The Articular Dislocation Angle of the First Metatarsophalangeal Joint

BACKGROUND: First metatarsophalangeal joint (MTPJ) subluxation or dislocation represents a common yet unquantified pathological feature in hallux valgus (HV). In this study, we introduce the concept of the articular dislocation angle (ADA), defined as the angle between a line perpendicular to the proximal articular surface of the proximal phalanx and a line perpendicular to the distal articular surface of the first metatarsal, to quantitatively evaluate the degree of joint subluxation or dislocation. Using the ADA concept, this study quantitatively investigated the relationships between HV-related angular parameters (hallux valgus angle [HVA], distal metatarsal articular angle [DMAA], distal articular set angle [DASA], and ADA), thereby precisely delineating the pathological deformity components contributing to HV. METHODS: We measured HVA, DMAA, DASA, and ADA on weight-bearing radiographs of 70 feet (52 patients) with HV to investigate their interrelationships and quantify their respective contributions to HV. The mean patient age was 58 years, 63 feet were in female patients, and all patients were Asian. Additionally, by comparing DMAA and DASA measurements obtained from weight-bearing radiographs versus computed tomography (CT) scans, we systematically evaluated the accuracy of measurements on weight-bearing radiographs. RESULTS: Radiographic data demonstrated that the relationship HVA = DMAA + DASA + ADA held true across all HV severity grades. Quantitative analysis revealed a progressive increase in the ADA contribution with increasing disease severity (mild: 43.3%; moderate: 52.6%; severe: 69.3%), while DMAA and DASA contributions showed corresponding decreases (DMAA: 40.7%, 38.1%, 28.1%, respectively; DASA: 16.0%, 9.3%, 2.5%). Comparative analysis indicated excellent agreement between CT and weight-bearing radiographs for DMAA measurements, whereas radiographs systematically underestimated DASA values versus CT (mean differences: mild, -5.1°; moderate, -5.4°; severe, -7.8°), with the discrepancy magnitude increasing with disease severity. CONCLUSIONS: ADA provides an accurate quantitative assessment of first MTPJ subluxation or dislocation severity. The introduction of the ADA establishes a clear geometric relationship among HVA, DMAA, and DASA, formulated as: HVA = DMAA + DASA + ADA. Preoperative application of this formula allows effective assessment of the pathological components contributing to HV, thereby assisting surgeons in developing appropriate surgical strategies. LEVEL OF EVIDENCE: Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.