Identification of important interactions between subchondral bone ...

2.2 Biology of the joint
CHAPTER 2: Introduction
In order to get insight into the underlying processes of OA, it is important to understand the
physiology and biology of healthy joints. Joints that are able to move freely are termed synovial
joints and together with muscles and bone they represent the machinery that enables us to move.
The knee is a synovial weight-bearing joint, which comprises many tissues that maintain the
mobile function; articular cartilage, subchondral bone, ligaments, capsule, synovial membrane and
supporting muscles (fig. 1A). The ends of the adjoining bones are covered with articular cartilage,
which functions as a shock-absorber and provides a friction-free surface that enables the bones
to move smoothly against each other. The joint capsule, tendons, muscles and ligaments provide
stability of the joint. The bones are connected by strong ligaments and the tendons provide the
attachment of muscles to the bone. The inside of the joint is covered with a synovial membrane,
which provides and holds the lubricating synovial fluid that enables the joint to move easily (see
fig. 1B) 13,14 .
Fig. 1. Illustration of a healthy synovial knee joint. The different components of a knee
joint are outlined in figure A and B. A) The components responsible for stability and
movement are shown. B) The components of the articular capsule, which provide an easy
friction, are shown in details. The figure was produced by Madsen, S.H.
2.2.1 Development of bone from a cartilage template
Although OA is a disease of the whole joint, this thesis focuses on bone and cartilage, which are
the most affected tissues in OA. A description of the development of the joint, with respect to
bone and cartilage, will help the reader to understand the tight connections between these two
tissues.
15