Distal Metatarsal Articular Angle and First Metatarsal Pronation: A Weightbearing CT Correlation Study

Background The distal metatarsal articular angle (DMAA) is a measurement used in surgical decision making of hallux valgus correction. However, it is difficult to measure on plain radiographs, is subject to projection bias, and its role in pathology is unclear. With the advent of weight-bearing CT (WBCT), our understanding of hallux valgus as a multiplanar deformity has evolved. The aim of this study was to investigate whether there is a relationship between the DMAA and pronation of the first metatarsal head in patients with hallux valgus. Methods This was a single-centre, retrospective analysis of 50 patients with hallux valgus deformity who had WBCTs obtained as part of routine pre-operative work-up. Patients with metatarsophalangeal joint arthritis, hindfoot deformity, and previous surgery were excluded. From the WBCT images, digital radiographs were created and the DMAA measured. Measurements were taken by 2 authors, each repeated twice and the average of all 4 measurements used in analysis. We also measured intermetatarsal angle (IMA), hallux valgus angle (HVA), and metatarsal pronation angle (MPA). Results There were 41 females and 9 males, with a mean age of 52.4 ± 15.8 years. The IMA was 14.5 ± 3.3 degrees, HVA was 29.3 ± 8.4 degrees, MPA was 11.7 ± 6.3 degrees, and DMAA was 15.5 ± 5.3 degrees. Intraclass correlation coefficient (ICC) for intra-observer reliability was 0.829 for assessor 1 and 0.910 for assessor 2. ICC for inter-observer reliability was 0.727. Pearson correlation revealed no link between IMA and DMAA, nor HVA and DMAA. However, there was a significant (albeit small) correlation between MPA and DMAA ( r = 0.337, P = .017). In the mixed effects model, MPA remained an independent predictor of DMAA (β = 0.276, 95% CI 0.153-0.399, P < .001). Conclusion There was reasonable reliability in measuring DMAA between authors on WBCT although there was variation in measurements. We found the DMAA appeared to increase with increasing metatarsal pronation. The DMAA may therefore be (in part) projection artefact secondary to metatarsal pronation, and surgeons should be aware of this during surgical planning.