Optimizing the correlation between first metatarsal head position and osteotomy plane angle in modified scarf osteotomy: a three-dimensional digital analysis

A modified Scarf osteotomy can not only correct hallux valgus deformity but also relieve lateral metatarsalgia by lowering the head of the first metatarsal. This effect is achieved by inclining the longitudinal osteotomy plane from dorsomedial to plantar-lateral. However, with the increase in the inclined angle, the correction ability for the intermetatarsal angle will weaken, which is critical for hallux valgus correction. The balance between intermetatarsal angle correction(IMAC) and the lowering of the first metatarsal head is crucial for good clinical benefits, and the inclined angle determines the balance. We built 11 three-dimensional models with weight-bearing CT of hallux valgus patients. Then, we simulated scarf osteotomy with different inclined longitudinal osteotomies and explored the exact mathematical relationship between the four variables: lateral displacements of the plantar fragment(LDPF), inclination of the longitudinal osteotomy plane༈ILOP༉, intermetatarsal angle, and plantar displacement of the metatarsal head༈PDMH༉. The degree of IMAC and the PDMH increased with the LDPF. With the increase in the inclination of ILOP, when LDPF is the same, the IMAC correction ability decreases, and PDMH increases. Scattering the LDPF and IMAC, they are both highly positively and linearly related. Scattering the LDPF and PDMH, the two were also positively and linearly related in all four different ILOP groups. The change in the slope of the lines was heterogeneous, the effects of ILOP on IMAC and PDMH were nonlinear. The optimal ILOP inclination angle was 15 degrees to 30 degrees, as observed by the fitted lines. Between 15 degrees and 30 degrees, the IMAC ability decreased less, while the PDMH was moderate and could produce clinical efficacy. The optimal inclination angle for longitudinal osteotomy was determined to be 15–30 degrees. The developed formula serves as a practical tool for preoperative planning.