Fig. 29.1
CT scan showing a typical terrible triad pattern of fracture, characterized by a fracture of the coronoid, a fracture of the radial head, and a posterior dislocation of the elbow
29.2 Pathogenesis
The terrible triad is the result of an indirect trauma to the elbow, characterized by an injury on the hand, with a combination of compression on the elbow, valgus stress, and forearm supination with respect to the humerus (Video 29.1). This dynamic of the trauma is usually the result of a fall to the ground onto an outstretched hand which is resting on the ground and with the elbow extended in an attempt to slow the fall.
Depending on several factors, the first of which being the degree of flexion and pronation of the elbow during the trauma, the elbow may suffer a spectrum of pathologies ranging from a “simple dislocation” (not associated with fractures) to a terrible triad. The basic principle of the interaction between the degree of flexion and pronation of the elbow and severity of the instability is relatively simple.
Tip
The elbow is more stable in flexion and pronation and less stable in extension and supination.
In extension and supination, the major stabilizers of the elbow resisting posterolateral dislocation are the soft tissues (lateral collateral ligament and anterior capsule). In flexion and pronation, the stability of the elbow is further reinforced by the bony stabilizers (coronoid and radial head). As a consequence, the elbow is more stable in flexion and pronation, while it is less stable in extension and supination.
A trauma in extension and supination is thus able to dislocate the elbow with lower kinetic energies without affecting the bony stabilizers of the elbow. If the elbow instead undergoes a trauma in this position of greater stability, in flexion and pronation, the dislocation occurs only after a fracture of the primary (coronoid) and secondary stabilizer of the elbow (radial head). This explains why the terrible triad occurs most frequently with high-energy trauma or with lower-energy trauma in patients affected by osteoporosis.
In addition to the degree of flexion and supination of the elbow, the degree of valgus stress and compression of the elbow also influences the type and severity of elbow instability. The greater the trauma component of valgus stress as compared to the compression, the greater the damage to the medial collateral ligament and radial head, versus the damage to the coronoid.
29.3 Principles of Biomechanics Applied to the Terrible Triad
Tip
The residual instability is a critical concept in understanding the treatment of a terrible triad.
The terrible triad is characterized by an impairment of primary and secondary stabilizers of the elbow, frequently resulting in a residual instability of the elbow. The residual instability is defined as the degree of instability after reduction of the terrible triad. The primary stabilizers of the elbow, pertaining to the terrible triad, are the joint capsule, the lateral collateral ligament, and the coronoid. The radial head, which under normal conditions is a secondary stabilizer, becomes a primary stabilizer in circumstances where the coronoid is deficient.
The degree of involvement of these structures, however, varies from case to case, significantly affecting the degree of residual instability and therefore the treatment and prognosis of the terrible triad. The concept of residual instability is vital and determines the outcome of our conservative or surgical treatment.
The degree of residual instability depends on the interaction of several elements:
Extent of the coronoid fracture
Extent of the radial head fracture
Potential for spontaneous healing of the primary and secondary stabilizers
29.4 Fracture of the Coronoid
The extent of the coronoid fracture is the most important factor in predicting the degree of residual instability of the elbow. The classification of Ragan-Morrey [1] is probably the most used and divides the fractures into three groups depending on the extent of the fracture of the coronoid (Fig. 29.2).
Fig. 29.2
CT scan showing a type I (a), type II (b), and type III (c) fracture of the coronoid according to the Regan-Morrey classification
29.4.1 Type I
Fracture of the apex (Fig. 29.2a). The elbow after reduction usually shows no signs of residual posterolateral instability, even with severe impairment of the radial head. If there is a complete lesion of the medial collateral ligament associated with a fracture of the radial head, a residual valgus instability can occur and must be diagnosed early. In case of type I coronoid fracture, the coronoid remains an efficient primary stabilizer of the elbow and the radial head a secondary stabilizer. That means that the function of the radial head is not mandatory for posterolateral stability. The elbow remains stable even under unfavorable conditions such as extension and supination. The stability of this condition allows the elbow to be mobilized early and to allow physiological healing of the lateral collateral ligament.
29.4.2 Type II
Tip
Terrible triad with a type II fracture of the coronoid is at high risk of inadequate treatment and subsequent complications.
Fracture affecting 50 % of the coronoid (Fig. 29.2b). The elbow has significant residual posterolateral instability in extension and supination, if the function of the radial head is compromised. Conservative treatment in this case must be weighed carefully and is closely linked to the function of the radial head, which in this context is a primary stabilizer. If the radial head is only partially affected by the fracture or has a fracture that is likely to heal well, conservative treatment may be indicated, but the patient must be followed closely with clinical and radiographic follow-up for early detection of residual posterolateral instability. A concomitant lesion of the medial collateral ligament increases the degree of residual instability by adding residual valgus instability.
In our experience, patients affected by a terrible triad with a type II fracture of the coronoid comprise the cases where there is the greatest risk of inadequate treatment and subsequent complications.
29.4.3 Type III
Fracture affecting more than 50 % of the coronoid (Fig. 29.2c) (base of the coronoid). In these cases, the elbow is unstable posterolaterally also at more than 45° of flexion and in pronation. The radial head is usually not able to provide sufficient stability to the elbow even after a proper reconstruction or prosthetic replacement. In these cases, coronoid reconstruction, suture of the external collateral ligament, and reconstruction or replacement of the radial head are recommended. In some cases, even proper surgical treatment cannot guarantee the absence of residual instability, and it is necessary to use an external fixator which is kept in place for 4–6 weeks.
29.5 Extent of the Fracture of the Radial Head
The extent of radial head involvement is of great importance since it can become the primary stabilizer in terrible triad cases. The radial head becomes a primary stabilizer against valgus instability in concomitant lesions of the medial collateral ligament and a stabilizer against posterior instability in the event of type II and III coronoid fractures.
In these cases, it is essential to restore the function of the radial head through reconstruction or prosthetic replacement. The function of the radial head is of such importance in the terrible triad such that our inclination is to prefer a prosthetic implant that provides excellent primary stability rather than a suboptimal reconstruction. For the same reason, we prefer an anatomic prosthesis which ensures better initial stability rather than bipolar prosthesis.
Tip
The radial head becomes a primary stabilizer against valgus instability in case of concomitant lesion of the medial collateral ligament and a crucial stabilizer against posterior instability in the event of type II and III coronoid fractures.
The study of the type of fracture of the radial head is vital to predict the degree of residual instability of the elbow affected by a terrible triad. The involvement of the anterosuperior aspect of the radial head, the comminution of the fragments, and a fracture with displacement of the neck are negative prognostic factors in which we recommend a prosthetic replacement. One or two fragments without involvement of the neck are potentially reducible and fixable with screws or a precontoured plate (Fig. 29.3). The final goal is to obtain good primary stability of the elbow, thus allowing early mobilization.
