Rotator cuff tears are common injuries, and proper healing of the shoulder muscle is often difficult.

“Chronic tears often result in fat accumulation within the rotator cuff muscles, resulting in negative clinical outcomes, including weakening and atrophy of the muscles,” says Manuel Schubert, M.D., resident in orthopaedic surgery at Michigan Medicine. “It’s believed that this process of fat infiltration makes rotator cuff muscle damage one of the most difficult to rehabilitate after injury.”

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Schubert explains that the process of fat accumulation following a tear appears to occur more frequently in rotator cuff muscles than other muscle groups.

But why this particular muscle set?

“We wanted to determine if there are cellular, molecular and genetic reasons for why rotator cuff muscles tend to develop this fat accumulation after injury,” he says.

In a new study, published in The Journal of Bone & Joint Surgery, Schubert and team used a mouse model to isolate specific stem cells, called satellite cells, in rotator cuff muscles, as well as calf muscles for comparison, to determine the extent of muscle and fat cells that develop from these satellite cells.

They then performed DNA-level studies to understand how the gene pathways of the muscles may differ.

“Even though the stem cells obtained from the rotator cuff muscles and the calf are thought to be the same kind of muscle stem cells, we wanted to determine if these cells are different in how their development is controlled,” Schubert says, “Which may provide insight for why fat tends to accumulate more in rotator cuff muscles.”

Key findings

The research team first obtained and isolated stem cells in each muscle to identify which cells develop into muscle cells and which develop into fat cells.