Study Design. A retrospective observational study. Objective. This study examined the role of skeletal muscles in the spinal alignment of lumbar degenerative kyphosis (LDK). Summary of Background Data. Adult spinal deformity (ASD) may present as LDK, which is unusual in that it typically lacks any coronal deformity. Methods. This retrospective study included 367 female patients with ASD. Demographic and radiographic data from LDKs were compared with those with other ASD. The LDK multifidus was also subjected to histological analysis. The muscle volume and strength were analyzed using whole-body dual x-ray absorptiometry and pulmonary function tests. Magnetic resonance images were used to determine the cross-sectional area (CSA) and fatty infiltration area (FIA) of the psoas and multifidus. Results. A total of 52 patients (15%) were classified as LDK. Compared with other ASDs, those with LDK had a significantly larger sagittal vertical axis (16.3±5.0 cm), and pelvic incidence minus lumbar lordosis (52.8°±9.2°). The LDK had significantly lower CSA and higher FIA of the multifidus, but not in the psoas (multifidus CSA 223±96 vs. 477±129 mm 2, P<0.001; FIA 82% vs. 31%, P<0.001). Eighty-six percent of LDK had a multifidus CSA of less than 300 mm 2, and 82% had an FIA of more than 80%. Whole-body dual x-ray absorptiometry showed no significant difference in the other body sections between LDK and other ASDs. The percent-predicted peak expiratory flow was within normal range in 82% of the LDK. LDK multifidus specimens revealed diffuse, nonuniform muscular fibers and growth of the interstitium and adipose tissue, with no muscular inflammation. We redefined LDK as drop body syndrome, a distinct form of ASD. Conclusion. Significantly decreased CSA and increased FIA of the multifidus were observed in LDK, whereas the muscle strength and volume of the other body sections were normal. These findings strongly suggest the presence of isolated lumbar extensor myopathy in LDK.
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