Causes / typical injury mechanism: Proximal tibiofibular joint injuries are rare entities that involve disruption or attenuation of the ligaments connecting the tibia and fibula proximally near the knee joint. Injuries can be of the more chronic variety, described as "instability," which is seen most commonly in individuals who carry diagnoses hallmarked by ligamentous laxity (ie, Ehlers-Danlos syndrome). Here, there may be excess motion at the joint, resulting in subluxation. In those with generalized ligamentous laxity, these injuries tend to be seen more in preadolescent females and tend to decrease in prevalence with skeletal maturity. For those who carry the diagnosis of a connective tissue disease, this may persist later into life.

More acutely, these injuries occur as dislocations where the relationship between the two bones is more significantly disrupted. This tends to occur most commonly with rotation or direct impact to a hyper-flexed knee, occurring both in athletes (wrestling, football, gymnastics, etc) as well as in traumatic injuries. Those occurring in conjunction with high-energy trauma are commonly associated with tibial plateau or tibial shaft fractures. Due to this, acute injuries tend to occur most commonly in the more active population ranging from 20-50 years of age. Considering the rarity of the injury and likely high "miss" rate, the actual prevalence has been poorly described in the current literature.

Classic history and presentation: In the ED, these injuries will most likely be encountered as acute dislocations, whereas chronic instability is more likely encountered in an outpatient clinic. The patient will commonly present with lateral-based knee pain over the proximal tibiofibular joint region, located on average 1 cm below the lateral knee joint line. Patients may be unable to tolerate weight-bearing or have pain with attempted knee extension. Acutely, this will likely follow an athletic or traumatic rotational injury with a flexed knee. The majority (around 85%) of dislocations occur in the anterolateral direction.

Risk factors: Individuals with ligamentous laxity more commonly experience the chronic variety of this injury. This includes those with benign general ligamentous laxity as well as those with connective tissue diseases such as Ehlers-Danlos syndrome. There has also been an association identified with those diagnosed with rheumatoid arthritis, chronic osteomyelitis of the region, below the knee amputees, and long distance runners.

Pathophysiology: Anatomically, the proximal tibiofibular joint is primarily stabilized by both anterior and posterior ligaments. There are 3 ligaments anteriorly and 2 posteriorly, making the connection more robust anteriorly. This explains why anterolateral dislocation is the most common presentation. Other less common presentations include posteromedial and superior dislocations.

This joint is secondarily stabilized by several structures including the joint capsule, surrounding ligaments (most notably the lateral collateral ligament [LCL]), and the biceps femoris tendon, which inserts on the fibular head. These secondary attachments are important to consider as the LCL and biceps femoris are both tight in flexion and loose in extension, which impacts injury mechanism, evaluation, and subsequent reduction.