Question

Let's discuss joints and the mechanics of the skeletal system. Pick a joint and identify the bones that are involved in that joint.

• The joint I picked is the ball and socket joint.
• What might be the lumps/bumps (named correctly) that are associated with that joint to act as the surface for the joint to move, as well as points for muscle attachment that may cause that joint to move?

Example: Knee joint is a hinge joint and is made up of the femur and the tibia. The patella (a sesamoid bone) plays a role by helping with muscle movement but doesn't actually form part of the knee joint. The fibula is located in the same area but doesn't form part of the knee joint. The joint surfaces are the condyles of the femur superiorly resting on the condyles of the tibia. Both the femur and the tibia have epicondyles which are for muscle attachments that help in movement of the knee joint. The tibia also has the tibial tuberosity which is for muscle attachment near the knee. The range of motion of the knee is flexion and extension.

Answer 1

Ball and Socket Joint

Shoulder Joint

This joint is formed between the Glenoid Cavity of the Scapula (Shoulder Blade) and the head of Humerus.This joint can show a variety of movements:

• Adduction
• Abduction
• Extension
• Flexion
• Circumduction
• Internal Rotation
• External Rotation

Like most synovial joints, the articulating surfaces are covered with hyaline cartilage. The head of the humerus is much larger than the glenoid fossa, giving the joint a wide range of movement at the cost of inherent instability. To reduce the disproportion in surfaces, the glenoid fossa is deepened by a fibrocartilage rim, called the glenoid labrum.

Factors that contribute to stability:

• Rotator cuff muscles – surround the shoulder joint, attaching to the tuberosities of the humerus, whilst also fusing with the joint capsule. The resting tone of these muscles act to compress the humeral head into the glenoid cavity.
• Glenoid labrum – a fibrocartilaginous ridge surrounding the glenoid cavity. It deepens the cavity and creates a seal with the head of humerus, reducing the risk of dislocation.
• Ligaments – act to reinforce the joint capsule, and form the coraco-acromial arch.
• Biceps tendon – it acts as a minor humeral head depressor, thereby contributing to stability.