Total shoulder replacement is a surgery typically conducted to alleviate symptoms of arthritis such as chronic pain, limited range-of-motion, and impaired daily functioning.
A common challenge in shoulder replacement is that the native glenoid bone is worn posteriorly. In such cases, the surgeon typically either implants a standard glenoid component malaligned relative to the scapula, or reams away healthy bone on the anterior side to achieve normal alignment, both of which can lead to complications. One of the most serious problems with shoulder replacement is loosening of the glenoid implant, which occurs in approximately 5 to 10 percent of cases. Such patients typically require revision surgery. Gregory Lewis, Ph.D., in collaboration with April Armstrong, M.D., shoulder surgeon, Penn State Hershey Bone and Joint Institute, is actively engaged in research to understand the internal micromechanics of replaced glenoids with the aim of decreasing the risk of glenoid loosening. New findings, presented at the 2012 Orthopaedics Research Society annual meeting, compare glenoid loosening and implant construct damage with a new type of posterior-augmented polyethylene implant to fill in the resulting gap from bone loss (versus a conventional implant). Lewis explains, “We used human cadaver specimens to test how well the two types of implants hold up under simulated, physiologic cyclic loading. At various cycle intervals, up to 100,000 cycles, [equivalent to twenty-five high-load shoulder movements per day for ten years], we measured external implant micromotions along with internal construct damage.
Micro-computed tomography (micro-CT) with twenty micrometer resolution was used to examine the cement mantle around each implant peg for possible damage and cracks.” Both implant types showed a gradual progression of cement mantle damage due to the cyclic loading. Micromotions were all less than one millimeter. However subsidence, or settling, of the implant into the bone increased with loading cycles and exceeded one millimeter in some specimens. Lewis noted that “All cement cracks were seen around the upper half of the pegs; the cement deeper in the vault appeared to remain intact. This gives us clues about where and how to make adjustments in implant design and cementing technique to decrease the chances of loosening.” The research that Lewis and Armstrong are conducting is likely to continue for years to come, as they continue to refine implant techniques and design concepts. Using the data they gather from this and other experiments, Lewis plans to construct more accurate computerized models of joint implants that allows better prediction of implant outcomes. Lewis notes that “Although we do not yet consider biological changes over time that may occur, our approach of micro-CT of cyclically loaded cadaver specimens allow us, for the first time, to examine the correlation between external implant loosening and internal micro-damage evolution in cemented glenoids. Ultimately, the goal is to improve glenoid implant design and cementing methods, so that fewer patients need revision surgery.”
Gregory S. Lewis, Ph.D.
Assistant professor, orthopaedics and rehabilitation
Graduate Study: Ph.D., mechanical engineering,
Penn State University, University Park, PA