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Kevin W. Farmer, MD
Courtesy of Kevin W. Farmer, MD

AAOS Now

Published 7/1/2014
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Sharon Johnson

Getting Reverse Shoulder Arthroplasties Just Right

OREF grant recipient aims to set new tensioning protocols

Reverse total shoulder arthroplasty (RTSA) was developed to bring back functionality for patients with cuff-deficient arthritic shoulders. Although the first reverse implant designs emerged in the early 1970s, it took 30-some years of perseverance on the part of forward-looking clinicians and researchers before the U.S. Food and Drug Administration approved RTSA for use in specific clinical situations in 2004.

Subsequent advances in technique and technology have brought greater precision to the procedure. Still, no objective standard for determining optimum soft-tissue tension across the glenosphere-humeral socket, which contributes to a stable and safe RTSA, currently exists. With support from the Orthopaedic Research and Education Foundation (OREF), Kevin W. Farmer, MD, hopes to change that.

Dr. Farmer, assistant professor in the department of orthopaedic surgery, sports medicine division at the University of Florida, Gainesville, Fla., said receiving a 2011 OREF Young Investigator Grant was pivotal to going forward with his research. “The OREF grant provided the support that allowed my research to happen,” he said.

Finding the right tension
In current RTSA practice, Dr. Farmer explained, soft-tissue tension is gauged through trial and retrial. This not only relies on subjective judgment but also adds time to the procedure. Given the growing popularity of RTSA, a study that aims to develop objective methods to determine optimal soft-tissue tension will benefit a growing number of patients and orthopaedists. In particular, general orthopaedic surgeons who perform the procedure less frequently than surgeons who specialize in shoulder care should find the results helpful.

Introducing greater objectivity and efficiency to RTSA may also decrease complications, according to Dr. Farmer. He hopes his OREF-supported research will help reduce the incidence of both dislocation, associated with too little soft-tissue tension, and acromial fracture and decreased motion, associated with too much tension.

From the lab to the OR
Phase one of the study was undertaken in the lab where researchers replaced three cadaver shoulders with an RTSA trial implant that used the same instrumentation technology developed for total hip arthroplasty.

Postprocedure, the researchers subjected the joints to range-of-motion testing and documented real-time force vector measurements. This phase ensured proper functioning of the instrumented implant, confirmed design specifications for custom instrumented glenosphere implants for use with participating patients, and delineated proper procedures for intraoperative testing.

Kevin W. Farmer, MD
Courtesy of Kevin W. Farmer, MD
Dr. Farmer inserts a custom, sensor-instrumented glenosphere trial prosthesis in a patient undergoing reverse total shoulder arthroplasty. With the custom implant in place, Drs. Farmer and Wright will test range of motion while the implant records force vector measurements for analysis.
Courtesy of Kevin W. Farmer, MD

“We found significant force changes throughout the range of motion that were principally different than what we had anticipated,” Dr. Farmer said. “The force in the deltoid changed significantly based on the position of the arm.”

In phase two, Dr. Farmer and his colleague, Thomas W. Wright, MD, performed RTSAs on 20 patients (men and women older than age 50) who consented to using a custom, sensor-instrumented glenosphere implant as a trial prosthesis. After the custom trial implant was in place, the researchers subjected the patients’ shoulders to the same range-of-motion testing used in phase one, while the implant recorded force vector measurements for analysis.

Then, Drs. Farmer and Wright used a hand-held tensiometer to record the same range-of-motion measurements as the instrumented glenosphere implant. Repeat testing with the hand-held instrument facilitated data confirmation and gave the team an opportunity to assess the use of alternate (and less costly) technology.

As of this writing, the team is in process of evaluating the study findings, including any information that may help to inform future care of participating patients.

A pebble in the pond
Dr. Farmer said he finds great satisfaction in answering open questions through research. Still greater satisfaction comes, he said, from applying new knowledge in clinical practice and in sharing new insights with other orthopaedic surgeons.

“I have already begun helping design an arthroscopic surgical skills curriculum utilizing our onsite surgical skills and cadaver labs. I may also help design a formal shoulder/elbow/sports education series. I look forward to continuing my involvement in research and the education of future generations of orthopaedic surgeons throughout my career.”

Sharon Johnson is a contributing writer for OREF and can be reached at communications@oref.org