The role of the sternum, costosternal articulations, intervertebral disc, and facets in thoracic sagittal plane biomechanics: a comparison of three different sequences of surgical release.

STUDY
DESIGN: Eighteen human torsos were used in three experiments (A, B, and C) to determine the changes in sagittal motion due to three different sequences of three surgical releases.
OBJECTIVES: To investigate the relative effects of releasing the intervertebral disc, the costosternal joint, the sternum, and the facet joints on sagittal thoracic motion and the consequences of altering the sequence of the releases. SUMMARY OF BACKGROUND DATA: The biomechanics of the thoracic spine are different from the cervical and lumbar spine particularly due to the ribs and sternum, which contribute to stability and control motion. The role of the sternum and costosternal articulation in the biomechanics of thoracic sagittal motion has not been well studied. The effects of releasing each of these structures, whether alone or with discectomy and/or facetectomy, is potentially relevant in the surgical correction of thoracic deformities, such as severe kyphosis, and in the biomechanics of thoracic fracture.
METHODS: In Experiment A, the release sequence was back to front: total facetectomy (T4-T8), then radical discectomy (T4-T8), then costosternal release, then sternal osteotomy. In Experiment B, the release sequence was front to back: sternal osteotomy, then costosternal release, then radical discectomy, and finally total facetectomy. In Experiment C, the release sequence was: radical discectomy, then sternal osteotomy, then costosternal release, then total facetectomy. The different sequences allowed separate analysis of each component and the synergistic patterns. In each of the three experiments, the torso was flexed then extended each time by an applied force (25 N) before and after each release. The extent of flexion and the extent of extension were measured each time and compared with the intact condition, after each release.
RESULTS: The results obtained for sternal osteotomy were combined with the results obtained for costosternal release to give "sternal release." Radical discectomy provided the greatest increase (P < 0.05) in range of motion (ROM) compared with the other two single releases, no matter what the sequence. For paired release combinations, the radical discectomy and sternal release (as in Experiments B and C) provided a significant (P < 0.05) increase in total sagittal ROM compared with the combination of radical discectomy and total facetectomy (Experiment A). In Experiment A, sternal release accounted for 42% of the total sagittal ROM compared with only 26% related to the total facetectomy (Experiment B). In general, all of the releases allowed more extension than flexion.
CONCLUSIONS: Sagittal plane motion in the thoracic spine is influenced by all three structures tested in this experiment. Overall, the radical discectomy provides the greatest increase in total ROM and in extension compared with the other two releases. The second most influential release is the combination of sternal osteotomy plus costosternal release (i.e., sternal release), particularly in extension (correction of kyphosis). When two releases are done in sequence, radical discectomy plus sternal release provides the greatest increase in total ROM and in extension. Overall, total facetectomy is the least effective release. These data have relevance for surgical strategies in the correction of thoracic kyphosis or lordosis and suggest a potential role for sternal osteotomy and costosternal release in severe and rigid upper thoracic kyphosis.