Last week I posted a blog about Bones, this is really only part of the story in understanding what can go wrong as many lameness problems are due to joints or muscle injury. This week I thought I would look in a bit more detail at joints.
The joints are where bones meet. Some of the 'meeting places' will have movement and some won't - like the bones in the skull.
There are also different types of joint, defined by the amount of movement they allow and their structure.
Fibrous Joints:- hold bones tightly together with short tough connective tissue. There is no movement. Examples include the bones of the skull.
Cartilaginous Joints:- are held together by cartilage! For example the spine.
Synovial Joints:- these contain a fluid filled cavity which ensures they move freely in one or more direction. Synovial fluid is the reason that joints should not be poulticed - the poultice will draw out the fluid which will adversely affect the joint!
Most joints are synovial. The ends of the bones are made up of layers of compact bone which are then covered in hyaline (articular) cartilage. This is smooth and ensures a smooth, slippery surface for easy movement. The cartilage also acts as a shock absorber and a joint lubricant. The spaces between the bones make up the joints and this is surrounded by the joint capsule. This capsule is made up of the synovial membrane which is a deep layer and produces the synovial fluid and the fibrous membrane on the surface.
Ligaments hold the bones together and stabilise the joint.
There are also different types of synovial joints, which are named according to the movement they can make:
- Hinge joints(ginglymus) - only extension and flexion - eg: the elbow.
- Pivot joints (trochoid)- allow shaking movements - eg: the atlas & axis (first 2 cervical vertebrae)
- Ball & Socket Joints (spheriod) - allow movement in any direction. One bone has a round head which fits into a cup on the other bone - eg: the shoulder
- Plane Joints (arthroidal) - allow sliding and gliding movement - eg: the bones within the knee or hock.
Types of Joint Movement
- Flexion of a joint decreases the angle of a joint eg: when bending the knee.
- Extension increases the angle of a joint eg: when straightening the leg.
- Rotating is a twisting motion around an axis (or central point).
- Adduction is the movement of a leg towards the body.
- Abduction is the movement of a leg away from the body.
- Gliding or sliding is the movement between the opposing surfaces of arthroidal joints.
Joints in Detail
Shoulder :- formed by the scapula and humerus this is a spheroid joint. There are few ligaments as the joint is stabilised by muscles. The muscles restrict the movement allowed to extension, flexion and a small amount of abduction and adduction.
Elbow: - is formed by the humerus and radius & ulna. This is a ginglymus joint which only allows flexion and extension. Collateral ligaments hold the elbow in position with muscles.
Knee(Carpus):- is made up of more than one joint and several carpal bones. It is also a ginglymus joint and has a large joint capsule which runs from the radius to the large metacarpal and surrounds all the structures of the carpus. The knee has extensive ligaments inside and outside, during flexion the ligaments relax and the foot will turn out slightly. This joint corresponds to the human wrist.
Fetlock:- is formed by the large metacarpal and the first (proximal) phalanx. Another ginglymus joint it is hyper-extended when in a normal standing position. The large joint capsule allows greater flexibility and this joint is supported by suspensory apparatus. This joint is subjected to the greatest stress of any of the joints of the limbs!
Pastern:- also called the proximal interphalangeal joint, it is formed by the first (proximal) and second (medial) phalanx . Again a ginglymus joint there is restricted action and it is also extended when the horse is in a normal standing position. The extensor tendons limit the flexion of this joint.
Coffin:- this is formed by the second (medial) and third (distal) phalanx and is located inside the hoof. Also called the distal interphalangeal joint it allows a great deal of movement. The suspensory navicular ligament supports the foot and the distal navicular ligament supports the distal sesamoid, attaching it to the distal phalanx. This joint is again in extension when the horse is standing! Over extension of this joint is prevented by the suspensory apparatus and the deep digital flexor tendon.
Hock:- is a complex joint , like the knee it is made up of a number of bones, but is another ginglymus. It is held in place by the medial and lateral collateral ligament. The calcaneus (the bone which sticks out) acts as a lever for the attachment of the tendons and it is attached to the remaining tarsal bones by ligaments. The front of the hock has fewer ligaments. The hock corresponds with the human ankle.
Stifle: this joint is in 2 parts. The femurorbital articulation is formed by the tibia and femur. It is held together by several ligaments. The fermuropatellar articulation is formed between the patella and the femur. This is a gingylmus joint allowing extension and flexion although extension is limited bu the ligaments. This joint corresponds with the human knee.
Hip: is formed by the femur and acetabulum and is a spheroid joint. The acetabulum is the cup shaped cavity. This joint also has extensive ligaments. The acetabulum has no articular cartilage! The universal movement which is possible with a spheroid joint is limited in the hip by the accessory ligament.
Sacro-iliac: this joint is formed between the axial and appendicular skeleton - the sacrum and wing of the ilium. This joint is held together by strong ligaments and there is limited movement possible to prevent damage to the nerves.
Vertebral column: the vertebrae are joined together by fibrocartilaginous discs between the vertebrae bodies. There is some movement allowed in any direction as the articular processes of each vertebrae form sliding joints. The largest are in the head region and they decrease in size until they become fused in the sacral area.
Next week I will look at Muscles in more detail, followed by Tendons and Ligaments!
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Until next time!