Biomechanics of long segment fixation: Hook patterns and rod strain

This is an in vitro study of the mechanical effects of varying hook attachm ent patterns in long segment kyphotic deformity. In such cases, the optimal implant bulk, fatigue life, and construct rigidity to reliably achieve fus ion are still unquantified. Rod strains were measured for multiple laminar hook patterns in a synthetic thoracic spine test bed. Stresses were calcula ted from strain data. The model displayed similar flexion bending stiffness to the thoracic spine. None of the hook patterns significantly changed ove rall construct stiffness. Greatest rod strains were seen when utilizing awa y-facing apical hooks. This model was too stiff to detect differences in co nstruct stiffness. Nonetheless, rod stress analysis showed that for multise gment thoracic constructs, particularly with fixed kyphosis, minimizing api cal hooks will minimize rod strain. If periapical hooks are necessary, orie nting the hooks toward the apex will minimize rod strain.