Radiography

Applications

Radiographs are the workhorse of knee imaging. Nearly any symptom or sign may be initially evaluated with an x-ray. Radiographs provide useful information across the entire spectrum of knee pathology, including congenital deformities, arthritis, trauma, oncology, sports injuries, metabolic disease, and arthroplasty evaluation.³⁶

Technique

A brief orientation to x-ray technology enhances an understanding of knee imaging. An x-ray tube converts electricity into a beam of high-energy photons. The x-ray beam is aimed through the knee. A cassette containing x-ray film is positioned opposite the x-ray beam on the other side of the patient. Photons that pass through the patient strike the film, exposing it. Tissue density is the primary determinant of whether a photon successfully reaches the film. A dense tissue such as bone absorbs or deflects most photons. So, few photons reach the film. Areas of unexposed film appear white, representing dense tissue. Less dense materials, such as the lung or fat, do not obstruct as many passing photons. Here the exposed film appears darker. Simplistically, a radiograph is a shadow formed by high-energy light outlining the patient’s anatomy.

Traditionally, an exposed film was developed and hung for interpretation. Over the past 20 years, new cassette designs have replaced film with an imaging plate, creating computed radiography (CR). After exposure, the imaging plate is run through a CR reader and the captured image is digitized. The imaging plate is subsequently reset and can be reused thousands of times. Digital radiography (DR) represents the next evolution of filmless image capture. DR dispenses with the cassette entirely and utilizes a flat panel detector. The detail and overall image quality of DR are superior to CR.

A remaining niche for true film radiography is the standing, frontal, long-standing view radiograph of the lower extremity. This may be requested for precise knee anatomic and mechanical axis measurements.

After an image has been acquired by a CR or DR reader, it is transmitted to the picture archiving and communication system (PACS) for interpretation. The advantages of PACS are manifold, including image manipulation (windowing, zooming, etc.), transmission (electronic), and storage (online, easily accessible). The time-honored file room has given way to a well-ventilated closet housing several hard-working computers.

Radiographic Views

Standard radiographic examination of the knee consists of three views: anteroposterior (AP), lateral, and axial (sunrise or Merchant). Tunnel, posteroanterior flexion weight-bearing (Rosenberg view) and oblique views may be performed for particular indications. In the setting of knee instability and ligamentous injury, stress radiographs may be performed. If a bilateral examination has been requested, each knee should be imaged separately.

Anteroposterior Radiograph

The AP view is obtained with the knee extended, the cassette behind the knee, and the central x-ray beam perpendicular to the cassette. A standing (weight-bearing) AP radiograph more accurately assesses the joint space than one obtained with the patient supine.^1,^2,^31,³⁸ For this reason, as well as to allow the estimation of valgus or varus angulation, weight-bearing images are preferable whenever possible (Fig. 5-1). Normal structures evaluated on every AP radiograph of the knee are the patella, the medial and lateral femoral condyles, the medial and lateral joint compartments, the tibial spines, the medial and lateral tibial plateaus, and the fibula. The AP view also provides a gross assessment of femoral tibial alignment (Fig. 5-2A). The lateral compartment is normally slightly wider than the medial.

Figure 5-1 Anteroposterior supine versus weight-bearing views. Severe medial joint space narrowing is more apparent on the weight-bearing view (A) compared with the supine view (B).

Figure 5-2 A, Normal anteroposterior view of the right knee. Femoral-tibial alignment is in 6 degrees of valgus angulation. The lateral compartment is normally slightly wider than the medial compartment. B, Lateral view of the knee. Blumensaat’s line (open arrow) represents the roof of the intercondylar notch. The physeal scar is indicated by the solid arrows. The patella is commonly located between these two lines, with the lower pole approximately at the level of Blumensaat’s line. The Insall-Salvati ratio is a more accurate method of assessing patellar height: the length of the patellar tendon (dotted line) divided by the greatest diagonal length of the patella (solid line) should be approximately 1 (0.8 to 1.2).

Lateral Radiograph

The lateral view is obtained with the knee flexed 30 degrees and the patient lying on the affected limb. The cassette is positioned under the lateral side of the knee, and the x-ray beam is directed perpendicular to the cassette. This view depicts the quadriceps tendon, the patella, the patellar tendons, the suprapatellar bursa, the distal femur, the proximal tibia, and the proximal fibula (Fig. 5-2B).

The medial femoral condyle is slightly larger than the lateral condyle. The lateral femoral condyle can be identified by the presence of the lateral femoral sulcus at the anterior aspect of its weight-bearing portion.³⁴ Blumensaat’s line represents the roof of the intercondylar notch. The closed physeal scar is also evident on the lateral view, and the patella should fall between it and Blumensaat’s line.

The tibial plateaus slope downward as they progress posteriorly, a fact that can aid in fracture identification. The plateaus may be differentiated by several clues. The higher of the two tibial spines belongs to the medial plateau. At its posterior extent, the medial tibial plateau projects most dorsally and is squared. In contrast, the posterior aspect of the lateral tibial plateau slopes smoothly downward with a rounded contour.

The quadriceps and patellar tendons are well evaluated on a lateral view. The distal quadriceps tendon attaches to the superior pole of the patella. The patellar tendon extends from the inferior pole of the patella to the tibial tubercle. Both structures are well demarcated by a posterior fat plane. They should be straight and of uniform thickness.

In the setting of a joint effusion and suspected occult intra-articular fracture, a cross-table lateral view is useful for evaluating for lipohemarthrosis. The view is obtained with the patient supine and the knee slightly elevated. The cassette is placed adjacent to the medial knee. This positioning, in contrast to the standard lateral view, is better tolerated by a traumatized patient. The presence of a fat fluid level indicates an intra-articular fracture (most commonly, the tibial plateau) and prompts further evaluation with computed tomography (CT) or magnetic resonance imaging (MRI) (Fig. 5-3A, B, and C).

Figure 5-3 Knee joint effusion. A, Lateral radiograph demonstrates an oval soft tissue density representing a joint effusion within the suprapatellar pouch posterior to the quadriceps tendon. B, Cross-table lateral view shows a fat-fluid level (arrows) indicative of an intra-articular fracture with lipohemarthrosis.

C, A sagittal T1-weighted magnetic resonance image obtained with the patient supine demonstrates high signal fat (asterisk) floating on top of intra-articular hemorrhage. An acute supracondylar fracture is also noted (arrows).

(From Torg JS, Pavlov H, Morris VB: Salter-Harris type III fracture of the medial femoral condyle occurring in the adolescent athlete. J Bone Joint Surg Am 63:586, 1981.)

The suprapatellar bursa is the proximal extension of the joint space. It may be identified on the standard lateral view as a slender, 1 to 2 mm, vertically oriented structure contained within the lucent area of fat formed by the anterior margin of the distal femur and the posterior margin of the quadriceps tendon.

Superoinferior positioning of the patella may be evaluated using the Insall-Salvati ratio. This is the ratio of the greatest length of the patella divided by the length of the patellar tendon. This ratio averages 1.17 and normally falls between 0.8 and 1.2 (see Fig. 5-2B). A long patellar tendon generates a ratio greater than 1.3, indicating a high patella (patella alta). Conversely, a short tendon accompanies a low patella and a ratio less than 0.8, termed patella baja.

The axial view of choice is the Merchant view.^15,^35,³⁷ The patient is placed in the supine position on the radiography table; the knees are flexed 45 degrees (using a fixed or adjustable platform), and the cassette is placed on the proximal part of the shins. Both knees are exposed simultaneously, with the x-ray beam directed toward the feet, inclined 30 degrees from the horizontal (Fig. 5-4A). This view provides an excellent assessment of patellofemoral alignment and is ideal for assessing the osseous patellofemoral articular surfaces (Fig. 5-4B). In contrast, a skyline (or sunrise) view is obtained with the patient prone and the knee in maximum flexion. This view demonstrates the posterior surface of the patella and the anterior surface of the femur, but the imaged femoral surface is not at the patellofemoral joint. Furthermore, accurate assessment of patellofemoral alignment is limited when the knee is flexed excessively.^7,¹⁵ Some patients have difficulty tolerating this position.

Figure 5-4 Merchant view. A, Technique. B, Normal Merchant view. Patellofemoral alignment is normal bilaterally, and the osseous structures and articular cortices are normal.

Supplemental Views

The tunnel view is a frontal view obtained with the knee flexed 60 degrees. It can be obtained AP with the patient in the supine position, or posteroanterior (PA) with the patient prone or kneeling on the cassette. The x-ray beam is directed perpendicular to the tibia. This view demonstrates the posterior aspect of the intercondylar notch, the inner posterior aspects of the medial and lateral femoral condyles, and the tibial spines and plateaus (Fig. 5-5A). It is ideal for evaluating patients with suspected osteochondritis dissecans (OCD), which tends to occur more posteriorly in the intercondylar notch (Fig. 5-5B, C, and D).

Figure 5-5 Tunnel view. A, Normal tunnel view demonstrates the posterior aspect of the femoral condyles, the tibial spines, the articular surfaces of the tibial plateau, and the intercondylar notch. B, Tunnel view from a different patient, demonstrating an ovoid area of lucency at the inner margin of the medial femoral condyle (arrow) suspicious for osteochondritis dessicans. C, Coronal and (D) sagittal proton density MRI confirms the large osteochondral defect (arrow) with a completely displaced osteochondral fragment located in the suprapatellar joint recess (arrow).

(Case provided courtesy of the MRI Department, Hospital for Special Surgery, New York, New York.)

The flexed, PA weight-bearing (Rosenberg) view is a modified tunnel view. It is obtained with the patient standing and the knee flexed 45 degrees. The patellae should touch the film cassette. The x-ray beam is centered at the level of the inferior pole of the patella and is directed 10 degrees caudad. It captures the joint space at the posterior aspect of the femorotibial joint. This view is valuable for evaluating arthritis. It detects joint space narrowing due to cartilage loss that often goes unappreciated or underestimated on a conventional AP weight-bearing view (Fig. 5-6A and B).^8,^9,^25,^47,⁴⁹ Comparisons of intraoperative and radiographic findings demonstrate that the flexed PA weight-bearing view has greater accuracy, sensitivity, and specificity than the conventional extension weight-bearing radiograph.⁴⁹

Figure 5-6 The flexed weight-bearing posteroanterior (PA) view. A, Routine standing anteroposterior (AP) film demonstrates moderate bilateral medial compartment joint space narrowing with proliferative changes (arrows). B, PA flexion view demonstrates the findings to be more severe with marked narrowing of bilateral medial joint compartments, complete loss of the joint space, and bone-on-bone apposition (arrows).

Oblique radiographs complement a routine examination. Occult fractures and tibiofibular arthritis may be more easily detected with oblique views than with routine AP radiographs. Bilateral oblique views are obtained at 45 degrees of internal and external rotation, with the patient supine, the affected knee extended, and the cassette behind the knee. The x-ray beam should be directed 5 degrees cephalad. Views should demonstrate the patella, the femoral condyles, the tibial plateaus, and the fibula. In external rotation, the tibia and fibula are superimposed on each other. With internal rotation, there is less superimposition between the tibia and fibula (Fig. 5-7A and B).

Figure 5-7 Internal (A) and external (B) rotational views of the knee in different patients. In external rotation, the tibia and fibula are superimposed on each other; with internal rotation, there should be less superimposition. A suprapatellar joint effusion is seen on the internally rotated view.

Various stress views have been described for evaluation of instability and include valgus and varus stress radiographs for evaluation of the collateral ligaments, and anterior drawer stress radiographs for evaluation of the anterior cruciate ligament. These require use of a mechanical stress device or lead gloves worn by the x-ray technologist and manually stress the knee joint. In current clinical practice, these views are seldom ordered, as MRI is considered the gold standard for evaluation of internal derangement, and because stress views often need to be performed with local anesthesia for pain control.

Special Considerations and Anatomic Variants

Two sesamoid bones commonly identified on knee radiographs are the fabella and the cyamella. The fabella is located within the lateral head of the gastrocnemius. It overlies the lateral femoral condyle on the frontal view and sits posterior to the distal femur on the lateral view. The cyamella lives within the popliteus tendon. On a frontal radiograph, it may be found at the insertion of the popliteus in the notch of the lateral femoral condyle (Fig. 5-8A, B, and C).

Figure 5-8 Fabella and cyamella. The fabella is a circular osseous density, a sesamoid, in the lateral head of the gastrocnemius muscle. A, On the anteroposterior (AP) view, it is superimposed on the lateral femoral condyle. B, On the lateral view, the fabella is posterior to the femoral condyles. C, A cyamella is a sesamoid bone in the popliteus tendon. On the AP view, the cyamella is seen within the notch at the lateral aspect of the lateral femoral condyle.

Normal variants also occur in the patella, which can have two or more osseous centers, referred to as a bipartite or multipartite patella (Fig. 5-9A and B). A bipartite patella is the more common variant, seen in 1% of the population. It is bilateral 50% of the time.³⁵ The smaller pieces of the patella are located superolaterally and should fit neatly together like pieces of a jigsaw puzzle. The width of a bipartite patella is usually greater than that of the contralateral patella when assessed on a tangential axial view. MRI depicts intact cartilage overlying a bipartite patella, whereas a fracture displays disrupted osteochondral integrity. These features help dismiss the suspicion of a fracture.³⁵ Rarely (<2% of the time), a bipartite patella can be symptomatic. MRI of the knee may demonstrate bone marrow edema on one or both sides of the synchondrosis.