Abstract
INTRODUCTION
Most total knees today are CR or PS, with lateral and medial condyles similar in shape. There is excellent durability, but a shortfall in functional outcomes compared with normals, evidenced by abnormal contact points and gait kinematics, and paradoxical sliding. However unicondylar, medial pivot, or bicruciate retaining, are preferred by patients, ascribed to AP stability or retention of anatomic structures (Pritchett; Zuiderbaan). Recently, Guided Motion knees have been shown to more closely reproduce anatomic kinematics (Walker; Willing; Amiri; Lin; Zumbrunn). As a design approach we proposed Design Criteria: reproduce the function of each anatomic stabilizing structure with bearing surfaces on the lateral and medial sides and intercondylar; resected cruciates because this is surgically preferred; avoid a cam-post because of central femur bone removal, soft tissue entrapment, noises, and damage (Pritchett; Nunley). Our hypothesis was that these criteria could produce a Guided Motion design with normal kinematics.
METHODS & MATERIALS
Numerous studies on stability and laxity showed the ACL was essential to controlling posterior femoral displacement on the tibia whether the knee was loaded or unloaded. Under load, the anterior upwards slope of the medial tibial plateau prevented anterior displacement (Griffen; Freeman; Pinskerova; Reynolds). The posterior cruciate and the downward lateral tibial slope produced lateral rollback in flexion. The Replica Guided Motion knee had 3 bearings (Fig 1). The lateral side was shallow and sloped posteriorly, with a posterior lip to prevent excess displacement. The medial anterior tibial and femoral slopes were increased as in the anatomic knee. In the intercondylar region, a saddle bearing replaced ACL function by controlling posterior femoral displacement. For testing, a typical PS design was used as comparison. A Knee Test Machine (Fig 2) flexed the knee, and applied axial compression, shear and torque to represent a range of functions. Bone shapes were reproduced by 3D printing and collaterals by elastomeric bands. Motion was recorded with a digital camera, and Geomagic to process data.
RESULTS
The kinematics of normal knees was the benchmark (Arno). The results for neutral path of motion, and the AP laxity about the neutral path, are shown (Fig 3). The PS showed symmetric motion, with anterior medial sliding and excessive constraint in low and high flexion. For the Replica, the medial condyle remained almost constant, but the lateral side rolled posteriorly with flexion, less than normal to prevent damage to the posterior lateral tibial plastic. The lateral side had similar anterior laxity to anatomic, but more than anatomic in late flexion. Based on 10 parameter motion scoring, the Replica was closer to normal than the PS, 82% cf 51%.
DISCUSSION
Functional outcomes after TKA are less than normal, TKA design being a likely factor. The approach shown here is intended to reproduce more anatomic kinematics of neutral path of motion and laxity. Such a Replica Guided Motion knee, based on an anatomic structure/stability approach, could reproduce close to normal kinematics even without the cruciates or a cam-post. This may result in improved functional outcomes, and a closer feeling of a normal knee.
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