Normal Knee

Technical Aspects

Standard radiographs of the knee are performed as the first-line examination in knee disorders. Radiographs include anteroposterior and lateral views of the entire knee, as well as axial views of the patellofemoral joint ( eFig. 24-1 ). More specialized views include tunnel views, standing views, and axial patellofemoral views in different degrees of flexion. On anteroposterior views, the medial and lateral femorotibial joint space width can be assessed. Occasionally, severe cartilage thinning may become apparent on standing views only; therefore, some authors prefer standing views to views obtained with the patient lying on the radiographic table. Axial views of the patella are often obtained in advanced degrees of flexion (sunrise view), but this allows only a limited assessment of patellar tracking because patellar subluxation typically occurs at lower degrees of flexion. Axial views at lower degrees of flexion are more helpful in this situation but are technically more difficult to obtain. Tunnel views allow the depiction of joint bodies with an intercondylar location that would not be visible on regular anteroposterior views because of superimposition. This view also allows the assessment of a more posterior part of the femoral condylar contour, which may be helpful in the depiction of osteochondral lesions.

Ultrasonography of the knee offers a relatively complete assessment of the knee with the exception of the intramedullary bone and cartilage. However, such examinations need a significant time contribution from a skilled examiner, which has limited the popularity of this technique in some countries. The extensor apparatus can be examined with the knee in slight flexion because the quadriceps and patellar tendon are taut in this position. In the suprapatellar area, the suprapatellar recess can be examined to determine whether it is distended by an effusion. It is important also to obtain views in the parapatellar area on the medial and lateral aspects at the suprapatellar area because, occasionally, fluid is localized off of the midline, especially to the lateral side. With the ultrasound transducer in the coronal plane along the medial aspect of the femur, the superficial and deep medial collateral ligament (MCL) can be seen ( Figs. 24-1 and 24-2 ). When the transducer is displaced to an inferior position over the tibial cortex, a coronal section through the pes anserinus can be obtained ( Fig. 24-3 ). The insertion of the pes anserinus is made up of the sartorius, gracilis, and semitendinosus tendons. With the ultrasound transducer placed in the coronal plane at the anterolateral aspect of the tibia, a small protuberance of the cortex, designated as the Gerdy tubercle, is easily depicted. The iliotibial band inserts onto the Gerdy tubercle ( Fig. 24-4 and eFig. 24-2 ). With the ultrasound probe placed in the coronal plane along the posterolateral knee, the sulcus popliteus and tendon are easily identifiable. The lateral collateral ligament (LCL) courses over the sulcus for the popliteus tendon, and, at its distal aspect, the LCL fuses with the biceps tendon (see eFigs. 24-3 and 24-4 ). With the ultrasound probe positioned in a coronal plane along the posterior third of the medial knee, the semimembranosus sulcus is identified. Posteromedially, Baker cyst, representing a congenital bursa, can be identified with an ultrasound with a connection extending between the medial gastrocnemius and semimembranosus tendons ( Fig. 24-5 and eFig. 24-5 ). From a posterior view, the intercondylar area also can be examined in the transverse and sagittal planes for an assessment of the cruciate ligaments. Finally, the small ligaments at the posterolateral corner can be assessed.

The knee can also be examined with CT arthrography. This technique may yield similar results to MRI for the menisci and potentially better results for cartilage assessment. The ligaments, however, are better visualized with MRI.

The knee is by far the most frequently examined joint with MRI. The role of MRI has been studied most extensively in the evaluation of menisci, ligaments, and cartilage. Its value has been widely established, and it is the standard technique for the assessment of meniscal and ligament injury. Imaging parameters show significant variations among institutions, indicating that many different approaches may be used to obtain an adequate MR study of the knee. Both high- and low-field systems produce acceptable results for routine MR imaging. As a basic rule, multiplanar imaging is performed with coronal, sagittal, and transverse images. The choice of sequences should include any combination of fluid-sensitive sequences with anatomic sequences. Fluid-sensitive sequences can be either fat-saturated proton density–weighted, or T2-weighted spin-echo images versus short tau inversion recovery images. Both are satisfactory, and choices will often depend on the quality of the sequence and most optimal imaging time with the specific MR system. Fluid-sensitive sequences are very sensitive to fluid collections, edema, and pathologic processes, in general, but tend to appear grainy given their poorer signal-to-noise ratio. Therefore, anatomic images are always included in a complete knee examination, and these may be either T1-weighted or proton density–weighted, spin-echo images. Fat saturation should not be used for the anatomic images because it increases noise and limits imaging detail. For the assessment of cartilage, 3D gradient-echo sequences are popular with some radiologists; however, many centers are satisfied with the assessment of cartilage on the fluid-sensitive sequences. A present trend is to consider isotropic 3D (e.g., 3DVista) imaging sets to potentially replace the conventional 2D images. For the assessment of tumor or osteomyelitis, intravenous contrast administration is recommended. The sequence of choice to assess for contrast enhancement is a fat-saturated, T1-weighted sequence. It is strongly recommended to obtain exactly the same sequence (positioning, parameters, fat saturation) before and after administration of a contrast agent to allow for optimal assessment of possible enhancement.