Patellar Injury and Dislocation Workup
- Author: Gerard A Malanga, MD; Chief Editor: Craig C Young, MD more...
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- Plain radiography
- Plain radiography is the most common diagnostic imaging study performed for patellofemoral dysfunction, and it is the least costly.
- A standard radiographic protocol for the screening for patellar dislocation includes anteroposterior, true lateral, and axial or sunrise views.
- Several variations of the axial view radiograph can be used, but the most widely accepted techniques are taken with the patient supine and the knee at 20-45° of flexion with and/or without external rotation of the tibia.
- A true lateral view can be used to assess for several possible problems. First, with a vertical position of the knee, it can be used to evaluate for patella alta or baja. Second, it can be used for a measurement of patella tilt. Some authors find this view provides more sensitivity for patellofemoral pain and previous subluxation than a measurement of tilt on an axial view. Third, it can help evaluate trochlear depth. In this view, examination of the proximal portion of the trochlear groove for dysplasia is possible. This area is especially important in patella motion in early flexion; this evaluation is not possible on an axial view.
- A Merchant view can be used to determine the severity of patellar subluxation and the presence of a maintained dislocation, although this second condition is not very common because most acute dislocations spontaneously reduce.
- Plain radiography is also very effective for identifying patellar, tibial, and femoral fractures. In a study of 214 patients with acute knee trauma, a single lateral view radiograph had 100% sensitivity in detecting fractures. Overall, plain radiography is an effective and inexpensive screening tool that is particularly useful in diagnosing fractures. However, radiography has limited value in assessing soft-tissue structures.
- CT scanning
- When CT scanning was tested against standard radiography for determining patellar subluxation, CT scanning was found to be significantly more sensitive.
- Stanciu et al found that CT scanning was roughly 1.5 times more sensitive in detecting patellar tracking anomalies than standard radiography. In Shea and Fulkerson also found that CT scanning was highly effective for detecting patellar subluxation. Using CT scanning, the authors accurately determined the severity of patellar tilt versus subluxation. In addition, based on CT scan findings, Shea and Fulkerson selected the patients for lateral retinacular release procedures (ie, patients in whom conservative treatment was unsuccessful). Finally, based on CT scan findings, they predicted the prognosis for patients undergoing surgery.
- CT scanning may also be performed as a dynamic study. This study is performed using a helical CT scanning process and a continuous 10-second exposure through the midtrochlea level; patellofemoral dynamics may be evaluated for their in vivo physiologic relationships.
- CT scanning has advantages over plain radiography because it is capable of imaging patellar cartilage.
- A disadvantage of CT scanning is that it is less sensitive than magnetic resonance imaging (MRI) in detecting lesions in cartilage. CT scanning often requires contrast medium for certain studies, and it also exposes patients to radiation.
- MRI is extremely sensitive for identifying soft-tissue anomalies of the knee. In one study, MRI helped physicians identify MPFL injuries occurring secondary to acute patellar dislocation. MRI can also be used to help determine if joint laxity is present in the absence of bony deformities. MRI can be very helpful in assessing the extensor mechanism and its relationship to the patellofemoral apparatus.[18, 19]
- Although far more expensive, MRI is more effective than CT scanning in determining if patellar chondral lesions are present and for determining cartilage thickness and volume. Assessment of patellar cartilage thickness and volume is important to evaluate for osteoarthritis, and it is useful to preoperatively calculate the effect of joint contact and load transmission after surgical procedures.
- T1-weighted images are not as sensitive as T2-weighted images. However, T2 imaging yields a high number of false-positive results, which may be due to the detection of very early cartilaginous lesions. Further tests need to be performed.
- The kinematic MRI method is quite suitable to determine patellar tracking and other functional aspects of the patellofemoral joint. Brossmann et al found kinematic MRI findings correlated well with arthroscopic findings in patellofemoral pathology. The clinical value of kinematic MRI is questionable for most patients, and these studies are not available at most centers.
- Axial views on MRI or CT scans may be used to evaluate the trochlea-tubercle distance. This is the horizontal distance in a vertical plane between the intercondylar notch and the tibial tubercle. Similar to the Q-angle, this evaluates the potential laterally directed vectors on the patella. Some authors suggest that a distance of 2 cm is specific, but not sensitive, for maltracking.
- Delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) may be a useful method to monitor cartilage degeneration in patients with recurrent patellar dislocation following conservative treatment. Watanabe et al assessed 3 groups of knees: group I, both knees of patients with bilateral RPD and dislocated side knees of patients with unilateral RPD; group II, nondislocated side knees of patients with unilateral RPD; and group III, both knees of healthy volunteers. The investigators analyzed postcontrast T1 [T1(Gd)] of cartilage at the medial and lateral facets of all 3 groups and found significant differences in the mean T1(Gd) among all groups for both medial and lateral facets. In addition, the T1(Gd) of cartilage for both medial and lateral facets had a significant correlation with the length of time between the initial dislocation and MRI (P < 0.05).
- Ultrasonography is not typically used in the assessment of patellar injury and dislocation.
- Ultrasonography is better than CT scanning for distinguishing between cysts, granulation tissue, metaplasia, mucinoid degeneration, and congenial defects of the patellar ligament. Knowing that ultrasonography is available to help determine the cause of anterior knee pain is important.
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