Posteriorly augmented glenoid components - computing how much bone is lost on insertion

Augmented glenoid component designs for type B2 erosions: a computational comparison by volume of bone removal and quality of remaining bone

These authors used a computational modeling was to compare the volume of glenoid bone that would need to be removed in the implantation of three different designs of posteriorly augmented glenoid components in the management of B2 genoid erosion. They 'virtually implanted' a full-wedge, a posterior-wedge, and a posterior-step in 3-D reconstructions of 16 patients with B2 glenoids, correcting retroversion to 0° and 10°.

Importantly the amount of bone removed with these implants ranged from 1500 to 3000 cubic millimeters. When correcting to 0°, the posterior-wedge implant removed less bone than the posterior-step and the full-wedge. 

The residual glenoid bone density with the posterior-wedge was significantly greater than with the posterior-step.

Comment: This is an interesting study, suggesting that bone removal is necessary for fitting posteriorly augmented glenoids to the pathoanatomy encountered in glenohumeral arthritis. 

We have posted this concern previouslyl

Their implant fitting was virtual and not actual: implants could be manipulated to rotate (clockwise/ counterclockwise and superoinferior) and to translate (anteroposterior and superoinferior). The instrumentation for fitting the glenoid bone to the back of the glenoid component would seem to be quite complex. While the authors suggest that it would be best to use computerized

preoperative planning software making use of 3D CT-based models of the glenoid with properly sized implants, the problem of developing instrumentation to implement the plan remains.

While the authors conclude that " Augmented components can provide a bone-preserving option for B2 glenoid management." that statement is only correct if B2 glenoid management involves correction of retroversion. Our approach is manage the B2 glenoid by conservatively reaming only enough to convert the biconcavity to a single concavity without attempting to change version. See also this related article. this one, and this one as well.

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Do special glenoid components solve the problem of glenoid bone deficiency?

Nonstandard glenoid components for bone deficiencies in shoulder arthroplasty

As in the prior post, these authors grapple with the issue of glenoid bone deficiency in total shoulder arthroplasty. In this article, they evaluated the clinical and radiographic outcomes in 38 patients having a primary or revision anatomic shoulder arthroplasty with one of 3 nonstandard glenoid components: a polyethylene component with an angled keel for posterior glenoid wear without posterior subluxation; a polyethylene component with 2 mm of extra thickness for central glenoid erosion; or a posteriorly augmented metal-backed glenoid component for posterior glenoid wear and posterior subluxation. The average clinical follow-up was 7.3 years (range, 2-19 years) or until revision surgery.

While overall the patients were improved, thirteen had moderate or severe subluxation preoperatively, and 11 had subluxation at follow-up. Three glenoid components had loosened and 3 were at risk for loosening at an average 5.5 years of follow-up. Seven patients had revision surgery: 4 for instability, 1 for osteolysis, 1 for component loosening with osteolysis, and 1 for a periprosthetic fracture. Three additional patients had removal of glenoid components, 2 for infection and 1 for loosening. Ten-year survival rate free of revision or removal of the angled keel component was 73%; of the extra thick (+2 mm) component; and of the posteriorly augmented metal-backed glenoid component, 31%.

Comment: This article again stresses the difficulty in managing deficient glenoid bone in performing total shoulder arthroplasty. If there is insufficient bony support the component is at risk for failure.

We refer to the combination of posterior humeral subluxation on the glenoid, glenoid retroversion and a biconcave glenoid as the bad arthritic triad (BAT).

We have used the ream and run procedure for carefully selected patients with this type of glenohumeral pathoanatomy, avoiding the risk of glenoid component failure in the bad arthritic triad (BAT).

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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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