Kinematics of a total arthroplasty of the ankle: Comparison to normal ankle motion

Although the concept of a total ankle arthroplasty has been advanced as a m ethod for treating severe ankle arthritis, the clinical experience with all of the models developed has been discouraging. Both the constrained design s, which maximize joint contact area by restricting the available motion, a nd the unconstrained designs, which allow more normal motion at the expense of higher contact stresses, uniformly result in implant loosening, pain, a nd clinical failure in 2 to 7 years. This has led to the recommendation aga inst the use of a total ankle arthroplasty except in very low-demand patien ts. Failure of ankle implants can be ascribed to either anatomic considerations (e.g. - the talus is too small to accommodate the stress transfers of a pr osthesis), or mechanical etiologies. Abnormal 3-dimensional motion of the a nkle following arthroplasty would fall into the latter category. This study examined the motion that occurs after implantation of an unconst rained-type total ankle arthroplasty. Using previously validated methodolog y, axially loaded ankle specimens were cycled through an are of plantarflex /dorsiflexion while measuring the resulting coupled internal/external and v arus/valgus rotations. The average coupled motions in prosthetic ankles were not significantly dif ferent than their intact controls. There was, however, a significantly incr eased amount of hysteresis (defined as the difference between the upper and lower pathways of coupled motion at any given sagittal position) that occu rred as the ankle was dorsiflexed and planter flexed. The increased hystere sis was seen in both the axial and coronal planes. This indicates that ther e was a greater permitted envelop of motion in the prosthetic ankles compar ed to normal ankles. It is hypothesized that this subtle change in ankle ki nematics caused by the arthroplasty leads to abnormal stress transfer at th e prosthesis-bone interface, thereby promoting early implant failure.