The coronoid process is a fan shaped flare approximately 1.5cm in height and 2.5cm wide at the base that projects unsupported medially from the ulna by an average of 7mm.1Over 50% of the anteromedial facet of the coronoid lacks support rendering it susceptible to injury with forced varus moments of the forearm. The coronoid is important not only as an osseous restraint but also as the site of insertion of the anterior bundle of the medial collateral ligament, anterior capsule and brachialis muscle. Injury to the coronoid cannot be thought of in isolation and consideration must be given to the mechanism of injury and to associated injuries around the elbow, in particular to the radial head, collateral ligaments, anterior capsule and olecranon. In 1989 Regan and Morrey presented a landmark paper raising awareness of the contribution of coronoid fractures to elbow instability.2 Concerns regarding ongoing elbow instability after fracture/dislocation have resulted in a trend toward more aggressive fixation of coronoid fractures. However many elbow surgeons are now concerned that overzealous fixation may contribute to post-traumatic stiffness.
While the importance of the coronoid process of the ulna to elbow stability has long been recognised, the complexity of injury patterns has, until recently, been underestimated. Fracture of the coronoid occurs in 2 to 15% of elbow dislocations mostly as a result of a shear injury from the trochlea as it dislocates. In the classic “terrible triad” injury the coronoid fracture is associated with fracture of the radial head and avulsion of the collateral ligaments. Injury is most easily appreciated from the appearance on a plain lateral radiograph of the elbow. Early classifications, such as that of Regan and Morrey, considered only the coronal plane injury that contributes to posterior elbow stability with axial loading. More subtle injury of the coronoid process is now recognised, that is medial oblique fracture sustained as a result of varus load and included in a newer classification by O’Driscoll.3 This injury is not easily appreciated on the lateral radiograph, but suspicion should be raised by the presence of a double crescent sign.4 On the AP radiograph there may be apparent obliteration of the medial joint space as a result of the subluxated ulna but the injury is most consistently diagnosed with three-dimensional reconstruction of a CT scan of the elbow. This injury is usually associated with an avulsion of the lateral ligament complex of the elbow and if the elbow is dislocated the medial collateral ligament is likely to have torn too.
The authors report that they have demonstrated that in an elbow with intact ligaments stability is maintained until there is loss of approximately 60% of the height of the coronoid, although when the radial head is absent loss of 40% of the height of the coronoid will lead to valgus external rotation instability. They further report that medial oblique fractures will not result in instability. The reader is lead to the assumption that fractures involving less than 40% of the height of the coronoid do not need to be fixed as long as injury to the radial head and both lateral ligament complex and anterior bundle of the medial collateral ligament are addressed. While this may be true for the terrible triad injury, most commonly associated with a coronal shear of the coronoid, the reader should be cautious interpreting the size of a coronoid fragment from radiological imaging and should have significant reservations in particular when applying this rule to the medial oblique fractures.5 This discreet group of injuries can be varied in severity and may include enough of the medial coronoid flare that without surgical reduction and fixation the ulna will inevitably subluxate on the trochlea. The effect of coupled movements that occur in vivo, in particular posterolateral and varus-posteromedial rotation, should not be ignored, and have been addressed in part in the study. However in vitro studies, that do not reproduce all the forces and freedom of movement of the elbow in vivo, may underestimate the degree of instability produced by more minor fractures. The reader should exercise judicious caution when translating information from the controlled environment of biomechanical studies into the clinic setting.
1. Matzon JL, Widmer BJ, Draganich LF, Mass DP, Phillips CS. Anatomy of the coronoid process. J Hand Surg Am 2006;31-8:1272-8.
2. Regan W, Morrey B. Fractures of the coronoid process of the ulna. J Bone Joint Surg [Am] 1989;71-9:1348-54.
3. O'Driscoll SW, Jupiter JB, Cohen MS, Ring D, McKee MD. Difficult elbow fractures: pearls and pitfalls. Instr Course Lect 2003;52:113-34.
4. Sanchez-Sotelo J, O'Driscoll SW, Morrey BF. Medial oblique compression fracture of the coronoid process of the ulna. J Shoulder Elbow Surg 2005;14-1:60-4.
5. Rafehi S, Lalone E, Johnson M, King GJ, Athwal GS. An anatomic study of coronoid cartilage thickness with special reference to fractures. J Shoulder Elbow Surg. Aug 2011 [EPub ahead of print]
Watts AC, Consultant Elbow and Upper Limb Surgeon
Trail IA, Consultant Upper Limb Surgeon
Wrightington Hospital, Hall Lane, Appley Bridge, Wigan, WN6 9EP