Skeletal System: Bones, Joints, Cartilage, Ligaments, Bursae
Our musculoskeletal system is made up of muscles, tendons, ligaments, bones, cartilage, joints and bursae. Our muscles work with the nervous system to contract when stimulated with impulses (messages through the receptor arc) from motor nerves. The muscles are attached to the bones with ligaments. Our skeletal system is made mostly of bones and cartilage. Bones attach to bones with cartilage or ligaments. Bursae are small fluid filled at friction points near joints to protect ligaments and tendons from rubbing against bare bones.
Our skeleton forms a strong, solid internal framework of bones for our body, yet our bones only make up about 14% of our total body weight. Bones get their elasticity from tough elastic ropelike fibers of collagen. The core of some small bones is called marrow, it is soft and jellylike. The hard outside of bones is reinforces by strong rods called osteons.Bones have special cells called osteoblasts that make new bone and osteoclasts that break up the old bone. Bones grow by getting longer on the ends called the epiphyseal plate or growth plate. Bones are made rigid by hard deposits of minerals like phosphate and calcium. The bones of the skeleton support our skin, give our body shape, protect and support our organs and make it possible for us to move by acting as single and double levers. Bones do not move on their own; muscles move our bones by pulling on them. Muscles cannot push against the bone, so muscles come in pairs, one muscle pulls the bone one way and the paired muscle pulls the bone back the other way. We have a total of 233 bones. Some bones come in pairs that are almost identical in size and shape — the bones in the left arm are the same as the bones in the right arm. There are also single bones in the median plane of our body — the vertebrae in our back and neck. However, since our bones are constantly being rebuilt as we get older, both their structure and form can change. Our bones can be rigidly connected to each other or joined by rubbery cartilage, or flexibly linked by muscular or ligamentous joints. An adult skeleton has 206 bones, although some people have extra bones in their spine (backbone). A baby’s skeleton has 300 bones or more. As the baby grows, some of the bones fuse such as the bones in the skull and the pelvis. Most girls and women have smaller skeletons than boys and men of the same age. There are two main parts of the skeleton—the axial skeleton and the appendicular skeleton.
The Axial Skeleton
The axial skeleton (trunk) is made up of the 80 bones in our upper body. Bones of the axial skeleton include:
- Skull (facial and cranial bones)
- Vertebrae in the spine (backbones)
- Sternum (breastbone)
Our arms and shoulders hang from the axial skeleton.
The Appendicular Skeleton
There are 126 bones in the arms, shoulders, hips, and legs. The appendicular skeleton is made up of our limbs or appendages—two arms and two legs—our pelvis and right and left shoulders. Our arms hang from our shoulders and legs attached to our hips.
Bones of the Upper Appendage (Arm)
- Shoulder girdle—scapula (shoulder blade), clavicle (collar bone)
- Humerus—long bone of the upper arm
- Radius—long bone of the forearm; connects with the humerus to form the elbow
- Ulna—long bone of the forearm; connects with the humerus to form the elbow
- Carpals—8 small bones of the wrist
- Metacarpals—small bones of the hand
- Phalanges—14 bones of the fingers (3 in each finger) and thumb (2 in the thumb)
Bones of the Lower Appendage (Leg)
- Pelvic Girdle—made up of the right and left hip bones which are joined in the back with the sacrum and in the front at the symphysis pubis
- Hipbone—made of the ilium, pubis and ischium
- Femur—long bone of the thigh and longest bone in the body; connects with pelvis to form and hip joint and the tibia and fibula to form the knee joint
- Tibia—long bone of the lower leg (shin bone); connects with the femur to form the knee
- Fibula—thinner, long bone of the lower leg
- Patella—kneecap (Learn more about knee anatomy)
- Tarsals—small bones of the hand
- Phalanges—bones of the toes (3 in each toe and 2 in the big toe)
Joints—also called articulations—are formed where the surfaces of two or more bones meet and articulate with each other. There are about 400 joints in the human body. Joints allow both movement and flexibility. Joints are classified by how much movement they allow—function—or what they are made of—structure. Joints are usually classified structurally by the tissue that connects them. The tissue could be cartilage, fibrous tissue, synovial fluid, or some combination of the three. Functionally, joints can be classified by the degree of movement possible, the number of bones involved, and the complexity of the joint. Most body joints allow us to move, and some only allow movement in certain ways. Fixed or immovable joints allow no movement. A dislocated joint happens when the bones of the joint are forced out-of-place, usually while playing sports but can also happen with accidents. There are 3 major functional joints and 3 major types of structural joints.
Types of Functional Joints
- immovable (synarthrosis) joints—the bones are held together by fibrous tissue so they don’t move at all; example is the skull bones
- slightly movable (amphiarthrosis) joints—the bones are held together by cartilage that allows only a little movement; examples are the joints in the spine
- freely movable (diarthrosis) joints—also called synovial joints, allow the most movement; examples are hip and knee joints
Types of Structural Joints
- fibrous: the articular surfaces (point on the bone’s surface where the two bones meet) are held together by fibrous connective tissue. Very little movement is possible. Examples of fibrous joints are sutures, syndesmoses, and gomphoses.
- cartilaginous (amphiarthroses): the bones in cartilaginous joints are held together by cartilage which allows slight movement.
- synchondroses-these are temporary joints where the cartilage converts to bone by the time we are adults. The growth plates of long bones are examples of this type of joint.
- symphyses-these joints have a pad of fibrocartilage separating the bones; an example is the symphysis pubis
- synovial-the bony surfaces on the ends of the bones are covered with articular cartilage and separated by a slippery, lubricating fluid called synovia. They bones are held together in the joint by ligaments lined with synovial membranes which produce the synovial fluid. These freely moving joints are mostly found in our arms and legs. Synovial joints also include:
- a joint cavity or joint space: space between the articulating surfaces; articulating surfaces are the bone surfaces that move against each other when the joint moves. The articulating surfaces are covered with a layer of hyaline cartilage that cushions and protects the bones. The synovial membrane defines the boundaries of the joint space—everything outside of the synovial membrane is outside the joint space. The synovial membrane is wrapped by layers of connective tissue that form the joint capsule.
- an articular capsule: a sac-like structure that surrounds the joint and has an outer layer lined with a synovial membrane (synovium) that makes the synovial fluid. Synovial fluid acts as a lubricant, forms a fluid seal and helps distribute the force placed on the joint.
- reinforcing ligaments: tough, fibrous connective tissues that connect the bones and reinforce the joint capsule. On the outside of the joint capsule are thick strap-like bands, called collateral ligaments. These ligaments direct the force that travels through the joint and keep the joint on track. Outside of these structures are the muscles that travel across the joint.
Types of Synovial Joints
Based on the type of movement the joint allows and its structure, synovial joints can be put into several categories.
- gliding (plane) joint: have flat or slightly curved articular surfaces and allow gliding movements. The way they are bound together by the ligaments may not allow movement in all directions. Examples of a gliding joint are the intertarsal and intercarpal joints of the hands and feet.
- hinge joint: have a convex (curved outward) part of bone that fits into the concave (curved inward) part of another bone. The action of the hinge joint is like that of a door hinge and motion is limited to bending and straightening. Our elbows and knees are examples of hinge joints.
- pivot (swivel) joint: have a bone with a rounded end fitting into a groove in another bone. Pivot joints allow one bone to pivot on the other bone. An example is head of the radius rotates within the groove of the ulna.
- condylar (ellipsoidal) joint: these joints have a bony surface that is oval-shaped fitting into a concave surface of another bone. These joints allow bending, straightening, abduction, adduction and circumduction. An example of condular joints are in the hands.
- saddle joint: these joints are simialr to condylar joints but allow more movement. The only saddle joints are in the thumb.
- ball-and-socket joint: these bones fit together like a ball in a socket: the round end of one bone fits into the concave socket of the other bone. The only ball-and-socket joints are the shoulders and hips.
Joint Range of Motion
Range-of-motion means how far and in what direction a joint can move. All joints have a normal range of motion–that is when they are healthy and normal they should be able to move a certain distance and direction. Range of motion is measured in angles using a goniometer. A joint has a limited range of motion when it cannot move to it’s full range. Limited motion can be cause be injury, a mechanical problem or a disease process. When you have a physical exam, your range of motion is checked to see if you have full or limited range of motion. Surgery can also cause limit the range of motion in a joint. These are degrees of normal ranges of motion:
- shoulder: bend from 0-90°; straighten 90-0 °; move away from the body 0-90° ; move towards the body 90-0° ; rotate away form the center of the body 0-90° ; rotate toward the center of the body 90-0°
- elbow: bend from 0-90°; straighten 90-0 °; move away from the body 0-90° ; move towards the body 90-0° ; rotate away form the center of the body 0-90° ; rotate toward the center of the body 90-0°
- wrist: bend from 0-90°; straighten 0-70 °; move away from the body 0-25° ; move towards the body 0-65°
- metacarpophalangeal fingers: bend down 0-90°; straighten 0-30°
- interphalangeal – proximal: bend 0-120°; straighten 120-0°
- interphalangeal – distal: bend 0-80°; straighten 80-0°
- metacarpophalangeal-thumb: bend 0-70°; straighten 60-0°; abduct 0-50°; adduct 40-0°
- interphalangeal – thumb: bend 0-90°; straighten 90-0°
- hip: bend from 0-125°; straighten 115-0°; move away from the body 0-45° ; move towards the body 45-0° ; rotate away form the center of the body 0-45° ; rotate toward the center of the body 45-0° ; abduction 0-25°; adduction 20-0°
- knee: bend from 0-130°; straighten 120-0°
- ankle: bend downward 0-50°; move upward 0-20°
- foot: turn inward 0-35°; turn outward 0-25°
- metatarsophalangeal toes: bend down 0-35°; bend up 0-80°
- interphalangeal: bend down 0-50°; bend up 50-80°
Joints of the Upper Appendage (Arm)
- Shoulder—links the arm to the trunk. It is located away from the trunk so the arm can move freely. The arm hangs vertically beside the trunk.
- Elbow—the elbow can bend from 15 – 180 °. When the elbow is bent, the shoulder and metacarpus are in the same plane.
- Carpal—wrist joint
- Phalangeal—finger joints
Joints of the Lower Appendage (Leg)
- Hip—links the leg to the trunk. The leg hangs vertically below the trunk. The hip joint is where the hipbone joins the femur. The hip joint can bend from 0-125 ° and straightened from 115 to 0 °.
- Knee—the knee can bend from 0 – 130 ° and extend from 120 – 0 °. The knee is formed by the tibia, fibular, femur and patella.
- Metatarsal—joints in the foot
- Phalangeal—joints in the toes
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