LOOSE-HINGE TOTAL ELBOW ARTHROPLASTY - AN EXPERIMENTAL-STUDY OF THE EFFECTS OF IMPLANT ALIGNMENT ON 3-DIMENSIONAL ELBOW KINEMATICS

A previous study suggested that the kinematics of a loose-hinge total elbow arthroplasty (TEA) are those of a truly semiconstrained joint. T his study addresses the effects of malposition of the implant. The thr ee-dimensional elbow kinematics during simulated active motion were st udied in six cadaver specimens using an electromagnetic tracking devic e. In addition to simulated active elbow flexion, flexion arcs were ob tained under an elbow varus or valgus moment, to calculate the structu ral varus-valgus laxities. The results after four different Coonrad-Mo rrey TEA positions of implantation were compared with those of the int act elbow. The flexion-extension amplitudes were not significantly dec reased after TEA implantation, except with external rotation of the ul nar component, which resulted in a loss of extension. In the intact el bow and after TEA implantation in any position, the mean varus-valgus deviations throughout elbow flexion were in a narrower range than the structural limits imposed by the ligaments (intact elbow) or the TEA h inge design. With internal malrotation of the humeral component over 1 0 degrees, however, the valgus structural limit was reached and, conve rsely, the varus limit with external rotation over 10 degrees. The cli nical improvement observed with the semiconstrained TEA is derived fro m the benefits of the less constrained articulation. The proximodistal changes of TEA implantation have no consequence on the kinematic patt ern. Rotational malpositioning of either humeral or ulnar component sh ould be avoided, the first because it changes the kinematic pattern to ward the structural limits of the implant and, therefore, may lead to excessive stresses at the bone-cement-implant interfaces and to early loosening, and the latter because it causes loss of extension.