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.

Cartilage (articular cartilage)

Cartilage is part of the passive musculoskeletal system. It is usually not vascularised and is supplied by diffusion when pressure changes during movement. It serves as a pressure-elastic and bending-elastic buffer. As such, it forms the hyaline cartilage that covers articulating bone areas. Physiologically, the articulating bone ends are covered with vascularised hyaline cartilage, which has a coefficient of friction that cannot be achieved with regular technical mass processes (coefficient of friction 0.003 to 0.02 and thus a power of ten below that of commercially available high-quality hydrodynamically lubricated bearings). The hyaline cartilage consists of around 90% chondrocytes and around 10% extracellular matrix, ECM (intermediate fibre and filler substances: unstructured basic substance and organised collagen fibre network). The collagen fibre structures form a dense three-dimensional network that can withstand tension. Proteoglycans (glucosaminoglucans, chondroitin sulphate, keratan sulphate) are integrated into this
by hyaluronic acid to form complexes. These are highly water-binding and therefore generate good pressure resistance. While the matrix constantly produced by the chondrocytes has a turnover of a few hundred days, that of the basic substance is 200-400 years. After the end of growth, it loses the ability to mitotic division, which is why the ability to regenerate and heal is extremely limited.

The fibre structure forms domains that deform under pressure and squeeze fluid into less stressed neighbouring domains. As this squeezing and redistribution process takes a certain, albeit short, amount of time, the dynamic properties of the cartilage are time-dependent: under very rapid loading, it becomes extremely hard, which equates to reduced elasticity and increased vulnerability.

Prolonged pressures in the same position are particularly damaging to the cartilage. In the short term, the cartilage in defect areas can reversibly „hypertrophy“ due to high-grade water binding, but this is reversible within 30 s after loading. The load-bearing capacity of the cartilage is in the order of 220 / cm² or about 220 bar, which is far from being achieved even with competitive running loads such as 10,000 m running with a magnitude of 50 kp/cm. If there are no other favourable factors, arthrosis as a result of many years of running can therefore be ruled out.

The cartilage is primarily nourished by diffusion during pressure changes, which is why a healthy level of movement with larger radii of motion is essential for the health of the cartilage. Secondarily, it is also supplied by the subchondral vessels of the underlying bone. If the vessels sclerose, usually as a result of tears, a source of nutrition is lost.

The cartilage is protected in the best possible way through physiological movement. The muscular
proprioceptors also contribute to this.

Physiologically, these are made of hyaline cartilage; if this is permanently damaged as part of arthrosis, it can be replaced by fibrous cartilage.

The cartilage types in detail:

elastic cartilage

The elastic cartilage is not found in the movement apparatus, but in the external auditory canal, the auricle and the auditory tube, the epiglottis and the small bronchi. It is rich in cells and contains elastic fibres, which are fibrillin fibres associated with elastin. Elastic cartilage is yellowish in colour, slightly elastic and can be deformed by pressure and does not tend to calcify.

hyaline cartilage

Hyaline cartilage is the most common type of cartilage in the human body. It is found not only in the cartilage coverings of the articulating bones, but also in the nose, larynx, trachea, bronchi and ribs. It is bluish-white transparent and, in addition to very low friction, offers a very good combination of elastic shock absorption and robustness. It is not permeated by nerves or vessels (arteries, veins, lymph) and consists of a matrix in which chondrocytes are embedded that hardly proliferate. Before the end of the growth phase, the cartilage is still supplied arterially from the subchondral bone, after which it is only supplied by diffusion during pressure changes. Hyaline cartilage contains approx. 90% extracellular matrix (collagen and non-collagen) and 1-10% chondrocytes. The extracellular matrix contains a high proportion of water due to the high water-binding capacity of the highly sulphated proteoglycans (consisting of chondroitin sulphate, hyaluronic acid and keratan sulphate). Aggrecan with more than 100 chondroitin sulphate and keratan sulphate chains is the most frequently occurring proteoglycan. The largest proportion of collagen is type 2, followed by types 5, 6, 9 and 11. The collagen network has limited elasticity and, in conjunction with the high proportion of bound water, creates the required combination of elasticity and compressive strength. During prolonged compression, water is pressed into neighbouring domains, which reduces the cartilage thickness and makes the cartilage more susceptible. However, it fills up again after the load is removed. The hyaline articular cartilage is divided into 4 zones, from superficial to deep:

  • tangential zone
  • Transition zone
  • Radial zone
  • Mineralisation zone

In the superficial tangential zone, the chondrocytes lie flat and are aligned parallel to the surface so that they roughly follow the alignment of the collagen fibres. Collagen fibrils predominate here, the proportion of chondrocytes and proteoglycans and thus of water is rather low. The surface towards the joint space forms a cell-free layer in the form of collagen fibrils (lamina splendens). The more profound transition zone tends to have round chondrocytes and a higher proteoglycan content. In the radial zone, columnar chondrocytes are then present, arranged orthogonally to the surface of the cartilage, the collagen fibres are arranged parallel to it. The radial zone is separated from the following calcified mineralisation zone by the „tide mark“, a zone with a presumably high proteoglycan content, in which the extracellular matrix contains many calcium phosphate crystals, which ensures good pressure transmission to the subchondral bone. The hyaline cartilage can withstand compressive forces up to 5 or 7 times the body weight and transmit them to the bone.

Fibrous cartilage

Fibrocartilage is a taut type of rather cell-poor cartilage with a high proportion of collagen type 1 and 2, which makes it very resistant to compression and tension. The mechanical resilience is lower than that of the complex hyaline cartilage. Fibrocartilage develops from connective tissue in which fibroblasts differentiate into chondrocytes under pressure and tensile stimuli. The collagen fibres are present in bundles, with chondrocytes between them in rows. Fibrocartilage occurs in the following places:

Fibrous cartilage is particularly suitable where shear forces occur; it is less suitable for tensile and compressive forces