Joint capsule and ligaments

The joint capsule (Capsula articularis) and the ligaments (Ligaments) surround each true joint of the body. The capsule encloses the joint space and produces the synovial fluid. The movements of a joint are regulated and controlled by it, and even more so by the ligaments. They influence the passive stability of a joint.

Function

A main task of the capsule is the production of sufficient and high-quality synovial fluid. This joint fluid allows for resistance-free and friction-free movement of the joint partners. Furthermore, synovia is crucial for the nourishment of the articular cartilage and the menisci and provides the necessary substances. Additionally, the capsule conveys important information about the position and movement of the joint (proprioception). Therefore, it is also richly innervated and equipped with many receptors. Typically, muscles are fused to the joint capsule. They tension it and thus prevent it from being pinched between the moving bones. The ligaments function to control and limit the range of motion and are responsible for the (passive) stability of the joints. They are also equipped with numerous receptors, which, however, only respond later and at higher loads.

Braus, Hermann, Braus 1921 147, labeled as public domain, details on Wikimedia Commons

Structure

The capsule is composed of two layers: an outer, more stable and stronger layer (Membrana fibrosa), which protects and supports the sensitive inner membrane.

The inner, finer layer, known as the membrana synovialis (which can be further subdivided), forms numerous folds. This is a method used by the body to increase an (active) surface area (e.g., brain convolutions). The innermost layer of the membrana synovialis connects with the tangential layer of the articular cartilage, while the outer layer of the membrana synovialis connects with the intracapsular periosteum (bone covering within the joint capsule). See also our blog on joint cartilage.

In areas of the membrana fibrosa that are regularly exposed to certain mechanical stresses and forces, the course of the collagen fibers adapts accordingly: the fibers orient themselves in the direction of the force acting on them (similar to the bone-trabeculae) and align themselves accordingly. As a result, the capsule often becomes thicker at these points, and these are referred to as ligaments. These are nothing more than an adaptation of the membrana fibrosa to mechanical stresses and a part of the joint capsule. These ligaments are usually flat and band-shaped.

There are also some ligaments that have no direct connection to the capsule (e.g., lateral collateral ligament at the knee). However, even these are always connected to the capsule by loose connective tissue. These bands usually have a distinctly rounder and more tendon-like structure.

A special case is the intracapsular ligaments, which lie within the joint capsule and have no connection to the membrana fibrosa but do have a connection to the membrana synovialis (e.g., cruciate ligaments).

The blood supply to the ligaments is less than that of the capsule. This means therapeutically that intracapsular ligaments heal and regenerate better and faster than extracapsular bands that have no direct connection to the capsule.

Knee joint capsule anterior, Henry Vandyke Carter Henry Gray, Gray345, labeled as public domain, details on Wikimedia Commons

Degeneration

As the aging process progresses and even more so after injuries, there is an increase of collagen type II, which grows into the capsule from the capsule-bone connection. This results in cartilaginization of the capsule (fibrous cartilage).

The tissue becomes firmer and less elastic, and the number of so-called soluble cross-links decreases while the number of non-soluble cross-links increases significantly. This shift is also observed in diabetes mellitus.

The water content of the capsule and ligaments decreases overall with age, resulting in further loss of elasticity and, as a direct consequence, reduced load-bearing capacity with age.

Immobilization also promotes the aforementioned changes, and they even become more pronounced. As a result, the length of an immobilized ligament increases more with respect to its load. The stability of the affected joint decreases as a result. The capsule becomes smaller due to changes in the membrana fibrosa, reducing mobility and range of motion.

Interestingly, the body has built-in protection mechanisms that usually prevent reaching the maximum load limits of connective tissue structures. According to studies, physiological everyday movements of the knee joint utilize only about 10% of the maximum load capacity of the anterior cruciate ligament and only about 30% of the ligament patella.

Knee joint capsule posterior, Henry Vandyke Carter Henry Gray, Gray346, labeled as public domain, details on Wikimedia Commons

What our joint capsule needs

During movements of a joint – and therefore the capsule – oxygen and nutrients travel from the extracapsular vessels to the intracapsular vessels of the membrana synovialis. They then enter the synovial fluid through diffusion and osmosis. The movement and constant alternation between tension and relaxation facilitate the transport of substances from the vessels into the joint interior and back. The ligaments need stress stimuli, which arise with each movement. Without these – e.g., after prolonged immobilization – the load-bearing capacity of a ligament decreases rapidly. Whether the load-bearing capacity of a healthy ligament can be improved at all remains unclear.

Hip joint capsule ligaments, Henry Vandyke Carter Henry Gray, Gray340, labeled as public domain

Training

Whether and to what extent training directly affects the quality of a healthy capsule and its ligaments is controversial/questionable. Studies have shown that training only slightly influences the thickness of ligaments and their stability. However, there is no doubt about the devastating effect of immobilization with massively reduced load-bearing capacity and reduced quality of the ligaments!!

As we know from our previous blogs – movement with loading and unloading promotes various positive effects: Blood circulation is increased, synovial fluid is produced more, the exchange of substances is promoted and improved, which all leads to an improvement in the quality of the functional unit. The improvement of technique and coordination also promotes proprioception with the neural system.

Henry Vandyke Carter creator QS:P170,Q955620 Henry Graycreator QS:P170,Q40319, Gray355, labeled as public domain, details on Wikimedia Commons

Therapy

It is assumed that stimulation of the capsule receptors reflexively leads to pain relief and a decrease in sympathetic activity in the central nervous system. This causes a normalization of physiological processes. Additionally, the treated joint's mobility is improved. In manual therapy as well as active exercise forms, the joint receptors are stimulated.

Furthermore, recent studies show that the formation of scar tissue is linked to a lack of physiological stress stimuli during the healing phases. It is important to control the stress stimuli throughout rehabilitation to prevent new ruptures. This is because the load-bearing capacity of the capsule and ligaments, as well as their proprioception, is significantly reduced!

The physiotherapists and osteopaths at BodyLab are aware of the anatomical and physiological conditions and know the therapeutic options available in case of injuries or complaints/problems. We are happy to advise and instruct you regarding training and exercise possibilities. If this is not yet (after operations or injuries) or no longer possible, the quality and function of the joint cartilage as a functional unit can be improved, and pain and complaints can be reduced, through passive joint techniques, sometimes under compression or traction.

Once again: Life means movement!

If you need us, we are happy to be there for you!

Your BodyLab Team – Your connective tissue specialists

Osteopathy and Physiotherapy | Rehabilitation and Training

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Image Credit

Henry Vandyke Carter Henry Gray, Gray334, labeled as public domain, details on Wikimedia Commons