THE ROLE OF ANTEROMEDIAL FORAMINOTOMY AND THE UNCOVERTEBRAL JOINTS INTHE STABILITY OF THE CERVICAL-SPINE - A BIOMECHANICAL STUDY

Study Design. The biomechanical role of the cervical uncovertebral joi nt was investigated using human cadaveric spines. Sequential resection of cervical uncovertebral joints, including clinical anteromedial for aminotomy, 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 cer vical spine and their effects on inter-body bone graft stability. Summ ary of Background Data. Although the biomechanical role of the cervica l uncovertebral joints has been considered to be that of a guiding mec hanism in flexion and:extension and a limiting mechanism in posterior translation and lateral bending, there have been no studies quantifyin g this role. According to results in quantitative anatomic studies, an atomic variations exist in uncovertebral joints, depending on the vert ebral level, articular angulation, and relative height of the joints. Methods. Fourteen human functional spinal units at C3-C4 and C6-C7 und erwent sequential uncovertebral joint resection, with each stage of re section followed by biomechanical testing. The uncovertebral joint was divided anatomically into three parts on each side: the posterior for aminal part, the posterior half, and the anterior half. The loading mo des included torsion, flexion,;; extension, and lateral bending. A sim ulated anterior bone graft construct was also tested after each uncove rtebral joint resection procedure. Results. Significant changes in sta bility were observed after sequential uncovertebral joint resection in all loading modes (P< 0.05). The biomechanical contribution of uncove rtebral joints decreased in the following order: the posterior foramin al part, the posterior half,and the anterior half. Unilateral and bila teral foraminotomy most affected the stability of the functional spina l unit during extension, causing a 30% and 36% decrease in stiffness o f the function spinal unit, respectively.The effect was less in torsio n and lateral bending. After sequential resection, there was a statist ically significant difference between decreases in torsional stiffness at C3-C4 and C6-C7 (P < 0.05). The stiffness of the simulated bone gr aft construct decreased progressively during flexion and lateral bendi ng after each foraminotomy ( P < 0.05). Increased bone graft height of 79% returned stability to the preforaminotomy level. Conclusions. Thi s is the first study to quantitate the biomechanical role of uncoverte bral joints in cervical segmental stability and the effect at each int ervertebral level. The effect differs because of anatomic variations i n uncovertebral joints. The major biomechanical function of uncoverteb ral joints includes the regulation of extension and lateral bending mo tion, followed by torsion, which is mainly provided by the posterior u ncovertebral joints. This study highlights the clinical assessment of additional segmental instability attributed to destruction of the unco vertebral joints during surgical procedures or by neoplastic lesions.