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pathology perspectives BY RUTH WERNER CRACKLE, POP, SNAP! JOINT DISRUPTIONS Voluntary movement begins with a thought. Our brain initiates it by sending a message down the spinal cord to motor neurons that stimulate muscular contraction, which then tugs on bony attachments. But we need our joints to translate that muscle contraction into movement. The leverage provided by all those hinges, balls-in- sockets, pivots, and gliders turns the impulse to "scratch your nose" into the graceful, effortless, almost spontaneous action that it is. The anatomical term for joint is articulation—it means having things connect in a sequence that works. The same term is used to refer to articulated buses and trains. And it applies to language as well: to be articulate means to be able to string words and ideas together in a chain that makes sense. Sometimes problems interfere with the sequence of translating force into movement at the points of leverage. This article will focus on three challenges to proper articulation: dislocation, dysplasia, and subluxation. ANATOMY REVIEW: SYNOVIAL JOINTS Synovial joints are composed of two or more bones with articular cartilage, a synovial membrane, a ligamentous capsule, and varying types and amounts of supporting fascia, ligaments, tendons, and muscles. Some joints have ligaments inside the capsule to tie bones together; others have a supporting structure called a labrum—a cuff of cartilage that helps to support and align the humerus at the shoulder and the femur at the hip. A similar structure can be found in the annular ligament that holds the radius in place as it rotates at the elbow. It is important to point out that while anatomy books show each of these structures as discreet and separate units, in a living body they are completely intertwined and blended through a three-dimensional matrix of connective tissues. Correct alignment is crucial for synovial joints to work effectively. When the bones in a joint are not in their optimal relationship, joint function is impaired at best and lost altogether at worst: range of motion, weight-bearing capacity, and pain-free, efficient movement is at risk. This can happen because of specific trauma, because of a congenital problem with the shape of the bones, or as a slowly progressive or chronic problem of instability without full dislocation. DISLOCATION A dislocation is a situation in which all joint integrity has been lost: the articulating surfaces are not in contact, and movement that uses the joint as a leverage point cannot happen. Most primary dislocations are the result of trauma, ranging from jamming a finger on a basketball to shattering the acetabulum in a motor vehicle accident. Dislocations are most common at the shallowest joints: the glenohumeral joint, the metacarpal- phalangeal joints, and the patella, which can slip out of the groove defined by the condyles of the femur. Obviously, a major dislocation is unlikely to happen without extensive damage to other tissues as well. The joint capsule and synovial lining can be stretched; supporting ligaments earn CE hours at your convenience: abmp's online education center, www.abmp.com 103