Radiographic evaluation of the knee for instability traditionally involved a measure- ment of the quadriceps angle (or Q angle), first described by Brattstörm [3]. Two lines are drawn, one from the ipsilateral anterior superior iliac spine (ASIS) to the center of the patella and the second from the center of the patella to the tibial tuber- cle (Fig. 10.1). Knees with Q angles greater than 20° are at higher risk for patellar instability and have also been associated with anterior knee pain [23]. Q angles are
Fig. 10.1 Frontal radiograph of the lower pelvis through the proximal tibias
demonstrates measurement of the Q angle. One line (solid white line) is drawn from the anterior superior iliac spine to the central patella, and the second line (dashed white line) is drawn from the central patella to the tibial tuberosity and then extended superiorly. The angle between the two lines is the Q angle
increased in various clinical scenarios, such as genu valgum, increased femoral anteversion, laterally positioned tibial tuberosity, and external tibial torsion. The reliability and validity of the Q angle remain controversial. Radiographs can evalu- ate static alignment of the patellofemoral joint, with patellar position and height seen on frontal and lateral radiographs respectively. Axial radiographs are obtained with the patients’ knees in some degree of flexion, which engages the patella in the trochlea. Axial views vary tremendously in terms of the information they provide, based on the degree of knee flexion they are taken at. At higher degrees of flexion, less information can be obtained regarding patella position relative to the trochlea (tilt and subluxation). True lateral radiographs are the most appropriate views to assess trochlear morphology.
After patellar dislocation, radiographs may reveal effusion and soft tissue swell- ing which dissipate, as well as lateral patellar tilt and subluxation. The typical medial patellar fleck of the bone is present on radiographs in about one third of acute dislocators [21].
Cross-Sectional Imaging (CT and MRI)
A more reliable and validated measurement which can be done on cross-sectional imaging is the tibial tuberosity-trochlear groove (TT-TG) distance [25]. Since patel- lar position and alignment vary on static CT and MR imaging exams, the TT-TG has become a commonly used surrogate marker of patellar alignment which is relatively stable between imaging modalities and in various pathologies and is thus used most often now to help determine treatment options [4, 5]. On MRI, it is measured from the deepest point of the trochlear sulcus to the midpoint of the patellar tendon at its insertion on the tibial tuberosity on a direct axial image [25, 26] (Fig. 10.2). An increased TT-TG distance has been shown to be associated with the presence of lateral patellar facet chondrosis [25] and patellar instability [2, 18]. The trochlear angle, the angle between the medial and lateral trochlear facets on axial image, can also be used to indicate propensity for dislocation, such as in a shallow trochlea with a higher trochlear angle (Fig. 10.3).
In the setting of transient lateral patellar dislocation, reduction often occurs prior to imaging, and the clinical presentation of acute pain and knee swelling can be nonspecific. For this reason, the presence of hallmark features on imaging is necessary to make or confirm the diagnosis of a patella dislocation. MRI of the knee is frequently performed to evaluate underlying anatomy, characteristic bone marrow contusions indicative of dislocation, location of disruption of the medial retinacu- lum and medial patellofemoral ligament (MPFL), and extent of any fractures, as well as to identify osteochondral injury and fragments [14] (Fig. 10.4).
When the patella dislocates laterally or on its way back in, the inferomedial aspect of the patella impacts against the anterior lateral aspect of the extra-articular lateral femoral condyle [8], causing bone marrow contusions which present as focal
a d
f b e
c
Fig. 10.2 Axial proton density-weighted images of the knee demonstrate measurement of the TT-TG distance. A vertical line is drawn through the tibial tubercle (panel a), a second vertical line is drawn through the center of the trochlear sulcus (panel b), and the distance between those two lines is measured (panel c), which is normal in this case (10.6 mm). In panels d–f, using the same method as in panels a–c, an abnormal TT-TG distance of 20.1 mm is seen
areas of bone edema (Fig. 10.5). The contusion on the lateral femoral condyle is more anterior, superior, and lateral than those seen in the pivot shift of anterior cru- ciate ligament tears [15, 27]. Inferomedial patellar osteochondral injuries are pres- ent in about 70% of patellar dislocation patients [9]. More rarely, a shear-type fracture may occur along the lateral femoral condyle [24]. With dislocations that occur at higher degrees of flexion, there is higher risk of injury to the lateral femoral condyle rather than the lateral trochlea.
Fig. 10.3 Axial radiograph of the knee demonstrates a shallow trochlear angle of 153.7° (normal <145°)
Fig. 10.4 Axial proton density-weighted image demonstrates a grade IV chondral injury at the patellar median ridge (black block arrow), with a displaced chondral fragment (white arrow) and large knee effusion visible in the same slice.
Chondral injury dictates early surgery
In the acute setting, the location (femoral vs. patellar) of MPFL/medial retinacu- lar complex tear can be difficult to discern as almost every patient will have some injury pattern on the patellar side. Femoral injury is indicated by proximal edema at the anterior aspect of the proximal medial collateral ligament. The MPFL itself is often difficult to discern [7].
Intra-articular bodies, which are seen in as many as 33% of patients after patellar dislocation [7, 9, 15], usually present as separated fragments of chondral tissue or osteochondral fragments from the medial patella or lateral femoral condyle.
The majority of patients who have dislocated and spontaneously relocated their patellae will have persistent patellar subluxation or tilt due to concomitant MPFL/
medial retinacular complex injury and joint effusion [7].
Fig. 10.5 Axial radiograph and fat-suppressed MRI sequences demonstrate classic findings of acute transient lateral patellar dislocation. Panel a shows a small fracture fragment (white arrow) at the medial patellar facet corresponding to an area of bone marrow contusion (white arrow) on MRI (panel b). In a different patient, another classic bone marrow contusion pattern is seen along the medial patellar facet (white arrow, panel c) and lateral femoral condyle (white arrow, panel d)
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b