Friday, January 8, 2016

Stemless humeral prosthesis - is there an advantage?

SPECT/CT demonstrates the osseointegrative response of a stemless shoulder prosthesis.

These authors evaluated 28 patients with primary osteoarthritis treated with a shoulder arthroplasty using a stemless humeral component. They studied the local metabolic bone activity around the implant using single-photon emission computed tomography integrated with multidetector computed tomography (SPECT/CT). They concluded that the primary osseointegration of a stemless shoulder prosthesis was almost completed 3 months after implantation.

Comment: The authors state that the motivation for pursuing this type of prosthetic humeral arthroplasty was to avoid stem-related problems, such as periprosthetic fractures and loosening
of the humeral component. They suggest that the stem-related complications may be due to bone loss as a result of stress shielding, a phenomenon they believe occurs in 9% of conventional stemmed 
shoulder arthroplasties. 

As we have pointed out before (see this link), stress shielding is an issue when the metal stem of a prosthesis fits tightly in the diaphysis so that loading bypasses the proximal humerus. However, this problem is avoided in current practice by the use of impaction grafting of a humeral component stem so that load is applied evenly to the proximal humerus (see this link).

We are often asked why we do not use stemless or resurfacing humeral components in our shoulder arthroplasty practice. The reasons are several: (1) without resection of the anatomic humeral head, access to the glenoid is blocked compromising the surgeons ability to do a 'perfect' glenoid arthroplasty, (2) without a stemmed prosthesis, the surgeon does not have the ability to adjust the position of the humeral articular surface, for example through use eccentric humeral heads to achieve stability in cases of functional posterior decentering (see this link), and (3) should a revision be necessary, the risks of removing an ingrowth prosthesis are avoided.

Shown below is a diagram of an impaction grafted stem and an x-ray of an impaction grafted stem 6 years after total shoulder arthroplasty with no evidence of stress shielding.