movement physiology: joint

yogabook / movement physiology / joint

A joint (lat. articulatio, abbr. art., plural art.) connects a bone with one or more other bones.

synovial joint

True joints (diarthrosis, articulatio synovialis) are those in which the articulating bones have a hyaline cartilage covering for low-wear gliding or rolling and the joint is hermetically sealed with a two-layer joint capsule, whose inner layer, the synovialis (also: synovial membrane), produces a lubricating fluid (synovia) on which the cartilage glides. The synovium can both produce and absorb synovia. The outer layer (membrana fibrosa) provides mechanical protection with taut connective tissue. The membrana fibrosa can have capsular ligaments, which are ligaments connected to the capsule or ligament-like thickenings of the capsule. In addition to capsular ligaments, ligaments can also be extracapsular (e.g. lateral collateral ligament of the knee joint) or intracapsular not connected to the capsule, such as the cruciate ligaments of the knee joint. These are then usually also surrounded by a synovium, which is connected to the capsule. The capsules of the joint lie loosely against the bones with reserve folds to allow the entire ROM. Inflammation in the joint can lead to an overproduction of synovia, haemorrhages to a haemarthrosis, which requires immediate treatment as the blood damages the cartilage.

synarthroses

There are several types of false joints (synarthrosis, adhesions):

  1. Synsarcosis: bones connected by muscles, in humans this is only the scapulothoracic gliding bearing
  2. Bone fusions as with coccyx and sacrum
  3. cartilaginous connections (Articulationes cartilagineae):
    • Synchondroses (bones connected via hyaline cartilage), such as rib cartilage, sternal bone
    • Symphysis (pubic symphysis)
  4. connective tissue connections (articulationes fibrosae)

Capsule (joint capsule)

Part of the passive musculoskeletal system that forms the outer boundary of a joint and encloses the articulating bone ends.
With its innermost layer, the synovia (synovial membrane), the joint capsule forms the synovial fluid of the same name, a fluid with a high hyaluronic acid content whose viscosity changes adversely with decreasing temperature.
The outer layer (membrana fibrosa) consists of staffy collagenous tissue and is connected to the periosteum of the bone. The joint capsule contains Ruffini corpuscles, Vater-Pacini corpuscles and Golgi tendon organs, which serve as sensors for proprioception. Free nerve endings serve as nociceptors for pain perception. Some joint capsules are reinforced in strips to form capsular ligaments (ligamenta capsularia). In addition to the limitation of movement by the capsule, there may be other ligaments both outside (extracapsular) and inside (intracapsular) the joint capsule. If more than two bones are involved in a joint, as in the case of the wrist, knee or ankle joint, the synovial membrane can delimit individual spaces. Capsules can be damaged by trauma and may not guide the bones sufficiently in the long term, resulting in joint instability. In these cases, there is often also a torn muscle fiber or torn ligament. If the capsule tears as a result of trauma, this can lead to hemarthrosis (bleeding into the capsule). Inflammatory processes can lead to swelling of the ligaments and loss of flexibility. The synovial membrane often reacts to injuries to the capsule by producing more synovial fluid.

Capsular ligament

A capsular ligament is usually a strip-shaped fibrous reinforcement of a joint capsule, i.e. a part of the joint capsule designed as a ligament.

Capsulitis (capsulitis)

An inflammatory change in the joint capsule that can potentially occur in various joints, associated in particular with pronounced pain and restricted flexibility. The most common is capsulitis adhaesiva of the shoulder joint (periarthropathia humeroscapularis). The hip joint is the second most commonly affected after the shoulder joint.

Ligament

A fibrous, barely stretchable strip of connective tissue that limits the relative movement of movable parts of the human musculoskeletal system. The main component of ligaments is collagen. Ligaments are found in the joints to set firm-elastic limits of movement between bones in joints and thus define the maximum geometric range of motion, which is then more or less restricted by muscles in the sense of soft-elastic limits of movement. They also act as retinaculi (restraining ligaments) to keep tendons close to joints or bones so that they – and their muscles – do not tend to run across three-dimensional space in the shortest possible distance. Retinaculi also limit the flexibility of the patella in the knee joint, as the patella would have a strong tendency to dislocate if the quadriceps were not tensed. If ligaments are subjected to excessive tensile stress, they become overstretched or even tear or rupture.

An overview of the ligaments, sorted according to the joint to which they belong, can be found here, else see the corresponding joint.

movements

Joints generally allow 1, 2 or 3-dimensional movements. Each dimension of movement (also: degree of freedom) contains two opposing directions. The movement dimensions together span the movement space (traffic space, ROM). The mobility in the individual dimensions is specified according to the neutral zero method, usually starting with flexion and ending with extension. If both are possible, there is a zero in the centre, e.g. 150° – 0° – 5° for 150° flexion and 5° extension or hyperextension in the knee joint. If there is a mobility restriction of any kind, a zero is shown at the end of the affected side and the achievable value in the centre, e.g. 150° – 10° – 0° for an extension deficit of 10° in the knee joint.

Dimension of movement / movement dimension

The entire range of movement (ROM, formerly also called traffic space) of a joint can be represented with independent dimensions so that each position has unique coordinates. A movement therefore corresponds to a curve in this space. A certain standard coordinate system is common, whose axes are given by vectors on the directions: frontal, cranial, lateral (dexter), which correspond to normals on the frontal plane, transverse plane and sagittal plane. Typically, the ROM is a convex set.

Limit of movement

The limit of movement is the point and type of stop of a movement based in the joint or the muscles spanning it, see also under terminal:

  1. soft-elastic: muscular
  2. firm-elastic: ligamentous
  3. hard-elastic: the bone or the cartilage covering of the bones

Terminal / end-degree (movement, position)

A joint position is called terminal when no further movement is possible in its direction, seen from the center of the ROM. This direction in which the movement limit occurs does not have to correspond strictly to a single movement dimension, but can be a combination of several. This definition is independent of the number of movement dimensions of a joint, so it applies to both the one-dimensionally mobile elbow joint and the three-dimensionally mobile glenohumeral joint or hip joint. This definition also does not require an exact breakdown of the movement into the movement dimensions. The limit of movement can be set physiologically by different structures and is perceived by the examiner in different ways during a passive examination:

  1. Muscles: soft-elastic end feeling (e.g. restriction of hip flexion with the knee joint extended by the hamstrings)
  2. Ligaments: firm-elastic end feeling (e.g. limitation of inversion and eversion of the ankle complex by various ligaments)
  3. the bony joint structure, i.e. the cartilage of the articulating joint partners: hard-elastic end feeling (e.g. touchdown of the olecranon to the humerus in the elbow joint on full extension)

The soft-elastic case, i.e. the limitation of movement by muscles, can be influenced to a significant extent by training, for example by stretching training, which increases the flexibility of the muscles through (positive) longitudinal muscle adaptation. On the other hand, unfavourably selected prolonged immobilization can lead to negative longitudinal muscle adaptation and a reduced number of serial sarcomeres and thus to reduced muscle flexibility. On the other hand, a muscle can adapt to heavy loads and increase in strength and circumference, which, depending on how it is performed, can also reduce the flexibility of the muscle. However, any type of sports training should not attempt to change the non-muscular limits: in the case of cartilage, this would by definition result in osteoarthritis/arthrosis; in the case of ligaments, it would result in ligament laxity, hyperextensibility or ligamentary insufficiency, which also is a predisposition to osteoarthritis/arthrosis due to the excessive displacement of the bones in relation to each other in terms of extent or type.

Firm-elastic

a flexibility limit in a joint that is given by ligaments is referred to as Firm-elastic, see also under final position.

hard-elastic

A limit of movement in a joint that is given by bones or their cartilage cover is referred to as hard-elastic, see also under end-degree position.

Soft-elastic

A limit of flexibility in a joint that is given by muscles is referred to as soft-elastic, see also end-degree position.

Hypermobility (joint)

Hypermobility of a joint is an angular excess flexibility in one or more movement dimensions of a joint. By definition, it is not instability and is not pathogenic, but can predispose to the development of instability. The term hypermobility is often used carelessly. Physiological flexibility in the joints is stated inconsistently by anatomists, but hyperextension of the knee and elbow joints is usually considered physiological up to a certain degree, which is stated differently by the authors. The diagnosis of hypermobility (excessive flexibility) is often made lightly and identified as the cause of existing complaints. In the absolute majority of cases, however, it is not a case of genuine hypermobility in the sense of a hypermobility syndrome – such cases are very rare and frequently occur in the context of Ehlers-Danlos syndrome or Marfan syndrome – but only of flexibility that exceeds the population average and is not in itself pathological or pathogenic.

instability (joint)

Congenital or developed ability of a joint to move in a dimension other than the physiological dimensions of movement. The cause is usually an insufficiency of the passive musculoskeletal system, i.e. the ligaments and the joint capsule. It can be caused by trauma, a lack of training stimuli (underuse), degenerative processes (causes: inadequate trophism, lack of training stimuli, overuse, physiological ageing) or pain (interferes with proprioception and therefore also with joint control) or be the result of illness or injury (causes: iatrogenic in arthoscopies, surgical intervention; everyday trauma, sports injuries). Instability is always pathological and usually also pathogenic. In the case of the knee, for example, insufficiency of the collateral ligaments can lead to varus and valgus movements in the knee joint as well as to transverse translation of the tibia in relation to the femur. In the first case, even more so than in the second, articulation is partially lost, meaning that the joint surfaces have noticeably to dramatically reduced contact, which leads to significantly increased stress on the bones and their cartilage coverings. This is a cardinal difference to hypermobility, in which the angular dimension of one or more physiological movement dimensions is increased, but the movement still takes place (at least to a large extent) on the intended cartilage surfaces.

Depending on the author, the term instability described above is also referred to as laxity and the term instability also requires that the affected person also subjectively has an unstable feeling in the joint.

Hyperextension

Hyperextension of a joint beyond the 180° angle, which is present in neutral zero. This can occur, for example, in the knee joint, elbow joint, metacarpophalangeal joints or other finger joints. In the toe joints, further extension than the stretched angle in standard anatomical position is part of the rolling movement of the foot when walking and is physiological. Any hyperextension can be associated with non-muscular discomfort, which must be interpreted as a sign of unphysiological stress on joint structures, which requires the position of the joint to be reduced at least to such an extent towards 180° that the discomfort disappears. A certain degree of hyperextensibility, particularly of knee joints and metacarpophalangeal joints, is common, especially in women. This can be due, among other things, to postural habits, which are partly due to the muscular equipment and adaptation to it, see also the FAQ. In other cases, a tendency to hyperextend is anatomically favored, such as hyperextension of the knee joints in the case of a „minus heel“, i.e. a flat heel bone. Pathologically hyperextensible knee joints are referred to as genu recurvatum, elbow joints as cubitus recurvatum.

In some cases, hyperextension of the elbow joint causes mediodorsal discomfort in the transition to the sulcus ulnaris.

Congruence

The property of two articulating bones or their cartilage coverings to fit together exactly, i.e. to have contact on a large part of the entire cartilage surface in every joint position.

The opposite of congruence is incongruence. Some joints are physiologically incongruent, but physiologically congruent joints can also become pathologically incongruent.

Incongruence

The property of two articulating bones or their cartilage coverings not to fit together exactly, i.e. in each joint position they only have contact on a small partial area of the entire cartilage surface and not in large areas. The knee joint, for example, is clearly incongruent with the strongly rounded bicondylar femur and the almost flat tibial plateau. There is far less incongruence in the hip joint, in which the surfaces of the two bones are minimally separated from each other in all directions, possibly due to the resulting better lubrication from the contact surface of the acetabulum with the femoral head. Apart from acquired cases, incongruence can also be congenital.

Incongruence arthrosis/osteoarthritis

An arthrosis/osteoarthritis that develops on the basis of incongruent joint partners that are physiologically congruent. Possible causes for this are

  1. subluxations, made possible, for example, by ligament insufficiencies (instabilities of the joint)
  2. In the case of the shoulder joint: rotator cuff lesions and ruptures. These in turn cause subluxations
  3. Traumas
  4. Incorrectly healed fractures that pass through the joint surfaces
  5. Fractures that dislocate the joint partners

Incongruence arthrosis is common in the ankle joint following injuries to the tibiofibular syndesmosis.