Fat Expansion Not Fat Infiltration of Muscle Post Rotator Cuff Tendon Tears of the Shoulder: Regenerative Engineering Implications.

Purpose: Rotator cuff (RC) tears affect many individuals around the globe. Ambiguity of rotator cuff repair surgical outcomes is currently a limitation that is associated with fat accumulation and atrophy in the rotator cuff muscles. To improve the efficacy of rotator cuff repairs, a deeper understanding of the root causes is required. Traditionally, the term "fat infiltration" has been used to described fatty changes in muscle after rotator cuff tears. This paper introduces the concept of fat expansion as a more appropriate description for the appearance of fatty rotator cuff tear pathological changes. Furthermore, the contribution of fibroadipogenic progenitor (FAP) cells to pathological changes associated with rotator cuff injuries is presented to characterize the molecular basis of impairment. Lastly, the field of regenerative engineering is discussed as a promising solution to the pathological changes associated with rotator cuff tears. Methods: The connection between fatty infiltration, fat expansion, fat accumulation, fibroadipogenic cells, and regenerative engineering in the context of rotator cuff tears was explored using the databases PubMed and Google Scholar. Results: Numerous articles have supported the role of muscle resident fibroadipogenic cells as a contributor to rotator cuff tear pathological changes. In addition, regenerative engineering solutions prove to improve the pathological changes associated with rotator cuff tears. Conclusion: The term fat expansion is more appropriate to describe fat accumulation associated with rotator cuff tears, and the employment of regenerative engineering treatment strategies improve the pathological changes associated with rotator cuff tears. Lay Summary: Fat accumulation after rotator cuff tears has been associated with post-operative complications. Infiltration or entering of adipocytes from the external muscle environment has historically been the reported cause of the rapid increase in fat and muscle atrophy observed after rotor cuff tears. This review will dismiss the use of the term fat infiltration and acknowledge the implications of muscle resident stem cells, known as fibroadipogenic (FAP) cells, to rotator cuff tear pathological changes. Additionally, regenerative engineering, a field which seeks to regenerate various tissues using biomaterial-based scaffolds and stem cells, will be discussed as a potential solution for pathological changes.