Jason R. Hull
Although the history and physical examination are the most important instruments in evaluating joint pain, musculoskeletal imaging remains an essential adjunct. Multiple imaging modalities are available to confirm a provisional diagnosis or narrow the differential diagnosis. Because many imaging techniques are costly and require radiation exposure for the patient, they should be used judiciously. Imaging should begin with a standard set of plain radiographs, with special radiographic views and more elaborate modalities used if indicated. Once a diagnosis is established and a treatment plan initiated, imaging also can be used to monitor disease progression or response to treatment.
Plain Radiography
Plain radiography is the modality of choice for initial confirmatory or screening imaging. Modern equipment and techniques have reduced the amount of ionizing radiation required for routine radiography. The equipment and processing are relatively inexpensive and widely accessible compared with other modalities. The osseous anatomy of the knee is simple and easily understood, making interpretation of the images straightforward.
Initial Radiographic Evaluation
To some degree the images included in a screening examination are a matter of preference and may even be tailored to answer specific questions raised during the history and physical examination. However, the initial radiographic examination should include the least amount of radiation necessary to adequately visualize the essential anatomic features of the knee:
The author prefers four views for the initial radiographic examination: (i) standing weight-bearing anteroposterior (AP) view, (ii) standing weight-bearing posteroanterior (PA) flexion view, (iii) lateral view, and (iv) axial view of the patella.
The standing AP view of the knee (Fig. 17-1A) is obtained with the patient standing with equal weight on both lower extremities, knees fully extended, and toes pointing straight ahead. The beam is positioned horizontally at the level of the distal pole of the patella. Inclusion of both knees on one image allows for careful side-to-side comparison for subtle abnormalities. Like a standard non–weight-bearing AP radiograph, the standing AP view allows inspection of many osseous landmarks of the knee, including the distal surfaces of the femoral condyles, medial and lateral epicondyles, medial and lateral tibial plateaus, tibial spines, and the head of the fibula. However, the weight-bearing radiograph more accurately represents the condition of the articular surface and is more likely to exhibit tibiofemoral joint space narrowing if pathology is present. With no mechanical load through the knee, non–weight-bearing technique allows relaxation through the joint and may project artificial widening of the joint space. In addition, much like physical evaluation, assessment of tibiofemoral alignment on a weight-bearing radiograph better illustrates the functional alignment of the joint.
With the exception of stationary standing, few weight-bearing activities occur with the knee in full extension. In addition, tibiofemoral contact area decreases and contact force increases as the knee flexes, making the posterior weight-bearing surfaces of the femoral condyles susceptible to early degeneration under pathologic conditions. The addition of a weight-bearing 45-degree PA flexion view allows for assessment of the posterior weight-bearing surfaces of the femoral condyles and provides a tangential view of the medial and lateral weight-bearing surfaces of the tibial plateau (Fig. 17-1B). This view is also obtained with the patient standing with equal weight on both lower extremities and toes pointing straight ahead, but the knees are flexed 45 degrees with the patellae touching the vertically oriented radiograph cassette. The beam is directed posteroanterior, angled 10 degrees caudad, and centered at the level of the distal pole of the patella. An additional advantage of this view is visualization of the intercondylar notch of the femur, similar to that observed with a traditional tunnel view.
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Figure 17-1 A: Standing anteroposterior view of the knees demonstrates significant narrowing of the lateral compartment joint space in the left knee with a well-maintained joint space of the contralateral right knee. B: Weight-bearing 45-degree posteroanterior flexion view reveals complete loss of the lateral compartment joint space in both knees. |
The lateral view of the knee (Fig. 17-2) is obtained with the patient lying with the lateral aspect of the affected knee against the radiograph table. With the knee flexed approximately 30 degrees, the x-ray beam is directed tangential to the surfaces of the tibial plateau, perpendicular to the radiograph cassette under the lateral aspect of the knee. The lateral view also visualizes most landmarks of the distal femur, proximal fibula, and proximal tibia, adding an orthogonal view for complete evaluation of osseous architecture. Additionally, the proximal and distal poles of the patella and relationships of the patellofemoral articulation can be assessed, as well as the soft tissues of the extensor mechanism.
A tangential axial view of the patella (Fig. 17-3) must be obtained to visualize the medial and lateral facets of the patella, alignment of the patella within the femoral sulcus, and the status of the patellofemoral joint space. Multiple techniques have been described for obtaining the axial view, differing only by angle of knee flexion at which the image is obtained. The Merchant view is probably the most popular and widely used method, owing to its clear representation of the medial and lateral patellar facets, patellofemoral joint space, and alignment. The view is obtained with patient's knees flexed 45 degrees over the end of the exam table. The x-ray beam is directed caudally through the patella, at an angle 30 degrees below horizontal, at a film cassette resting on the patient's shins in a position perpendicular to the x-ray beam.
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Figure 17-2 Lateral view of the knee. The Insall-Salvati ratio (T/P) can be used to determine the height of the patella relative to the sulcus of the distal femur. The ratio of the length of the patellar tendon (T) to the diagonal length of the patella (P) should differ by no more than 20%. |
Image Interpretation
Evaluation of any plain radiograph must be performed in a routine, systematic fashion. The patient's name, date of examination, and laterality (right versus left) of the image
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must be verified. The clinician should briefly review the image for quality control to make sure all pertinent anatomy is visualized, paying particular attention to image penetration, joint position, and orientation of the x-ray beam.
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Figure 17-3 Merchant view of the knee. A: To measure the congruence angle, line DB is established as a reference line that bisects the sulcus angle ABC. A second line is drawn from the apex (ridge) of the patella to the center of the sulcus. Congruence angle DBE is positive if it opens lateral to reference line DB, and measurements greater than approximately 15 degrees indicate lateral patellofemoral subluxation. B: The lateral patellofemoral angle is formed by a tangent across the medial and lateral condyles and a line parallel to the lateral facet of the patella. An angle that opens medially represents excessive lateral patellar tilt. |
The clinician must review the extent of the patient's soft tissue and osseous anatomy visible on an image. Like the components of a thorough history, the findings peripheral to the knee joint may provide valuable diagnostic clues and historical data that could alter the diagnosis or treatment plan. The presence of significant vascular calcification indicates the presence of peripheral vascular disease and may preclude the use of a tourniquet should surgical intervention be warranted. Evidence of posttraumatic or postsurgical changes may be evident in the femoral or tibial diaphyses near the margins of the image, requiring additional images for complete evaluation. Rarely, occult soft tissue masses and neoplasms may be identified as subtle soft tissue densities adjacent or peripheral to the knee joint. Likewise, osseous neoplasms may be identified, either as an incidental finding or as the primary source of the patient's symptoms.
Osseous Structures
Evaluation of the osseous structures of the knee begins with identification of all major osseous landmarks (Table 17-1) and assessment of general architecture. The lateral femoral condyle is normally smaller than the medial condyle and exhibits a more acute radius of curvature posteriorly. The medial and lateral tibial plateaus are also asymmetric, with the larger medial plateau demonstrating a concave contour on the lateral projection and the lateral plateau exhibiting slight convexity. Both tibial surfaces slope posteriorly approximately 10 degrees. The patella, best visualized on the axial view, consists of asymmetric medial and lateral facets.
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The medial facet is smaller and slightly convex, and the larger lateral facet is concave in the coronal plane. Gross abnormalities in size, shape, contour, or spatial relationships of major osseous structures may indicate the presence of congenital, developmental, or posttraumatic pathology.
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TABLE 17-1 Radiographic Landmarks and Anatomy of the Knee |
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Osseous structures must be inspected for intraosseous abnormalities indicative of metabolic or neoplastic processes. Bone quality and mineralization can provide diagnostic clues, with generalized osteopenia present in the setting of inflammatory arthritides as a result of intense regional hyperemia. Conversely, degenerative conditions are associated with increased mineralization in the subchondral region as a result of increased local stresses. All joint surfaces should be examined for focal defects, wear patterns, and other signs of remodeling such as osteophyte formation.
Alignment
The anatomic axis of the knee is represented by the intersection of lines drawn parallel to the long axes of the femur and tibia and is typically between five and seven degrees of valgus. In most patients, the standing AP view allows for adequate assessment of tibiofemoral alignment in the coronal plane. However, patients with pre-existing congenital, posttraumatic, or postsurgical deformities of the femur or tibia require an AP long-leg hip to ankle radiograph for accurate assessment of the anatomic axis (Fig. 17-4). The mechanical weight-bearing axis of the knee, illustrated by a straight line drawn from the center of the femoral head to the center of the ankle joint, can also be determined on the long-leg radiograph. A line passing through the middle third of the proximal tibia represents a neutral mechanical axis, whereas lines passing through the medial third and lateral third represent varus and valgus mechanical axes, respectively.
The relationship of the patella to the distal femur must be evaluated on both the lateral and axial images of the knee. The height of the patella relative to the distal femur is evaluated on the lateral radiograph using the Insall-Salvati ratio (Fig. 17-2). Normal patellar height is represented by a ratio of one, whereas ratios of 0.8 and 1.2 represent patella baja and patella alta, respectively. The relationship of the patella to the sulcus of the distal femur is evaluated on the axial view of the patella. Patellofemoral subluxation can be evaluated objectively by calculating the congruence angle (Fig. 17-3A). When the apex of the patella lies lateral to the center of the femoral sulcus and the congruence angle is >15 degrees, lateral patellofemoral subluxation is present. The lateral patellofemoral angle assesses patellar tilt (Fig. 17-3B) and opens medially in the presence of abnormal lateral tilt.
Articular Cartilage and Joint Space
Erosion of articular cartilage, expressed as joint space narrowing on plain radiographs, is the final common pathway for many disease processes of the knee joint and must be accurately assessed in the medial, lateral, and patellofemoral compartments. Many disease processes produce predictable patterns of joint space narrowing. For example, inflammatory conditions result in generalized enzymatic destruction of articular cartilage and diffuse joint space narrowing on plain radiographs. Conversely, osteoarthritis begins as a localized mechanical process that leads to focal cartilage loss and joint space narrowing.
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Figure 17-4 Anteroposterior long-leg hip-to-ankle radiograph. The mechanical axis of the limb is determined by a line between the center of the femoral head and the center of the ankle. |
As stated previously, a standing AP radiograph is more predictive of tibiofemoral articular cartilage loss than a non–weight-bearing view. However, the weightbearing 45-degree PA flexion view is the most sensitive plain radiographic method for detection of tibiofemoral joint space narrowing (Fig. 17-1). With this technique, the x-ray beam is tangential to the weight-bearing surfaces of the femoral condyles and tibial plateau and perpendicular to the desired joint space measurement, therefore providing accurate assessment of the actual joint space. Because the patella does not enter the femoral sulcus until the knee is flexed 30 degrees, axial views of the patellofemoral joint must be obtained at flexion angles >30 degrees to visualize the patellofemoral articulation. The traditional skyline view is obtained with the knee flexed >90 degrees, and in this degree of flexion the patella contacts the femur distal to the femoral sulcus. The patellofemoral joint space is well visualized on the Merchant view, obtained at 45 degrees of knee flexion (Fig. 17-3).
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Soft Tissues
Differential absorption of the x-ray beam by adjacent tissues of varying composition allows visualization of many peripheral soft tissue features. The extensor mechanism is the only normal periarticular soft tissue structure routinely visualized on plain radiographs. On the lateral view, adjacent adipose tissue delineates the linear soft tissue densities of the quadriceps and patellar tendons proximal and distal to the patella, respectively. Many pathologic processes can be diagnosed by their characteristic appearance as soft tissue densities. On the lateral image, joint effusion is an easily identified soft tissue mass deep to the quadriceps tendon. Popliteal cysts are also fluid-filled masses occasionally identified as soft tissue densities on the lateral radiograph. Intra-articular and extra-articular soft tissue structures are commonly visualized secondary to calcification. Intra-articular findings include chondrocalcinosis from deposition of calcium pyrophosphate dihydrate crystals in articular cartilage and menisci. Osteocartilaginous loose bodies are associated with many conditions affecting the articular surface, including trauma, chondromalacia patella, osteochondritis desiccans, and osteoarthritis. They may be identified anywhere within the knee joint including the femoral notch, suprapatellar pouch, medial and lateral gutters, and the posterior recesses of the knee. Extra-articular soft tissue calcifications commonly occur in tendinous and ligamentous structures. Calcification at the origin of the medial collateral ligament (Pellegrini-Stieda sign) signifies prior injury to the medial collateral ligament. Notable soft tissue calcifications in the region of the popliteal vessels may suggest need for further evaluation with ultrasonography to rule out aneurysm and pseudoaneurysm of the popliteal artery.
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Figure 17-5 Images depicting a high-grade articular cartilage defect in the distal weight-bearing surface of the medial femoral condyle.A: Sagittal fat-suppressed three-dimensional spoiled gradient-echo image demonstrating fluid, which is low signal intensity, within the lesion (arrow). B: Coronal short time inversion recovery (STIR) image of the same lesion. Note the adjacent edema in the subchondral bone (arrow). |
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) has become a powerful complement to plain radiography for evaluation of the painful knee. Although costly, MRI has many advantages over other imaging modalities and has supplanted many older imaging techniques. MRI provides multiplanar capability and unrivaled image contrast with spatial resolution comparable with that of computed tomography (CT). The ability to enhance tissue contrast by variation of scanning parameters allows for comprehensive evaluation of all osseous and soft tissue structures of the knee, including articular cartilage, without exposing the patient to ionizing radiation.
The utility of MRI for evaluation of overuse and acute injuries to soft tissue structures of the knee, including tendons, capsuloligamentous structures, and the menisci, is unrivaled and is well documented in the orthopaedic and sports medicine literature. The reader is referred to other sources for a comprehensive review of these topics.
MRI can be particularly useful in the early diagnosis of many pathologic processes of bone before they are detectable by plain radiography. Osteonecrosis, stress fracture, osteomyelitis, and neoplasm may all share a stage when patients present with significant symptoms but normal plain
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radiographs. All of these conditions cause early accumulation of marrow edema at the site of the lesion. MRI capitalizes on these early changes in local tissue characteristics, which are manifested as areas of high signal on T2-weighted images.
Assessment of isolated articular cartilage injuries has improved with the implementation of imaging sequences designed specifically for articular cartilage. The two most commonly used sequences are the T1 fat-suppressed three-dimensional (3D) spoiled gradient-echo technique and the T2 fast spin echo technique. The 3D gradient echo sequence displays articular cartilage as a smooth band of hyperintense tissue along the cortical margin of the articular surface (Fig. 17-5A). On the T2 fast spin echo sequence, articular cartilage is a smooth band of intermediate signal intensity. Articular defects in this sequence are visualized much as an arthrogram, with hyperintense fluid filling the defect and interrupting the normally smooth contour. Both techniques exhibit a high degree of accuracy, with the best results in the patellofemoral joint where the hyaline cartilage is thickest. The short time inversion recovery (STIR) sequence is also a commonly used sequence and produces images similar to the fast spin echo technique (Fig. 17-5B).
Interest in MRI evaluation of articular cartilage of the knee has increased in recent years, mostly because of the development of articular cartilage restoration procedures. The improved imaging techniques help identify appropriate candidates for reconstruction, aid in preoperative planning, and provide a reliable method for following postoperative progress and response to treatment without the need for repeat arthroscopy. Imaging of the articular surface can also help with patient selection for corrective osteotomies about the knee, with or without the need for cartilage restoration. Imaging can offer preoperative prognostic information to patients undergoing arthroscopy for other reasons, as the presence of articular surface defects has a negative impact on clinical outcome.
Suggested Readings
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Jungius KP, Schmid MR, Zanetti M, et al. Cartilaginous defects of the femorotibial joint: accuracy of coronal short inversion time inversion-recovery MR sequence. Radiology. 2006; 240:482-488.
Leach RE, Gregg T, Siber FJ. Weight bearing radiography in osteoarthritis of the knee. Radiologica. 1970; 97:265-268.
McGrory JE, Trousdale RT, Pagnano MW, et al. Preoperative hip to ankle radiographs in total knee arthroplasty. Clin Orthop Rel Res. 2002; 404:196-202.
Merchant AC, Mercer RL, Jacobsen RH, et al. Roentgenographic analysis of patellofemoral congruence. J Bone Joint Surg Am. 1974; 56:1391-1396.
Rosenberg TD, Paulos LE, Parker RD, et al. The forty-five degree posteroanterior flexion weightbearing radiograph of the knee. J Bone Joint Surg Am. 1988; 70:1479-1483.