A majority of the acetabular shells used today are designed to be press-fit into the acetabulum. Adequate initial stability of the press-fit implant is required to achieve biologic fixation, which provides long-term stability for the implant. Amongst other clinical factors, shell seating and initial stability are driven by the interaction between the implant's outer geometry and the prepared bone cavity. The goal of this study was to compare the seating and initial stability of commercially available hemispherical and rim-loading designs. The hemispherical test group (n=6) consisted of 66mm Trident Hemispherical shells (Stryker, Mahwah NJ) and the rim-loading test group (n=6) consisted of 66mm Trident PSL shells (Stryker, Mahwah NJ). The Trident PSL shell outer geometry is hemispherical at the dome and has a series of normalizations near the rim. The Trident Hemispherical shell outer geometry is completely hemispherical. Both shells are clinically successful and feature identical arc-deposited roughened CpTi with HA coatings on their outer geometry. Hemispherical cavities were machined in 20pcf polyurethane foam blocks (Pacific Research Laboratories, WA) to replicate the press-fit prescribed in each shell's surgical protocol. The cavity for the hemispherical design was machined to 65mm (1mm-under ream) and the cavity for the rim-loading design was machined to 67mm (1mm- over ream). Note that the rim-loading design features ∼2mm build-up of material at the rim when compared to the hemispherical design. The shells were seated into the foam blocks using a drop tower (Instron Dynatup 9250G, Instron Corporation, Norwood, MA) by applying 7 impacts of 6.58J/ea,. The number and energy of impacts are clinically relevant value obtained from surgeon data collection through a validated measurement technique. Seating height was measured from the shell rim to the cavity hemispherical equator (top surface foam block) using a height gage, thus, a low value indicates a deeply seated shell. A straight torque out bar was assembled to the threads at the shell dome hole and a linear load was applied with a MTS Mechanical Test Frame (MTS Corporation, Eden Prairie, MN) to create an angular displacement rate of 0.1 degrees/second about the shell center. Yield moment of the shell-cavity interface, representing failure of fixation, was calculated from the output of force, linear, displacement, and time. Two sample T-tests were conducted to determine statistical significance.Introduction
Materials and Methods
During activities of daily living (ADL), varus moments are experienced in the knee, which can result in frontal plane rotation, or liftoff, of the lateral femoral condyle with respect to the tibial plateau. An understanding of this rotation is valuable as it could potentially lead to contact between the femoral component and polyethylene post of a total knee replacement (TKR). Therefore, the purpose of this study was 1) to assess how much frontal plane rotation was achieved due to varus moments imposed on a total stabilized (TS) TKR from the stair ascent activity, and 2) to determine whether a TS TKR could withstand the contact stresses imposed by the varus loading for 1 million cycles without the post fracturing or plastically deforming. A PS femoral component paired with a TS polyethylene insert and baseplate (Triathlon, Stryker, Mahwah, NJ) were aligned on a multi-axis testing system (MTS Systems Corp, Eden Prairie, MN) (Figure 1). Size 1 components were used as they represented the worst-case size for testing. The femoral component was fixed at 60 degrees of flexion, representing an angle of peak varus moment during stair ascent [1]. The peak varus moment used in this study was determined by scaling the data from In order to evaluate the frontal plane rotation achieved due to the varus moment with minimal influence from other loads, an FEA model of the physical test setup was used to determine the lowest joint compressive load that would allow testing to be stable. Given this, testing was completed with a constant joint compressive load of 1500 N (33% of that reported by Lastly, a validation test was run on a component with the polyethylene post notched at the medial distal aspect. The post fractured during testing indicating that the test could induce the clinical failure mode of interest.INTRODUCTION
METHODS
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
