Background: Hallux rigidus and metatarsus primus elevatus (MPE) are associated, but their causal relationship remains unknown. Several surgical approaches for treating hallux rigidus are available. We evaluated morphological characteristics of hallux rigidus with different grades to determine the optimal surgical approach. The amount of degenerative change in the metatarsophalangeal joint on the preoperative roentgenograms was graded on a scale of 1–3. We analyzed the morphology of hallux rigidus using X-ray image mapping developed by our team. Methods: This study involved weight-bearing, dorsoplantar, and lateral foot X-rays of 36 feet from 26 patients underwent surgery for hallux rigidus (Group R) at our institution, and 26 normal feet (Group N). A two-dimensional coordinate system was used to analyze the sharps of these feet by converting each dot on the radiographs into X and Y coordinates. Diagrams of the feet from each group were drawn for comparison. Feet with grades 2 (Group R2) and 3 (Group R3) hallux rigidus and normal feet were compared by Kruskal–Wallis test. Results: Mapping revealed that the tip of distal phalanges of the second, and third toes in Group R medially shifted (P < 0.05) in dorsoplantar image of the feet, and that the medial point, a part of the talus, navicular, cuneiform, and first metatarsal bone in Group R, shifted lower (P < 0.05) in lateral feet images of the feet. Multiple comparisons revealed a significant navicular bone depression in grade 3 hallux rigidus compared with normal feet. A significant difference was observed between Group N and R3 but not between Group R2 and N or R3. Conclusions: X-ray morphological analysis of the foot revealed MPE in Group R. Elevation gradually increased as hallux rigidus grade worsened. Therefore, osteotomy combined with cheilectomy, whereby the first metatarsal bone can be tilted toward the plantar side, are useful for treating a higher-grade hallux rigidus. Level of evidence: Level III, comparative study.
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