Anteromedial Facet Fractures of the Coronoid

J Whitcomb Pollock, M.D., MSc, FRCSC
University of Ottawa
The Ottawa Hospital
Ottawa, ON

Introduction
Elbow dislocations occur in approximately six out of every 100,000 people during their lifetime. The elbow is the second most commonly dislocated joint in the adult upper limb1. Dislocations constitute 10 to 25% of all injuries to the elbow, and are most prevalent in young adults2. Isolated coronoid fractures are uncommon and are usually associated with injuries to the collateral ligaments3. Although the coronoid is considered one of the primary stabilizers of the elbow, there are only limited clinical and biomechanical studies investigating these injuries. Interest in the coronoid has recently increased due to the recognized importance of anteromedial coronoid facet fractures.

 

Anatomy and Biomechanics
The elbow is composed of three articulations: 1) the ulnohumeral joint, 2) the radiocapitellar joint, and 3) the proximal radioulnar joint. The ulnohumeral joint, along with the LCL and MCL, is considered a primary stabilizer of the elbow4,5. The coronoid makes up the distal aspect of the ulnohumeral joint and articulates with the trochlea. It consists of an anterior coronoid process and anteromedial facet. The anterior coronoid process provides attachments for the anterior capsule and the brachialis muscle, and acts as a buttress to posteriorly directed forces, thereby resisting posterior ulnohumeral displacement6. Biomechanical studies have shown that 50% of the coronoid is required to provide normal kinematics and posterior and varus stability7.

The anteromedial facet has a medial projection, making it susceptible to injury during an elbow dislocation8 and it has been reported to provide posteromedial stability of the ulnohumeral joint9. Fractures of the anteromedial facet have been linked to posteromedial rotational instability (PMRI) of the elbow and, if untreated, are thought to rapidly lead to posttraumatic arthritis10-14. A recent biomechanical study demonstrated that small anteromedial coronoid fractures affect elbow kinematics9. This study suggested that internal fixation of anteromedial coronoid facet fractures larger than 2.5 mm should be considered and that LCL repair alone is insufficient to restore kinematics in most patients with this injury.

The LCL consists of the lateral ulnar collateral ligament (LUCL), the radial collateral ligament (RCL), and the annular ligament (AL). The LUCL originates on the lateral epicondyle and inserts onto the supinator crest of the proximal ulna. The LCL is considered the primary soft tissue stabilizer of varus angulation and posterolateral rotation15.

The MCL consists of three distinct bundles: anterior, posterior and transverse. The anterior bundle is considered the most important ligamentous stabilizer against valgus stress, posteromedial instability (PMRI) and internal rotation of the ulna. It originates on the medial epicondyle and inserts onto the sublime tubercle which is located on the anteromedial base of the coronoid process. The posteromedial bundle of the MCL has been shown to contribute to varus and internal rotational stability of the elbow16.

Fracture Classification
Regan and Morrey17 classified coronoid fractures into three types (1-3) based on height in the coronal plane. Type 1 fractures involve the anterior tip of the coronoid, type 2 up to 50% of the coronoid height, and type 3 are fractures involving more than 50%. This classification system continues to be useful and is commonly employed clinically. However, it does not consider fractures which occur in the sagittal plan, particularly those involving the anteromedial facet of the coronoid.

O' Driscoll et al.11 introduced a more comprehensive classification which includes the anteromedial facet and is based on anatomical location, fracture size, and mechanism of injury (Figure 1). Type 1 fractures involve the tip of the coronoid process and are divided into two subtypes based on size of the fracture: subtype I fractures are smaller than 2 mm and subtype II fractures are larger than 2 mm. Type 2 fractures involve the anteromedial facet and are divided, based on anatomical location, into three subtypes: subtype I involves the rim; subtype II includes the rim and tip; and subtype III involves the rim and the sublime tubercle with or without the tip. Basal coronoid fractures make up Type 3, involve at least 50% of the height of the coronoid and are divided into two subtypes depending on whether or not the fracture involves the base of the olecranon.

POLLOCKFigure_1

Figure 1. O'Driscoll Coronoid Fracture Classification.  Type 1 fractures involve the tip (A).  Type 2 fractures involve the anteromedial facet of the coronoid and are subdivided into three subtypes: subtype I involves the rim, subtype II involves the rim and tip, subtype III involves the rim and sublime tubercle with or without the tip (B).  Type 3 fractures involve the base and body of the coronoid.

Mechanism of Injury
Anteromedial coronoid (AMC) fractures (O'Driscoll Type 2) are thought to occur by falling on an outstretched arm, with the forearm in pronation, and varus/posteromedial rotation with axial loading through the elbow. These fractures are commonly accompanied by injuries to the LCL and posterior bundle of the MCL. The trochlea causes a shear or depression fracture of the anteromedial facet of the coronoid. Injury to the anterior bundle of the MCL can also occur with AMC fractures, particularly with subtype III. The combined ligamentous and osseous injury can result in posteromedial rotatory instability (PMRI), articular incongruity, alteration in joint contact patterns, and arthrosis11.

Evaluation and Treatment Options
A detailed neurovascular examination must be performed before and after reduction of a dislocated elbow. The shoulder and wrist (particularly the DRUJ) should be evaluated for associated injury. Although rare, compartment syndrome and vascular injury can occur and should be ruled out. Radiographic signs of these fractures are often subtle, such as loss of a parallel medial ulnohumeral joint line, or varus mal-alignment. A double crescent sign which represents a depressed anteromedial facet of the coronoid, may be seen on a lateral radiograph (Figure 2)11. The radiocapitellar joint can also be widened with a LCL injury. Computer tomography (CT) is recommended, with a low threshold, if there are any clinical or radiographic concerns18. CT can clarify fracture patterns, identify osteochondral fragments within the joint and assist in selecting the surgical approach and type of internal fixation.

The literature on AMC fractures is very limited and the exact treatment indications, and optimal surgical techniques and fixation have not yet been established. Currently, recommendations are based on a single biomechanical study9, small clinical series10 and expert opinion13,14,19. Internal fixation of AMC fractures and collateral ligament repair are recommended in order to avoid rapid degeneration of the ulnohumeral joint10,18. Based on biomechanical evidence, internal fixation of AMC facet fractures larger than 2.5 mm should be considered, as isolated repair of the LCL will not restore kinematics in the majority of patients with this injury. It may be possible to manage small (2.5 mm) subtype I AMC fractures, with an intact MCL, non-operatively using a strict rehabilitation protocol.

POLLOCKFigure_2

Figure 2. AP and lateral radiographs of a left elbow.  The double arch sign can be seen on the lateral view (solid white arrows).  The AP view clearly demonstrates an anteromedial coronoid fracture involving the rim and sublime tubercle (Type 2, subtype III).

 

A posterior incision is recommended for the surgical treatment of anteromedial coronoid fractures to allow access to both the medial and lateral sides of the elbow. A medial fasciocutaneus flap should be developed, followed by the identification and release of the ulnar nerve. A Taylor approach, through the bed of the ulnar nerve (between the heads of flexor carpi ulnaris), provides the most complete exposure to the anteromedial coronoid, the sublime tubercle and the MCL. If necessary, this approach can be extended proximally by releasing the flexor-pronator muscle origin off the supracondylar ridge and reflecting the anterior capsule off the humerus. The most difficult aspect of this approach is preserving the origin of the MCL on the medial epicondyle and the insertion on the sublime tubercle. An injured MCL should be repaired. While the optimal fixation of anteromedial coronoid fractures has not yet been determined, buttress plate or screw fixation has been suggested. Once the coronoid is secured, the LCL should be repaired through either an anterolateral EDC split or posterolateral Kocher approach. In order to restore stability, it is essential to reestablish the isometric origin of the LCL on the lateral epicondyle. Non-comminuted AMC fractures involving the rim and tip can be addressed with arthroscopic assisted percutaneous fixation. However, this method requires considerable expertise with elbow arthroscopy and clinical studies are needed to determine the efficacy, safety and applicability of this technique. Avoiding the extensive anteromedial dissection required to access the coronoid is an important potential advantage of arthroscopic fixation.

POLLOCKFigure_3a POLLOCKFigure_3b

Figure 3.  AP and lateral radiographs of the left elbow following trans-osseous LCL repair, ORIF of the anteromedial coronoid and radial head.

Treatment Outcomes
Reports on the outcomes of anteromedial coronoid fractures are very limited. Doornberg and Ring10 reported a retrospective review of 67 coronoid fracture dislocations of the elbow. Eleven of those patients developed varus posteromedial instability, all of which were associated with anteromedial facet fractures of the coronoid. In a second study, Doornberg and Ring reported on 18 patients with AMC fractures with an average follow-up of 26 months. Of these, six patients had malalignment of the anteromedial facet, and all developed varus subluxation, arthrosis of the elbow and fair or poor function. The remaining 12 patients, treated with stable anatomical fixation of the AMC, had a good or excellent functional result. Based on these two studies and other case reports, repair of the LCL and ORIF of anteromedial coronoid fracture is recommended, as well as repair of the MCL when necessary.

Summary
Unrecognized or untreated AMC fractures can have devastating outcomes and can potentially result in career-ending injuries. For this reason, it is important to have a high level of suspicion for these fractures. CT should be considered for most if not all of these injuries. Further research is required to determine the outcome of non-operative and operative management. Treatment indications, protocols and optimal surgical techniques for AMC fractures have not yet been established. Preoperative planning, proficient knowledge of elbow anatomy, and patience are required for successful treatment of these difficult injuries.

References

  1. Morrey B.F. The elbow and its disorders. 3rd ed. Philadelphia: Saunders; 2000.
  2. Josefsson P.O., Nilsson B.E. Incidence of elbow dislocation. Acta Orthop Scand 1986 Dec;57(6):537-8.
  3. Hildebrand K.A., Patterson S.D., King G.J. Acute elbow dislocations: simple and complex. [Review] [68 refs]. Orthopedic Clinics of North America 1999 Jan;30(1):63-79.
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  15. Dunning C.E., Zarzour Z.D., Patterson S.D., Johnson J.A., King G.J. Ligamentous stabilizers against posterolateral rotatory instability of the elbow. J Bone Joint Surg Am 2001 Dec;83-A(12):1823-8.
  16. Pollock J.W., Brownhill J., Ferreira L.M., McDonald C.P., Johnson J.A., King G.J. Effect of the posterior bundle of the medial collateral ligament on elbow stability. J Hand Surg Am 2009 Jan;34(1):116-23.
  17. Regan W., Morrey B.F. Classification and treatment of coronoid process fractures. Orthopedics 1992 Jul;15(7):845-8.
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