This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The olecranon process may be fractured either close to its extremity near the insertion of the triceps tendon, through approximately the middle of the greater sigmoid cavity, or toward the coronoid process.
The second is the more common. The fracture which occurs nearer the insertion of the triceps is liable to occur from muscular action, the triceps contracting and tearing off the piece of bone into which it is inserted. The shape of the process should be noted. In the bottom of the greater sigmoid cavity near where the process joins the shaft it is constricted and weakened by a groove which sometimes passes nearly or quite across its surface. This is the weakest point and is most often the site of fracture.
The triceps muscle inserts not only into the upper surface of the olecranon but also along its sides. In addition it sends off a fibrous expansion to each side ; the one to the medial condyle is thin, but the one to the lateral condyle forms a broad, tough, fibrous band which stretches from the olecranon to the lateral condyle and passes down over the anconeus to be attached to the outer edge of the upper fourth of the ulna (Fig. 314). In cases of fracture the fragment is only slightly displaced upward by the contraction of the triceps. The reason is that the fibrous expansion of the triceps usually is not sufficiently torn to allow of the retraction of the fragment. The amount of separation of the fragments is directly proportional to the amount of tearing of the lateral fibrous expansion of the triceps tendon. By extending the forearm the triceps is relaxed and by pushing the fragment down crepitus can often be elicited.
Fracture of the olecranon process is usually treated with the elbow slightly flexed. Complete extension is not commonly employed. The slight flexion allows for the effusion into the joint and leaves the arm sufficiently extended to relax the triceps.
Fracture of the coronoid process does occur but it is exceedingly rare. The brachi-alis anticus does not insert into its tip, but at the lower part of its anterior surface. The fracture is most liable to occur in cases of luxation, the process being knocked off as the humerus comes forward.