Category: Ankle Introduction/Purpose: Contemporary total ankle arthroplasty (TAA) techniques reconstruct the tibiotalar articulation while preserving the medial and lateral malleoli. The malleoli place anatomic constraints on the design of the talar component of the TAA system. If these constraints are not respected, then proper placement of the talar component may be compromised intra- operatively, or unintended contact between the prosthesis and bone may occur post-operatively. The goal of this study was therefore to quantify the medial and lateral malleolar boundaries. Methods: The orientation of the talus from a frontal view was quantified based on CT scans of left leg, non-arthritic specimens (n=89; 52M/37F) (Figure 1a). The talar dome was identified as the portion of the talus superior to the talar neck. The frontal profile of the dome defined by a plane positioned through the medial and lateral high points was extracted for each specimen. Statistical shape analysis was performed to identify the modes of variation of the frontal profile. Medial and lateral lines were fit to each profile, and resulting angles relative to the superior-inferior axis were measured. A paired student t-test (P<.05) was used to assess differences between the medial and lateral malleolus. Results: Figure 1b shows the average frontal profile of the talar dome, as well as ±1 and ±2 standard deviations. The average medial and lateral taper angles of the talus were 15.5° ± 5.9° and 7.6° ± 2.9°, respectively, with the lateral taper angle being significantly smaller than the medial taper angle (P<.001). The medial taper increased with talus size (as measured by the medial- lateral width of the dome) whereas the lateral taper decreased with talus size (Figure 1c), though both regressions were weak (R2 < 0.1). No statistically significant difference in taper angles was found between genders. Conclusion: Restoration of the bicondylar articulation geometry of the tibiotalar joint is an important design goal for TAA. One aspect of this geometry is the anatomic constraints imposed by the medial and lateral malleoli, to minimize unexpected impingement of the TAA prostheses with surrounding anatomic structures. Here, we have quantified those constraints through analysis of the medial and lateral taper of the talus, showing an increased taper angle on the medial side of the talus as compared to the lateral side.

Category: Midfoot/Forefoot Introduction/Purpose: A metatarsal osteotomy is sometimes used to correct hallux valgus deformities in the foot, by placing an angular bone graft or spacer in the osteotomy wedge held together using an osteotomy plate. Information on loads through the osteotomy is essential to ensure that the device is adequately tested to perform safely. This study used a validated musculoskeletal computer model of the lower extremities [J Biomech Eng. 2015] with a detailed foot section [Carbes, ISB 2011, Brussels], for evaluating loads across the 1st metatarsal osteotomy during the stance phase of walking gait. This study also evaluates the effect of anatomic variability (osteotomy location relative to anatomical landmarks) and surgical variability (plane of the osteotomy cut) on the peak loading across the osteotomy. Methods: The foot section of the model included 26 bones and major soft tissue structures (Figure 1a), resulting in more than 75 forces acting on the 1st metatarsal. Four healthy patient specific models were analyzed (body weight: 774±73 N; foot length: 246±16 mm), for five osteotomy planes. The osteotomy plane perpendicular to the long axis of the 1st metatarsal (posterior- anterior) was considered a neutral-N osteotomy. Four additional planes were considered to capture surgical variability of 5 degrees in abduction-AB, adduction-AD, dorsiflexion-DF and plantarflexion-PF. Compressive (Fx) and shear (Fy, Fz) forces as well as three moments: varus/valgus (Mx), flexion/extension (My) and internal/external rotation (Mz) were predicted in body weight (BW) multiples. Anatomic variability (standard deviation in peak values between patients, averaged across all osteotomies) and surgical variability (Standard deviation in peak values between osteotomies, averaged across all patients) were computed for the peak force and moment components. Results: Similar trends for peak forces and moments were predicted for all patients. An average of 2.2xBW peak force was predicted along the long axis of the 1st metatarsal (Fx) at 50% gait position (Figure 1b, 1c). The predicted shear forces (Fy, Fz) were much lower than the axial compressive force (Fx). The average moment predictions were similar among all five osteotomy planes, with peak moments predicted in the internal/external direction (My) at 43% and 55% of gait positions. Thus, load variability due to patient anatomy and surgical cuts were studied for Fx and My (Figure 1c). Load variability between patients of 18% and 21% were predicted for the Fx and My, respectively. The highest surgical variability of 7% and 3% were predicted for Fx and My. Conclusion: A validated musculoskeletal computer model can be used to inform physiological loading across the 1st metatarsal osteotomy throughout gait, which can be used to evaluate the device safety. The model further helps to understand the influence of anatomical and surgical variability on the peak loading passing through the 1st metatarsal osteotomy, providing guidance in surgical planning. Although peak loading passing through the 1st metatarsal osteotomy is affected by both anatomical as well as surgical variability, it was influenced more by anatomical variability than surgical. This modeling approach can be used to evaluate loading for patients with numerous specific disorders.

Category: Ankle Introduction/Purpose: Highly crosslinked polyethylene (HXLPE) was developed for its superior wear properties in comparison to conventional polyethylene (CPE). The higher dose irradiation required for HXLPE may also cause embrittlement, which reduces fatigue resistance and leads to surface cracking or fracture of the polyethylene bearing. Concern over fatigue resistance has prevented widespread adoption of HXLPE for use in in total ankle arthroplasty (TAA). The aim of this study was to determine whether HXLPE has sufficient fatigue strength for total ankle arthroplasty under simulated physiologically relevant motion profiles and loading in the ankle. Methods: A bicondylar, semi-constrained HXLPE TAA design was subjected to 10 million cycles (Mc) of fatigue testing under loading conditions representative of a walking gait. Kinetics and kinematics of gait were incorporated into a computational model (Dassault Systemes / SIMULIA, Johnston, RI), for prediction of peak stresses on the HXLPE insert. Based on predicted peak stresses, worst case component size and loading configuration were identified. Ten samples were tested on a closed loop servohydraulic test frame (MTS Systems Corp., Minneapolis, MN) for 10Mc. Testing was conducted to a peak load of 5600 N (1259lbs), representing approximately 5 times body weight for a 240 lb individual. Following testing, all samples were evaluated for evidence of polyethylene fracture or surface cracking. Results: Peak stresses in the HXLPE insert occurred during heel off, closely corresponding to both peak axial force and dorsiflexion during gait. The smallest sized component had the highest polyethylene insert stresses, whereas larger sized components had more material to bear the same load, resulting in up to a 30% decrease in stress. All 10 specimens completed 10Mc of testing at 5 times body weight without fracture or surface cracking of the polyethylene insert. Conclusion: HXLPE has sufficient fatigue strength to withstand 10Mc of loading at 5 times body weight at the point of peak stresses during gait in total ankle arthroplasty, and therefore, may be mechanically strong enough to withstand the demands of the ankle. Further clinical evidence is necessary to determine if these results translate to adequate fatigue strength with clinical use of HXLPE.