Biomechanical evaluation of cervical spine stabilization methods using a porcine model

Reduction of Lower Cervical Facet Dislocation: A Review of All Techniques

Objective: The surgical treatment of lower cervical facet dislocation is controversial. Great advancements on reduction techniques for lower cervical facet dislocation have been made in the past decades. However, there is no article reviewing all the reduction techniques yet. The aim is to review the evolution and advancements of the reduction techniques for lower cervical facet dislocation.Methods: The application of all reduction techniques for lower cervical facet dislocation, including closed reduction, anterior-only, posterior-only, and combined approach reduction, is reviewed and discussed. Recent advancements on the novel techniques of reduction are also described. The principles of various techniques for reduction of cervical facet dislocation are described in detail.Results: All reduction techniques are useful. The anterior-only surgical approach appears to be the most popular approach. Moreover, many novel or modified reduction and fixation methods have been introduced in recent years.Conclusion: The selection of surgical approach depends on a combination of factors, including surgeon preference, patient factors, injury morphology, and inherent advantages and disadvantages of any given approach.

Effect of correction surgery for cervical kyphosis on compensatory mechanisms in overall spinopelvic sagittal alignment

PURPOSE

Progression of kyphotic deformity at the middle/lower cervical spine can cause difficulty with horizontal gaze, so compensation at other spinopelvic parts may occur. However, the precise mechanism remains unclear. The present study investigated the effect of correction surgery for cervical kyphosis on the compensatory mechanisms in overall spinopelvic sagittal alignment.

METHODS

Forty-one patients, comprising 23 males and 18 females (mean age 67 years), underwent correction surgery for cervical kyphosis using the posterior screw-rod system. Spinopelvic lateral radiographs in the standing position were taken before and after surgery. C0-1 angle, C1-2 angle, clivo-axial angle (CAA), C2-7 angle, thoracic kyphosis, lumbar lordosis, pelvic incidence, pelvic tilt, and sacral slope were measured. Correlations between C2-7 angle and these parameters before surgery, and correlations between the correction angle of cervical kyphosis and postoperative changes of these parameters were evaluated.

RESULTS

Negative correlations were found between the C2-7 angle and CAA (R = -0.640, p < 0.01), and C2-7 angle and C0-1 angle (R = -0.762, p < 0.001) before surgery. Negative correlations were found between the correction angle of C2-7 and change of CAA (R = -0.718, p < 0.001), and between the correction angle of C2-7 and change of C0-1 angle (R = -0.672, p < 0.01) after surgery.

CONCLUSIONS

The present study demonstrated that C0-1 angle and CAA are more important in the compensatory mechanism for kyphotic deformity at the middle/lower cervical spine compared to downward parameters. That is, to maintain horizontal gaze, lordosis increases at the cranio-cervical junction with greater kyphosis at the middle/lower cervical spine. Correction of cervical kyphosis in the middle/lower cervical spine resulted in normalization of the C0-1 angle and CAA because the compensatory mechanism at the cranio-cervical junction for obtaining horizontal gaze was no longer necessary after surgical intervention.

Surgical Management of Old Lower Cervical Dislocations With Locked Facet

STUDY DESIGN

Prospective study.

OBJECTIVE

To evaluate the role of the canal and vertebrae sagittal diameter (C/V) ratio in the treatment of old dislocations of the lower cervical spine.

SUMMARY OF BACKGROUND DATA

Few studies have reported the management of old dislocations of the lower cervical spine. Conservative treatments including the use of a Halo vest, neck brace, and prolonged traction have been problematic. Operative treatment consisted of a primary or staged reduction and fusion using an anterior, posterior, or combined approach.

METHODS

Fourteen consecutive patients with old dislocations of the lower cervical spine were included in this series. The preoperative C/V value was calculated based on the measurement on the neutral sagittal computed tomography at the most narrow place of the dislocated segments. Closed reduction was attempted in 9 patients with moderate stenosis (C/V>0.5). Five patients with severe stenosis (C/V≤0.5) were treated with a primary combined anterior and posterior operation. Patient's radiographic information, pain, and neurological function were assessed and recorded before and after surgery.

RESULTS

Closed reduction followed by anterior cervical discectomy and fusion was performed in 3 of 9 patients with moderate stenosis. Eleven patients underwent circumferential release, posterior reduction, and fixation followed by anterior fusion. No severe complications were found. The average operative time was 138±43 minutes. The average blood loss was 239±140 mL. The postoperative C/V value was significantly increased. The neurological status improved at least one grade in all 13 neurologically impaired cases except for 2 who had complete spinal cord injuries. Bony fusion was obtained in all patients at 1-year follow-up.

CONCLUSIONS

The C/V value plays an important role in determining surgical solutions for old lower cervical dislocations with locked facets. Favorable clinical outcomes can be achieved using closed reduction and surgical procedures with anterior or anterior plus posterior approaches.

360-degree cervical spinal arthrodesis for treatment of pediatric cervical spinal tuberculosis with kyphosis

BACKGROUND

There is limited evidence to guide treatment for pediatric cervical spinal tuberculosis with kyphosis (PCSTK). This study retrospectively evaluates the safety, feasibility and efficacy of 360-degree arthrodesis combined with anterior debridement and decompression for treating PCSTK, while simultaneously emphasizing the role of posterior fixation for the correction and maintenance of the kyphosis angle.

METHODS

From May 2006 to December 2012, a total of 12 children with PCSTK underwent 360-degree cervical spinal arthrodesis followed by debridement of focus and decompression of the spinal cord. Data on the angle of kyphosis correction, visual analogue scale scores of pain, the American Spinal Injury Association scoring system of nerve function scores, erythrocyte sedimentation rate (ESR) and body weight were collected at certain periods. Clinical efficacy was evaluated by statistical analysis based on collected data.

RESULTS

Average follow-up period was 34.3 ± 8.6 months. No postoperative complications related to the instrumentation occurred, and neurologic function improved in various degrees. Preoperative kyphosis angle was 41.4 ± 5.2°, and significantly decreased to -4.9 ± 4.9° after surgery. The correction of kyphosis and loss of correction were 47.1 ± 4.9° and 0.6 ± 1.4°, respectively. Average pretreatment ESR was 49.8 ± 13.2 mm/h, which normalized (8.5 ± 0.6 mm/h) within three months in all patients. Average preoperative visual analogue scale was 6.6 ± 1.6, which decreased to 2.3 ± 1.4 postoperatively and 0.3 ± 0.5 during the final follow-up. Mean preoperative body weight was 25.9 ± 5.1 kg, and body weight was 33.5 ± 4.8 kg at the third month of post-operation. Bone healing was achieved in all patients after a mean period of 5.4 months.

CONCLUSIONS

360-degree arthrodesis combined with anterior debridement and decompression is a safe and effective method for the treatment of PCSTK. For the correction and maintenance of the kyphosis angle, additional posterior fixation is recommended.

C5 nerve palsy after posterior reconstruction surgery: predictive risk factors of the incidence and critical range of correction for kyphosis

PURPOSE

It has been reported that the incidence of post-operative segmental nerve palsy, such as C5 palsy, is higher in posterior reconstruction surgery than in conventional laminoplasty. Correction of kyphosis may be related to such a complication. The aim of this study was to elucidate the risk factors of the incidence of post-operative C5 palsy, and the critical range of sagittal realignment in posterior instrumentation surgery.

METHODS

Eighty-eight patients (mean age 64.0 years) were involved. The types of the disease were; 33 spondylosis with kyphosis, 27 rheumatoid arthritis, 17 athetoid cerebral palsy and 11 others. The patients were divided into two groups; Group P: patients with post-operative C5 palsy, and Group NP: patients without C5 palsy. The correction angle of kyphosis, and pre-operative diameter of C4/5 foramen on CT were evaluated between the two groups. Multivariate logistic regression analysis was used to determine the critical range of realignment and the risk factors affecting the incidence of post-operative C5 palsy.

RESULTS

Seventeen (19.3 %) of the 88 patients developed C5 palsy. The correction angle of kyphosis in Group P (15.7°) was significantly larger than that in Group NP (4.5°). In Group P, pre-operative diameters of intervertebral foramen at C4/5 (3.2 mm) were significantly smaller than those in Group NP (4.1 mm). The multivariate analysis demonstrated that the risk factors were the correction angle and pre-operative diameter of the C4/5 intervertebral foramen. The logistic regression model showed a correction angle exceeding 20° was critical for developing the palsy when C4/5 foraminal diameter reaches 4.1 mm, and there is a higher risk when the C4/5 foraminal diameter is less than 2.7 mm regardless of any correction.

CONCLUSIONS

This study has indicated the risk factors of post-operative C5 palsy and the critical range of realignment of the cervical spine after posterior instrumented surgery.

Biomechanical evaluation of a biomimetic spinal construct

Background

Laboratory spinal biomechanical tests using human cadaveric or animal spines have limitations in terms of disease transmission, high sample variability, decay and fatigue during extended testing protocols. Therefore, a synthetic biomimetic spine model may be an acceptable substitute. The goal of current study is to evaluate the properties of a synthetic biomimetic spine model; also to assess the mechanical performance of lateral plating following lateral interbody fusion.

Methods

Three L3/4 synthetic spinal motion segments were examined using a validated pure moment testing system. Moments (±7.5 Nm) were applied in flexion-extension (FE), lateral bending (LB) and axial rotation (AR) at 1Hz for total 10000 cycles in MTS Bionix. An additional test was performed 12 hours after 10000 cycles. A ±10 Nm cycle was also performed to allow provide comparison to the literature. For implantation evaluation, each model was tested in the 4 following conditions: 1) intact, 2) lateral cage alone, 3) lateral cage and plate 4) anterior cage and plate. Results were analysed using ANOVA with post-hoc Tukey’s HSD test.

Results

Range of motion (ROM) exhibited logarithmic growth with cycle number (increases of 16%, 37.5% and 24.3% in AR, FE and LB respectively). No signification difference (p > 0.1) was detected between 4 cycles, 10000 cycles and 12 hour rest stages. All measured parameters were comparable to that of reported cadaveric values. The ROM for a lateral cage and plate construct was not significantly different to the anterior lumbar interbody construct for FE (p = 1.00), LB (p = 0.995) and AR (p = 0.837).

Conclusions

Based on anatomical and biomechanical similarities, the synthetic spine tested here provides a reasonable model to represent the human lumbar spine. Repeated testing did not dramatically alter biomechanics which may allow non-destructive testing between many different procedures and devices without the worry of carry over effects. Small intra-specimen variability and lack of biohazard makes this an attractive alternative for in vitro spine biomechanical testing. It also proved an acceptable surrogate for biomechanical testing, confirming that a lateral lumbar interbody cage and plate construct reduces ROM to a similar degree as anterior lumbar interbody cage and plate constructs.

Outcomes of surgical intervention for cervical spondylotic myelopathy accompanying local kyphosis (comparison between laminoplasty alone and posterior reconstruction surgery using the screw-rod system)

PURPOSE

The surgical strategy for cervical spondylotic myelopathy (CSM) accompanying local kyphosis is controversial. The purpose of the present study was to compare and evaluate the outcomes of two types of surgery for CSM accompanying local kyphosis: (1) laminoplasty alone (LP) and (2) posterior reconstruction surgery (PR) in which we corrected the local kyphosis using a pedicle screw or lateral mass screw.

METHODS

Sixty patients who presented with local kyphosis exceeding 5° were enrolled. LP and PR were each performed on a group of 30 of these patients; 30 CSM patients without local kyphosis, who had undergone LP, were used as controls. The follow-up period was 2 years or longer. Preoperative local kyphosis angles in LP and PR were 8.3° ± 4.4° and 8.8° ± 5.7°, respectively. Preoperative C2-7 angles in LP, PR and controls were -1.7° ± 9.6°, -0.4° ± 7.2° and -12.0° ± 5.6°, respectively. The recovery rate of the JOA score, local kyphosis angle and C2-7 angle at post-op and follow-up were compared between the groups.

RESULTS

The recovery rate of the JOA score in the LP group (32.6 %) was significantly worse than that in the PR group (44.5 %) and that of controls (53.8 %). Local kyphosis angles in the PR and LP groups at follow-up were 4.0° ± 8.6° and 8.0° ± 6.0°, respectively. However, although the C2-7 angle at follow-up was improved to -11.1° ± 12.7° in PR, and maintained at -11.6° ± 6.2° in controls, it deteriorated to 0.5° ± 12.7° in LP.

CONCLUSIONS

The present study is the first to compare the outcomes between LP alone and PR for CSM accompanying local kyphosis. It revealed that PR resulted in a better clinical outcome than did LP alone. This result may be due to reduction of local kyphosis, stabilization of the unstable segment, and/or the maintenance of C2-7 angle until follow-up in the PR group.

Distractive flexion injuries of the subaxial cervical spine treated with a posterior procedure using cervical pedicle screws or a combined anterior and posterior procedure

The purpose of this study was to compare the clinical and radiographic outcomes of patients with distractive flexion (DF) injuries of the subaxial cervical spine who had undergone a posterior procedure using cervical pedicle screw (CPS) fixation with those who had undergone a combined anterior and posterior procedure. Recommendations for the surgical treatment of DF injuries of the subaxial cervical spine remain controversial. There are few clinical reports of posterior CPS fixation for DF injuries. We retrospectively reviewed the clinical records and radiographs of 50 consecutive patients with DF injuries of the subaxial cervical spine treated at the Imakiire General Hospital. Group 1 consisted of 24 patients who underwent posterior wiring fixation and fusion with additional anterior decompression and fusion. Group 2 consisted of 26 patients who underwent posterior decompression and fusion with CPS fixation. Group 1 had a significantly longer operation time (295.4 minutes) than Group 2 (163.3 minutes). Group 1 had significantly higher blood loss (689.1g) than Group 2 (313.7 g). No patient in Group 1 or 2 developed postoperative neurological worsening. The mean loss of kyphotic correction was 1.6° and 0.1° in Groups 1 and 2, respectively, and the loss of kyphotic correction in Group 2 was significantly less than that of Group 1. We suggest that posterior procedures with CPS fixation are reasonable for the management of cervical DF injuries.

Biomechanical and radiographic evaluation of an ovine model for the human lumbar spine

While various species of animal models have been used in preclinical investigations of spinal implant devices to assess their biological adaptation and biomechanical performance, few studies have made comprehensive comparisons to validate their suitability of modelling the human spine. The purpose of this study was to assess essential biomechanical behaviours and disc morphology of the ovine lumbar model. Flexibility testing was conducted on the spines (L3—L4 and L4—L5) of nine skeletally matured sheep. Segmental rotation and intradiscal pressure were measured and load sharing between the intervertebral disc and posterior elements were calculated on the basis of a simplified parallel spring model. Following the tests, the spinal segments were sectioned into a series of sagittal slabs, and transverse radiographs of these slabs were taken to evaluate the variation in the disc height and end-plate curvature. Comparing the biomechanical and radiographic results with published data on the human lumbar spine, good comparability between the ovine and cadaveric lumbar spines was found in terms of the general disc shape and in most of the biomechanical parameters including the range of motion, neutral zone, and load sharing between the intervertebral disc and posterior elements. A few distinctive differences were also found between the two, including flatter sagittal alignment, smaller disc dimensions, and greater lateral bending motion in the ovine model.

Anterior versus combined anterior and posterior fixation/fusion in the treatment of distraction-flexion injury in the lower cervical spine

This study compared the results of combined anterior and posterior fixation/fusion with those of anterior fixation/fusion alone through a retrospective review of 50 patients with a distraction flexion injury of the cervical spine. Group A (n=28) had unilateral facet joint subluxation or dislocation (Allen stage I or II) and anterior fixation/fusion alone. Group B (n=10) had bilateral dislocation (Allen stage III) and anterior fixation/fusion alone. Group C (n=5) had unilateral subluxation or dislocation and combined anterior and posterior fixation/fusion. Group D (n=7) had bilateral dislocation or total dislocation (Allen stage III or IV) and combined anterior and posterior fixation/fusion. The following parameters were analyzed: the change in the vertebral height and Cobb's angle, neurologic recovery, fusion time, fusion rate, surgery time, and the rate of complications. The mean fusion time was 3.75+/-2.10, 6.00+/-2.82, 3.60+/-1.34, and 3.85+/-2.26 months in groups A, B, C, and D, respectively. Group B had a significantly longer mean fusion time than groups A and D (Mann-Whitney U-test, P=0,012, P=0.014). There was a significant difference in the operation time between groups A and B and groups C and D. There were no significant differences in the changes in vertebral height and Cobb's angle, fusion rate, and neurologic recovery. The complications encountered were three cases of distal screw loosening in group A (n=2) and B (n=1), and three cases of delayed union in group A (n=2) and B (n=1). There were no complications in groups C and D. In those with a bilateral dislocation, the fusion time was increased when only anterior fixation/fusion had been performed but the clinical results, such as neurologic recovery and complications, were similar in the four groups. Overall, anterior fixation/fusion alone in a bilateral dislocation is recommended as an alternative method.

Comparative biomechanical study of cervical spine stabilisation by cage alone, cage with plate, or plate-cage: a porcine model

PURPOSE

To compare stability and subsidence associated with 3 types of cervical spine stabilisation.

METHODS

The C3 to C4 vertebrae of 28 Polish pigs were used. Pigs with intact vertebrae (group 1) underwent standard anterior cervical discectomy (group 2), followed by stabilisation using a cage alone (group 3), a cage with plate (group 4), or a plate-cage (group 5). Cervical spine stability and subsidence were compared in all 5 groups.

RESULTS

Stability was significantly increased after stabilisation by a cage with plate or a plate-cage, but not by a cage alone. The difference between stabilisation by a cage with plate and a plate-cage was not significant. Subsidence was maximal after the cage-alone stabilisation (3.1 mm), being 1.6 mm after the cage-with-plate and plate-cage stabilisations.

CONCLUSION

Additional plating as a supplement to anterior interbody cervical cage stabilisation significantly improves segmental stability and subsidence.

Anterior C2-C3 fixation with screws: proposal of a new technique and comparative mechanical assays

The technical difficulties involved in the anterior fixation of the C2-C3 vertebral segment by means of plates and screws, related to retraction of the structures around the vertebral segment, appropriate exposure of the site and positioning of the screws and plate, motivated the development of a new modality of fixation of this segment using only screws. Fixation of the C2-C3 vertebral segment according to the technique proposed requires less exposure of the vertebral segment and does not involve the technical difficulties of standard fixation with plates and screws. In order to study the mechanical properties of this new modality of vertebral fixation, mechanical tests were performed comparing the proposed technique (fixation solely with screws positioned in the craniocaudal direction) and routinely used fixation (H plate and screws). The tests were performed using 80 cervical spine segments from Landrace pigs aged 5 months. The vertebral segments fixed by the two techniques were divided into experimental groups of ten specimens each and submitted to mechanical tests of flexion, extension, lateral bending and rotation in a universal testing machine. The mechanical properties used to compare the results were the load necessary to produce a pre-established deformation and stiffness. No significant differences were observed between the values obtained for the production of the pre-established deformation in the flexion and rotation tests. In the extension and lateral bending tests, the mean values obtained for vertebral segments fixed only with screws were significantly higher. Analysis of stiffness showed no significant difference in the flexion, rotation and lateral bending tests, whereas in the extension tests, the mean values for the group fixed only with screws were significantly higher. The results of the mechanical tests performed showed that fixation of the C2-C3 segment only with screws was not inferior from a mechanical point of view when compared to fixation with H plates of the Orozco type.

Biomechanical Evaluation of the Pullout Strength of Cervical Screws

OBJECTIVE

In the process of anterior cervical fusion, little is known about the biomechanics of anterior cervical screw pullout. In this study, three different aspects of cervical screw fixation were evaluated: self-tapping (ST) versus self-drilling (SD) screws, the effect of screw geometry (length, diameter, thread pitch), and the use of rescue screws.

METHODS

Nine screws consisting of different diameters, lengths, and thread pitch (cancellous and cortical) were tested in peak pullout force in an artificial bone model using an MTS 858 Mini Bionix test system. Rescue screws (4.5 mm) were then inserted in the failed holes of 4.0-mm screws and extracted to determine their holding strength.

RESULTS

Length of screws and thread pitch both had a significant effect on the pullout force. Each 1 mm of increased screw length translates to 16 N of increased force to pullout in the foam bone model. Pullout strength did not vary significantly according to screw diameter or between SD and ST screws. However, the SD screw has an advantage because it can decrease the length of surgery. A decrease in pullout force of between 43% and 70% was found when using rescue screws.

CONCLUSIONS

In situations in which the use of rescue/salvage screws is required, the surgeon should anticipate a significant decrease in the holding force compared with the original screw. Future directions for research include an evaluation of pullout force for screw and plate constructs.

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.

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.

Anwendung eines Bandscheibenersatzimplantats aus einer neuartigen porösen TiO2/Glas-Keramik – Teil 1: Einsatz in der Schafs-Halswirbelsäule und radiologische Verlaufsuntersuchungen / Application of a Stand-Alone Interbody Fusion Cage Based on a Novel Porous TiO2/glass Composite – Part 1: Implantation in the Sheep Cervical Spine and Radiological Evaluation

Zur Beurteilung des radiologischen, biomechanischen und histologischen Einwachsverhaltens neuer Materialien, Implantate und Cages für die zervikale interkorporelle Fusion, bieten sich Tiermodelle und hier insbesondere das Schafs-Halswirbelsäulenmodell an.

In biomechanischen In-vitro-Versuchen an humanen Kadaver-Halswirbelsäulen wurden erste Erfahrungen hinsichtlich Primärstabilität eines Cage aus einer neuartigen, porösen TiO

Zur entsprechenden In-vivo-Beurteilung fusionierten wir 10 Schafs-Halswirbelsäulen in den Höhen C2/3 und C4/5 jeweils mit PMMA und einem Ecopore-Keramik-Cage und führten nativradiologische, sowie computertomographische Verlaufsuntersuchungen direkt post-operativ und alle 4 Wochen in den folgenden 2 bzw. 4 Monaten durch. Neben der Etablierung des Tiermodells, wurden die radiologischen Veränderungen im Verlauf und die Fusion der operierten Segmente analysiert. Darüberhinaus wurden Messungen der entsprechenden Bandscheibenfachhöhen (DSH) und Intervertebralwinkel (IVA) durchgeführt und verglichen.

Nach Einbringen der Implantate in die Bandscheibenfächer nahm zunächst in beiden Gruppen die mittlere Bandscheibenfachhöhe und der Intervertebralwinkel zu (34,8%; 53,9%). In den folgenden Monaten verringerte sich die Bandscheibenfachhöhe nicht signifikant, deutlicher nach Ecopore-Fusion als nach PMMA-Interposition bis auf Werte unterhalb der Ausgangswerte. Ebenso nahm der Intervertebralwinkel im postoperativen Verlauf, deutlicher in der Ecopore-Gruppe als in der PMMA-Gruppe, ab (p < 0,05). Diese Veränderungen im Sinne einer Einsinterung der Implantate, konnte in den radiologischen Verlaufskontrollen morphologisch bestätigt werden. Die radiologisch beurteilbare Fusion, d.h. solide knöcherne Überbauung des operierten Segments, war nach Implantation eines Ecopore-Cage ausgeprägter (83%) als nach PMMA-Interposition (50%) (nicht statistisch signifikant).

In diesem ersten Teil unserer In-vivo-Untersuchungen zu dem Einsatz des neuartigen Cage-Materials wurde die Anwendung im Spondylodesemodell der Schafs-Halswirbelsäule aufgezeigt. Es zeigten sich radiologische Unterschiede, in Bezug auf die ausgeprägtere Einsinterung des Ecopore-Cage und die deutlichere, nachweisbare Fusion des mit dem neuen Material operierten Segments. In dem ersten Teil dieser Studie wurden die radiologischen Veränderungen der fusionierten Segmente über mehrere Monate dargestellt und morphologisch analysiert, bevor die biomechanischen Analysen und Vergleiche in einem weiteren Teil präsentiert werden sollen.

 

Animals are becoming more and more common as in vitro and in vivo models for the human spine. Especially the sheep cervical spine is stated to be of good comparability and usefulness in the evaluation of in vivo radiological, biomechanical and histological behaviour of new bone replacement materials, implants and cages for cervical spine interbody fusion.

In preceding biomechanical in vitro examination human cervical spine specimens were tested after fusion with either a cubical stand-alone interbody fusion cage manufactured from a new porous TiO

Immediately after placement of both implants in the disc spaces the mean DSH and IVA increased (34.8% and 53.9%, respectively). During the following months DSH decreased to a greater extent in the Ecopore-segments than in the PMMA-segments, even to a value below the initial value (p > 0,05). Similarly, the IVA decreased in both groups in the postoperative time lapse, but more distinct in the Ecopore-segments (p < 0,05). These changes in terms of a subsidence of the implants, were confirmed morphologically in the radiological examination in the course. The radiologically evaluated fusion, i.e. bony bridging of the operated segments, was more pronounced after implantation of an Ecopore-cage (83%), than after PMMA interposition (50%), but did not gain statistical significance.

In this first in vivo examination of our new porous ceramic bone replacement material we showed its application in the spondylodesis model of the sheep cervical spine. Distinct radiological changes regarding evident subsidence and detectable fusion of the segments, operated on with the new biomaterial, were seen. We demonstrated the radiological changes of the fused segments during several months and analysed them morphologically, before the biomechanical evaluation will be presented in a subsequent publication.

Biomechanical analysis of anterior cervical spine plate fixation systems with unicortical and bicortical screw purchase

Anterior plate fixation with unicortical screw purchase does not involve the risk of posterior cortex penetration and possible injuries of the spinal cord. However, there are very few biomechanical data about the immediate stability of non-locking plate fixation with unicortical or bicortical screw placement. The aim of the present study was to evaluate the immediate biomechanical properties in terms of flexibility of a non-locking anterior plate system with 4.5-mm screw fixation and unicortical or bicortical screw purchase applied to a single destabilized cervical spine motion segment. Using fresh cadaveric cervical spine specimens C3-C7, multidirectional flexibility was measured at the level C4-C5 before and after destabilization and fixation with an anterior plate with either unicortical or bicortical screw purchase. The results showed that fixed cervical spine segments with anterior plate and bicortical screw purchase were more rigid than intact specimens in all modes of testing. The difference was statistically significant for flexion and extension ( P<0.001). Plate fixation with unicortical screw purchase had statistically significant decreased ranges of motion compared to the intact specimen only in extension. Neither unicortical nor bicortical screw purchase decreased the range of motion significantly in axial rotation compared to the intact specimens. This in vitro study documented that neither unicortical nor bicortical screw purchase with non-locking plate fixation can increase stability in all modes of testing, in axial rotation in particular. Direct comparison between the group with uni- and that with bicortical screw fixation did not reveal significant differences, and therefore no advantage was shown for either type of screw fixation. Therefore, we demonstrated that both uni- and bicortical screw purchase with non-locking plate fixation can decrease immediate flexibility of the tested motion segment, with better results for bicortical purchase. No significant differences were found comparing the two groups of screw fixation. These data suggest that unicortical screw fixation can be used for anterior plate fixation with a comparable immediate stability to bicortical screw fixation.

Biomechanical comparison of cervical spine reconstructive techniques after a multilevel corpectomy of the cervical spine

STUDY DESIGN

An in vitro biomechanical study of several reconstructive techniques after a two-level cervical corpectomy.

OBJECTIVES

To evaluate, compare, and quantitate the stability of several reconstructive strategies (anterior, posterior, or anterior/posterior with or without instrumentation) after a multilevel cervical corpectomy.

SUMMARY OF BACKGROUND DATA

Several clinical and biomechanical studies have questioned the stability of stand-alone long-segment anterior plate fixation after a multilevel (>or=2) corpectomy. The large cantilever forces generated within the stabilized construct, particularly at the caudal screw-bone interface, have led to plate and screw dislodgement and the need for further surgical intervention. The addition of posterior segmental instrumentation has been shown to improve overall stability and decrease local stresses on the anterior fusion construct (graft and plate).

MATERIALS AND METHODS

Seven fresh-frozen cadaveric human cervical spines (C1-T1) were harvested. The C1-C2 and C7-T1 vertebral bodies were embedded in poly-methylmethacrylate (PMMA). Three VICON cameras tracked three-dimensional segmental motions at the ends of the fusion construct after a two-level corpectomy and placement of a strut graft with or without instrumentation. Pure moments (flexion/extension, lateral bending, and axial rotation) were applied to the C1 level of each specimen. The motion segments were loaded to a maximum of 2 Nm using dead weights. Testing was first performed on the intact specimens. Then, a two-level corpectomy at the C4 and C5 levels was performed. A PMMA strut graft was then placed into the corpectomy site. Biomechanical testing was then repeated among three different reconstruction techniques: 1) anterior cervical locking plate (PEAK; Depuy-Acromed, Raynham, MA) with dual unicortical screw fixation at C3 and C6; 2) posterior cervical instrumentation (Summit; Depuy-Acromed) using a 3.0-mm rod with segmental lateral mass screw fixation from C3 to C6; and 3) a combined anterior-posterior instrumentation using the anterior PEAK plate and posterior Summit rod system.

RESULTS

In all pure moments tested (flexion/extension/lateral bending/axial rotation) the combined anterior-posterior instrumentation reconstruction model and the posterior-only instrumentation model were significantly more rigid than the anterior-only instrumentation model (P < 0.05). Interestingly, no statistically significant difference was noted between the combined anterior plate/posterior instrumentation model and the posterior instrumentation-only model.

CONCLUSION

The biomechanical results obtained suggest that posterior segmental instrumentation confers significant stability to a multilevel cervical corpectomy regardless of the presence or absence of anterior instrumentation. In cases in which the stability of a multilevel reconstruction procedure is tenuous, the surgeon should strongly consider the placement of segmental posterior instrumentation to significantly improve the overall stability of the fusion construct.

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.

Comparison of anterior and posterior approaches in cervical spinal cord injuries

This study reports the results of 52 patients with unstable cervical spine injuries and associated spinal cord injuries randomized to either anterior or posterior stabilization and fusion. All patients had achieved reduction and had unstable injuries that were thought to require surgical stabilization. Patients requiring a specific approach for either reduction or decompression were not included. Frankel grades and ASIA motor index scores were followed in each patient as well as fusion status, changes in alignment, and pain at final follow-up. Neurologic improvement was noted in each group with no significant differences. In the anterior group, 70% improved at least 1 Frankel grade and 57% improved 1 Frankel grade in the posterior group. There were two nonunions in the anterior group (90% fusion) and none in the posterior group (100% fusion), although this was not statistically different. Seven patients in each group complained of pain at the final follow-up. There were no significant differences in fusion rates, alignment, neurologic recovery, or long-term complaints of pain in patients treated with either anterior or posterior fusion and instrumentation.

Anterior cervical dynamic ABC plating with single level corpectomy and fusion in forty-two patients

STUDY DESIGN

Forty-two consecutive patients undergoing dynamic ABC plated one-level ACF utilizing iliac crest autograft (38 patients) and fibula allografts (four patients) were evaluated. The unique ABC slotted plate design allows for up to 10 mm of cephalad and 10 mm of caudad plate migration.

OBJECTIVES

To evaluate the incidence and etiology of complications in forty-two patients undergoing anterior cervical dynamic ABC plating (Aesculap, Tuttlingen, Germany), during one level anterior corpectomy with fusion (ACF).

SETTING

New York, USA.

METHODS

Serial dynamic X-ray and 2 Dimensional CAT Scan (2D-CT) studies, obtained 3, 6, and up to 12 months postoperatively, in 42 patients documented the presence of fusion or complications including plate or graft extrusion or pseudarthrosis.

RESULTS

Four (9.5%) of 42 patients developed postoperative plate or graft-related complications during the average follow up interval of 34 months. One patient, with a plate/graft extrusion, required a second two level ACF with posterior wiring and fusion (PWF). Two patients with pseudarthroses and one patient with a delayed iliac crest strut fracture required secondary PWF.

CONCLUSIONS

Effective arthrodesis and a low incidence of complications following one level ACF performed utilizing dynamic ABC plates were attributed to reduced stress shielding and greater graft compression afforded by the unique plate design. Applying dynamic ABC plates for one level ACF was biomechanically advantageous with low morbidity.

Delayed iliac crest autograft fractures following plated single-level anterior cervical corpectomy with fusion

Recurrent cervical pain following single-level plated anterior corpectomy with fusion signaled delayed mid-iliac crest autograft fractures in four of 56 patients. Single-level anterior corpectomy with fusion used 15 fixed plates (Sofamor Danek) and 41 dynamic ABC plates (Aesculap). Patients in the fixed-plate and dynamic-plate groups averaged 46 and 48 years of age, respectively. Fusion was confirmed on both dynamic radiograph and two-dimensional CT studies an average of 4.5 months postoperatively. Two (13%) fixed-plated patients developed mid-iliac crest strut fractures 1 and 2 years postoperatively, whereas two (5%) dynamic-plated patients showed similar fractures 6 and 9 months following surgery. Immobilization in CTO orthoses resulted in fusion in one case, whereas three patients required secondary posterior wiring/fusion. Recurrent pain signaled delayed autograft strut fractures in four of 56 patients undergoing plated single-level anterior corpectomy with fusion.

Anterior dynamic plates in complex cervical reconstructive surgeries

The aim of this work was to evaluate the efficacy of dynamic anterior cervical plates following one-level anterior corpectomy with fusion (ACF) (i.e., C5-7) and two- to four-level ACF with posterior wiring and fusion (PWF). Dynamic plates (ABC Aesculap, Tuttlingen, Germany), allowing for 10 mm of cephalad and 10 mm of caudad plate migration, were applied for one-level ACF (28 patients) and multilevel ACF/PWF (20 patients). Two (7%) dynamic plates failed after one-level ACF. One pseudarthrosis and one plate extrusion occurred, both in morbidly obese patients. No multilevel ACF/PWF failed. The 7% failure rate for dynamic plates applied for one-level ACF is somewhat high, whereas the 0% failure rate for multilevel ACF/PWF with halo immobilization proves more promising. Perhaps PWF with halo stabilization for morbidly obese individuals undergoing one-level ACF would avoid future plate-related problems.

Surgical options for the treatment of cervical spondylotic myelopathy

Cervical spondylotic myelopathy is a disease of the cervical spinal cord that results from circumferential compression of the degenerative cervical spine, often in a congenitally narrow spinal canal. Surgical recommendations must be based on patient characteristics, symptoms, function, and neuroradiologic findings. ACDF is an excellent option for one- or two-level spondylosis without retrovertebral disease. Anterior corpectomy and strut grafting may provide an improved decompression and is ideal for patients with kyphosis or neck pain. Laminectomy historically yields poor results from late deformity and late neurologic deterioration but yields improved results with good surgical technique. Laminoplasty was developed to address cervical stenosis of three or more segments and compares favorable with anterior corpectomy and fusion for neurologic recovery. Laminoplasty has a lower complication rate than corpectomy and strut grafting but has a higher incidence of postoperative axial symptoms.

The Cervical Spine Study Group anterior cervical plate nomenclature

The authors review historical and biomechanical aspects of anterior cervical plate (ACP) systems. They propose a novel classification system for ACPs based on the biomechanical and graft-loading properties of these systems.

A retrospective review of the literature comprising both clinical and laboratory investigations regarding the ACP system was undertaken. Comparison of each system is considered in the context of the biomechanical attributes and graft-loading properties of each type of plate. Salient characteristics reviewed include restriction of screw backout, screw-angle variability, and mobility at the screw–plate interface. A new classification system for ACPs is proposed that primarily considers the ability of the construct to restrict screw backout, as well as the properties of the plate–screw interface—that is, the capacity for rotational or translational movement.

A new classification system is presented that provides unified, biomechanically descriptive nomenclature. Using this nomenclature, the ACP devices currently available and those developed in the future can be uniformly categorized.

Combined anterior and posterior instrumentation in severe fracture-dislocation of the lower cervical spine with help of navigation: a case report

The authors report a case of severe fracture-dislocation of C6-C7 with dural and spinal cord damage causing quadriplegia. The patient was treated surgically with circumferential stabilisation. Intense spinal instrumentation with pedicle screw fixation in addition to anterior decompression and plate fixation was essential for restoring the original vertebral column. This technique of internal fixation provided a more rigid anchor, however the risk to the neurovascular structures could not be completely eliminated. Therefore, the combined anterior and posterior stabilisation assisted with a navigation system was a safer and reasonable surgical treatment for this patient with severe cervical injuries.

Reoperation rates for acute graft extrusion and pseudarthrosis after one-level anterior corpectomy and fusion with and without plate instrumentation: etiology and corrective management

BACKGROUND

Reoperation rates after one-level anterior cervical corpectomy with fusion (ACF) performed without and with plates need further evaluation.

METHODS

Reoperation rates for graft extrusion and symptomatic pseudarthrosis were analyzed following 48 nonplated (1989-1996) and 35 plated (1997-2000) one-level ACF. Preoperatively, patients typically exhibited mild/moderate myelopathy attributed to spondylostenosis and ossification of the posterior longitudinal ligament (OPLL). Thirty-five ACF were performed with plates: 3 Orion, 12 Atlantis, and 20 ABC Aesculap plates. Fusion was documented on both dynamic X-rays and 2- or 3-dimensional CT studies 3 and 6 months postoperatively, or until fusion occurred. Follow-up averaged 82 months for the nonplated patients, and 21 months for the plated patients.

RESULTS

Out of 48 nonplated patients, 3 developed immediate graft extrusions within 24 hours of surgery requiring graft replacement. Another 2 exhibited symptomatic pseudarthrosis 6 months postoperatively, and required secondary posterior wiring with fusion (PWF). In comparison, 1 of the 35 patients with plated one-level ACF developed plate displacement 6 weeks postoperatively, while 3 exhibited symptomatic pseudarthrosis 6 months after surgery, and required secondary posterior wiring and fusion (PWF).

CONCLUSIONS

Comparison of one-level ACF performed with and without plates showed that plating did not appear to reduce pseudarthrosis or graft extrusion rates.

Anterior approaches to cervical spondylosis and ossification of the posterior longitudinal ligament: review of operative technique and assessment of 65 multilevel circumferential procedures

BACKGROUND

Multilevel anterior cervical corpectomy with fusion (ACF) offers direct resection of spondylostenosis and ossification of the posterior longitudinal ligament (OPLL) with immediate stabilization. Ideal candidates for multilevel ACF include younger patients (<65 years of age), or older individuals (>65 years of age) with loss or reversal of the cervical lordosis (kyphosis).

METHODS

Sixty-five patients, averaging 56 years of age and including 40 males and 25 females, with multilevel MR- and CT-documented spondylostenosis and OPLL were studied. Preoperatively, patients exhibited moderate to severe myelopathy (average Nurick grade 3.8), and were managed with 2- to 4-level ACF with posterior wiring and fusion (PWF) procedures with halo application. The first 22 patients had no plate instrumentation, the next 22 had constrained (Orion) plates applied followed sequentially by the application of 13 semi-constrained (Atlantis) plates, and finally, 8 dynamic (ABC Aesculap) plates.

RESULTS

Patients improved an average of three postoperative Nurick grades. None exhibited new cord injuries, whereas three had transient C5 root paresis. Graft/plate or vertebral fracture with extrusion were observed in 3/22 nonplated patients, 2/22 constrained-plated patients, 3/13 semi-constrained-plated patients, and 0/8 dynamic-plated patients. Fusion was documented on dynamic radiographs and 2D-CT or 3D-CT studies obtained 3 and 6 months postoperatively, or later where indicated.

CONCLUSIONS

Multilevel ACF/PWF offers direct resection of spondylostenosis and OPLL with immediate maximal stabilization. Thus far, no graft/plate or vertebral body fracture or extrusions have been seen with dynamic plates, whereas the absence of plating and constrained and semi-constrained plating systems have failed.

Biomechanical comparison of anterior cervical plating and combined anterior/lateral mass plating

BACKGROUND CONTEXT

Previous studies showed anterior plates of older design to be inadequate for stabilizing the cervical spine in all loading directions. No studies have investigated enhancement in stability obtained by combining anterior and posterior plates.

PURPOSE

To determine which modes of loading are stabilized by anterior plating after a cervical burst fracture and to determine whether adding posterior plating further significantly stabilizes the construct.

STUDY DESIGN/SETTING

A repeated-measures in vitro biomechanical flexibility experiment was performed to investigate how surgical destabilization and subsequent addition of hardware components alter spinal stability.

PATIENT SAMPLE

Six human cadaveric specimens were studied.

OUTCOME MEASURES

Angular range of motion (ROM) and neutral zone (NZ) were quantified during flexion, extension, lateral bending, and axial rotation.

METHODS

Nonconstraining, nondestructive torques were applied while recording three-dimensional motion optoelectronically. Specimens were tested intact, destabilized by simulated burst fracture with posterior distraction, plated anteriorly with a unicortical locking system, and plated with a combined anterior/posterior construct.

RESULTS

The anterior plate significantly (p<.05) reduced the ROM relative to normal in all modes of loading and significantly reduced the NZ in flexion and extension. Addition of the posterior plates further significantly reduced the ROM in all modes of loading and reduced the NZ in lateral bending.

CONCLUSIONS

Anterior plating systems are capable of substantially stabilizing the cervical spine in all modes of loading after a burst fracture. The combined approach adds significant stability over anterior plating alone in treating this injury but may be unnecessary clinically. Further study is needed to assess the added clinical benefits of the combined approach and associated risks.

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

STUDY DESIGN

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

OBJECTIVES

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

SUMMARY OF BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The management of one-level anterior cervical corpectomy with fusion using Atlantis hybrid plates: preliminary experience

To limit high pseudarthrosis rates encountered after cervical procedures for adjacent two-level disease, one-level anterior corpectomy with fusion was performed using the newly available Atlantis hybrid plates (fixed screws placed superiorly and variable screws placed inferiorly). Eight one-level anterior corpectomies with fusion were performed using iliac autografts and Atlantis hybrid plates. Nurick and Medical Outcomes Trust Short Form 36 scores were recorded before operation (Nurick grade 2.3) and 6 months after operation. Fusion status was followed an average of 9 months (range, 8 to 11 months). Fusion was confirmed an average of 6 months after operation in seven patients, when the mean Nurick grade was 0.2 and all had improved on the eight Medical Outcomes Trust Short Form 36 health scales. Atlantis plates contributed to successful fusion in seven of eight patients undergoing one-level anterior corpectomy with fusion using Atlantis hybrid plates.

A biomechanical comparison of the Rogers interspinous and the Lovely-Carl tension band wiring techniques for fixation of the cervical spine

OBJECT

The authors conducted a biomechanical study in which they compared the uses of the Rogers interspinous and the Lovely-Carl tension band wiring techniques for internal fixation of the cervical spine.

METHOD

An extensive biomechanical evaluation (stiffness in positive and negative rotations around the x, y, and z axes; range of motion in flexion-extension, bilateral axial rotation, and bilateral bending; and neutral zone in flexion-extension, bilateral axial rotation, and lateral bending to the right and to the left) was performed in two groups of intact calf cervical spines. After these initial tests, all specimens were subjected to a distractive flexion Stage 3 ligamentous lesion. Group 1 specimens then underwent surgical fixation by the Rogers technique, and Group 2 specimens underwent surgery by using the Lovely-Carl technique. After fixation, specimens were again submitted to the same biomechanical evaluation. The percentage increase or decrease between the pre- and postoperative parameters was calculated. These values were considered quantitative indicators of the efficacy of the techniques, and the efficacy of the two techniques was compared.

CONCLUSIONS

Analysis of the findings demonstrated that in the spines treated with the Lovely-Carl technique less restriction of movement was produced without affecting stiffness, compared with those treated with the Rogers technique, thus making the Lovely-Carl technique clinically less useful.

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.

Reconstruction after multilevel corpectomy in the cervical spine. A sagittal plane biomechanical study

STUDY DESIGN

An in vitro biomechanical study of reconstruction techniques used after multilevel cervical corpectomy.

OBJECTIVES

To determine the biomechanical behavior of the cervical spine after a multilevel corpectomy and reconstruction with a strut graft and supplementation of the graft with anterior and posterior plates.

SUMMARY OF BACKGROUND DATA

Reconstruction of the spine after multilevel corpectomy represents a significant challenge, with nonunion or graft dislodgment being relatively common. Anterior and posterior plate fixation have increased the possibilities for supplemental stabilization. Although some clinical studies have been performed to examine multilevel corpectomies reconstructed with plates, biomechanical studies are few and are limited to single-segment models.

METHODS

Flexibility testing was performed on 11 intact cervical spine preparations. Flexibility testing was also conducted on the spine preparations after reconstruction with a strut graft, after supplementation of the graft with an anterior plate, and after supplementation of the graft with lateral mass plates. Physiologic moments were applied dynamically, and the three-dimensional motion of the specimen was recorded with stereophoto-grammetry. Failure testing was performed on the plated specimens in compression. Load displacement curves and failure modes were analyzed.

RESULTS

The range of motion after reconstruction compared with the control was decreased 24% after strut grafting, 43% after application of an anterior plate, and 62% after application of posterior plates. Similarly, flexibility coefficients showed that the posterior plate technique was the least flexible, followed by the anterior plate technique, with the graft alone being the most flexible reconstruction construct. Load to initial failure tended to be higher in posterior than in anterior plate specimens, and screw pullout was the predominant failure mode.

CONCLUSIONS

The application of plates to the cervical spine as an adjunct to bone graft may improve the surgeon's ability to stabilize the spine after multilevel corpectomy. Understanding the biomechanics of these devices and the potential mode of failure is important in their use.

Traumatic instabilities of the cervical spine caused by high-speed axial compression in a human model. An in vitro biomechanical study

STUDY DESIGN

Traumatic injury of the cervical spine was produced on human cadavers and evaluated with instability tests and radiographs.

OBJECTIVE

To relate traumatic injuries of the cervical spine to instability and patterns of traumatic injury to different levels of impact energy.

SUMMARY OF BACKGROUND DATA

Data from young human cadavers are rare in traumatic models of the cervical spine, and instabilities caused by axial compression with different impacts remain unknown.

METHODS

Fourteen cervical spine specimens (C2-C4) obtained from fresh human cadavers were divided evenly into two groups and subjected to axial compressive impact with 30 J and 50 J impact energy, respectively. Pure moments in flexion-extension, left/right lateral bending, and left/right axial rotation were applied to each specimen before and after trauma. The maximum moment was 2.0 Nm in each case. Ranges of motion and neutral zones were measured using stereophotogrammetry.

RESULTS

Ranges of motion and neutral zones for both groups increased after trauma. No bony injury was observed on the radiographs after trauma with 30 J, but motions increased significantly in flexion, extension, and axial rotation. All specimens showed bony injuries after trauma with 50 J, whereas motions continued to increase significantly in all directions. The relative neutral zone values were larger than the corresponding range of motion values, except in flexion-extension after trauma with 50 J.

CONCLUSIONS

The injury patterns of the cervical spine were associated with impact energy, and a high level of impact energy could produce either three-column injury or anterior middle-column injury. Instabilities of the cervical spine caused by compressive trauma increased with the level of impact energy. The neutral zone was more sensitive than the range of motion in representing spinal instability, whereas instability testing was more sensitive than radiographs in evaluating traumatic injury of cervical spine.

The effect of locking fixation screws on the stability of anterior cervical plating

STUDY DESIGN

Current anterior cervical plate systems were tested with locked and unlocked fixation screws and with unicortical and bicortical fixation screws to determine fixation rigidity and pull-off strengths.

OBJECTIVES

To evaluate the effects of screw-plate locking and screw length on fixation strength and stability of anterior cervical plates.

SUMMARY OF BACKGROUND DATA

New plate systems provide for rigid locking of the screw-plate interface, theoretically increasing construct rigidity, allowing unicortical fixation, and preventing screw back-out. There are few data on the effects of locking screws on the stability of anterior cervical plating.

METHODS

Eighty fresh lamb vertebrae (C3-T1) were used. Test systems included: Cervical Spine Locking Plate (CSLP; Synthes, Paoli, PA, Orion plate (Sofamor-Danek, Memphis, TN), and Acroplate (AcroMed, Cleveland, OH). The CSLP and Orion plates were tested with fixation screws, locked and unlocked, and the AcroMed plate with unicortical and bicortical screw purchase. Biomechanical testing of the screw-plate constructs was performed to determine the initial bone-plate rigidity and pull-off strength. A 2.5-Nm cyclic bending moment was then applied to additional constructs for 10(5) cycles, and these constructs retested.

RESULTS

Locked CSLP and Orion constructs were more rigid than all unlocked unicortical systems initially and after cyclic loading (P < 0.05). After cycling, the rigidity of all unlocked unicortical constructs decreased significantly (P < 0.05). There was no significant difference in pull-off strengths between the CSLP, the Orion, and the unicortical AcroMed plate. However, all had significantly less pull-off strength than the AcroMed plate with bicortical screws. A negative correlation was observed between initial pull-off strength and sagittal vertebral body diameter.

CONCLUSIONS

Locking screws significantly increased the rigidity of the tested screw-plate systems initially and after cyclic loading. Because pull-off strength was affected by the vertebral body diameter, use of longer unicortical screws may be clinically beneficial in the patient with larger cervical vertebrae.

Cervical corpectomy: report of 185 cases and review of the literature

OBJECT

This study was conducted to determine the indications, safety, efficacy, and complication rate associated with performing corpectomy to achieve anterior decompression of neural elements or for removing anterior lesions.

METHODS

Between 1987 and 1998, 185 patients underwent cervical corpectomy for the treatment of degenerative spondylitic disease (81 cases), ossification of posterior longitudinal ligament (16 cases), correction of postoperative kyphosis (31 cases), trauma (39 cases), tumor (10 cases), and infection (eight cases). Ninety-nine patients presented with myelopathy, 48 with radiculomyelopathy, 24 with radicular pain, and 14 with neck muscle pain. Eighty-seven patients underwent a one-level corpectomy; 45 of these patients underwent a discectomy at a different level. Seventy patients underwent a two-level corpectomy; 27 of these patients underwent a discectomy at a different level. Twenty-eight patients underwent a three-level corpectomy. Autograft (iliac crest) was used in 141 cases and allograft (fibula) in 44 cases. All but six patients underwent fixation with an anterior plate-screw system. There were no operative deaths. During the procedure the vertebral artery was injured in four patients and preserved in two of them. No neurological sequelae were encountered. Postoperative hoarseness, transient dysphagia, and pain at the graft site were transitory and successfully managed. The fusion rate was 98.8%. Six patients experienced transient deterioration after surgery but they improved. No patient experienced permanent neurological deterioration and 160 (86.5%) improved.

CONCLUSIONS

Corpectomy has an important role in the management of various degenerative, traumatic, neoplastic, or infectious disorders of cervical spine. Following treatment in this series, radiculopathy always improved and myelopathy was reversed in most patients.

Pedicle screw fixation for nontraumatic lesions of the cervical spine

STUDY DESIGN

This retrospective study was conducted to analyze the clinical results in 45 patients with nontraumatic lesions of the cervical spine treated by pedicle screw fixation.

OBJECTIVES

To evaluate the effectiveness of pedicle screw fixation in reconstructive surgery for nontraumatic cervical spinal disorders.

SUMMARY OF BACKGROUND DATA

Pedicle screw fixation for hangman's fracture of the axis and traumatic lesions of the middle and lower cervical spine has been reported; however, there have been no reports on pedicle screw fixation for nontraumatic lesions of the cervical spine.

METHODS

Forty-five patients with nontraumatic lesions of the cervical spine underwent reconstructive surgery including pedicle screw fixation and fusion. Five patients underwent occipitocervical fixation for the lesion of the upper cervical spine, and one patient underwent separate occipitocervical fixation and cervicothoracic fixation. Cervical or cervicothoracic fixation was performed in 39 patients. Twenty-six of these patients underwent simultaneous laminectomy or laminoplasty. Supplemental anterior surgery was conducted for 15 patients.

RESULTS

Solid fusion was obtained in all patients except eight with metastatic vertebral tumors who did not receive bone graft. Correction of kyphosis was adequate. There were no neurovascular complications, except one case of transient radiculopathy caused by screw threads.

CONCLUSIONS

Pedicle screw fixation is a useful procedure for posterior reconstruction of the cervical spine. This procedure does not require the lamina for stabilization, and should be especially valuable for simultaneous posterior decompression and fusion. The risk to neurovascular structures, however, cannot be completely eliminated.