Abstract
Introduction
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.
Materials & Methods
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:
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Gaussian best fit spherical diameter of the entire cavity (Dimension A)
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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.
Results
The deviation was calculated by subtracting the desired diameter from the observed diameter, therefore, a negative value would indicate an undersized cavity. The average diametrical deviation for the 38 and 52mm reamers for dimension A was −0.22 ± 0.07 and −0.01 ± 0.11 respectively for the Smooth Cut Reamer. The average diametrical deviation for the 38 and 52mm reamers for dimension A was −0.60 ± 0.24 and −0.72 ± 0.21 respectively for the Conventional Reamer. The average diametrical deviation for the 38 and 52mm reamers for dimension B was −0.97 ± 0.05 and −0.54 ± 0.11 respectively for the Smooth Cut Reamer. The average diametrical deviation for the 38 and 52mm reamers for dimension B was −1.35 ± 0.28 and −1.53 ± 0.27 respectively for the Conventional Reamer.
Discussion
This study evaluated the accuracy of two different acetabular reamer designs. Results indicate that the Smooth Cut Reamers produce a cavity that is larger and more accurate to the indicated size of the reamer as shown by the reduced diametrical deviation at the rim (p-value < 0.05) and average spherical diameter (p-value < 0.05). Further investigation is warranted to determine if the variation in cavity geometry impacts shell seating and initial stability.
