AbstractThe role of an anomalous tibialis posterior (TP) tendon in the etiology of hallux valgus (HV) deformity was investigated in four stages: clinical, anatomical, neurological, and operative. In the clinical stage, the patients were instructed to planter flex and invert the foot to contract the TP tendon. Attempts to correct the deformity passively were not possible and resulted in, moderate pain in all patients (197 feet). When the foot was in dorsiflexion and everted (TP tendon was relaxed) the deformity was easily corrected without any pain in 196 feet (99.5%). In the second stage (anatomical), 10 cadavers (20 feet) with HV deformity and 10 cadavers (20 feet) without any foot deformity were dissected. An abnormal expansion of TP tendon into the oblique part of adductor hallucis was found consistently in all HV feet. The tendinous expansion was absent in all normal feet studied. Also when traction was applied to TP tendon, an increase in the metatarsophalangeal (MP) angle of the big toe was observed in all HV feet, but not in the control group. In the third group (neurological), faradic stimulation was applied to the TP muscle in 7 patients with HV deformity and 7 without deformity. The MP angle was increased in HV patients, but no change was observed in the control group. In the fourth stage, 11 patients (18 feet) surgically operated for HV deformity by proximal metatarsal osteotomy and excision of the band anchoring TP tendon to the oblique part of adductor hallucis. The results were excellent in 10 patients (17 feet) even after a minimum 2.5 year follow‐up. Also, in two patients who were operated by different procedures with poor results, the TP tendon was lengthened by Z‐plasty and they were completely pain free.Our data show the dynamic role of anomalous expansions of the TP tendon into the oblique part of adductor hallucis muscle in HV patients. We suggest this expansion be excised in addition to other operative procedures selected for the surgical treatment of HV patients. © 1994 Wiley‐Lis