the synovium is the synovial membrane lining the joint space between two articular surfaces. it consists of a bursa that surrounds and seals the joint. within it, synovial fluid serves as a lubricant, chondrocytes nutrient delivery system, and waste removal system.
contents
synovial capsule membranes secrete hyaluronic acid, which forms the proteoglycan aggrecan that allows for water retention, and lubricin, a boundary-layer lubricant. both reduce friction between opposing cartilage in joints.
aggrecan has highly negatively charged moieties that attract water:
in addition, macrophages/monocytes are the main cell type. they remove waste in the fluid and joints by phagocytosis. there should be almost no blood or neutrophils.
removal
synovial fluid is naturally drained through interstitial diffusion that eventually leads to lymphatic drainage.
normal intrasynovial pressure
at rest, joints are collapsed because of the subatmospheric pressure within the joints. this pressure is maintained by Starling forces and lymphatic drainage that maintain interstitial filtration pressures.
cracking the joints actually reduces the pressure further, forcing dissolved gases within the synovial fluid to rapidly diffuse down the lower pressure gradient. consequently, a bubble forms and collapses, producing the clicking sound you no doubt hear from me regularly.
normal range of motion does not significantly increase intrasynovial pressure because of the compliance provided by the synovial fluid.
abnormal intrasynovial pressure
this normally results from either mechanical trauma or inflammation. either can trigger macrophages to release cytokines that initiate the inflammatory process.
this leads to vasodilation, edema, and neutrophilic infiltration. hyaluronic acid depolymerizes from the increase of reactive oxygen species from neutrophils, decreasing viscosity. the accumulation of cells and decrease in synovial fluid results in an increased concentration that contributes to high intrasynovial pressure. an inflamed joint has decreased compliance to pressure:
this decreased compliance leads to synovial effusion. this effusion still stays within the capsule, but can overwhelm lymphatic drainage. as pressure increases and drainage decreases, the synovium becomes cyst-like.
this synovial cyst can eventually build enough pressure up to rupture and decompress.
a Baker’s cyst is the accumulation of joint fluid behind the knee. if the joint capsule is displaced, it can herniate behind the knee. it is commonly associated with a tear in the meniscal cartilage or degenerative rheumatoid arthritis. it can be asymptomatic.
