ABSTRACT
The ideal treatment for unstable thoracolumbar fractures remains controversial with posterior reduction and stabilization, anterior reduction and stabilization, combined posterior and anterior reduction and stabilization, and even nonoperative management advocated. Short segment posterior osteosynthesis of these fractures has less comorbidities compared with the other operative approaches but settles into kyphosis over time. Biomechanical comparison of the divergent bridge construct versus the parallel tension band construct was performed for anteriorly destabilized T11-L1 spine segments using three different models: (1) finite element analysis (FEA), (2) a synthetic model, and (3) a human cadaveric model. Outcomes measured were construct stiffness and ultimate failure load. Our objective was to determine if the divergent pedicle screw bridge construct would provide more resistance to kyphotic deforming forces. All three modalities showed greater stiffness with the divergent bridge construct. The FEA calculated a stiffness of 21.6 N/m for the tension band construct versus 34.1 N/m for the divergent bridge construct. The synthetic model resulted in a mean stiffness of 17.3 N/m for parallel tension band versus 20.6 N/m for the divergent bridge (p = 0.03), whereas the cadaveric model had an average stiffness of 15.2 N/m in the parallel tension band compared with 18.4 N/m for the divergent bridge (p = 0.02). Ultimate failure load with the cadaveric model was found to be 622 N for the divergent bridge construct versus 419 N (p = 0.15) for the parallel tension band construct. This study confirms our clinical experience that the short posterior divergent bridge construct provides greater stiffness for the management of unstable thoracolumbar fractures.
ABSTRACT
STUDY DESIGN: A retrospective analysis of prospectively collected data on a cohort of 19 myelomeningocele patients undergoing spino-pelvic deformity surgery. OBJECTIVE: To examine if greater curve correction with third generation spinal implants correlate with improved pressure distribution and resolution, or prevention of skin ulcerations in myelomeningocele patients. SUMMARY OF BACKGROUND DATA: Children born with myelomeningocele have often complex spino-pelvic deformities leading to skin ulcerations. METHODS: A cohort of 19 consecutive wheelchair dependent patients with myelodisplastic spinal deformities, who underwent spinal surgery, was prospectively followed with regular pressure mappings for a minimum of 2 years. Standard spino-pelvic radiologic measurements were obtained. Sitting pressure mappings were obtained over the study period using the Force Sensitive Applications from Vista Medical (Winnipeg, Manitoba, Canada). Statistical analysis was done using SAS (SAS Institute Inc, Cary, NC). Paired t-test and Wilcox on Signed Rank test was used where applicable. Significance was taken to be P<0.05. RESULTS: Surgery significantly corrected radiographic parameters, specifically, Cobb angle (52%), pelvic obliquity (89%), and to a lesser degree pelvic tilt. Stratifying the data based on fixation type showed that the M-W construct was able to significantly correct pelvic obliquity. While significant changes in radiographic variables were observed after surgery, this was not the case with the various pressure mapping variables. Only minor changes after surgery were observed in the average pressure, maximum pressure, and variable coefficient of pressure. What was observed was an improvement in the overall distribution from anterior/posterior and right/left. While the values only approached statistical significance (P=0.053) for right/left, however, this did not appear to be clinically significant regarding skin ulceration. CONCLUSION: Despite significant surgical corrections in radiographic parameters, these resulted in small changes in pressure distributions and do not appear to influence skin ulceration in the myelomeningocele patient. Pressure mapping may not be a useful tool in predicting outcome of spinal surgery. Factors which were proven to influence pressure distribution are the sagittal pelvic orientation and also achieving coronal spine balance.