Joint Anatomy

Joints (also called articulations) are the places where two skeletal element meet. The typical joints that we think of are highly movable joints known as synovial joints. But there are also immovable joints such as the sutures in the skull, in which two bones knit together by fibrous tissue that is eventually replaced by bone.  As well, there are slightly movable joints such as the pubic symphysis or the joints between the vertebrae in the spine.

Synovial Joints

A synovial joint has a fluid filled space, the synovial cavity which is created because the area where the two bones meet is enclosed in a fibrous capsule. The inner surface of the joint capsule is lined by a membrane called the synovium. The synovium produces synovial fluid which fills the space of the synovial cavity.

The ends of the bone that meet within the synovial cavity are covered by a layer of hyaline cartilage called the articular cartilage.  In the dried bones of the skeleton that we examined last week, the very smooth surfaces of the condyles on the heads of the femur or humerus are places that in the living bone were covered with articular cartilage.  The smooth articular cartilage combined with the lubrication of the synovial fluid provide a low-friction surface for movement between the bones in a joint.

Blood vessels do not penetrate into cartilage, and it has a limited ability for repair.  This is particularly true for articular cartilage.  Damage to articular cartilage occurs in arthritis.

Joints are stabilized by ligaments. A ligament is a piece of fibrous connective tissue that links bone to bone, while a tendon is a piece of fibrous connective tissue that links muscle to bone. Note that what we are calling the patellar tendon is really a combination of both tendon and ligament.  Its upper part extends from the quadriceps muscles, while its lower part extends between the bone of the patella and the bone of the tibia.  The knee is stabilized by two external ligaments (the tibial collateral ligament and the fibular collateral ligament) and two ligaments that are inside the joint capsule (the anterior cruciate ligament and the posterior cruciate ligament).  The cruciate ligaments are so named because they cross each other.

The table below groups the different structures found in the knee joint according to whether they are bone, cartilage, or ligament.

Knee Joint:  Material to Study

We will study pictures of a model of a knee joint in which the capsule is removed to show the internal structures. First, watch the two videos from Acland's Video Atlas of Anatomy to understand the relationship between the cartilage, ligaments, and fibrous capsule of the knee joint.  Then identify the structures in the anterior and posterior views of the knee joint model.