Ogden1 described 2 types of PTFJs, horizontal and oblique, with the latter being considered less stable because of less rotational mobility. The horizontal configuration is defined as <20° of inclination of joint surface in relation to the horizontal plane, and the oblique variation is defined as >20° of inclination of the joint surface in relation to the horizontal plane.1
BIOMECHANICS AND FUNCTION
The primary function of the PTFJ is to dissipate torsional loads applied to the ankle, attenuate lateral tibial bending moments, and transmit axial loads from weight bearing on the extremity.1 The degree of knee flexion, ankle dorsiflexion, and tibial rotation all play an important role in PTFJ biomechanics. In knee flexion, the proximal fibula moves anteriorly because of the relative laxity of the LCL and biceps femoris tendons. In knee extension, the LCL and biceps femoris tighten, pulling the proximal fibula posteriorly.1 Because the LCL and biceps tendon are both relaxed and less supportive during knee flexion, the PTFJ is more prone to injury with a flexed knee. The ankle plays an important role in the biomechanics of the PTFJ because it contains the distal syndesmosis, where both the tibia and fibula are firmly attached distally. During ankle dorsiflexion, the fibula must externally rotate to accommodate a wider anterior talus.1 In regard to tibia rotation, Scott and colleagues9 demonstrated the relationship between tibial rotation and fibular translation. With internal tibial rotation, the fibular head translated posteriorly and with external tibial rotation, the fibula translated anteriorly. The greatest translational motion was seen during loading of the knee into varus and during external tibial rotation at all flexion angles.
CLINICAL CONDITIONS
Ogden10 classified instability of the PTFJ into 4 main groups: anterolateral dislocation, posteromedial dislocation, superior dislocation, and atraumatic subluxation. Injury to the PTFJ usually occurs in younger, athletic patients during sports that require violent twisting motions such as soccer, basketball, dance, skiing, horseback riding, parachute jumping, jet skiing, and judo. Patients with generalized ligamentous laxity have been described as at increased risk for joint instability.10,11
ACUTE DISLOCATION
The most common injury to the PTFJ is an anterolateral dislocation and involves injury to both the anterior and posterior capsular ligaments, and occasionally the LCL.10 Anterolateral dislocation is usually the result of a fall on a hyperflexed knee with the foot inverted and plantarflexed.11 While most anterolateral dislocations are the result of indirect sports trauma, several have been associated with other types of skeletal injuries such as fracture-dislocation of the hip, crush injury of the proximal and distal ends of the tibia, fracture-dislocation of the ankle, proximal tibial fracture, and fracture-dislocation of the distal femoral epiphysis.10 Ogden10 described the mechanism as follows: (1) sudden inversion and plantar flexion of the foot causing tension in the peroneal muscle group, extensor digitorum longus, and extensor halluces longus, which applies a forward dislocating force to the proximal end of the fibula; (2) simultaneous flexion of the knee, relaxing the biceps tendon and LCL; and (3) twisting of the body over the knee, transmitting energy along the femur to the tibia, exerting a relative external rotatory torque of the tibia on the foot, which is already fixed in inversion. Steps (2) and (3) spring the proximal end of the fibula laterally while the contracting muscles of (1) pull the fibula anteriorly.
Posteromedial dislocation is the second most common type of acute PTFJ dislocation. Posteromedial dislocations usually involve direct trauma and are associated with peroneal nerve injuries.1,2,10 The mechanism of dislocation results in tearing of the anterior and posterior PTFJ capsular ligaments, followed by injury to the LCL and other surrounding ligaments. This allows the biceps femoris to draw the unsupported proximal part of the fibula posteromedially along the posterolateral tibial metaphysis.7
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