Biomechanical Contribution of the Rib Cage to Thoracic Stability

STUDY DESIGN: In vitro assessment of rib cage biomechanics in the region of true ribs with the ribs intact then sequentially resected in 5 steps. OBJECTIVE: To determine the contribution of the rib cage to thoracic spine stability and kinematics. SUMMARY OF BACKGROUND DATA: Previous in vitro studies of rib cage biomechanics have used animal spines or human cadaveric spines with ribs left unsecured, limiting the ability of the ribs to contribute to stability. METHODS: Eight upper thoracic specimens that included 4 ribs and sternum were tested in special fixtures that disallowed relative movement of the distal ribs and their vertebrae. While applying 7.5 Nm pure moments in 3 planes, angular motion at the middle motion segment was studied in intact specimens and then (1) after splitting the sternum, (2) after removing the sternum, (3) after removing 50% of ribs, (4) after removing 75% of ribs, and (5) after disarticulating and completely removing ribs. RESULTS: During flexion/extension, the sternum and anterior rib cage most contributed to stability. During lateral bending, the posterior rib cage most contributed to stability. During axial rotation, stability was directly related to the proportion of ribs remaining intact. On average, intact ribs accounted for 78% of thoracic stability. An intact rib cage shifted the axis of rotation unpredictably, but its position remained consistent after partial resection of the ribs. During lateral bending, coupled axial rotation was mild and unaffected by ribs. CONCLUSION: Because of testing methodology, the rib cage accounted for a greater percentage of thoracic stability than previously estimated. Different rib cage structures resisted motion in different loading planes.