Structure and Function of the Joints and Ligaments in Dogs
Synovial joints – These joints are the most common type of joint, and they provide free movement between the bones they link. They are typical of nearly all limb joints, such as the knee, elbow and wrist. Their name comes from the lubricating substance (synovial fluid) that is within the joint cavity. Tough, fibrous tissue encloses the area between the bone ends and is called the joint capsule.
What Are the Joints and Ligaments?
A joint is an articulation, the junction between two or more bones of the skeleton. Some joints have no movement, some allow only slight movement, and some are freely moveable.
A ligament is a tough band of white, fibrous, slightly elastic tissue. Ligaments are an essential part of the skeletal joints because they bind the ends of bones together to prevent dislocation and excessive movement that might cause breakage.
Where Are the Joints and Ligaments Located?
Joints are found throughout the body wherever two bones meet.
In the skeletal system, ligaments often stretch across the joints to connect different bones. Ligaments are also found in the abdomen where they support many internal organs, including the uterus, the bladder, the liver, and the diaphragm and help to hold these organs in position.
What Is the General Structure of the Joints and Ligaments?
A joint consists of bones, muscles, ligaments, cartilage and a lubricating fluid all enclosed by a tough joint capsule. Bones are anchored by ligaments that permit a certain amount of movement in specific directions. Many joints are surrounded by a joint capsule, which contains the joint lubricant, synovial fluid. The ends of the bones are covered by cartilage, creating a smooth surface that helps the joint move easily and helps to absorb any concussion as body weight is placed on the joint.
There are three types of joints:
Fibrous joints – These joints are united by fibrous tissue. The amount of movement of these joints depends in most cases on the length of fibers uniting the bones. The sutures of the skull are an example of fibrous joints. These bones are close together and have essentially no movement.
Cartilaginous joints – These joints are united by cartilage and allow some movement. The joints formed between each vertebra in the spine are an example. The intervertebral disc is actually cartilage, which acts as a cushion or pad between the adjacent vertebrae.
Joints receive blood from articular arteries that arise from vessels around the joint. Articular veins accompany the arteries and both are located in the joint capsule. Joints have a rich nerve supply, with many nerve endings in the articular capsule. The nerves within joints transmit a sense of position or proprioception information via the nerves back to the brain.
Ligaments are composed largely of long parallel or spiral collagenous fibers, but they also possess yellow elastic fibers. Ligaments may be intracapsular or extracapsular, depending on whether they are inside or outside the joint capsule, or part of the capsule itself.
What Are the Functions of the Joints and Ligaments?
The primary functions of joints are to provide motion and flexibility to the skeletal frame and to act as shock absorbers.
Ligaments bind the bone ends together to prevent dislocation and excessive movement that might cause breakage. Ligaments also support many internal organs, including the uterus, the bladder, the liver, and the diaphragm.
What Are the Common Diseases of the Joints and Ligaments?
Carpal hyperextension injuries cause a breakdown of the ligaments that support the back of the carpal joint in the wrist, resulting in collapse of the wrist. The foreleg rests closer to the floor, rather than in the normal upright position. It is usually the result of landing on the front legs after jumping from a significant height.
Coxofemoral luxation is dislocation of the hip. With this type of dislocation, the head of the femur comes out of the socket of the pelvis (acetabulum). The dislocation is usually the result of trauma and results in a non-weight bearing lameness of the affected limb.
Degenerative joint disease (DJD) is degenerative arthritis of one or more joints that usually occurs from wear and tear on the joints. DJD affects the smooth articular cartilage of the joint, which ordinarily covers the bone and is responsible for the smooth, non-painful motion of joints. When this cartilages degenerates and becomes worn, rough bony surfaces are exposed and rub together. Pain and lack of joint mobility then occur.
Hip dysplasia is the abnormal development of the hip joint that occurs primarily in large breed dogs. Progressive lameness, stiffness and pain often arise in the hind legs with this disease.
Rupture of the cruciate ligament is a common problem in dogs. This ligament is located within the stifle (knee) joint and acts to stabilize the position of the femur as it comes to rest on the tibia. Acute traumatic tearing of this ligament can occur or the ligament may slowly degenerate until it is so weakened that tearing occurs with little trauma.
Septic arthritis is a bacterial infection within a joint It can lead to severe erosion of the joint surface and may progress to systemic illness.
What Types of Diagnostic Tests Are Used to Evaluate the Joints and Ligaments?
A thorough orthopedic examination with palpation of the affected area and testing of range of motion is very useful in identifying possible ligament and joint problems.
X-rays provide a view of the joint and can detect arthritis, dislocation, joint effusion, hip dysplasia and other abnormalities of joints. Numerous X-ray techniques and views have been developed for assessing joints. Ligaments do not show up on plain X-rays.
Routine laboratory tests such as complete blood count, biochemistry profile and urinalysis may detect evidence of infection and other organ abnormalities.
Certain immune tests and serologic blood tests may be done to search for immune and infectious causes of joint disease.
Arthrography is a type of contrast X-ray study where dye is injected into the joint and then an X-ray is taken. It is particularly helpful in assessing the shoulder joint of dogs.
Arthroscopy involves the passage of a small, rigid scope into the joint. It is used most often for examining the knee and shoulder joints. Many other joints are too small for insertion of arthroscopes.
Arthrocentesis is the aspiration of fluid from joint for sake of microscopic examination and bacterial culturing. It is most useful when abnormal amounts of fluid are present in the joint(s).
Advanced imaging techniques, such as MRI and CT scans, may also be used to assess joints and ligaments.
Some joint and ligament diseases require surgical exploration of the site to define or diagnosis the specific abnormality present.