Cementless unicondylar knee implants are intended to offer surgeons the potential of a faster and less invasive surgery experience in comparison to cemented procedures. However, initial 8 week fixation with micromotion less than 150µm is crucial to their survivorship1 to avoid loosening2. Test methods by Davignon et al3 for micromotion were used to assess fixation of the MAKO UKR Tritanium (MAKO) (Stryker, NJ) and the Oxford Cementless UKR (Biomet, IN). Data was analyzed to determine the activities of daily living (ADL) that generate the highest forces and displacements4, 5. Stair ascent with 3.2BW compressive posterior tibial load was identified to be an ADL which may cause the most micromotion5. Based on previous studies6, 10,000 cycles was set as the run-time. The AP and IE profiles were scaled back to 60% for the Oxford samples to prevent the congruent insert from dislocating. A four-axis test machine (MTS, MN) was used. The largest size UKRs were prepared per manufacturer's surgical technique. Baseplates were inserted into Sawbones (Pacific Research, WA) blocks1. Femoral components were cemented to arbors. The medial compartment was tested, and the lateral implants were attached to balance the loads. Five tests were conducted for each implant with a new Sawbones and insert for each test per the test method3. The ARAMIS System (GOM, Germany) was used to measure relative motion between the baseplate and the Sawbones at three anteromedial locations (Fig. 1). Peak-Peak (P-P) micromotion was calculated in the compressive and A/P directions. FEA studies replicating the most extreme static loading positions for MAKO micromotion were conducted to compare with the physical test results using ANSYS14.5 (ANSYS, PA).Introduction
Methods
The MAKO Surgical Rio Robotic Arm utilizes the pre-op CT images to plan positioning of the uni-condylar and patella-femoral components in order to achieve the most desirable kinematics for the knee joint. We hypothesize that the anatomic matching surfaces and the cruciate retaining design of the Restoris knee will best replicate normal knee kinematics. We tested the healthy cadaveric knee versus the MAKO knee and the most common TKR designs in order to evaluate and compare the kinematic properties. Six healthy male left knees were dissected to leave only the knee capsule and the quadriceps tendon intact. The femur and the tibia were cut 20cm from the joint line and potted with cement into a metal housing. The knee was attached to a crouching machine capable of moving the knee joint though its normal human kinematics from extension to maximum flexion, validated in previous studies. Forces applied to the quadriceps tendon allowed the knee to flex and extend physiologically, and springs attached to the posterior were substituted as the hamstrings at a rate of half the force exerted by the quadriceps as shown in the literature. Three dimensional visual targets attached to the bones were tracked by computer software capable of recreating the positions of the bones in any given flexion angle. A cruciate retaining and posterior stabilized TKR design were chosen to represent the TKRs most commonly available in the market today. The intact knee, MAKO implanted knee, CR and PS TKR designs were tested in sequence on the same specimens. The computer software analyzed the normal distance between the bone surfaces and plotted the locations of contact which could then be quantitatively compared for each given scenario [Fig. 1].Introduction
Methods
The treatment of osteoarthritis using artificial knee joints is expected to expand further over the next decade. Increasingly, patients expect quicker rehabilitation, improved performance, and high durability. However, economic limitations require a reduced cost for each procedure, as well as early intervention and even preventative measures. The major goal of implant design needs to be a restoration of normal knee mechanics, whether by maximum preservation of tissues, or by guiding surfaces which replicate their function. In this paper it is proposed that total knees should exhibit anatomic knee mechanics, namely medial stability – lateral mobility. Many studies in the past have shown that the neutral path of motion of the anatomic knee, is that the medial side remains relatively immobile in the AP direction, which will impart a feeling of stability, while the lateral side shows posterior femoral displacement with flexion, to obtain a high range of flexion. There is considerable rotational laxity about this neutral path to accommodate a range of positions and activities. Recent studies carried out in our laboratory using an up-and-down crouching machine, and other test machines, have conformed this mechanical behaviour. To further elaborate, we tested eight young male subjects in a 7T MRI machine, where compressive and shear loads were applied. AP displacements occurred laterally but not medially. We attributed this behaviour to the medial meniscus and the tibial bearing geometry under weight-bearing conditions. On the basis of these various studies, we developed a method for the design of Guided Motion knees, which would be implanted without the cruciates, and which would restore anatomic knee mechanics. The method started with the femoral component, where the medial side had features to provide a continuous radius anteriorly, and distally to 75 degrees flexion when a post-cam would contact. This feature would prevent paradoxical anterior femoral sliding in early flexion. Multiple femoral positions were then defined for accommodating anatomic motion, in particular limited AP motion on the medial side, but posterior displacement laterally. Tibial bearing surfaces were generated accordingly. Tests were carried out on the crouching machine and on a Desktop TKR Test machine to compare the TKR motion with anatomic. Although not accurate in all respects, the Guided Motion designs were closer than models of standard TKR’s today. Such Guided Motion designs hold the promise for restoring anatomic knee mechanics and a normal feeling knee.
The objective was to develop a simple, rapid, and low-cost method for evaluating proposed new Total Knee (TKA) designs, and then to use the method to evaluate three different TKA models with different kinematic characteristics. In a previous study, we reported on the use of an Up-and-Down Crouching Machine, where the neutral path of motion for knee specimens were measured, and then TKR models were implanted and the tests repeated. These experiments showed that standard CR and PS designs behaved more like an ACL deficient knee, whereas Guided Motion knees produced motion similar to that of the intact specimens. However the method was time consuming, technically demanding, and expensive, and hence is suitable for designs which had already passed through a screening method. The latter was the subject of this present study, called the Desktop TKR Test Machine. The principle of the testing protocol on the machine, called Holistic Testing, was that a spectrum of compressive, shear and torque forces were applied to a knee, to represent a complete spectrum of daily and sporting activities. The resulting femoraltibial positions were measured, both the Neutral Path of Motion and the Laxity about the neutral path. The motions were displayed as both the motion of the transverse femoral axis on the tibial surface, and by the centers of the lateral and medial contact patches. Eight knee specimens were tested first, to act as a reference target for evaluating TKR models. Knee models were designed in the computer and made in a hard low-friction plastic using SLA and stereolithography. A typical Posterior-Stabilized (PS) TKA did not display the normal external femoral rotation with flexion, and also showed abnormal anterior sliding on the medial side prior to cam-post engagement. Guided Motion designs included a Medial Pivot type, and a Medial Pivot with a cam-post. Both of these had a dished medial side and a shallow lateral side, to more accurately reproduce anatomic motion characteristics. The guidedmotion design with the cam-post produced a neutral path and laxity more similar to that of normal. It was concluded that the test method satisfied the objective in terms of being a useful test method for rapid evaluation of new proposed TKR designs. The method was able to identify designs which showed motion and stability characteristics closer to the normal anatomic knee.
