Pronation on Weightbearing Radiographs is Not Associated with Pronation Calculated from Weightbearing CT Scans

CATEGORY: Bunion; Other INTRODUCTION/PURPOSE: In hallux valgus (HV), first metatarsal pronation is increasingly recognized as an important aspect of the deformity. Weightbearing CT (WBCT) scans have allowed investigators to quantify pronation of the first metatarsal. Yamaguchi et al. (J Orthop Sci 2015) proposed that pronation of the first metatarsal can be estimated by looking at the shape of the lateral aspect of the first metatarsal head on weightbearing AP radiographs of the foot, which would provide surgeons with a simple and readily available guide for evaluation of pronation. The purpose of this study was to compare pronation determined from the shape of the lateral head of the first metatarsal on AP weightbearing radiographs in HV patients with pronation calculated from WBCT scans. METHODS: Patients were included in this study if they had preoperative (n=22 patients) or 5-month postoperative (n=22 patients) WBCT scans and corresponding weightbearing AP radiographs of the affected foot. Pronation on weightbearing AP radiographs was grouped into four groups 0–9°, 10–19°, 20–30°, and >30° as described by Wagner and Wagner (Foot Ankle Clin 2018) by two independent observers. Pronation of the first metatarsal on WBCT scans was measured using a technique developed by Campbell et al. (Foot Ankle Int 2018), which calculates pronation of the first metatarsal in reference to the second metatarsal using a 3D CAD model, and using the alpha angle as described by Kim et al. (Foot Ankle Int 2015) and was categorized into the same four groups described for the radiographs. Weighted kappa coefficients (k) were used to determine interobserver reliability and agreement between pronation groups on radiographs and WBCT scans. Intraclass correlation coefficients were used to determine reliability of the alpha angle measurements on WBCT scans. After dividing patients into plain radiograph pronation groups, differences in mean WBCT pronation of the first metatarsal was compared using Kruskal-Wallis tests. RESULTS: Agreement between the two observers for preoperative and postoperative pronation on radiographs was moderate (k=.529,P<.001) and fair (k=.382,P=0.001), respectively. Intraclass correlation coefficients for preoperative and postoperative alpha angles were good (ICCs > 0.80, P<.001). Pronation of the first metatarsal on radiographs agreed with pronation on WBCT scans using a CAD method in 7 of 22 cases preoperatively (31.8%) and 8 of 22 cases postoperatively (36.4%). However, statistically, this was represented no agreement by more than chance between preoperative or postoperative pronation on WBCT scans and radiographs (k=.041, P=.774 and k=.059, P=.595, respectively) (Figures 1A-B). Similarly, pronation of the first metatarsal on radiographs did not agree with pronation on WBCT scans using the alpha angle measure either preoperatively or postoperatively (k=.070, P=.474 and k=.063, P=.364, respectively). Preoperatively, in the plain radiograph pronation groups of 10-19° (n=9), 20- 29° (n=11), and >30° (n=2), mean pronation calculated from WBCT scans using a 3D CAD method was 21.1°, 25.2°,and 20.9°, respectively (P=.752), and mean pronation calculated using the alpha angle was 13.2°, 15.8°, and 22.3°, respectively (P=.308). Postoperatively, in the plain radiograph pronation groups of 0-9° (n=9), 10-19° (n=6), and 20-29° (n=7), mean pronation calculated from WBCT scans using a 3D CAD method was 22.3°, 22.5°, and 21.0°, respectively (P=.976), and mean pronation calculated using the alpha angle was 3.1°, 5.7°, and 6.4°, respectively (P=.252). CONCLUSION:: Pronation of the first metatarsal measured on weightbearing AP radiographs using the lateral first metatarsal head had only fair to moderate interobserver agreement and did not statistically significantly agree with pronation measured on WBCT scans using either a 3D CAD model or the alpha angle. Additionally, mean pronation on WBCT scans did not significantly differ by any magnitude between the plain radiograph pronation groups, which indicates that there was no association between pronation measured on WBCT scans and plain radiographs. These results suggest that surgeons should use caution when trying to derive first metatarsal pronation from AP weightbearing radiographs for surgical planning purposes.