Mechanical Axis Method to Determine First Intermetatarsal Angle and Tibial Sesamoid Position

Utilizing the mechanical axis can decrease load on the joint and be beneficial when analyzing bony deformities and planning surgical correction with osteotomies. The aim of this study was to identify the normal mechanical axes of the first and second metatarsals and use them to obtain the first/second mechanical intermetatarsal angle (mIMA). The mechanical axis of the first metatarsal was used to obtain the mechanical tibial sesamoid position (mTSP), which provides a mechanical relationship with the sesamoid apparatus. The angular difference between the anatomic and mechanical axis lines (anatomic-mechanical angle [AMA]) was determined for the first metatarsal and for the second metatarsal. The commonly used first/second anatomic intermetatarsal angle (aIMA) and anatomic tibial sesamoid position (aTSP) were also obtained and compared with the first/second mIMA and mTSP. In this retrospective analysis, radiographs of 50 normal feet (40 patients) were assessed. Pearson's correlation coefficients were used to measure reliability between obtained measurements. Mean first/second aIMA was 8.6 ± 3.0 degrees, and first/second mIMA was 8.6 ± 2.6 degrees. First metatarsal AMA was 1.1 ± 1.0 degrees; second metatarsal AMA was 2.0 ± 1.6 degrees. The mTSP was 2.8 ± 1.1, and aTSP was 2.9 ± 1.0. The TSP median was 3 (range, 1-5). Using the mechanical axis method to obtain the first/second mIMA and the mTSP is reproducible and not affected by anatomic changes to the shape of the metatarsal. Unlike the anatomical axis, the mechanical axis does not change, therefore we recommend using the mechanical axis during surgical planning and when obtaining preoperative and postoperative measurements for the long bones of the foot, particularly for forefoot conditions such as hallux valgus.