The effects of hook pattern and kyphotic angulation on mechanical strengthand apical rod strain in a long-segment posterior construct using a synthetic model

Study Design, Synthetic spine models were used to compare the effects of ho ok pattern and kyphotic angulation on stiffness and rod strain in long-segm ent posterior spinal constructs. Objectives. To examine the biomechanical effects of hook patterns and kypho tic angulation on long-segment posterior spinal constructs. Summary of Background Data, Kyphotic deformities managed by increasing rod diameter and hence construct stiff ness have sh own decreased postoperative loss of correction and hardware complications. The biomechanical effects o f hook pattern and kyphosis are unknown. Methods, Spine models of 0 degrees, 27 degrees, and 54 degrees sagittal con tour, composed of polypropylene vertebral blocks and isoprene elastomer int ervertebral spacers, representing T3-T12, were used for biomechanical testi ng of long-segment posterior spinal constructs. Models were instrumented wi th 6.35-mm titanium rods and one of the following hook configurations: 20-h ook compression, 16-hook compression, 16-hook claw apex-empty, 16-hook claw apex-full, or 8-hook claw. Construct stiffness and rod strain during axial compression were determined. Results. The compression-hook patterns provided at least a 45% increase in construct stiffness (P = 0.013) and a 22% decrease in rod strain (P < 0.000 1) compared with those obtained with the claw-hook pattern with the best bi omechanical performance. When analyzing all five hook patterns, there was a 19% decrease in construct stiffness and 27% increase in rod strain when pr ogressing from straight alignment to 27<degrees> of sagittal contour (P < 0 .0001). Progressing from straight alignment to 54<degrees> decreased constr uct stiffness by 48% and increased rod strain by 55% (P < 0.0001). Construc t stiffness was inversely cor; related to rod strain in all five hook patte rns (R-2 = 0.82-0.98, P < 0.001). Conclusions. Using compressive-hook patterns and decreasing the kyphotic:de formity significantly increases construct stiffness and decreases rod strai n.