PubMed · 6342723
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D B Gilboe. 1983. Centric relation: functional anatomy.. https://pubmed.ncbi.nlm.nih.gov/6342723/
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STUDY DESIGN: The biomechanical influence of in situ setting hydroxyapatite cement was examined for use in pedicle screw revision surgery. Pull-out testing of control and pedicle screws augmented with hydroxyapatite cement was performed in human cadaver vertebrae. OBJECTIVES: To determine the immediate effect of using hydroxyapatite cement to augment revision pedicle screws after failure of the primary pedicle screw fixation. SUMMARY OF BACKGROUND DATA: The potential problems associated with using polymethylmethacrylate to augment revision pedicular instrumentation have prompted the search for other solutions. The introduction of resorbable hydroxyapatite pastes may have provided new biocompatible solutions for pedicle screw revision. METHODS: Ten human cadaver vertebrae were instrumented with 6.0-mm pedicle screws in each pedicle. The screws were loaded to failure in axial tension (pull-out). The failed pedicles then were instrumented with 7.0-mm pedicle screws, either augmented with hydroxyapatite cement or nonaugmented, which also were loaded to failure. Finally, the nonaugmented 7.0-mm screw hole was reinstrumented with a hydroxyapatite cement-augmented, 7.0-mm pedicle screw and loaded to failure. RESULTS: The pull-out strength of the 7.0-mm, hydroxyapatite cement-augmented screws was 325% (P = 2.9 x 10(-5)) of that of the 6.0-mm control screws, whereas the strength of the 7.0-mm nonaugmented screws was only 73% (P = 2.0 x 10(-2)) of that of the 6.0-mm control screws. The 7.0-mm screws augmented with hydroxyapatite cement also were able to salvage 7.0-mm pull-out sites to 384% (P = 6.9E-5) of the pull-out strength of the 7.0-mm nonaugmented screws. CONCLUSIONS: Hydroxyapatite cement may be a mechanically viable alternative to polymethyl methacrylate for augmenting revision pedicular instrumentation and should be considered for future experimental, animal, and clinical testing.
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.
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