Learning Modules - Medical Gross Anatomy Introduction to Joints - Page 1 of 22

Transcription

1 Introduction to Joints - Page 1 of 22 Joints - Introduction When you think about your joints, you most likely think immediately of your knees, shoulders, hips, elbows, etc. Certainly these are large and important joints, but there are many other joints in the body that you may have never realized existed. For example, the union of the parallel borders of the radius and the ulna and the junction between a tooth and its socket are both types of joints. A joint, or articulation, is defined as the junction between two or more bones of the skeleton. Joints can be classified in one of two ways: by the movement they permit or by the tissue joining the bones of the joint. Each of these systems of classification provides useful information about the joint, but the systems do not necessarily correspond.

2 Introduction to Joints - Page 2 of 22 Joints - Classification by Movement Joints can be classified by how much movement they allow. Synarthroses: Immovable joints. Specific examples of synarthroses are suture joints (the joints in the skull) and synchondroses (the type of joint found in growth plates). Amphiarthroses: Slightly moveable joints. A specific example of an amphiarthrosis is a symphysis (such as the joint between two vertebrae). Diarthroses: Freely moveable joints. Specific examples of diarthroses are typical synovial joints such as the shoulder and wrist. We will discuss these specific examples in more detail later in the module. An important concept to remember is that joint strength and flexibility are opposed. Greater joint strength comes at the cost of less flexibility and vice versa. The movement allowed at a particular joint depends on the shape of the bones and articular surfaces, the ligaments crossing the joint, and the muscles crossing the joint.

3 Introduction to Joints - Page 3 of 22 Joints - Classification by Tissue Joining Bones Joints can also be classified by the type of tissue connecting the bones of the joint. Fibrous Joints: The bones of these joints are connected by fibrous ligaments only. Cartilaginous Joints: The bones involved in cartilaginous joints are joined by some type of cartilage. Synovial Joints: These joints are freely moveable and are characterized by a joint cavity between the bones that has a synovial membrane and is lubricated with synovial fluid. We will discuss each of these types of joints in more detail on the following screens.

4 Introduction to Joints - Page 4 of 22 Fibrous Joints Fibrous joints are connected only by fibrous ligaments. A ligament is dense connective tissue that connects bone to bone (as opposed to tendons, which connect muscles to bones). Ligaments are named based on their position or based on the bones they attach. There are 3 distinct types of fibrous joints: 1. Suture Joints 2. Gomphoses 3. Syndesmoses

5 Introduction to Joints - Page 5 of 22 Fibrous Joints - Sutures Suture joints are one type of fibrous joint. They are the type of joint that connect the flat bones of the skull which meet in a tooth-like pattern. The fibrous tissue of these joints is continuous with the periosteum, and the joints are synarthroses (immovable). Suture joints undergo changes throughout childhood and into early adulthood. At birth, the sutures have broad spaces of fibrous tissue called fontanelles. These joints close down and undergo ossification (becoming fused with bone) beginning in childhood and continuing into a person's 20's. A joint that has ossified over time is called a synostosis.

6 Introduction to Joints - Page 6 of 22 Fibrous Joints - Gomphoses Gomphoses are another type of fibrous joint. They are the type of joint that anchors the teeth in the alveolar processes (sockets) of the mandible and the maxilla. These joints are amphiarthroses (slightly moveable), although the normal movement of this joint is not what we consider a "loose tooth". Gomphoses (even in adult teeth) allow microscopic movements that allow us to sense how hard we are biting and if we have food stuck in our teeth.

7 Introduction to Joints - Page 7 of 22 Fibrous Joints - Syndesmoses Syndesmoses are the third type of fibrous joint. In this type of joint, apposed bones are joined by a fibrous membrane (interosseous membrane) or a ligament. These joints are amphiarthroses (slightly moveable) and are maintained as fibrous unions throughout life. (Syndesmoses do not become synostoses.) Examples of syndesmoses are the attachment of the borders of the radius and ulna, which are connected with an interosseus membrane, the attachment of the borders of the tibia and fibula, which are connected with an interosseous membrane, and the inferior tibiofibular joint which is connected by a ligament.

8 Introduction to Joints - Page 8 of 22 Cartilaginous Joints The next broad classification of joints we will discuss are cartilaginous joints. Cartilaginous joints are joined by either hyaline cartilage or fibrocartilage. The cartilage of cartilaginous joints is avascular and anervous except at the margins. Hyaline cartilage is slippery and strong when compressed, but has little tensile strength (strength against being stretched). Fibrocartilage, on the other hand, is tough and strong both when compressed and when stretched (high tensile strength). There are two distinct types of cartilaginous joints: 1. Synchondroses 2. Symphyses

9 Introduction to Joints - Page 9 of 22 Cartilaginous Joints - Synchondroses Synchondroses are a temporary type of cartilaginous joint that are seen at epiphyseal plates (growth plates) during development. They allow for growth of long bones by flexibly joining the epiphysis, or end, of a growing long bone to the diaphysis, or shaft, of a long bone. This type of joint is made of hyaline cartilage that is eventually replaced by bone after growth is completed (synchondroses become synostoses). You may have heard that children are particularly susceptible to breaking bones at their growth plates. This makes sense because that region of the bone is actually hyaline cartilage, which has little tensile strength. Synchondroses are also found at the union between the first rib and the sternum.

10 Introduction to Joints - Page 10 of 22 Cartilaginous Joints - Symphyses Symphyses are the second type of cartilaginous joint. These are permanent cartilage unions characterized by fibrocartilage disks separating bones covered by hyaline cartilage. These are strong amphiarthroses (slightly moveable joints). Examples of symphyses are the pubic symphysis and the joints between vertebral segments with their tough intervertebral disks.

11 Introduction to Joints - Page 11 of 22 Synovial Joints Synovial joints are the most common, most moveable and most complex type of joint. Thus, we will spend a little more time on this joint type. Essentially, all synovial joints are diarthroses (freely moveable) thereby giving us the flexibility to walk, throw a ball, write, etc. Synovial joints have a general structure that is characteristic of this joint type. All synovial joints have: 1. A joint cavity between the bones 2. A synovial membrane lining 3. Articular cartilage Some synovial joints also have accessory structures.

12 Introduction to Joints - Page 12 of 22 Synovial Joints - Characteristics The following are characteristic of synovial joints: 1. Joint Cavity - The space between articulating bones that is lined with the synovial membrane. The joint capsule surrounding the joint cavity provides support for the delicate synovial membrane where it is not in contact with bone. 2. Synovial Membrane Lining - This structure secretes synovial fluid which lubricates and nourishes the joint. (Syn=like, ovial=egg-white) 3. Articular Cartilage - This covers the ends of bones that articulate with each other. It consists of hyaline cartilage lubricated with synovial fluid. It is very slick and smooth to reduce friction in the joint. 4. Accessory Structures - These do not necessarily appear in all synovial joints, but play key roles when they are present. We will discuss these structures more in the next screen.

13 Introduction to Joints - Page 13 of 22 Synovial Joints - Accessory Structures There are several types of accessory structures that may be present in synovial joints. 1. Accessory ligaments - these connect bone to bone and stabilize the joint by limiting motion in unwanted directions. Accessory ligaments may be capsular - a thickening of the joint capsule itself; extracapsular-outside the joint capsule; or intracapsular-inside the joint capsule. 2. Articular disks or menisci - these intervene between joint spaces. Examples are the lateral and medial menisci in the knee and the articular discs in the sternoclavicular joints. 3. Muscles and tendons - these can be very important for the integrity of many joints. Examples are the rotator cuff muscles in the shoulder which support the humerus and keep it in the glenoid fossa, and the popliteus muscle in the knee.

14 Introduction to Joints - Page 14 of 22 Synovial Joints - Subclassifications There are six subclassifications of synovial joints based on their structure and how much movement they allow. In order from the least amount of movement to the most amount of movement allowed they are: 1. plane joints 2. hinge joints 3. pivot joints 4. condyloid joints 5. saddle joints 6. ball and socket joints We will discuss each subclass of the synovial joints and give examples in the next several screens.

15 Introduction to Joints - Page 15 of 22 Synovial Joints - Plane Joints Plane joints allow only gliding or sliding motions. This motion can be in any direction of a single plane. Thus, it is a uniaxial joint. The articular surfaces in plane joints are flat or slightly curved and the movement is restricted by a tight fibrous capsule. Examples of plane joints include the: 1. facet joints (those between articulating surfaces on the vertebrae) 2. intercarpal joints 3. carpometacarpal joints 4. intermetacarpal joints 5. intermetatarsal joints 6. acromioclavicular joints

16 Introduction to Joints - Page 16 of 22 Synovial Joints - Hinge Joints Hinge (a.k.a. ginglymus) joints allow flexion and extension only. The movement at hinge joints is around one axis that is perpendicular to the bones of the joint. Hinge joints are another example of uniaxial joints. In these joints, the capsule is thin and flexible on the surfaces where bending occurs, but is reinforced with strong laterally placed collateral ligaments. Examples of hinge joints are the: 1. elbow 2. knee 3. interphalangeal joints (the joints between finger and toe segments)

17 Introduction to Joints - Page 17 of 22 Synovial Joints-Pivot Joints Pivot (a.k.a. trochoidal) joints are also uniaxial joints. They allow rotary movement around one axis that is longitudinal through the bone. In this type of joint, a process of one bone rotates in a ring formed by the other bone and a ligament. Examples of pivot joints are: 1. Proximal radioulnar joint 2. Atlas-axis joint (the joint between the first and second cervical vertebrae)

18 Introduction to Joints - Page 18 of 22 Synovial Joints - Condyloid Joints Condyloid joints allow flexion, extension, abduction, adduction and circumduction (which is a combination of flexion, extension, abduction, and adduction). These joints consist of oval surfaces which allow for movement in two planes perpendicular to each other. Thus, this is a biaxial joint. Rotation at this joint is not allowed due to the shape of the articulating surfaces. Examples of condyloid joints are: 1. The wrist 2. Metacarpophalangeal joints of the fingers and metatarsophalangeal joints of the toes. Note: If the difference between circumduction and rotation is confusing for you, try this: With your arm pointed straight ahead of you, trace a large circle in the air with your finger (using your whole arm)-this is circumduction of the shoulder. Now, with the same arm, pretend as though you are turning a screwdriver with your elbow straight-this is rotation of the shoulder. Also, remember that the metacarpophalangeal joints are capable of circumduction (tracing out circles), but cannot do rotation.

19 Introduction to Joints - Page 19 of 22 Synovial Joints-Saddle Joints Saddle (a.k.a. sellar) joints are also biaxial joints, but here, the articulating surfaces are concavoconvex (one bone shaped like a saddle and the other shaped like a horse's back). These joints allow flexion, extension, abduction, adduction and circumduction. An example of a saddle joint is the carpometacarpal joint of the thumb.

20 Introduction to Joints - Page 20 of 22 Synovial Joints - Ball and Socket Joints Ball and socket joints are multiaxial, allowing movement in an almost infinite number of axes through the ball of the joint. These joints allow flexion, extension, abduction, adduction, circumduction and rotation. Ball and socket joints allow for more flexibility than any other type of joint. Examples are: 1. The hip 2. The shoulder

21 Introduction to Joints - Page 21 of 22 Joints - Blood Supply Joints are relatively poorly supplied with blood vessels. In general, the blood supply comes from collateral branches of larger vessels. Many joints are surrounded by systems of anastomoses that allow blood flow around the joint regardless of the position of the limb.

22 Introduction to Joints - Page 22 of 22 Joints - Nerve Supply In contrast to blood supply, joints are very well supplied with sensory nerves. The major sensation from joints is proprioception, which allows us to know what position our limbs are in. We also have pain sensation in joints carried by branches of larger nerves in the area. Thus, joint pain is often referred to the skin overlying the joint or the muscles around the joint. The major point to remember about joint innervation is Hilton's Law: A nerve innervating muscles that act across a joint must also supply sensory fibers to that joint. So, for example, the femoral nerve which supplies the quadriceps muscles also sends sensory branches to the knee joint.