The stepped glenoid component

In their classic article, Stepped Augmented Glenoid Component in Anatomic Total Shoulder Arthroplasty for B2 and B3 Glenoid Pathology, the authors demonstrate that a stepped augmented glenoid component can restore premorbid glenoid anatomy in patients with asymmetric biconcave glenoid bone loss (Walch B2), with short-term clinical and radiographic results equivalent to those for patients without glenoid bone loss (Walch A1) treated with a non-augmented component. 

They found a greater risk of osteolysis around the central peg in patients with moderate-to-severe B3 glenoid pathology with this stepped augmented glenoid component. As demonstrated in the technique guide, use of this component involves reaming of the posterior glenoid to fit the step, which may diminish the bony support for the back of the component.

Should this component fail, the posterior bone stock available for conversion to a reverse total shoulder may be compromised. 

A recent paper, Total Shoulder Arthroplasty for Glenohumeral Arthritis Associated with Posterior Glenoid Bone Loss: Midterm Results of an All-Polyethylene, Posteriorly Augmented, Stepped Glenoid Component,  presented a 5 year followup of 35 shoulders receiving a stepped glenoid for the treatment of glenohumeral osteoarthritis with posterior glenoid bone loss. The average preoperative glenoid retroversion was 21.6˚. Although postoperative CT scans were obtained, the postoperative glenoid retroversion was not presented.
Two patients (6%) experienced prosthetic instability requiring revision.

The average Lazarus score (0 no radiolucency to 5 gross loosening) was 0.72. The average Yian score (0 no radiolucency to 18 radiolucent line around entire component) was 2.6. There was an increase in Lazarus score and decrease in Wirth score between 2- and 5-year follow-up. The severity of radiographic loosening correlated with patient-reported pain levels. 

The authors point out that these results are not inferior to those achieved with standard glenoid components in the treatment of glenohumeral arthritis with posterior bone loss.

Comment: Since posterior reaming removes posterior bone and the stepped component adds posterior polyethylene, it would be of interest to know the net change in glenoid retroversion in these patients.

Two figures from this series seem to suggest that substantial retroversion remains after insertion of the stepped component.

As pointed out in the two posts referenced below, future research is needed to determine the clinical value and means of "correcting" glenoid retroversion.

What happens when glenoid version and inclination are "corrected"?

Glenoid version: acceptors and correctors

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Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link).
The total shoulder arthroplasty (see this link).
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link).
Shoulder rehabilitation exercises (see this link).

The variable biconcave glenoid, a challenge for fitting posteriorly augmented and step-cut components

Quantification of the position, orientation, and surface area of bone loss in type B2 glenoids

These authors evaluated 55 type B2 glenoids using computed tomography to create three-dimensional reconstructions.

They found the maximal erosion was usually posterior - inferior (8 o'clock in the right shoulder and 4 o'clock in the left shoulder), but there was a substantial degree of variability.  The line of erosion was curved in 35% of the cases. The pathological concavity accounted for an average of 44% ±  12% of the overall glenoid surface. The pathological concavity was flatter (radius 37mm ± 8) in comparison to the normal glenoid (radius 34mm ± 7). 

Comment: It would be of interest to know what percent of the total population of arthroplasty shoulders these 55 B2 glenoids represented.

The authors point out that the challenges in fitting the variable B2 pathology (such as that shown above) with posteriorly augmented or step-cut glenoid components.

Reaming and subsequent bone removal to accommodate posteriorly built-up glenoid components carries the risk of excess bone removal, reducing the quality and quantity of the remaining glenoid bone and leading to potentially compromised implant stability.

In our approach to prosthetic glenoid arthroplasty, we do not attempt to correct glenoid version, but rather we ream the glenoid bone as conservatively as possible to a single concavity, preserving glenoid bone stock.

As suggested in another recent related article "Glenoid component loosening in total shoulder arthroplasty may be prevented by component placement on a congruent and adequate bony surface."

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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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