A 52-Year-Old Man with a History of Stiffness and Pain in the Shoulder

The patient was diagnosed with partial dissociation of the humeral head component from the stem.

Reference: Lee AH, Dilisio MF, Warner JJP. Late partial dissociation of a humeral head Morse taper associated with periprosthetic proximal humeral fracture: a case report. JBJS Case Connect. 2016 Apr-Jun;6(2):e34.

We report on a rare mode of failure of a shoulder arthroplasty through the Morse taper linkage nine and a half years after the index procedure. Modular humeral components have allowed a more anatomic placement of the humeral component that more effectively restores native patient anatomy. The linkage of the humeral head to the stem in most of the popular arthroplasty designs involves a Morse taper that allows the head to engage into the stem. Morse tapers and reverse Morse tapers have essentially become the universal linkage mechanism to mate the head to the stem in modern modular shoulder arthroplasty systems. We are not aware of any alternative commercially available head-stem linkage mechanism in the shoulder arthroplasty market. While dissociation of the Morse taper linkage has been reported in shoulder arthroplasty, nearly all of these reports have occurred in the first six weeks and were assumed to have been caused by surgeon error in seating the Morse taper mechanism.

A biomechanical study utilizing the Biomet Total Shoulder prosthesis demonstrated that the mean force required to dissociate the humeral head was 2926 N, and that fluid contamination of the taper could also prevent proper engagement of the taper. The present case, however, utilized the Zimmer Anatomical Total Shoulder System, which is designed with an eccentric Morse taper (unlike the Biomet stem). In addition, the stem of the Zimmer system is built ex vivo and has an outflow on the female end of the taper for fluid to escape, thus diminishing the likelihood of fluid contamination.

In the hip, Morse tapers are utilized to link the femoral head to the stem, and there have been several case studies of the disassembly of the modular femoral head component following attempted closed reduction. However, the hip is different from the shoulder in that more uniform compressive loads are applied to the proximal aspect of the femur, in contrast to the more physiologically unstable glenohumeral joint, which sees substantially more shear. These biomechanical differences may be relevant when comparing the Morse taper in hip arthroplasty with the Morse taper in shoulder arthroplasty.

We hypothesize that the patient’s fall resulted in an impaction force that was applied directly to the Morse taper and surrounding bone, resulting in disengagement of the humeral head from the stem, a periprosthetic proximal humeral fracture, and associated rupture of the fibrous capsule. It is possible that the head component was not fully seated; however, our initial and follow-up radiographs had demonstrated correct positioning and appropriate sizing. While corrosion has been well studied in hip arthroplasty, relatively little is known about its effects in the shoulder. Loads transmitted through the shoulder are lower than those in the hip; thus, we expect less force transmission through the humeral prosthesis.

At the six-month follow-up, the patient was doing well, and we expect that the implant will remain in its current position, given the patient’s baseline functional status. The loose component may result in increased metallosis, which may lead to increasing pain and discomfort; however, we are aware of only one such instance following total shoulder arthroplasty in the literature, which occurred within two years postoperatively.

Our report indicates that glenoid loosening and humeral stem loosening are not the only two modes of late hardware failure in total shoulder arthroplasty. Late dissociation of the humeral head from the stem after shoulder arthroplasty is possible and can be caused by a fracture. The shoulder surgeon should be mindful of this potential complication when evaluating periprosthetic proximal humeral fractures and should pay attention to detail in the evaluation of such patients.

Reference: Lee AH, Dilisio MF, Warner JJP. Late partial dissociation of a humeral head Morse taper associated with periprosthetic proximal humeral fracture: a case report. JBJS Case Connect. 2016 Apr-Jun;6(2):e34.