INTRODUCTION: In vivo intervertebral stiffness cannot be determined directly because MRI and X‐ray images can only provide an estimate of stiffness based on degeneration, and spinal manipulation techniques and intraoperative measurement devices can only obtain the mechanical properties of the entire system. An alternative method to measure intervertebral stiffness might be structural vibration testing, which measures the response of a system to an applied vibration as a function of frequency, and allows determination of modal parameters such as resonance frequencies (ratio between stiffness and mass), vibration modes (pattern of motion) and damping. The objective of this study was to determine if structural vibration testing can reveal the resonance frequencies of flexion‐extension, lateroflexion and axial rotation of lumbar motion segments, and to examine whether resonance frequencies are sensitive to alterations in intervertebral stiffness.
METHODS: A shaker was used to vibrate the upper vertebra of 16 motion segments, while the response was obtained in anteroposterior and mediolateral direction from accelerometers on the transverse processes, the spinous process and the ventral side of the upper vertebra. Measurements were performed in three conditions: intact, after dissection of the ligaments, and after puncturing the annulus fibrosus.
RESULTS: All segments showed clear resonance peaks for flexion‐extension, lateroflexion, and axial rotation. Dissection of the ligaments did not affect the resonance frequencies, but puncturing the annulus significantly reduced the resonance frequency of axial rotation.
DISCUSSION: These results indicate that structural vibration testing can be utilized to assess the modal parameters of lumbar motion segments. Based on these results, vibration testing might be a promising method to study the mechanical properties of larger spinal sections and ultimately patients suffering from low back disorders.