Long term acetabular component fixation is dependent on bone ingrowth, which is affected by initial stability and the contact area between the bone and acetabular component. Mismatch between the component and cavity size has been shown to be one reason for component loosening. Furthermore, the potential of acetabular fracture during insertion of oversized components is larger than line-to-line components. An ideal cavity preparation would be a true hemispherical cavity that can provide maximum contact area between the shell and bone while also achieving adequate press fit for implant initial stability. The goal of this study was to characterize the cavity morphology produced by a commercially available reamer and compare it to a new reamer design. 36mm and 52mm reamers (n=6) were selected from conventional reamers (Stryker, Mahwah, NJ), which have successful clinical history exceeding 20 years, and Smooth Cut Reamers (Tecomet, Warsaw, IN and Stryker, Mahwah, NJ), which are a new design. Hemispherical cavities were created in 30 pcf polyurethane foam blocks (Pacific Research Laboratories, WA) using a custom software for the Mako System (Stryker, Mahwah, NJ), with new reamers of both designs. A reamer 2mm smaller in diameter than the final size was used to create a pilot cavity to replicate a clinically relevant reaming scenario. The resulting cavities were scanned using a Triple Scan high resolution 3D Scanner (ATOS, Purchase, NY) to generate 3D models of each cavity. The models were then post processed, and the following dimensions were collected:
Gaussian best fit spherical diameter of the entire cavity (Dimension A) Gaussian best fit diameter at the rim of the cavity (measured at a distance of 0.25mm from the top surface of the foam block) (Dimension B) One-sided two sample T-tests were conducted to determine statistical significance.Introduction
Materials & Methods
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
