Saturday, May 3, 2014

Glenoid loosening in total shoulders - a biomechanical analysis - metal backs

Load transfer after cemented total shoulder arthroplasty.

These authors again point out that glenoid loosening is the primary reason for failure after a total shoulder arthroplasty accounting for 32% of the complications after this procedure. They reinforce our prior work on the importance of the rocking horse mechanism of glenoid component loosening from eccentric loading as described here.

They sought to determine the load transfer and stress distribution after the introduction of a glenoid implant. 

They found that in the intact glenoid, for both centered and eccentric loading, the majority of stress was distributed in the cancellous bone and then gradually to the cortical bone lining the scapular body.

The introduction of a cemented TSA implant significantly altered the load transfer pattern in the glenoid. With a glenoid implant, the load was redistributed so that the cement and implant carried a considerable amount of the load transferring the load to the medial cortical bone by passing the cancellous bone, leading to stress-shielding of the bone beneath the prosthesis.

Their model confirmed our view that compressive stress on the posterior side and tensile stress on the anterior side of the glenoid produce the ‘‘rocking-horse’’ phenomenon, with higher stresses in the cement layer, suggesting that cement failure is likely with eccentric loading.

Finally, they found that with metal backed components, there were lower stresses in the cancellous bone suggesting stress shielding caused by the high stiffness of the metal backing,  supporting clinical observations of osteolysis and high rates of loosening in metal-backed glenoid implants. We've discussed the problems of metal backs quite a bit before as shown here.

Comment: This study again highlights the fact that 'better initial fixation' of an implant may cause the joint forces to bypass the bone beneath the implant, leading to bone resorption and failure in the longer term. In a soon to be published study, we found that the rate of revision was over three times higher for metal-backed glenoids (14.0%) than for all polyethylene components (3.8%) (p<.0001). Nevertheless, new designs of metal backed glenoid components appear on the market each year.

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