Biomechanics and Tribology of Total Ankle Replacement

Claire Brockett, PhD email address: CLBrockett@gmail.com     Department of Mechanical Engineering, INSIGNEO Institute for in Silico Medicine, University of Sheffield, UK

This article discusses the biomechanics and tribology of total ankle replacements considering the influence of implant design and generation on functional outcome, before discussing the interplay between biomechanics and tribology in the clinical success of total ankle replacement. It reflects on what we know and highlights areas for further research, as well as identifying factors to consider in clinical practice.

Keywords

Total ankle replacement; Tribology; Biomechanics

Key points

1. Total ankle replacement has been used since the 1970s but clinical outcomes tend to be less successful than other lower limb joint replacement.
2. Range of motion in ankle replacement is typically lower than "normal" control groups but better than ankle fusion and restores function beyond the arthritic ankle.
3. Tribology of total ankle replacement has a critical role in clinical survivorship.
4. Limited experimental studies to date but demonstrate effect of design and biomechanics on wear performance of a total ankle replacement.
5. Retrieval analysis gives insight into biomechanics and tribology of total ankle replacement and indicates adverse loading conditions that are yet to be modeled experimentally.

Introduction

Osteoarthritis (OA) of the ankle is a degenerative condition, which affects approximately 1% of the adult population.1 It is frequently associated with previous trauma (severe or recurrent ankle sprain, joint fracture) and typically presents in younger patients than arthritis of the hip or knee. It has a significant impact on patient quality of life.2 Early-stage interventions may include orthotics and steroidal injections. Surgical interventions, such as total ankle replacement (TAR) and ankle fusion, are often offered as a final option, with ankle fusion still considered the clinical gold standard due to better clinical survivorship. However, recent ankle replacement devices have demonstrated improved performance and offer better biomechanical function compared with ankle fusion, so they are increasing in popularity.3 This article will give a brief overview of the design development of ankle replacement with specific consideration of biomechanical and tribological performance.

History of Total Ankle Replacement

Total ankle replacement was developed in the 1970s and used the materials and technology that had been applied to total hip and knee replacement in the decades before. Initial approaches did not give full consideration of the natural biomechanics and geometry of the ankle and were often highly constrained or not sufficiently constrained, leading to failure at the fixation interface and damage to the surrounding soft tissues.4 Often these ankle replacements demonstrated promising initial outcomes but high failure rates were observed beyond 2 years, which led to the intervention being abandoned in clinic for nearly a decade.5,6 Second-generation implants sought to address earlier problems by using cementless fixation, which enabled smaller devices and less bone resection than cemented implants, creating semiconstrained, more anatomically representative bearing surfaces and maintaining ligamentous tension.4 Typically these designs used a 2-component or 3-component implant with metallic tibial and talar components and a polyethylene insert that was either mobile or fixed (Fig. 1). Until the last 5 years, a mobile bearing was the more preferred design in Europe because it was considered to give more rotational and translational freedom, aiming to reduce stress at the fixation interface and increasing tolerance to surgical positioning.7 Conversely, fixed-bearing total ankle replacements have long been the preferred choice in the United States, with the devices considered to provide stability and mitigate risk of polyethylene insert dislocation.8 These devices showed a significant improvement in clinical outcome but survivorship was still typically around 80% at 10 years post implant, much lower than the successful outcomes of hip or knee replacement.4 In addition to the design of the bearing surface, there continues to exist a wide variety of fixation features across different total ankle replacements ranging from long tibial stems, screw fixation, and smaller bars/lugs. Each of these has different proposed benefits including improved stability, early fixation, and more physiologic stress distribution to the bone but also have concerns including stress shielding, implant loosening, and elevated contact stress due to reduced contact area, respectively, so an optimized fixation solution seems to not yet be achieved.9 More recent designs have reflected improvements in materials technology, such as the use of cross-linked polyethylene,10 as well as advances in surgical instrumentation to improve implant positioning and soft tissue balancing.11 These devices are currently quite early in their clinical use, so literature is typically outlining short-term to medium-term performance (up to 5 years), which often also includes surgeon learning-curves12 but the overall picture for these new devices is promising. Clinical failure in total ankle replacement (requiring revision surgery) is often associated with aseptic loosening and osteolysis13-15-with osteolytic lesions considered to be associated with strain within the bone, stress shielding, and immune response to wear debris16-hence, the biomechanics and biotribology of ankle replacements are important factors in clinical outcome and patient satisfaction.

Current Evidence: Biomechanics

To consider the biomechanics of total ankle replacement, we first need to consider the joints the device replaces and their contribution to overall motion at the ankle. In a total ankle replacement, metallic bearing components are fixed to the tibia and talus, with the polyethylene insert located between these 2 bearings (Fig. 217). Several cointerventions, including fixation of the syndesmosis or sub-talar fusion,18 have been adopted during TAR surgery; however, compared with primary total ankle replacement, the incidence of cointervention is rare.
Fig. 1 A mobile-bearing (A) and fixed-bearing (B) total ankle replacements-nominally based on MatOrtho BOX and Wright Medical Infinity designs, respectively (current generation devices).8 
(Image courtesy of Dr Alexandra Smyth.) Motion of the whole ankle complex occurs primarily in the sagittal plane, with dorsi/plantar-flexion contributing a maximum range of motion of 65° to 75°, most of which occurs at the talocrural joint.19 Rotation and inversion-eversion are considered to occur at both the talocrural and sub-talar joints, although the sub-talar joint contributes more to the overall range of motion. Conventional gait analysis enables us to quantify the motion of the overall ankle joint complex but is unable to separate the motions of the sub-talar and talocrural joints. Hence, the biomechanical data presented in this article will largely explore whole joint function of different ankle replacements.
Fig. 2 Exemplar of a total ankle replacement with tibial axis indicated.17 
(Image by Ashley Stratton-Powell, licenced under CC BY-NC-SA 2.5) One of design requirements of total ankle replacement is the preservation of range of motion, compared with ankle arthrodesis. Early biomechanical studies of first-generation total ankle replacements tended to indicate no improvement in range of motion or cadence postoperatively, although this may well be related to the challenges of those early devices.4,19 Studies exploring second-generation and third-generation ankle replacements have tended to compare devices against the alternative clinical treatment-ankle arthrodesis-and typically demonstrate improved range of motion, particularly within the sagittal plane for the TAR cohorts.20,21 Biomechanical studies suggest that patients with TAR are able to perform daily activities more efficiently than patients with fusion. Increased plantarflexion and higher ankle power in TAR cohorts indicates a better ability to propel the foot forward.22 Conversely, ankle fusion-due to the rigidity of the ankle joint complex-exhibits an increased moment arm through the foot, meaning dorsiflexion moments are distributed across other joints of the foot, which may contribute to the adjacent joint arthritis that is sometimes observed.23 Many of the gait studies and models of the ankle use rigid single-segment foot models, which do not isolate the true motion of the tibiotalar joint and therefore also include any compensatory mechanisms of adjacent joints. Additionally, clinical biomechanics studies rely on skin markers to derive motion of bones that are not adjacent to the skin-it is simply not possible to mount a marker on the skin over the talus. Therefore, we rely on multisegment models to model the predicted behavior of the talus, based on the motion of other joints in the foot and lower limb. Few multisegment models...