Synovium

The synovium (plural: synoviums, not synovia) or synovial membrane is a soft tissue lining of joints bursae and tendon sheaths and seems to be responsible for the secretion and absorption of synovial fluid components, whereby the synovial fluid arises from leakage of the intimal capillaries ^1-4.

Gross anatomy

The synovial membrane consists of an intimal layer and a thicker layer of loose connective tissue, the subintima, which contains fewer cells and is composed of fatty, fibrous or loose areolar tissue. The intima is well innervated and vascularizedvascularised containing different types of cells including fibroblasts, macrophages, adipocytes, nerve fibres, vascular endothelial cells as well as lymphocytes ^1-4.

The synovium forms the inner layer of the joint capsule in diarthrodial joints and faces the tendon within the tendon sheath. It forms the inner lining in bursae.

Histology

The synovial intima features two types of synoviocytes: macrophagic (type A) and fibroblastic (type B) cells. The macrophagic type is mobile being able to remove and degrade waste and cell debris. The fibroblast-like type possesses dendritic processes forming a network within the surface of the synovial membrane and are involved in the production of matrix components such as hyaluronan, fibronectin and collagen ³.

Radiographic features

The synovium will be mostly imaged and assessed with MRI and ultrasound in the setting of inflammatory conditions such as arthritis, tenosynovitis and bursitis in which it becomes thickened and increased amounts of synovial fluid will be present.

Ultrasound

The normal synovium should neither show hypoechoic synovial hyperplasia nor vascularity on colour and power Doppler, not even in the presence of effusion ⁵.

Conversely, ultrasound will show hypoechoic synovial hyperplasia and increased vascularity on colour or power Doppler in the setting of synovitis ⁵.

MRI

Suggested MR imaging for assessment of synovial disease include T1 weighted images before and after intravenous contrast as well as T2 weighted fat saturated or STIR images in at least two different planes.

Signal characteristics of normal synovium are ⁴:

• T1: intermediate

• T2: hyperintense

• T2FS/PDFS: hyperintense

Contrast administration facilitates the differentiation between the synovial fluid and the synovium, which thickens and enhances avidly in case of inflammation or synovitis.

Development

Within the joints, the synovial membrane is last fully differentiated and the least specializedspecialised if compared by the skeletal elements and capsular structures.

History and etymology

The word synovium is derived from ‘synovia’, which is thought to have been coined by Paracelsus ^ref.

Related pathology

The following pathologies and diseases are related to the synovial membrane ⁴:

• synovitis

• osteoarthritis

• rheumatoid arthritis, juvenile idiopathic arthritis, septic arthritis

• crystal deposition disease

• pigmented villonodular synovitis,

  siderotic synovitis

• synovial sarcoma

• synovial hyperplasia

• tenosynovial giant cell tumour

See also

• synovitis