A joint (arthroses) is the joining between two or more bones and can be movable or immovable. They are held together by fibrous tissue that can be condensed to form specialised ligaments giving the joint more stability. Joints are classified into types depending on their general morphology.
There are 3 main types of joint: fibrous (synarthrosis), cartilaginous (amphiarthrosis) and synovial (diarthrosis).
Fibrous joints are found between two bones connected by fibrous tissue (collagen). There are 3 different types of fibrous joint; sutures, gomphoses and syndesmosis.
Cartilaginous joints are two bones connected by fibrocartilage or hyaline cartilage. There are two types of cartilaginous joints: synchondroses and symphyses.
Synovial joints are movable joints and have a large range of motion. The adjacent joint surfaces are covered with a thin layer of hyaline cartilage and separated from each other by a narrow joint cavity. A synovial membrane seals the joint cavity and secretes a viscous, lubricating and nutrient fluid, called synovial fluid, into the joint cavity. The presence of the hyaline cartilage and the synovial fluid allows the bone surfaces to slide over each other relatively free from friction. Around the outside of the joint is a tough fibrous joint capsule. This attaches to the articular margins of the adjacent bones and is often thickened to form joint ligaments, which together with the surrounding muscles hold the bones in position.
Within the cavity of a synovial joint, between the opposing bones, can be a fibrocartilaginous meniscus or disc. These act to make the articular surfaces more congruent and act as a shock absorber.
Each synovial joint can be described as being uniaxial (moves in one plane), biaxial (moves in two planes at right angles to each other), or multiaxial (moves in multiple planes). There are 6 types of synovial joint each classified by the type of movement that occurs at the joint.
There are a number of terms used to describe the movement that occurs at different joints. They are important to understand because they are also used when describing muscle actions.
Range of motion is the term used to describe the limits of which a particular joint can move. The range of motion is affected by a number of factors including the shape of the articular surfaces, the strength and laxity of the surrounding capsule and ligaments, and the action and strength of the surrounding muscles.
The temporomandibular joint is often described as a hinge joint but it also functions as a condyloid (ellipsoid) and gliding joint. The joint is an articulation between the condyle of the mandible and the concave surface made up of the mandibular fossa of the temporal bone and the articular tubercle of the maxilla. The joint cavity is split into separate superior and inferior cavities by the presence of an articular disc. The articular capsule and the lateral temporomandibular and sphenomandibular ligaments support the joint.
The range of motion the mandible is capable of is: depression/elevation (opening and closing the mouth), protrusion/retraction (carrying the mandible forwards and backwards) and a small amount of lateral movement (side-to-side movement).
The shoulder is a synovial ball and socket joint between the shallow glenoid cavity of the scapula and the hemispherical head of the humerus. The glenoid cavity is deepened by the glenoid labrum, a ring of fibrocartilage attached to the boundaries of glenoid fossa. Even so, the fossa still remains relatively shallow and because of this the shoulder joint has the largest range of motion in the body.
The non-articular surfaces of the joint are lined by a synovial membrane, which communicates with the subscapular bursa. The tendon of long head of biceps enters the joint space through the bicipital groove and lies in a tubular synovial sheath as it passes posteriorly and medially across the superior aspect of humeral head. Because of this flexibility the joint must rely on the coordinated actions of the rotator cuff muscles (subscapularis, supraspinatus, infraspinatus and teres minor) to maintain its stability as well as the tendon of biceps brachii. The glenohumeral ligaments (superior, medial, inferior) and joint capsule only act to stabilize the joint when tightened, which only occurs at the limits of the ranges of motion.
The range of motion the shoulder joint is capable of is: adduction/abduction, flexion/extension, circumduction and medial/lateral rotation.
The elbow joint is a compound synovial joint because it is composed of three articulations; the humeroulnar, humeroradial and proximal radioulnar articulations, bound together by one joint capsule. It is formed between the trochlea of the humerus and the trochlea notch of the ulna and between the capitulum of the humerus and the head of the radius. The component between the humeroulnar joint acts as a hinge joint and the humeroradial component is a pivot joint.
The complexity of the opposing articular surfaces as well as the strong collateral ligaments restrict its motion and maintains its stability. The ligaments of the elbow joint include the medial ulnar collateral, and the lateral radial collateral ligaments and the annular ligaments. The synovial membrane lines the capsule, between the capsule and the membrane are three fat pads.
The range of motion the elbow joint is capable of is: flexion/extension and rotation (pronation and supination).
The hip joint is a synovial ball and socket joint between the concave lunate articular surface of the acetabulum of the hip bone (os coxae) and the convex head of the femur. The lunate acetabular articular surface is horseshoe-shaped and completed inferiorly by the transverse acetabular ligament. The acetabulum is made deeper by the fibrocartilaginous acetabular labrum attached to its rim.
The articular surface of the femoral head is covered in hyaline articular cartilage except where it is interrupted at the pit for the attachment of the ligamentum teres (ligament of the head of the femur).
The joint is enclosed in a very strong fibrous capsule that is reinforced by three thickenings, the iliofemoral, pubofemoral and ischiofemoral ligaments.The joint is lined by synovial membrane, which covers the capsule and the labrum and forms a sleeve around the ligament of the head of the femur.
The range of motion the hip joint is capable of is: adduction/abduction, flexion/extension, circumduction and medial/lateral rotation.
The knee joint is the largest synovial articulation in the body. It is a combination of articulations between the medial and lateral femoral condyles, the medial and lateral tibial condyles and the posterior surface of the patella. The capsule and synovial membrane invest the entire joint to create a single synovial cavity.
Within the knee joint, attached to the tibial condyles are two fibrocartilaginous discs, the medial and lateral menisci. The menisci increase the congruence between the articulating surfaces of the tibia and femur. The knee has several strong intracapsular and extracapsular ligaments. The intracapsular ligaments include the strong anterior and posterior cruciate ligaments. The extracapsular ligaments include the medial and lateral collateral ligaments.
The menisci along with the ligaments and various tendinous and muscular attachments surrounding the joint, contribute to its stability.
The knee joint is usually defined as a hinge joint however when flexed it is also capable of rotation and glide. The range of actions that the knee joint is capable of are: flexion/extension and rotation.
Rheumatoid arthritis is a systemic disease which means it can affect other parts of the body such as the heart, lungs, and eyes.