Synovium

For Young Learners

Structure and Location

Inside your knee there is a sealed space, and that space is lined with a thin membrane called the synovium. It is only a few cells thick in places.

The lining does not cover the smooth cartilage surfaces. It wraps around everything else: the inside of the capsule, the folds and pockets, and even sleeves around the ligaments in the middle of the joint.

Function

The synovium makes synovial fluid. There is normally only a small amount, a couple of millilitres, roughly a teaspoon at most. It is thick, clear and slightly yellow, and it feels like egg white.

The fluid does two things. It makes the surfaces slippery, and it feeds the cartilage, which has no blood supply of its own.

When a knee is injured or irritated, the lining makes much more fluid than usual. That is the swelling people feel. The swelling is the lining reacting, rather than the damage itself.

For Patients

Structure and Location

The synovial membrane lines the inside of the joint capsule and every surface within the joint except the articular cartilage and the exposed surfaces of the menisci. It forms folds and recesses, including a large pouch above the kneecap that extends several centimetres up the thigh.

It also forms a sleeve around the cruciate ligaments, which is why those ligaments sit inside the joint but outside the fluid filled space.

Within the joint there are folds called plicae, which are remnants of development. They are present in many people and usually cause no trouble.

Function

The membrane produces synovial fluid, normally one to four millilitres in a healthy knee. The fluid lubricates the joint and carries nutrients to the cartilage, which has no blood supply.

The membrane itself is well supplied with blood vessels and nerves, in contrast to the cartilage it serves.

Clinical Relevance

Almost all knee swelling is synovial fluid, produced in excess by an irritated lining. The timing is informative. Swelling within a few hours of injury suggests blood in the joint, which points to a more significant structural injury. Swelling over a day or two suggests the lining reacting.

Because the lining is richly supplied with nerves, it is a genuine source of pain, unlike cartilage. Much of the discomfort in a swollen or arthritic knee comes from here.

The large pouch above the kneecap is why a swollen knee looks full at the front and above, and why clinicians sweep downwards over that area when testing for fluid.

Conditions affecting the synovium are covered in the conditions section.

For Medical Students

Structure and Location

The synovial membrane lines the fibrous capsule and all intracapsular surfaces other than articular cartilage and the free surfaces of the menisci. It is one to three cell layers thick, comprising type A macrophage like synoviocytes and type B fibroblast like synoviocytes, overlying a vascular subintima.

The suprapatellar bursa communicates with the joint in most adults, extending the cavity several centimetres proximally. The membrane reflects around the cruciate ligaments, rendering them intracapsular but extrasynovial.

Developmental folds persist as plicae, most commonly suprapatellar, medial and infrapatellar.

Function

Type B synoviocytes secrete hyaluronan and lubricin, which confer boundary lubrication. Type A cells are phagocytic and clear debris. Normal synovial fluid volume in the knee is approximately one to four millilitres, is a dialysate of plasma with added hyaluronan, and is acellular to near acellular.

The fluid provides the sole route of nutrition for articular cartilage, exchange being driven by cyclical loading.

Clinical Relevance

Effusion represents increased synovial fluid production and is the common final pathway for intra-articular pathology of almost any cause. Onset within two hours implies haemarthrosis and raises the probability of cruciate rupture, osteochondral fracture or peripheral meniscal tear. Onset over twenty four hours implies a reactive effusion.

Aspiration is both diagnostic and therapeutic. Fat globules in a haemarthrosis suggest an intra-articular fracture. Cell count, Gram stain, culture and polarised microscopy distinguish septic from crystal and inflammatory causes.

The richly innervated and vascularised synovium is a principal source of pain and of the inflammatory mediators that drive arthrogenic muscle inhibition, which is why effusion control is an early rehabilitation target.

Synovial pathology itself is covered in the conditions section.