What’s the Mechanism Behind Shoulder Replacement Loosening?

Total shoulder replacement for severe joint osteoarthritis has come a long way since its first use. But it isn’t perfect yet. Ten to 15 per cent of patients receiving this implant later develop loosening of the glenoid component (socket side of the implant).

The reason for glenoid component loosening is the focus of this study from the Cleveland Clinic in Ohio. The type of implant used was a polyethylene (plastic) press-fit (not glued in but pressed against the bone so that more bone would fill in around it) design. A peg in the center of the socket implant helps hold it in place until bone in-fill takes place.

Loosening is observed on X-rays as osteolysis (bone loss as the body resorbs bone cells). What causes this? Is it more likely to develop the more time goes by after shoulder replacement? Is it linked with the shape of the glenoid side of the shoulder before surgery? For example, retroversion (tipped back) position of the socket may contribute to the osteoarthritic degenerative changes in the first place and may also be linked with osteolysis leading to loosening.

To find out, 66 patients who received the DePuy Anchor Peg Glenoid Component were followed up. All surgeries to implant the shoulder replacement were done by one surgeon. When there was too much retroversion, the surgeon did his best to correct the problem. He did this by shaving or cutting away excess bone along the front of the glenoid. The goal was to create a glenoid (socket) that was perpendicular to the scapula (body of the shoulder blade where the glenoid is located).

In some cases, the surgeon was unable to obtain a perfect fit with complete correction. To get complete correction of glenoid retroversion would require taking off too much bone. It was better to remove as much bone as possible to re-align the glenoid but without taking so much that it was no longer possible to insert the glenoid component.

To measure results, patients completed a health survey and answered questions about satisfaction after surgery. X-rays were taken before and after surgery and compared. About half of the patients had CT scans ordered when X-rays were not enough to fully see the glenoid. These imaging studies were repeated postoperatively and made it possible for the surgeon to evaluate the bone, assess alignment (e.g., retroversion, accuracy of glenoid component to scapula interface), and look for any signs of implant loosening around the central peg.

It turned out that pre- and postoperative glenoid retroversion was the main factor in osteolysis and peg loosening. More than 15 degrees of tilt still remaining after surgery contributed to early loosening. It is believed (but has not been conclusively proven yet) that the retroversion causes increased load on the humeral head.

The surgeon concluded by suggesting that whenever possible, glenoid retroversion should be corrected to within 15 degrees before placing the implant into the shoulder. This also means that many things must be taken into account when preparing for shoulder replacement.

The surgeon is advised to use advanced imaging studies to look for pathology (i.e., problems with alignment, presence of bone deficiency). Both X-rays and CT scans should be taken pre-operatively (CT scans before surgery were not standard practice at the time of this study). And special care must be taken when selecting the implant used in order to accommodate for pathology and alignment problems observed preoperatively.