Curve Progression After the Termination of Bracing for Adolescent Idiopathic Scoliosis: Usefulness of Combining the Proximal Femur Maturity Index (PFMI) and Risser Staging

Abstract

Background The brace therapy for adolescent idiopathic scoliosis (AIS) typically ends upon the end of growth. However, determining the timing of growth cessation can be challenging. The purpose of this study was to evaluate the utility of the proximal femur maturity index (PFMI), which can be assessed simultaneously with Risser staging without requiring additional radiation exposure, in determining the appropriate timing to terminate bracing. To achieve this, we investigated the relationship between skeletal maturity at the end of bracing, post-bracing curve progression, and height growth in patients who had been successfully treated with a brace. Methods Between April 2010 and March 2021, a total of 84 female patients with AIS who started bracing at our hospital with an initial Cobb angle of 20-40 degrees were included. All patients were followed for at least one year after brace termination. Height and radiographic parameters (Risser staging, PFMI, Cobb angle) were retrospectively collected. Results At the end of the bracing period, patients were categorized into Risser stage 4 (85.7%) and 5 (14.3%). By the last follow-up, patients with Risser stage 4 experienced an average main curve progression of 1.8°, whereas those with Risser stage 5 had an average progression of -0.3° (P = 0.03). Patients with Risser stage 4 were further divided into PFMI grade 5 (59.7%) and 6 (40.3%). Significant curve progression was observed in patients with PFMI grade 5 (average: 3.0°) compared to grade 6 (average: -0.6°) (P < 0.0001). The mean height growth was 1.9 cm/year for PFMI grade 5, and 0.3 cm/year for grade 6, with significant differences between these groups (P < 0.001). Conclusions PFMI allowed further categorization within Risser stage 4: PFMI grade 5 indicated remaining growth potential and risk of postbracing curve progression, whereas grade 6 indicated growth cessation. The combined use of Risser staging and PFMI, both evaluable through the same whole-spine radiograph, may provide a more accurate prediction of growth cessation.

Keywords: adolescent idiopathic scoliosis (ais); height growth; proximal femur maturity index; risser staging; skeletal maturation.

Figure 1. Flow chart of enrollment

Figure 2. Relationship among Risser staging, proximal femur maturity index (PFMI) and chronological age

The X-axis represents chronological age, and the Y-axis represents height growth. The number under each stage represents the average chronological age.

Figure 3. Postbracing curve progression

a: Risser staging, b: proximal femur maturity index (PFMI) When brace treatment was terminated at Risser stage 4 or PFMI grade 5, the curve was significantly progressed compared to Risser stage 5 and PFMI grade 6, respectively. Statistical analysis was performed by unpaired t-test.

Figure 4. The ratio of proximal femur maturity index (PFMI) in Risser stage 4 and 5 at the end of brace

a: Risser stage 4, b: Risser stage 5

Figure 5. Average annual height growth

a, b: There was a significant difference between Risser stage 4 and 5, as well as proximal femur maturity index (PFMI) grade 5 and 6. Statistical analysis was performed by one-way ANOVA. c: Patients who terminated bracing at Risser stage 4, the mean height growth after brace termination was PFMI grade 6: 0.2±0.4 cm, grade 5: 1.3±1.3 cm with significance (unpaired t-test).