New method for determining apparent axial center of rotation of lumbar and thoracic spine segments.

One main question in spinal kinematics is the determination of the spine's apparent axial center of rotation. Previous research on this topic has yielded contradictory results. The objective of this study was to determine the apparent axial center of rotation for seven lumbar and six thoracic spinal segments by developing and validating a new method. A custom six-degree-of-freedom device, allowing full range of motion, was used with motion recording and analysis software. This system tracked a grid of markers on a specimen when rotational torque was applied in both clockwise and counterclock wise directions at 3.53 Nm, 7.05 Nm, 10.58 Nm, and 14.10 Nm. The area encompassing the apparent axial center of rotation was determined by identifying the five markers with the least amount of motion. The marker angular displacement was calculated as the angle between a virtual line drawn between two points in the initial and final torque conditions. Rotation in both directions was averaged. The lumbar and thoracic spinal segments averaged an apparent axial center of rotation at the posterior border of the vertebral endplates and the anterior border of the spinal canal, with average clockwise to counterclockwise angular displacement ratios of 0.87 and 0.97, respectively.

Relative contribution of acromioclavicular joint capsule and coracoclavicular ligaments to acromioclavicular stability.

HYPOTHESIS: We hypothesized that both the AC joint capsule and CC ligaments are biomechanically robust structures in the anterior-posterior (AP) and superior-inferior (SI) planes with low loads, and that these ligaments provide essential function in AC joint stability. MATERIALS AND METHODS: Anterior-posterior (AP) and superior-inferior (SI) AC joint translations were quantified in 6 cadaver matched pairs with AC joint compressions of 10N, 20N and 30N, and with translational loads of 10N and 15N. Either the AC joint capsule or CC ligaments were transected, and measurements were then repeated. Biomechanical characteristics of the remaining AC joint capsule or CC ligaments were compared. RESULTS: There were significant increases in AP translation with the cut AC joint capsule, and significant increases in SI translation with the cut CC ligaments (P < 0.0001). Compression significantly decreased translation (P < 0.0001). DISCUSSION: Our study is supported by, and further develops, recent studies and anatomical knowledge. It offers two interpreted pieces of information for the sports medicine physician to consider for reconstruction of the AC joint. First, resection of the distal clavicle may have a detrimental effect. Second, repair of the AC joint capsule, in addition to the customarily repaired CC ligaments, appears to have a beneficial effect. CONCLUSION: The AC joint capsule is a robust anatomical structure that contributes significantly to the AC joint stability, especially in the AP plane. Compression increases stability. LEVEL OF EVIDENCE: Basic science study.