BIOMECHANICS SPINE Volume 36, Number 26, pp E1686–E1693 ©2011, Lippincott Williams & Wilkins E1686 www.spinejournal.com December 2011 Biomechanical Contribution of the Rib Cage to Thoracic Stability Leonardo B. C. Brasiliense, MD ,* Bruno C. R. Lazaro, MD,* Phillip M. Reyes, BSE,* Seref Dogan, MD,*† Nicholas Theodore, MD ,* and Neil R. Crawford, PhD* 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. T he rib cage comprises the sternum, ribs, interconnect- ing joints, and soft tissue; not only does it encircle and protect many vital organs, but it also provides addi- tional mechanical support to the spine. Many reports of the stabilizing benefit conferred by the rib cage have been anec- dotal. Some studies have reported the association of indirect sternal fractures with thoracic spine injuries. 1 ,2 Others have noted different clinical courses in patients with thoracic frac- tures depending on whether the rib cage is intact. 3 On the basis of these findings, it has been hypothesized that the rib cage could represent a “fourth-column” of stability in the tho- racic spine. 4 Studies of canine thoracic spines provide further evidence of the stabilizing effect of the rib cage. 5 ,6 However, although certain animal models may adequately simulate the range of motion (ROM) responses of other segments of the spine in normal and injured conditions, 7 the biomechani- cal effect of anatomical differences in the rib cage between quadrupeds and bipeds remains undetermined. Watkins et al 8 studied rib cage biomechanics using human cadaveric spines, demonstrating that the rib cage was responsible for 36% of thoracic spine stability. However, all previous canine and human cadaveric studies may have underestimated the con- tribution of the rib cage because the ribs and sternum were never incorporated in the testing design, with loads applied solely on the spinal column. These studies therefore repre- sent one extreme in which the stabilizing contribution of ribs occurs only through their posterior connection at the spine and any remaining anterior resistance the distally severed tis- sues can still exert. We hypothesized that if the ribs were given a better oppor- tunity to contribute to stability, simulating more realisti- cally the anterior muscle and connective tissue attachments at the ribs, their contribution would actually be greater than 36%. We therefore used a new strategy to assess the stability imparted by the rib cage in the thoracic spine at the true ribs (as opposed to floating or false ribs). Special fixturing allowed the test forces applied to the segment to be evenly distributed to both ribs and spine, more accurately representing the in vivo biomechanics of these structures. This study assessed the stabilizing and kinematic effects of the rib cage by analyzing From the *Spinal Biomechanics Laboratory, Division of Neurological Surgery, Barrow Neurological Institute, Phoenix, Arizona; and †Department of Neurosurgery, Uludag University School of Medicine, 16059, Gorukle-Bursa, Turkey. Acknowledgment date: December 17, 2009. Acceptance date: March 10, 2011. The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and reprint requests to Neil R. Crawford, PhD, c/o Neuroscience Publications, Barrow Neurological Institute, Phoenix AZ 85013; E-mail: ncrawfo@chw.edu Key words: thoracic spine, rib cage, biomechanics, kinematics. Spine 2011;36:E1686–E1693 DOI: 10.1097/BRS.0b013e318219ce84 Copyright © 2011 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.