COPYRIGHT © 2004 BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED  A Comparison of the Microarchitectural Bone Adaptations of the Concave and Convex Thoracic Spinal Facets in Idiopathic Scoliosis BY KEVIN G. SHEA, MD, T YLER FORD, BS, ROY D. BLOEBAUM, PHD, JACQUES D’ASTOUS, MD, AND HOWARD KING, MD Investigation performed at St. Luke’s Children’s Hospital, Boise, Idaho, the Shriners Hospital for Children, Salt Lake City, and the Bone and Joint Research Laboratory, Veterans Administration Hospital, Salt Lake City, Utah Background: A limited number of studies have assessed the changes in bone microarchitecture in spinal facets with use of light microscopy but not with use of electron microscopy techniques. The purpose of this study was to analyze the facets in patients with scoliosis to determine whether there are differences in the bone microarchitecture of con- tralateral facets at the same anatomic level. Methods: In eight patients undergoing posterior spinal arthrodesis for the treatment of idiopathic scoliosis, biopsy specimens of facet pairs at matched anatomic levels were obtained from three locations: (1) the curve apex, (2) one level cephalad to the apex, and (3) one level caudad to the apex. The facets were analyzed for cortical bone porosity and thickness with use of scanning electron microscopy and National Institutes of Health imaging software. The con- cave and convex facets were compared with use of a paired t test. Results: The mean porosity (and standard deviation) for the concave and convex facets was 16.5% ± 5.8% and 24.1% ± 6.2%, respectively. Those on the convex side were significantly more porous than those on the concave side (p ≤ 0.03). The mean cortical width for the concave and convex facets was 798 ± 266 μm and 377 ± 124 μm, re- spectively. The concave facets had a significantly thicker cortex than did the convex facets (p < 0.01). Conclusions: These results suggest that scoliotic deformities apply eccentric forces to spinal facets and that the concave and convex portions of the curve are subject to compression and tension forces, respectively. This analysis complements previous investigations of bone microarchitecture in animal models with use of a known compression- tension environment, and it suggests that the spinal facets remodel in a manner consistent with Wolff’s law. Clinical Relevance: Future studies of human spinal facets in scoliosis offer the opportunity to further define the mi- croarchitectural response of human bone. Additional study will be necessary to determine whether these eccentric microarchitectural changes represent a secondary response to abnormal loading in the spine or whether an underly- ing pathological process in bone is a primary factor in the generation of scoliotic deformities. Further understanding of bone-remodeling in scoliosis may help to validate animal models and provide insight into the pathophysiology of scoliosis. espite extensive research, relatively little is known about the etiology of adolescent idiopathic scoliosis. Multiple theories have been proposed, and its etiology is probably multifactorial in nature 1,2 . Recent studies have sug- gested that genetic factors play a role in some, but not all, forms of scoliosis 3-5 . A limited number of studies have evaluated gross and microarchitectural changes in the bone of patients with scoli- osis 6-8 . Enneking and Harrington performed histological stud- ies with the light microscope on inferior articular processes in patients with scoliosis 8 . They evaluated chondrogenesis, osteo- genesis, subchondral bone formation, and cartilage degener- ation. Stilwell 7 studied the mechanisms of bone-remodeling under conditions of chronic asymmetrical loading using an D