The role of anteromedial foraminotomy and the uncovertebral joints in the stability of the cervical spine. A biomechanical study.

Study Design: The biomechanical role of the cervical uncovertebral joint was investigated using human cadaveric spines. Sequential resection of cervical uncovertebral joints, including clinical anteromedial foraminotomy, was conducted, followed by biomechanical testing after each stage of resection.

Objectives: To clarify the biomechanical role of uncovertebral joints and clinical anteromedial foraminotomy in the cervical spine and their effects on interbody bone graft stability.

Summary Of Background Data: Although the biomechanical role of the cervical uncovertebral joints has been considered to be that of a guiding mechanism in flexion and extension and a limiting mechanism in posterior translation and lateral bending, there have been no studies quantifying this role. According to results in quantitative anatomic studies, anatomic variations exist in uncovertebral joints, depending on the vertebral level, articular angulation, and relative height of the joints.

Methods: Fourteen human functional spinal units at C3-C4 and C6-C7 underwent sequential uncovertebral joint resection, with each stage of resection followed by biomechanical testing. The uncovertebral joint was divided anatomically into three parts on each side: the posterior foraminal part, the posterior half, and the anterior half. The loading modes included torsion, flexion, extension, and lateral bending. A simulated anterior bone graft construct was also tested after each uncovertebral joint resection procedure.

Results: Significant changes in stability were observed after sequential uncovertebral joint resection in all loading modes (P < 0.05). The biomechanical contribution of uncovertebral joints decreased in the following order: the posterior foraminal part, the posterior half, and the anterior half. Unilateral and bilateral foraminotomy most affected the stability of the functional spinal unit during extension, causing a 30% and 36% decrease in stiffness of the functional spinal unit, respectively. The effect was less in torsion and lateral bending. After sequential resection, there was a statistically significant difference between decreases in torsional stiffness at C3-C4 and C6-C7 (P < 0.05). The stiffness of the simulated bone graft construct decreased progressively during flexion and lateral bending after each foraminotomy (P < 0.05). Increased bone graft height of 79% returned stability to the preforaminotomy level.

Conclusions: This is the first study to quantitate the biomechanical role of uncovertebral joints in cervical segmental stability and the effect at each intervertebral level. The effect differs because of anatomic variations in uncovertebral joints. The major biomechanical function of uncovertebral joints includes the regulation of extension and lateral bending motion, followed by torsion, which is mainly provided by the posterior uncovertebral joints. This study highlights the clinical assessment of additional segmental instability attributed to destruction of the uncovertebral joints during surgical procedures or by neoplastic lesions.