# A Review Study on Total Ankle Replacement
## 2 TAR System Study
### Bearing Type  
Both fixed and mobile-bearing systems exist for TAR implants. The main difference between these two designs is rotation, where the fixed-bearing systems have rotation dictated by the tibial component, while mobile-bearing systems allows translation and rotational movement [39]. Research on implant migration in fixed-bearing systems with a modular stem has shown that the implant continues to migrate for up to 2 years, with an average migration of up to 2 mm. This has shown to be greater in males than females, possibly due to having greater compressive forces in their ankle. This is consistent with previous studies in the knee showing that the prosthesis often needs to \"settle\", which can take up to 2 years after the initial operation [40]. A prospective study carried out between 2017 and 2020 compared preoperative and postoperative gait patterns in patients undergoing TAR with fixed bearing implants. Ankle range of motion was maintained from midstance to pre-swing during gait postoperatively. Further, TAR was shown to improve kinematic motion of the hindfoot joints. However, post TAR kinematics were not restored to that of controls without ankle arthritis. The authors therefore recommend that multi-segment foot modeling is relevant when assessing the outcome of TAR [41]. Another study evaluated polyethylene wear in fixed bearing systems where the polyethylene insert is fixed to the tibial component. Fourteen polyethylene liners were removed from uncemented, fixed bearing total ankle replacements for analysis of wear patterns. The liners were analyzed for burnishing, scratching, mechanical damage, pitting and embedded particles and showed increased wear posteriorly compared to anteriorly. It was theorized that the constraints for anterior to posterior translation of the talus may contribute to this pattern. The heterogeneity in damage supports a non-uniform stress distribution, which could lead to edge loading and eventual failure [42]. Mobile-bearing systems have three unconstrained tibial, talar and polyethylene components, with the theoretical advantage of better stress distribution. However, it can be challenging to determine the ankle axis of rotation and the rotation between tibial and tal components has been shown to have wide variation after mobile-bearing TAR. A study carried out on mobile-bearing TAR showed measurable motion between the mobile-bearing polyethylene insert and the tibial component, emphasizing the importance of axial rotation [43]. A study in the United Kingdom on patient outcomes after uncemented, mobile bearing TAR found equivalent functional and radiographic outcomes in patients receiving TAR for osteoarthritis, post traumatic arthritis and rheumatoid arthritis. Moreover, there was no statistically significant correlation between preoperative deformity and postoperative function. Any preoperative anterior translation was corrected in all groups postoperatively with good patient-reported outcomes. A mobile-bearing design may therefore be more forgiving when compared to a fixed-bearing design if there is incomplete component alignment [44]. A concern specific to mobile bearing implants is the reported high rates of polyethylene failure and periprosthetic osteolysis. A study on 99 HINTEGRA mobile bearing TAR, a prosthesis used commonly in Europe and Canada, found radiographic osteolysis in 37.4% at mean 40 month follow up. Sixty seven percent of these had progression of their radiographic osteolysis, but not all of these were clinically symptomatic. Similarly, mean 159 month follow up of STAR outcomes showed osteolytic lesions in 50% of patients despite 93.9% implant survival rate at 10 years [45]. Therefore the relationship between these radiographic findings and clinical outcomes remains to be elucidated. Current data shows favorable patient-reported and clinical outcomes for both mobile and fixed bearing TAR designs, without significant clinical difference.