Monday, November 4, 2013

The challenges of metal backed glenoids - why do they fail more often?

There has been a huge interest in metal backed glenoid components. To review some of our prior posts on this topic, enter 'metal' into the "search this blog" box to the right. A recent article, "Bone Remodeling of the Scapula after a Total Shoulder Arthroplasty" helps explain the high failure rate reported in the literature. The authors evaluated the changes in the bone adaptation of the scapula after shoulder arthroplasty. Five finite element models of the implanted scapula were developed considering the implantation of three anatomical, cemented, all-polyethylene components; an anatomical, cementless, metal-backed component; and a reverse, all-metal component. They found that metal-backed components were associated with greater stress shielding,  regardless of the bone quality.

An important consideration is the Young's modulus of the materials involved. This quantity, also known as the tensile modulus or elastic is a measure of the stiffness of a material.

Here are some representative values of the materials of interest in total shoulder arthroplasty. Young's modulus is expressed in Giga Pascals (GPa).

Cancellous Bone 0.4
Ultra high molecular weight polyethylene 0.5
PMMA bone cement 2
Cortical Bone 8
Titanium 112
Cobalt chrome 200

Thus with a metal backed glenoid there are two critical mismatches of Young's modulus: (1) that of metal (100-200) to bone (0.4-8) and (2) that of metal (100-200) to polyethylene (0.5). The first may account for the stress shielding and the second may account for the increased rate of polyethylene failure and polyethylene-metal dissociation seen with metal backed components. By contrast, all polyethylene components have better modulus matches with bone. PMMA bone cement also has a similar modulus to that of bone and polyethylene.

What this means is that when the component is loaded, the materials of similar Young's modulus deform similarly, whereas those with mismatched Young's modulus do not, resulting in relative displacement at the interface. This issue may be of lesser concern in reverse total shoulders because the loading of these prostheses is concentric, whereas it is recognized that much of the loading in anatomic total shoulders is off-center, or eccentric.

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