5.JOINTS

• Joints link the bones of the skeletal system into a functional whole that supports the body, permits effective movement, and protects the softer organs.
• Joints such as the shoulder, elbow, and knee are remarkable specimens of biological design, being self-lubricating, almost friction-less, and able to bear heavy loads and withstand compression while executing smooth and precise movements.
• It is equally important that other joints be less movable or even immobile, as they are better able to support the body and protect delicate organs.
• The vertebral column is only moderately mobile, as it must allow for flexibility of the torso and yet protect the delicate spinal cord and support much of the body’s weight.
• Bones of the cranium must protect the brain and sense organs, but need not allow for movement, hence they are locked together by immobile joints, the sutures studied in chapter 8.
• In everyday life, people take the greatest notice of the most freely mobile joints of the limbs, and it is here that people feel most severely compromised by such disabling diseases as arthritis.
• Much of the work of physical therapists focuses on limb mobility.
• Bony joints, or synostosis, are immobile joints formed when the gap between two bones ossifies and they become a single bone.
• Fibrous joints, also called synarthrosis, are points at which adjacent bones are bound by collagen fibers that emerge from one bone, cross the space between them, and penetrate into the other.
• Serrate sutures are analogous to a dovetail wood joint.
• Lap (squamous) sutures occur where two bones have overlapping beveled edges, like a miter joint in carpentry.
• There are four major categories of joints: bony, fibrous, cartilaginous, and synovial.
• Sutures are immobile or only slightly mobile fibrous joints that closely bind the bones of the skull to each other; they occur nowhere else.
• On the surface, a lap suture appears as a relatively smooth (nonserrated) line.
• Serrate sutures appear as wavy lines along which the adjoining bones firmly interlock with each other by their serrated margins, like pieces of a jigsaw puzzle.
• There are three types of fibrous joints: sutures, gomphoses, and syndesmoses.
• Sutures can be classified as serrate, lap, and plane sutures.
• Joints are the points where bones connect, and they serve various functions.
• Examples of serrate sutures include the coronal, sagittal, and lambdoid sutures that border the parietal bones.
• This chapter surveys all types of joints, from the utterly immobile to the most mobile, but with an emphasis on the latter.
• A moving bone has a relatively stationary axis of rotation that passes through the bone in a direction perpendicular to the plane of movement.
• The articulations of the phalanges, for example, can bend through a broad arc, but their movement is limited by ligaments that join them.
• In many cases, joint movement is limited by the shapes of the bone surfaces.
• Gymnasts, dancers, and acrobats increase the ROM of their synovial joints by gradually stretching their ligaments during training.
• The knee can extend to about 180°, but its motion is limited by a cruciate ligament and other knee ligaments.
• The shoulder joint, for example, has an axis of rotation that passes from anterior to posterior when the arm is raised to one side of the body, and it passes through the shoulder from lateral to medial when the arm is lifted to point at something straight in front of the body.
• In solid geometry, there are three mutually perpendicular axes, x, y, and z, which correspond to the transverse, frontal, and sagittal planes of the body.
• The ROM of a joint is determined by the structure of the articular surfaces of the bones, the strength and tautness of ligaments and joint capsules, and the action of the muscles and tendons.
• “Double-jointed” people have unusually large ROMs at some joints, not because the joint is actually double or fundamentally different from normal in its anatomy, but because the ligaments are unusually long or slack.
• The ROM of a joint can also be increased by gradually stretching the muscles and tendons.
• The survey of joint anatomy and movements provides a foundation for the study of muscle actions in chapter 10.
• The proximal radioulnar joint, not involved in the hinge, is a joint where the edge of the disclike head of the radius fits into the radial notch of the ulna, held in place by the anular ligament.
• The coxal joints bear much of the body’s weight and have deep sockets.
• The coxal (hip) joint is the point where the head of the femur inserts into the acetabulum of the hip bone.
• The subdeltoid, subacromial, subcoracoid, and subscapular bursae are located at the shoulder.
• The elbow joint includes two joints that form the elbow hinge—the humeroulnar and humeroradial—and one joint, the radioulnar, not involved in the hinge.
• The olecranon bursa is a prominent bursa at the elbow to ease the movement of tendons over the joint.
• The transverse humeral ligament extends from the greater to the lesser tubercle of the humerus and forms a tunnel housing the tendon from the long head of the biceps.
• The elbow is a hinge joint composed of two articulations: the humeroulnar joint where the trochlea of the humerus joins the trochlear notch of the ulna, and the humeroradial joint where the capitulum of the humerus meets the head of the radius.
• The deltoid is the large muscle that caps the shoulder, and the other bursae are named for parts of the scapula described in section 8.4a.