Classification and Movements of Synovial Joints
1. Terminology and Definition :
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| sliding & gliding movement of joint surface |
Male surface:
An articulating surface which is larger in surface area and always convex in all directions .
Female surface:
An articulating surface which is smaller and concave in all directions .
Simple joints:
Joints with only two articulating surfaces, i.e. male and female.
Compound joints:
Joint possessing more than one pair of articulating surfaces.
Degrees of freedom:
Number of axes at which the bone in a joint can move.
Uniaxial joint:
Movement of bone at a joint is limited to one axis, i.e. with one degree of freedom, e.g. interphalangeal joints.
Biaxial joint:
joint with two degrees of freedom, e.g. wrist joint.
Multi-axial:
Three axes along with intermediate positions also, e.g. shoulder joint .
Translatory motion:
Sliding movements of one articulating surface over the other.
2. Movements and Mechanism of Joints .
A. Angular movement:
Movement leading to diminution or increase in angle between two adjoining bones. They are of two types:-
(a) Flexion and extension:
Bending and straightening of joint respectively.
(b) Abduction and adduction:
Movement away and towards the median plane respectively.
Circumduction:
When a long bone circumscribes a conical space.
B. Rotation:
Bone moves around a longitudinal axis across the joint.
(a) Adjunct rotation:
independent rotations of joint ., e.g. locking of knee joint.
(b) Conjunct rotation:
rotations which accompany other movements as in 1st carpometacarpal joint.
3. Shape of Articular Surface
The common shapes of the articular surface are:
(a) Ovoid:
When concave-female ovoids. When convex-male ovoids.
(b) Sellar/saddle-shaped:
These are convex in one plane, concave in the perpendicular plane.
4. Mechanical Axis of a Bone and Movement of a Bone
It is a reference point around which joint mechanics can be studied and around which the most habitual conjunct rotation occurs.
Spin:
Simple rotation around the bone’s stationary mechanical axis.
Swing:
Any other displacement of the bone and its mechanical axis apart from spin is termed a swing. Swing may be pure or impure (swing + element of spin).
Ovoid of motion:
This represents the imaginary surface which would include all possible paths of a point on the mechanical axis at some distance from its related joint.
Cardinal swing:
When the mechanical axis moves in the shortest pathway when bone moves.
Arcuate swing:
When the mechanical axis moves in the longest pathway along with the bony movement.
Co-spin:
When the effect of adjunct rotation is additive to the rotation.
Anti-spin:
Adjunct rotation which has a nullifying effect on rotation. Basic components of movements of the synovial joints are: (1) Spin, (2) Sliding, and (3) Rolling.
1. Spin:
It occurs around a fixed mechanical axis.
2. Slide-
During sliding movement, the mechanical axis of the joint and both ends of a moving bone move in the same direction. The transverse axis of movement is not fixed and it undergoes gliding or translation or linear movement.
3. Rolling:
In rolling movement, one end of the mechanical axis moves in a particular direction and the other end moves in opposite direction. The transverse axis of movement is almost fixed. The resultant movement is rolling along an arc. Rolling and sliding occur together in knee joint.
Joint Positions
[1]. Close packed position:
When the joint surfaces become completely congruent, their area of contact is maximal and they are tightly compressed. In this position fibrous capsule and ligaments are maximally spiralized and tense; no further movement is possible; surfaces cannot be separated by disruptive forces; articular surfaces are liable to trauma .
Close packed positions of the joints
Loose packed position of the joints:
All other positions of incongruency. Examples: Least packed position.
Shoulder – semi-abduction .
Hip – semiflexion .
Knee – semiflexion .
Ankle – plantar flexion.
Limitation of Movement
Factors
• Reflex contraction of antagonistic muscles.
• Due to stimulations of mechanoreceptors in articular tissue.
• Ligaments tension;
• Approximation of soft parts.
MECHANISM OF LUBRICATION OF A SYNOVIAL JOINT
1. Synovial fluid secreted by synovial membrane is sticky and viscous due to hyaluronic acid (a mucopolysaccharide). It serves the main function of lubrication of the joint, but also nourishes the articular cartilage.
2. Hyaline cartilage covering the articular surfaces possesses inherent slipperiness, like that of the ice.
3. Intra-articular fibrocartilages , articular discs or menisci, complete or incomplete, help in spreading the synovial fluid throughout the joint cavity, but particularly between the articular surfaces, e.g. temporomandibular joint .
4. Haversian fatty pads (Haversian glands) occupy extra spaces in the joint cavity between the incongruous bony surfaces. All of them are covered with synovial membrane, and perhaps function as swabs to spread the synovial fluid.
5. Bursa is a synovial fluid filled bag in relation to joints and bones, to prevent friction. The inflammation of bursa is called bursitis.
BLOOD SUPPLY OF SYNOVIAL JOINTS
The articular and epiphysial branches given off by the neighbouring arteries form a periarticular arterial plexus. Numerous vessels from this plexus pierce the fibrous capsule and form a rich vascular plexus in the deeper parts of synovial membrane.
The blood vessels of the synovial membrane terminate around the articular margins in a fringe of looped anastomoses termed the circulus vasculosus (circulus articularis vasculosus). It supplies capsule, synovial membrane, and the epiphysis.
The articular cartilage is avascular. After epiphysial fusion, communications between circulus vasculosus and the end arteries of metaphysis are established, thus minimizing the chances of osteomyelitis in the metaphysis.
NERVE SUPPLY OF SYNOVIAL JOINTS
1. The capsule and ligaments possess a rich nerve supply, which makes them acutely sensitive to pain. The synovial membrane has a poor nerve supply and is relatively insensitive to pain. The articular cartilage is non-nervous and totally insensitive.
Articular nerves contain sensory and autonomic fibres. Some of the sensory fibres are proprioceptive in nature; these are sensitive to position and movement, and are concerned with the reflex control of posture and locomotion. Other sensory fibres are sensitive to pain.
Autonomic fibres are vasomotor or vasosensory. The joint pain is often diffuse, and may be associated with nausea, vomiting, slowing of pulse, and fall in blood pressure. The pain commonly causes reflex contraction of muscles which fix the joint in a position of maximum comfort. Like visceral pain, the joint pain is also referred to uninvolved joints.
2. The principles of distribution of nerves to joints were first described by Hilton (1891). Hilton’s law states that a motor nerve to the muscle acting on joint tends to give a branch to that joint (capsule) and another branch to the skin covering the joint.
The concept of innervation of a joint was further elucidated by Gardner (1948) who observed that each nerve innervates a specific region of the capsule, and that the part of the capsule which is rendered taut by a given muscle is innervated by the nerve supplying its antagonists. Thus the pattern of innervation is concerned with the maintenance of an efficient stability at the joint .
LYMPHATIC DRAINAGE OF SYNOVIAL JOINTS
Lymphatics form a plexus in the subintima of the synovial membrane, and drain along the blood vessels to the regional deep nodes.
STABILITY OF SYNOVIAL JOINTS
The various factors maintaining stability at a joint are described here in order of their importance.
1. Muscles: The tone of different groups of muscles acting on the joint is the most important and indispensable factor in maintaining the stability. Without muscles, the knee and shoulder would be unstable, and arches of the foot would collapse.
2. Ligaments: Are important in preventing any over-movement, and in guarding against sudden accidental stresses. However, they do not help against a continuous strain, because once stretched, they tend to remain elongated. In this respect the elastic ligaments (ligamenta flava and ligaments of the joints of auditory ossicles) are superior to the common type of white fibrous ligaments.
3. Bones: Bones help in maintaining stability only in firm type of joints, like the hip and ankle. Otherwise in most of the joints (shoulder, knee, sacroiliac, etc.) their role is negligible.
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