The Role of the Sternum, Costosternal Articulations, Intervertebral Disc, and Facets in Thoracic Sagittal Plane Biomechanics and Deformity Correction: A Comparison of Three Different Sequences of Surgery

Introduction: Thoracic biomechanics are unique due to the ribs and sternum. The role of the sternum and costosternal articulations in thoracic fracture or in deformity correction has not been well studied. The effects of releasing these structures are potentially relevant to the correction of severe thoracic deformities. The purpose of this study is to investigate the relative effects of the intervertebral disc, the sternum, the costosternal joints and the facet joints on sagittal thoracic motion and the consequences of altering the sequence of surgical releases.

Methods: Eighteen human torsos were tested sequentially performing total facetectomy (T4-8), radical discectomy (T4-8) or transverse sternal osteotomy with costosternal release (sternal release). The surgical releases were done in three different sequences, allowing analysis of individual components and different pairs of releases, simulating various clinical scenarios. In Experiment A, the release sequence was back to front: facetectomy (T4-8), then discectomy (T4-8), then sternal release. In Experiment B, the release sequence was front to back: sternal release, then discectomy, then facetectomy. In Experiment C it was disc first: discectomy, then sternal release, then facetectomy. The torso was flexed and extended by an applied 25N force 6 times before and after each release. The extent of both angular flexion and angular extension were compared to the intact condition, and after each cumulative release. The length of the sternum was also measured at the limit of flexion and extension.

Results: Radical discectomy provided the greatest increase in total range of motion (ROM) as compared to the other two single releases (P<0 .05). Sternal release allowed significantly more extension and total ROM than facetectomy (P<0.05). In Experiment A, the sternal alone accounted for 42% of sagittal motion, indicating a significant stabilizing effect even when disc facets were disrupted. For paired releases, discectomy provided (P<0.05) compared to combination facetectomy. general, all releases flexion. The average increase in length was 1.55 cm.

Conclusions: Sagittal plane thoracic motion is influenced by all structures tested. The radical discectomy provides the greatest increase in total ROM and in extension when compared to the other two releases. The sternal release allowed significantly (P<0 .05) more ROM than the facetectomy, suggesting sternal complex provides stability. Overall, total multilevel facetectomy is least effective release. When two releases were combined, radical discectomy plus release provided greatest increase in and extension. These data are relevant for thoracic fractures surgical strategies to correct kyphosis or lordosis, a potential role osteotomy severe upper reconstruction trauma.