Corpus: Joint

An articulation is a movable connection between two or more bony or cartilaginous skeletal elements.

The study of joints is called arthrology.

2.1. ...according to joint type

The joints of the human body can be divided into "true" (discontinuous) and "non-genuine" (continuous) joints.

2.1.1. Real joints (diarthroses)

True joints or diarthroses or synovial joints have a discontinuity (interruption) between the bones involved in the joint. This gap is called the joint space. It separates the articular surfaces, which are covered by articular cartilage and referred to as the joint head and socket. From the outside, the joint is surrounded by a tight joint capsule. It may have reinforcements in some places, which are known as joint or capsular ligaments. The joint capsule consists of two layers:

• an outer layer, the membrana fibrosa, made of tight, collagenous connective tissue. This layer merges into the periosteum.
• an inner layer, the synovial membrane or synovial membrane, which has an epithelial-like appearance.

The joint capsule defines an all-round enclosed cavity, the joint cavity, which is filled with a viscous fluid (synovia).

Examples of real joints are:

• Upper ankle joint (articulatio talocruralis)
• Lower ankle joint (articulatio subtalaris and articulatio talocalcaneonavicularis)
• Knee joint (articulatio genus)
• Shoulder joint (articulatio humeri)
• hip joint (articulatio coxae)
• and many more

Real joints often have auxiliary structures that support their function. These include

• Ligamenta (ligaments)
• Disci or menisci articulares (inter-articular discs)
• Labra articularia (joint lips)
• Bursae synoviales (synovial bursa)
• Recessus articulares (joint pockets)

Amphiarthroses are a special type of true joint. These are real joints, but their mobility is severely restricted.

2.1.2. Non-genuine joints (synarthroses)

Non-genuine joints or synarthroses are continuous cartilaginous or connective tissue bone connections that have no interruption (gap) and therefore only have limited mobility. However, they play an important role as growth zones in the body. They include

• Cartilaginous joints (articulationes cartilagineae)
  • Synchondroses: connection via hyaline cartilage, e.g. on the sternum
  • Symphysis: connection via fibrous cartilage, e.g. symphysis pubica, symphysis intervertebralis
• Connective tissue joints (articulationes fibrosae)
  • Syndesmoses: ligamentous, e.g. syndesmosis tibiofibularis
  • Gomphoses: wedging, e.g. teeth in the tooth socket (articulatio dentoalveolaris)

Synarthroses are only preserved if they are prevented from ossifying by constant movement, otherwise a synostosis develops. For example, the connective tissue sutures between the skull bones ossify with increasing age.

If shear stresses cause gaps to form in symphyses, this is referred to as hemiarthrosis (e.g. unco-vertebral joint).

2.1.3. Pathological joints (pseudarthroses)

False joints (pseudarthroses) are a special case. They are not joints in the proper sense, but more or less mobile, pathological interruptions of the normal bone structure that can occur in the event of inadequate fracture healing.

2.2. ...according to joint shape or mobility

The true joints can be categorised according to the shape of the joint surfaces:

• Spherical joint (articulatio spheroidea)
  • nut joint (enarthrosis spheroidea)
• Hinge joint (articulatio cylindroidea or ginglymus)
• Angular joint (trochoginglymus)
• Saddle joint (articulatio sellaris)
• Condylar joint (articulatio condylaris)
• Swivel joint (articulatio trochoidea)
  • Wheel joint
  • Cone joint
• Egg joint or ellipsoid joint (articulatio ellipsoidea)
• Flat joint (articulatio plana)

2.3. ...by structure

• Articulatio simplex: Simple joint between two bones
• Articulatio composita: Composite joint between several bones and with more than two joint parts.

2.4. ...according to degrees of freedom

• Uniaxial joints (e.g. hinge joints)
• Biaxial joints (e.g. egg joints, saddle joints)
• Triaxial joints (e.g. ball joints)

2.5. ...according to biomechanical significance

• Main joint
• Secondary joint

2.6. ...according to development

• Separation joint
• attachment joint

Joints have joint sensors in the joint capsule that transmit position and pain information to the CNS. In addition to the proprioceptors of the surrounding musculature, they serve to control movement and prevent overloading.

Joints are exposed to high biomechanical loads and are therefore frequently affected by injuries to the musculoskeletal system. Traumatic, but also non-traumatic joint damage usually manifests itself as joint pain ("arthralgia") at rest or during exercise, as well as joint swelling.

Inflammation of a joint is known as arthritis, while degenerative, primarily non-inflammatory changes are known as arthrosis.