"BONE BRUISES" OF THE KNEE: A REVIEW

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1 "BONE BRUISES" OF THE KNEE: A REVIEW Chad E. Mathis, M.D. Ken Noonan, M.D. Kosmas Kayes, M.D. ABSTRACT Magnetic resonance (MR) imaging is often used - to assess the location and degree of ligamentous wm. ims,17/1 or cartilage damage in the knee following acute traumatic injury. Occasionally, these studies will also document abnormal signal within the adjacent subchondral bone. These "bone bruises" are X considered incidental findings by some, while others suggest possible clinical significance. The purpose of this paper is to review the literature and consolidate current thinking on the radiographic characteristics, classification, histopathology and natural history of bone bruises. INTRODUCTION Over the past decade, magnetic resonance (MR) gg. imaging has become the modality of choice to assess acute knee injuries. When the physical exam is equivocal, MR imaging may assist the surgeon in the evaluation of meniscal and ligamentous injury, especially injury to the anterior cruciate ligament (ACL). Another Figure 1. Coronal TI-weighted image si finding on MR imaging that may indicate significant signal intensity within the marrow consi injury to the knee is the subchondral osseous contusion known as a "bone bruise." A bone bruise is best diagnosed by correlating the increased signal intensity seen on T2-weighted images with the decreased signal intensity seen on Tl-weighted images. T2-weighted images reflect shifts in free water (i.e. edema, hemorrhage, and inflammatory response) and therefore are useful for determining the acuity of injury. Tl-weighted images demonstrate fat content of the bone marrow and offer less resolution of the bone bruise, but are useful in concert with T2-weighted images in determining location and morphology of the injury (Figure 1)10. With current technology, a STIR (Short Time to Inversion Recovery) image (Figure 2)... can be obtained which combines the advantages of Ti and T2-weighted images; the MR image undergoes fat i _ nullification, and bone marrow edema is easily detected as an area of high signal intensity. hows an area of decreased ;istent with a bone bruise. Figure 2. Coronal STIR (fat suppressiorn) image of the same cut as Department of Orthopaedic Surgery Figure 1 shows an area of increased sij.gnal intensity in the lateral Indiana University School of Medicine femoral condyle. Note the improved ressolution of the bone bruise Indianapolis, Indiana compared to Figure The Iowa Orthopaedic Journal

2 'Bone Bruises" ofthe Knee: A Review CLASSIFICATION OF BONE BRUISES Mink and Deutsch6 were the first to classify bone bruises. They separated these lesions into four groups: bone bruises, stress fractures, femoral and tibial fractures, and osteochondral fractures. The first two groups were classified solely on history since their appearance on MR imaging is similar. The latter two groups were found to have the characteristic MR findings of a bone bruise and the fractures for which they were classified. Lynch et al.5 proposed a different classification system with only two types. Type 1 is a bone bruise without cortical disruption and a type 2 bone bruise has associated cortical disruption. However, the classification system that is most widely used and has the most clinical significance is a modification of the Mink and Deutsch system. In the classification of Vellet et al.16, bone bruises are classified as either reticular or geographic based on the pattern of osseous injury. The reticular pattern is described by changes in signal intensity that resemble the strands of a net and are distant from the subchondral bone. The geographic pattern has signal intensity changes that are more discrete and focal and are contiguous with the subchondal bone. It has been suggested that this modification of the Mink and Deutsch classification may have prognostic significance as well'6. LOCATION AND PATTERN OF BONE BRUISES Mink and Deutsch6 first examined the location of bone bruises and their associated injury patterns. Their series of 30 bone bruises in 27 knees included 17 in the distal femur, six in the proximal tibia, six in both the femur and tibia (three knees), and one in the proximal fibula. Rosen et al.9 later noted that 83 percent of bone bruises associated with an ACL injury were located in the lateral compartment. Of these, 50 percent involved the lateral femoral condyle and 50 percent involved the lateral tibial plateau. Additionally, 30 percent (19 of 64) of these patients had similar signals in more than one area. Speer et al.10 also found that 83 percent of patients with ACL rupture had a bone bruise in the lateral compartment and noted, more specifically, involvement of the terminal sulcus of the lateral femoral condyle. They showed that the subchondral plate was always involved and the highest signal intensity occurred in knees with injury to the terminal sulcus of the lateral femoral condyle. They felt that a common mechanism of injury was present since 95 percent of those patients with a bone bruise on MR imaging also had a bone or soft tissue injury to the posterolateral aspect of the joint. Speer's "MRI triad," which includes an ACL rupture, an osseous lesion of the terminal sulcus of the lateral femoral condyle, and a bone or soft tissue injury to the posterolateral corner, is a valuable asset to the clinician and radiologist in the diagnosis of ACL rupture'0. The location and pattern of bone bruises in each compartment of the knee were further studied by Vellet et al.16, who developed their own classification system. The lateral compartment accounted for 65 of the 72 femoral condyle lesions and 64 of the 87 bone bruises of the tibial plateau. The relationship of the MR image pattern to the location of the bone bruise was also recorded. The reticular pattern was the most prevalent and occurred with equal frequency in the medial and lateral compartments. On the other hand, 91 percent of the bone bruises with a geographic pattern occurred in the lateral compartment. Also, no geographic bone bruise occurred without a reticular bone bruise. In the lateral femoral condyle, 42 percent displayed a geographic pattern while only 28 percent of the medial femoral condyle lesions were geographic. Of the proximal tibia bone bruises, only 8 percent of lateral compartment and 4 percent of the medial compartment lesions had a geographic pattern. Finally, Spindler et al.12 also found that the majority of bone bruises that were associated with ACL tears were located in the lateral femoral condyle. He further noted no association between the location of a bone bruise, as seen on MRI, and meniscal tear. MECHANISM OF INJURY AND ASSOCIATED INJURIES In patients presenting with acute knee hemarthrosis, Vellet et al. found that in 78 per cent (65 of 83) of cases the mechanism of injury included deceleration, rotation, or valgus stress. When specifically evaluating geographic patterns of bone bruises, 62 per cent (18 of 29) of cases were caused by these mechanisms. The bone bruise occurred at the site of impaction and the difference between a geographic and a reticular pattern was the surface area over which the injury was distributed'6. Based on their findings of posterolateral injury in 95 per cent of cases, Speer et al.10 proposed three possible mechanisms of injury to the terminal sulcus of the lateral femoral condyle. The first mechanism is a pivot shift of the posterolateral tibial rim and meniscus. The second is a hyperextension injury of the anterolateral tibial rim and meniscus. Finally, the third is an injury sustained from reduction of a pivot shift event of the anterolateral tibial rim and meniscus. Evaluating the mechanism of injury of ACL rupture in skiers, Speer et al." found a difference between the mechanism of injury in skiers and other high load athletes. They found that only 40 per cent of bone bruises in skiers with ACL Volume

3 C. E. Mathis, K Noonan, K Kayes ruptures occurred in the lateral femoral condyle" compared to the 83 per cent incidence reported in their previous study10. They also found that the tibial plateau lesions were located in the posterocentral aspect of the knee as well as the posterolateral corner10"11. Based on the pattern and distribution of bone bruises, they were able to postulate that the mechanism of ACL injury in skiers involved a violent rotation of the tibia on the femur with more flexion of the knees". Disruption of the fibers on MR imaging has long been the radiographic hallmark of ACL rupture. However, the relative obliquity of the fibers of the ACL in the sagittal plane sometimes make the distinction of partial versus complete rupture difficult, and an associated finding may assist the clinician in making the diagnosis. Robertson et al.8 found that the most sensitive and specific sign of ACL rupture was discontinuity of the ACL fibers in the sagittal and axial planes. They also found that the presence of a posterolateral bone bruise to be specific, but not sensitive, for ACL disruption. Mink and Deutsch6 tabulated the bone bruises according to mechanism and the presence of an associated injury. They found seven isolated bone burises and 23 associated with a twisting injury. Of those with a twisting injury, five had a collateral ligament tear and 16 had an ACL tear. Spindler et al.12 also noted that there was a statistically significant association between ACL rupture and bone bruises. In this series, 80 per cent of bone bruises of the lateral femoral condyle and posterior lateral tibial plateau occurred in association with an ACL injury. Their proposed mechanism of injury was anterior subluxation of the tibia with impaction on the anterior aspect of the femur and the posterior aspect of the tibia'2. In similar studies of patients with bone bruises found on MR imaging, Lee and Yao4 reported nine ACL tears in 22 patients with bone bruises and Lynch et al.5 reported 41 ACL tears in 87 patients. Vellet et al.16 noted that 79 per cent of patients with a bone bruise had a complete ACL tear, compared to a control group of knee injuries without a bone bruise where only 56 per cent had a complete ACL tear. Rosen et al.9 in a study of acute ACL injuries further delineated the location of bone bruises. They noted that 85 per cent of 75 patients with an acute ACL rupture had bony changes on MR imaging. Tung et al.15 found bone bruises in 44 per cent of knees with arthroscopically diagnosed ACL ruptures and in only 9 per cent of knees with intact ligaments. They went on to state further that the location, rather than the presence of a bone bruise, was more indicative of ACL rupture. HISTOLOGY OF BONE BRUISES Attempts to correlate MR imaging and histopathology is at the heart of the dilemma of bone bruises. Rosen et al.9 were the first to attempt to find a correlation between what was seen on the MR image and histology. They performed a single bone biopsy at the time of surgery for ACL reconstruction. Grossly, there was hemorrhage at the site compared to the expected pale yellow color of normal marrow. The findings of the biopsy revealed edema and hemorrhage initially thought to be consistent with recent fracture. They stated, however, that after further review of the specimen there were no areas of trabecular compression or fracture. Coen et al.1 noted softening of the articular cartilage over a geographic bone bruise when examined arthroscopically. Histological evidence of bone bruises is lacking and remains a potential area for future research. A canine knee model was used by Thompson et al.'4 to examine the short term effects of bone bruises. In this study, patellofemoral joints of canines were subjected to various transarticular loads and were grossly and histologically examined at intervals of two, 12, and 24 weeks. Histologically, there were partial thickness clefts in the cartilage surface and fractures across the interface of the zone of calcified cartilage and bone at the early time intervals. At six months, the partial thickness clefts progressed to full thickness clefts, cartilage fibrillation, cloning, and loss of matrix proteoglycan staining. They concluded that, despite the initial normal radiographic and arthroscopic appearance of bone and cartilage, subchondral changes consistent with a bone bruise on magnetic resonance imaging may lead to eventual osteoarthritic changes'4. THE NAT1URAL HISTORY OF BONE BRUISES Assuming that the MR finding of a "bone bruise" represents an actual contusion, it is likely that they uniformly heal with time. Tung et al.15 first noticed the relationship between delayed MR imaging and the decreased incidence of bone bruises. In a study of 50 MR images in patients with known ACL rupture, 22 of 30 patients who had their study performed within nine weeks of injury had signal changes consistent with a bone bruise, while none of the 20 studies performed after nine weeks following injury had positive findings. Graf et al.3 reported similar results with a 71 per cent incidence of bone bruises on MR images taken within six weeks of ACL injury. Nawata et al.7 classified MR images taken within a month of injury as acute, those taken between one and 12 months as subacute, and those done after one year as chronic. Seventy per cent of patients in the acute group had evidence of a bone 114 The Iowa Orthopaedic Journal

4 "Bone Bruises" ofthe Knee: A Review Figure 3. Sagittal Tl-weighted images through the center of the klee. Note the weli defined PCL fibers and the absence of ACL fibers. bruise on MRI. Only 31 per cent of the subacute and 5 per cent of the chronic group had similar changes. The lesions in the subacute and chronic groups were smaller than those of the acute group, reflecting resolution of the edema and hemorrhage. The question of whether the force required to cause a subchondral bone bruise exceeds a threshold beyond which the articular cartilage can repair or recover was asked by Rosen et a19. It had been postulated that these lesions were benign and they completely resolved on follow-up MR imaging6f. However, Rosen et al. noted on follow-up MR images evidence of subchondral sclerosis, cartilage thinning, cartilage loss, osteochondral defects, and cortical impaction even when the bone bruise resolved9. Stein et al. reported that lesions on the lateral tibial plateau were more likely to remain sclerotic compared to other joint surface lesions. They also noted that there was no difference in outcome of a bone bruise in the presence or absence of a meniscal tear. They concluded that at least a portion of patients will have persistent and long term effects of articular cartilage damage secondary to sustaining a force significant enough to produce a bone bruise in combination with an ACL tear'3. Vellet et al.16 selected a cohort of 21 patients with both a geographic bone bruise of the lateral femoral condyle and an associated reticular bone bruise of the posterior lateral tibial condyle. Follow-up MR imaging at six months showed that 14 of the 21 geographic lesions had corresponding osteochondral sequelae, and complete resolution without apparent sequelae of the reticular bone bruises. Cartilage loss was seen in ten of the 21 follow-up MR images, two patients had osteosclerosis, three had apparent cartilage thinning, and three had an osteochondral defect16. Engebretsen et al.2 also reported a difference in outcome based on the pattern of bone bruise with two-thirds of the geographic lesions and none of the reticular lesions demonstrating osteochondral sequelae. No study has specifically addressed the treatment of bone bruises. However, Rosen et al.9 suggested delayed weight bearing in patients with such lesions. SELECTED CASE REPORTS Case 1. A thirty-nine year old female presented with right knee pain of one week's duration. The past medical history was unremarkable and she denied any recent trauma. Examination and plain radiographs were normal. The physician then elected to obtain an MRI which revealed several loose bodies and an area seen on both Ti-weighted (Figure 1) and STIR images (Figure 2) consistent with a large bone bruise of the lateral femoral condyle. Case 2. A twenty-five year old male presented four days after suffering a twisting injury to his right knee. On exam he had a small effusion, a positive Lachman's test and full range of motion of the knee. An MRI of the knee was obtained. The axial Ti-weighted images through the center of the knee showed an ACL tear (Figure 3). The fat suppressed images showed a subchondral tibial bone bruise in the lateral compartment (Figure 4). Case 3. A twenty-six year old healthy female presented with complaints of knee pain and locking after suffering a twisting injury to the knee. An MRI of the knee revealed a medial meniscal tear (Figure 5) and a bone bruise of the entire medial tibial plateau (Figure 6). The ACL and PCL were intact. Volume

5 C. E. Mathis, K Noonan, K Kayes Figure 4. Coronal STIR image showing a discrete area of increased signal intensity within the subchondral bone of the lateral tibial plateau defining the bone bruise (arrows). f?n: Figure 6. Coronal STIR image showing a large geographic bone bruise involving the medial tibial plateau. CONCLUSION The addition of MR imaging has increased the abil- *ity of the surgeon to diagnose ACL rupture and meniscal f ~~tears, and with this has come the ability to see damage to the surrounding structures including occult osseous lesions. Bone bruises, especially posterolateral bone bruises, are common in patients with ACL rupture. More importantly, the presence of a bone bruise may *also represent significant articular cartilage damage potentially altering the natural history of an isolated ACL tear. It is conceivable that variations in outcome between conservatively managed ACL tears may be due to articular cartilage injury seen with associated bone bruises. Perhaps the presence of an associated geo- -~~ V1~~~ graphic bone bruise indicates a worse prognosis necessitating differences in treatment and rehabilitation. It Figure 5. Sagittal T2-weighted image demonstrating a medial may be prudent to proceed slowly with weight bearing meniscal tear as an area of increased signal intensity within the in these patients, especially those with geographic bone substance of the meniscus. bruises, to possibly limit the extent of cartilage damage or loss. 116 The Iowa Orthopaedic Journal

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