Plain Radiographs are Insufficient for Evaluation of First Metatarsal Pronation in Patients Undergoing Surgery for Hallux Valgus

Abstract

Category: Midfoot/Forefoot; Bunion Introduction/Purpose: Previous studies using weight-bearing CT (WBCT) have suggested that up to 30% of patients with hallux valgus may have excessive pronation of the first metatarsal when defined as a metatarsal pronation angle (MPA) of greater than 16 degrees (Najefi et al.). The MPA is calculated on coronal slices of a CT or MRI, but axial imaging is not the norm in the assessment of these patients. It is however unclear how accurate radiographs are at determining pronation. This study aimed to assess the degree of pre-operative first metatarsal pronation in patients undergoing surgery for hallux valgus using radiographs and WBCT. Methods: This was a retrospective review of patients undergoing scarf osteotomy for hallux valgus correction. Patients were included if they had both plain radiographs and WBCT. Patients were excluded if they had any prior surgery to the foot in question. Pre-operative weightbearing plain radiographs of the foot were analyzed and the classification by Okuda et al. was used to determine if pronation was present using the shape of the lateral edge of first metatarsal head. Pre-operative WBCTs were reviewed and MPA calculated. Twenty-two feet were included. Results: In total 10 feet (45.5%) had an ‘angular’ first metatarsal head, 9 (40.9%) had an ‘intermediate’ head shape, and 3 (13.6%) had a ‘round’ head on plain radiographic assessment. The overall mean MPA was 16.25 degrees (range 2.1-30), and 12 (54.5%) feet had an MPA of greater than 16 degrees (excessively pronated). There was a strong correlation between increasing MPA and increasing roundness of the metatarsal head (Eta = 0.952). The mean MPA was 14.4 degrees in the angular head group, 17.0 degrees in the intermediate group, and 20.1 degrees in the round head group. In the intermediate and round groups 9 feet (75%) had greater pronation than the normal range. By contrast, only 3 (30%) of feet in the round head group had pronation greater than 16 degrees. Conclusion: In this surgical population of hallux valgus patients, the overall prevalence of excessive first metatarsal pronation was greater than in general cohorts of hallux valgus patients. Patients with intermediate and round shaped heads on plain radiographs tended to have greater first metatarsal pronation than those with angular shaped heads, although the correlation was not perfect. This suggests that plain radiographic assessment of metatarsal rotation may not be accurate as there may be discrepancies due to changes in foot position or angle of beam while acquiring the radiographs. Although further work is still required to determine whether there is clinical benefit from addressing the pronation, axial imaging (such as with WBCT) may be useful in the intermediate and round groups for accurate assessment of pronation and to determine if it is significant. This may be especially relevant in surgical candidates in whom osteotomies such as a scarf are being considered, where pronation cannot be addressed.