Thursday, April 14, 2016

Stemless total shoulder arthroplasty - safety, efficacy and utility

Clinical and Radiographic Outcomes of the Simpliciti Canal-Sparing Shoulder Arthroplasty System: A Prospective Two-Year Multicenter Study.

These authors report the minimum two year followup on 149 of 157 patients with glenohumeral arthritis from 14 different centers treated with a press-fit, porous-coated, metaphyseal fixed humeral implant between July 2011 and November 2012 in a U.S. Food and Drug Administration (FDA) Investigational Device Exemption (IDE)-approved protocol.

The mean age and sex-adjusted Constant, SST, and ASES scores improved from 56% preoperatively to 104% at two years (p < 0.0001), from 4 points preoperatively to 11 points at two years (p < 0.0001), and from 38 points preoperatively to 92 points at two years (p < 0.0001), respectively. 

The mean forward elevation improved from 103° ± 27° to 147° ± 24° (p < 0.0001) and the mean external rotation, from 31° ± 20° to 56° ± 15° (p < 0.0001). The mean strength in elevation, as recorded with a dynamometer, improved from 12.5 to 15.7 lb (5.7 to 7.1 kg) (p < 0.0001), and the mean pain level, as measured with a visual analog scale, decreased from 5.9 to 0.5 (p < 0.0001). 

There were three postoperative complications that resulted in revision surgery: infection, glenoid component loosening, and failure of a subscapularis repair. There was no evidence of migration, subsidence, osteolysis, or loosening of the humeral components or surviving glenoid components.

Comment: These authors are to be congratulated on a well-done prospective FDA approved multicenter trial with 16 shoulder reconstruction specialists to investigate the new device. See this link. In the hands of these surgeons and in these patients the device demonstrated safety and efficacy.

They point out that humeral complications in shoulders with stemmed humeral components are rare, but can include periprosthetic humeral fractures during and following total shoulder arthroplasty, proximal humeral bone loss due to stress shielding, humeral stem loosening, and osteolysis. They also point out that removal of a well-fixed humeral stem can be difficult and can result in additional destruction of proximal humeral bone. They assert that a stemless humeral component could provide a method for (1) making revision easier by preservation of the humeral bone and (2) improving anatomic positioning of the humeral head; however neither of these assertions is tested in this paper. It is not known whether this approach saves operating time or has a lower cost in comparison to standard humeral implants.

It is of note that the application of the the stemless implant was limited; it was used for less than 1/3 (31.4% (157)) of the 500 primary anatomic total shoulder arthroplasties performed by the study surgeons during the enrollment period. Reasons for non-use were many: low preoperative Constant score, diagnoses other than osteoarthritis or posttraumatic arthritis, patients with risk of falls, prior open surgery, cortisone use, physical activities that could affect the outcome of surgery, or lack of sufficient quality bone to seat and support implant (the surgeon attempted to compress the neck cut surface with his/her thumb; bone that was easily compressed with minimal force was considered insufficient for implantation of this device).

In contrast to this stemless device, we find that an impaction-grafted humeral component not only preserves but adds bone to the humerus, enables positioning of the humeral articular surface in the desired location (including an eccentric offset when needed as shown in this link) and is applicable to essentially the full range of shoulder arthroplasty indications, rather than being subject to the restrictions for a stemless prosthesis described in this article.

We use a chrome cobalt humeral head prosthesis connected to a stem (titanium alloy) the tapered body of which fits inside the humerus.

We note that the humeral canal may be cylindrical or tapered.
and that the cross sectional geometry varies 

We agree that trying to fit a prosthesis by reaming the inside of the bone may substantially weaken it.

and that trying to force a tight fit risks fracture.

Our preferred method for securing the stem within the humeral canal is to use impaction grafting with bone harvested from the arthritic humeral head to conform the inner surface of the bone to the prosthesis. Some have likened this fitting of the patient's bone the prosthesis to the fitting of the traveler to the bed by the inn keeper Procrustes.
As a result, the tapered stem is securely fixed with a biological press fit that safely distributes the load from the prosthesis to the humerus, avoiding stress shielding which has been noted with short stem prostheses (see this link).

The amount of bone removed with an impaction grafted stem (below left) seems no greater than that with a stemless head prosthesis (below right).


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