Ligament

Last revised by Yacoob Omar Carrim on 19 May 2022

Ligaments are connective tissue structures that arch over joints connecting one bone to another bone with primary functions as stabilizers of articulations. The scientific study of ligaments is called syndesmology.

Ligaments are ultimately formed from numerous ligamental fibrils. The fibrils combine to form fibers, which in turn form fascicles that combine to form a ligament 1. Many ligaments are covered by an epiligament which is a vascular covering that blends into the ligament at their bony attachments known as insertions 2.

Histologically, ligaments and tendons share similar structures and extracellular matrices with only small cellular differences 1. The main resident cells in ligaments are a type of fibroblast called "ligamentocytes" 1,2.

Ligaments are about 70-80% collagenous fibers, of which ~90% are type I collagen fibers and ~10% type III collagen fibers, with tiny amounts of several other collagen types contributing. Elastin is another key structural protein and water makes up anything between 60 and 80% of their volume (similar to tendons). The arrangement of the collagen fibers is less organized in ligaments than in tendons with a predominant "weaving" pattern 1,2.

The extracellular matrix of ligaments is based around ground substances, including proteoglycans, glycosaminoglycans (GAGs) and glycoproteins, which are hydrophilic, meaning that the water content of ligaments is high. This hydration is important, permitting better flexibility when under compression and aiding structural integrity, stability and optimal collagenous maintenance 1.

A key function of ligaments is as a passive stabilizer of the articulation, acting to direct joints through their full range of movement when under a tensile stress. They demonstrate non-linear anisotropic behavior. When the applied load is low, ligaments are fairly elastic. However, as the loads increase, compliance drops and almost linear stiffness is achieved. Loading a ligament past this point leads inevitably to loading failure i.e. ligamentous rupture 2.

Ligaments are now also thought to have a role in proprioception, the ability to sense the position of one's joints in space. As ligaments are stretched, nervous feedback results, which in turn causes a muscular contraction contributing to proprioception 2.

 

Ligaments are linear structures. Injury shows as a loss of the linear structure with hypoechoic foci. Contralateral evaluation of ligaments under tension is useful.

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