Anterior lengthening in scoliosis occurs only in the disc and is similar in different types of scoliosis

Abstract

BACKGROUND CONTEXT: Relative anterior spinal overgrowth was proposed as a generalized growth disturbance and a potential initiator of adolescent idiopathic scoliosis (AIS). However, anterior lengthening has also been observed in neuromuscular (NM) scoliosis and was shown to be restricted to the apical areas and located in the intervertebral discs, not in the bone. This suggests that relative anterior spinal overgrowth does not rightfully describe anterior lengthening in scoliosis, as it seems not a generalized active growth phenomenon, nor specific to AIS. PURPOSE: To determine if compensatory curves in congenital scoliosis exhibit a mechanism of anterior lengthening without changes in the vertebral body, similar to curves in AIS and NM scoliosis. STUDY DESIGN/SETTING: Cross-sectional. PATIENT SAMPLE: CT-scans were included of patients in whom a short segment congenital malformation had led to a long thoracic compensatory curve without bony abnormality. Based on data of other scoliosis types, the calculated required sample size was n=12 to detect equivalence of vertebral bodies as compared with nonscoliotic controls. Out of 143 congenital scoliosis patients, 18 fit the criteria and compared with 30 nonscoliotic controls, 30 AIS and 30 NM scoliosis patients. OUTCOME MEASURES: The anterior-posterior length discrepancy (AP%) of the total curve and for vertebral bodies and intervertebral discs separately. METHODS: Of each vertebral body and intervertebral disc in the compensatory curve, the anterior and posterior length was measured on CT-scans in the exact mid-sagittal plane, corrected for deformity in all three planes. The AP% was calculated for the total compensatory curve (Cobb-to-Cobb) and for the vertebral bodies and the intervertebral discs separately. Positive AP% indicated that the anterior side was longer than the posterior side. RESULTS: The total AP% of the compensatory curve in congenital scoliosis showed lordosis (+1.8%) that differed from the kyphosis in nonscoliotic controls (-3.0%; p<.001) and was comparable to the major curve in AIS (+1.2%) and NM scoliosis (+0.5%). This anterior lengthening was not located in the bone; the vertebral body AP% showed kyphosis (-3.2%), similar to nonscoliotic controls (-3.4%) as well as AIS (-2.5%) and NM scoliosis (-4.5%; p=1.000). However, the disc AP% showed lordosis (+24.3%), which sharply contrasts to the kyphotic discs of controls (-1.5%; p<.001), but was similar to AIS (+17.5%) and NM scoliosis (+20.5%). CONCLUSIONS: The current study on compensatory curves in congenital scoliosis confirms that anterior lengthening is part of the three-dimensional deformity in different types of scoliosis and is exclusively located in the intervertebral discs. The bony vertebral bodies maintain their kyphotic shape, which indicates that there is no active anterior bony overgrowth. Anterior lengthening appears to be a passive result of any scoliotic deformity, rather than being related to the specific cause of AIS.