Clinical Outcomes of Single Excision Versus Kidner Procedure for Type II Accessory Navicular Associated with Flatfoot in Adults: Does accessory navicular induce flexible flatfoot?

Background Patients with type II accessory navicular (AN) originally complain of the medial pain of foot. With increasing frequency, some of them has been recognized flexible flatfoot (FFF) at the first weightbearing radiographic examination. Posterior tibial tendon (PTT) dysfunction is widely accepted as a significant contributor to FFF. However, the PTT was not affected in these patients. The relationship between AN and FFF remains controversial. The contribution of AN to FFF was designed in this study. Methods Adult patients who complained of medial pain and bone eminence between January 2014 and January 2020 were included. 61 patients were confirmed to have the AN with flatfoot and randomly divided into two operative groups. The AN was excised in Group A, and the PTT was reconstructed to the navicular region with an anchor in Group B. Preoperative and postoperative evaluations were performed, including clinical evaluations, the American Orthopedic Foot and Ankle Society (AOFAS) mid-foot scale, a visual analog scale (VAS) and radiographic assessments of Meary’s angle, Pitch angle, talonavicular coverage, Kite’s angle and naviculocuboid overlap. PTT decline angle (PDA) and AN-Navicular joint inclination angle (ANJCA) in the lateral view were designed to evaluate the effect of AN on FFF. Results Fifty-six patients (56 feet) were included in this study because 5 patients were excluded. The mean follow-up period was 22.29 months with single excision (Group A) and 20.86 months with Kidner procedure (Group B). The AOFAS mid-foot score improved from 70.39±7.78 pre-operationally to 89.46±7.06 at the last follow-up in Group A and from 67.14±8.14 pre-operationally to 89.64±6.88 at the last follow-up in Group B. The VAS score decreased from 2.82±0.39 and 2.86±0.36 to 0.89±0.31 and 0.79±0.42, respectively. The radiographic results representing flatfoot significantly increased in the two groups. In the lateral view, PDA significantly increased after the operation, and the effect of PTT on the arch upward was induced by pull angulations and shorter distances. Conclusion The FFF with AN may be induced by AN and its synchondrosis. The weakened plantar ligament of synchondrosis was impaired under chronic tension and shear forces may be implicated as the etiologic biomechanical mechanism. AN excision or it with PTT reconstruction could release the pain and benefit the PTT pulling sufficiency.