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Canine joints are also known as articulations, which are the point where two or more bones meet. These may allow movement (e.g. elbow joint) or may provide stability (e.g. skull) depending on the type of joint.
Joint Type Classification; 3 main groups of joints, named due to their characteristic structural features.
- Synovial joints - (previously known as a diarthrosis) facilitates movement. They have a joint space between the adjoining bones (synovial cavity) and include most of the limb joints
- Cartilaginous joint -(previously known as a amphiarthroses) allows a slight movement such as a stretching or compression e.g. between vertebrae and characterised by flattened discs of fibrocartilage connecting the articular surfaces
- Fibrous joint - (previously known as a synarthroses) are immovable joints that include syndesmoses, sutures and gomphoses
Integrating knowledge of the design and functional components of synovial joints to injury types and healing times, empowers clinical decision making for effective treatment pathways.
Types of Synovial Joints
The limb synovial joints permit the most movement. You have a simple joint (2 articular surfaces within a joint capsule) or a compound joint (more than 2 articular surfaces in a joint capsule).
1. Hinge; movement occurs primarily in a single plane and permits flexion and extension, with a very limited degree of rotation, e.g. elbow, stifle, interphalangeal joints
2. Ball and socket; also known as a spheroidal joint and permits movement around 3 axis, e.g. hip and shoulder joint
- Flexion / extension
- Abduction / adduction
- Internal / external rotation (also known as medial and lateral rotation)
The dog has specific design features to ensure the canine ball and socket hip and shoulder joints physiologically function predominantly as a hinge joint, unlike in humans
3. Pivot; also known as a trochoid joint, is when the movement is around the longitudinal axis of the bone, for example the proximal radioulnar joint between the radius and ulna bones
4. Plane; is where the articular surfaces are flat and permit a slight gliding movement
Structures of Synovial Joints
- Articulating surfaces of the bone ends are covered with hyaline cartilage
- Joint cavity filled with synovial fluid. In this cavity exists a negative hydrostatic pressure, which contributes to stabilising the joint
- A pair of collateral ligaments which consist of dense, fibrous tissue and are thickenings of the fibrous joint capsule. These are positioned and aligned on the medial and lateral aspects of the joint to optimise forward saggital plane and stabilise the joint
- A fibrous joint capsule is composed of an inner synovial membrane and an outer fibrous membrane
- Synovial fluid to lubricate the contact surfaces of synovial joints
Joint capsule consists of 2 parts;
The outside fibrous layer;
- Dense connective tissue composed mainly of white fibrous tissue containing some yellow elastic fibres. It provides some stability to the joint and is continuous with the periosteum of the bone
- Mainly composed of collagenous fibres
- Contains afferent pain fibres of the joint capsule
- Insertion of the joint capsule and articular ligaments are similar to Sharpeys' Fibres, where there is a gradual transition to allow for a more even distribution of forces throughout the different tissues
The inside synovial layer;
- This structure is delicate and lines the synovial cavity = synovial membrane
- Synovial membrane is a vascular connective tissue that lines the inner surface of the joint capsule and produces the synovial fluid. It consists of 2 sub layers called the Intima layer and the Subintima layer
- Intima layer is an incomplete cellular lining on the inner layer of the membrane. The cells are called synoviocytes (Type A and B)
- Subintima layer is the deeper layer which can be adipose, areolar or fibrous. This layer contains numerous blood vessels and lymphatics
Articular Cartilage Structure + Function
Hyaline articular cartilage is milky white in colour in thicker areas and is translucent with a bluish tinge in thinner regions.
Hyaline cartilage consists of more specialised Type II collagen than fibrocartilage Type I collagen and is divided into 4 layers; tangential superficial layer, intermediate transitional layer, radiate layer and calcified cartilage layer.
The matrix of hyaline articular cartilage is a complex of collagenous fibrils and ground substance. Collagen fibrils provide strength to the cartilage and the ground substance contains proteoglycans and glycoproteins.
- Articular cartilage facilitates bone on bone movement
- Cartilage is avascular and nutrition occurs by diffusion of the nutrients in the synovial fluid
- Joint movement facilitates this process by creating intermittent pressures that pump fluid through the cartilage
- Cartilage contains no nerve endings and therefore is not capable of pain
Lubrication and shock absorption in a joint
Synovial membrane lubrication is a process called boundary lubrication and relies heavily on the Hyaluronic Acid (HA) in the joint fluid. HA sticks to the surface of the membrane and allows it to slide over other soft tissues in the joint. This is important since the soft tissues form a major part of the frictionless resistance to joint movement.
Cartilage-on-cartilage lubrication is a process independent of HA. Boundary lubrication is active at low loads, but hydrostatic lubrication (also called squeeze film lubrication) replaces this at high loads. The cartilage surfaces are kept apart by a fluid film (made up of joint fluid and interstitial fluid) which weeps from the cartilage.
Shock absorption in joints occurs through the subchondral bone plate and the periarticular soft tissues, not through the cartilage.
Movements of Synovial Joints
Joint movements are powered by muscle contraction of muscles that pass across the joint and are known as active movements.
- Flexion (folding) is movement between 2 or more bones so the angle between them is reduced.
- Extension (straightening) is the movement 2 or more bones where the angle becomes increased.
- Flexion and extension in the dog occurs in the sagittal plane unless it is described as otherwise (left side flexion / lateral flexion) of the vertebral spine.
- Abduction = to take the limb away from the body
- Adduction = to take the limb towards the body
- Medial rotation = rotation of the limb along its axis towards the median plane
- Lateral rotation= rotation of the limb around it's long axis in the direction away from the median plane
- Pronation of the antebrachium (forearm) = medial rotation so the palmar surface faces ventrally (prone - downwards)
- Supination of the forearm = lateral rotation of the paw so the palmar surface rotates from the medial position a dorsal position (upwards)
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