Which parameters predict correction of the intermetatarsal angle after first metatarsophalangeal fusion?

Fusion of the first metatarsophalangeal joint (MTPJ) is a commonly performed surgical procedure. Although the effect of first MTPJ fusion on reduction of Intermetatarsal angle (IMA) is well described, contributing factors remain unclear. The aim of this study was to identity predictive parameters for IMA reduction. Fifty-one patients (68 feet) who underwent a first MTPJ fusion and had an IMA greater than fourteen degrees were assessed retrospectively. The average age was 68 (31.4-79.3) years. Sixteen demographic and radiographic variables were evaluated using a multivariate regression analysis for association with change in IMA after surgery. The mean preoperative IMA was 16.11 (range, 14.0-22.5) degrees with a mean reduction of 4.95 (range, 0-17) degrees after surgery. Multivariate regression analysis revealed three significant independent predictors of the change in IMA. Increased preoperative IMA (β = .663, CI = .419, .908, P <.001), increased preoperative translation at base of MT1 (β = .490, CI = 0.005, .974, P = 0.039), and less postoperative translation in the fusion (β= -0.693, CI= -1.054, -.331, P= 0.002) significantly increased the amount of IMA reduction. Pre-operative IMA and translation at the base of the first metatarsal were positive predictors for correction of IMA after first MTPJ fusion. Translation at the level of the MTP I fusion was a negative predictor for the amount of IMA correction. Based on these findings, we recommend minimizing the lateral translation of the proximal phalanx relative to the metatarsal head to optimize IMA correction after MTPJ fusion.