The shoulder joint is considered a 'ball and socket' joint, however, the 'socket' (the glenoid fossa of the scapula) is quite shallow and small, covering at most only a third of the 'ball' (the head of the humerus). The socket is deepened by the glenoid labrum . The glenoid labrum is a firm, white structure that form a ring around the glenoid cavity (the cup of the ball and socket shoulder joint).
The labrum is made of fibrous cartilage tissue, which is similar to the meniscus in the knee.
The labrum is triangular in shape in cross-section and thus deepens the socket of the shoulder. It also provides a 'suction-cup' effect to the joint. These two functions means that it is extremely important in providing a form of stability to the shoulder joint.
If the labrum is torn the shoulder can become unstable. This is what happens in shoulder dislocations and instability . Different injury mechanisms cause different tears in various sites around the labrum . These are given specific descriptive names (below). We also define the site of the tears by describing the glenoid face as a clock, where the top is 12 o'clock, 3 o'clock is at the front and 9 o'clock is at the back (for ease of use this applies to both left and right shoulders).
When a tear is a combination of Bankart , Reverse Bankart tear and SLAP tear , it is known as a 270 degree tear:
Chronic, longstanding labral tears can give rise to paralabral cysts. This is where the tear becomes a one-way valve so joint synovial fluid seeps out of the shoulder joint and not back. Depending on the site of the cyst it can press on surrounding nerves such as the Suprascapular Nerve or Axillary Nerve (Quadrilateral space syndrome).
Sub labral foramen, is an anatomical variant where the labrum can be 'lifted up' between 12 and 3 o'clock. It should not be confused with a labral tear, as its edge is clearly round and smooth and not red and ragged.