Kinematic Differences in Posterior Stabilized Total Knees Determined by a Holistic Experimental Evaluation Method

Abstract

Posterior stabilized (PS) total knees are regarded as one classification, although there are considerable differences in condylar and cam-post geometries which may affect their kinematic outputs. Evaluation methods for kinematics have included Oxford type machines [1], robots [2], knee simulators [3,4], computer models [5], and loading rigs specifically designed to measure laxity in line with the ASTM standard on constraint [6].We developed a desktop machine for kinematic evaluation of total knee models under a full range of loading conditions and flexion angles, and proposed that the data from the anatomic knee be used as the benchmark [7].Our hypotheses were that current PS designs will show a large variation in kinematic output, that these would differ from anatomic characteristics, and that an asymmetric guided motion design could potentially restore anatomic kinematics.The test machine applied combinations of compressive loads, shear forces and torques, at a full range of flexion angles, representing a spectrum of functional activities. Forces of 1,000 N compression, 200 N shear, and 5 Nm torque were applied by computer controlled pneumatic actuators.Springs of the correct stiffness represented soft tissue restraints [8]. The five loading conditions were applied sequentially at incremental flexion angles specified in Fig. 1 ranging from 0 – 120 degrees. Fig. 1 shows a schematic of loading and constraint parameters of the machine.The sequence of loading conditions was: Compression load only, compression and anterior shear, compression and posterior shear, compression and internal torque, and compression and external torque.By digitization of fiducial points on the femoral and tibial components at each test condition, and subsequent computer modeling, the femoral-tibial and kinematic data were determined. The test parts were metal-poly components, and for a new asymmetric guided motion design, a stereo-lithographic model in a hard, low-friction plastic was used, with a Krytox AT fluoro ether grease used for lubrication. All TKR components used were left knee components. Three current PS designs were tested together with a Guided Motion PS design intended to reproduce characteristics of anatomic motion.Both qualitative (Fig. 2) and quantitative (Figs. 3 and 4) data were acquired from 3D computer analysis.The first design (PS1) showed uniform posterior displacement of the contact points after 90 deg flexion. The anterior and posterior (AP) shear contacts showed moderate AP sliding which could allow some paradoxical motion. There was restricted internal-external rotation after 90 deg flexion due to restraint of the cam-post and the posterior plastic lip.PS2 showed a progressive posterior displacement with flexion, but the possibility of large AP sliding due to shear forces. There was adequate internal-external rotation at all flexion angles. PS3 showed contact points which were posterior, even on the posterior edges, under most test conditions. The square post showed corner contacts and restraint of rotation. The three designs showed symmetric lateral and medial motion.The fourth design was a Guided Motion with high medial conformity, low lateral conformity, and a rounded cam-post to reproduce the characteristics of the normal knee. There was progressive lateral rollback with flexion, 3–4 mm posterior medial rollback in high flexion, as with the anatomic knee, small AP medial sliding with shear forces, and adequate internal-external rotation.The test method proved to be simple and convenient to use and provided data which would evaluate any TKR design compared with an anatomic benchmark. The method is also an effective tool for comparing designs of varying constraint against one another to evaluate the performance characteristics of a specific PS as compared to known clinical designs.Current PS designs showed variable geometric characteristics which would impact function: an asymmetric Guided Motion design could better reproduce the motion characteristics of the normal anatomic knee. In a previous study using a similar method, kinematic characteristics of the normal anatomic knee were determined, to be applied as a benchmark for TKA design [7]. These anatomic characteristics for our specific loading conditions were: for the neutral path of motion during flexion, small AP displacements on the medial side, and continuous posterior displacement on the lateral side; small AP laxities medially and 3–8 mm laxity laterally; internal - external rotational laxities of 13–25 degrees with instant centers of rotation on the medial side.