I had the great pleasure to present the work of my fellow researchers to the Korean Shoulder and Elbow Society Annual Scientific Meeting in Seoul last week. Dr. Wolbert van den Hoorn and his team have been investigating the neural control of the deltoid muscle function, and they have found some fascinating insights into its function.
Reverse Total Shoulder Replacement has been a revolution for treating end-stage shoulder conditions. The Reverse Shoulder Replacement was invented by Professor Paul Grammont to solve end-stage rotator cuff disease. By the time we are 70, 50% of us will have a tear in our rotator cuff tendons in our shoulders. For many, this is an asymptomatic condition; however, for some, it is interminable discomfort, leaving them unable to sleep at night due to aching pain and, during the day, struggling to work or enjoy what they like to do for recreation. Reverse Shoulder Replacement surgery gives these people back their life, and whereas once they thought about their shoulder all day and all night, they find they no longer think about their shoulder.
For a Reverse Shoulder Replacement to work well, the deltoid muscle function is key. Without the deltoid muscle, the Reverse Shoulder Replacement just doesn’t work for people. There are some people who, despite all the best surgical planning and expertise, find that their Reverse Shoulder Replacement just doesn’t function the way it should, and in many cases, it’s the deltoid muscle that isn’t working the way it should. We would like to know why.
Wolly and his team have set out to find out why it is that, in some people after a Reverse Shoulder Replacement, their deltoid muscle doesn’t do what it should. We plan to study the function of the deltoid muscle in patients who have had a Reverse Shoulder Replacement with electromyography. Before we study these people, we first have to study the deltoid in normal people with electromyography, and it is this work that I was able to present in Korea.
When we think about a muscle activating, we mostly think about telling that muscle to contract and it contracting, but that’s not quite the way it works. A muscle doesn’t actually contract as a single unit. Muscle contraction is directly controlled at the level of spinal motor neurons—the "final common pathways" described by Sherrington in 1906. Essentially, the central nervous system controls groups of motor neurons rather than the entire muscle.
The deltoid is better thought of as having three parts to it: anterior, lateral, and posterior. Wolly and his team studied the deltoid muscle with electromyography in normal volunteers and found that, on average, the deltoid muscle has 18 separate motor units controlling its contraction, and these motor units were coordinated by what we call distinct common neural drives—one for each part of the deltoid. Rather than functioning independently, these common neural drives overlap in their control of the deltoid. For example, some motor units in the posterior and anterior deltoid segments received a dual input from the same common neural drive.
The take-home learning is that deltoid muscle control is much more nuanced than we might have assumed. What we hope to learn is why, in some patients after Reverse Shoulder Replacement, the deltoid muscle doesn’t function the way it should. Once we work that out, it should be possible to get it to function properly—perhaps by changing our surgical planning and/or by changing our postoperative rehab program.
I would like to shout out to my fellow researchers:
Our student team: Ella Hill and Fred Dupuis
The Researcher Team: Francois Hug, Kylie Tucker, and Wolbert van den Hoorn
The Surgical Team: Ashish Gupta and Mark Ross