Posterior augmented glenoid implants - cortical bone support and bone sacrifice

Posterior augmented glenoid implants require less bone removal and generate lower stresses: a finite element analysis.

These authors used finite element models of 3 arthritic scapulae with varying severities of posterior glenoid wear to compare 4 different implant configurations: standard glenoid implant in neutral alignment with asymmetric reaming, standard glenoid implant in retroversion, glenoid implant augmented with a posterior wedge in neutral alignment, and glenoid implant augmented with a posterior step in neutral alignment.

Contact between the humeral and glenoid components was simulated, and a compressive force of 625 N (representing 85% body weight) was applied through the humeral head into the glenoid surface.

They found that asymmetric reaming for the standard implant in neutral version required the most bone removal, resulted in the lowest percentage of back surface supported by cortical bone, and generated strain levels that risked damage to the most bone volume. In comparison to an asymmetric reaming for the standard implant in neutral version, a wedged implant removed less bone or insertion of a standard component in retroversion, had a significantly greater percentage of the back surface supported by cortical bone, and generated strain levels that risked damage to significantly less bone volume.

The results below compare the authors' wedge component (left), with a standard component inserted in retroversion (Std-R), with a stepped glenoid (Step) and a standard glenoid inserted after version 'correction'.

They concluded that the percentage of the back surface of the glenoid supported by cortical bone was associated with durability of the cement mantle;  preservation of cortical bone may have the advantage of maintaining the structural integrity of cement mantle.

Comment: Like the authors, we are concerned about preserving glenoid cortical bone and about the amount of bone that can be sacrificed with the insertion of posteriorly augmented glenoid components as diagrammed below. If such a component were to fail, there would be a substantial bone defect.

We have found that 'correction' of glenoid version is not necessary for the restoration of posterior stability and thus will ream the glenoid only as necessary to achieve a single concavity as shown below. So rather that using a guide pin inserted at a certain angle, we use a reamer that permits adjustment of the angle of reaming.

If a glenoid component is elected this is inserted in a position of retroversion (below left), preserving bone in contrast to approaches that sacrifice bone to 'correct' retroversion (below right). The humerus is centered in the glenoid by soft tissue balancing and an eccentric humeral head component if necessary.

As an example demonstrating that correction of glenoid version is not necessary, here's the case of a very active lady in her 60's with glenoid dysplasia. 

Because of her activity level and glenoid retroversion of over 50 degrees we managed her shoulder with a ream and run procedure (one year post surgery films below). 

She is back to kayaking and skiing without concern for polyethylene failure.

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Posterior Augmented Glenoid - is there an advantage over the standard glenoid component?

Preliminary Results of a Posterior Augmented Glenoid Compared to an all Polyethylene Standard Glenoid in Anatomic Total Shoulder Arthroplasty

These authors report on 24 patients having total shoulder arthroplasty using a posteriorly augmented glenoid for arthritis with posterior glenoid wear. The degree of posterior wear and retroversion before surgery are not presented.

At two years after surgery, 60% of the shoulders had a periglenoid radiolucent line with an average radiographic line score of 1.10.

One glenoid was radiographically loose. 

Two shoulders demonstrated superior subluxation. 

Three were anteriorly subluxated.

Comment: These results speak to the challenges inherent in the use of posteriorly augmented components.

One of the rarely discussed concerns is the effects of using thick posterior polyethylene to manage the   posteriorly directed loads applied when the arm is elevated to the functional position of forward elevation, which is known to create the risk of functional decentering.

This is best explained by noting that when the arm is at the side with a posteriorly augmented glenoid, the net humeral joint reaction force (red arrow) is centered.

However, when the arm is elevated to a functional position, the net humeral joint reaction force (red arrow) is directed posteriorly against the posteriorly augmented polyethylene, subjecting it to the risk of cold flow.

 Furthermore, the point of application of the net humeral joint reaction force of the elevated arm creates an increased glenoid loosening moment (blue line), when the posterior polyethylene is thick.

It is possible that these mechanisms contributed to the development of lucent lines and instability with posteriorly augmented glenoid components in the series presented.


Our approach to the retroverted glenoid is simple - see this link..

(1) We do not rely on preoperative CT scans because they cannot image the shoulder in the functional  position of forward elevation. Instead we prefer the simple standardized axillary view taken with the arm elevated 90 degrees in the plane of the scapula as shown below (this shoulder demonstrates the bad arthritic triad).

(2) We do not use preoperative planning software or patient specific drill guides, but rather ream the glenoid conservatively without trying to 'normalize' glenoid version as shown in this link.

(3) Finally, in a total shoulder arthroplasty for a retroverted glenoid, we place a standard glenoid component on the conservatively reamed glenoid, using an anteriorly eccentric humeral humeral head component if necessary to achieve centering of the articulation. 

This type of reconstruction is shown below on an axillary view taken with the arm in the functional position of forward elevation. Note the centered humeral head and the lack of glenoid lucent lines after two years of implantation.

This approach perseveres the maximal amount of glenoid bone in contrast to what is required to fit the bone to a more complex back side geometry as explained in this link.

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Augmented glenoid component - requires removal of posterior glenoid bone

There is a lot to be learned about evaluation and management of the arthritic glenoid as shown in a recent article. In this article the x-rays below illustrate some of these. The figure below shows the implantation of a 7 mm posteriorly augmented glenoid component  'recommended' by a three dimensional imaging and templating system. Note that this implantation requires removal of about half of the dense bone in the posterior glenoid.

This bone removal is also shown in the figures below copyrighted by Steve Lippitt comparing the bone loss with the StepTech to that of the conservative glenoid reaming approach that we embrace (using anteriorly augmented humeral head components and rotator interval plication if necessary to manage posterior instability).

      

 

It is apparent that if a stepped posterior glenoid component fails, the bone loss will greatly complicate any additional reconstruction.

Here are the eccentric heads used in conduction with conservative reaming.

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Eccentric anterior reaming vs posterior augmented glenoid components - is this the right way to answer the question?

Posterior Glenoid Wear in Total Shoulder Arthroplasty: Eccentric Anterior Reaming Is Superior to Posterior Augment

These authors compared the use of augmented glenoid components to eccentric reaming with standard glenoid components in a model of posterior glenoid wear with a 12°-posterior glenoid defect  created in 12 composite scapulae.

In the posterior augment group, glenoid version was corrected to 8° and an 8°-augmented polyethylene glenoid component was placed. 

In the eccentric reaming group, anterior glenoid reaming was performed to neutral version and a standard polyethylene glenoid component was placed.

Specimens were cyclically loaded in the superoinferior direction to 100,000 cycles.

Surviving specimens in the posterior augment group showed greater displacement than the eccentric reaming group during superior edge loading and during inferior edge loading.

A greater number of specimens in the eccentric reaming group were able to achieve the final 100,000 time without catastrophic fracture than those in the posterior augment group.

They concluded that "When addressing posterior glenoid wear in surrogate scapula models, use of angle-backed augmented glenoid components results in accelerated implant loosening compared with neutral-version glenoid after eccentric reaming, as shown by increased implant edge displacement at analogous times."

Comment: While we are no fans of posteriorly augmented or stepped glenoid components, we feel misled by a title like "Posterior Glenoid Wear in Total Shoulder Arthroplasty: Eccentric Anterior Reaming Is Superior to Posterior Augment" - thinking that this is a clinical outcome study that grappled with the wide variety of glenod pathoanatomy we encounter in people - and then find out that this is a study of the properties of glenoid components placed in plastic bones that do not duplicate the bone of an arthritic shoulder and then are loaded in the superior / inferior direction which would seem to be of much less interest than the response to posteriorly directed loads which challenges the stability of retroverted glenoids.

Of interest is the statement in the discussion: " Initial testing for this study was performed with cadaveric scapulae but was abandoned because of the substantial variability between samples, which had a greater effect on implant stability than prosthesis design." Folks, we live in a world where there is substantial variability among patients and we need to do studies that recognize that fact.

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You may be interested in some of our most visited web pages including:shoulder arthritis, total shoulder, ream and run, reverse total shoulder, CTA arthroplasty, and rotator cuff surgery as well as the 'ream and run essentials'

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