Page 67 - ASOP ROT Study Guide

10 . 2 Stability
Three factors contribute to elbow stability:
♦ ligaments,
♦ muscles, and
♦ shape.
As mentioned before, the radius is bound to the ulna by a "sling" of ligaments. This is called the
annular ligament. It forms a cuff in which the radius can rotate to function as a uniaxial pivot
joint. Holding the forearm to the arm, are two collateral ligaments, one on the radial and one on
the ulnar side. The ulnar or medial collateral ligament is, like its namesake in the knee, the more
well developed. The capsule of the elbow joint is a looser, thinner structure than the ligaments and
does not contribute much to stability.
The long muscles crossing the elbow joint contribute to its stability. On the anterior side of the
elbow there are flexor muscles which flex the joint and on the posterior side of the elbow there are
extensor muscles which extend the joint.
In profile, the olecranon process resembles the end of a spanner (Figure 32). It is well designed to
resist forces (particularly in an anteroposterior or posteroanterior direction) as it grasps the
trochlea like a nut. However, only a moderate amount of stability is present to resist forces from
side to side in a medial or lateral direction.

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10 . 3 Axes of Movement
The elbow joint allows movement through two axes:
♦ a horizontal axis in the coronal plane, and
♦ a vertical axis along the length of the forearm
Movement around the first axis allows flexion and extension in the sagittal plane. Movement about
the second axis down the length of the forearm, allows the radius to rotate. As it does so its distal
end migrates around the distal end of the ulna. This action allows the hand to be turned over. The
position when the palm is facing upwards is called "supination'' and when the palm is facing
downwards is called "pronation". In this position the radius has rotated around the ulna along the
long axis of the forearm and is now lying on top of the ulna instead of by its side.