Congruency of the TN Joint and Subtalar Joint Middle Facet Under With and Without Weightbearing: A Preliminary Report

Introduction/Purpose: Peritalar subluxation is a key feature of both flatfoot and cavovarus deformities. Our preliminary studies have found that on WBCT there is 20% subluxation of the middle facet of the subtalar joint in normal controls, and >35% in patients with flexible flatfoot deformities; In the spherical talonavicular joint there is physiological uncovering of the talar head regardless of whether the joint is bearing weight or not, since the articular area of the talar head is 1.2 times of the navicular. We hypothesize that there may be a tendency to overestimate the pathological peritalar subluxation on both XR and WBCT. This study aimed to investigate the congruency of each peritalar joint using a weightbearing 3D CT scan remodeling technique. Methods: Five below-knee-amputated fresh frozen cadaveric feet were used (no history of surgery, trauma, arthritis, and deformities). Each specimen was CT scanned using both non-weightbearing and weightbearing protocols. Segmentation on Materialise Mimics software was used to remodel each peritalar bone three dimensionally. Congruency of the restored talonavicular joint and subtalar middle facet was evaluated in the GeoMagic. The total articular surface area for each bone was reconstructed, calculated and recorded. The articulation of each joint with or without weightbearing was restored for analyzing joint uncoverage. Paired t-test (P value ≤0.05) was used to compare the coverage % differences. Results: In the anterior and middle facets of the subtalar joint, There was 16.40% uncoverage of the calcaneus under non- weightbearing, and 30.68% of uncoverage under weightbearing; 17.94% uncoverage of the talus under non-weightbearing, and 24.89% of uncoverage under weightbearing. In the posterior facet of the subtalar joint, the total articular surface on the talus side (683.96±112.07 mm2) was 1.13 times larger than the calcaneus side (606.78±107.23 mm2). In the talonavicular joint, the total articular surface on the talus side (714.18±124.97 mm2) was 1.28 times larger than the navicular side (556.76±97.65 mm2). Conclusion: Our preliminary study in this group of normal cadaver feet has found that in both the talonavicular and middle facet of the subtalar joints, there is physiological uncovering or subluxation regardless of whether the joint is bearing weight or not. Further investigation with a larger sample size is in process.