Lower Limb II: Knee

Knee

The vulnerability of the knee, the largest joint in the body, to direct trauma makes knee injuries very common throughout life. Most acute injury to the knee is sustained during adolescence and adulthood, with motor vehicle accidents and athletic activities being the major causing factors. Fractures are much more common than dislocations, but injuries to the cartilaginous and soft-tissue structures, such as tears of the menisci and ligaments, are the most common types of injuries, particularly in older adolescents and younger adults. The symptoms accompanying knee trauma vary according to the specific site of injury and thus constitute important indications of the type of injury. However, clinical history and physical examination are rarely sufficient for making a precise diagnosis. Radiologic examination plays a determining role in diagnosing the various traumatic conditions involving the knee joint.

Anatomic-Radiologic Considerations

Conventional radiographs are the first-line approach to the traumatized knee, and often they are sufficient for evaluating many traumatic conditions of the joint. However, the great incidence of cartilaginous and softtissue injuries, occurring either as isolated conditions or in association with fractures, requires the use of ancillary imaging techniques for adequate evaluation of the joint capsule, articular cartilage, menisci, and ligaments.

The standard radiographic examination usually consists of obtaining radiographs of the knee in four projections: the anteroposterior, the lateral, and the tunnel projections, as well as an axial view of the patella. The anteroposterior radiograph of the knee allows sufficient evaluation of many of the most important aspects of the distal femur and proximal tibia: the medial and lateral femoral and tibial condyles, the medial and lateral tibial plateaus and tibial spines, and the medial and lateral joint compartments and the head of the fibula (Fig. 9.1). However, the patella is not well demonstrated on this view because it is superimposed on the distal femur. Proper evaluation of this structure requires a lateral projection (Fig. 9.2) on which the relationship of the patella and femur can also be assessed. Proximal (superior) displacement of the patella is called patella alta; distal (inferior) displacement is called patella baja. The length of the patella is measured from its upper pole (base) to the apex. The length of the patellar ligament is measured from its proximal attachment, just above the apex, to the notch on the proximal margin of the tibial tubercle. These two measurements are approximately equal and the normal variation does not exceed 20% (Fig. 9.3). In addition to imaging the patella in profile, the lateral radiograph of the knee allows evaluation of the femoropatellar compartment, the suprapatellar bursa (pouch), and the quadriceps tendon. The femoral condyles overlap on this projection, and the tibial plateaus are demonstrated in profile. Occasionally, a cross-table lateral view of the knee—obtained with the patient supine, the affected leg extended, and the central beam directed horizontally—may be required to demonstrate the intracapsular fat-fluid level (fat-blood interface [FBI] sign of lipohemarthrosis; see Fig. 4.38B). An angled posteroanterior projection of the knee, known as the tunnel (or notch) view, is also obtained as part of the standard radiographic examination (Fig. 9.4). This view is useful in visualizing the posterior aspect of the femoral condyles, the intercondylar notch, and the intercondylar eminence of the tibia.

To demonstrate an axial view of the patella, various techniques are available. The one most commonly used provides what has been called the sunrise view (Fig. 9.5). However, the degree of flexion required to obtain this view results in depressing the patella more deeply within the intercondylar fossa; consequently, the articular surfaces of the femoropatellar joint are not well demonstrated, and subtle subluxations of the patella may not be detected. To overcome this limitation, Merchant and colleagues have described a technique for obtaining an axial view of the patella that demonstrates the femoropatellar joint to better advantage (Fig. 9.6). It is particularly effective in detecting subluxations of the patella because it allows specific measurements to be made of the normal relations of the patella to the femoral condyles. Subtle abnormalities in these relations may not be seen on the standard axial view because of the degree of knee flexion required for that view, which prevents the patella from subluxing.

The measurements of the femoropatellar relations obtainable from Merchant axial projection concern the sulcus angle and the congruence angle (Fig. 9.7). Normally, the sulcus angle, which is described by the highest points of the femoral condyles and the deepest point of the intercondylar sulcus, measures approximately 138 degrees. By dissecting this angle with two lines—a reference line drawn from the apex of the patella to the deepest point of the sulcus and a second line from the lowest point of the patellar articular ridge to the deepest point of the sulcus—Merchant and colleagues were able to determine the degree of congruence, or the congruence angle, of the femoropatellar joint. When the deepest point of the patellar articular ridge fell medial to the reference line, the angle formed was assigned a negative value; when it fell lateral to the reference line, the angle was designated with a positive value. In 100 normal subjects included in their study, the average congruence angle was −6 degrees. An angle of +16 degrees or greater was found to be associated with various patellofemoral disorders, particularly lateral patellar subluxation (see Fig. 9.46). On occasion, patellofemoral disorders that are more difficult to diagnose may require, as Ficat and Hungerford recommended, additional tangential views obtained with 30, 60, and 90 degrees of knee flexion.

FIGURE 9.1 Anteroposterior view. (A) For the anteroposterior view of the knee, the patient is supine, with the knee fully extended and the leg in the neutral position. The central beam is directed vertically to the knee with a 5- to 7-degree cephalad angulation. (B) The radiograph in this projection sufficiently demonstrates the medial and lateral femoral and tibial condyles, the tibial plateaus and spines, and both the medial and lateral joint compartments. The patella is seen en face as an oval structure between the femoral condyles.

FIGURE 9.2 Lateral view. (A) For the lateral view of the knee, the patient is lying flat on the same side as the affected knee, which is flexed approximately 25 to 30 degrees. The central beam is directed vertically toward the medial aspect of the knee joint with an approximately 5- to 7-degree cephalad angulation. (B) The radiograph in this projection demonstrates the patella in profile, as well as the femoropatellar joint compartment and a faint outline of the quadriceps tendon. The femoral condyles are seen overlapping, and the tibial plateaus are imaged in profile. Note the slight posterior tilt of the tibial plateaus, which normally measures approximately 10 degrees.

FIGURE 9.3 Femoropatellar relationship. The length of the patella and the patellar ligament are approximately equal; normal variability does not exceed 20%.

FIGURE 9.4 Tunnel view. (A) For the tunnel (or notch) projection of the knee, the patient is prone with the knee flexed approximately 40 degrees, with the foot supported by a cylindrical sponge. The central beam is directed caudally toward the knee joint at a 40-degree angle from the vertical. (B) The radiograph in this projection demonstrates the posterior aspect of the femoral condyles, the intercondylar notch, and the intercondylar eminence of the tibia.

FIGURE 9.5 Sunrise view. (A) For an axial (sunrise) view of the patella, the patient is prone, with the knee flexed 115 degrees. The central beam is directed toward the patella with approximately 15-degree cephalad angulation. (B) The radiograph in this projection demonstrates a tangential (axial) view of the patella. Note the deep position of this structure in the intercondylar fossa. The femoropatellar joint compartment is well demonstrated.

FIGURE 9.6 Merchant view. (A) For the Merchant axial view of the patella, the patient is supine on the table, with the knee flexed approximately 45 degrees at the table’s edge. A device keeping the knee at this angle also holds the film cassette. The central beam is directed caudally through the patella at a 60-degree angle from the vertical. (B) On the radiograph obtained in this projection, the articular facets of the patella and femur are well demonstrated.

Among the ancillary techniques available for the evaluation of injuries to the knee, arthrography, computed tomography (CT), and magnetic resonance imaging (MRI) provide crucial information. CT is especially useful in the evaluation of complex fractures of the distal femur, the tibial plateaus, and the patella. In fractures of the tibial plateaus, it is effective in determining the amount of depression of the articular surface and in identifying small comminuted fragments that may be displaced into the joint, as well as comminution about the tibial spines, which may indicate avulsion of the cruciate ligaments. Tomography, by its ability to demonstrate the integrity of the anterior cortex, is also helpful in planning a surgical approach to the treatment of tibial plateau fractures.

FIGURE 9.7 Sulcus and congruence angles. Two specific measurements can be obtained from the Merchant axial view: the sulcus angle and the congruence angle. The sulcus angle, formed by lines extending from the deepest point of the intercondylar sulcus (a) medially and laterally to the tops of the femoral condyles, normally measures approximately 138 degrees. To determine the congruence angle, the sulcus angle is bisected to establish a reference line (ba), which is drawn to connect the apex of the patella (b) with the deepest point of the sulcus (a). In normal subjects, this line is close to vertical. A second line (ca) is then drawn from the lowest point on the articular ridge of the patella (c) to the deepest point of the sulcus (a). The angle formed by this line and the reference line is the congruence angle. If the lowest point on the patellar articular ridge is lateral to the reference line, then the congruence angle has a positive value; if it is medial to the reference line, as in the present example, then the angle has a negative value. In Merchant’s study, the average congruence angle in normal subjects was −6 degrees (standard deviation [SD], ±11 degrees). (Modified from Merchant AC, Mercer RL, Jacobsen RH, Cool CR. Roentgenographic analysis of patello-femoral congruence. J Bone Joint Surg [Am] 1974;56A:1391-1396.)

Arthrography used to be the procedure of choice in evaluating injuries to the soft-tissue structures of the knee, such as the joint capsule, menisci, and ligaments (Fig. 9.8). It is still valuable in examination of the
articular cartilage, particularly when subtle chondral or osteochondral fracture is suspected, or when confirmation of the presence or absence of osteochondral bodies in the knee joint is required in suspected osteochondritis dissecans. However, in the evaluation of the menisci, cruciate ligaments, and collateral ligaments, arthrographic examination has been almost completely replaced by MRI.

FIGURE 9.8 Arthrography of the knee. For arthrographic examination of the knee, the patient is supine on the radiographic table, with both legs fully extended and in the neutral position. The patella is pulled laterally and rotated anteriorly, and the joint is entered from the lateral aspect at the midpoint of the patella. Before injection of contrast, the joint should be aspirated to avoid dilution of the contrast agent by joint fluid. For a double-contrast study, 40 to 50 mL of room air is injected into the joint, followed by 5 to 7 mL of positive contrast agent (usually 60% diatrizoate meglumine mixed with 0.3 mL of epinephrine 1:1,000, which delays absorption of the contrast). Radiographs are then obtained in the prone position using the spotfilm technique (see Fig. 9.10).

The medial and lateral menisci (or semilunar cartilages) of the knee are crescent-shaped fibrocartilaginous structures attached, respectively, to the medial and lateral aspects of the superior articular surface of the tibia (Fig. 9.9). Normally, the medial meniscus is visualized on arthrography as a triangular structure intimately attached to the joint capsule and tibial (medial) collateral ligament; its smooth borders are coated by positive contrast agent and surrounded by injected air. The normal arthrogram shows no air or contrast within the substance of the meniscus or at its periphery (Fig. 9.10A-C). Although the lateral meniscus is structurally very similar to the medial meniscus, it has a very important distinguishing feature. The popliteal muscle’s tendon and its sheath pass through a portion of the posterior horn of the lateral meniscus, separating it from the joint capsule. This anatomic site, known as the popliteal hiatus, gives an arthrographic impression of separation of the periphery of the lateral meniscus from the capsule; it should not be mistaken for a tear (Fig. 9.10D,E). An important fact to remember is that not all areas of the menisci are well demonstrated by knee arthrography. Only the parts seen tangentially can be assessed accurately. For example, the posterior part of the posterior horn of the lateral meniscus constitutes a blind spot because it extends deeply into the knee joint (see Fig. 9.9).

The cruciate ligaments of the knee are also structures commonly subject to injury (Fig. 9.11). In the evaluation of these ligaments, arthrography was the procedure of choice before the MRI era and is even now occasionally performed. The radiograph is obtained to best advantage in the lateral projection with 60 to 80 degrees of knee flexion and with the examiner applying pressure to the posterior aspect of the proximal tibia. When tensed, the anterior cruciate ligament (ACL) normally projects as a straight line extending from the intercondylar notch to a point approximately 8 mm posterior to the anterior margin of the tibia. The posterior cruciate ligament is seen as a straight or slightly bulging line extending to the posterior margin of the tibial plateau (Fig. 9.12).

In the past decade, MRI of the knee has gained wide acceptance in the diagnosis of traumatic abnormalities and currently is the method of choice in evaluating various knee structures, particularly the menisci, cruciate ligaments, and collateral ligaments. Routinely, T1-weighted and T2-weighted images are obtained in the sagittal, coronal, and axial planes. The sagittal plane is generally the most effective for evaluation of the cruciate ligaments, menisci, patellar ligament, and quadriceps tendon. Coronal sections are needed for evaluation of the medial and lateral collateral ligaments, as well as the menisci. The axial plane is best to evaluate the patellofemoral joint compartment. The axial plane is also helpful in evaluating the popliteal cysts and their relationship to the surrounding structures of the popliteal fossa.

MR arthrography (MRa) is effective in evaluating residual or recurrent meniscal tears after meniscal surgery. It is also a valuable technique to demonstrate loose intraarticular chondral or osteochondral bodies, synovial plicae, and to evaluate the stability of various osteochondral lesions, including osteochondritis dissecans and osteochondral fracture. MRa of the knee is performed by injecting up to 40 mL of diluted gadolinium solution into the joint using the same technique as described for conventional knee arthrography (see Fig. 9.8). Coronal, sagittal, and axial images are obtained, most commonly with fat-suppressed T1- (or proton density) and T2-weighted sequences.

The menisci are seen on MRI as wedge-shaped or bow tie-shaped structures of uniformly low signal intensity in practically all pulse sequences (Fig. 9.13). The anterior and posterior cruciate ligaments, like the menisci, are seen as low-signal intensity structures on all spin echo sequences. The ACL is straight and fan shaped (slightly wider at its femoral attachment) and demonstrates low-to-intermediate signal intensity (Fig. 9.14A). The posterior cruciate ligament is arcuate in shape when the knee is in extension or mild flexion and becomes increasingly taut as the knee is flexed. Normally, it has very low signal intensity (Fig. 9.14B). Anteriorly to the posterior cruciate ligament, one can observe a small bulge produced by the anterior meniscofemoral ligament, also known as the ligament of Humphrey (Fig. 9.14B,C). Posteriorly, a small bulge is created by the posterior meniscofemoral ligament, known as the ligament of Wrisberg (Fig. 9.14D,E).

The medial collateral ligament consists of two components: superficial and deep. The superficial component, which is the principal medial stabilizer of the knee, arises from the medial femoral epicondyle just below the adductor tubercle and inserts into the medial aspect of the tibia, approximately 5 cm below the joint line. The deep layer of the medial collateral ligament, which is considered part of the fibrous capsule, attaches loosely to the peripheral margin of the body of the medial meniscus. The lateral collateral ligament attaches to the lateral epicondyle of the femur superiorly just above the popliteus groove, in which region it merges with the outer surface of the capsule. From here, it extends inferiorly and posteriorly to attach to the anterior portion of the apex of the fibular head. Both collateral ligaments are best demonstrated on the images obtained in the coronal plane. Like the menisci and cruciate ligaments, they also display low signal intensity (Fig. 9.15).

During evaluation of MRI of the knee, it is helpful to use a checklist as provided in Table 9.1.

Evaluation of knee instability caused by ligament injuries may require obtaining stress views. These techniques are most commonly performed in cases of suspected injury to the medial collateral ligament (Fig. 9.16; see also Fig. 9.83). They are less frequently performed during the evaluation of insufficiency of the anterior and posterior cruciate ligaments (Fig. 9.17). These examinations should preferably be performed under local anesthesia.

Arteriography and venography may need to be used in the evaluation of concomitant injury to the vascular system, although recently more often MR angiography is performed for this purpose. CT is effective in the evaluation of tibial plateau fractures, and it is occasionally used to evaluate injury to

the cartilage and soft tissues, particularly the menisci and cruciate ligaments. CT used in conjunction with arthrography (computed arthrotomography) is useful in the evaluation of osteochondritis dissecans (see Fig. 9.60C,D) and in detecting nonopaque osteochondral bodies in the knee joint.

FIGURE 9.9 Tibial plateau. In the topography of the tibial plateau, the medial meniscus is a C-shaped fibrocartilaginous structure with anterior horn attached anteriorly to the intercondylar eminence of the tibia and with posterior horn inserted into the intercondylar area in front of the attachment of the posterior cruciate ligament. The anterior horn of the lateral meniscus, which is an O-shaped structure, is attached in front of the lateral intercondylar tubercle, and the posterior horn inserts medially into the lateral intercondylar tubercle, in front of the attachment of the posterior horn of the medial meniscus.

FIGURE 9.10 Arthrography of the knee. Multiple spot films obtained during arthrographic examination of the knee demonstrate the normal appearance of the medial (A-C) and lateral (D,E) semilunar cartilages. The contrast-outlined margins of the medial meniscus show its triangular shape. The posterior horn (A) is longer than the body (B) and the anterior horn (C), and the free edge of the meniscus is sharply pointed. Features of the normal lateral meniscus include the gap of the popliteal hiatus, which separates the meniscus from the joint capsule (D). The posterior horn reattaches to the capsule more posteriorly (E). No contrast should be seen within the substance of any aspect of the menisci.

FIGURE 9.11 The cruciate ligaments. In the topography of the cruciate ligaments of the knee, the ACL arises on the medial surface of the lateral femoral condyle at the intercondylar notch (A) and attaches on the anterior portion of the intercondylar eminence of the tibia (C) (see also Fig. 9.9). The posterior cruciate ligament originates on the lateral surface of the medial femoral condyle within the intercondylar notch (B) and inserts on the posterior surface of the intercondylar eminence (D) (see also Fig. 9.9). Neither cruciate ligament is attached to the tibial tubercles.

FIGURE 9.12 Arthrography of the cruciate ligaments. Double-contrast arthrogram of the knee demonstrates the normal appearance of the cruciate ligaments. Note the angle formed by their projectional intersection and their taut appearance. Each ligament can be traced from its origin in the femur to its insertion in the tibia. The boundaries of the cruciate ligaments are sharply outlined because the contrast medium coats their synovial reflexions. The cruciate ligaments are extrasynovial structures; only the anterior surface of the ACL and the posterior surface of the posterior cruciate ligament are covered by synovium.

FIGURE 9.13 Appearance of normal menisci on MRI. (A) Anterior and posterior horns of the medial meniscus as seen on sagittal T2*-weighted MPGR sequence (flip angle 30 degrees). (B) Anterior and posterior horns of the lateral meniscus as seen on sagittal T2*-weighted MPGR sequence (flip angle 30 degrees). (C) Body of the medial meniscus as seen on sagittal spin echo T1-weighted sequence. (D) Anterior and posterior horns of the lateral meniscus as seen on sagittal spin echo T1-weighted sequence. (E) Schematic representation of topography of the medial and lateral menisci and surrounding structures as seen in the midplane of the coronal MRI. (Modified from Firooznia H, Golimbu C, Rafii M. MR imaging of the menisci: fundamentals of anatomy and pathology. Magn Reson Imaging Clin N Am 1994;2:325-347.)

FIGURE 9.14 Cruciate ligaments. Spin echo MR images of the normal cruciate ligaments. (A) Sagittal proton density-weighted image demonstrates the anterior margin of the ACL, straight and well defined, representing the anteromedial bundle; the posterior margin is ill-defined because of the oblique orientation of the ligament and it represents the posterolateral bundle. (B) Oblique coronal T2-weighted image depicts the ACL from the origin in the lateral femoral condyle to the insertion in the tibia (arrows). (C) The posterior cruciate ligament is seen in its entirety, in one sagittal image, from the femoral to the tibial attachments. Observe the small bulge anteriorly produced by the anterior meniscofemoral ligament (arrow). (D) In this sagittal section, the anterior meniscofemoral ligament of Humphrey is very prominent, simulating a loose body or meniscal fragment (arrow). (E) Here, meniscofemoral ligaments, both anterior (Humphrey) and posterior (Wrisberg), are prominent.

FIGURE 9.15 Collateral ligaments. (A) Coronal T2-weighted fat-saturated MR image of the normal medial collateral ligament. The superficial fibers of the medial collateral ligament are well defined in this section through the intercondylar notch (arrows). The insertion of the posterior cruciate ligament in the inner aspect of the medial femoral condyle is well demonstrated. The menisci are seen as small triangles of low signal intensity. (B) Coronal T2-weighted fat-saturated image demonstrates the superficial (long arrows) and deep (arrowheads) fibers of the medial collateral ligament. Note the deep crural fascia (short arrow) and the tibial arm of the semimembranosus tendon (T). (C,D) Coronal T2-weighted fat-saturated images of the lateral (fibular) collateral ligament (arrow). On this posterior sections, note the meniscofemoral ligament, which extends from the posterior horn of the lateral meniscus to the inner surface of the medial femoral condyle (arrowheads). The lateral and medial menisci and posterior cruciate ligament are well demonstrated.

For a summary of the preceding discussion in tabular form, see Tables 9.2 and 9.3 and Figure 9.18.

Injury to the Knee

Fractures About the Knee

Fractures of the Distal Femur

Most often sustained in motor vehicle accidents or falls from heights, fractures of the distal femur are classified according to the site and extension of the fracture line as supracondylar, condylar, and intercondylar. Supracondylar fractures can be further classified as nondisplaced, impacted, displaced, and comminuted (Fig. 9.19). These injuries are usually well demonstrated on the standard anteroposterior and lateral radiographs of the knee (Fig. 9.20); however, in rare instances, an oblique view of the knee may be needed to evaluate an obliquely oriented fracture line. Tomography used to be required in cases of comminution for a full evaluation of the fracture lines and localization of the fragments (Fig. 9.21), although currently helical CT with multiplanar and three-dimensional (3D) reformation has surpassed conventional tomographic technique (Fig. 9.22).

Fractures of the Proximal Tibia

The medial and lateral tibial plateaus are the most common sites of fractures of the proximal tibia. Because they usually result when the knee is struck by a moving vehicle, they are also called fender or bumper fractures; some, however, may be the result of twisting falls. The Hohl classification gives an overview of six different types of tibial plateau fractures and is useful in correlating the various types of injuries with the applied forces causing them (Fig. 9.23). In the Hohl classification, pure abduction injury results in a nondisplaced split fracture of the lateral tibial plateau (type I) (Fig. 9.24). When axial compression is combined with abduction force,

local central depression (type II) and local split depression (type III) fractures occur (Fig. 9.25). Total depression fractures (type IV), which are more commonly seen in the medial tibial plateau because of its anatomic configuration (absence of the fibula), are characterized by the lack of comminution of the articular surface. Type V fractures in the Hohl classification, which are infrequently encountered, are local split fractures without central depression involving the anterior or posterior aspects of the tibial plateau. Comminuted fractures involving both tibial plateaus and having a Y or T configuration (type VI) usually result from vertical compression, such as a fall on the extended leg (Fig. 9.26). Types III and VI are frequently associated with fracture of the proximal fibula. In our institution, we use the Schatzker classification of tibial plateau fractures which, similar to the Hohl classification, arranges tibial plateau fractures into VI types but according to involvement of the medial or lateral plateau (Fig. 9.27).

TABLE 9.1 Checklist for Evaluation of Magnetic Resonance Imaging of the Knee

*Osseous Structures*
	Femoral condyles (c, s, a) Tibial plateau (c, s) Gerdy tubercle (s, a) Patella (c, s, a) Proximal fibula (c, s, a)
*Cartilaginous Structures*
	Articular cartilage (c, s, a)
*Joints*
	Femorotibial (c, s) Femoropatellar (s, a)
*Menisci*
	Medial (c, s) Lateral (c, s)
*Ligaments*
	Medial collateral—deep and superficial fibers (c) Lateral collateral complex—biceps femoris tendon, lateral collateral ligament proper, iliotibial band (c) Anterior cruciate—anteromedial and posterolateral bundles (c, s) Posterior cruciate (c, s) Meniscofemoral—Humphry (anterior) and Wrisberg (posterior) (c, s) Transverse (s) Patellar (“tendon”) (s) Patellar retinaculae—medial and lateral (a) Arcuate (c, a) Popliteofibular (c, s) Fabellofibular (c)
*Muscles and Their Tendons*
	Quadriceps (s, a) Popliteus (c, s) Plantaris (a) Biceps femoris (c) Semimembranosus (s, a) Semitendinosus (s, a) Gracilis (s, a) Sartorius (s, a) Gastrocnemius (s, a) Soleus (s, a)
*Bursae*
	Popliteal (Baker)—between the tendons of the medial head of gastrocnemius and semimembranosus (s, a) Prepatellar (s, a) Deep infrapatellar (s, a) Pes anserinus (c) Semimembranosus—tibial collateral ligament (c)
*Other Structures*
	Synovial plicae (c, a) Infrapatellar plica (s) Hoffa fat pad (s, a) Popliteus hiatus (c) Popliteal artery and vein (a) Lateral geniculate artery (c) Tibial and peroneal nerves (a)
The best imaging planes for visualization of listed structures are given in parenthesis; c, coronal; s, sagittal; a, axial.

FIGURE 9.16 Valgus stress. For a stress film of the knee evaluating the medial collateral ligament, the patient is supine, with the knee flexed approximately 15 to 20 degrees. The leg is placed in the device, and the pressure plate is applied against the lateral aspect of the knee. (The arrows show the direction of the applied stresses.) Radiographs are then obtained in the anteroposterior projection (see Fig. 9.83B).

FIGURE 9.17 Anterior-drawer stress. For a stress film of the knee evaluating the ACL, the patient is placed in the device on his or her side, with the knee flexed 90 degrees. The pressure plate is applied against the anterior aspect of the knee. (The arrows show the direction of the applied stresses.) Radiographs are then obtained in the lateral projection.

TABLE 9.2 Standard and Special Radiographic Projections for Evaluating Injury to the Knee

Projection		Demonstration
Anteroposterior		Medial and lateral joint compartments Varus and valgus deformities Fractures of:
			Medial and lateral femoral condyles Medial and lateral tibial plateaus Tibial spines Proximal fibula
		Osteochondral fracture Osteochondritis dissecans (late stage) Spontaneous osteonecrosis Pellegrini-Stieda lesion
	Overpenetrated	Bipartiite or multipartite patella Fractures of patella
	Stress	Tear of collateral ligaments
Lateral		Femoropatellar joint compartment Patella in profile Suprapatellar bursa Fractures of:
			Distal femur Proximal tibia Patella
		Dislocations Sinding-Larsen-Johansson disease^a Osgood-Schlatter disease^a Osteochondral fracture Osteochondritis dissecans (late stage) Spontaneous osteonecrosis Joint effusion Tears of:
			Quadriceps tendon Patellar ligament
	Stress	Tears of cruciate ligaments
	Cross-table	FBI sign of lipohemarthrosis
Tunnel (posteroanterior)		Posterior aspect of femoral condyles Intercondylar notch Intercondylar eminence of tibia
Axial (sunrise and Merchant)		Articular facets of patella^b Sulcus angle^b Congruence angle^b Fractures of patella Subluxation and dislocation of patella^b
^aThese conditions are best demonstrated using a low-kilovoltage/soft-tissue technique. ^bThese features are better demonstrated on Merchant axial view.
FBI, fat-blood interface.

TABLE 9.3 Ancillary Imaging Techniques for Evaluating Injury to the Knee

Technique	Demonstration
Arthrography (usually doublecontrast; occasionally singlecontrast using air only)	Meniscal tears Injuries to:
		Cruciate ligaments Medial collateral ligament Quadriceps tendon Patellar ligament Joint capsule
	Chondral and osteochondral fractures Osteochondritis dissecans (early and late stages) Osteochondral bodies in joint Subtle abnormalities of articular cartilage
CT and computed arthrotomography	Spontaneous osteonecrosis Injuries to:
		Articular cartilage Cruciate ligaments Menisci
	Osteochondral bodies in joint Osteochondritis dissecans
Radionuclide imaging (scintigraphy, bone scan)	Subtle fractures not demonstrated on standard studies Early and late stages of:
		Osteochondritis dissecans Spontaneous osteonecrosis
Angiography (arteriography, venography)	Concomitant injury to arteries and veins
MRI	Same as arthrography, CT, and radionuclide imaging
MRa	Residual or recurrent meniscal tears Complications after meniscal surgery Loose intraarticular bodies Synovial plicae Stability of osteochondral lesions Tears of collateral ligaments Tears of cruciate ligaments
MR angiography	Same as angiography
MRI, magnetic resonance imaging; CT, computed tomography; MRa, magnetic resonance arthrography; MR, magnetic resonance.

FIGURE 9.18 Spectrum of radiologic imaging techniques for evaluating injury to the knee. The radiographic projections or radiologic techniques indicated throughout the diagram are only those that are the most effective in demonstrating the respective traumatic conditions. ^#Almost completely replaced by CT. AP, anteroposterior; CT, computed tomography.

FIGURE 9.19 Classification of distal femur fractures. Fractures of the distal femur can be classified according to the site and extension of the injury as supracondylar, condylar, and intercondylar fractures.

FIGURE 9.20 Supracondylar fracture. A 58-year-old man was injured in a motorcycle accident. Anteroposterior (A) and lateral (B) radiographs of the knee demonstrate a comminuted supracondylar fracture of the distal femur. The extension of the fracture lines and the position of the fragments can be assessed adequately on these standard studies.

FIGURE 9.21 Supracondylar fracture. A 22-year-old racing car driver was injured in an accident on the track. (A) Anteroposterior view of the right knee shows a comminuted fracture of the distal femur. Tomography was performed, and sections in the anteroposterior (B) and lateral (C) projections demonstrate intraarticular extension of the fracture lines, with split of the condyles and posterior displacement of the distal fragments. The multiple comminuted fragments can be localized.

FIGURE 9.22 CT and 3D CT of supracondylar fracture. A 54-year-old woman was injured in a motor vehicle accident. (A) Anteroposterior radiograph of the right knee shows markedly comminuted supracondylar fracture of the femur. (B,C) Coronal and sagittal reformatted CT images show displacement of various fracture fragments. 3D CT reconstructed images, (D) oblique and (E) viewed from the posterior aspect, depict the position and orientation of displaced fracture fragments in more comprehensive fashion.

FIGURE 9.23 The Hohl classification of fractures of the tibial plateau. (Modified from Hohl M. Tibial condylar fractures. J Bone Joint Surg [Am] 1967;49A:1455-1467.)

FIGURE 9.24 Fracture of the tibial plateau. A 30-year-old man was hit by a car while he was crossing the street. Anteroposterior radiograph (A) and tomogram (B) show a split fracture of the lateral tibial plateau (Hohl type I).

FIGURE 9.25 Fracture of the tibial plateau. Anteroposterior radiograph of the knee shows the appearance of a tibial plateau fracture, which is a combination of wedge and central depression fractures involving the lateral tibial condyle (Hohl type III).

Fractures of the tibial plateau may not be obvious on the routine radiographic examination of the knee, particularly if there is no depression (Fig. 9.28A,B). In such cases, however, the cross-table lateral projection often reveals the FBI sign, which indicates the presence of an intraarticular fracture (Fig. 9.28C). Demonstration of an obscure fracture line may require oblique projections.

FIGURE 9.26 Fracture of the tibial plateau. Anteroposterior radiograph (A) and lateral tomogram (B) demonstrate the characteristic appearance of the Y-type bicondylar tibial fracture (Hohl type VI).

The role of CT in evaluation of tibial plateau fractures has been well established. CT provides optimal visualization of the plateau depression, defects, and split fragments. It also proved to be accurate in assessing depressed and split fractures when they involved the anterior and posterior border of the plateau and in demonstrating the extent of fracture comminution. Particularly useful are reformatted images in various planes and 3D reconstruction (Figs. 9.29, 9.30, 9.31). Recently, Kode and coworkers suggested that MRI was equivalent or superior to two-dimensional (2D) CT reformation for the depiction of tibial plateau fracture configuration (Figs. 9.32 and 9.33). The multiplanar capabilities of MRI may facilitate 3D perception and, in addition, this technique permits assessment of the associated injuries to the ligaments and menisci that are not visible on CT scans (Fig. 9.34).

FIGURE 9.27 The Schatzker classification of fractures of the tibial plateau. (Modified from Koval JK, Helfet DI. Tibial plateau fractures: evaluation and treatment. J Am Acad Orthop Surg 1995;3:86-93.)

FIGURE 9.28 Fracture of the tibial plateau. While crossing the street, a 38-year-old woman was struck by a car. Anteroposterior (A) and lateral (B) radiographs show substantial joint effusion, but the fracture line is not clearly seen. (C) Cross-table lateral view demonstrates the FBI sign, indicating intraarticular extension of the fracture.

FIGURE 9.29 CT of fracture of the tibial plateau. A 23-year-old man was injured in a motorcycle accident. The conventional radiographs of the right knee (not shown here) demonstrated a fracture of the tibial plateau. (A) Axial CT section through the proximal tibia shows a comminuted fracture of the medial tibial plateau. (B) Sagittal reformatted image shows that the anterior part of the plateau is mainly affected. (C) Coronal reformatted image demonstrates comminution and depression. (D) Anterior view of the 3D reconstructed image in addition to depression of the medial anterior tibial plateau shows associated fracture of the proximal fibula. (E) Bird’s eye view of the 3D reconstructed image shows the spatial orientation of the fracture lines.

FIGURE 9.30 CT of fracture of the tibial plateau. A 22-year-old man fell down from a tall ladder and injured his right knee. The conventional radiographs demonstrated fracture of the tibial plateau. (A) Coronal reformatted CT scan shows extension of the lateral tibial plateau fracture into the tibial shaft. (B) Posterior view of the 3D reconstruction shows the fracture line, but the interfragmental split is not well demonstrated. (C) Anterior view of the 3D reconstruction shows the split better. (D) Bird’s eye view of the 3D CT scan effectively demonstrates the details of the split and comminution of the tibial plateau.

An important feature of tibial plateau fractures is their association with injury to ligaments and the menisci. The structures most at risk are the medial collateral and the ACLs (see Fig. 9.11) and the lateral meniscus (see Fig. 9.9), because lateral tibial plateau fractures usually result from valgus stress (Fig. 9.35). Moreover, damage to the ACL may be associated with avulsion of the lateral tibial spine or the anterior intercondylar eminence. Stress views and MRI usually reveal these associated abnormalities. If clinical examination and radiologic studies, including stress views, show ligamentous structures to be intact, then nondisplaced fractures of the tibial plateau can be treated conservatively. In depression-type fractures, however, Larson recommends open reduction in patients whose fractures show 8 mm of articular depression. Generally, surgery is indicated for fractures of the tibial plateau showing articular depression of 10 mm or more.

Complications. The most frequent complications of fractures of the distal femur and the proximal tibia are malunion and posttraumatic arthritis.

Segond Fracture

The Segond fracture consists of a small-fragment avulsion fracture from the lateral aspect of the proximal tibia just below the level of the tibial plateau, best demonstrated on anteroposterior radiograph of the knee (Fig. 9.36). The mechanism of this injury is internal rotation of the leg associated with varus stress on a flexed knee that creates tension on the lateral capsule and lateral capsular ligament. This, in turn, causes an avulsion fracture at the insertion of this ligament on the lateral tibial plateau. This injury may be associated with capsular tear, a tear of the ACL, and lateral meniscus tear, resulting in chronic anterolateral knee instability (Fig. 9.37).

Recently, Hall and Hochman described a reverse Segond-type fracture affecting medial tibial plateau, associated with tears of the posterior cruciate ligament, medial collateral ligament, and medial meniscus (Fig. 9.38). The mechanism of this injury and the constellation of radiographic findings are the reverse of that seen with the classic Segond injury complex. The avulsion fracture of the medial tibial plateau is caused by a valgus stress and external rotation of a flexed knee.

Fractures of the Patella

Fractures of the patella, which may result from a direct blow to the anterior aspect of the knee or from indirect tension forces generated by the quadriceps tendon, constitute approximately 1% of all skeletal injuries. Generally, patellar fractures may be longitudinal (vertical), transverse,

or comminuted (Fig. 9.39). In the most commonly encountered patellar injury, seen in 60% of cases, the fracture line is transverse or slightly oblique, involving the midportion of the patella. In evaluation of such injury, it is important to recognize what has been called the bipartite or multipartite patella. This anomaly represents a developmental variant of the accessory ossification center or centers of the superolateral margin of the patella and should not be mistaken for a fracture (Fig. 9.40). CT may help distinguish this developmental anomaly from patellar fracture. As an aid to avoid misdiagnosing a bipartite or multipartite patella as a fracture, it is important to keep in mind that the accessory ossification centers are invariably in the upper lateral quadrant of the patella and, if the apparent fragments are put together, they do not form a normal patella. Fracture fragments, however, form a normal patella if they are replaced. Injury to the patella is usually sufficiently demonstrated on the overpenetrated anteroposterior and lateral radiographs of the knee (Figs. 9.41, 9.42, 9.43).

FIGURE 9.31 CT of fracture of the tibial plateau. Coronal (A) and sagittal (B) CT reformatted images show a Hohl type III (displaced, local spit depression) fracture of the lateral tibial plateau. (C) 3D reconstructed image (posterior view) more realistically depicts the features of this injury.

FIGURE 9.32 MRI of fracture of the tibial plateau. (A) T2-weighted (spin echo, TR 2000/TE 80 msec) coronal image shows a broad-based band of low signal intensity traversing the lateral tibial plateau (long arrows). Extensive soft-tissue edema is seen superficial to the iliotibial band (small arrows). (B) Proton density-weighted (spin echo, TR 2000/TE 20 msec) sagittal image shows central localized depression of the tibial plateau (arrow). The degree of comminution and depression is well depicted. (From Bloem JL, Sartoris DJ, eds. MRI and CT of the musculoskeletal system. A text-atlas. Baltimore: Williams Wilkins; 1992.)

FIGURE 9.33 MRI of fracture of the tibial plateau. (A) Coronal gradient echo (MGPR) image shows a tibial plateau fracture (arrowheads). (B) Sagittal gradient echo (MGPR) image demonstrates the anterior extension of the fracture and evulsion of the tibial spines (arrowheads). (From Berquist TH, ed. MRI of the musculoskeletal system, 3rd ed. Philadelphia: Lippincott-Raven Publishers; 1997.)

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