DIAGNOSTIC IMAGING
Plain radiographs in the anteroposterior (AP) and true lateral views are useful as first-line investigations in suspected PTFJ dislocation. Comparable AP and lateral radiographs of each knee are highly recommended to detect findings that suggest dislocation.2 Abnormal findings include increased interosseous space, medial or lateral displacement of the fibula on the anteroposterior view, and anterior or posterior displacement of the fibula head on lateral view as shown in Figure 2.19,20
Resnick and colleagues2 proposed the use of the linear sloping radiodensity that defines the posteromedial corner of the lateral tibial condyle as an indicator of anterolateral or posteromedial PTFJ dislocation. However, this application is limited because of the PTFJ’s highly variable morphology.8 In a recent study conducted by Hey and colleagues,21 5968 (2984 patients) knee radiographs were retrospectively collected and subjected to radiographical measurements and statistical analysis. The tibiofibular overlap method had a specificity of 94.1% and 84.5% when diagnosing PTFJ dislocations on the AP and lateral views, respectively.21
If a diagnosis of PTFJ is suspected but not clearly established based on radiography, computed tomography with comparison views of the contralateral knee are recommended to confirm the diagnosis.17,22 This becomes more critical in cases of suspected subluxation/chronic PTFJ instability. Additionally, magnetic resonance imaging (MRI) can be used to assess chronic PTFJ instability. Recently, Burke and colleagues23 performed a 10-year retrospective case series that included 7 patients with chronic PTFJ instability and included MRI as part of their evaluation. The MRI abnormalities in these patients included periarticular soft tissue edema, including in the proximal soleus muscle (n = 5), periarticular ganglion or ganglia (n = 4), tibiofibular ligament edema (n = 4), subchondral marrow edema (n = 3), posterior tibiofibular ligament thickening (n = 2), subcortical cyst at a ligament insertion (n = 2), partial-thickness tear of the anterior tibiofibular ligament (n = 1), and tibiofibular joint effusion (n = 1).
OSTEOARTHRITIS
Routine knee radiographs can show PTFJ joint space narrowing, sclerosis, marginal osteophytes, and local osteopenia as conventional components of osteoarthritis of any joint. Serial radiographs have also been described as effective in evaluating progressive degenerative changes of the PTFJ.14 An MRI will show osteophyte formation, subchondral cysts, subchondral sclerosis, joint effusion, joint space narrowing, and is highly sensitive for detecting degenerative changes in cartilage, as well as identifying other possible pathologies such as synovial cysts or pigmented villonodular synovitis. Chronic PTFJ instability appears to predispose to tibiofibular osteoarthritis as reported by Burke and colleagues,23 who found a particularly high incidence (42.9%) of osteoarthritis in patients with chronic PTFJ instability. Additionally, Veth and colleagues14 found degenerative changes in 8 of 19 patients presenting with PTFJ dislocations.
TREATMENT
ACUTE DISLOCATION
Prompt recognition and treatment of any acute PTFJ dislocation are necessary to avoid long-term instability and other possible sequelae.11 Treatment consists of reduction followed by restriction of weight-bearing.11 Traditionally, the knee is immobilized with a cast in extension for 3 to 4 weeks followed by knee mobilization and progressive range of motion exercises,24 but there is some controversy regarding complete immobilization.11,25,26
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