Sunday, September 15, 2013

a model of a wedge-shaped glenoid component - does it answer our questions?

Augmented wedge-shaped glenoid component for the correction of glenoid retroversion: a finite element analysis

These authors performed a finite element analysis of a 15 degree wedged glenoid component in a model based on the CT scan of an arthritic retroverted shoulder. A compressive force of 625 N was applied through the humeral head. The model assumed a 1-mm-thick cement mantle applied on the posterior surface of the glenoid and pegs. Implant-to-cement and cement-to-bone interfaces were treated as perfectly bonded to simulate a well-fixed postoperative condition.

When the model simulated the insertion of a standard glenoid component in retroversion (E in the figures below), the result was  high compressive stresses and decreased cyclic fatigue life predictions for trabecular bone.

When the model simulated the insertion of a wedged glenoid component (F in the figures below) the stresses were decreased and greater bone fatigue life was predicted. 

While the authors conclude that "A wedged glenoid implant is a viable option to correct severe arthritic retroversion, reducing the need for eccentric reaming and the risk for implant failure.", there are some caveats that need to be applied before one assumes that these results have clinical importance. 
First, the loads were applied in compression whereas it is commonly recognized that humeral loading is usually eccentric with greater loads on the posterior aspect of the glenoid. 
Second, the model assumes that the reconstruction keeps the head centered on the glenoid, wheras it has not been shown clinically that wedged glenoid components provide stability for a posteriorly subluxated arthritic head. 
Third, the authors model a 'cement mantle' between the component and bone - while this was previously a common approach, it is now recognized that a 1 mm wafer of cement is brittle and subject to fatigue failure. Modern technique avoids cement between the component and bone except in the peg holes. 
Fourth, while the model predicted that the fatigue life for trabelular bone was lower for retroverted glenoids, it is not clear that trabecular bone fatigue is a common mode of failure for total shoulders. 
Finally, the model assumed "relative conformity between the articular surfaces of the humeral head and the glenoid implant" serving to reduce peak polyethylene pressures. However, with eccentric loading the thicker posterior polyethylene may be at risk for cold flow.

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