Biomechanical study of the development of scoliosis, using a thoracolumbar spine model

Predictors for spinal deformity following resection of intramedullary tumor via posterior approach: a systematic review and meta-analysis

PURPOSE

The present study aimed to identify the clinical predictive factors for worsened spinal deformity (SD) following surgical resection via posterior approach for primary intramedullary tumors.

METHODS

A systematic search was performed using PubMed, Web of Science, and Scopus databases to extract potential references. Observational studies reporting predictive factors for worsened SD following surgical resection via posterior approach for primary intramedullary tumors were included. The odds ratio (OR) was calculated for dichotomous parameters.

RESULTS

Four retrospective cohort studies were included in the meta-analysis. They were comprised of two groups of patients; those who developed SD (n = 87) and those who did not (n = 227). For patients with IMSCTs, age under 25 years as well as age under 13 years were the demographic variables associated with postoperative SD (odds ratio [OR] 3.92; p = 0.0002 and OR 4.22; p = 0.003). In both the fusion and the non-fusion subgroups, preoperative spinal deformity strongly predicted postoperative SD (OR 11.94; p < 0.001), with the risk highly elevated among the non-fusion patients (OR 24.64; p < 0.0002). Thoracolumbar junction involvement was also found to be a predictor of postoperative SD for patients with IMSCT (OR 2.89; p = 0.02).

CONCLUSION

This study highlights the importance of considering age, preoperative spinal deformity, and thoracolumbar junction involvement as predictors of postoperative spinal deformity following surgical resection for IMSCT. These findings may provide guidance for the management of these patients, including the development of preoperative planning strategies and the selection of the most appropriate surgical approach for high-risk patients.

Failure analysis of C-5 after total disc replacement with ProDisc-C at 1 and 2 levels and in combination with a fusion cage: finite-element and biomechanical models

OBJECT

The purpose of this study was to evaluate the failure risk of cervical vertebrae after total disc replacement with a keel-design prosthesis (ProDisc-C), taking into consideration the effects of vertebral body height, multilevel replacement, and the association with an adjacent fusion cage. Although promising clinical results have been reported for the ProDisc-C, some clinical studies have reported vertebral body–splitting fractures at single- and multilevel arthroplasty sites. This implant has central keels to provide solid initial stability, and some authors associate the potential risk of vertebral body failure with the keel design, especially in patients with small vertebral body height or when the implant is used at multiple levels.

METHODS

The study was performed using a specimen-specific C4–6 cervical-segment finite-element model to assess the compressive strains on the C-5 vertebral body for each cervical segment configuration, and synthetic polyurethane models to experimentally predict the compressive load at failure for 3 vertebral body heights.

RESULTS

The use of a keeled ProDisc-C prosthesis at multiple levels or in combination with a fusion cage increases by a factor of 2–3 the compressive strains at the C-5 vertebral body relative to single-level arthroplasty. All implanted segment configurations tested demonstrated a continuum of the load at failure and the vertebral body height, but no significant differences were found between the 3 vertebral body heights in each segment configuration.

CONCLUSIONS

The use of a keeled ProDisc-C prosthesis at 2 adjacent levels or combined with a fusion cage presented the lowest load-at-failure values, 2 times higher on average than the ones occurring during physiological tasks. This fact indicates an identical and limited risk of vertebral body failure for these 2 segment configurations, whereas vertebral body height appears to slightly affect this risk. However, for some tasks that place higher physical demands on the neck, beyond what was represented by our models, there may also be risk of microdamage initiation, which is not present in the single-level arthroplasty.

Comparative fixation methods of cervical disc arthroplasty versus conventional methods of anterior cervical arthrodesis: serration, teeth, keels, or screws?

Object

Using a synthetic vertebral model, the authors quantified the comparative fixation strengths and failure mechanisms of 6 cervical disc arthroplasty devices versus 2 conventional methods of cervical arthrodesis, highlighting biomechanical advantages of prosthetic endplate fixation properties.

Methods

Eight cervical implant configurations were evaluated in the current investigation: 1) PCM Low Profile; 2) PCM V-Teeth; 3) PCM Modular Flange; 4) PCM Fixed Flange; 5) Prestige LP; 6) Kineflex/C disc; 7) anterior cervical plate + interbody cage; and 8) tricortical iliac crest. All PCM treatments contained a serrated implant surface (0.4 mm). The PCM V-Teeth and Prestige contained 2 additional rows of teeth, which were 1 mm and 2 mm high, respectively. The PCM Modular and Fixed Flanged devices and anterior cervical plate were augmented with 4 vertebral screws. Eight pullout tests were performed for each of the 8 conditions by using a synthetic fixation model consisting of solid rigid polyurethane foam blocks. Biomechanical testing was conducted using an 858 Bionix test system configured with an unconstrained testing platform. Implants were positioned between testing blocks, using a compressive preload of −267 N. Tensile load-to-failure testing was performed at 2.5 mm/second, with quantification of peak load at failure (in Newtons), implant surface area (in square millimeters), and failure mechanisms.

Results

The mean loads at failure for the 8 implants were as follows: 257.4 ± 28.54 for the PCM Low Profile; 308.8 ± 15.31 for PCM V-Teeth; 496.36 ± 40.01 for PCM Modular Flange; 528.03± 127.8 for PCM Fixed Flange; 306.4 ± 31.3 for Prestige LP; 286.9 ± 18.4 for Kineflex/C disc; 635.53 ± 112.62 for anterior cervical plate + interbody cage; and 161.61 ± 16.58 for tricortical iliac crest. The anterior plate exhibited the highest load at failure compared with all other treatments (p < 0.05). The PCM Modular and Fixed Flange PCM constructs in which screw fixation was used exhibited higher pullout loads than all other treatments except the anterior plate (p < 0.05). The PCM VTeeth and Prestige and Kineflex/C implants exhibited higher pullout loads than the PCM Low Profile and tricortical iliac crest (p < 0.05). Tricortical iliac crest exhibited the lowest pullout strength, which was different from all other treatments (p < 0.05). The surface area of endplate contact, measuring 300 mm2 (PCM treatments), 275 mm2 (Prestige LP), 250 mm2 (Kineflex/C disc), 180 mm2 (plate + cage), and 235 mm2 (tricortical iliac crest), did not correlate with pullout strength (p > 0.05). The PCM, Prestige, and Kineflex constructs, which did not use screw fixation, all failed by direct pullout. Screw fixation devices, including anterior plates, led to test block fracture, and tricortical iliac crest failed by direct pullout.

Conclusions

These results demonstrate a continuum of fixation strength based on prosthetic endplate design. Disc arthroplasty constructs implanted using vertebral body screw fixation exhibited the highest pullout strength. Prosthetic endplates containing toothed ridges (≥ 1 mm) or keels placed second in fixation strength, whereas endplates containing serrated edges exhibited the lowest fixation strength. All treatments exhibited greater fixation strength than conventional tricortical iliac crest. The current study offers insights into the benefits of various prosthetic endplate designs, which may potentially improve acute fixation following cervical disc arthroplasty.

Correlation of a preoperative grading scale with progressive spinal deformity following surgery for intramedullary spinal cord tumors in children

OBJECT

Postoperative progressive spinal deformity often complicates functional outcome after resection of pediatric intramedullary spinal cord tumors (IMSCTs). The authors propose a preoperative grading scale that correlates with the postoperative development of progressive spinal deformity requiring subsequent fusion.

METHODS

The data obtained in 164 patients who underwent resection of an IMSCT at a single institution were retrospectively collected and analyzed to determine the development of progressive spinal deformity requiring fusion. A grading scale (range of scores I-V) was created based on the presence or absence of 4 preoperative variables: preoperative scoliosis, involvement of the thorocolumbar junction, age<13 years, and number of surgeries for an IMSCT. The grading scale was then retrospectively applied to this series of 164 children to assess the correlation of variables with subsequent spinal deformity.

RESULTS

Nine patients presented with Grade I status, 41 patients with Grade II, 58 patients with Grade III, 44 patients with Grade IV, and 12 patients with Grade V. Overall, 44 patients (27%) developed progressive spinal deformity requiring fusion at a mean follow-up of 5 years after surgery. A higher preoperative grade was associated with an increasing need for subsequent fusion for progressive spinal deformity (Grade I [0%], Grade II [5%], Grade III [26%], Grade IV [40%], and Grade V status [75%]).

CONCLUSIONS

Application of this grading scheme to a series of resected pediatric IMSCTs has demonstrated its correlation with the incidence of postoperative progressive spinal deformity requiring fusion. The application of a standardized grading scheme will assist in the process of surgical decision making and postoperative evaluation.

Biomechanical study of anterior spinal instrumentation configurations

The biomechanical impact of the surgical instrumentation configuration for spine surgery is hard to evaluate by the surgeons in pre-operative situation. This study was performed to evaluate different configurations of the anterior instrumentation of the spine, with simulated post-operative conditions, to recommend configurations to the surgeons. Four biomechanical parameters of the anterior instrumentation with simulated post-operative conditions have been studied. They were the screw diameter (5.5-7.5 mm) and its angle (0 degrees - 22.5 degrees), the bone grip of the screw (mono-bi cortical) and the amount of instrumented levels (5-8). Eight configurations were tested using an experimental plan with instrumented synthetic spinal models. A follower load was applied and the models were loaded in flexion, torsion and lateral bending. At 5 Nm, average final stiffness was greater in flexion (0.92 Nm/degrees) than in lateral bending (0.56 Nm/degrees) and than in torsion (0.26 Nm/degrees). The screw angle was the parameter influencing the most the final stiffness and the coupling behaviors. It has a significant effect (p < or = 0.05) on increasing the final stiffness for a 22.5 degrees screw angle in flexion and for a coronal screw angle (0 degrees) in lateral bending. The bi-cortical bone grip of the screw significantly increased the initial stiffness in flexion and lateral bending. Mathematical models representing the behavior of an instrumented spinal model have been used to identify optimal instrumentation configurations. A variation of the angle of the screw from 22.5 degrees to 0 degrees gave a global final stiffness diminution of 13% and a global coupling diminution of 40%. The screw angle was the most important parameter affecting the stiffness and the coupling of the instrumented spine with simulated post-operative conditions. Information about the effect of four different biomechanical parameters will be helpful in preoperative situations to guide surgeons in their clinical choices.