraction osteotomy can be used alone or in combination with other techniques as some patients may require multistage procedures with anterior and posterior spinal reconstruction to obtain stable sagittal correction. All deformities in these patients were kyphotic in nature with only mild elements of scoliosis or coronal plane deformity. This is unlike lumbar and thoracic curves where the kyphosis is frequently associated with scoliosis.

Kinematics of progressive circumferential ligament resection (decompression) in conjunction with cervical disc arthroplasty in a spondylotic spine model

STUDY DESIGN

Benchtop biomechanics study examining kinematic effects of progressive resection in a human cadaveric spine model.

OBJECTIVE

To determine the effects of posterior longitudinal ligament (PLL) resection, unilateral and bilateral foraminotomy, and laminectomy on cervical intervertebral rotation and translation after cervical disc arthroplasty (CDA).

SUMMARY OF BACKGROUND DATA

Although the clinical results after CDA have been studied, there remain unanswered questions regarding the surgical techniques used at the time of device insertion. For example, it is unclear whether a surgeon should retain or resect the PLL and uncinate processes at the time of primary surgical intervention. Further, the effect of a subsequent posterior decompression (foraminotomy or laminectomy) on the stability of a motion segment containing a disc arthroplasty is unknown.

METHODS

Three-dimensional intervertebral motion was measured by biplanar videography in human cadaveric spines at C4-C5 or at C5-C6 subjected to a 1.5-Nm moment applied to induce motion in the sagittal plane. Coupled motions were not constrained. After measuring intact spine motion, disc arthroplasty with bilateral ventral foraminotomy was performed without PLL resection. Sequentially, rotations and translations were measured after PLL resection, unilateral foraminotomy, bilateral foraminotomy, and laminectomy.

RESULTS

CDA with bilateral ventral foraminotomy increased sagittal rotation by 0.4 degrees (16%) compared with the intact spine. The addition of PLL resection increased rotation by 0.5 degrees (14% increase). Unilateral and bilateral foraminotomy had negligible effects on sagittal rotation or anteroposterior (AP) translation. Laminectomy resulted in an additional sagittal plane rotation of 2 degrees. The sagittal-plane interverterbal rotation resultant after all interventions was 6 degrees , with 1.5 mm of AP translation occurring only.

CONCLUSION

Given that a greater degree of motion was seen with PLL resection combined with ventral foraminotomy, we recommend that PLL resection be performed when performing CDA. In our benchtop model, unilateral and bilateral posterior foraminotomies were not associated with the creation of significant sagittal rotational or AP translational instability.

Clinical and radiologic outcome of laminar screw at C2 and C7 for posterior instrumentation--review of 25 cases and comparison of C2 and C7 intralaminar screw fixation

BACKGROUND

The aim of this study is 2-fold: to analyze a clinical case series in which we used laminar screws for cervical posterior instrumentation and to describe the difference between C2 and C7 laminar screws in terms of technique and anatomy.

METHODS

Data were obtained from 25 patients who underwent cervical posterior fixation with intralaminar screws at C2 or C7. C2 intralaminar screw instrumentation was used for 7 patients requiring occipitocervical fixation (basilar invagination [3 patients], C1 unstable bursting fracture [1 patient], C1-C2 instability with occipital assimilation [2 patients], and dystopic os odontoideum [1 patient]), 13 patients with C1-C2 instability, 1 patient with C2-C3 subluxation, and 4 patients undergoing C7 fixation due to pseudoarthrosis or cervical instability after trauma. A total of 34 laminar screws were placed including 1 thoracic laminar screw, and the patients were assessed both clinically and radiographically.

RESULTS

There were no instances where a screw violated the spinal canal nor any hardware fractures noted during the follow-up period. As for perioperative complications, there were 2 cases of postoperative wound infection, 1 case of dural laceration during dissection, and 2 cases of partial dorsal laminar breach. However, there was no neurologic compromise in any of the cases. The fusion success rate was 100%.

CONCLUSION

These preliminary results support the use of intralaminar screws for posterior instrumentation at C2 and C7.

Early results from posterior cervical fusion with a screw-rod system

PURPOSE

We performed 65 cases of posterior fusion surgery for cervical and/or high thoracic lesions using a polyaxial screw-rod system.

PATIENTS AND METHODS

A total of 486 screws were implanted in 65 patients.

RESULTS

Fixation of the screws was carried out over an average of 2.9 spinal segments. Upon evaluation by postoperative CT scans, twelve (2.5%) screws had suboptimal trajectories but two of these revealed radiculopathy in one patient and required screw repositioning. No vascular sequelae resulted. There has been no segmental motion in any of the cases to date. As for other complications, there was one case of dural tearing and two cases of lateral mass fractures. There were no infections or other wound healing problems or hardware failures. No patients had neurological deterioration after surgery. There were statistically significant improvements in the mean Neck Disability Index (NDI) scores and Visual Analogue Scale (VAS) scores in the preoperative and late postoperative follow-up evaluations. Although further studies are required to establish the long-term results of fusion rates and clinical outcomes.

CONCLUSION

We cautiously suggest that the posterior polyaxial screw-rod system can be safely used as a primary or additional fusion method in this risky region. The successful and safe use of this method is dependent on a precise preoperative surgical plan and tactics for ensuring safe screw fixation.

Biomechanical comparison of instrumented and uninstrumented multilevel cervical discectomy versus corpectomy

STUDY DESIGN

In vitro flexibility test comparing biomechanics of cervical corpectomy versus discectomy with and without instrumentation.

OBJECTIVES

To evaluate whether the additional effort required to perform multilevel discectomies instead of corpectomies is worthwhile biomechanically.

SUMMARY OF BACKGROUND DATA

Both cervical corpectomy and discectomy have been shown to be effective clinically. No previous biomechanical comparison exists.

METHODS

Fourteen human cadaveric cervical spines were studied: 1) intact, 2) after discectomy and wedge grafting at C4-C5, C5-C6, and C6-C7 (Group 1) or corpectomy and strut grafting of C5 and C6 (Group 2), 3) after attaching a locking metal plate from C4-C7, and 4) after adding posterior locking lateral mass screw/rod instrumentation across C4-C7. Non-constraining, nondestructive torques induced flexion, extension, lateral bending, and axial rotation (maximum, 1.5 Nm) while angular motion was measured stereophotogrammetrically.

RESULTS

Discectomy and grafting did not alter the range of motion (ROM) significantly from normal during any loading mode (P > 0.11). Corpectomy and grafting allowed a significantly greater range of motion than normal during flexion, lateral bending, and axial rotation (P < 0.05). Addition of an anterior plate reduced ROM to significantly less than normal during all loading modes in both groups (P < 0.005). Addition of posterior instrumentation further reduced ROM significantly in both groups (P < 0.01). There was no significant difference in ROM between corpectomy and discectomy groups in any loading mode whether uninstrumented (P > 0.18), anteriorly plated (P > 0.33), or anteriorly and posteriorly instrumented (P > 0.30).

CONCLUSIONS

Less difference in stability was observed than was predicted between specimens receiving multilevel discectomy versus multilevel corpectomy, regardless of whether specimens were left unplated, plated anteriorly, or fixated with combined anterior/posterior instrumentation.

Mechanical comparison of posterior instrumentation constructs for spinal fixation across the cervicothoracic junction

STUDY DESIGN

A biomechanical study comparing 4 different posterior implant configurations for instrumentation across the cervicothoracic junction.

OBJECTIVE

To compare mechanical parameters during flexion bending and axial rotation testing among 4 different posterior cervicothoracic rod-and-screw constructs.

SUMMARY OF BACKGROUND DATA

Several posterior rod constructs are available for instrumentation across the cervicothoracic junction. No studies have examined the mechanical properties of constructs of varying rod diameters and rod connector types.

METHODS

Four different rod-and-screw-based constructs for posterior cervicothoracic instrumentation underwent flexion bending or axial rotation testing. Stiffness, ultimate and yield forces (torques) were compared using an analysis of variance.

RESULTS

Significantly lower stiffness, ultimate and yield force (torque) was observed with a 3.5-mm rod construct compared with the 3 other constructs. No significant differences were demonstrated between a dual diameter rod (3.5 mm to 5.5 mm) and a solid domino connector extending between 3.5-mm and 5.5-mm rods. A hinged domino connector construct between 3.5-mm and 5.5-mm rods had similar stiffness but lower ultimate and yield force from either the dual diameter rod or the solid domino construct.

CONCLUSIONS

The present results demonstrate that the 3.5-mm rod-and-screw construct is the weakest configuration for posterior fixation across the cervicothoracic junction. The dual diameter rod and fixed domino connector constructs were the strongest and demonstrated similar values for yield and ultimate force.

A Comparative Biomechanical Analysis of Spinal Instability and Instrumentation of the Cervicothoracic Junction

OBJECTIVE

Stabilization of the cervicothoracic junction is challenging but commonly required in patients with traumatic, neoplastic, congenital, and postlaminectomy conditions. Although extensive research has been performed on stabilization of the cervical spine, there remains a paucity of published data on instrumentation at the cervicothoracic junction. Using 2-column, 3-column, and corpectomy instability models, a biomechanical analysis was performed on the effects of increasing the number of posterior segmental fixation points and/or anterior column reconstruction at the cervicothoracic junction.

METHODS

Multidirectional flexibility testing was performed utilizing a 6-degree-of-freedom spine simulator and 7 fresh-frozen human cadaveric spines (occiput-T6). After intact spine analysis, each specimen was destabilized and reconstructed as follows: (1) C7/T1 2-column injury with posterior instrumentation; (2) C7/T1 3-column injury with posterior instrumentation; (3) C7/T1 3-column injury with anterior interbody cage/plate and posterior instrumentation; and (4) C7/T1 3-column injury plus C7 corpectomy with anterior cage/plate and posterior instrumentation. All reconstruction groups were tested with posterior instrumentation (screws connected by dual-diameter rods) from C5-T1, C5-T2, and C5-T3.

RESULTS

For 2-column injuries, there were no statistically significant differences in flexibility (P>0.05), although there was a trend toward reduced flexibility with increasing levels of thoracic fixation. For 3-column injuries, posterior fixation alone resulted in excessive flexibility in flexion/extension even with instrumentation to T3 (P<0.05). With the addition of anterior column instrumentation, there were no observed differences in flexion/extension and lateral bending. For axial rotation, instrumentation to T1 alone demonstrated increased motion relative to the intact spine (P<0.05). The 3-column injury with corpectomy model demonstrated similar flexibility properties to the 3-column injury model.

CONCLUSIONS

With 3-column instability posterior segmental fixation alone from C5-T3 was inadequate, and the addition of anterior instrumentation restored flexibility to the intact condition. There was a strong trend toward reduced flexibility with increasing levels of thoracic fixation in all instability models.

THE USE OF RIGID INTERNAL FIXATION IN THE SURGICAL MANAGEMENT OF CERVICAL SPONDYLOSIS

IN THE SURGICAL management of cervical spondylosis, the application of rigid internal fixation can enhance the immediate stability of the cervical spine. The sophistication of such internal fixation systems and the indications for their use are continuously evolving. A sound understanding of regional anatomy, biomechanics, and kinematics within the cervical spine is essential for the safe and effective application of internal fixation. Numerous options currently exist for anterior cervical plating systems; some lock the screws to the plate rigidly (constrained), whereas others allow for some rotational or translational motion between the screw and plate (semiconstrained). The role of anterior fixation in single and multilevel fusions is still the subject of some controversy. Long anterior cervical reconstructions may require additional posterior fixation to reliably promote fusion. Rigid fixation in the posterior cervical spine can be achieved with lateral mass screws or pedicle screws. Although lateral mass screws provide excellent fixation within the subaxial cervical spine, the regional anatomy of C2 and C7 often make it difficult to place such screws, and pedicle screws at these levels are advocated. Pedicle screws achieve fixation into both the anterior and posterior column and are arguably the most stable form of rigid internal fixation within the cervical spine. Familiarity with these internal fixation techniques can be an extremely valuable tool for the spine surgeon managing these degenerative disorders of the cervical spine.

Cervical pedicle screws vs. lateral mass screws: uniplanar fatigue analysis and residual pullout strengths

BACKGROUND CONTEXT

Although successful clinical use of cervical pedicle screws has been reported, anatomical studies have shown the possibility for serious iatrogenic injury. However, there are only a limited number of reports on the biomechanical properties of these screws which evaluate the potential benefits of their application.

PURPOSE

To investigate if the pull-out strengths after cyclic uniplanar loading of cervical pedicle screws are superior to lateral mass screws.

STUDY DESIGN

An in vitro biomechanical study.

METHODS

Twenty fresh-frozen disarticulated human vertebrae (C3-C7) were randomized to receive both a 3.5 mm cervical pedicle screw and lateral mass screw. The screws were cyclically loaded 200 times in the sagittal plane. The amount of displacement was recorded every 50 cycles. After cyclical loading, the screws were pulled and tensile load to failure was recorded. Bone density was measured in each specimen and maximum screw insertion torque was recorded for each screw.

RESULTS

During loading the two screw types showed similar stability initially, however the lateral mass screws rapidly loosened compared to the pedicle screws. The rate of loosening in the lateral mass screws was widely variable, while the performance of the pedicle screws was very consistent. The pullout strengths were significantly higher for the cervical pedicle screws (1214 N vs. 332 N) and 40% failed by fracture of the pedicle rather than screw pullout. Pedicle screw pullout strengths correlated with both screw insertion torque and specimen bone density.

CONCLUSIONS

Cervical pedicle screws demonstrated a significantly lower rate of loosening at the bone-screw interface, as well as higher strength after fatigue testing. These biomechanical strengths may justify their use in certain limited clinical applications.

The evolution of spinal instrumentation for the management of occipital cervical and cervicothoracic junctional injuries

STUDY DESIGN/SETTING

Independent computer-based literature review of articles pertaining to instrumentation and fusion of junctional injuries of the cervical spine.

OBJECTIVE

To review and discuss the evolution of instrumentation techniques and systems used in the treatment of cervical spine junctional injuries.

SUMMARY OF BACKGROUND DATA

Instrumentation of junctional injuries of the cervical spine has been limited historically by failure to achieve rigid internal fixation in multiple planes. The evolution of these techniques has required increased insight into the morphology and unique biomechanics of the structures to be instrumented.

METHODS

Computer-based literature search of Ovid and PubMed databases.

RESULTS

Extensive literature search yielded insights into the evolution of systems initially based on onlay bone graft combined with wiring techniques. Such techniques have come to include systems incorporating rigid, longitudinal struts that accommodate multiplanar screws placed in the lateral masses, pedicles, transarticular regions, and occipital bone.

CONCLUSIONS

Despite a rapid evolution of techniques and instrumentation technologies, it remains incumbent on the physician to provide the patient with a surgical procedure that balances the likelihood of a favorable outcome with the risk inherent in the implementation of the procedure.

Effect of constrained posterior screw and rod systems for primary stability: biomechanical in vitro comparison of various instrumentations in a single-level corpectomy model

Cervical corpectomy is a frequently used technique for a wide variety of spinal disorders. The most commonly used approach is anterior, either with or without plating. The results for single-level corpectomy are better than in multilevel procedures. Nevertheless, hardware- or graft-related complications are observed. In the past, constrained implant systems were developed and showed encouraging stability, especially for posterior screw and rod systems in the lumbar spine. In the cervical spine, few reports about the primary stability of constrained systems exist. Therefore, in the present study we evaluated the primary stability of posterior screw and rod systems, constrained and non-constrained, in comparison with anterior plating and circumferential instrumentations in a non-destructive set-up, by loading six human cadaver cervical spines with pure moments in a spine tester. Range of motion and neutral zone were measured for lateral bending, flexion/extension and axial rotation. The testing sequence consisted of: (1) stable testing; (2) testing after destabilization and cage insertion; (3a) additional non-constrained screw and rod system with lateral mass screws, (3b) with pedicle screws instead of lateral mass screws; (4a) constrained screw and rod system with lateral mass screws, (4b) with pedicle screws instead of lateral mass screws; (5) 360 degrees set-up; (6) anterior plate. The stability of the anterior plate was comparable to that of the non-constrained system, except for lateral bending. The primary stability of the non-constrained system could be enhanced by the use of pedicle screws, in contrast to the constrained system, for which a higher primary stability was still found in axial rotation and flexion/extension. For the constrained system, the achievable higher stability could obviate the need to use pedicle screws in low instabilities. Another benefit could be fewer hardware-related complications, higher fusion rate, larger range of instabilities to be treated by one implant system, less restrictive postoperative treatment and possibly better clinical outcome. From a biomechanical standpoint, in regard to primary stability the constrained systems, therefore, seem to be beneficial. Whether this leads to differences in clinical outcome has to be evaluated in clinical trials.

Biomechanical analysis of rigid stabilization techniques for three-column injury in the lower cervical spine

STUDY DESIGN

Comparison of nondestructive multidirectional flexibility in groups of specimens receiving two different posterior instrumentation constructs with or without anterior plating.

OBJECTIVE

To compare stability after a three-column injury stabilized posteriorly by lateral mass screws-rods at C5-C6 and pedicle screws-rods at C7 ("LLP") or by pedicle screws-rods at C5-C6-C7 ("PPP"), and to compare posterior, anterior, and combined anterior-posterior fixation.

SUMMARY OF BACKGROUND DATA

Pedicle screws resist pullout better than lateral mass screws, but little research has compared the stability of pedicle screws to that of lateral mass screws used within constructs.

METHODS

Fourteen human cadaveric C4-T1 specimens were tested intact, posteriorly instrumented (7 LLP and 7 PPP), anteriorly instrumented, or with combined (anterior-posterior) instrumentation. Nonconstraining, nondestructive torques induced flexion, extension, lateral bending, and axial rotation while angular motion was recorded optically.

RESULTS

Posterior, anterior, and combined instrumentation each significantly improved stability (P < 0.05). Combined fixation provided significantly better stability than either anterior or posterior instrumentation alone. In no loading mode and in no testing condition was any parameter significantly different between LLP and PPP. Posterior instrumentation provided significantly better stability than anterior instrumentation.

CONCLUSIONS

Anterior plate and posterior screw-rod fixation alone improve stability in a two-level, three-column cervical injury model. Combined fixation further improves stability. There is little discernible difference in immediate postoperative stability between posterior rod constructs combining lateral mass and pedicle screws and those using only pedicle screws.

Posterior cervicothoracic instrumentation in spine tumors

STUDY DESIGN

We retrospectively review 32 patients who underwent posterior fixation for cervicothoracic junctional tumors. All patients possessed unstable or potential after surgery unstable spines as a result of either their tumors or the surgery performed. We examined cervicothoracic spine stability, maintenance of alignment, and associated complications.

OBJECTIVES

To review our experience with 3 different posterior osteosynthesis systems applied to the cervico-thoracic junction for spinal tumors. Our review includes surgical outcomes and complications. The evolution through 3 different systems between 1994 and 1997 reflects our attempts to improve accuracy in light of variable facet and pedicle interspaces. Our goal is not to compare the efficacy of the systems but to assess the efficiency of cervicothoracic facet and transpedicular screw and plate or rod fixation. However, we will comment on why the evolution occurred. The 3 different systems share a similar characteristic. Each system employs posterior cervical facet screw fixation and thoracic trans-pedicular screw fixation.

SUMMARY OF BACKGROUND DATA

Spinal disorders involving the cervicothoracic junction and specific instrumentation to this region have been sparsely described in the literature.

METHODS

Between June 1994 and June 2000, 32 patients underwent surgery for spinal tumors involving the cervicothoracic junction at our institution. There were 27 males and 5 females. The ages ranged from 17 to 72 years with a mean age of 52 years. A total of 32 cervicothoracic instrumentations were performed. We used the R. Roy-Camille thoracolumbar plate in 20 patients, the cervico-thoracic plate in 8, and the Agora rod system in 4. In all, 96 lateral mass screws were implanted from C4 to C6, 54 into C7, and 180 pedicle screws from T1 to T8. Nineteen patients had lung cancer with vertebral body invasion (Pancoast tumors), 11 had metastasis to the cervicothoracic junction, 1 had a chondrosarcoma, and 1 had myeloma. In a first group consisting of 19 patients, a combination of anterior and extended posterior surgical approaches allowed complete en bloc resection of the tumors, including all invaded vertebrae. Four total vertebrectomies and 15 partial vertebrectomies were performed. A second group of 13 patients had only posterior palliative stabilizing procedures with laminectomy and cervicothoracic fixation.

RESULTS

The follow-up period varied from 3 to 54 months, average 15 months. The average duration of survival for patients who underwent partial or total vertebrectomy was 16 months (range 3-54 months). The average duration of survival for patients who underwent palliative decompression and stabilization was 11 months (range 5-19 months). No changes in the sagittal alignment occurred during the immediate postoperative period for 30 patients. However, 2 mechanical failures occurred. Two patients experienced a clinically significant early increase in thoracic kyphosis and required revision of the posterior instrumentation. A 21-month minimum follow-up was available for 6 patients, in whom all implants were stable. We noted no screw, plate, or rod breakage in this series. No neurologic complications, including root impingement or spinal cord injury, or vertebral artery injury occurred related to screw insertion into either the thoracic pedicles (180 screws) or the cervical lateral masses (96 screws in C4-C5-C6 and 54 screws in C7).

CONCLUSIONS

Posterior plate or rod and screw fixation is a good method of treatment for cervicothoracic instability in spine tumors. Facet screw fixation in the cervical spine with Roy-Camille drilling technique and transpedicular screw fixation in the thoracic spine provides an efficacious means by which to stabilize the cervicothoracic junction. This stabilization technique was effective even in cases of high postoperative instability, such as with partial or total vertebrectomy. This screw-type stabilization is clinically effective and well documented. The evolution through 3 different systems reflects our attempts to improve accuracy in light of variable facet and pedicle interspaces. Importantly, posterior instrumentation will not interfere with subsequent laminectomy or more extreme surgical procedures.

Biomechanical evaluation of cervical lateral mass fixation: a comparison of the Roy-Camille and Magerl screw techniques

Object. The purpose of this study was to assess human cervical spine pullout force after lateral mass fixation involving two different techniques: the Roy-Camille and the Magerl techniques. Although such comparisons have been conducted previously, because of the heterogeneity of results and the importance of this procedure in clinical practice, it is essential to have data derived from a prospective and randomized biomechanical study involving a sufficient sample of human cervical spines. The authors also evaluated the influence of the sex, the vertebral level, the bone mineral density (BMD), the length of bone purchase, and the thickness of the anterior cortical purchase.

Methods. Twenty-one adult cervical spines were harvested from fresh human cadavers. Computerized tomography was performed before and after placing 3.5-mm titanium lateral mass screws from C-3 to C-6. Pullout forces were evaluated using a material testing machine. The load was applied until the pullout of the screw was observed. A total of 152 pullout tests were available, 76 for each type of screw fixation. The statistical analysis was mainly performed using the Kaplan—Meier survival method.

The mean pullout force was 266 ± 124 N for the Roy-Camille technique and 231 ± 94 N for the Magerl technique (p < 0.025). For the C3–4 specimen group, Roy-Camille screws were demonstrated to exert a significantly higher resistance to pullout forces (299 ± 114 N) compared with Magerl screws (242 ± 97 N), whereas no difference was found between the two techniques for the C5–6 specimen group (Roy-Camille 236 ± 122 N and Magerl 220 ± 86 N). Independent of the procedure, pullout strengths were greater at the C3–4 level (271 ± 114 N) than the C5–6 level (228 ± 105 N) (p < 0.05).

No significant correlation between the cancellous BMD, the thickness of the anterior cortical purchase, the length of bone purchase, and maximal pullout forces was found for either technique.

Conclusions. The difference between pullout forces associated with the Roy-Camille and the Magerl techniques was not as significant as has been previously suggested in the literature. It was interesting to note the influence of the vertebral level: Roy-Camille screws demonstrated greater pullout strength (23%) at the C3–4 vertebral level than Magerl screws but no significant difference between the techniques was observed at C5–6.

Evolution of posterior cervical and occipitocervical fusion and instrumentation

The past several decades have been the setting for a remarkable evolution of spinal instrumentation technology. The advancements that have been made have allowed previously complex disorders of the cervical spine, the atlantoaxial articulation, and the occipitocervical junction to be managed more effectively with direct methods of internal fixation and arthrodesis. This has resulted in improvements in patient outcomes and fusion success rates. The improved strength of instrumentation constructs allows minimal, if any, external bracing, obviating the need for a halo orthosis in many cases. In this paper the authors review key events that have occurred in neuroimaging, biomechanical testing, and the development of fusion and instrumentation constructs.

Pedicle screws enhance primary stability in multilevel cervical corpectomies: biomechanical in vitro comparison of different implants including constrained and nonconstrained posterior instumentations

STUDY DESIGN

6 human cervical spines were tested in vitro in a biomechanical nondestructive set-up to compare different anterior, posterior and combined instrumentations after a corpectomy C4-C6.

OBJECTIVES

To evaluate the primary three-dimensional stability of the different instrumentations.

SUMMARY OF BACKGROUND DATA

The clinical results after stabilization of multilevel corpectomies are often disappointing. Higher biomechanical stability could enhance the rate of successful outcomes. The best instrumentation for these high-grade instabilities has yet to be found.

METHODS

Six human cervical specimens were loaded nondestructively with pure moments and unconstrained motion at C3/7 was measured. The six specimens were instrumented with each of the following fixation techniques: 1. Cage 2. Nonconstrained posterior screw and rod system with lateral mass (NC-LM) 3. and pedicle screws (NC-P) 4. Constrained posterior screw and rod system with lateral mass (C-LM) and 5. pedicle screws (C-P) 6. Circumferential (C-P and anterior plate) 7. Anterior plate (OAP).

RESULTS

For flexion/extension and axial rotation the circumferential instrumentation showed lowest ROM values, followed by C-P. The use of pedicle screws showed only a lower ROM when using the constrained system. No difference was found between the two screw types in the nonconstrained system. The anterior plating had the lowest stabilizing effect of all instrumentations, except for the cage alone.

CONCLUSIONS

Usage of pedicle screws enhances primary stability only when using an constrained screw and rod system. In axial rotation the nonconstrained system showed no distinct difference compared to the intact state, independent of the screw type.

Biomechanical evaluation of posterior screw fixation in cadaveric cervical spines

Sixteen fresh-frozen spines from cadavers (C4-T1) were randomized on the basis of dual energy xray absorptiometry analysis of bone mineral density. The specimens were subjected to physiologic loads (<or= 1.5 N-m) in a device that applied pure unconstrained, flexion and extension, lateral bending, and torsional moments. After a major injury, including a wide C6 laminectomy, bilateral capsulectomies, and 65% facetectomy at C6-C7, four constructs were applied to each specimen in a balanced sequence, and the stability tests were repeated. All the constructs were identical posterior cervical rods secured to the spine with lateral mass screws in C5 and pedicle screws in C7. The constructs differed only in the presence or absence of a transverse connector between the rods, presence or absence of lateral mass screws in C6, and unicortical or bicortical lateral mass screws. Insertional screw torque was higher in specimens with greater bone mineral density. Pedicle screws had greater torque than lateral mass screws. Posterior cervical spine rod fixation provided an equivalent stability by use of either unicortical or bicortical lateral mass screws. A transverse stabilizer between the rods reduces the number of lateral mass screws needed in posterior cervical spinal rod systems. Repeated stability tests, even when done with physiologic loads, may compromise the screw-bone interface. This effect is most pronounced in specimens that have low bone mineral density.

Laminectomy for cervical myelopathy

STUDY DESIGN

Cervical laminectomy with or without fusion, or laminoplasty, successfully address congenital or acquired stenosis, multilevel spondylosis, ossification of the posterior longitudinal ligament (OPLL), and ossification of the yellow ligament (OYL). To optimize surgical results, however, these procedures should be applied to carefully selected patients.

OBJECTIVES

To determine the clinical, neurodiagnostic, appropriate posterior cervical approaches to be employed in patients presenting with MR- and CT-documented multilevel cervical disease. To limit perioperative morbidity, dorsal decompressions with or without fusions should be performed utilizing awake intubation and positioning and continuous intraoperative somatosensory-evoked potential monitoring.

SETTING

United States of America.

METHODS

The clinical, neurodiagnostic, and varied dorsal decompressive techniques employed to address pathology are reviewed. Techniques, including laminectomy, laminoforaminotomy, and laminoplasty are described. Where preoperative dynamic X-rays document instability, simultaneous fusions employing wiring or lateral mass plate/screw or rod/screw techniques may be employed. Nevertheless, careful patient selection remains one of the most critical factors to operative success as older individuals with prohibitive comorbidities or fixed long-term neurological deficits should not undergo these procedures.

RESULTS

Short- and long-term outcomes following dorsal decompressions with or without fusions vary. Those with myelopathy over 65 years of age often do well in the short-term, but demonstrate greater long-term deterioration. Factors that correlated with greater susceptibility to deterioration include advanced age (>70 years at the time of the first surgery), severe original myelopathy, and recent trauma.

CONCLUSIONS

Success rates of laminectomy with or without fusion, or laminoplasty may be successfully employed to address multilevel cervical pathology in a carefully selected population of patients.

Biomechanical evaluation of new posterior occipitocervical instrumentation system

Twelve fresh-frozen cadaveric occipitocervical specimens were randomized based on dual energy xray absorptiometry analysis of bone mineral density. The specimens were subjected to physiologic loads in a device that applied pure unconstrained flexion and extension, lateral bending, and axial rotational moments. The spines were tested intact and after major injury simulating transoral decompression of the dens. Biomechanical testing using pure moments with physiologic loads (< 1.5 N-m) was used to compare stability of posterior occipitocervical plates and screws, loop and cable construct, and new cervical rod and screw system. The injury created significantly less stiffness and greater range of motion and neutral zone at C1-C2 in flexion and extension and lateral bending and greater range of motion and neutral zone in axial rotation than the intact state. In lateral bending, the new rod construct had significantly lower mean values for range of motion than the loop and the plate construct. In axial rotation, the rod construct had a significantly higher mean value for stiffness than the other two devices and a significantly lower mean value for range of motion than the loop. The new rod-based instrumentation system for occipitocervical fixation is biomechanically equivalent or superior to a plate and screw construct and a rod and cable system.

Posterior stabilization at the cervicothoracic junction: a biomechanical study

STUDY DESIGN

This study biomechanically evaluated three fixation devices for stability with posterior two- and three-column injuries.

OBJECTIVES

To find an effective means of posteriorly stabilizing injuries at the cervicothoracic junction.

SUMMARY OF BACKGROUND DATA

The cervicothoracic spine is complex anatomically and has been a difficult challenge in approach and stabilization of traumatic and degenerative disorders.

METHODS

Twenty-one human cadaveric spines (C3-T3) were loaded in flexion, extension, lateral bending, and axial torsion. A posterior two-column injury was created at C7-T1. One of three posterior fixation systems was applied (two rod-screw systems, one plate-screw system, all with screws at C5, C6 and T1, T2). The spines were tested again. A three-column injury was created by transecting the remaining anterior structures; the spines were tested a final time.

RESULTS

In flexion-extension, there were no significant differences in stiffness between intact and instrumented two-column injury specimens for all systems; the instrumented three-column injury was significantly (P < 0.05) less stiff than intact specimens in extension. Ranges of motion and neutral zones decreased from intact to instrumented two-column injuries and increased from intact to three-column constructs. In lateral bending and axial rotation, all systems were stiffer than intact spines for both injuries; ranges of motion and neutral zones were reduced for both injuries compared with intact specimens.

CONCLUSION

All three systems stabilize the cervicothoracic junction with a posterior two-column injury in flexion-extension, lateral bending, and axial rotation; none was adequate for a three-column injury, particularly in extension. A three-column injury at this level would warrant supplemental anterior fixation.

Posterior approaches in the management of cervical spondylosis and ossification of the posterior longitudinal ligament

BACKGROUND

If the cervical lordotic curvature has been well preserved, spondylostenosis or ossification of the posterior longitudinal ligament, with or without instability, may be approached posteriorly in selected older patients (over 65 years of age). Posterior surgical alternatives include the laminectomy with or without fusion, or laminoplasty. However, in younger patients or in geriatric patients with predominantly anterior disease with kyphosis, direct anterior surgical procedures yield better results.

METHODS

Laminectomy with medial facetectomy and foraminotomy is classically performed in cases in which stability is preserved. However, posterior stabilization using either facet wiring or lateral mass fusion may be warranted. Although some consider the "open door" laminoplasty a reasonable alternative for dorsal decompression, limitations include restricted access to the hinged side, a potential for "closing of the door," and it does not offer a "real" fusion.

RESULTS

Postoperative neurologic improvement may approximate an 85% incidence of good to excellent results. However, where a posterior decompression has been chosen, particularly in younger individuals with or without a lordotic curvature, or in older patients with kyphosis, they will fail to significantly improve, and will be susceptible to early neurologic deterioration.

CONCLUSIONS

Posterior approaches to cervical disease may be successful in geriatric individuals in whom the cervical lordotic curvature has been well preserved. However, it is inappropriate for either older or younger patients with predominantly anterior disease, for whom direct anterior decompression with or without posterior stabilization is indicated. In those patients with significant ventral ossification of the posterior longitudinal ligament (OPLL), direct anterior resection will result in improved neurologic outcomes, whereas posterior decompression will fail to achieve a similar degree of neurologic recovery. Furthermore, dorsal decompression of OPLL may promote a more rapid progression of OPLL growth and concomitant neurologic deterioration.

Anterior cervical plating reverses load transfer through multilevel strut-grafts

STUDY DESIGN

In vitro biomechanical study using a programmable testing apparatus that replicated physiologic flexion/extension cervical spine motion and loading mechanics.

OBJECTIVE

To determine the influence of anterior plating on multilevel cervical strut-graft mechanics in vitro.

SUMMARY OF BACKGROUND DATA

The addition of anterior instrumentation does not prevent construct failure in multilevel cervical corpectomy.

METHODS

Six fresh human cadaveric cervical spines (C2-T1) were tested in the four following sequential conditions: harvested, C4-C6 corpectomy, strut-grafted, and strut-grafted with an anterior cervical plate. A force-sensing strut-graft was used to measure compression/tension, flexion/extension and lateral bending moments, and axial torsion. Parameters of stiffness, vertebral motion, and strut-graft loads were compared to determine differences between the four spine conditions.

RESULTS

Application of the anterior plate significantly increased the global stiffness (P < 0.01) and decreased the local motion (P < or = 0.01) of the instrumented levels (C3-C7). Flexion of the strut-grafted spine loaded the strut-graft, whereas extension unloaded the strut-graft. With the anterior plate, flexion of the plated spine unloaded the strut-graft. Extension significantly loaded the strut-graft more than similar degrees of flexion in the strut-grafted condition (P = 0.01). Strut-graft loading end limits of 225 N were reached with a mean 7.5 degrees extension in the plated spines.

CONCLUSIONS

Anterior multilevel cervical plating effectively increases stiffness and decreases local cervical motion after corpectomy. However, anterior cervical plating also reverses graft loads and excessively loads the graft in extension, which may promote pistoning and failure of multilevel constructs.

Loosening at the screw-vertebra junction in multilevel anterior cervical plate constructs

STUDY DESIGN

An in vitro biomechanical study of one-level and three-level corpectomy and anterior cervical plate models.

OBJECTIVE

To investigate the failure of the screw-vertebra interfaces in one- and three-level corpectomy models.

SUMMARY AND BACKGROUND DATA

Although there are several biomechanical studies of strength and stability of anterior cervical plating, there has been no investigation into clinically observed failures.

METHODS

One- and three-level models (corpectomy, strut graft, and anterior plate) were constructed from eight cadaveric specimens (C2-T1). Multidirectional flexibility tests (1.0 Nm moments) performed before and after fatigue (1000 cycles, 1.0 Nm flexion-extension, 0.14 Hz) documented the screw-vertebra motions at upper and lower ends. Ranges of motion and neutral zones were determined. Analysis of variance was used to evaluate significant differences between the upper and lower ends of the plates and changes caused by fatigue loading (P < 0.05).

RESULTS

Extension motion at the lower ends was more than at the upper ends in both models. Fatigue increased three-level model ranges of motion at the lower end by 171% in flexion, 164% in extension, 153% in lateral bending, and 115% in axial rotation. Similar increases were observed in neutral zones. Fatigue loading produced no significant changes in one-level models.

CONCLUSION

There was excessive screw-vertebra motion caused by fatigue at the lower end of the three-level corpectomy model. These findings of loosening may explain clinically observed failures at the caudal end of long anterior cervical plate constructs. Longer screws, larger diameter screws, and supplemental posterior fixation may decrease screw-vertebra loosening.

The in vitro effects of instrumentation on multilevel cervical strut-graft mechanics

STUDY DESIGN

Biomechanical study using a programmable testing apparatus that replicated physiologic flexion-extension cervical spine motion, and loading mechanics.

OBJECTIVES

To determine the influence of anterior, posterior, or combined plating on multilevel cervical strut-graft mechanics in vitro.

SUMMARY OF BACKGROUND DATA

The addition of instrumentation does not prevent construct failure in multilevel (more than two levels) cervical corpectomy.

METHODS

Six fresh human cadaveric cervical spines (C2-T1) were tested in six sequential conditions that included harvested (H), C4-6 corpectomy, strut grafted, strut grafted with an anterior cervical plate (SGAP), strut grafted with posterior plates (SGPP), and strut grafted with combined anterior and posterior plates (SGAPP). A customized force-sensing strut graft (FSSG) was used to measure axial compression-tension, flexion-extension and lateral bending moments, and axial torsion. Parameters of stiffness, segmental vertebral motion, and strut-graft loads were compared, to determine differences among the spine conditions.

RESULTS

Flexion of the strut-grafted spine loaded the FSSG, and extension motion unloaded the FSSG. With the anterior plate, flexion of the SGAP spine significantly unloaded the FSSG; extension loaded the FSSG more than flexion of the unplated spine (P = 0.03). The opposite occurred with the posterior plates (SGPP), where flexion of the spine significantly loaded the FSSG (more than the strut grafted spine) and extension unloaded the FSSG (P < 0.03). The combined construct (SGAPP) counteracted the tension band effect of the individual plates and demonstrated significantly less overall FSSG load change than either plate alone (P = 0.03).

CONCLUSIONS

Multilevel cervical instrumentation effectively increases stiffness after corpectomy. However, anterior or posterior plating alone excessively loads the graft with small degrees of motion, which may promote pistoning and failure of multilevel constructs.