The nature of the annular ligament: a scoping review and histological analysis using a dual approach.

Background: The annular ligament (ANL) is a crucial structure of the elbow joint, playing a vital role in maintaining stability and function of the radial head (RH). Despite its known anatomical and stabilizing roles, the histological composition of the ANL remains unclear. The first article to describe its nature introduced the concept of a wrap-around ligament. A wrap-around ligament is defined as "any tendon that bends around a bony pulley or threads through a fibrous 1 en route to its insertion". These ligaments are often fibrocartilaginous due to an adaptation to compressive forces. During pronation and supination the ANL is compressed against the RH. Therefore, the authors hypothesize that the ANL contains (fibro)cartilaginous tissue where the ligament is compressed against and experiences friction from the RH. Understanding the precise histological nature of the ANL is essential for surgical procedures involving the RH, such as fracture osteosynthesis, in which hardware placement may lead to complications. Methods: This study consisted of 2 components: a scoping review and a histological analysis. First, a scoping review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews. A systematic literature search was performed using Embase, Medline, Web of Science Core Collection, Google Scholar, and Cochrane Library. Published literature was screened, and articles were included if they addressed the ANL and its histology or tissue composition. In addition to the review, a histological analysis was performed on 3 ANLs from postmortem human specimens. Results: A total of 2,453 articles were identified, of which 7 met the eligibility criteria. The literature search revealed discrepancies regarding the presence and location of fibrocartilage and synovial linings within the ANL. Histological examination of 3 postmortem human specimens samples showed (fibro)cartilage cells in 1 ligament and chondrification in another. These features were located in the central part of the ANL, where it articulates with and is compressed against the RH. The third ligament showed no fibrocartilage cells. All 3 ANLs exhibited a synovial lining on the lateral side where the ligament attached to the ulna, but not in the center part of the ligament, between the articulating surfaces of the ANL and the RH. Conclusion: The ANL is a wrap-around ligament and contains (fibro)cartilaginous tissue. There was no synovial lining present on the articulating surface of the ANL between the ANL and the RH.