chapter 4:imaging the patellofemoral joint
The lateral radiograph may be taken in the lateral decubitus position or standing with the knee extended and flexed (30 degrees) (Fig. 4.2). This places the patellar tendon under tension and demonstrates the functional relationship of the patella to the tibia and, more important, of the patellar facets to the femur. The beam is centered on the tibiofemoral joint line. This degree of flexion should correspond to that of the tangential view.
The lateral view adds considerably to our knowledge of the patella. Patellar height and thickness can be measured. One can observe the general morphology and determine if there is subchondral sclerosis, evidence of arthrosis, or calcification in the quadriceps or patellar tendon. Lund and Nilsson (3) noted a shallow excavation in the patella of most patients with proven "chondromalacia patellae," which may be noted on the lateral radiograph. One can also determine the vertical distance of the patella from the tibia.
Maldague and Malghem (4‑6) have described the interpretation of a precise lateral radiograph of the knee with regard to patellar alignment. This interpretation, however, depends on an exact lateral of the knee in which the posterior and distal femoral condyles are overlapped. Consistent reproduction of the precise lateral requires fluoroscopy. An experienced radiologic technician, however, can obtain precise laterals in the majority of patients. This requires palpation of the posterior condyles and experience in lining up the posterior femoral condyles on the radiograph cassette.
One major advantage of the lateral view is that it provides information regarding patellar rotation (tilt) with the patient in a weightbearing position. The lateral view may be taken in full extension and again at the desired degree of knee flexion. In most patients, this will be 30 degrees of knee flexion or, to correlate with a standard Merchant view, at 45 degrees of knee flexion. Thus, the clinician is able to get a more functional view of patellar rotation in weightbearing. Murray has pointed out that the lateral view is particularly sensitive in detecting patella dislocators(7)
Figures 4.3, A to C show what can be seen with a precise lateral. Note that the normally aligned patella demonstrates a central ridge that is posterior to the lateral facet line (Fig. 4.3, A). When the patella is rotated (tilted), one may notice overlap of the central ridge and lateral facet lines (Fig. 4.3, B). With more extreme rotation of the patella, usually associated with some subluxation (lateral translation), the central ridge may actually rotate anterior to the lateral facet, giving a picture as seen in Figure 4.3, C. The precise lateral view, therefore, becomes an excellent image for determining rotation of the patella. Unfortunately, the lateral view really does not help much in evaluating translation of the patella (as considered separately from tilt). Translation (subluxation) will be better evaluated on the axial view.
The lateral view is also very helpful in evaluating trochlear morphology (7). The center of the trochlea will be seen as the most posterior line (small arrow), while the medial (large arrow) and lateral (open arrow) condyles may be seen separately (Fig. 4.4). Normally, the lateral trochlear line should terminate well proximal to the medial trochlear line without crossing the central trochlear line. With experience, the clinician will learn to appreciate morphology of the trochlea better on the lateral view than is possible on any other view, including the axial. This is because the axial view only gives one portion of the trochlea, whereas the entire trochlea may be seen on the lateral view with full excursion of the medial and lateral condyles and the center of the trochlea. Thus, it is possible on the lateral view to obtain an impression of the entire trochlea with regard to its depth and proximal configuration. This information is extremely important as it relates directly to patellofemoral stability.
Blumensaat (8) has described the normal relationship of the patella to an anterior prolongation of a dense line that marks the ventral border of the intercondylar fossa (Blumensaat's line) (Fig. 4.5). Elevation of the distal pole of the patella above this line with the knee flexed 30 degrees has been widely accepted as indicative of patella alta. Insall and Salvati (9) have demonstrated that Blumensaat's line is not an accurate measurement for the position of the normal patella at 30 degrees of flexion. This has been confirmed by Jacobsen and Berthensen (10) on bilateral knee radiographs in 50 asymptomatic volunteers. Their method is simple and does not depend upon exact positioning of the degree of flexion. It consists of determining the ratio of patellar tendon length (LT) to greatest diagonal length of the patella (LP). Both series (9, 10) indicate an upper limit of normal of 1.2 (at the 90% level). Grelsamer and Meadows (11), however, showed that the Insall‑Salvati ratio lacks sensitivity because of variations in patella morphology. A long, distal, non‑articulating portion in a patella with a short articular area will give an inaccurate impression in some patients when the Insall‑Salvati ratio is used (Figs. 4.6, A to D). Grelsamer described a "morphology ratio" relating the patellar articular length to the overall patellar length. Blackburne and Peel (12) proposed another modification of this technique using the tibial articular surface as the reference level for patellar height (Fig. 4.7). These researchers measured the patellar articular length and determined its ratio to the distance between the tibial articular surface and the patellar articular surface. This ratio is 0.8 in normal knees flexed at 30 degrees. Caton described a similar method (13).
Bernageau (14) emphasized the proximal trochlea and its relationship to the patella (Fig. 4.8). Ultimately, the clinician must identify whether or not the patella is correctly aligned, and whether there is smooth entry of the patella into the trochlea. The Bernageau technique makes a lot of sense in this regard, and is currently the author's preference.
Inside Chapter 4: