Clinical History: A 23 year-old female presents with medial knee pain following a twisting injury. (1a) A single fat-suppressed proton density-weighted coronal image is provided. What is the diagnosis?
Transient lateral patellar dislocation.
Although you may feel that being asked to make the specific diagnosis of lateral patellar dislocation from a single image is unrealistic, the coronal view in fact reveals a classic and highly characteristic appearance, allowing the diagnosis to be made with confidence.
The marrow edema and medial patellofemoral ligament (MPFL) injury pattern above are virtually pathognomonic of a transient lateral patellar dislocation, as little else will cause such an appearance. Given the lack of history of direct trauma, a reliable diagnosis can be made.
The pattern of bone bruising seen in a transient lateral patellar dislocation is easy to understand if one considers the mechanism of injury. The injury typically occurs from a twisting event with the knee in mild flexion (less than 30°). The patella becomes unstable and undergoes a transient, violent lateral displacement. During this dislocation phase of injury, shearing forces can damage the articular surfaces of either the patella or the lateral femoral condyle. With knee extension and subsequent reduction, the patella bounces back into position and in doing so, the medial patella impacts against the non-weightbearing anterolateral aspect of the lateral femoral condyle, resulting in the characteristic marrow edema pattern.
The convex articular surface of the patella places it at risk for chondral injury in either the dislocation or the reduction phases of injury. Femoral condylar chondral injuries occur during the dislocation phase due to impaction shearing forces of the patella upon the flexed femur, typically occurring at the weightbearing surface. Because as noted above, the femoral bone bruise occurs as the patella moves forward during reduction, bone bruises at the lateral femoral condyle always course anteriorly from the site of any femoral chondral injury that may be present. The trochlea, due to its large surface area and concave contour, is rarely subject to chondral injury in patellar dislocation.
The above video demonstrates the mechanism of injury in patellar dislocation. Courtesy of Daniel Bodor, MD, Radsource.
Because of the transient and brief nature of lateral patellar dislocations, the diagnosis is frequently unrecognized by both patients and clinicians.1 Pain is often described as medial because of soft tissue injuries that occur to the medial retinaculum and/or MPFL. Evaluation of both of these soft tissue structures is critical when one examines a MRI following lateral patellar dislocation, as the extent of soft-tissue injury influences the use of operative repair. Trauma itself rarely causes patellofemoral dislocations without predisposing factors such as trochlear dysplasia, patella alta and lateralization of the tibial tuberosity. MRI plays a crucial role in quantification and characterization of these predisposing anatomic variations which are key to addressing the patient?s patellar instability operatively. Nearly 50% of all first-time dislocators will sustain further dislocations. Recurrent dislocations and chronic patellofemoral joint instability will lead to significant cartilage damage and severe arthritis if left untreated. The primary aim of surgery is to repair the knee damage and to correct the anomalies that are predisposing to chronic instability.
Anatomy and Function
The most important soft tissue passive stabilizers involved in patellofemoral dislocation injuries have traditionally been referred to as the MPFL and the medial retinaculum. Despite the presence of numerous detailed anatomical dissection and MRI based studies of the anatomy2,3, there is much variability in the description of these structures within the literature. Most, however, agree regarding the importance of the MPFL and its role as the strongest restraint to lateral patellar displacement. The MPFL is composed of a stronger transverse band and a more variable and weaker oblique decussation. The transverse band attaches to the upper pole of the patella, and this component originates on a bony groove that lies between the adductor tubercle and the medial epicondyle, slightly posterior to the epicondyle. The oblique decussation of the MPFL blends with fibers of the superficial MCL. Other structures that blend in this region include the medial patellotibial ligament, the medial patellomeniscal ligament, the investing fascia, and the medial joint capsule. In effect all three medial layers of the knee thus comprise the medial retinaculum, which is by itself not a discrete, single structure. Anatomically, the transverse band of the MPFL is also a component of the medial retinaculum, but for purposes of MRI interpretation, the general convention is to describe abnormalities of the transverse band as being MPFL injuries, whereas more distal injuries which involve multiple layers are generally referred to as abnormalities of the medial retinaculum.
As with the anatomy, there is considerable variability in both the surgical and radiology literature regarding the location of soft tissue injuries in patients following patellar dislocation. There is agreement, however, that the MPFL is almost always injured with lateral patellar dislocations4. In our experience, injuries to the transverse band most often involve the femoral attachment, whereas injuries to the oblique decussation and associated medial retinacular structures are more common at the patellar attachment. The femoral attachment of the transverse band of the MPFL is not always discretely visible, and therefore secondary signs on MRI such as fluid, edema, and soft tissue thickening at the attachment indicate MPFL injury.
The most important active stabilizer of the patella is the vastus medialis oblique (VMO). The VMO blends distally with the MPFL to attach to the medial border of the patella along its upper two thirds. The VMO is active, not only in full extension but also at 30 degrees and up to 100 degrees of flexion. It acts as a powerful extensor of the knee. VMO dysplasia may play a role in patellofemoral instability. Fluid extending into the VMO myotendinous junction is generally seen in the setting of coexisting MPFL/retinacular tears.
The osteocartilaginous anatomy of the patellofemoral joint provides additional static stabilization to the joint. Trochlear geometry, including slope of the lateral wall and depth, is an important factor. Predisposing factors to patellofemoral dislocation include patella alta, excessive lateral distance between the tibial tubercle and trochlear groove and a congenitally shallow trochlear groove5, any of which significantly increase a patient’s likelihood for dislocation. Less common predisposing factors to be aware of include laterally tilted patella, VMO dysplasia and generalized joint laxity.
Patella alta is considered an important factor in patellar instability. The decreased patellar contact area decreases stability particularly in shallow degrees of flexion and thus predisposes to lateral patellar subluxation. There are several different techniques described in the literature to assess patella alta and many of these are reviewed in detail in the MRI Web Clinic, August 20106. The Insall-Salvati index is the most widely accepted measurement and is easily performed on radiographs and MRI examinations.
Excessive lateralization of the tibial tuberosity allows the patella to be pulled laterally in flexion and is considered to be a risk factor for instability. The distance from the tibial tubercle to the trochlear groove (TT-TG) provides a quantitative evaluation of excessive lateralization of the tibial tuberosity.7
The normal trochlea is concave with medial and lateral facets articulating with the patellar facets. Flattening or developmental dysplasia of the trochlea leads to loss of normal patellar tracking and can predispose to lateral patellar dislocation with flexion. Techniques for quantifying trochlear dysplasia with MRI have been standardized and shown to be accurate and reproducible in assessing lateral trochlear inclination, facet asymmetry, and trochlear depth8,9. The discrimination between low-grade and high-grade dysplasia is important because prognosis and treatment may depend upon the severity of trochlear dysplasia. These measurements are not routinely recorded in the MRI report, but in select situations may be helpful in quantifying low-grade versus high-grade dysplasia.
Trochlear depth of less than 3 mm on MRI has a sensitivity of 100% and a specificity of 96% for trochlear dysplasia.10
MRI Findings in Transient Patellofemoral Dislocation
Bone bruises at the anterolateral aspect of the lateral femoral condyle and at the inferomedial patella are the most constant findings in patients who have sustained a recent patellar dislocation. Osteochondral injuries of the inferomedial patella are seen in up to 70% of patellofemoral dislocations. Osteochondral injuries to the inferomedial patella may be the result of impaction during dislocation or shearing with reduction. Femoral osteochondral injuries, when present, typically involve the lateral weightbearing surface. In either case, careful search for possible displaced osteochondral fragments is necessary, as surgery is often indicated in such patients.
Abnormalities of the medial retinaculum and MPFL are seen in 82-100% of MRI examinations following patellar dislocation. Characterization of the type of medial restraint injury is crucial for surgical planning. Tears of the medial retinaculum and the MPFL are commonly present at both the patellar and femoral attachments, though tears of the transverse band of the MPFL are more likely at the femoral attachment. Femoral avulsion of the MPFL is a predictor of chronic instability and may be a surgical indication in some patients with acute injury. Persistent abnormalities of patellar alignment and abnormalities of trochlear morphology are also common, and should be carefully described.
MRI findings of Chronic Patellofemoral Instability and Recurrent Dislocations
Patients who experience multiple patellar dislocations are more likely to have anatomical variants of the trochlea, patellar alta, or tibial tubercle lateralization. Of course, if medial soft tissue restraints are disrupted at the first dislocation, the loss of such restraints makes future dislocations more likely. Areas of scarring and osteochondral lesions become more common as the frequency of patellar dislocation increases.
Surgical Indications and Repair Techniques
Nonoperative treatment is generally recommended in first time dislocators unless there are MRI findings of severe predisposing dysplasia and the presence of a chondral or osteochondral body. Such patients are generally treated with immobilization for 3 to 6 weeks. Physical therapy is directed to increasing range of motion and to strengthening the VMO and quadriceps muscles. The patients are then J-braced for 3 to 6 months for all sports activities.
Surgical indications following patellar dislocation include the presence of a chondral or osteochondral body, significant rupture of the medial stabilizers most importantly the MPFL, a persistent laterally subluxed patella, or a second dislocation injury in a patient with malalignment or dysplasia. It is reported in surgical literature that 50-75% of recurrent dislocators, have some form of dysplasia, malalignment or generalized joint laxity.
Surgical repair is most commonly directed to MPFL reconstruction and to distal realignment in the subset of patients who have significant osseous malalignment. The latter distinction is important to recognize among both radiologists and surgeons. In patients without osseous malalignment, MPFL reconstruction and plication of the medial restraints is reported to decrease re-dislocation rates to 5%. However, in the setting of osseous malalignment, MPFL reconstruction alone leads to higher rates of recurrent instability. In general realignment surgery such as tibial tubercle transfer should be strongly considered in patients with TT-TG > 15mm (borderline) and typically should be performed in patients with TT-TG > 20mm. It is therefore recommended that radiologists include measurement of TT-TG in reports on patients who undergo MRI for patellar instability.
There are numerous techniques reported in literature and commonly used to reconstruct the medial sided soft tissue constraints. One of the more common MPFL reconstruction procedures uses a single hamstring tendon graft passed through the medial intermuscular septum at the adductor magnus insertion fixed by an anchor in the femoral condyle and sutured or anchored to the superomedial pole of the patella11. MRI can provide valuable information regarding the status of such repairs in patients who experience recurrent dislocation following surgery.
Surgical realignment procedures include medialization of the tibial tubercle particularly in patients with a TT-TG distance greater than 20mm. Patients with patella alta may also benefit from tibial tuberosity advancement.
Trochleoplasty is rarely performed in this country and is reserved for significant dysplasias or when other surgical options have been insufficient in restoring patellofemoral stability. The most common trochleoplasty procedure described is a combination of lateral trochlear osteotomy and bone graft to heighten the lateral trochlea.
Recent literature does not encourage the use of lateral release, since this can increase patellar instability. The lateral retinaculum provides significant additive support to the medial stabilizers. Long term follow up studies in patients who have undergone a lateral release have shown an increased incidence of patellofemoral arthritis.
Lateral patellar dislocation is a common clinical entity with a characteristic MR appearance. Because the diagnosis of lateral patellar dislocation is often unsuspected, MR provides valuable diagnostic information in such cases. Additionally, MR’s ability to delineate the extent of injury and predisposing factors is important in patient care and surgical planning.
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