Hallux valgus is the most common foot deformity in foot and ankle surgery, which can seriously affect the patients’ shoe wearing and even walking [23]. As a surgical treatment of hallux valgus, Scarf osteotomy is the first Z-shaped osteotomy of the longitudinal axis of the first metatarsal bone reported by Meyer in 1926. In 1991, Weil described this Z-shaped osteotomy using the word “Scarf” for the first time, and in 2000, Weil et al. popularized this procedure in clinical practice [24]. At present, Scarf osteotomy has become one of the commonly used surgical methods for treating hallux valgus. Simple Scarf osteotomy, i.e., Z-shaped osteotomy, has significant advantages in the treatment of hallux valgus, characterized by strong corrective ability, good stability, high clinical application value and high safety [9, 11, 25]. Postoperative deformity recurrence is the most common complication after surgical treatment. Scarf osteotomy can not only effectively correct deformities, but also preserve the range of motion of the first metatarsophalangeal joint, leaving space for fusion surgery to deal with recurrence [26]. Compared with other types of osteotomies, Scarf osteotomy can achieve comprehensive correction, so it can offer strong corrective ability and good corrective effect, effectively restoring the normal structure and function of the foot. Meanwhile, it can reduce the risk of postoperative joint stiffness and cause less surgical trauma, allowing patients to recover faster and suffer less postoperative pain. Moreover, Scarf osteotomy can also improve the appearance of the foot by adjusting the foot structure, providing better stability and reducing the risk of recurrence [27–28]. It is also noteworthy that Scarf osteotomy has greatly reduced the incidence of postoperative complications, such as metastatic plantar pain, bone non-healing, and metatarsal necrosis. To date, a number of modified and combined Scarf osteotomies for the treatment of hallux valgus have been reported. For example, Zheng Wenyuan et al. [22] reported that the first metatarsal joint Swanson prosthesis joint replacement combined with the first metatarsal osteotomy and bone grafting could effectively correct hallux valgus. However, there is still limited research on the conventional double-screw translational Scarf osteotomy. Therefore, in this study, we aimed to conduct finite element analysis to explore the stress changes in various parts of the foot and the screws after Scarf osteotomy under different translational displacements, in order to develop more accurate diagnosis and treatment plans and provide guidance and theoretical basis for clinical practice.
In this study, we compared the preoperative results obtained from the FreeStep plantar pressure testing system with the data obtained from finite element analysis. It was found that the plantar stress distribution of the finite element model was similar to that derived from the test data (i.e., similar stress distribution positions and numerical values). The stress on the forefoot accounted for 75% of the overall stress in the finite element model, and accounted for 71% of the overall stress in the static analysis cloud map. In both systems, the pressure was mainly distributed in the forefoot. In the finite element model, the maximum stress was found at the heel, which is similar in position to point M (the maximum pressure point) on the static analysis cloud map, and their values are also similar. This fully confirms the validity of the established finite element model of hallux valgus.
In the simulation of the Scarf osteotomy with four different translational displacements, it was found that, under the same loading and boundary conditions, the Von mises stress on the first metatarsal bone was the highest when the bone fragment was pushed outwards for 6mm. Under this setting, the Von mises stress on the first metatarsal bone achieved the most significant increase, accounting for 65% of the total stress of all metatarsal bones, and was 9 times of the preoperative value. Scarf osteotomy with 6mm displacement can restore the stress on the first metatarsal bone, relieve the stress on lateral metatarsal bones, and improve the stress distribution of the forefoot, therefore effectively correcting the hallux valgus deformity and improving the symptoms of metastatic plantar pain. At the same time, the maximum von Mises stresses on the second and third metatarsal bones under the 6mm setting were reduced by 16% and 11% respectively compared to the preoperative values, and were the lowest among the four settings. This suggests that the 6mm setting can effectively reduce the stress on the second and third metatarsal bones, which is the key to reducing metastatic pain and preventing the occurrence of metastatic pain at the second and third metatarsal bones after surgery. In addition, the stress on the fifth metatarsal bone was decreased by 14%. Before surgery, the total stress on the first and fifth metatarsal bones accounted for 44% of the total stress of all metatarsals, while in the 6mm Scarf osteotomy model, the total stress on the first and fifth metatarsal bones accounted for 76% of the total stress of all metatarsal bones, indicating an increase of 32%. This is compliant with the normal physiological condition that the first and fifth metatarsal bones should be the main load-bearing positions of the foot. Overall, Scarf osteotomy can increase the stress on the first metatarsal bone, relieve the stress on lateral metatarsal bones, change the force-bearing position and level on the forefoot, and reduce the probability of postoperative metastatic plantar pain. After the surgery, the patient’s plantar pressure distribution tends to return normal. Precise and customized Scarf osteotomy is conducive to enhancing osteotomy accuracy, making the plantar pressure distribution more reasonably, and shortening the surgical and postoperative recovery time.
With respect to the stress on the screws, it was found that the maximum Von mises stress was ranged 44.664–78.961 MPa under four different settings, all within the yield strength of titanium alloy, therefore able to ensure effective and stable fixation [29]. Under the 6mm setting, the maximum von Mises stress on the screws was the lowest (40.995 MPa). By observing the screw stress cloud map, it can be found that there was stress concentration at the screw head in the other three settings rather than the 6mm setting, which means that the 6mm setting can yield more uniform and reasonable stress distribution, and probably better stability. This is very important for maintaining the postoperative stability of the body. Monitoring and analyzing the stress and stress distribution on the screws can effectively prevent problems like screw loosening and deformation, providing more reliable support for clinical practice.
Some limitations of this study should be pointed out. First, we only performed static analysis but not dynamic analysis. Although our finite element model has been validated through plantar pressure test, it cannot completely replace in vivo research for the reason that the accuracy of biomaterial parameters and connections between anatomical structures cannot be guaranteed.