Joints, Ligaments and Tendons: Trading Off Movement and Stability

Your shoulder can rotate almost any direction. Your knee mostly bends and straightens. This is not an accident of anatomy. Every joint trades some amount of movement for some amount of stability, and the tissues surrounding it are built to enforce exactly that trade off.

A joint is simply the place where two bones meet, but different joints are built very differently depending on the job they need to do. The shoulder is shaped like a shallow socket, allowing an enormous range of movement in almost every direction, but that shallow shape makes it inherently less stable. The hip is shaped like a much deeper socket, allowing less movement but far greater built in stability. This is the fundamental trade off nearly every joint in the body makes in some way. Ligaments: bone to bone Ligaments are tough, fibrous bands that connect one bone directly to another across a joint. Their job is to prevent the joint from moving further than it safely should in a particular direction, acting like a set of built in limits. A joint with naturally poor bony stability, such as the shoulder, relies more heavily on its surrounding ligaments to keep it in place, which is part of why the shoulder is more prone to ligament injury than the hip. Cartilage: cushioning the contact Where two bones actually meet inside a joint, their surfaces are covered with a smooth layer of cartilage rather than bare bone. Cartilage reduces friction, allowing the joint to move smoothly, and absorbs some of the mechanical load passed through the joint during movement. Unlike bone, cartilage has a very limited blood supply, which is a large part of why cartilage damage tends to heal slowly and incompletely compared with a bone fracture. Tendons: muscle to bone While ligaments connect bone to bone, tendons connect muscle to bone, transmitting the force a muscle generates into movement at the joint. A tendon is not simply a passive rope. It stores and releases a small amount of elastic energy with each movement, which is part of why a warmed up tendon performs more efficiently than a cold one. Why some joints move more than others The overall range and type of motion available at a joint depends on the combined shape of the bones, the tension of the surrounding ligaments, and the arrangement of muscles and tendons crossing it. A hinge joint, such as the elbow, mainly bends and straightens in one plane. A ball and socket joint, such as the hip or shoulder, allows movement in multiple directions at once. Neither design is better in an absolute sense. Each represents a different balance point between mobility and stability, suited to the demands placed on that particular joint. What a doctor checks A doctor examining a joint problem is, in effect, working out which of these structures, bone, ligament, cartilage or tendon, is behind the pain or restricted movement. Range of movement. A doctor moves the joint through its normal directions, comparing both sides of the body and noting exactly where movement becomes painful or limited. Specific stress tests. Certain manoeuvres apply gentle, targeted stress to a particular ligament or tendon, revealing instability or pain specific to that one structure. Imaging. An X ray shows bone and joint alignment, while an MRI can show soft tissues such as ligaments, cartilage and tendons that an X ray cannot capture. When this system is affected When a ligament, tendon or the cartilage lining a joint is damaged by injury or gradual wear, the joint can become painful, swollen, or unstable, even when the bones themselves remain undamaged. This is explored further in the lesson "Joint Pain and Swelling".

A joint's range of motion and its stability sit at opposite ends of a trade off, and the shape of the joint together with its surrounding ligaments, cartilage and tendons determines where on that trade off a particular joint sits.