Experimental evaluation of seven different spinal fracture internal fixation devices using nonfailure stability testing. The load-sharing and unstable-mechanism concepts.

Fracture site immobilization capabilities of seven internal spine fixation systems were experimentally evaluated: Harrington double distraction (plain, supplemented with Edwards sleeves, supplemented with sublaminar wires), Luque rectangle, Kaneda device, transfixed Kaneda device, and Steffee plates. Stability evaluation involved comparing the three-dimensional motion that occurred across an experimentally created L1 slice fracture, and the load resistance of the implants when subjected to axial, flexion, extension, lateral bending, and torsional loads. Each implant was tested on 15 different vertebral segments from 200-250-lb calves. All implants load-shared with the fractured vertebral column to varying degrees. All except the Steffee plate system showed an obvious biomechanical weakness in one or more load modes. The unstable 4R-4bar mechanism configuration of bilevel spinal implants was identified. Mechanism configurations allow free movement with little or no resistance to the applied load until load sharing by the spinal column stops the collapse and resists the applied load.