Knee
Ne Siang Chew
Philip Robinson
John H. Harris Jr.
GENERAL CONSIDERATIONS
The knee is probably more commonly injured than any other joint of the body and is the most vulnerable joint from the standpoint of athletic injury. Extensive soft tissue damage may exist without a recognizable radiographic sign. Thus, a normal-appearing radiographic examination of the knee does not exclude the possibility of significant injury to the ligaments surrounding, or within, the knee joint. Magnetic resonance imaging (MRI) is the imaging modality of choice in the diagnosis of acute (and chronic) soft tissue, chondral, and occult skeletal injuries of the knee. However, because the definitive evaluation and management of patients with soft tissue and occult skeletal injuries of the knee are not within the purview of emergent care and because MRI is currently not a generally accepted part of the emergency imaging of the appendicular skeleton, the reader is referred to the cited references.
The use of multidetector computed tomography (MDCT) is useful in the search of undisplaced fractures and is now an integral in the evaluation of traumatized knees, especially in the depiction of complex fracture anatomy. MDCT provides fast-volume imaging, multiplanar reconstructions (MPRs) with near isotropic viewing, threedimensional (3-D) images, and thick-slice (wedge) MPRs, which offer surgeons a detailed road map for preoperative planning. Computed tomography (CT) has also allowed for grading of trauma in knees especially in tibial plateau type fractures (Schatzker classification), permitting patients to be managed more effectively. The potential benefits and disadvantages of MDCT over plain radiography are summarized in Table 20.1.
In children, particularly, pain caused by an abnormality of the hip may be referred to the knee. It is important that the hip be specifically examined when a child complains of knee pain that seems to be disproportionate to the clinical and radiographic evaluation of the knee or if a young child refuses to bear weight on one leg. In adults, a nontraumatic swollen, hot, tender knee should prompt consideration of bacterial pyarthrosis.
GENERAL ANATOMY OF THE KNEE JOINT
Key radiologic anatomy pertinent to interpretation of trauma radiographs is summarized in the next paragraphs.
The Femur
The distal femur is composed of two bulbous bony projections: The medial and lateral femoral condyles. On the sagittal projection or lateral radiograph, the medial condyle can be discerned from the lateral condyle by its morphology (Figs. 20.1 and 20.2); the medial condyle is larger and has a convex articular surface. Both femoral condyles contain condylopatellar sulci, minor indentations that divide the condyles in a sagittal oblique plane.
The medial condylopatellar sulcus divides the condyle into an anterior one-third and a posterior twothird segment. The lateral condylopatellar sulcus divides the condyle into two approximate halves. Aside from these minor indentations, the condyles should be smooth.
The medial condylopatellar sulcus divides the condyle into an anterior one-third and a posterior twothird segment. The lateral condylopatellar sulcus divides the condyle into two approximate halves. Aside from these minor indentations, the condyles should be smooth.
TABLE 20.1 Benefits and Disadvantages of the Use of Multidetector Computed Tomography over Plain Radiography in the Traumatized Knee | ||||
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 Figure 20.1. Recognizing the structures of the knee on the lateral radiographic view. b, bursa; e, eminence; fem, femoral; t, tendon; pl, plateau. |
 Figure 20.2. A magnified lateral view of the knee. The yellow dotted line outlines the larger, more convex medial femoral condyle. The medial condylopatellar sulcus is denoted by the yellow arrowhead. The blue dotted line outlines the lateral femoral condyle. The lateral condylopatellar sulcus is denoted by the blue arrowhead. |
The medial and lateral condyles are separated inferiorly by the intercondylar notch (Figs. 20.3 and 20.4).
 Figure 20.3. Recognizing the normal structures on the frontal/anteroposterior (AP) projection of the knee. |
The Tibia
On the lateral projection, the medial tibial plateau is concave medially with a pointed dorsal corner (Fig. 20.5; see also Figs. 20.3 and 20.4 for accompanying frontal projection). The lateral tibial plateau is relatively flat with a rounded dorsal corner. Disruption to these contours should raise the suspicion for a fracture.
 Figure 20.4. Normal frontal/anteroposterior (AP) radiographic appearance of an adult knee. |
The medial and lateral tibial plateaus are separated by an intercondylar eminence (Fig. 20.6). The intercondylar eminence is devoid of cartilage and mainly serves as a footprint for ligamentous attachments. It is a triangular area with a base anteriorly and an apex posteriorly. Posteriorly near the apex arise two bony projections: The slightly anterior medial and a posterior lateral tibial spines/tubercles/eminences (Fig. 20.6). The anterior intercondylar eminence serves as the root attachments for the anterior medial and lateral menisci and footprint for the anterior cruciate ligament (ACL). The posterior intercondylar eminence functions as root attachments for the posterior medial and lateral menisci and the footprint for the posterior cruciate ligament (PCL). The medial tibial spine is more anterior than its lateral counterpart and can be discerned on the lateral radiograph.
 Figure 20.5. A magnified view of the lateral radiograph of the knee. The purple dotted line denotes the lateral tibial spine, the blue dotted line denotes the lateral tibial condyle, and the yellow dotted line denotes the medial tibial plateau. |
The ACL inserts between the medial and lateral tibial spines, 10 to 14 mm behind the anterior border of the tibia (Fig. 20.7). Presence of an anteriorly placed bony fragment in the intercondylar notch is suspicious for distal ACL avulsion (Fig. 20.7C). The PCL attaches in a depression between the two tibial plateaus dorsally. Focal discontinuity of the posterior tibial plateau articular surface should suggest distal PCL avulsion (Fig. 20.8).
The anterior tibial tubercle is a bony protuberance located on the anterior aspect of the proximal tibia. It acts as the distal attachment for the infrapatellar tendon.
Gerdy tubercle is a bony protuberance at the anterior lateral tibial condyle, serving as the distal attachment for the iliotibial band (ITB).
The Fibula
The proximal fibula comprises a head and styloid process that act as attachment sites for posterolateral corner structures of the knee (Fig. 20.9). The styloid process is the pointed aspect of the fibula head. Knowledge of anatomical ligamentous attachment landmarks can help the reader discern the type of ligamentous injury that has occurred. The posterosuperior medial facet of the fibular styloid process is occupied by the popliteofibular ligament attachment. The fibular styloid process/fibular head apex serves as the distal attachment for the arcuate, fabellofibular, and popliteofibular ligaments, collectively known as the arcuate complex. The arcuate ligament is deep to that of the fabellofibular ligament. The fabellofibular ligament insertion lies medial to the direct arm of the short head of biceps femoris (not shown). The lateral collateral ligament and long head of biceps femoris are further lateral and are attached to the lateral margin of the fibular head.
 Figure 20.6. A: An axial section through the menisci and intercondylar eminences. The intercondylar eminences are devoid of cartilage and serve as root attachments for the menisci and footprints to the ACL and PCL. lig, ligament. B: 3-D reconstruction of the tibia and fibula. The medial and lateral tibial spines are depicted. Note the more anterior medial tibial spine. |
 Figure 20.7. A: A 3-Tesla coronal T1 MRI demonstrates the distal insertion of the ACL. The ACL inserts between the yellow arrowheads, between the lateral tibial spine (LTS) and the medial tibial spine (MTS). B: A 3-Tesla sagittal PD (proton density) MRI image depicting the normal location of the distal anterior cruciate ligament (ACL) insertion, located 10 to 14 mm posterior to the anterior tibial border (yellow triangle). (continued) |
 Figure 20.7. (continued) C: Presence of an anteriorly placed bony fragment (black arrow) in the intercondylar notch in a rugby player with an effusion in the setting of acute trauma. The arrowheads demonstrate a deepened lateral sulcus (deep notch sign), compatible with pivot-shift type injury mechanism, relating to an ACL injury. D: A sagittal T2 fat-saturated MRI of the same patient shows an avulsion of the ACL footprint. The black arrow points to avulsed cortex of the intercondylar eminence, attached to the distal fibers of the ACL. The ACL, depicted by black arrowheads, is redundant. E: A corresponding sagittal reformatted CT shows the avulsed bony footprint of the ACL. |
 Figure 20.8. A: Discontinuity of the posterior tibial plateau secondary to a distal posterior cruciate ligament (PCL) avulsion injury in a football player. B: Corresponding sagittal T1 image of the same patient demonstrating a bony avulsion (black arrowhead) of the distal PCL footprint. |
 Figure 20.9. A: Ligamentous attachments to the fibula. The popliteofibular ligament attaches to the posterosuperior facet of the fibula. The fibula apex acts as the attachment for the arcuate and fabellofibular ligaments. The lateral collateral ligament and long head of biceps femoris are attached further laterally at the lateral margin of the fibula head. (continued) |
 Figure 20.9. (continued) B: Individual attachments of the posterolateral corner structures on the fibular styloid and head. C: A para-axial representation of the relationship between the posterolateral corner ligamentous attachments. lig, ligament. |
Identification of an avulsion fracture of the fibular head is important because it may ref lect injury to the arcuate complex (Figs. 20.10 and 20.11). Such injuries may lead to posterolateral instability, characterized clinically by posterior subluxation and external rotation of the tibial plateau relative to the femur. Failure to recognize this type of injury may cause chronic instability and failure of cruciate ligament reconstruction.
 Figure 20.10. A: Tiny avulsion fracture of the fibular styloid. Although a tiny fracture, such fractures may herald more significant posterolateral corner ligamentous injury. See the corresponding MRI image. B: Corresponding coronal STIR MRI image: The white arrowheads depict marrow edema secondary to the avulsion. The black arrow points to an edematous, partially torn arcuate ligament. Notice the arched appearance of the arcuate ligament. |
 Figure 20.11. A: The black arrows depict an avulsion of the fibula head. Notice that compared to Figure 20.10, the site of the avulsion is more inferolateral, a feature that may indicate a lateral collateral ligament and biceps femoris avulsion. See the corresponding MRI image. B: Coronal T1 MRI image with a black arrow identifying the avulsed fibular head, which is still attached to the combined biceps femoris and lateral collateral ligament. |
The Proximal Tibiofibular Joint
The proximal tibiofibular joint is the articulation of the lateral tibial condyle and the fibula head, located posterolaterally in relation to the tibia. Bound by a joint capsule and connected via anterior and posterior ligaments, the relationship between the tibia and fibula should be constant. Overcoverage or undercoverage of the fibula head on anteroposterior (AP) radiographs should raise the suspicion of proximal tibiofibular joint dislocation. A useful landmark in identifying the correct position of the fibula head and in excluding proximal tibiofibular joint dislocation or subluxation is the posterior tibial groove. The posterior tibial groove, located posteromedially within the lateral tibial condyle, can be recognized by following the lateral tibial spine inferiorly along the posterior aspect of the tibia on the lateral radiograph. Radiographically, this can be seen as an oblique radiodense line. On lateral projections, this line should intersect the fibula head (Fig. 20.12). Anterior and lateral deviation of the fibula head on lateral and AP radiographs respectively signify anterolateral dislocation, whereas posterior and medial deviation of the fibula on lateral and AP radiographs respectively indicate posteromedial dislocation.
The Patella
The patella (Figs. 20.1, 20.2, 20.3, 20.4 and 20.13) is the largest sesamoid bone in the human body, lying within the substance of the quadriceps mechanism. This flat, triangular structure, with a distal apex, articulates with the trochlear groove of the femur.
The patella has two main surfaces or facets: The medial and lateral facets, separated by a central ridge. Knowledge of patella facetal morphology can aid orientation. There are three main variations of patella morphology, that is, Wiberg types I to III (Table 20.2).
The most common patella morphology is the Wiberg type II patella (Fig. 20.13). On the axial plane, the lateral facet is generally flatter, wider, and deeper than its medial counterpart. The medial
facet can be easily recognized; it contains a vertical bridge that divides it from a smaller, more medial “odd” facet. The odd facet only contacts the femoral condyle in flexion.
facet can be easily recognized; it contains a vertical bridge that divides it from a smaller, more medial “odd” facet. The odd facet only contacts the femoral condyle in flexion.
 Figure 20.12. A: The normal posterior tibial groove. Follow the dotted yellow line, denoting the lateral tibial spine proximally, which becomes the posterior tibial groove, demonstrated by the short bold black arrows. The posterior tibial groove is the radiodense line, which in normality should intersect the fibula head. Any deviation, together with a relevant clinical history, should raise suspicion of a dislocation injury. B: 3-D paracoronal depiction of the posterior tibial groove (purple arrowheads). |
With the knee slightly flexed, the craniocaudal patella tendon length should be equal to the length of the craniocaudal length of the patella (Insall-Salvati ratio) (Fig. 20.14A). Variation in this ratio of up to 0.2 is acceptable but beyond this, one should suspect injury to the extensor mechanism, especially when there is a relevant clinical history and soft tissue swelling. In particular, if the apparent patella tendon length is 20% more than that of the bony patella (patella alta, high-riding patella) (Fig. 20.14B), a patella tendon rupture should be assumed. Conversely, for the reversed scenario with patella baja (low-riding patella), then a quadriceps tendon rupture is suspected.
RADIOGRAPHIC ANATOMY ACCORDING TO PROJECTION
The radiographic anatomy of the normal adult knee is seen in Figure 20.15. In the frontal projection (Fig. 20.15A), the patella is obscured by the density of the superimposed intercondylar portion of the femur. Therefore, the AP projection of the knee usually has little significance in the evaluation of the patella. The medial and lateral intercondylar spines are well seen within the concavity of the intercondylar notch of the femur (Fig. 20.3). On a properly positioned frontal projection of the knee made with the central beam passing approximately 1 cm inferior to the inferior pole of the patella, the medial and lateral compartments of the joint space and the contiguous surfaces of the femoral condyles and the tibial plateau are clearly demonstrated.
TABLE 20.2 Patella Morphology According to Wiberg | ||||||||
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 Figure 20.13. A: An axial 3-D CT reconstruction of a Wiberg type II patella. The yellow arrow points to the lateral facet, the short white arrow points to the median ridge, and the purple arrow points to the vertical bridge that divides the medial facet. B: An axial 3-D CT view of the patella and its facets of a 70-year-old female patient. The patella morphology corresponds to a Wiberg type II classification. LF, lateral facet; MFP, medial facet proper; OF, odd facet. |
 Figure 20.14. A: The Insall-Salvati ratio, which is the ratio of the craniocaudal patella tendon length (A) to the patella length (B), should be 1.0 ± 0.2. B: Patella alta in a patient with cerebral palsy, with no history of trauma. Inferior fragmentation of the patella is thought to be related to chronic traction/stress. The Insall-Salvati ratio is much greater than 1.2. |
 Figure 20.15. A: Normal adult knee in the frontal AP projection. B: Normal adult knee seen in lateral projection. Lateral radiograph of a normal adult knee. The positioning is nearly perfect, as judged by the superimposition of the femoral condyles. The slight concavity (black arrow) is the lateral (condylar) sulcus. The normal lucency of the suprapatellar recess (asterisk) identifies the deep surface of the quadriceps tendon (white arrow). The infrapatellar space (open arrows) is also normally lucent. C: Normal adult knee seen in externally oblique projection. D: Normal adult knee seen in internally rotated oblique projection. E: Axial (“sunrise”) view of the normal patella. The lateral femoral condyle (black arrowheads) and the long lateral facet (white arrows) of the patella are less steep than their medial counterparts. |
In the true lateral projection (Fig. 20.15B), the articulating surface of the femoral condyles should be closely superimposed and the patellofemoral space clearly evident. The suprapatellar recess of the joint space should be easily identified on a properly exposed lateral radiograph. The infrapatellar portion of the joint space, lying anterior and inferior to the femoral condyles, should be visible as a rectangular area of relative radiolucency (Hoffa fat pad).
External (Fig. 20.15C) and internal (Fig. 20.15D) oblique projections provide a perspective of the femoral condyles and tibial tuberosities not visible on the straight frontal and lateral projections. Additionally, the patella is partially projected free of the femur, making possible the evaluation of its medial and lateral margins.
The axial (tangential, “sunrise”) view of the patella is seen in Figure 20.15E. In this projection, the patellofemoral compartment and the posterior surface of the patella are well delineated. The medial femoral condyle is larger and more prominent than the lateral, and the slope of the smaller medial facet of the posterior surface of the patella is steeper than that of the larger lateral facet. This is the anatomic basis for the fact that dislocation of the patella practically always occurs laterally. The axial projection may be obtained with the patient either prone or supine. In either event, the leg is flexed on the thigh approximately 45 degrees, and the central x-ray beam is directed posterior to the patella through the patellofemoral compartment.
RADIOGRAPHIC EXAMINATION
The number of radiographic projections deemed necessary to make a definitive diagnosis in knee trauma varies according to institutions. Verma et al. attests to the use of a single lateral radiograph for fracture screening; this being 100% sensitive in depicting knee fractures with a 100% negative predictive value. In many American institutions, the routine radiographic examination of the knee consists of a frontal/AP, lateral, and each oblique projections (Fig. 20.15). Although the Ottawa study recommends only an AP, lateral, and one oblique projection as the routine radiographic examination, some authors advocate a four-view examination for the acutely injured knee in light of the potential patient inconvenience, administrative, and legal costs that could result from a missed fracture on the threefilm examination. In the United Kingdom, however, it is common practice to perform two orthogonal views (AP and lateral) in the initial assessment of knee trauma. This practice is compatible with Eisenberg et al.’s recommendations derived from a prospective large veteran association study. Eisenberg concluded that AP and lateral views were optimum for diagnosis for compensable service-related disabilities, including fractures. In our institution, if patients are still symptomatic and non-weight-bearing on reassessment, further cross-sectional imaging either with MRI or CT is performed.
If injury to the patella is suspected, an axial (tangential) view of the patella (Fig. 20.15E) should be obtained at the time of the initial radiographic examination. The axial projection of the patella may be obtained with the patient either prone or supine. When clinically necessary, it is possible, with gentlecontrolled positioning, to flex the knee sufficiently to obtain an axial view of the patella.
The open-joint (“tunnel”) projection (Fig. 20.16) is designed to provide optimum visualization of the intercondylar eminences and the intercondylar notch. It frequently provides the best visualization of foci of osteochondritis dissecans and of loose bodies (osteochondromatosis) within the joint space.
CT with 3-D reformation is useful in confirming a suspected fracture or to unambiguously demonstrate the extent and spatial relationships of a comminuted fracture of the proximal tibia or distal femur.
 Figure 20.16. Intercondylar (“tunnel,” “open joint”) projection of the knee. Arrows indicate the intercondylar spines. |
RADIOGRAPHIC MANIFESTATIONS OF TRAUMA
General Radiographic Signs
The primary and secondary radiographic signs that accompany trauma of the knee are summarized in the Table 20.3.
We discuss the secondary signs and their origins in the next paragraphs.
Subcutaneous emphysema (Fig. 20.17) appears as irregularly distributed focal areas of decreased density in the soft tissues. The morphology of a knee joint effusion is illustrated in Figures 20.18 and 20.19 (compare with a normal suprapatellar recess/bursa in Fig. 20.15). The suprapatellar recess/bursa is a potential space bounded anteriorly by the suprapatellar fat pad and posteriorly by the prefemoral fat pad. These fat pads, containing fat, are lucent on plain radiographs. Normally, the suprapatellar recess/bursa measures less than 5 mm in AP diameter. When filled with fluid (effusion) or blood (hemarthrosis), a radiodense suprapatellar recess becomes evident, effacing both anterior suprapatellar and posterior prefemoral fat pads. The anatomy and plain radiographic findings of a knee joint effusion are illustrated in an MRI of a patient with an ACL rupture and small resultant knee joint effusion, depicted in Figure 20.18B. The principal effect of an effusion that causes distention of the knee joint capsule is to displace the quadriceps femoris muscle, the suprapatellar tendon, and the patella anteriorly. In addition, a large effusion usually causes the patella to be rotated around its coronal axis so that it is canted anteroinferiorly (Fig. 20.19).
TABLE 20.3 Radiographic and Computed Tomography Manifestation of Knee Joint Trauma | |
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