A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous

joint capsule In anatomy, a joint capsule or articular capsule is an envelope surrounding a synovial joint.periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulating surfaces. This joint unites long bones and permits free bone movement and greater mobility. The synovial cavity/joint is filled with synovial fluid. The joint capsule is made up of an outer layer of fibrous membrane, which keeps the bones together structurally, and an inner layer, the

synovial membrane The synovial membrane (also known as the synovial stratum, synovium or stratum synoviale) is a specialized connective tissue that lines the inner surface of capsules of synovial joints and tendon sheath A tendon sheath is a layer of synovial m ...

, which seals in the synovial fluid. They are the most common and most movable type of

joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw ...

in the body of a mammal. As with most other joints, synovial joints achieve movement at the point of contact of the articulating

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...

Structure

Synovial joints contain the following structures: * Synovial cavity: all diarthroses have the characteristic space between the bones that is filled with synovial fluid *

Joint capsule In anatomy, a joint capsule or articular capsule is an envelope surrounding a synovial joint.fibrous membrane that may contain ligaments and (2) the inner

that secretes the lubricating, shock absorbing, and joint-nourishing synovial fluid; the joint capsule is highly innervated, but without blood and lymph vessels, and receives nutrition from the surrounding blood supply via either diffusion (a slow process) or by

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...

, a far more efficient process achieved through exercise. * Articular cartilage: the bones of a synovial joint are covered by this layer of hyaline cartilage that lines the epiphyses of joint end of bone with a smooth, slippery surface that does not bind them together; articular cartilage functions to absorb shock and reduce friction during movement. Many, but not all, synovial joints also contain additional structures: * Articular discs or menisci - the

fibrocartilage Fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions. It owes its inflexibility and toughness to the former of these constituents, and its elasticity to the latter. It is the only type of ...

pads between opposing surfaces in a joint * Articular fat pads -

adipose tissue Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular ...

pads that protect the articular cartilage, as seen in the infrapatellar fat pad in the knee *

Tendons A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...

- cords of dense regular connective tissue composed of parallel bundles of collagen fibers * Accessory ligaments (extracapsular and intracapsular) - the fibers of some fibrous membranes are arranged in parallel bundles of dense regular connective tissue that are highly adapted for resisting strains to prevent extreme movements that may damage the articulation * Bursae - saclike structures that are situated strategically to alleviate friction in some joints (shoulder and knee) that are filled with fluid that is similar to synovial fluid The bone surrounding the joint on the proximal side is sometimes called the ''plafond'', especially in the talocrural joint. A damage to this occurs in a Gosselin fracture.

Blood supply

The blood supply of a synovial joint is derived from the arteries sharing in the

anastomosis An anastomosis (, plural anastomoses) is a connection or opening between two things (especially cavities or passages) that are normally diverging or branching, such as between blood vessels, leaf veins, or streams. Such a connection may be normal ...

around the joint.

Types

There are seven types of synovial joints. Some are relatively immobile, but are more stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury. In ascending order of mobility, they are:

Function

The movements possible with synovial joints are: *

abduction Abduction may refer to: Media Film and television * "Abduction" (''The Outer Limits''), a 2001 television episode * " Abduction" (''Death Note'') a Japanese animation television series * " Abductions" (''Totally Spies!''), a 2002 episode of an ...

: movement away from the mid-line of the body *

adduction Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relati ...

: movement toward the mid-line of the body * extension: straightening limbs at a joint * flexion: bending the limbs at a joint *

rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

: a circular movement around a fixed point

Clinical significance

The ''joint space'' equals the distance between the involved bones of the joint. A ''joint space narrowing'' is a sign of either (or both)

osteoarthritis Osteoarthritis (OA) is a type of degenerative joint disease that results from breakdown of joint cartilage and underlying bone which affects 1 in 7 adults in the United States. It is believed to be the fourth leading cause of disability in the ...

and inflammatory degeneration. The normal joint space is at least 2 mm in the hip (at the superior

acetabulum The acetabulum (), also called the cotyloid cavity, is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint. Structure There are three bones of the ''os coxae'' (hip bone) tha ...

), at least 3 mm in the

knee In humans and other primates, the knee joins the thigh with the human leg, leg and consists of two joints: one between the femur and tibia (tibiofemoral joint), and one between the femur and patella (patellofemoral joint). It is the largest join ...

, and 4–5 mm in the shoulder joint., in turn citing: For the temporomandibular joint, a joint space of between 1.5 and 4 mm is regarded as normal. Joint space narrowing is therefore a component of several

radiographic classifications of osteoarthritis Radiographic systems to classify osteoarthritis vary by which joint is being investigated. In osteoarthritis, the choice of treatment is based on pain and decreased function, but radiography can be useful before surgery in order to prepare for the ...

. In

rheumatoid arthritis Rheumatoid arthritis (RA) is a long-term autoimmune disorder that primarily affects synovial joint, joints. It typically results in warm, swollen, and painful joints. Pain and stiffness often worsen following rest. Most commonly, the wrist and ...

, the clinical manifestations are primarily synovial inflammation and joint damage. The fibroblast-like synoviocytes, highly specialized mesenchymal cells found in the

, have an active and prominent role in the pathogenic processes in the rheumatic joints. Therapies that target these cells are emerging as promising therapeutic tools, raising hope for future applications in rheumatoid arthritis.

References

Tortora & Derrickson () ''Principles of Anatomy & Physiology'' (12th ed.). Wiley & Sons Rogers, Kara (2010) ''Bone and Muscle: Structure, Force, and Motion'
p.157
/ref> Sharkey, John (2008) ''The Concise Book of Neuromuscular Therapy'
p.33
/ref> Moini (2011) ''Introduction to Pathology for the Physical Therapist Assistant'
pp.231-2
/ref> Bruce Abernethy (2005) ''The Biophysical Foundations Of Human Movement'' pp.23
331
/ref> {{DEFAULTSORT:Synovial Joint Joints