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Basic Anatomy for the Artist

Lesson #4

The Synovial Joint of the Knee

I have chosen the knee joint as an example of synovial structure because the landmarks are very visible and it is a most important region in figure work. A simple synovial joint is dicussed first.

A simple synovial joint (as those between the three bones in each of the fingers) has a convex surface on one bone fitting against a concave surface on a second bone. The bone ends are covered by a thin layer of articular cartilage. The cartilage which has no blood or nerve supply is kept alive by the synovial fluid which fills the joint space. This fluid is produced by the synovial membrane which lines the capsule of the joint. The whole joint is enclosed by a capsule of fibrous tissue, and in many cases ligaments composed of fibrous tissue and some elastic fibers form also to join the bones together to add strength and guard against bone displacement. For the artist this means that at joint areas there are the specialized enlarged ends of bones to consider in the form as well as the build up of the soft tissue.

In the knee joint, the lower end of the femur has two convex surfaces called the medial and lateral condyles. They are convex front to back as well as side to side, a rocker shape. They fit against two shallow surfaces on the top of the tibia which are called the tibial medial and lateral condyles. These condyles are all covered by a layer of cartilage. On the upper surface of the tibia are semilunar cartilages around the rims of the condylesand part of the surfaces which create an extra cushion for absorbing weight and shock. These are a factor also in the cruciate ligaments, joining the bones together within the joint space.

At the front of the joint is the patella, a sesamoid bone (seed) which has developed within the tendon of the quadriceps (four heads) femoris. It is the only muscle on the front of the thigh. The posterior part of the patella is wedge-shaped to fit between the condyles. As the knee bends and straightens the patella is accommodated in the furrow. The tendon of the quadriceps femoris inserts into the tibial tuberosity, a
large V-shaped mass of bone on the front of the tibia. The part of the tendon from the lower edge of the patella to the tuberosity is called the patellar tendon. Like all tendons it remains approximately the same length during knee action.

Quadriceps Femoris

Quadriceps Femoris is the only muscle on the front of the thigh and the only one which can extend the leg. When its fleshy parts shorten they pull on the tendon which is inserting into the tibia tuberosity, and pull the tibia in alignment with the femur. Quadriceps is composed of four muscles which have a common insertion.

• Vastus Intermedius lies deep and is attached to the front of the femur.
• Rectus Femoris spans the hip loint and has its origin from the anterior inferior spine of the pelvis and the rim of the socket for the head of the femur. The socket is called the acetabulum. The muscle bundles are short and bipennate in design. When they shorten they create a bulging form on the front of the thigh. The precise flat shape of the tendon above the patella can usually be seen.
• Vastus medialis and Vastus Lateralis arise from the back of the femur along the linea aspera. They surround the femur as they wrap around it, medially and laterally, to insert into the patella at the front. The forms of these two muscles are very apparent on the inside and outside of the leg. The medialis inserts into two-thirds of the medial border of the patella and also a small part of its base (the upper border). The lateralis inserts into the rest of the base and a little of its lateral side.

This muscle is greatly developed in ballet dancers, and in athletes who use their legs in the extended action as in walking and kicking.

THE VERTEBRAL COLUMN AND THE VERTEBRAE

The vertebral column is the central axis of the back of the trunk. The column has four regions:

• seven cervical vertebrae
• twelve thoracic vertebrae
• five lumbar vertebrae
• the sacrum and the coccyx

Except for the first two cervical vertebrae, the cervical, thoracic and lumbar all have cushions between them of fibro-glatinous substance called the nucleus pulposus. This is surrounded by a tough fibrous capsule which binds the two vertebrae together. These are called articular discs and act as shock absorbers as well as allowing for action between the vertebrae. The fibers of the capsule are arranged in
layers, and they run diagonally to each other. This gives both strength and mobility to the joint. Each vertebrae is weight supporting and the vertebrae increase in size down to the fifth lumbar. At this point weight is transferred to the sacrum and then out to the two hip bones. From that point it is transferred to the legs if one is standing or to the two ischial tuberosities of the pelvis on which one sits.

Each vertebrae is different to some degree but there is a common pattern. The weight bearing part is called the body and consists of a small cylindrical block of bone greater in width than in height. It is covered on its upper and lower surfaces with cartilage. From the back of this solid cylinder an arch of bone projects which creates a hole. When the vertebrae are placed on top of each other these holes create a continous canal, the vertebral canal, in which the spinal cord is enclosed. Where each vertabrae meets the next vertebrae a small area of the spinal cord is exposed at the intervertebral foramen, and it is through this foramen that each segmental nerve emerges to supply its segment of the body. A transverse process of bone projects from either side of the arch, and a spinous process from the back. These serve as levers for muscle attachments, and these muscles by contracting cause the column to bend and also rotate in actions required by the body.

See you next time!