Amir H. Fayyazi

Screw angulation affects bone-screw stresses and bone graft load sharing in anterior cervical corpectomy fusion with a rigid screw-plate construct: a finite element model study

BACKGROUND CONTEXT Anterior corpectomy and reconstruction with bone graft and a rigid screw-plate construct is an established procedure for treatment of cervical neural compression. Despite its reliability in relieving symptoms, there is a high rate of construct failure, especially in multilevel cases. PURPOSE There has been no study evaluating the biomechanical effects of screw angulation on construct stability; this study investigates the C4-C7 construct stability and load-sharing properties among varying screw angulations in a rigid plate-screw construct. STUDY DESIGN A finite element model of a two-level cervical corpectomy with static anterior cervical plate. METHODS A three-dimensional finite element (FE) model of an intact C3-T1 segment was developed and validated. From this intact model, a fusion model (two-level [C5, C6] anterior corpectomy) was developed and validated. After corpectomy, allograft interbody fusion with a rigid anterior screw-plate construct was created from C4 to C7. Five additional FE models were developed from the fusion model corresponding to five different combinations of screw angulations within the vertebral bodies (C4, C7): (0 degrees, 0 degrees), (5 degrees, 5 degrees), (10 degrees, 10 degrees), (15 degrees, 15 degrees), and (15 degrees, 0 degrees). The fifth fusion model was termed as a hybrid fusion model. RESULTS The stability of a two-level corpectomy reconstruction is not dependent on the position of the screws. Despite the locked screw-plate interface, some degree of load sharing is transmitted to the graft. The load seen by the graft and the shear stress at the bone-screw junction is dependent on the angle of the screws with respect to the end plate. Higher stresses are seen at more divergent angles, particularly at the lower level of the construct. CONCLUSION This study suggests that screw divergence from the end plates not only increases load transmission to the graft but also predisposes the screws to higher shear forces after corpectomy reconstruction. In particular, the inferior screw demonstrated larger stress than the upper-level screws. In the proposed hybrid fusion model, lower stresses on the bone graft, end plates, and bone-screw interface were recorded, inferring lower construct failure (end-plate fractures and screw pullout) potential at the inferior construct end.

Radiostereometric Analysis of Postoperative Motion After Application of Dynesys Dynamic Posterior Stabilization System for Treatment of Degenerative Spondylolisthesis

Study Design Prospective case series Objective This was designed to precisely measure motion after posterior dynamic stabilization using Dynesys instrumentation. Summary of Background Data The Dynesys posterior dynamic stabilization system, which stabilizes the spinal segment while potentially decreasing the risk of adjacent segment disease, is undergoing evaluation by the US Food and Drug Administration for treatment of degenerative spondylolisthesis without fusion. Evaluation of adjacent segment disease requires precise characterization of motion on the surgical level. Unfortunately, routine clinical radiographic techniques are imprecise and unreliable for full characterization of spinal segment motion. Radiostereometric analysis, which is very precise and reliable for in vivo measurement of motion, was used to examine spinal segment motion after dynamic stabilization with Dynesys. Methods Six patients (age 59±7 y) underwent posterior decompression followed by posterior stabilization using Dynesys instrumentation (4 one-level, 2 two-levels). Three to 5 tantalum beads were placed in each vertebral body. Postoperative biplanar radiographs were obtained in flexion, extension, right, and left lateral bending, and 3-dimensional reconstruction was performed using radiostereometric analysis at 3, 6, 12, and 24 months postoperatively. The translations and rotations of the superior vertebral body were measured relative to the inferior vertebral body. Results Over the 24-month follow-up period, mean flexion, extension, left, and right lateral bending of the motion segments were noted to be 1.0 degrees, 2.4 degrees, 0.6 degrees, and 0.6 degrees or less, respectively. There were no statistically significant changes in the degree of motion. During follow-up, no significant changes in neutral position of the device were noted in any of the 3 planes, and minimal translation was noted in the postoperative period. Conclusions The Dynesys dynamic instrumentation system seems to stabilize degenerative spondylolisthesis. As expected in the degenerative lumbar spine, the segmental motion of the implanted level in this study was limited and considerably less than normal spinal motion.

Correlation of Radiostereometric Measured Cervical Range of Motion With Clinical Radiographic Findings After Anterior Cervical Discectomy and Fusion

Study Design. Prospective clinical study. Objective. To evaluate the correlation between clinical radiographic findings and sagittal range of motion (ROM) measured using radiostereometric analysis (RSA) after anterior cervical discectomy and fusion (ACDF). Summary of Background Data. Evaluation of fusion after ACDF continues to be difficult. Radiographic films including flexion/extension views are routinely used for this purpose. Unfortunately, routine radiographs are insensitive in demonstrating pseudarthrosis. RSA is an accurate technique that can be used in evaluation of segmental motion in vivo and can potentially be used in evaluation of spinal fusion. Methods. Sixteen patients who underwent multi-level ACDF were enrolled in this study. The procedure was performed in the routine fashion; cervical plates were utilized in each case. Intraoperatively, 3 to 5 tantalum beads were inserted into each vertebral body. At the 1-year follow-up period, sagittal ROM of the operated segments was measured with RSA. In addition, each segment was clinically evaluated for evidence of radiographic fusion by using a 3-point grading system (fused, uncertain, pseudarthrosis) and by measuring the interspinous widening on flexion/extension films. The correlation between the radiographic findings and RSA measured sagittal ROM was evaluated. Results. Fourteen 2-level and two 3-level procedures representing 31 motion segments were analyzed. The average sagittal ROM of all segments as measured by RSA was 1.3 ± 1.4°. The sagittal ROM of the segments with less than 2 mm of interspinous widening on clinical flexion/extension radiographs was measured at 1.1° ± 1.0° with RSA, whereas the sagittal ROM of the segments with greater than 2 mm of interspinous widening was measured at 3.4° ± 2.9°; a significant correlation was noted between the 2-point grading method and the sagittal ROM (Pearson coefficient, r = 0.504, P = 0.004). Using the 3-point grading system, there were 20 levels graded as fused (0.8° ± 0.9°), 6 levels were graded as uncertain (1.7° ± 1.0°), and 4 levels were graded as pseudarthrosis (3.5° ± 2.7°). The pseudarthrosis group showed significantly greater motion than the fusion group (P = 0.005); a significant correlation was noted between the 3-point grading method and the sagittal ROM (Pearson coefficient, r = 0.561, P = 0.001). Conclusion. In this study, we evaluated the utility of RSA in evaluating segmental motion after ACDF and demonstrated a significant difference between segments that demonstrated radiographic evidence of fusion when compared with segments that demonstrated evidence of pseudarthrosis. RSA appears to be a quantitative technique capable of assisting in the evaluation of fusion.

Anterior cervical interbody constructs: effect of a repetitive compressive force on the endplate

Graft subsidence following anterior cervical reconstruction can result in the loss of sagittal balance and recurring foraminal stenosis. This study examined the implant–endplate interface using a cyclic fatigue loading protocol in an attempt to model the subsidence seen in vivo. The superior endplate from 30 cervical vertebrae (C3 to T1) were harvested and biomechanically tested in axial compression with one of three implants: Fibular allograft; titanium mesh cage packed with cancellous chips; and trabecular metal. Each construct was cyclically loaded from 50 to 250 N for 10,000 cycles. Nondestructive cyclic loading of the cervical endplate–implant construct resulted in a stiffer construct independent of the type of the interbody implant tested. The trabecular metal construct demonstrated significantly more axial stability and significantly less subsidence in comparison to the titanium mesh construct. Although the allograft construct resulted in more subsidence than the trabecular metal construct, the difference was not significant and no difference was found when comparing axial stability. For all constructs, the majority of the subsidence during the cyclic testing occurred during the first 500 cycles and was followed by a more gradual settling in the remaining 9,500 cycles.

An ex vivo exothermal and mechanical evaluation of two-solution bone cements in vertebroplasty.

BACKGROUND CONTEXT Previous ex vivo studies showed that the properties of commercial cements modified for use in vertebroplasty are not optimal and are associated with several drawbacks, including high exothermic reaction, low cement viscosity and consequent extravasation, and unpredictable wait time after cement preparation. Additionally, strength and stiffness restoration are controversial varying with the cement type, volume injected, and technique used. PURPOSE To investigate maximum polymerization temperatures and mechanical performance of novel two-solution bone cement (TSBC) modified by the addition of cross-linked poly(methyl methacrylate) nanospheres (η-TSBC) and microspheres (μ-TSBC) in a cadaver vertebroplasty model in comparison to a commercially available cement (KyphX). To study the viability of application of these novel cement formulations in the treatment of vertebral compression fractures. STUDY DESIGN/SETTING Ex vivo biomechanical and exothermal evaluation of TSBCs using cadaveric vertebral bodies (VBs). METHODS Thirty-one cadaveric vertebrae (age, 74±2 years; T score, -1.5±0.5) were disarticulated. Thirteen vertebrae were assigned into three groups and instrumented with thermocouples positioned midbody along the intersection of the midsagittal and midcoronal axes, as well as along the intersection of the midsagittal axis and posterior VB wall. After equilibration at 37°C, 5 mL of cement was injected and temperatures were recorded for 1 hour. The groups were injected with η-TSBC, μ-TSBC, or KyphX. The remaining 18 vertebrae were biomechanically tested. After randomization into three groups, each specimen was fractured in compression and stabilized with 5 mL of each cement type. Each specimen was then retested in axial compression. RESULTS Temperatures in the central region of the vertebrae were significantly lower (p<.05) when injected with η-TSBC (44°C) in comparison to KyphX (75°C) and μ-TSBC (64°C). A significant difference was not detected between the pre- and postcementing strength (p>.05) of the three groups. There was no significant difference between the average values of stiffness among the cements (p>.05), however there was a significant difference between intact and treated stiffness (p<.05). CONCLUSIONS The TSBC cements decreased the local temperature within the VB while providing similar mechanical strength when compared with vertebrae treated with KyphX.

Comparison of Cobb technique, quantitative motion analysis, and radiostereometric analysis in measurement of segmental range of motions after lumbar total disc arthroplasty.

Study Design Prospective clinical study. Objective To compare the amount of segmental motion in the sagittal plane after lumbar total disc arthroplasty (TDA) measured by using the Cobb technique, quantitative motion analysis (QMA), and radiostereometric analysis (RSA). Summary of Background Data The aim of TDA is preservation of motion and therefore essential to properly quantify the motion. Clinically, segmental motion is measured by using the Cobb technique, which involves either the endplates or the implant as radiographic landmarks. This technique has been reported to have large intraobserver and interobserver variability. QMA and RSA are in vivo techniques that can measure the segmental motion with accuracy, but have not been compared with each other or compared with the Cobb technique in the literature. Methods Ten patients (6 males and 4 females, 47±7 y) with lumbar disc degeneration were surgically treated with ProDisc-L (Synthes Inc). Intraoperatively, tantalum beads were inserted into each vertebra and patients were followed postoperatively at 1 month, 1 year, and 2 years. At each follow-up time-point, biplanar flexion/extension radiographs were obtained and sagittal range of motion (ROM) of the index level was calculated by using the RSA technique. Clinical flexion/extension radiographs were also obtained and the sagittal ROM at the same level was calculated by using a modified Cobb technique. The clinical films were additionally analyzed by Medical Metrics for sagittal ROM using QMA. The results of the 3 measurement techniques were statistically analyzed and compared in pairwise fashion. Results A significant difference (P=0.02) was observed between the Cobb technique (5.9±4.9) and RSA (3.5±2.4). A trend (P=0.069) was also seen between QMA (5.7±4.7) and RSA. On paired-samples correlation, a significantly high correlation was seen between Cobb technique and QMA (r=0.868, P<0.001). A larger variability was seen when using the Cobb technique or QMA in comparison to the RSA. Conclusions Sagittal ROM after TDA was similar between QMA and digital Cobb technique. A larger variability was seen between these techniques and RSA.

Assessment of Magnetic Resonance Imaging Artifact Following Cervical Total Disc Arthroplasty.

Background Cervical disc arthroplasty has become a technique for the treatment of cervical degenerative disc disease. Clinically, the need to accurately assess the neural elements at the operative and adjacent levels is critical postoperatively. The purpose of this study was to quantitatively and qualitatively measure the amount of MRI artifact produced by various cervical total disc replacements. Methods T1 and T2-weighted turbo spin-echo MRI sequences were collected on the cervical spine (C2-T1) of a 68 year-old unembalmed male cadaver. A discectomy was performed at C5-6, followed by successive implantation of six different total disc replacements. The scans were quantitatively evaluated by three of the authors. The volume of artifact was measured using image analysis software. Qualitative analysis of the adjacent and index neural elements was performed. Results The artifact in the T2 weighted images was noted to be 58.6±7.3 cm3 for Prestige ST, 14.2±1.3 cm3 for ProDisc-C, 7.5±0.8 cm3 for Discover, 8.0±0.3 cm3 for Prestige LP, 6.6±0.7 cm3 for Bryan, and 7.3±0.6 cm3 for ProDisc-C titanium prototype. Acceptable intraobserver and excellent interobsever correlation was demonstrated using Pearson Correlation and Concordance Correlation Coefficient analysis. The adjacent and implanted level neural elements (spinal cord and neuroforamina) were easily visualized on the T2 weighted images after the implantation of titanium devices (ProDisc-C titanium prototype, Discover, Prestige LP and Bryan). After implantation of a cobalt chrome implant (ProDisc-C), the adjacent level neural elements were easily visualized but the implanted level could not be fully visualized due to distortion of the images. The quality of the distortion was least favorable after the implantation of the stainless steel implant (Prestige ST), where neither the adjacent nor the index level could be fully visualized. Conclusion The volume of the artifact seen following cervical total disc arthroplasty is highly dependent upon the material property of the implant. Quantitative analysis described in this study demonstrated sufficiently low intraobserver and interobserver variability to be considered a reliable technique.

An in vivo kinematic comparison of dynamic lumbar stabilization to lumbar discectomy and posterior lumbar fusion using radiostereometric analysis

Background Biomechanical studies have shown that dynamic stabilization restores the neutral zone and stabilizes the motion segment. Unfortunately, there are limitations to clinical measurement of lumbar motion segments when using routine radiographs. Radiostereometric analysis is a 3-dimensional technique and can measure the spinal motion segment more accurately than techniques using plain film radiographs. The purpose of this study was measure and compare the range of motion after dynamic stabilization, posterior lumbar fusion (PLF), and lumbar discectomy. Methods Four patients who underwent lumbar decompression and dynamic stabilization (Dynesys; Zimmer Spine, Inc., Warsaw, Indiana) for treatment of lumbar spondylosis were compared with 4 patients with a similar diagnosis who were treated by PLF and pedicle screw fixation (PLF group) and 8 patients who had undergone lumbar microdiscectomy (discectomy group) for treatment of radiculopathy. During the surgical procedure, 3 to 5 tantalum beads were placed into each of the operative segments. The patients were followed up postoperatively at 1 month, 1 year, and 2 years. At each follow-up time point, segmental motions (flexion, extension, and total sagittal range of motion [SROM]) were measured by radiostereometric analysis. Results Flexion, extension, and SROM measured 1.0° ± 0.9°, 1.5° ± 1.3°, and 2.3° ± 1.2°, respectively, in the Dynesys group; 1.0° ± 0.6°, 1.1° ± 0.9°, and 1.5° ± 0.6°, respectively, in the PLF group; and 2.9° ± 2.4°, 2.3° ± 1.5°, and 4.7° ± 2.2°, respectively, in the discectomy group. No significant difference in motion was seen between the Dynesys and PLF groups or between the Dynesys and discectomy groups in extension. Significant differences in motions were seen between the PLF and discectomy groups and between the Dynesys and discectomy groups in flexion (P = .007) and SROM (P = .002). There was no significant change in the measured motions over time. Conclusions In this study a significantly lower amount of motion was seen after dynamic stabilization and PLF when compared with discectomy. A future study with a larger cohort is necessary to examine what effect, if any, these motions have on clinical outcomes.

An in vitro study examining a novel suction curette device for lumbar discectomy compared with standard manual discectomy

OBJECTIVE This purpose of this study was to objectively evaluate and assess the efficacy and efficiency of discectomy and endplate preparation during transforaminal lumbar interbody fusion (TLIF) using traditional manual instrumentation versus a novel suction discectomy curette. Transforaminal lumbar interbody fusion is the most widely used approach for lumbar arthrodesis, and its success depends on the ability to achieve fusion. Complete preparation of intervertebral disc space (removal of the nucleus, endplate cartilage, and margin of inner annulus) is the surgical goal. Performing an adequate discectomy requires numerous instrument passes, increasing surgical time and the risk of complications. METHODS Four experienced spinal surgeons performed transforaminal discectomies from T-12 to S-1 on 5 whole-body cadavers. Each level (n = 26) was randomly assigned to either a control group using traditional instruments (12 levels) or to a suction curette group (14 levels). The time required to perform the discectomy and the number of passes through the annulus were recorded. Motion segments were dissected and analyzed by digital photogrammetric analysis. The intervertebral disc and the discectomy cross-sectional areas were measured on both superior and inferior images of each dissected surgical level. Areas were divided into 4 quadrants based on a midsagittal and midcoronal axis and analyzed for regional efficiency. In addition, a cross-sectional area of bony endplate (the area still covered with cartilage) and an area of endplate perforation were evaluated. RESULTS There was no significant difference in surgical time between the techniques (7:51 ± 2:43 minutes in the manual discectomy [MD] group and 7:06 ± 3:33 minutes in the suction curette discectomy [SD] group). There were significantly fewer (p < 0.01) instrument passes in the SD group (13 passes) compared with the MD group (43 passes). For both techniques, the amount of disc removed depended upon the anatomical region, with the posterior-contralateral side having the least amount of disc material removed. There was significantly less (p < 0.01) disc material removed in the MD group (38%) compared with the SD group (48%). The amount of disc material removed was significantly more (p < 0.05) in each quadrant when comparing the SD and MD groups, with the anterior regions showing the largest difference. For both techniques, the preparation of the endplate within the discectomy area resulted in a mostly cartilaginous interface (50% MD, 48% SD); a smaller amount of bony interface area (31% MD, 38% SD); and a smaller amount of perforation to the interface area (19% MD, 13% SD). There were no significant differences between the groups in terms of endplate preparation. CONCLUSIONS The improved discectomy observed with the suction curette device could potentially improve the clinical fusion rate.