Bi-Radial Curvature Morphology of the Healthy Talus

Abstract

Category: Ankle Introduction/Purpose: In order to design an implant for the ankle joint that mimics normal joint motion, the condylar geometry of the talus must be anatomically accurate. Previous attempts to describe the curvature of the talus have typically involved fitting single-radius arcs to the medial and lateral facets. The purpose of this investigation was to determine if the curvature of the medial and lateral sides of the talus can be more accurately described by dividing the condyles into anterior and posterior regions, thus creating bi-radial curves for both the medial and lateral sides of the talus. Methods: After IRB approval, 18 subjects (9 male, 9 female; mean age 34.5 years) underwent weight-bearing CT scans at mid- stance of simulated gait. All subjects were deemed to have a healthy right ankle by the surgeon investigator. CT images were used to generate 3D models of each talus. A coordinate system was defined and the articular surface of the talus was separated into four sections: medial-anterior (MA), medial-posterior (MP), lateral-anterior (LA) and lateral-posterior (LP). The curvature of each section was defined by selecting points on the articular surface at 10° intervals. The extent of each radius was 30° of arc and the magnitude of each radius was selected to minimize the gaps between the radii and the spline curve to define a best-fit bi-radial approximation to the spline curve using geometry that could be easily used to define the articular surface of the talus. Ratios of the aforementioned radii were calculated. Results: The average MA, MP, LA and LP radii were 18.3 mm, 26.6 mm, 21.5 mm and 25.1 mm, respectively. The medial (A/P), lateral (A/P), anterior (M/L) and posterior (M/L) radii ratios were 0.70, 0.87, 0.88 and 1.07, respectively. The anterior and posterior ratios were compared using a paired t-test and found to be statistically different (P=.019). Further, the data were compared against a hypothesized value of 1 using a one-tailed one-sample t-test. The anterior ratio was significantly lower than 1 (P=.014) while the posterior ratio was significantly greater (P=.037). On the lateral side, 83.3% of the subjects exhibited a larger posterior radius than anterior radius. Only one subject (5.6%) had a larger anterior radius than posterior radius on the medial side. Conclusion: This study shows that the radius increases in the sagittal plane from the anterior portion to the posterior portion of both the medial and lateral sides of the talus. Furthermore, the MA radius is smaller than the LA radius. Conversely, the MP radius is larger than the LP radius. These results substantiate the validity of an implant design that incorporates a condylar radius ratio that is smaller for the anterior dorsiflexion surface and greater for the posterior plantar flexion surface. Implants with more accurate anatomical geometry may allow for more normal kinematics and potentially prolong the life of the implant.