Instability of the First Ray and Hallux Valgus in Patients with Adult Acquired Flatfoot Deformity (AAFD)

Abstract

Category: Bunion Introduction/Purpose: Longitudinal arch collapse and first ray instability represent landmarks for adult acquired flatfoot deformity (AAFD), and have been linked to the development and progression of hallux valgus (HV). Radiographic evaluation of first ray instability is usually marked by increased angulation between the first and second metatarsals. The 1-2 intermetatarsal angle (IMA) is also an important aspect in the staging of HV deformity. Weightbearing CT imaging (WBCT) provides three-dimensional evaluation of dynamic deformities such as AAFD and HV. The purpose of this study was to assess the correlation between hallux valgus severity and foot collapse indicators using WBCT measurements, in patients with AAFD. We hypothesized that a flattening of the longitudinal arch, increased hindfoot valgus and forefoot abduction would correlate with greater IMA and HV angles. Methods: In this retrospective comparative study, 108 patients with stage II AAFD, 36 men and 72 women, with a mean age of 54.4 (range, 20-78) years, had their WBCTs evaluated by 2 blinded and independent board-certified foot and ankle orthopedic surgeons. The readers assessed multiple variables related to the severity of the hallux valgus and flatfoot deformities including: 1-2 intermetarsal angle, hallux valgus angle, talocalcaneal angle in the axial plane, talus-first metatarsal angle in the axial and sagittal planes, hindfoot alignment angle, hindfoot moment arm, navicular- and medial cuneiform-floor distance and the talonavicular uncoverage angle. Intra- and interobserver reliability were calculated by Pearson or Spearman’s correlation and intraclass correlation coefficient, respectively. A multiple regression analysis was used to evaluate the correlation between the variables indicative of AAFD and the severity of HV. P-values less than 0.05 were considered significant. Results: The intra- and interobserver reliability ranged from (0.65-0.99). Means and standard deviations for IM and HV angles were 11.3°±3.7° and 17.6°±13.4°, respectively. These angles significantly correlated with each other (p<0.0001). Most of the AAFD measurements evaluated were significantly associated with either increased IM or HV angles. IM angle correlated with increased talocalcaneal (26.0°±10.3°, p<0.0001), talus-first metatarsal (19.0°±13.6°, p=0.0001), and hindfoot alignment angles (22.3°±12.9°, p= 0.0049). HV angle correlated with medial cuneiform-floor distance (15.1mm±5.5 mm, p=0.0183), talus-first metatarsal angle in the axial plane (p=0.0004) and sagittal plane (15.7°±8.8°, p=0.0351), talonavicular uncoverage angle (17.8°±13.9°, p=0.0035). Hindfoot moment arm and navicular-floor distance were the only AAFD measurements that did not correlate with IM or HV angles. Conclusion: To the best of our knowledge this is the first study to confirm the association between AAFD, first ray instability and hallux valgus deformity using WBCT images. Our study results demonstrated that stage II flatfoot patients indeed have increased intermetatarsal and hallux valgus angles. Measurements traditionally used for staging the severity of AAFD showed significant positive correlation with increased IM or HV angles. Even though cause and effect cannot be determined with certainty, foot and ankle surgeons should consider these findings during evaluation and surgical planning of patients with AAFD.