Bone bruises: Their patterns and significance

Bone Bruises: Their Patterns and Significance Arthur H. Newberg and Steven M. Wetzner MR imaging is sensitive in the detection of occult stress and posttraumatic fractures in and around the knee joint. In some patients, the pain from these bony injuries can mimic that of meniscal tears. The abnormality of the bone may explain completely the patient's symptoms and obviate the need for any further work-up. The types of injuries detected by MRI include bone bruises, stress or insufficiency fractures, and osteochondral fractures. Bone bruises or contusions are characterized by a diffuse or localized pattern of low signal intensity on Tl-weighted images w i t h o u t a defined fracture. Blood, edema, hyperemia, and perhaps microfracture of the trabeculae may all contribute to the marrow signal alterations. Anterior cruciate ligament injuries often are accompanied by a characteristic bone contusion pattern, such as hemorrhage or edema in the posterior aspect of the lateral tibial plateau, as w e l l as the anterior aspect of the lateral femoral condyle. Copyright 9 1994 by W.B. Saunders Company

TH THE increasing application of MRI in patients with acute musculoskeletal trauma, more occult traumatic lesions of bone are identified. ~ MRI has been shown to be sensitive in the detection of occult bone lesions, and it can detect and help assess both occult and traumatic bone lesions. In some patients, the pain from these bony injuries can mimic that of meniscal tears. The abnormality of the bone may completely explain the symptomatology and obviate the need for any further work-up in these cases. A clinically significant bony injury may initially and possibly always be radiographically occult. The prevalence of occult posttraumatic injuries has been shown to be approximately 72% .2 In a normal knee MRI examination, the tightly packed lameUar bone of the cortex can be seen as a peripheral, smooth black line. The subcortical and medullary spaces are composed of red or fatty marrow and trabeculae. Red marrow is of intermediate signal intensity on short and long echo time (TE) spin-echo sequences, slightly brighter than muscle but darker than subcutaneous fat. Fatty marrow has high signal intensity on T1- and intermediateweighted images, darkens on T2 weighting, and behaves similar to subcutaneous fat on all sequences. Cancellous bone is the weight-bearing structure that suffers in fatigue fractures, stress fractures, and acute traumatic injuries of the knee.

W

From Tufts University School of Medicine, Department of Radiology, New England Baptist Hospital Boston, MA. Address reprintrequests to Arthur H. Newberg, MD, Department of Radiology, New England Baptist Hospital 125 Parker Hill Ave, Boston, MA 02120. Copyright 9 1994 by W.B. Saunders Company 0887-2171/94/1505-000555.00/0 396

The MRI findings in knee trauma do not depend on direct visualization of the trabeculae either at the time of initial injury or during repair. Rather, MRI depends on imaging the marrow and its evolution in response to acute injury. With fracture of trabeculae, one might expect hemorrhage and edema in the tissues surrounding the injury. MRI is exquisitely sensitive in the detection of abnormal amounts of water in tissues, either because of edema from inflammation or the separation of serum from the cellular components of blood. This characteristic probably explains the apparent increased sensitivity of MRI over plain radiographs in the early detection of bone bruises) If the MRI reader identifies a bony injury, there are clinical applications, in as much as injuries to bone require reduced weight bearing for an extended period for proper healing. In addition, persistent pain on weight bearing after successful meniscal or ligamentous repair often poses a difficult diagnostic dilemma. Detection of bony injury in this setting can be helpful in examining postoperative patients. TECHNIQUE

Numerous imaging sequences can be used in the patient with knee pain. The sequences selected should be based on personal experience and the capabilities of the magnet. However, some type of T2- or T2*-weighted data set is recommended in patients with acute knee injuries because certain bone injury patterns can be helpful to the reader in recognizing associated intra-articular knee pathology. Occult fractures are of diminished signal intensity on Tl-weighted images and of increased signal intensity on T2- and T2*-weighted images. Re-

Seminars in Ultrasound, CT, and MRI, Vol 15, No 5 (October), 1994: pp 396-409

BONE BRUISES

cently, the routine use of fast spin-echo sequences with fat saturation (FSE-FS) was begun in patients referred for knee MRI within 4 weeks of an injury. T2-weighted FSE-FS imaging is a sensitive and rapid method of identifying and assessing the degree of bone trauma around the knee. 4 This technique increases the conspicuity between normal bone marrow and foci of bone bruises. Bone marrow fat has higher signal intensity on T2-weighted FSE images than on conventional T2-weighted spin-echo images because of the effects of contrast averaging (Fig 1). When a fat saturation pulse is added to the FSE sequence, the signal intensity of fat is suppressed. This increases the contrast between the normal bone marrow and areas of intramedullary hemorrhage (Fig 2). It has been shown that conventional T2-weighted spin-echo and FSE techniques consistently underestimate the peripheral margins of marrow involvement. 4 T2-weighted FSE images obtained without FS are less sensitive in showing bone bruises because marrow fat tends to have higher signal intensity on FSE sequences without FS in comparison with conventional T2-weighted spinecho images with similar TE. Kapelov et al 5 found that only 62% of the lesions seen on FSE-FS images also were identified on conventional T2-weighted spin-echo data sets. FSE-FS sequences save time because repetition time (TR) and TE can be reduced without loss of acceptable tissue contrast. Shorter examination

397

Fig 2, Coronal FSE-FS image showing bone marrow suppression, except for a bone contusion in the lateral femoral condyle.

times usually result in improved cooperation by the patient, especially in those with excess anxiety or patients who are in pain. In addition, shorter imaging sequences help to reduce motion artifacts. Short TI inversion recovery (STIR) techniques may also be considered; however, there are limitations including a relatively long imaging time for the acquisition of a limited number of image slices, vulnerability to motion artifact, and poor signal-noise ratio. 5 In an acute fracture, associated fluid or hemorrhage shows increased signal intensity on a

Fig 1. Comparison of FSE and FS. (A) Coronal FSE proton-density image fails to show a bone bruise. (B) Coronal FSE with FS clearly shows an area of bone bruise in the lateral femoral condyle (arrow).

398

NEWBERG AND WETZNER

T2- or T2*-weighted data set. Fractures with diffuse areas of low signal on Tl-weighted sequences show increased signal with long TR and TE settings. This finding reflects the prolonged T2 values in edematous or hemorrhagic marrow. STIR sequences are more sensitive than gradient-echo T2* data sets in identifying subacute fractures with associated edema (Fig 3). However, T2 sequences may be helpful in showing acute fracture morphology when excessive marrow hemorrhage obscures detail on Tl-weighted or STIR images. Chronic fractures may remain low in signal intensity with variable TR and TE imaging parameters. 6 BONE BRUISES: MECHANISM AND CLASSIFICATION

The dominant mechanism of injury identified in patients with occult fractures is similar to that in patients who do not have such fractures. Most of these lesions in the lateral compartment are consistent with induced valgus forces at the time of injury, regardless of whether the injury is associated with rotational or deceleration forces (Fig 4). Such factors also would account for the high prevalence of associated ligamentous injuries. It is common for patients with collateral ligament injury to continue to

9 Fig 3. Sagittal STIR image showing a bone contusion in the lateral femoral condyle of an 18-year-old female varsity Division I college soccer player. The bone marrow is dark except for the intense area of brightening probably caused by hemorrhage. The patient returned to competition in approximately 3 weeks.

Fig 4. Coronal T1 image in a ig-year-old man with a lateral femoral condyle bone bruise (arrow) associated with a lateral meniscus tear, lateral ligamentous disruption, and a medial collateral ligament (MCL) sprain. (Reprinted with permission, z2)

have pain with weight bearing even after knee bracing (Fig 5). Assuming the menisci are normal on the MRI study, bone abnormalities adjacent to the weight-bearing cortex may be responsible for the persistent pain. 7 The invariable association of bone bruises involving the lateral femoral condyle with occult posterior tibial fractures suggests that anterior

Fig 5. Coronal T2-weighted FSE image shows an MCL tear (large arrowhead) associated with a bone contusion in the lateral femoral condyle (arrowheads).

BONE BRUISES

tibial subluxation results in tibiofemoral impaction at the point of impact. Occult, posttraumatic lesions, termed bone bruises, have been believed to be self-limiting, benign abnormalities with no sequelae. Yao and Lee 7 were the first to describe occult intraosseous fracture. They showed a case of linear subchondral fracture with surrounding edema that resolved on a follow-up MRI performed 6 weeks later. Although healing of microtrabecular fractures may occur, the less resiliant chondral surface could theoretically undergo chondrolysis and death proportionate to a force's impact and distribution. After trauma, the alterations identified in the bone marrow signal are probably related to blood, hyperemia, and perhaps microfracture of the trabeculae. Vellet's study2 shows that posttraumatic occult subcortical fractures are a heterogeneous group of lesions associated with a high prevalence of osteochondral sequelae. A bone bruise is defined on T1 images as a traumatically involved, geographic, and nonlinear area of signal loss involving the subcortical bone (Fig 6). On T2-weighted images, most or all of the lesions have increased signal intensity. 8A bone bruise can be seen on the contralateral side of a collateral ligament injury or in the lateral femoral condyle after lateral patellar

399

dislocation. It has been shown that all patients with an acute patellar dislocation show bone m a r r o w i n j u r y . 9 In acute patellar dislocation, the patella hits the lateral femoral condyle causing the b o n e bruise (Fig 7). In approximately one third of cases, bone marrow changes also are identified in the patella itself. Bone bruises have been classified into three types6: In a type I lesion, there is a loss of signal intensity on short TE images that is located primarily within the medullary cavity of the bone, usually involving both the epiphyseal and metaphyseal regions without cortical interruption (Fig 8). Type II is defined as a loss of signal intensity on short TE images that is associated with an interruption of the black cortical line (Fig 9). Type III is defined as a signal intensity loss on Short TE images that is restricted primarily to the region of bone immediately adjacent to the cortex Without a definite cortical interruption (Fig 10). Vellet et aF offer a more complex classification of bone injuries based on the definitions of reticular and geographic injuries (Table 1). Reticular injuries are regions of reticular, serpiginous stranding of diminished signal intensity on T1 weighting within the high signal intensity of the epiphyseal or metaphyseal marrow (Fig 11). "Such lesions may b e associated

Fig6. A 15-year-old boy complained of bilateral knee pain after several days of football practice. The radiographs were normal. {A) Coronal T1 and (B) coronal fat-suppressed images show the characteristic reticular pattern of a type I bone bruise identified in both the media| femoral condyle and the medial tibial plateau. Similar findings were observed on an MR| examination of the contralateral knee.

400

NEWBERG AND WETZNER

Table 1. Classification of Bone Injuries Occult subcortical fracture Reticular (bruises) Geographic--Type I Geographic---Type II Linear Stress fracture Linear Globular Osteochondral lesion Chondral Displaced Impacted Tibial plateau fracture Femoral shaft fracture Based on Veilet et al.2

with focal cortical impaction remote from such fractures. ''2 Geographic injuries are occult subcortical fractures characterized by their contiguity to the subjacent cortical bone that may show focal cortical impaction. Geographic I lesions are coalescent with the exception of their periphery, which may show evidence of reticulation. Geographic II lesions are crescentic and coalescent, demarcating a circumscribed central zone of marrow fat. Linear lesions are discrete linear regions of T1 diminished signal and unassociated with evidence of significant perifocal reticulation. Impaction occurs in conjunction with

Fig 8. Sagittal T1 image showing loss of normal fatty marrow signal secondary to bone bruises (arrows) in a patient who sustained an ACL tear.

geographic fractures. They show variable degrees of depression of the articular cortical osteochondral surface (Fig 12). STRESS FRACTURES

Stress or insufficiency fractures around the knee may be a source of the patient's pain and

t

V84

.... ~,~i

i~ 84

Fig 7. Axial FSE images in a 19-year-old female college athlete with recent patella subluxation and a bone contusion, (A) Laterally subluxed patella. (B) Lateral femoral condyle bone bruise characterized by bright signal intensity in the marrow (arrow). (Reprinted with permission. ~)

BONE BRUISES

401

Fig 9. (A) Coronal T1 image shows a nondisplaced Sailer III fracture of the lateral tibial epiphysis and metaphysis in a 15-year-old boy who sustained a soccer injury. Plain films initially and at 6 weeks were normal. (B) Coronal T2 image shows surrounding areas of bone edema or hemorrhage characterized by high-intensity signal alterations. Also note a lateral femoral condyle bone bruise. (Reprinted with permission.:)

Fig 10. Sagiltal FSE proton-density image illustrating a well-demarcated type III bone contusion in the lateral femoral condyle (arrows).

yet present initially with normal plain radiographs. However, the radionuclide bone scan often shows an area of nonspecific increased tracer uptake. Observations on the MRI appearance of stress fractures show that the lesions are characterized by a linear zone of decreased T1 signal surrounded by a broader, poorly defined, low signal-intensityarea (Fig 13). The linear component remains dark on T2-weighted images, but the surrounding zone of presumed edema becomes brighten s Stress fractures usually present with a linear area of diminished signal intensity coursing through the cancellous bone marrow (Fig 14). The lack of a soft tissue mass, absence of cortical destruction, and presence of normal surrounding bone marrow should, in most cases, effectively exclude the diagnosis of a bone tumor. Occasionally, in more acute phases of a stress response, the MRI appearance may be similar to that of a bone bruise. 3

402

NEWBERG AND WETZNER

as reticular (serpentine), geographic, amorphous, or even discrete linear areas of low signal intensity in the subcortical bone that do not extend through the cortex. 8 Occult bone injuries (bone marrow contusion and edema) have been depicted in the lateral compartment in 85% to 97% of patients with acute ACL injury. Osteochondral injuries of the lateral femoral condyle occur less frequently (20%) but are more specific for concurrent ACL injury than lateral compartment contusion. It is uncertain how long these bone bruises are present after the acute injury. Tung et al 1~ found that 73% of patients with an ACL tear who underwent MRI within 9 weeks of the injury had a bone bruise. However, none of their patients showed a bone marrow injury pattern if the examination was performed 9 or more weeks after the knee insult. 1~ The ACL serves as the primary restraint to anterior tibial translation and as a secondary stabilizer to resist internal tibial rotation. Therefore, the ACL would predictably be torn when the femur is rotated externally in relation to a fixed lower extremity, or when the tibia is

Fig 11. Coronal spin-echo T1 image in a 31-year-old woman who suffered a ski injury. Note the serpiginous areas of low signal intensity (arrows) adjacent to a well-defined, low signal fracture line (arrowheads), (Reprinted with permission, zz)

In osteochondral lesions, the chondral component may be difficult to detect, in as much as articular cartilage is difficult to evaluate on routine MRI sequences. Additional T2 or T2* data sets may be needed to enhance the contrast. Occasionally a large chondral defect is detectable as an articular defect filled in by fluid signal intensity. Only minimal, if any, subchondral bone marrow signal changes may be detectable (Figs 15 and 16). ANTERIOR CRUCIATE LIGAMENT TEARS

Anterior cruciate ligament (ACL) tears may be accompanied by osseous and meniscal injuries. Secondary MRI signs of an ACL deficient knee include bowing of the posterior cruciate ligament (PCL), anterior translation of the tibia, an uncovered lateral meniscus, and bone bruises, t~ Occult fractures seen with ACL tears are of the bone bruise type. These are defined

Fig 12. A 39-year-old man hyperextended his knee during a basketball game. Coronal T1 sequence through the posterior aspect of the knee shows a minimally impacted lateral tibial plateau fracture (arrows). The radiographs were normal and the patient was treated conservatively, (Reprinted with permission. ~)

BONE BRUISES

403

Fig 13. An 84-year-old woman with knee pain. {A) Coronal T2*-weighted image shows a medial tibiel metaphysis insufficiency fracture (arrow). (B) Whole body bone scan shows increased uptake in the medial tibial metaphysis. The radiographs were normal. (Reprinted with permission. 22)

Fig 14. An 89-year-old woman with knee pain. (A) Coronal and (B) sagittal spin-echo proton-density data sets show a tibial metaphysis insufficiency fracture. Plain films became positive 3 weeks later. (Reprinted with permission, z2)

404

NEWBERG AND WETZNER

i

Fig 15. Sagittal FSE (A) proton-density and (B) T2-weighted images show a large chondral defect on the medial femoral condyle (arrows). The large joint effusion fills the chondral defect, thereby enhancing its visualization.

rotated internally relative to the femur. In ACL tears, there is relative external rotation of the femur over the fixed tibia, called the pivot shift; reproduction of this abnormal motion at physical examination is a standard clinical test for ACL tears. If the pivot shift phenomenon occurs with enough force, a unique pattern of occult fractures occurs in the middle portion of the lateral femoral condyle and the posterior aspect of the lateral tibial plateau because the

bones are compressed against one another ("kissing contusion") (Fig 17).11 During internal rotation in valgus stress, the posterior aspect of the lateral tibia can impact on the lateral femoral condyle, producing bone marrow or osteochondral injuries within the lateral compartment. When the knee returns to a normal position, the sites of bone marrow injury are no longer adjacent to one another. 12 Murphy 13 identified bone impaction sites in

Fig 16. Axial FSE images show (A) an articular defect in the lateral femoral sulcus (arrows). (B) The T2-weighted FSE image shows an osteochondral loose body in the medial suprapatellar bursa to better advantage because of the presence of a large effusion with bright signal intensity (arrowhead).

BONE BRUISES

405

Fig 17. Dual-echo sagittal FSE images in a patient who sustained an ACL tear. (A) Lateral femoral condyle and posterior lateral tibial plateau (arrows) show the characteristic bone bruise pattern associated with an ACL tear. (B) Torn ACL on sagittal image (arrow). (Reprinted with permission.~)

the posterolateral tibial plateau in 94% of his study group and in the lateral femoral condyle in 91%. The bone lesions show regions of low signal intensity in the subchondral medullary bone on Tl-weighted images and have increased medullary signal on T2-weighted images (Fig 18). Partial ACL tears usually show no

bone marrow signal changes. Bone signal abnormalities were seen in 100% of patients with ACL tears in Murphy's study. 13 In fact, Kaplan et a111 studied 100 consecutive patients with an MRI diagnosis of ACL tear. All patients were examined within 4 weeks of their injury. The investigators "were unable to find one case of a

Fig 18. A 19-year-old man sustained an injury while playing hor FSE sagittal data set shows bone marrow edema/ hemorrhage in the lateral femoral condyle ~andlateral posterior tibial plateau (arrows). Other images showed an ACL tear. (Reprinted with permission,zz)

406

Fig 19. A 20-year-old Division I college football linebacker injured during a game. Sagittal Tl-weighted image illustrates a prominent and deep lateral femoral suIcus (arrow). At arthroscopy, the ACL was found to be completely transected. (Reprinted with permission, zz)

posterior lateral tibial plateau fracture without an associated ACL tear. ''11 An occult fracture of the posterolateral tibia with or without an associated fracture in the

NEWBERG AND WETZNER

lateral femoral condyle ("kissing sequestrum") is a relatively frequent finding in acute ACL tears and, when present, is highly suggestive of such an associated tear. 11,14High signal intensity is not a constant finding on gradient-echo images because of the signal loss from local field inhomogeneities where marrow is in direct contact with trabecular bone. In normal knees, there is an indentation along the articular surface of the anterior aspect of the lateral femoral condyle. Normally, this lateral pateliofemoral sulcus measures approximately 0.45 mm. In patients with an ACL tear, this sulcus may measure from 1 mm to 5 mm in depth. The lateral condylopateUar sulcus is located directly over the anterior horn of the lateral meniscus. It has been shown that the mean depth of the sulcus in patients with an ACL tear is greater than in normal controls. A depth of more than 1.5 mm is a reliable, indirect sign of a torn ACL (Fig 19). 15 Deepening of the sulcus may be related to an impaction fracture. It seems that abnormalities of the cartilage, cortex, and subchondral bone of the lateral femoral condyle sulcus show a spectrum of changes in ACL injuries, ranging from occult bone bruises to localized chondral and transchondral impaction fractures. The detection of occult fractures may cause

\;!

i

ii

Fig 20. A 17-year-old woman suffered a bicycle accident. (A) Anteroposterior radiograph of the knee with slight internal rotation shows the characteristic bony avulsion seen in the Segond fracture (open arrow). (B) Coronal proton-density-weighted image shows diminished signal intensity in the lateral tibial metaphysis. The fracture fragment probably is seen but not without the help of the accompanying plain films.

BONE BRUISES

407

sionally may be detectable in the posterolateral tibial plateau suggestive of an ACL tear. POSTERIOR CRUCIATE LIGAMENT INJURIES

Fig 21. Coronal fat-suppressed image showing a lateral proximal tibia with a Segond fracture fragment (arrow) and localized bone marrow signal alterations adjacent to the fracture fragment. (Photo courtesy of Dr David Titelbaum, Brockton, MA.)

one to pay closer attention to the ligamentous structures. In these cases, if the ACL is not clearly torn, additional imaging sequences should be performed to establish with certainty the integrity of the A C E In situations in which the ACL is difficult to visualize, or when the distinction between a partial or complete tear is uncertain, the presence of bone changes typical of complete ACL tears suggests that ACL insufficiency is present) 3 A recent report 16 suggests that subtle plain film abnormalities occa-

The posterior cruciate ligament is torn less often than t h e ACL. However, this injury also may have associated bone marrow injury. Sonin et a117found that in 17 (36%) of 47 patients with PCL tear there was a bone marrow injury. The anterior tibial plateau was the most commonly affected site. The bruise occurs when the anterior tibial plateau impacts against the femur with forced posterior displacement. For example, this may be seen in an automobile accident when the flexed knee is driven against the dashboard. 17 SEGOND FRACTURE

The Segond fracture was first described in 1879 when Paul Segond showed that internal rotation of the tibia with the knee in flexion results in tension on the lateral joint capsule of the knee at its midpoint, where a fibrous band, the lateral capsular ligament, produces an avulsion fracture of the lateral tibia. 18 The Segond fracture, or lateral capsular sign, is found in 6% to 13% of patients with anterolateral rotational instability of the knee. 19 This fracture is important because a relatively small and often hard to see fragment is an indicator of significant intraarticular pathology because almost all patients

Fig 22, Coronal FSE proton-density images from two different patients with tibial plateau fractures. (A) A vertical fracture line is identified in the lateral tibial plateau (arrowhead). (B) There is a medial tibial plateau fracture extending to the medial tibial metaphysis. The radiographs did not show these fractures.

408

NEWBERG AND WETZNER

Fig 23. A 50-year-old woman sustained a skiing injury. (A) Plain anterioposterior radiograph shows a depression of the lateral tibial plateau (arrow). (B) Coronal T2 FSE-FS image shows the fracture and the extensive area of bone marrow hemorrhage (arrow), (C) Sagittal T2-weighted FSE image clearly shows the central depressed fracture fragment of the lateral tibial plateau (arrows).

have an ACL tear and up to two thirds may have an associated meniscal injury (Fig 20). Plain films are essential for diagnosing the Segond fracture, which has a remarkably uniform appearance; it is typically elliptical (10 m m x 3 ram) and displaced 3 mm from the tibial metaphysis. Unfortunately, if plain films are not available at the time of the knee MRI examination, one may not identify the fracture fragment on the coronal sequences. In one study2~ of 12 patients, the characteristic cortical fragment was seen in only 4 patients. However, in all patients, abnormal signal intensity was detected in the marrow adjacent to the tibial rim at the

tibial insertion of the capsular ligament, as were irregularity and edema of the capsule (Fig 21). The differential diagnosis of the Segond fracture includes an avulsion of the iliotibial tract from Gerdy's tubercle and a fibular head avulsion by the biceps femoris or fibular collateral ligament. TIBIAE PLATEAU FRACTURES

Tibial plateau fractures, historically referred to as bumper or fender fractures, result from violent injury to the knee. Although they may be caused when the bumper of an automobile strikes a pedestrian, today they are more often

BONE BRUISES

409

the result of an axial load injury and may be seen in severe skiing injuries or other falls in which there is a valgus force applied to the knee. The MRI examination is extremely useful and accurate in the assessment of the cortical and medullary bone injury (Fig 22). The indications for surgical open reduction include depression of the articular surface of more than 5 mm or instability of more than 10~ To plan treatment, both tomography and CT have been used. 21 However, MRI is more valuable in determining the presence and degree of plateau

depression, separation of fracture fragments, and severity of comminution (Fig 23). In addition, MRI has the added advantage of evaluating any other associated intra-articular pathology. MRI has contributed greatly to our understanding of the mechanism and sequelae of the acutely traumatized knee. The MRI examination and its technique and interpretation, as well as attention to detail, allow the interpreter to make a meaningful contribution to the care of the injured patient.

REFERENCES 1. Berger PE, Ofstein RA, Jackson DW, et al: MRI demonstration of radiographically occult fractures: What have we been missing? Radiographics 9:407-436, 1989 2. Vellet AD, Marks PH, Fowler PJ, et al: Occult posttraumatic osteochondral lesions of the knee: Prevalence, classification, and short term sequelae evaluated with MR imaging. Radiology 170:271-276, 1991 3. Stoller DW: Magnetic Resonance Imaging in Orthopaedics and Sports Medicine. Philadelphia, PA, Lippincott, 1993, pp 140-146, 339-360 4. Kapelov SR, Teresi LM, Bradley WG, et al: Bone contusion of the knee: Increased lesion detection with fast spin-echo MR imaging with spectroscopic fat saturation. Radiology 189:901-904, 1993 5. Mirowitz SA: Fast scanning and fat-suppression MR imaging of musculoskeletal disorders. AIR Am J Roentgenol 161:1147-1157, 1993 6. Lynch TCP, Crues JV III, Morgan FW, et al: Bone abnormalities of the knee: Prevalence and significance at MR imaging. Radiology 171:761-766, 1989 7. Yao L, Lee JK: Occult intraosseous fracture: Detection with MR imaging. Radiology 167:749-751, 1988 8. Mink JH, Deutsch AL: Occult cartilage and bone injuries of the knee: Detection, classification, and assessment with MR imaging. Radiology 170:823-829, 1989 9. Virolainen H, Visuri T, Kuusela T: Acute dislocation of the patella: MR findings. Radiology 189:243-246, 1993 10. Tung GA, Davis LM, Wiggins ME, et al: Tears of the anterior cruciate ligament: Primary and secondary signs at MR imaging. Radiology 188:661-667, 1993 11. Kaplan PA, Walker CW, Kilcoyne RF, et al: Occult fracture patterns of the knee associated with anterior cruciate ligament tears: Assessment with MR imaging. Radiology 183:835-838, 1992 12. Remer EM, Fitzgerald SW, Friedman H, et al: Anterior cruciate ligament injury: MR imaging diagnosis and patterns of injury. Radiographics 12:901-915, 1992

13. Murphy BJ, Smith R L Uribe JW, et al: Bone signal abnormalities in the posterolateral tibia and lateral femoral condyle in complete tears of the anterior cruciate ligament: A specific sign? Radiology 182:221-224, 1992 14. McCauley TR, Moses M, Kier R, et al: MR diagnosis of tears of anterior cruciate ligament of the knee: Importance of ancillary findings. AJR Am J Roentgenol 162:115119, 1994 15. Cobby M J, Schweitzer ME, Resnick D: The deep lateral femoral notch: An indirect sign of a torn anterior cruciate ligament. Radiology 184:855-858, 1992 16. Stallenberg B, Gevenois PA, Sintzoff SA Jr, et al: Fracture of the posterior aspect of the lateral tibial plateau: Radiographic sign of anterior cruciate ligament tear. Radiology 187:821-825, 1993 17. Sonin AH, Fitzgerald SW, Friedman H, et al: Posterior cruciate ligament injury: MR imaging diagnosis and patterns of injury. Radiology 190:455-458, 1994 18. Dietz GW, Wilcox DM, Montgomery JB: Segond tibial condyle fracture: Lateral capsular ligament avulsion. Radiology 159:467-469, 1986 19. Goldman AB, Pavlov H, Rubenstein D: The Segond fracture of the proximal tibia: A small avulsion that reflects major ligamentous damage. A JR Am J Roentgeno1151:11631167, 1988 20. Weber WN, Neumann CH, Barakos JA, et al: Lateral tibial rim (Segond) fractures: MR imaging characteristics. Radiology 180:731-734, 1991 21. Rafii M, Firooznia H, Golimbu C, et al: Computed tomography of tibial plateau fractures. AJR Am J Roentgenol 142:1181-1186, 1984 22. Newberg AH: Bone bruises: Patterns and significance, in Weissman BN (ed): Syllabus: A Categorical Course in Musculosketal Radiology. Advanced Imaging of Joints: Theory and Practice. Oak Brook, IL, RSNA Publishing, 1993, pp 219-224