Marcus Richter

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.

Computer-assisted surgery in posterior instrumentation of the cervical spine: an in-vitro feasibility study.

Abstract Transarticular C1/2 screws are widely used in posterior cervical spine instrumentation. The use of pedicle screws in the cervical spine remains uncommon. Due to superior biomechanical stability compared to lateral mass screws, pedicle screws can be used, especially for patients with poor bone quality or defects in the anterior column. Nevertheless there are potential risks of iatrogenic damage to the spinal cord, nerve roots or the vertebral artery associated with both posterior cervical spine instrumentation techniques. Therefore, the aim of this study was to evaluate whether C1/2 transarticular screws as well as transpedicular screws in C3 and C4 can be applied safely and with high accuracy using a computer-assisted surgery (CAS) system. We used 13 human cadaver C0-C5 spine segments. We installed 1.4-mm Kirschner wires transarticular in C1/2, using a specially designed guide, and drilled 2.5-mm pedicle holes in C3 and C4 with the assistance of the CAS system. Hole positions were evaluated by palpation, CT and dissection. Forty-eight (92%) of the 52 drilled pedicles were correctly positioned after palpation, imaging and dissection. The vertebral artery was not injured in any specimen. All of the 26 C1/2 Kirschner wires were placed properly after imaging and dissection evaluations. No injury to vascular or bony structures was observed. C1/2 transarticular screws as well as transpedicular screws in the cervical spine can be applied safely and with high accuracy using a CAS system in vitro. Therefore, this technique may be used in a clinical setting, as it offers improved accuracy and reduced radiation dose for the patient and the medical staff. Nevertheless, users should take note of known sources of possible faults causing inaccuracies in order to prevent iatrogenic damage. Small pedicles, with a diameter of less than 4.0 mm, may not be suitable for pedicle screws.

The Tokuhashi Score: Significant Predictive Value for the Life Expectancy of Patients With Breast Cancer With Spinal Metastases

Study Design. Retrospective study of 55 consecutive patients with spinal metastases secondary to breast cancer who underwent surgery. Objective. To evaluate the predictive value of the Tokuhashi score for life expectancy in patients with breast cancer with spinal metastases. Summary of Background Data. The score, composed of 6 parameters each rated from 0 to 2, has been proposed by Tokuhashi and colleagues for the prognostic assessment of patients with spinal metastases. Methods. A total of 55 patients surgically treated for vertebral metastases secondary to breast cancer were studied. The score was calculated for each patient and, according to Tokuhashi, the patients were divided into 3 groups with different life expectancy according to their total number of scoring points. In a second step, the grouping for prognosis was modified to get a better correlation of the predicted and definitive survival. Results. Applying the Tokuhashi score for the estimation of life expectancy of patients with breast cancer with vertebral metastases provided very reliable results. However, the original analysis by Tokuhashi showed a limited correlation between predicted and real survival for each prognostic group. Therefore, our patients were divided into modified prognostic groups regarding their total number of scoring points, leading to a higher significance of the predicted prognosis in each group (P < 0.0001), and a better correlation of the predicted and real survival. Conclusion. The modified Tokuhashi score assists in decision making based on reliable estimators of life expectancy in patients with spinal metastases secondary to breast cancer.

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.

Revision of cannulated and perforated cement-augmented pedicle screws: a biomechanical study in human cadavers.

Study Design. Biomechanical investigation of primary and revised cement-augmented pedicle screws in comparison with unaugmented screws. Objective. To evaluate revision of cannulated pedicle screws and investigate cement-augmented and nonaugmented screws biomechanically, testing the torque of primary screws and axial pullout force of revised screws in cadaver vertebrae. Summary of Background Data. Cement augmentation increases the pullout force and stability of pedicle screws in vertebrae with low bone mineral density, but surgeons are concerned about complications during revision. Methods. Bone mineral density was measured using quantitative computed tomography (CT) in 23 osteoporotic thoracolumbar junction vertebrae from human cadavers. Cannulated pedicle screws, augmented with bone cement (on right) or unaugmented (left), were inserted into each vertebra. After CT control, extraction torque was measured and the pedicles were reinstrumented with larger-diameter screws. The right screws were augmented again, with another CT control, before pullout testing. Results. Mean vertebral bone density was 52.6 mg/cm3. No major screw malpositioning was observed on primary CTs. Cement leakage was observed anterolaterally and into the spinal canal. Mean maximal torque in augmented screws (1.2 Nm, SD: 0.6) differed significantly from nonaugmented screws (0.8 Nm, SD: 0.6). Screw removal did not lead to vertebral destruction. No relevant changes due to positioning or leakage were observed on CT after revision procedures compared with primary findings. Maximal pullout force in revised augmented screws (713.2 N, SD: 254.6) differed significantly compared with nonaugmented screws (554.0 N, SD: 296.5). Bone damage was observed in several vertebrae during pullout force testing in augmented screws. Conclusion. Revision of cement-augmented pedicle screws was feasible without bone destruction, and larger-diameter screws can be used in revision procedures. The pullout force after revision was significantly better in cement-augmented screws. During pullout testing, the cement-bone interface broke before the screw-cement interface in several vertebrae, fracturing the pedicles.

Load-displacement properties of the normal and injured lower cervical spine in vitro

Abstract The objective of this study was to determine which discoligamentous structures of the lower cervical spine provide significant stability with regard to different loading conditions. Accordingly, the load-displacement properties of the normal and injured lower cervical spine were tested in vitro. Four artificially created stages of increasing discoligamentous instability of the segment C5/6 were compared to the normal C5/6 segment. Six fresh human cadaver spine segments C4-C7 were tested in flexion/extension, axial rotation, and lateral bending using pure moments of ± 2.5 Nm without axial preload. Five conditions were investigated consecutively: (1) the intact functional spinal unit (FSU) C5/6; (2) the FSU C5/6 with the anterior longitudinal ligament and the intertransverse ligaments sectioned; (3) the FSU C5/6 with an additional 10-mm-deep incision of the anterior half of the anulus fibrosus and the disc; (4) the FSU C5/6 with additionally sectioned ligamenta flava as well as interspinous and supraspinous ligaments; (5) the FSU C5/6 with additional capsulotomy of the facet joints. In flexion/extension, significant differences were observed concerning range of motion (ROM) and neutral zone (NZ) for all four stages of instability compared to the intact FSU. In axial rotation, only the stage 4 instability showed a significantly increased ROM and NZ compared to the intact FSU. For lateral bending, no significant differences were observed. Based on these data, we conclude that flexion/extension is the most sensitive load-direction for the tested discoligamentous instabilities.

Biomechanical evaluation of a new modular rod-screw implant system for posterior instrumentation of the occipito-cervical spine: in-vitro comparison with two established implant systems

Abstract Posterior instrumentation of the occipito-cervical spine has become an established procedure in a variety of indications. The use of rod-screw systems improved posterior instrumentation as it allows optimal screw positioning adapted to the individual anatomic situation. However, there are still some drawbacks concerning the different implant designs. Therefore, a new modular rod-screw implant system has been developed to overcome some of the drawbacks of established systems. The aim of this study was to evaluate whether posterior internal fixation of the occipito-cervical spine with the new implant system improves primary biomechanical stability. Three different internal fixation systems were compared in this study: the CerviFix System, the Olerud Cervical Rod Spinal System and the newly developed Neon Occipito Cervical System. Eight human cervical spine C0/C5 specimens were instrumented from C0 to C4 with occipital fixation, transarticular screws in C1/C2 and lateral mass or pedicle screws in C3 and C4. The specimens were tested in flexion/extension, axial rotation, and lateral bending using pure moments of ± 2.5 Nm without axial preload. After testing the intact spine, the different instrumentations were tested after destabilising C0/C2 and C3/C4. Primary stability was significantly increased, in all load cases, with the new modular implant system compared to the other implant systems. Pedicle screw instrumentation tended to be more stable compared to lateral mass screws; nevertheless, significant differences were observed only for lateral bending. As the experimental design precluded any cyclic testing, the data represent only the primary stability of the implants. In summary, this study showed that posterior instrumentation of the cervical spine using the new Neon Occipito Cervical System improves primary biomechanical stability compared to the CerviFix System and the Olerud Cervical Rod Spinal 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.