The elbow is a complex and unique joint, termed a ginglymoid joint, whose role is to provide improved range of motion with respect to placing our hand in positions for use in daily activities.
With the inherent increased range of motion that the elbow possesses, the unfortunate trade off is that it is prone to sprains following trauma, which if severe may result in an episode of instability.
Instability occurs when the bones that make up a joint (in this case three; the humerus, ulna and radius) demonstrate abnormally increased gliding/mobility over each other, which may be incomplete (known as a subluxation) or complete, with the bones no longer opposing (or articulating with) each other, termed a dislocation.
A subluxation may go undetected, however the diagnosis of a dislocation is usually clinically obvious seeing that patients are typically in extreme pain and thus usually seek immediate treatment prior to obtaining an MRI. An MRI may be performed following a dislocation in order to assess for any post-traumatic cartilage defects, which may warrant urgent surgical intervention, as well as the extent of ligamentous disruption and any fractures not seen with a conventional X-ray.
Figure 1. MRI of a child’s elbow demonstrates bright signal (single arrow) involving the medial collateral ligament (along the inner side of the elbow) following a fall. The abnormal laxity of the ligament on this side allows for the bones on the lateral (outer) side of the joint to collide (in this case the distal humerus and radius; double arrows) and thus result in contusions (“bruises”) of the bone marrow
Figure 2. MRI of a 15 year old male following a sporting injury demonstrates extensive bright signal within the soft tissues along the anterior (front) aspect of the elbow, in keeping with tearing of the joint capsule, which is typical for an episode of instability.
Figure 3. In this same patient, further images demonstrate disruption of the medial collateral ligament (arrows) by way of a fracture from its origin from the humerus. This pattern of injury is a more severe version of that seen in the first case and would be consistent with an episode of dislocation.
Developmental Issues: Osteochondritis dissecans
Developmental issues of the elbow may occur, such as osteochondritis dissecans. In this case, repetitive microtrauma in susceptible patients may lead to focal areas of interrupted blood supply of the cartilage and bone plate that from the joint surface. This most commonly involves the capitellum of the humerus to form a fragment that is separate from the underlying main bone structure
Figure 4. In the example provided above, this is partially detached, however the fragment overall remains in its expected position (osteochondritis dissecans in situ).
With further trauma and/or interrupted blood supply, progressive cell loss due to cell death, known as avascular necrosis, may render the above described fragment non-viable and thus lose the ability to partially heal and remain in contact with the underlying bone.
In a separate companion case below, we see that the fragment has now separated from the capitellum, no longer located adjacent to the underlying main bone. The capitellum demonstrates loss of its normal rounded contour, as if a piece of it has been removed with an ice cream scooper, which is a typical finding in this condition. This absence of the bone and cartilage plate is known as an osteochondral defect (osteo = bone; chondral= cartilage).
Figure 5. The fragment has now separated from the capitellum, no longer located adjacent to the underlying main bone.
Surgical retrieval is warranted, with or without fixation to its former site, depending on the size of the fragment and whether it is assessed as being potentially viable.
Fixation of the fragment, where possible, allows for healing and remodelling and thus restoration of smooth surfaces of the joint in order to prevent arthritis.