Load-carrying capacity of the human cervical spine in compression is increased under a follower load

Study Design. An experimental approach was used to rest human cadaveric cervical spine specimens, Objective. To assess the response of the cervical spine to a compressive follower load applied along a path that approximates the tangent to the curve of the cervical spine. Summary of Background Data. The compressive load on the human cervical spine is estimated to range from 120 to 1200 N during activities of daily living. Ex vivo experiments show it buckles at approximately 10 N. Differences between the estimated in vivo loads and the ex vivo load-carrying capacity have not been satisfactorily explained. Methods. A new experimental technique was developed for applying a compressive follower load of physiologic magnitudes up to 250 N, The experimental technique applied loads that minimized the internal sheer forces and bending moments, loading the specimen in nearly pure compression. Results. A compressive vertical load applied in the neutral and forward-flexed postures caused large changes in cervical lordosis at small load magnitudes, The specimen collapsed in extension or flexion at a load of less than 40 N, In sharp contrast, the cervical spine supported a load of up to 250 N without damage or instability in both the sagittal and frontal planes when the load path was tangential to the spinal curve. The cervical spine was significantly less flexible under a compressive follower load compared with the hypermobility demonstrated under a compressive vertical load (P < 0.05). Conclusion. The load-carrying capacity of the ligamentous cervical spine sharply increased under a compressive follower load. This experiment explains how a whole cervical spine can be lordotic and yet withstand the large compressive loads estimated in vivo without damage or instability.