Radiographic Indirect Parameters for First Metatarsal Rotation do not Correspond to Real Measurements in Hallux Valgus

CATEGORY: Midfoot/Forefoot; Bunion INTRODUCTION/PURPOSE: Subjective radiographic findings are commonly used in Hallux Valgus (HV) to dictate deformity aspects and surgical treatment. Head roundness, distal metatarsal metaphyseal angle (DMMA) and sesamoid positioning are being employed as predictors of ray pronation, intrinsic deformities, and soft tissue imbalance despite the lack of these observations' reliability. Estimation of first metatarsal rotation based on different head shapes has been proposed, including classifications for its severity. The objective of this study was to demonstrate if first metatarsal rotation measured by the alpha angle in weight-bearing computerized tomography (WBCT) images correlates with its head shape. We hypothesized that alpha angle values would not reliably reflect an indirect classification for rotation, and other variables would contribute to miscalculation. METHODS: In this IRB-approved, case-control retrospective study, we analyzed 26 hallux valgus feet (19 patients) and 20 control feet (16 patients) through conventional radiographs (XR) and WBCT images. Two blinded fellowship-trained orthopedic foot and ankle surgeon performed the measurements. Head format, roundness classifications (0, 1, 2 or 3), head plantar surface diameter, hallux valgus angle (HVA), intermetatarsal angle (IMA), DDMA, sesamoid station, sesamoid rotation, sesamoid arthritis, and metatarsal rotation (alpha angle) were evaluated. Interclass Correlations Coefficients (ICC) were performed for interrater reliability. Normative data were analyzed by ANOVA and comparison among groups and methods by Student's T-test. A multivariate regression analysis was executed to evaluate which of the measurements influenced the rotation classification and a partition prediction model constructed to find how the variables contributed to the grading system. Statistical significancy was considered for p-values of less than 0.05. RESULTS: All ICCs were found above 0.80 within both XR and WBCT readings. HV patients and controls were analogous regarding age and body mass index. Similarity was also found in WBCT and XR for traditional HV angles, considering both groups. Mean values were higher in HV patients than controls when evaluating alpha angle (11.51 [9.42-13.60] to 4.23 [1.84-6.62], 95%CI), head diameter (22.35 [21.52-23.18] to 21.01 [20.07-21.96]) and sesamoid rotation angle (26.72 [24.09-29.34] to 4.56 [1.63-7.50]). HVA and IMA were poorly correlated to alpha angles (values below 0.11). WBCT assessment downgraded the head roundness classification in comparison to XR. Changes in this classification were explained chiefly by the sesamoid station in the axial plane (R2: 0.37), where stations 4 to 7 were found to be strong predictors of roundness classification 2 and 3. The alpha angle had a low influence in head roundness classification (R2: 0.15). CONCLUSION: Metatarsal rotation cannot be reliably predicted through head roundness. Glenosesamoid arthritis and sesamoid subluxation alter the first metatarsal head format and diameter, blurring roundness evaluation. Sesamoid stations from 4 to 7 were strong predictors of higher roundness classifications. Measurements using different readers and methods (XR and WBCT) were reliable. First ray rotation (alpha angle) values did not influence head roundness classification strongly nor were correlated to HV severity.