Test methods

The reference method used the talar-first metatarsal angle in lateral foot weight-bearing radiograph. This angle has been used to identify the foot deformity in patients with pes planus and pes cavus. The talar-first metatarsal angle is formed between a line drawn along the longitudinal axis of the talus and the first metatarsal bone (Fig. 1A). The arch of foot is classified as pes cavus when convex upward angle >4 degrees, a normal arched foot has convex upward to downward angle within 4 degrees, and pes planus has convex downward angle >4 degrees [7,13]. The talar-first metatarsal angle compared with symptoms of the patient shows sensitivity and specificity of 88% and 94% respectively in pes cavus, and 100% and 87% respectively in pes planus [14]. This angle has a good intra-observer reliability (intraclass correlation coefficient [ICC]=0.96) and inter-observer reliability (ICC=0.69) [8]. The talar-first metatarsal angle in radiograph was measured at a single time using a goniometer by the one experienced foot and ankle orthopaedist who blinded for the clinical diagnosis.

Talar-first metatarsal angle (A), Harris imprint index (B), Chippaux-Smirak index (b/a×100), and Staheli index (b/c) (C).

The footprints were performed by the Harris mat footprint method and documented in the medical record. The 3 index tests of HII, CSI, and SI were conducted with computer-assisted program. (1) The HII measured a distance from the origin of two perpendicular lines in centimeter. The y-axis is a midline foot axis from tip of 2nd toe to the midpoint of hindfoot and the x-axis is a perpendicular line drawn medially and laterally at the widest and narrowest point of the arch (Fig. 1B). HII score is based on distance from the origin of the x-axis where the lateral side of arch is negative and medial side of arch is positive. HII score defined as pes cavus is (-4, -3, -2), normal arched foot is (-1, 0, +1), and pes planus is (+2, +3, +4) [5,9]. (2) The CSI is the ratio between the narrowest width of the midfoot (line b) to the widest width of the forefoot (line a) (CSI=b/a×100; Fig. 1C). CSI≤24% is classified as pes cavus, 25% to 45% as normal arched foot and ≥46% as pes planus [9]. (3) The SI is the ratio between the narrowest width of the midfoot (line b) to the widest width of the hindfoot (line c) (SI=b/c; Fig. 1C). SI≤0.4 is categorized as pes cavus, 0.5–0.7 normal arched foot and ≥0.8 as pes planus [9,15]. All 3 index tests were measured two times with an interval of 2 weeks to determine the intra-observer reliability by the rehabilitation physician. Furthermore, the orthotist who blinded to the results of the other tests measured the index tests at one time to calculate the inter-observer reliability.