Musculoskeletal pain and the immune system

When you experience musculoskeletal pain—back, shoulder, knee, wrist, neck pain, etc.—while biomechanical factors determine the location, the nature of the inflammatory reaction established why it hurts the way it does. A paper just published in the American Journal of Sports Medicine examines this in the context of tendinopathy (pain in the muscle tendons that can occur from the neck down to the feet).

"The authors previously identified increased levels of inflammatory cytokines in an early human model of tendinopathy and sought to extend these studies to the cellular analysis of tissue."

In this study they set out to...

"...characterize inflammatory cell subtypes in early human tendinopathy, the authors explored the phenotype and quantification of inflammatory cells in torn and control tendon samples."

The authors biopsied specimens of torn supraspinatus tendon, matched intact subscapularis tendon specimens collected from 20 patients undergoing arthroscopic shoulder surgery, and control specimens from 10 patients undergoing arthroscopic stabilization surgery. They quantified the inflammatory white blood cells present: macrophages, T cells, mast cells, and vascular endothelium (CD34). What did the data show?

"Subscapularis tendon samples obtained from patients with a torn supraspinatus tendon exhibited significantly greater macrophage, mast cell, and T-cell expression compared with either torn supraspinatus samples or control subscapularis-derived tissue."

There is a clinical gem embedded in the authors' conclusion. These inflammatory white blood cells express the immune system's 'innate', cell-mediated (versus humoral, antibody) activity. This is modified by numerous factors that can cause the inflammation to be out of proportion to the mechanical aspects of the injury. These factors influence the conditions and circumstances that modify the pain, and can determine whether it subsides or becomes chronic.

"This study provides evidence for an inflammatory cell infiltrate in early mild/moderate human tendinopathy. In particular, the authors demonstrate significant infiltration of mast cells and macrophages, suggesting a role for innate immune pathways in the events that mediate early tendinopathy."

Clinicians and patients alike should consider the role of the immune system in addition to the biomechanical factors whenever there is musculoskeletal pain.