Abstract
Background:
Total ankle replacements (TAR) are not as successful as total hip or total knee replacements. A three-time increased revision rate is reported in registry data [1]. Therefrom, wear associated revisions are frequent [2]. However, there is little knowledge about the wear behavior of TAR. This may be partly related to the fact, that currently no standard for wear testing of TAR exists.
The aim of this study is to define a biomechanical valid, force-controlled test specification for level walking of TAR.
Material and Methods:
Basic requirements for force-controlled testing of TAR is the definition joint flexion, as well as active forces and torques acting on the joint and the definition of the ligamental stabilization of the joint.
To specify flexion of the ankle, gait analysis was performed on patients treated with a TAR (HINTEGRA, Smith & Nephew) using skin mounted markers. Data about in-vivo forces is missing for TAR. Hence, determination of active forces and torques was based on mathematical models as described in the literature.
A new testing device (figure 1) has been developed to measure ligamental stabilization of the ankle joint. Measurements were performed on 10 paired cadaver feet (n = 20). Measurements were performed in different flexion angles when applying anterior-posterior forces (± 160N) and internal-external torques (± 2,5 Nm) between the talus and the tibia.
Results:
Three different testing scenarios for level walking were defined, representing the rehabilitation of the patient:
- 1.
Level walking after implantation with reduced kinematics & kinetics
- 2.
Level walking with an increasing range of motion (ROM)
- 3.
Level walking with kinematics & kinetics based on the loading in the native/healthy joint
These definitions were based on the following findings: Gait analysis showed a reduced flexion for patients provided with TAR (ROM 14,9° TAR vs. 23,1° native side). Axial forces were defined to be 3.6 times in the native and 2.5 times body weight in TAR. Internal-external torques in total of 6.5 Nm and 10.1 Nm and AP-forces of 375N and 598N were defined for TAR and for the native joint, respectively.
Laxity of the ankle joint was found to be directly related to the flexion of the ankle joint. Restraints of the joint during simulation were defined for every testing scenario with regard to the flexion angle and the acting AP-forces and IE torques.
Conclusion:
Previous, published wear testing scenarios used self-defined displacement controlled methods. However, it is not known how TAR moves dynamically, especially regarding AP-motion. Therefore, the validity of these tests needs to be questioned. The new developed testing scenarios enable a biomechanically valid wear testing of TAR.
In this study determined forces and torques have been shown to be comparable to total knee replacements (TKR). However, the flexion is massively decreased compared to TKR. Additionally, the TAR has a smaller contact area compared to TKR. Therefore increased contact pressures during simulation have to be assumed. Based on this study, wear-testing of TAR has been performed (see abstract: Wear performance of total ankle replacements).
