Is there an anatomic basis for the different behavior of Lenke types 1AR and 1AL in idiopathic scoliosis? A study on facet joint tropism influence.

Abstract

AIS type 1 Curves are sub-classified based on the tilt of L4 as 1AR and 1AL. These curves are different w.r.t their curve behavior, progression and level selection. Presently there is no known anatomic etiology for the different behavior. Facet tropism (FT) is defined as the asymmetry between the facet angle of the left and right facet joints. The purpose of this study was to evaluate the correlation between facet tropism in the lumbar segments and occurrence of type 1AR and 1AL curves in AIS patients. AIS patients with diagnosis of type 1 AR and 1AL right thoracic AIS curves who underwent posterior instrumented fusion were queried from a single institutions' database. Patients needed to have an MRI of their entire spine to be included. L2-3, L3-4 and L4-5 Facet angles (FA, angle made by the facet line with the mid-sagittal line at respected vertebral level) were calculated. FT was classified as follows: ≤ 5° (minimal), 6- 10° (mild) and ≥ 11° (severe). 1AR and 1AL curves were compared for FA, FT and FT grade at each lumbar segmental levels. One hundred nineteen patients were included (77 females, mean age-13.85 years, mean BMI- 21.63, 73 1AL and 46 1AR). The mean thoracic Cobb was 52.5 ± 9.8°, thoracic kyphosis was 28.12 ± 12° and lumbar lordosis was 53.48 ± 12.6°. L3-4 FA on the right side was more coronally oriented in 1AR curves compared to 1AL curves (37° vs. 31°, p = 0.04). On comparing FT at each level, 1AR curves had a higher FT at L3-4 (1.5° vs. -2.3°, p = 0.01) and L4-5 levels (5.8° vs. -0.28°, p < 0.001) compared to 1AL patients. Similarly, 1AR patients had significantly more patients with severe FT at L3-4 (34.8% vs. 13.7%, p = 0.02) and at L4-5 (17.3% vs. 6.8%, p = 0.01) compared to 1ALcurves. L3-4 joints are more coronally oriented in 1AR curves compared to 1AL curves. 1AR patients displayed higher FT at L3-4 and L4-5 compared to 1AL patients. 1AR curves also reveal a higher percentage of severe FT at L3-4 and L4-5 levels. This may influence the curve behavior and progression in these two curve types.