Materials and Methods

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.

Methods

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).