Biomechanics of long segment fixation: Hook patterns and rod strain

This is an in vitro study of the mechanical effects of varying hook attachment patterns in long segment kyphotic deformity. In such cases, the optimal implant bulk, fatigue life, and construct rigidity to reliably achieve fusion are still unquantified. Rod strains were measured for multiple laminar hook patterns in a synthetic thoracic spine test bed. Stresses were calculated from strain data. The model displayed similar flexion bending stiffness to the thoracic spine. None of the hook patterns significantly changed overall construct stiffness. Greatest rod strains were seen when utilizing away-facing apical hooks. This model was too stiff to detect differences in construct stiffness. Nonetheless, rod stress analysis showed that for multisegment thoracic constructs, particularly with fixed kyphosis, minimizing apical hooks will minimize rod strain. If periapical hooks are necessary, orienting the hooks toward the apex will minimize rod strain.