Date of Award

12-2014

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Bioengineering

Advisor

Dr. John DesJardins, PhD

Committee Member

Dr. Martine LaBerge, PhD

Committee Member

Dr. Gregory Colbath, MD

Abstract

Loosening of the glenoid component via a 'rocking horse' phenomenon, whereby the glenoid loosens as a result of edge loading of the implant during mild off-axis motion, is a common complication of total shoulder arthroplasties (TSAs). Recently, a partial glenoid resurfacing implant, known as a glenoid inlay, has become available and due to its geometry has the potential to exhibit less rocking-horse loosening during humeral articulation than the standard glenoid onlay systems currently used in TSA. The inlay design is implanted centrally on the glenoid to match the surrounding anatomy with a fit that leaves it flush with the surrounding cartilage and is hypothesized to lessen the risk of rocking-horse loosening during physiologic activity. The purpose of this research study was to examine the contact pressures and implant stability associated with fatigue loading of the glenoid inlay and onlay systems during physiologic loading and motion in a cadaveric model. In this study, n=16 specimens (eight matched pair shoulders) were selected for testing. After potting the scapulae and humeri in resin, the articulating surface of the glenoid was positioned perpendicular to the floor, with the humerus secured for testing in an abduction angle of 60°. Normal biomechanical testing of the specimens was carried out on a custom shoulder testing system, using a materials testing machine and dynamic and fatigue materials testing software that articulated the humerus with respect to the glenoid. A flexible Tekscan sensor was reproducibly positioned in the glenohumeral joint to record the contact pressure distribution and area. A ± 5 mm displacement-controlled anterior/posterior humeral motion was induced to produce glenoid edge loading while an 88.9 N compressive joint load was applied across the joint. TSAs were then performed on all shoulders followed by post implantation CT, with one of each matched pair being implanted with the onlay glenoid implant and the other with the inlay glenoid implant. DJO SurgicalTM provided the TuronTM onlay shoulder system while Arthrosurface, Inc. provided the HemiCAP® inlay shoulder system. Biomechanical testing of the specimens was again carried out under the same conditions as mentioned previously, followed by ± 5 mm of anterior/posterior cyclic fatigue testing with a joint compressive load of 333.6 N was performed to 4000 cycles or until clinical loosening was observed. Comparing native tissue to implant surface pressure, all results showed higher pressures on both the inlay and onlay implants than the native tissue examined pre-implantation. Specimens implanted with onlay implants experienced much higher pressures on the edge of the glenoid than those with inlays. In comparison, glenoid edge pressure on specimens with the inlay implanted were dissipated by the native tissue still present on the glenoid edge. This is a potential explanation for the dramatic difference in visible loosening of the implants seen during fatigue testing. The inlay implant did not show any evidence of loosening after 4000 cycles of fatigue loading, however all onlays exhibited visible loosening (≥ 1 mm gap formation behind implant) in under 2000 cycles. This significant difference in loosening (p<0.001) is hypothesized to be the due to the observed load-sharing with the mostly native tissue still present on the glenoid edges of specimens implanted with the inlay, in contrast with specimens implanted with the onlay, where the polyethylene implant edges received all of the load.

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Engineering Commons

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