René Schmidt

Posterior Atlantoaxial Fixation: Biomechanical in Vitro Comparison of Six Different Techniques

Study Design. Six different techniques for atlantoaxial fixation were biomechanically compared in vitro by nondestructive testing. Objective. To evaluate the immediate three-dimensional stability of a new atlas claw combined with transarticular screws and alternative techniques for transarticular screw fixation in comparison with established techniques. Summary of Background Data. Posterior transarticular screw fixation in combination with wire–bone graft constructs is frequently used for C1–C2 fixation. Sublaminar wire passage carries the potential risk of neurologic complication. Transarticular screw fixation is technically demanding and, for anatomic reasons, not always feasible. Methods. Six human cervical specimens were loaded nondestructively with pure moments, and unconstrained motion at C1–C2 was measured. The six specimens were instrumented with each of the following fixation techniques: Gallie fixation, transarticular screws and Gallie fixation, transarticular screws, transarticular screws and a new atlas claw, isthmic screws in the axis and the atlas claw, and lateral mass screws in the atlas and isthmic screws in the axis connected with rods. Results. The transarticular screws restricted lateral bending and axial rotation best. The three-point fixations (transarticular + Gallie and transarticular + claw) additionally restricted flexion–extension, with lowest values for transarticular screws and the atlas claw. The alternative techniques were not as stable as the three-point fixations, but more stable than the Gallie fixation. Conclusions. Biomechanically, the three-point fixation with transarticular screws and the atlas claw provides a rigid internal fixation that is not dependent on bone graft and sublaminar wiring. In cases wherein transarticular screws are not feasible, the isthmic screws and claw or the lateral mass screws and isthmic screws are biomechanical alternatives with less immediate stability.

Cervical pedicle screws: conventional versus computer-assisted placement of cannulated screws.

Study Design. Prospective clinical study with postoperative radiologic control of pedicle screw placement in the cervical spine. Objectives. To evaluate whether cervical pedicle screws can be placed safely in a conventional technique when using cannulated screws and separate stab incisions. Also, to evaluate if accuracy and safety of pedicle screw placement can be improved using a computer-assisted surgery (CAS) system (VectorVision®; BrainLAB AG, Heimstetten, Germany). Summary of Background Data. Pedicle screws are rarely used in the cervical spine compared to the use in lumbar and thoracic spine. The main reason is probably the potential risk of iatrogenic damage to the spinal cord, nerve roots, or vertebral artery caused by screw misplacement as well as the more demanding technique of pedicle screw placement in the cervical spine. Methods. A total of 52 consecutive patients with posterior cervical or cervicothoracic instrumentations using pedicle screws were evaluated prospectively. For the first 20 patients, 93 pedicle screws were implanted using the conventional technique with the image intensifier in the lateral view, and for the next 32 patients (167 screws), a CAS system was additionally used. For registration of the vertebra, surface-matching algorithms were used. For evaluation of screw placement, postoperative computerized tomography with multiplanar reconstructions in the screw axis was performed for each screw. Results. No implant-related complications were observed. No neurologic or vascular complications were found related to pedicle screws. The rate of pedicle perforations was 8.6% (8 screws) in the conventional group and 3.0% (5 screws) in the CAS group, and in all cases, less than 2-mm displacement. None of the screws with pedicle perforation had to be revised as a result of nonsufficient biomechanical stability or compression of neural/vascular structures. Conclusions. Transpedicular screws in the cervical spine and cervicothoracic junction can be applied safely and with high accuracy in a conventional technique. Cannulated screws and the use of separate stab incisions from C3–C6 with a trocar system allow for reduced screw misplacement rates. The CAS system leads to significantly reduced screw misplacement rates. Therefore, because of the potential risk of injury to the vertebral artery and neural elements, the use of a CAS system seems to be beneficial, especially for pedicle instrumentation C3–C6.

Cement leakage during vertebroplasty: an underestimated problem?

Abstract.Overall, vertebroplasty has a low complication rate. Nevertheless, severe complications can occur. The majority of these are related to cement extrusion. The rate of cement leakage is often obtained by X-ray, with only a single leak registration per vertebra. Detection rate of leaks in comparison with CT and inter-observer reliability for X-ray is, in large parts, unknown. We conducted this study to determine the value of fluoroscopy and X-ray used to detect cement leakage as compared to CT scans. Intraoperative findings in lateral fluoroscopy by the surgeon, and postoperative findings in X-rays by two orthopaedic surgeons, were compared with CT scans for the same study group. Multiple cement leakage was considered, and agreement rate was determined. The detection rate for leaks was 34% for lateral X-ray and 48% for lateral and AP view. Additional AP views only enhanced the detection of leaks in the segmental veins. The agreement rate between fluoroscopy/X-ray and CT scans ranged between 66% and 74%, while inter-observer reliability showed only fair agreement. The rate of cement leaks in vertebroplasty is high if multiple leaks are considered in CT scans. Detection rates using X-rays are low and complicated by only fair inter-observer agreement. Leaks in the basivertebral veins are frequently misinterpreted and can lead to severe complications. Therefore, CT scans should be obtained to calculate the exact leakage rate and to assess persistent or new pain occurring postoperatively.

Adjacent segment mobility after rigid and semirigid instrumentation of the lumbar spine.

Study Design. Retrospective radiographic analysis of lumbar spine range of motion (ROM) after monosegmental fusion and posterior dynamic stabilization at the level L4–L5. Objective. Comparison of segmental ROM at the index level and the cranial and caudal adjacent levels and of global lumbar spine ROM after monosegmental fusion and posterior dynamic stabilization. Summary of Background Data. The postulated advantage of nonfusion technology compared with fusion is based on the assumption that preservation of motion at the treated segment reduces the incidence of adjacent segment effects. Therefore, it is imperative to provide evidence that dynamic stabilization devices avoid hypermobility at the adjacent segments because this might substantiate a protective effect on the adjacent segments. Methods. Twenty-six patients with low back pain and claudication due to degenerative instability at the level L4–L5 with concomitant spinal stenosis were treated either with decompression and Dynesys (n = 11) or with decompression and fusion (n = 15). All patients underwent flexion/extension radiographs before surgery and at latest follow-up. ROM was assessed at the index level (L4–L5), the cranial/caudal adjacent levels (L3–L4/L5–S1), and at the lumbar spine from L2 to S1. Results. There was a significant reduction of the global ROM of the lumbar spine (L2–S1) and the segmental ROM at the index level (L4–L5) in the fusion group, whereas adjacent level ROM did not change significantly. In the Dynesys group, no significant changes of global lumbar spine ROM (L2–S1) and segmental ROM (index level and cranial/caudal adjacent levels) were seen. Conclusion. This study shows that neither monosegmental instrumented fusion nor monosegmental posterior dynamic stabilization with Dynesys alter the ROM of the cranial and caudal adjacent levels. Consequently, monosegmental posterior dynamic stabilization with Dynesys has no effect with regard to adjacent segment mobility compared with monosegmental fusion.

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.

In vitro study of accuracy of cervical pedicle screw insertion using an electronic conductivity device (ATPS part III)

Reconstruction of the highly unstable, anteriorly decompressed cervical spine poses biomechanical challenges to current stabilization strategies, including circumferential instrumented fusion, to prevent failure. To avoid secondary posterior surgery, particularly in the elderly population, while increasing primary construct rigidity of anterior-only reconstructions, the authors introduced the concept of anterior transpedicular screw (ATPS) fixation and plating. We demonstrated its morphological feasibility, its superior biomechanical pull-out characteristics compared with vertebral body screws and the accuracy of inserting ATPS using a manual fluoroscopically assisted technique. Although accuracy was high, showing non-critical breaches in the axial and sagittal plane in 78 and 96%, further research was indicated refining technique and increasing accuracy. In light of first clinical case series, the authors analyzed the impact of using an electronic conductivity device (ECD, PediGuard) on the accuracy of ATPS insertion. As there exist only experiences in thoracolumbar surgery the versatility of the ECD was also assessed for posterior cervical pedicle screw fixation (pCPS). 30 ATPS and 30 pCPS were inserted alternately into the C3–T1 vertebra of five fresh-frozen specimen. Fluoroscopic assistance was only used for the entry point selection, pedicle tract preparation was done using the ECD. Preoperative CT scans were assessed for sclerosis at the pedicle entrance or core, and vertebrae with dense pedicles were excluded. Pre- and postoperative reconstructed CT scans were analyzed for pedicle screw positions according to a previously established grading system. Statistical analysis revealed an astonishingly high accuracy for the ATPS group with no critical screw position (0%) in axial or sagittal plane. In the pCPS group, 88.9% of screws inserted showed non-critical screw position, while 11.1% showed critical pedicle perforations. The usage of an ECD for posterior and anterior pedicle screw tract preparation with the exclusion of dense cortical pedicles was shown to be a successful and clinically sound concept with high-accuracy rates for ATPS and pCPS. In concert with fluoroscopic guidance and pedicle axis views, application of an ECD and exclusion of dense cortical pedicles might increase comfort and safety with the clinical use of pCPS. In addition, we presented a reasonable laboratory setting for the clinical introduction of an ATPS-plate system.

Evaluation of Lumbar Spine Motion With Dynamic X-ray–a Reliability Analysis

Study Design. Radiologic evaluation of lumbar range of motion (ROM) with dynamic radiograph. Objectives. To calculate 95% confidence intervals (CIs) for the measurement error accompanying different methods, different observers, and different levels of training when measuring sagittal plane segmental ROM in lumbar spine. In addition, to compare the 95% CI with frequently common statistical methods of reliability analysis. Summary of Background Data. Dynamic radiographs are commonly used for ROM calculation of the lumbar spine. Yet, the reliability of different measurement methods still remains unclear. Methods. In 24 patients, levels L4–L5 and L5–S1 were measured with the Cobb and superimposition methods on flexion-extension radiographs. There were 2 experienced and 1 inexperienced observer that performed the measurements. The 95% CIs were compared with the corresponding Pearson correlation coefficient and P value (t test). Results. The 95% CI of the superimposition method was ±4.0° for the experienced and ±4.7° for the inexperienced observer. The corresponding values for the Cobb method was ±4.2° for the experienced and ±6.8° for the inexperienced observer. The 95% CI for the measurement error became even worse when different methods or observers were compared, whereas a method constancy revealed superior reliability than observer constancy in experienced observers. Conclusions. For lumbar ROM measurement with dynamic radiograph, the superimposition method seems to be more reliable than the Cobb method. Study protocols dealing with ROM measurement have to calculate the 95% CI of the measurement method used because clinically valid conclusions can only be drawn with respect to these intervals.

Limitations of the Cervical Porcine Spine in Evaluating Spinal Implants in Comparison With Human Cervical Spinal Segments : A Biomechanical In Vitro Comparison of Porcine and Human Cervical Spine Specimens With Different Instrumentation Techniques

Study Design. Porcine and human cervical spine specimens were in vitro biomechanically compared with different instrumentation techniques. Objectives. To evaluate whether subaxial porcine cervical spines are a valid model for implant testing in a single level corpectomy. Summary of Background Data. Biomechanical in vitro tests are widely used for implant tests, mainly with human spine specimens. The availability of human cadavers is limited and the properties of the specimen regarding age, bone mineral density, and grade of degenerative changes is inhomogeneous. Methods. Six porcine and six human cervical specimens were loaded nondestructively with pure moments: 1) in an intact state; 2) after a corpectomy of C5 and substitution by a cage with integrated force sensor; 3) after additional instrumentation with a posterior screw and rod system with: a) lateral mass and b) pedicle screws; 4) after instrumentation with an anterior plate; and 5) with a circumferential instrumentation. The unconstrained motion and the axial loads occurring in the corpectomy gap were measured, as well as the bone mineral density of the specimen before testing. Results. The range of motion in the intact state, as well as for the different instrumentations, was comparable for flexion-extension. In lateral bending and axial rotation, marked differences in the intact state as well as for pedicle screw instrumentations occurred. Conclusions. The subaxial porcine cervical spine is a potential model in flexion-extension because of its biomechanical similarity. For lateral bending and axial rotation, the marked differences severly restrict the comparability.

The stabilizing potential of anterior, posterior and combined techniques for the reconstruction of a 2-level cervical corpectomy model: biomechanical study and first results of ATPS prototyping

Clinical studies reported frequent failure with anterior instrumented multilevel cervical corpectomies. Hence, posterior augmentation was recommended but necessitates a second approach. Thus, an author group evaluated the feasibility, pull-out characteristics, and accuracy of anterior transpedicular screw (ATPS) fixation. Although first success with clinical application of ATPS has already been reported, no data exist on biomechanical characteristics of an ATPS-plate system enabling transpedicular end-level fixation in advanced instabilities. Therefore, we evaluated biomechanical qualities of an ATPS prototype C4–C7 for reduction of range of motion (ROM) and primary stability in a non-destructive setup among five constructs: anterior plate, posterior all-lateral mass screw construct, posterior construct with lateral mass screws C5 + C6 and end-level fixation using pedicle screws unilaterally or bilaterally, and a 360° construct. 12 human spines C3–T1 were divided into two groups. Four constructs were tested in group 1 and three in group 2; the ATPS prototypes were tested in both groups. Specimens were subjected to flexibility test in a spine motion tester at intact state and after 2-level corpectomy C5–C6 with subsequent reconstruction using a distractable cage and one of the osteosynthesis mentioned above. ROM in flexion–extension, axial rotation, and lateral bending was reported as normalized values. All instrumentations but the anterior plate showed significant reduction of ROM for all directions compared to the intact state. The 360° construct outperformed all others in terms of reducing ROM. While there were no significant differences between the 360° and posterior constructs in flexion–extension and lateral bending, the 360° constructs were significantly more stable in axial rotation. Concerning primary stability of ATPS prototypes, there were no significant differences compared to posterior-only constructs in flexion–extension and axial rotation. The 360° construct showed significant differences to the ATPS prototypes in flexion–extension, while no significant differences existed in axial rotation. But in lateral bending, the ATPS prototype and the anterior plate performed significantly worse than the posterior constructs. ATPS was shown to confer increased primary stability compared to the anterior plate in flexion–extension and axial rotation with the latter yielding significance. We showed that primary stability after 2-level corpectomy reconstruction using ATPS prototypes compared favorably to posterior systems and superior to anterior plates. From the biomechanical point, the 360° instrumentation was shown the most efficient for reconstruction of 2-level corpectomies. Further studies will elucidate whether fatigue testing will enhance the benefit of transpedicular anchorage with posterior constructs and ATPS.

Dynamic stabilization of the lumbar spine.

This study analyzed the outcome of patients treated with total disk replacement and posterior dynamic stabilization. For pathologies of different origin, dynamic stabilization of the lumbar spine is a novel alternative to fusion surgery. Although a physiological reconstruction of the sagittal profile was not always achieved, improvement was seen in all subscales of the clinical outcome measures in both treatment groups. Posterior dynamic stabilization and total disk replacement are promising alternatives to fusion with acceptable morbidity for strictly defined indications.