Hamard M1, Boudabbous S1, Tscholl PM2
1 Departement of Radiology, University Hospitals Geneva, Geneva, Switzerland
2 Department of Orthopedic and Trauma Surgery, University Hospitals Geneva, Geneva, Switzerland
Clinical imaging in patellofemoral disorders is fundamental for the understanding of the pathology, and to find the adequate treatment option. Anatomical risk factors such as trochlear dysplasia, patella alta, lateralized tibiale tubercle (measured by the tibiale tubercle trochlear groove distance), torsional or coronal lower limb alignment that are the origine of patellar maltracking or even patellar dislocation, can be assessed with high reliability on conventional radiographs combined with MR imaging. The conservative and surgical treatment will highly rely on these findings. The importance of clinical imaging in unspecific anterior knee pain is less clear. It is mostly performed to exclude structural injuries, however is rarely reliable to diagnose the painful structure. Critical appraisal of morphological risk factors is necessary especially in the treatment of anterior knee pain, since fortuitous pathological findings are frequent.
Une connaissance clinique adéquate est nécessaire pour une prise en charge optimale du syndrome fémoro-patellaire et pour l’interprétation de l’imagerie. Les anomalies anatomiques qui favorisent ce syndrome sont de nos jours bien investiguées par le couple radiographie conventionnelle-IRM. Ces anomalies sont nombreuses incluant la dysplasie trochléaire, la patella alta, une latéralisation du tubercule tibial (mesurée par la distance dite TAGT), anomalie d’alignement frontal ou de torsion des membres inférieurs; toutes ces anomalies sont à l’origine d’une mobilité inadéquate de la patella voir une luxation. Le traitement pourra être conservateur ou chirurgical et dépendra étroitement des trouvailles cliniques et radiologiques. En dehors de cette pathologie, le rôle clinco-radiologique est mal défini pour toute douleur antérieure du genou. Il a pour objectif principal d’éliminer d’éliminer des anomalies structurelles sans toujours trouver une corrélation avec la douleur. Une analyse critique et synthétique est nécessaire aussi bien pour les anomalies anatomiques trouvées ainsi que pour le choix du traitement.
Anterior knee pain (AKP) is one of the most common complaints in athletes, especially performing pivoting and jumping sports. Many structures around the patella can become symptomatic, mainly by structural overuse with or without patellar maltracking, poor muscle control or functional instability of the lower limb. The most frequently painful structures are the medial and lateral patellofemoral retinacula, the entheses of the patellar and less frequently the quadriceps tendon, and the intra-articular but extra-synovial fat pads. These structures, although visible on clinical imaging, might not always appear pathologic if painful, however are frequently associated with morphological risk factors. The analysis of the frequently painful structures and the anatomy predisposing to patellar maltracking requires a systematic approach on multi-modal imaging by radiologists and clinicians for the best understanding of the underlying causes of anterior knee pain.
The indication, advantages and disadvantages of the different imaging modalities will be here discussed in the context of anterior knee pain.
Patellofemoral dysplasia – radiological signs and measurements
Conventional radiography (CR)
CR allows detection and a rapid and global appreciation of the bony structure of the knee, the morphology of the patellofemoral joint and subchondral degenerative changes. The evaluation of AKP requires a weight-bearing antero-posterior (AP) view to evaluate the femoro-tibial knee joint, a strict lateral (L) view with overlapping posterior medial and lateral femoral condyles and an axial imaging in 30° of knee flexion (Table 1).
The most important morphological risk factors for patellar maltracking and patellar dislocation are based on the studies by Henry [1,2] and David Dejour  and are measured on the strict L view (trochlear dysplasia and patella alta). The tibial tubercle trochlear groove distance (TT-TG), initially measured on axial CR  is currently exclusively measured on multi-slice imaging (CT or MRI).
Trochlear dysplasia has according to Dejour et al  three radiographic features that are associated with a flat or even convex trochlea: the crossing sign (when the line of the trochlear groove crosses the highest point of the lateral trochlear facet), the double contour (when there is hypoplasia of the medial femoral condyle and the medial trochlear facet) and the supratrochlear spur (Figure 1). Only strict L view permit to evaluate this classification. Important to note, that trochlear dysplasia cannot be diagnosed on axial CR .
Patella alta is the second main morphological risk factor which can be visualized on the L view. Several indices have been proposed to evaluate the position of the patella, whereby the Insall-Savati (InSa-I)  and Caton-Deschamps index (CD-I)  are the most frequently used on CR, CT and MRI (Figure 2A) . Patella alta is defined by a InSa-I of >1.3-1.4 or a CD-I by >1.3, although the cut-off might vary in literature. Furthermore, surgical correction might be indicated only if CD-I is above 1.4, hence relativizing the cut-off values of patella alta .
Patellar dysplasia can be appreciated on axial CR according to the Wiberg classification. Patellae with a very short medial and therefore vertical facet (Wiberg type 3) are associated with trochlear dysplasia. This classification, however, has only limited clinical relevance . An infero-medial patellar spur is highly associated with trochlear dysplasia and can be a sign of patellar instability (Figure 3) .
Long leg standing CR can be indicated to evaluate associated valgus, which has been found to be an additional risk factor for patellar instability recently .
Multi-slices imaging modalities (MRI/CT)
Computed tomography (CT) and magnetic resonance imaging (MRI) are both nowadays widely available and permit multiplanar reconstructions in every planes. Whereas CT scan acquisition is much faster, irradiation has been minimized using cone-beam CT (CBCT) and the resolution of bony structure is more accurate, there is no irradiation with MRI and cartilage and soft-tissue are better visualized. MRI therefore is the gold-standard in patellofemoral assessment, especially in the early evaluation after primary patellar dislocation.
Whereas both imaging modalities can detect the main predisposing factors for patellar instability, cut-off values may differ due to the different landmarks (bony on CT versus cartilaginous-tendinous landmarks on MRI for trochlear morphology) and to the different flexion angle of the knee during acquisition of the images. Whereas CT are always performed in full extension, MRI are performed at about 10-15° of knee flexion due to the position in the knee coil, highly influencing the TT-TG value or patellar tilt.
Measuring trochlear dysplasia on 2D imaging has been introduced by David Dejour in 1998 on CT scan , and has led to the four type classification, from type A having a shallow trochlea to type D with a convex trochlea and the cliff pattern due to the hypoplastic medial trochlea and the absent trochlear groove (Figure 1). The classification of trochlear dysplasia on CT and MRI might differ due to the different osseous and cartilaginous trochlear surface, and also according the level of the axial image. Therefore, the trochlea needs to be assessed from proximal to distal for complete appreciation of its morphology and not solely on a single cut . There are several more measurement methods on axial imaging, such as the lateral trochlear inclination angle , sulcus angle , trochlear depth  and trochlear facet/condyle asymmetry (14) all able to describe trochlear dysplasia, however with only limited clinical relevance.
The most common patellar height ratios used on CR (InSa-I and CD-I) can also be measured on MRI (Figure 2B), however require a marginal correction of 0.1 for InSa-I and CD-I according to a comparative CR/CT/MRI study [15,16]. However, it might be difficult to find a single sagittal cut on CT or MRI which represents the entire patellar tendon and the patella, making the measurement of the InSa-I sometimes impossible . More recently, the ratio of the overlap between the patellar and the trochlear cartilage has been proposed by Biedert  and slightly modified by others [18,19]. This type of ratio however is highly depending on knee flexion angle, and on patellar position and needs therefore to be analysed carefully. The patellar tendon length with a cut-off value of 52 mm has gained only little attention in literature, probably due to the high interpersonal variability.
The tibial tubercle to trochlear groove (TT-TG) distance is a measurement for a distal lateral patellar vector force and represents the distance between the trochlear groove and the tibial tubercle and is increased above 15 mm and generally associated with trochlear dysplasia if above 20 mm . It can nicely be measured on CT by superposing two axial slices (Figure 4) or measured with adequate tools on MRI. Improved inter-rater reliability has been shown using the centre of the patellar tendon on the tibial tubercle (PT-TG). Some difficulties may arise in high-grade trochlear dysplasia where the trochlear groove is found only distally. Due to the distally lateral orientation of the trochlear groove, the TT-TG will be underestimated in these cases . The measurement results of CT and MRI cannot be compared. The flexed knee position on MRI decreases the TT-TG significantly compared to the fully extended knee on CT which undergoes the “screw-home mechanism” close to extension (Figure 5A and B). Therefore, TT-TG might vary up to 8-10 mm between CT and MRI. Due to the dynamic susceptibility of the TT-TG in full extension and the improved interrater reliability using soft-tissue landmarks, MRI in a standard knee coil should be preferred.
The patellar tilt, the lateral decline of the patella, described by Dejour  as one of the 4 major risk factors and pathologic if > 20°, is thought to be rather a consequence of trochlear dysplasia, patellar height, the TT-TG and probably also the patellar shape than an independent risk factor for patellar instability, and has only limited clinical evidence as a single measurement .
The femoral torsion is a further risk factor for patellar instability, which can be measured only on CT and MRI. It stands for the angle of the femoral neck to the posterior condylar line of the distal femur (Figure 6). For the MRI, the knee coil has to be removed, and the imaging is performed in full extension of the knee. The exact measurement and their cut-off values especially for surgical correction remain highly debated in literature , because of the wide range of values seen in healthy patients  and its several measurement techniques [25-28]. Usually, excessive femoral anterior torsion is considered > 25°. Since clinical examination may not be able to evaluate femoral torsion consistently, femoral and tibial torsion measurements on MRI (or CT) should be standard procedure for the assessment of patellofemoral instability.
Radiological signs of patellar dislocation
Acute patellar dislocation can be diagnosed on CR if the patella is locked, which is rare, or after reduction with a osteochondral fracture seen in the parapatellar recessus or a defect in the medial patellar facet on the axial CR view. A positive Holmgren sign, with a clearly delimited level showing liphemarthrosis, can be visible as well on lateral view, showing osseous injury. CT scan may show additionally condensed bone zones on the lateral femoral condyle and the inferomedial patella due to the impaction of the lateral patellar dislocation. MRI is able to show this bone bruise for several month, can visualize the entire size of the osteochondral lesion – which is almost exclusively seen in first episode of patellar dislocation (12-15%) – and MPFL rupture/tear which is present in almost all patients after primary patellar dislocation (Figure 7). In recurrent patellar dislocation, signs of MPFL rupture may be absent. Associated injuries occur are rarely seen, such as anterior cruciate ligament injuries, or medical collateral ligament tear.
Imaging findings in anterior knee pain and associated pathologies
The painful structures in anterior knee pain are almost exclusively the soft-tissues. The MRI (and ultrasonography) is therefore the imaging of choice:
Jumper’s knee, is an exercise related painful condition most frequently located at the distal tip of the patella. It mainly affects young athletes involved in jumping sports, that either perform a structural over use or a relative overuse in combination with either patella alta, or mild trochlear dysplasia . Most frequently, CR and MR imaging shows no pathologic findings. However, in the case of degenerative tendinopathy, the best imaging modality to assess this pathology is the MRI, which highlights proximal thickening of the patellar tendon of >7 mm, or intra-tendinous signal abnormalities near its central or medial insertion to the patella in chronic cases (Figure 8).  In only few cases, this tendinopathy affects the superficial fibers that stretch from the patellar tendon directly to the quadriceps tendon (galea aponeurotica).
The superolateral impingement syndrome is characterized by a friction of the infrapatellar and lateral fat pad between posterior side of patellar tendon and the anterior side of lateral femoral condyle [30,31]. It seems to be associated with patella alta, genu valgum or an excessive TT-TG. On MRI, oedema of the superolateral Hoffa fat pad and focal patellar tendinopathy can be seen (Figure 9). Due to the high-riding and often tilted patella, associated cartilage injury of the lateral trochlear and patellar facet are not unfrequent.
The pre-femoral fat pad impingement syndrome is caused by irritation of the pre-femoral fat pad that lies anterior to the distal femur, between the subquadricipital recessus and the anterior cortex of the distal femur. It is often associated with patella alta and femoropatellar maltracking (Figure 10). MRI shows oedema in the prefemoral fat pad in his central or inferolateral part.
The mediopatellar plica syndrome is usually presented as a diffuse anterior knee pain, with or without clicking or sometimes even a sensation of locking. It only can be visualized on MRI as a thin plicature of the capsule medial to the patella (Figure 11). There is no sign on imaging that proofs a conflict. Patient’s history, clinical imaging and its size lead to the diagnosis. Consecutive cartilage damage of the medial patellar facet is rare. The superopatellar plica or the intercondylar plica are other embryonic remnants, however much more rarely symptomatic.
Degenerative cartilage lesions are very frequently seen on MR imaging, most often however asymptomatic. Since there are no nerve endings in the cartilage, the origin of pain most probably lies in the subchondral bone. MR imaging (or in some selected cases where arthro-CT scan might be necessary) can graduate and localize the extent of the lesion according to the Outerbridge classification.
Patella bipartita describes the presence of an unfused ossicle usually found on the superolateral aspect of the patella (Figure 12), and can be found in 1-2% of the population. It usually is asymptomatic, however direct trauma to the patella can induce pain at fibrous scar, which can be visible on MRI.
- CR and MR imaging is mandatory to diagnose patellar dislocation, to visualize associated injuries and to evaluate patellofemoral anatomy known as the major risk factor for recurrent instability.
- Trochlear morphology, patellar height and torsional alignment also highly influences patellar tracking, that can be the origin of anterior knee pain.
- Its understanding in combination with clinical examination is the mainstay of a successful treatment.
Dr. med. Marion Hamard
Département de Radiologie
Hôpitaux Universitaires Genève
Rue Gabrielle-Perret-Gentil 4
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