Biomechanical effect of pedicle screw distribution in AIS instrumentation using a segmental translation technique: computer modeling and simulation

BACKGROUND: Efforts to select the appropriate number of implants in adolescent idiopathic scoliosis (AIS) instrumentation are hampered by a lack of biomechanical studies. The objective was to biomechanically evaluate screw density at different regions in the curve for AIS correction to test the hypothesis that alternative screw patterns do not compromise anticipated correction in AIS when using a segmental translation technique. METHODS: Instrumentation simulations were computationally performed for 10 AIS cases. We simulated simultaneous concave and convex segmental translation for a reference screw pattern (bilateral polyaxial pedicle screws with dorsal height adjustability at every level fused) and four alternative patterns; screws were dropped respectively on convex or concave side at alternate levels or at the periapical levels (21 to 25% fewer screws). Predicted deformity correction and screw forces were compared. RESULTS: Final simulated Cobb angle differences with the alternative screw patterns varied between 1 degrees to 5 degrees (39 simulations) and 8 degrees (1 simulation) compared to the reference maximal density screw pattern. Thoracic kyphosis and apical vertebral rotation were within 2 degrees of the reference screw pattern. Screw forces were 76 +/- 43 N, 96 +/- 58 N, 90 +/- 54 N, 82 +/- 33 N, and 79 +/- 42 N, respectively, for the reference screw pattern and screw dropouts at convex alternate levels, concave alternate levels, convex periapical levels, and concave periapical levels. Bone-screw forces for the alternative patterns were higher than the reference pattern (p 0.28). Alternate dropout screw forces were higher than periapical dropouts (p < 0.05). CONCLUSIONS: Using a simultaneous segmental translation technique, deformity correction can be achieved with 23% fewer screws than maximal density screw pattern, but resulted in 25% higher bone-screw forces. Screw dropouts could be either on the convex side or on the concave side at alternate levels or at periapical levels. Periapical screw dropouts may more likely result in lower bone-screw force increase than alternate level screw dropouts

Current Evidence Regarding the Treatment of Pediatric Lumbar Spondylolisthesis: A Report From the Scoliosis Research Society Evidence Based Medicine Committee.

© 2017 Scoliosis Research Society Study Design Structured literature review. Objectives The Scoliosis Research Society requested an assessment of the current state of peer-reviewed evidence regarding pediatric lumbar spondylolisthesis to identify what is known and what research remains essential to further understanding. Summary of Background Data Pediatric lumbar spondylolisthesis is common, yet no formal synthesis of the published literature regarding treatment has been previously performed. Methods A comprehensive literature search was performed. From 6600 initial citations with abstract, 663 articles underwent full-text review. The best available evidence regarding surgical and medical/interventional treatment was provided by 51 studies. None of the studies were graded Level I or II evidence. Eighteen of the studies were Level III, representing the current best available evidence. Thirty-three of the studies were Level IV. Results Although studies suggest a benign course for “low grade” (\u3c50% slip) isthmic spondylolisthesis, extensive literature suggests that a substantial number of patients present for treatment with pain and activity limitations. Pain resolution and return to activity is common with both medical/interventional and operative treatment. The role of medical/interventional bracing is not well established. Uninstrumented posterolateral fusion has been reported to produce good clinical results, but concerns regarding nonunion exist. Risk of slip progression is a specific concern in the “high grade” or dysplastic type. Although medical/interventional observation has been reported to be reasonable in a small series of asymptomatic high-grade slip patients, surgical treatment is commonly recommended to prevent progression. There is Level III evidence that instrumentation and reduction lowers the risk of nonunion, and that circumferential fusion is superior to posterior-only or anterior-only fusion. There is Level III evidence that patients with a higher slip angle are more likely to fail medical/interventional treatment of high-grade spondylolisthesis. Conclusions The current “best available” evidence to guide the treatment of pediatric spondylolisthesis is presented. Level of Evidence Level III; review of Level III studies