Tuesday, September 29, 2015

Total shoulder arthroplasty with an all polyethylene glenoid component & the importance of proper register.

Radiographic and clinical outcomes of total shoulder arthroplasty with an all-polyethylene pegged bone ingrowth glenoid component: prospective short- to medium-term follow-up

These authors reviewed a series primary total shoulders implanted with all polyethylene glenoid components fixed with a bone ingrowth central peg and three cemented peripheral pegs. The glenoid was reamed minimally to preserve subchondral bone, which was given priority above correcting retroversion. If the amount of retroversion was deemed unacceptable for this technique, a reverse arthroplasty was performed. Neither bone graft nor posterior capsulorrhaphy was employed.

Within 12 months, 4 shoulders were revised and excluded from final analyses. Seven patients did not complete their questionnaires. 

Only 1 of the 80 remaining shoulders was revised for aseptic glenoid loosening. At final follow-up, 81.6% had a radiolucency grade of 0 or 1. Nearly 90% had a glenoid seating grade of A or B. Grade 2 or 3 bone around the central peg was seen in 88.2%. 

Preoperative glenoid  morphology was evaluated as Walch A1 in 19 shoulders, A2 in 19, B1 in 18, B2 in 18, and C in 2. Interestingly, no statistical association existed between Walch glenoid types and radiolucency grades, bone grades around the central peg, perfect radiolucency grade, seating grade, and grade 3 bone around the central peg. 

These results demonstrate a high degree of success with a bone ingrowth all-polyethylene glenoid component, of which there are now several on the market. The results are such that one wonders why there continues to be interest in metal+bone glenoid components, which have been shown to have a higher failure rate. It is of further interest that these results were obtained without specific attempts to change glenoid version and that the results were independent of glenoid type. These findings suggest that bone grafting, eccentric reaming, and posteriorly augmented glenoid components are not commonly needed in performing total shoulder arthroplasty.

The figures below illustrate the concept of what we refer to as register: the relationship of the center of the glenoid component to the center of rotation of the humeral head. The presence of a radiographic marker in the central peg of the glenoid component makes it easy to draw a line representing the centerline of the glenoid component. The center of rotation of the humeral head is the center of a circle fit to the head's articular surface.  See the examples below and note the relationship of the glenoid centerline to the center of rotation of the head's articular surface.  In the first three, the centerline passes below the head center. Suboptimal register can result from high positioning of the humeral component, low positioning of the glenoid component on the scapula, or secondary rotator cuff deficiency.

When the humeral head center sits superior to the glenoid center line, the humeral metal articular surface tends to rest on the superior lip of the glenoid polyethylene with the risk of wear and cold flow leading to instability. Furthermore, when the head is superior to the glenoid centerline, the articulation is subject to posterior instability when the arm is flexed. 

 By contrast, here is an x-ray showing the desired register. As shown below, we strive to place the glenoid component slightly above the equator of the glenoid bone and strive to seat the humeral prosthesis fully.

Register can easily be checked at the time of surgery by translating the head posteriorly and observing the relationship of the center of the head to the center of the glenoid.  If the head sits too high in the glenoid, the prosthesis can removed and repositioned. 


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