The Variation Between Unilateral and Bilateral Weight Bearing in Hindfoot Alignment Evaluation: Part II: A Prospective In Vivo Kinematic and Plantar Pressure Study

Abstract

Category: Hindfoot; Midfoot/Forefoot Introduction/Purpose: When AP, lateral, and hindfoot views weight bearing XR's were taken, patients were frequently asked to stand on the foot being examined with the other foot lifted off the floor. However, we have found that alignments of the hind and midfoot during unilateral standing are different from those when bearing weight on both feet. This study compared the kinematics and plantar pressures between unilateral and bilateral standing positions in a group of asymptomatic volunteers. Methods: Ten participants (20 feet) without symptoms, previous trauma and surgical history in both lower limbs were included. Each participant first stood comfortably on both feet, then lifted the non-examined foot off the floor. Each person switched between the above two positions three times. Kinematic data was collected using an eight-camera Optitrack 3D motion capture system to reflect changes in alignments of the ankle, subtalar, and midtarsal joints. Plantar pressure distribution was assessed using a pressure platform. Paired t-tests were used to compare the metrics between the two standing positions, with statistical significance level set at P < 0.05. Results: From bilateral to unilateral weightbearing, 14 out of 21 kinematic angular parameters showed medium to high level of statistical significance (p=0.001-0.046) reflecting differences in the relative positions of the ankle, subtalar, and midtarsal joints in the two weightbearing positions resulting from pronation of the hindfoot and midfoot; the average plantar pressure change underneath the hallux (0.1616N), toes 2-5 (0.0907N), metatarsals 2-4 (0.2481N, 0.3204N, 0.2649N), midfoot (0.1794N), medial and lateral heel (0.259N, 0.246N) all showed statistically significant increase. There was a significantly higher plantar pressure increase on the medial than the lateral side of the feet. This corresponded to both the kinematic changes, and our findings in another study investigating the alignment change between the positions radiographically. Conclusion: The hindfoot and midfoot are more pronated in unilateral weightbearing than in bilateral weightbearing which may mask the severity of the deformity in a flexible cavovarus foot but increase that in a flexible flatfoot. Modification of the traditionally used unilateral weightbearing position in obtaining clinical radiographs should be considered. Bilateral WBCT scans may be a better alternative, taking advantage of allowing both feet to bearing weight naturally without concerning about one foot blocking the other during the radiographic imaging.