A Biomechanical Analysis of Load Sharing and Load Transfer in the Cervical Spine

The objective of this research was to determine the amount of load transferred from a cervical fusion mass to anterior cervical instrumentation and to compare and correlate implant stiffness with increased fatigue life, lower instrumentation failure, and higher fusion rates. The ASTM standard cervical unilateral construct test setup for screws or bolts (F1717-96) was used. Four different systems were used. Each implant was tested with and without the addition of a titanium mesh Harms cage to determine the stiffness of the anterior construct. A graph was plotted to illustrate the percent increase in stiffness with a Harms cage (%) versus the stiffness of the system (N/mm). With the addition of the Harms cage, all systems showed a significant increase in stiffness (p = 3.4 x 10-6). An inverse relationship was found between the compressive stiffness of the construct and its relative increase in stiffness with the addition of a load-sharing device. Anterior cages provide are a loadsharing device in the cervical spine and decrease the load on cervical plates and rods by 94%.