Predictive value of lateral-bending, push-prone, and fulcrum-bending radiographs in adolescent idiopathic scoliosis

INTRODUCTION: The advent of pedicle screws which provide distraction and derotation has led to higher correction of major curves. Newer methods have been devised to evaluate preoperative coronal flexibility, including lateral-bending (LB), push-prone (PP) and fulcrum-bending (FB) radiographs. Documentation of a consistent radiographic method predictive of correction rate has not been established. OBJECTIVE: To determine the most predictive radiographic method for evaluating spine flexibility and correction by comparing the correction rate (CR), flexibility rate (FR) and correction index (CI) of the Cobb's angle using the different radiographic methods. METHODS: Preoperative radiographs of 20 patients who underwent spinal fusion for adolescent scoliosis were obtained using the LB, PP and FB method and compared with postoperative radiographs. RESULTS: Comparing the mean Cobb angles using the different methods to that of postoperative standing showed that only the FB method is not significantly different from the latter (p=0.669). There was a significant difference between the Cobb's angle measured on the LB and PP and that measured on postoperative standing (p=0.043, p=0.008). Comparing the mean flexibility of the different methods with the mean CR also showed that the mean FR of LB (p=0.007) and PP (p=0.00013) were significantly different from the CR while that of FB is not significantly different from the CR (p=0.687). CONCLUSION: The FB radiograph demonstrated no statistical difference compared to postoperative radiograph, FR, and CI.

A biomechanical study on single rod spinal instrumentation system in an unstable thoracolumbar injury model: A finite element analysis

OBJECTIVES: To develop three dimensional computer models of the anterior thoracolumbar spine implants or constructs (the novel single rod-screw implant and the standard implants) and to evaluate its biomechanical properties through a graphically reconstructed testing standard. METHODS: We developed a finite element modeling technique based on actual geometry of the implant constructs and mechanical property data from standard biomechanical studies on anterior thoracolumbar spinal instrumentation systems. Seven constructs were mounted on simulated vertebral bodies. Axial load sharing was measured through a range of applied axial loads from 100 N to 1600 N. The static destructive tests were conducted. The bending strength of each construct was calculated with a full length corpectomy graft in place, simulating reconstruction of the anterior column, and with no graft in place, simulating catastrophic graft failure. RESULTS: Static testing parameter demonstrated highly significant differences between devices. The plate construct formed the highest subset in bending strength of 1000-1100 N, whereas the single rod showed the lowest value of 300-400N. However, the bending strengths of single rod and dual rod both without bone grafts were not significantly different. With the graft in place, bending strength of the constructs significantly increased beyond the maximum set of load of 1600N, underlying the importance of the graft in overall construct strength. CONCLUSION: The 3-D finite element models for anterior thoracolumbar instrumentation system were designed with mechanical properties comparable to the actual biomechanical testing results. Although single rod construct has the lowest value, its bending strength is comparable to the standard dual rod system under static axial loading. Bone graft contributed to overall construct stiffness.

Comparison of the hybrid locking plate, standard dynamic compression plate, and standard dynamic compression plate augmented with bone cement for fixation of osteoporotic humeral shaft fractures: A cadaveric biomechanical study

BACKGROUND: Studies comparing the relative strength of polymethylmethacrylate (PMMA) augmented fixation, standard plating and locked compression plate (LCP) system are few. The use of either the bone cement-augmented dynamic compression plate or the Hybrid LCP constructs may provide an additional tool for the treatment of fractures in patients with osteoporosis. METHODS: Eighteen (18) osteoporotic cadaveric humeral bones were assigned randomly to each of three groups (Dynamic Compression Plate [DCP], DCP augmented with bone cement, and the Hybrid LCP system) and tested in anterior-posterior bending and torsion/external rotation. The load to failure values were obtained and the results for each specimen compared. RESULTS: Significant differences were observed between the standard DCP and Hybrid LCP group (p-value=0.012), and in the cement-augmented and Hybrid LCP group (p-value=0.099) in torsion/external rotation loading. No significant difference was observed between the standard DCP and bone-cement augmented group (p-value=0.248). No significant difference was observed among the three groups in terms of stiffness (p-value=0.3868) in the four-point anterior-posterior bending modality. Screw pull-out of the implant was observed only in the regular DCP group in torsion/external rotation loading stress. CONCLUSION: Significant differences were seen between the three constructs in torsion/external rotation but not in anterior-posterior four-point bending. Bone failure, but not screw pull-out, was seen in the Hybrid LCP and bone cement-augmented DCP groups in torsion. This study showed that the LCP system and the bone cement-augmented constructs may provide greater screw purchase to the osteoporotic humerus.