Thursday, February 4, 2016

Total shoulder arthroplasty: glenoid component wear and rim failure

Analysis of severely fractured glenoid components: clinical consequences of biomechanics, design, and materials selection on implant performance

These authors studied 16 retrieved glenoid components that had failed because of severe fracture. The fractures primarily occurred along the exterior rim. Fourier transform infrared analysis and fractography revealed significant oxidative embrittlement for all gamma-sterilized glenoids (gamma-sterilized Hylamer; gamma-sterilized UHMWPE) leading to crack inception and subsequent fracture. By contrast, fatigue striations and internal flaws with little oxidation detected were evident on the fracture surfaces of the gas plasma-sterilized, remelted, highly cross-linked UHMWPE [HXL]), suggesting that brittle fracture resulted from a combination of elevated contact stress due to nonconforming surfaces, internal flaws, and reduced resistance to fatigue crack growth.

Comment: This article demonstrates the frailty of the glenoid component under rim loading. This frailty can manifest in terms of (1) rocking horse loosening, (2) cold flow deformity of the rim or (3) fracture. Avoiding rim loading requires careful attention to optimizing the relationships between the humeral and glenoid surfaces as well as the stability of the articulation.

Three prior articles are among those of interest in this regard. Their abstracts are reproduced here

Intrinsic stability of unused and retrieved polyethylene glenoid components.
The surface geometry of polyethylene components can be altered by in vivo use. The purpose of this investigation is to document the effects of these changes on the intrinsic stability provided by the glenoid component. We validated a method of measuring the intrinsic stability of glenoid components as indicated by the balance stability angle (the maximal angle between the glenoid centerline and the resultant humeral force before dislocation of the humeral head occurs). We compared observed values with those predicted for unused glenoid components for which the geometry was known. We then applied this method to retrieved glenoid components in which the surface geometry had been altered by in vivo use. The balance stability angles measured in retrieved glenoids were often substantially reduced: 11 of 24 glenoids had diminished balance stability angles of at least 30% in at least one direction. We concluded that the surface geometry of polyethylene glenoid components can be altered by in vivo use in a manner that may compromise their contribution to glenohumeral stability.

Observations on retrieved polyethylene glenoid components.
It is recognized that retrieved polyethylene hip and knee components may undergo substantial changes from their preimplantation form. Little information is available, however, regarding retrieved polyethylene glenoid components. We report on 39 glenoid components removed at an average of 2.5 years after implantation. Of components for which clinical data were available, the commonest reason for the revision arthroplasty was loosening of the glenoid component; many components also showed glenohumeral instability. The articular surface contours of most of the components were altered. Of components, 28 had obvious erosion of the rim, 27 had surface irregularities, 11 were fractured, and 9 had central wear. These observations in retrieved glenoid components point to the potential of polyethylene for deformation in vivo, especially when the mechanics of the arthroplasty are compromised.

Alterations in surface geometry in retrieved polyethylene glenoid component.
This study characterized and quantified the changes found in retrieved glenoid polyethylene components found at revision total shoulder arthroplasty (TSA). Twenty components obtained at revision TSA were evaluated, all from a system (Global, DePuy, Warsaw, IN) with a glenoid radius of curvature 3 mm greater than that of the humeral head. Laser surface scanning provided three-dimensional analysis of the surface of the glenoid component. Scans of unused components of similar sizes enabled determination of the changes occurring after implantation. Alterations in radius of curvature were noted in every glenoid. All showed loss of the balance stability angle (BSA, the maximal angle that the net humeral joint reaction force can make with the glenoid center line before the humeral head would dislocate) of at least 5 degrees in one or more directions. Increase in BSA in one direction was seen in 11 of the components. In five of these, the increase was associated with a reduction of the local radius to match that of the humeral component. Glenoid surface morphology and stability can be changed by in vivo use. While correlation with clinical instability in the patients from whom the implants were obtained was not possible, many of the observed changes in surface morphology are of sufficient magnitude to compromise the contribution of the glenoid surface to shoulder stability. Three patterns of wear were identified: "humeral" that showed loss of the mismatch between the humeral and glenoid radii of curvature (5 of 20 components), "diffuse" that showed broad surface irregularity (18 of 20), and "rim" wear with loss of the polyethylene rim of the component (14 of 20). More than one type of wear was possible within a single glenoid.


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