Spinous process strength

Study Design. Mechanical testing of cadaveric lumbar spines and dual energy radiograph absorptiometry scanning were performed. Objectives. To devise a technique to measure the strength of lumbar spinous processes and to determine the bone mineral density of the vertebrae used. Summary of Background Data. The spinous process has been identified as the weakest part of the anatomy to which a flexible fixation device can be attached. It was unknown if the spinous processes could withstand the forces applied by the device. Methods. A hook was fitted to the spinous process of 32 lumbar vertebrae. A custom-built rig was designed to secure a vertebra to a materials testing machine. A loop of cord was passed over a bar mounted on the crosshead of the machine and around the two bollards of the hook. As the crosshead was raised, a tension was applied to the cord. Each vertebra was tested to failure. The bone mineral density of each vertebra was then measured using dual energy radiograph absorptiometry. Results. Failure of the specimens occurred by failure of the spinous process, pedicles, or vertebral body. The logarithm (base 10) of the load (N) at which failure occurred was 2.53 ± 0.3, which corresponded to a mean failure load of 339 N. The bone mineral density of each vertebral body varied between 0.263 and 0.997 g/cm². A significant linear correlation was found between bone strength and bone mineral density (P < 0.0001). Conclusions. Specimens with a bone mineral density in the range of 0.263-0.997 g/cm² failed at a mean load of 339 N when the load was applied through the spinous process hook of a flexible fixation device.