Characterizing First Tarsometatarsal Joint Instability After Third-Generation Minimally Invasive Bunionectomy: A Coverage Mapping Assessment

Research Type: Level 2 - Prospective comparative study, Meta-analysis of Level 2 studies or Level 1 studies with inconsistent results Introduction/Purpose: Third-generation minimally invasive (MIS) bunionectomy requires lateral shifting of the first metatarsal (M1) head, which levers the 1st-tarsometatarsal (1st-TMT) joint and pushes it into adduction. The force across the 1st-TMT joint seems to overcome the ligamentous stabilizers, allowing the metatarsal to displace. Previous studies reported postoperative 1st-TMT “slippage” radiographically in the transverse plane, with medial subluxation of the M1 base. However, there is no comprehensive understanding of what occurs at this joint postoperatively. This study used coverage maps to characterize the change in congruency of the 1st-TMT joint following MIS bunionectomy. We hypothesized that the proximal fragment of M1 may undergo a rotational shift rather than pure translation and that the procedure would uncover the dorsal-lateral region of the 1st-TMT. Methods: This retrospective cohort study used data from a prospectively collected foot and ankle registry to examine patients with hallux valgus (HV) who underwent MIS bunionectomy. Pre- and postoperative weightbearing CT (WBCT) scans were collected from twelve patients with visible radiographic subluxation in the transverse plane (11 females, one male). The medial cuneiform and first metatarsal were segmented in both scans using Bonelogic Disior. The preoperative medial cuneiform coordinate system was calculated using an automated foot and ankle coordinate toolbox in Matlab and used to define the dorsal, plantar, medial, and lateral regions. The postoperative medial cuneiform was aligned with the preoperative bone to define identical articulation areas and create regional surfaces. A custom Matlab script calculated normal distances between the segmentation faces of the two bones and a 3-mm threshold to define joint coverage. One-sample t-tests compared coverage changes of the total area and regional areas to zero. Results: A significant decrease in total 1ST-TMT joint coverage was observed following MIS bunionectomy, shifting away from the dorsal-lateral and plantar-lateral regions towards the dorsal-medial region (Figure 1). Postoperatively, the total coverage of the 1ST-TMT joint area significantly decreased by an average of 16±17 mm2 (p=0.007). Regarding regional analyses, the dorsal-lateral region changed the most after MIS bunionectomy, with a significant decrease of 17±12 mm2 and a relative reduction of 32±18% (p < 0.0001) from preop. The plantar-lateral region significantly decreased by 6±10 mm2 with a relative change of 9±10% (p=0.0097), and the plantar-medial region increased by 7±9 mm2 with a relative change of 10±7% (p=0.0005). There were no significant changes in the dorsal-medial region (p=0.43). Conclusion: Our study suggests that MIS bunionectomy may significantly change the 1ST-TMT joint congruency, exhibiting an instability pattern involving a rotational mechanism rather than a pure medial translation of the first metatarsal base. Notably, the uncoverage was significantly higher in the dorsal-lateral aspect of the 1ST-TMT joint, as observed at the distal surface of the medial cuneiform, with relative preservation of the plantar surface. After MIS bunionectomy, the 1ST-metatarsal base appears to shift into pronation around a plantar fulcrum. Future studies are warranted to assess mid-to-long-term effects of 1ST-TMT uncoverage, including bunion recurrence and degenerative changes. Figure 1 – Differences in percent coverage of the first tarsometatarsal joint on the distal surface of the medial cuneiform between the preoperative and postoperative condition following MIS bunionectomy. * Indicates statistical significance of the differences compared to the theoretical mean of zero for the one-sample ttests. Coverage maps of representative patient changes from preop and postop shown for the dorsal-medial (DM), dorsal-lateral (DL), plantar-medial (PM), and plantar-lateral (PL) regions.