Saturday, November 5, 2016

Issues with posteriorly agumented glenoid components

Two studies with two year followup on posteriorly augmented glenoid components are presented here.


These authors report on 14 shoulders having shoulder arthroplasty with an augmented glenoid component to treat posterior glenoid bone deficiency associated with advanced osteoarthritis.

According to a modified Neer result rating system, 36% of patients had an excellent result, 50% a satisfactory result, and 14% an unsatisfactory result.

At the most recent radiographs, four of the 12 shoulders that could be evaluated radiographically had more than mild glenohumeral subluxation. Three shoulders had moderate posterior subluxation, one shoulder had severe anterior subluxation. All of the shoulders with postoperative subluxation had some degree of subluxation present prior to surgery. 

The shoulder with severe anterior subluxation postoperatively had moderate posterior subluxation prior to surgery. This patient had Parkinsonism and sustained anterior instability resulting in dislocation, treated with closed reduction, followed by immobilization and physical therapy. 

There was no periprosthetic lucency in four shoulders, periprosthetic lucency was grade 1 in seven shoulders and grade 5 in one shoulder, and the glenoid component had shifted in position. This shoulder also had moderate posterior subluxation, and this glenoid component was considered radiographically loose.

The authors concluded, "Our results suggest patients undergoing total shoulder arthroplasty with an asymmetric glenoid component for osteoarthritis achieve satisfactory mid-term pain relief and improvement in function; however, instability is not always corrected. The advantage of this component seems marginal, and its use has been discontinued."

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.