Subacromial Bursa: A Neglected Tissue Is Gaining More and More Attention in Clinical and Experimental Research

Abstract

The subacromial bursa has long been demolded as friction-reducing tissue, which is often linked to shoulder pain and, therefore, partially removed during shoulder surgery. Currently, the discovery of the stem cell potential of resident bursa-derived cells shed a new light on the subacromial bursa. In the meanwhile, this neglected tissue is gaining more attention as to how it can augment the regenerative properties of adjacent tissues such as rotator cuff tendons. Specifically, the tight fibrovascular network, a high growth factor content, and the large progenitor potential of bursa-derived cells could complement the deficits that a nearby rotator cuff injury might experience due to the fact of its low endogenous regeneration potential. This review deals with the question of whether bursal inflammation is only a pain generator or could also be an initiator of healing. Furthermore, several experimental models highlight potential therapeutic targets to overcome bursal inflammation and, thus, pain. More evidence is needed to fully elucidate a direct interplay between subacromial bursa and rotator cuff tendons. Increasing attention to tendon repair will help to guide future research and answer open questions such that novel treatment strategies could harvest the subacromial bursa's potential to support healing of nearby rotator cuff injuries.

Keywords: augmentation; bursitis; inflammation; pain; progenitor cells; subacromial bursa.

Figure 1

Anatomy of a left human shoulder joint. The subacromial/subdeltoid bursa lays between the rotator cuff, the deltoid, and the acromion. Due to the close location to the rotator cuff tendons, the contribution of the subacromial bursa in the development or healing of tendon pathologies seems obvious. This graphic was created with an image from Servier Medical Art.

Figure 2

Arthroscopic view from a posterior portal during arthroscopy of a right shoulder showing (a) a normal subacromial/subdeltoid bursa (B) and healthy tendon (T) and (b) a severe hypertrophic subacromial bursitis (B) and fraying of the supraspinatus tendon (T).

Figure 3

Overview of historical steps towards increased experimental research around the subacromial bursa: number of PubMed listed publications from 1934 to 2021 mentioning subacromial bursa OR subacromial bursitis (total) combined with the search terms “Histology”, “Cells”, and “Augmentation”.

Figure 4

Possible bursa–tendon interaction: Inflammation through trauma, impingement, recurrent pathological strain, or infection leads to the development of bursitis, whereas bursitis might be cured by anti-inflammatory drugs, pro-resolving mediators, and/or physiological strain. Bursitis is characterized in particular by the presence of immune cells such as monocytes/macrophages and lymphocytes (NK cells, B cells, and T cells), pro-inflammatory cytokines (e.g., SDF-1, IL-1β, and TNF-α), matrix metalloproteinases (MMPs), pain mediators (e.g., COX-2, NGF, and substance P), and chondral metaplasia. Compared to that, healthy bursae contain lower numbers of immune cells and predominate by growth factors and mesenchymal stem cells (MSCs) and, thus, might maintain tendon homeostasis. In bursa–tendon interplay it is hypothesized that growth factors and MSCs in the bursa can promote the healing of tendinopathies, including rotator cuff tears, and it has to be elucidated if pro-inflammatory cells and mediators might help to initiate healing cascades at the tendon rupture side. Vice versa, it is still debatable if bursitis is the cause or consequence of pathological conditions in the tendon. This graphic was created with images from Servier Medical Art.