Saturday, April 6, 2013

Glenoid component liftoff with posterior superior loading

Liftoff resistance of augmented glenoid components during cyclic fatigue loading in the posterior-superior direction

This study was funded by the company making the stepped glenoid design and all of the authors receive payments from the company.

The authors point out that most patients with glenohumeral osteoarthritis have posterior bone loss and that this posterior bone loss is associated with an increased risk of posterior instability and glenoid component failure. The authors and others have advocated the use of a posteriorly augmented glenoid component to lessen these risks, although the superiority of this approach over conventional means remains undocumented in clinical practice.

One of the concerns about posteriorly augmented glenoids is that they provide an increased lever arm for loosening with eccentric loading. The authors test the hypothesis is that a stepped augmented glenoid component will have less mechanical liftoff than augmented components of varying designs without a step. In a Saw Bones model, they compared four different prototypes in a model where a 170-lb compressive load and 4 mm of posterior-superior translation of the humeral head was applied for 100,000 cycles while anterior glenoid liftoff was measured. Each design changed the angle of the glenoid by 13 degrees. 

They found that the stepped glenoid component had significantly lower liftoff values than the other augmented designs. However, the stepped glenoid had approximately twice the liftoff of the non-augmented glenoid.

The concerns regarding the use of the stepped glenoid include (1) the risk of liftoff with eccentric loading, as investigated in this study, (2) the potentially increased risk of cold flow of the thickened posterior aspect of the component with years of eccentric loading and (3) the challenge of effecting an accurate match between the complex back side geometry of the stepped glenoid and the glenoid bone in the clinical setting.

Until long term clinical followup of this design becomes available, we continue to use a standard non-augmented glenoid component for total shoulders in the face of glenoid retroversion or a ream and run procedure. We do not attempt to correct glenoid retroversion with bone grafts or augmented components. Any tendency for posterior instability is managed with eccentric humeral heads and or rotator interval plication.

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