MS Thesis Defense: Gabriel Landi

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"Volumetric Wear Analysis of Polyethylene Orthopedic Liners Using Point Clouds"

Abstract

In vivo wear of ultra-high molecular weight polyethylene (UHMWPE) orthopedic bearings represents a major problem in shoulder and knee arthroplasty, with 16% of the latter failing due to the complication itself, and even more due to associated reasons for retrieval, notably including aseptic loosening. However, quantification of UHMWPE wear has been limited to linear and in vitro volumetric analyses, obscuring the understanding of the amount of clinically relevant material loss. This study proposes new methods to measure articular, rim, and backside surfaces of retrieved shoulder and knee UHMWPE liners, centered around the coordinate measuring machine (CMM)-based collection of high density point clouds. Subsequently, the clouds are algorithmically manipulated using thresholding, least squares fitting, interpolation, and smoothing to regenerate unworn surfaces, which are compared to the worn surfaces to identify and calculate volumetric wear. Utilizing gravimetric analysis as a standard, both methods operated within an error of 6 mm3. 
 
As a proof of concept for the methods’ potential clinical utility, they’re applied to characterize the relationship between in vivo wear and surgical, design, and material-based factors. Fifty reverse total shoulder arthroplasty and twenty total knee arthroplasty with varying retrieval reasons, in vivo durations, and manufacturers were analyzed. From the former, as already studied in knee and hip arthroplasty, it was found that irradiation crosslinking in conjunction with thermal stabilization or antioxidant additive decreases in vivo articular and rim wear rates with statistical significance (p = 0.0310, p = 0.0447); however, no relevant differences were found between manufacturer designs. The latter suggested possible differences in frontside versus backside wear and medial versus lateral wear, but a larger sample size is necessary to make statistically-based claims. Ultimately, as they relate to failure analysis and postmarket surveillance, these results suggest the potential clinical utility of generalized strategies that take point clouds as inputs—regardless of their collection method and implant type—and quantify in vivo material loss. Future efforts seek to understand more nuanced wear relationships, such as the effect of mechanical alignment on wear for knees, or the effects of center of rotation lateralization and neck shaft angle on wear for shoulders.

Thesis Committee

• Douglas Van Citters (Chair)
• Solomon Diamond
• John-Erik Bell, MD (DHMC)

Contact

For more information, contact Thayer Registrar at thayer.registrar@dartmouth.edu.