Using a 3D optical scanner to quantify wear depth in hip prostheses

The surgical replacement of osteoarthritic joints with artificial devices is becoming increasingly popular and successful due to advancements in material technology. Currently, the greatest challenge in hip replacement is increasing the life expectancy of the implant above the current average of 15-20 years. Premature failures caused by the wearing of the implant bearing surface has been cited as the main reason for the limited life span. Therefore, new material technologies for bearing surfaces with enhanced wear properties have been developed. For instance, the advent of highly cross-linked polyethylene has enabled a reduction in the thickness of the liner component in hip replacement devices. By reducing liner thickness, the contact area of the mating bearing surfaces can be increased, which can help reduce dislocations and restore natural gait.

However, a main concern with decreasing the thickness of highly cross-linked polyethylene liners is the potential susceptibility for uneven wear and detrimental cupping. Thus, accurate techniques are needed to visualize and measure wear depth in the polymer liners. Presently, available techniques such as coordinate measuring machines and laser scanning are expensive, have a limited number of sampling points, and require long scanning times. Therefore, researchers have been looking for alternatives and have identified optical technology as a promising solution.

Boise State University researchers led by Katherine Hollar and Professor Trevor Lujan in collaboration with Daniel Ferguson from Global Inspection Solution in the United States developed a 3D optical scanning technique for mapping and visualizing wear pattern distribution in hip prostheses. The authors first validated the accuracy and precision of this technique by using reference blocks with known wear depths. This procedure was then used to measure the surface wear of a hip resurfacing implant for canines with a highly cross-linked polyethylene liner that was subjected to over a million cycles of loading. Their work is published in the research journal, Wear.

The authors obtained an average accuracy and precision of 2.1 µm and 1.4 µm, respectively. This corresponded to errors below 10% for wear depths greater than 20 µm. Also, the 3D optical scanner generated repeatable colorimetric maps of wear depth in less than 20 minutes, which is much faster than other conventional techniques. Furthermore, the generated colorimetric maps enabled identification of localized regions with greater wear depth and revealed liners with asymmetrical wear patterns.

“This is the first study to successfully validate a 3D optical scanning procedure to visualize wear in hip implants that use highly cross-linked polyethylene liners. Because of the excellent accuracy, resolution, cost, and speed of 3D optical scanners, we feel the imaging methodology developed in this study can become the test standard for measuring wear depth in hip replacements and other joint replacement devices,” said Dr. Lujan.

The novel imaging methodology presented in this study can be used to evaluate the risk of premature failures in new bearing materials and device designs, and therefore can help improve the life expectancy of joint replacement devices.