MRI of Acute Bone Bruises: Timing of the Appearance of Findings in a Swine Model

Transcription

1 Musculoskeletal Imaging Original Research lankenbaker et al. MRI of cute one ruises Musculoskeletal Imaging Original Research Donna G. lankenbaker 1 rthur. De Smet 1 Ray Vanderby 2 Ronald P. McCabe 2 Stephanie. Koplin 3 lankenbaker DG, De Smet, Vanderby R, McCabe RP, Koplin S Keywords: bone bruise, knee, MRI DOI: /JR Received June 6, 2007; accepted after revision July 10, Department of Radiology, University of Wisconsin School of Medicine and Public Health, E3/311 Clinical Science Center, 600 Highland ve., Madison, WI ddress correspondence to D. G. lankenbaker (dg.blankenbaker@hosp.wisc.edu). 2 Department of Orthopedics and Rehabilitation, University of Wisconsin School of Medicine and Public Health, Madison, WI. 3 Department of Pathology, University of Wisconsin School of Medicine and Public Health, Madison, WI. WE This is a Web exclusive article. JR 2008; 190:W1 W X/08/1901 W1 merican Roentgen Ray Society MRI of cute one ruises: Timing of the ppearance of Findings in a Swine Model OJECTIVE. We developed an animal model of bone injury to determine the timing of the MR appearance of bone bruises and to follow these changes over time. materials and METHODS. We performed serial MRI of 16 knees of eight swine with one nontraumatized knee as a control and 15 knees traumatized by direct patellar impact injuries using a force-calibrated device. ll knees were imaged on a 1.5-T scanner using an 8-channel phased-array coil with T1-weighted, fat-saturated T2-weighted and STIR sequences. Scanning was performed at 1, 6, 12, and 30 hours after impact injury. Two radiologists independently reviewed each MR examination to identify MR signal intensity changes in the patellae and adjacent femoral condyles. RESULTS. In the 15 traumatized knees, bone bruises were noted in 93% of the patellae on T1-weighted images and in 87% of the patellae on fluid-sensitive MR images at 1 hour after injury and in 100% of the patellae at 6 hours. T1-weighted images were insensitive for detection of bone bruises in the femoral condyles. one bruises in the femoral condyles were seen on fluid-sensitive MR sequences as early as 1 hour after injury, with an increasing frequency over the 30-hour period. CONCLUSION. one bruises can be seen as soon as 1 hour after trauma but may not be seen until 30 hours after trauma. Fluid-sensitive (fat-saturated, T2-weighted, and STIR) MR sequences are more sensitive than T1-weighted images in the detection of bone bruises. P osttraumatic bone injuries, often called bone bruises or contusions, have been noted on MRI since 1988 [1]. bone bruise is seen on MRI as an area of low signal intensity on T1-weighted images and high signal intensity on fluid-sensitive sequences in the marrow [1 3]. These MRI findings are thought to represent marrow edema and, which may result from injury to the marrow itself from transmitted shock waves or from trabecular bone fractures [4, 5]. The limited histologic evidence that is available from biopsies of patients confirms the presence of marrow edema,, and trabecular fractures in these bone bruises [6]. lthough posttraumatic bone bruises are common, the timing of the appearance of these bruises in the acute phase has had only limited study. Recently, we performed MRI on an athlete 2 hours after traumatic hip subluxation, and the findings revealed a posterior labral tear with hip effusion but no bone abnormality (Fig. 1). subsequent MRI performed 3 months later to confirm healing of the labral tear revealed an anterosuperior bone marrow abnormality (Fig. 1). We were uncertain as to whether the abnormality at 3 months after the injury was a persistent delayed-onset bone bruise or an area of osteonecrosis. Clinical and MR follow-up documented clearing of the MR abnormality within 6 months and completely normal hip function and radiographic appearance at 2 years after the injury. ecause of the clinical importance of this differential diagnosis, we developed an animal model for a bone bruise and used this model to determine the timing of MR changes in the bone marrow after bone injury. Materials and Methods Experimental Model Our institutional animal research committee gave protocol approval for this study. ll policies on humane care and use of laboratory animals were followed. Ten crossbred female swine from a commercial vendor (University of Wisconsin rlington Farms) (mean weight, 51 kg; range, kg; JR:190, January 2008 W1

2 lankenbaker et al. mean age, 11 weeks; age range, weeks) were used for this study. dolescent swine were chosen on the basis of size so they could be moved multiple times while sedated. We decided to use the swine knee to develop our model for bone bruises because of the size of this joint in a swine and its morphologic similarity to the human knee. We specifically selected the patellofemoral joint for study because striking the anterior surface of the patella allowed more precise measurement of the impacting force than could be achieved in a model that attempted to reproduce the clinically common femorotibial bone bruises such as seen in anterior cruciate ligament injuries. Selection of the patella as the point of impact was also advantageous because it provided a site with a direct contusion, the patella, and a site with transmitted osteochondral forces, the femoral trochlea. efore the start of our study, two swine knees were scanned without any knee trauma to learn the normal MR appearance of the nontraumatized swine Fig year-old male athlete with acute traumatic hip injury., Coronal T2-weighted fat-saturated image obtained 2 hours after acute injury does not show any bone marrow edema. Posterior labral tear was noted on axial T2- weighted imaging (not shown)., Coronal T2-weighted fat-saturated image obtained 3 months after injury shows focal area of abnormally increased T2 (arrow). Sagittal T1-weighted images (not shown) confirmed anterior location of this signal abnormality. attery ir pressure source all valve Safety switch Trigger switch High flow valve Digital pressure gauge Pressure regulator Pressure reservoir Ch1 Ch2 Tektronix TDS1012 Digital Oscilloscope with 256-M memory djustable stop Impactor cceleration chamber knee. In addition, the impacting device was tested on two swine knees for standardization and calibration. Method for Impacting Patella efore beginning the MR portion of the study, we designed a pneumatically accelerated mass system to impact the swine patella. This concept was our adaptation of the system used by tkinson and Haut [7]. The pneumatic system allowed more flexibility than a drop-weight system to control the final energy and velocity of the accelerated mass. y adjusting the acceleration chamber length (via adjustable stops) and the mass of the impactor, a very wide range of impact energies and final velocities was available. We used a high-velocity impact to minimize the possibility of strain-induced fracture of the patella. The basic system (Fig. 2) consisted of a cylindric impactor mass (252 g) with polytetrafluoroethylene bearings (Fig. 3), an acceleration chamber fitted with optical sensors to measure final velocity, an electrically operated high-flow-rate Signal conditioning electronics Pressure relief holes Patella interface Custom load cell Optical velocity sensor rass impactor mass valve, and a pressure chamber with a digital pressure gauge to monitor chamber pressure. Pressure was applied to the pressure chamber via a pressure regulator and a ball valve and was read via the digital gauge to ± 0.1 psi (0.69 kpa). Pressure was released by a small switch and a battery. Once released, the mass accelerated down the acceleration chamber, where it interrupted two light beams separated by a precise distance, thereby allowing an accurate measure of the final velocity. The impactor then hit a custom-fabricated load cell positioned between the knee and the impactor. Using custom signal conditioning, load versus time data were relayed to a digital oscilloscope (TDS1012, Tektronix) and saved for later analysis. nalysis of the data was done with Microsoft Excel, and the resulting load data were plotted versus time. The pneumatically accelerated mass system was then tested on one swine before beginning the study (two knees) to determine the force that could be applied without visible fracturing (shattering) of Polytetrafluoroethylene slide bearings Polyoxymethylene nose piece Impactor load cell interface Load cell cable support stop Load cell patella interface 17-kN custom load cell Fig. 2 Schematic of pneumatically accelerated mass system designed to impact swine patellae. Fig. 3 Schematic of acceleration chamber fitted with optical sensors to measure final velocity. W2 JR:190, January 2008

3 MRI of cute one ruises the patella. To place the device accurately, a skin incision was made directly over the patella, which allowed palpation and marking of the patella. The size of the impaction device was approximately one half the size of the swine patella. efore animal testing, a certified animal care technician sedated the swine with tiletamine hydrochloride and zolazepam hydrochloride 7 mg/ kg (Telazol IM, Fort Dodge nimal Health), xylazine 2.2 mg/kg IM (Xyla-Ject, Phoenix Pharmaceutical), atropine sulfate 0.05 mg/kg IM. IV access was established in each swine s ear vein. Propofol 2 8 mg/kg was used if additional sedation was needed for intubation. Maintenance of anesthesia throughout the procedure was by inhaled 1 3% isoflurane via a endotracheal tube. dose of buprenorphine mg/kg IM was given before the start of bone injury followed by a second dose given at 1 6 hours after trauma as needed to maintain analgesia at the site of surgery until the animal was sacrificed. fter the initial development of the device for impacting the patella, we then studied both knees of eight swine, of which 15 were traumatized and one used as our control. ll 15 knees were incised for accurate placement of the impaction device. oth the patella and distal femur were evaluated for bone injury due to both direct and indirect injury causing the bone bruise. MRI Studies fter the patellar impaction injury, the animals were immediately taken to the MR scanner for imaging. ll 16 animal knees were imaged on a 1.5-T magnet using an 8-channel phased-array coil. ll animals were positioned supine with the coil centered over the knee joint. This was the best position for imaging because the coil was centered over the knees and because of the anesthesia. The sedation and anesthesia protocols used for imaging were identical to those used during the creation of the knee injury. The following sequences were performed: axial and sagittal T1 (TR/TE, 500/20), fat-saturated fast spin-echo T2 (2,200/76; echotrain length, 3), and fast spin-echo STIR (3,000/48; TLE 1: MR bnormalities in Swine one ruise Model Patella Location 1 Hour inversion time, 160). Coronal fast spin-echo STIR (3,000/50; inversion time, 160) sequences were also performed along the long axis of the femur. 16-cm field of view; matrix, ; number of excitations, 2; and 3-mm thickness with 0.5-mm gap were used for all sequences. Scanning was performed immediately (completed within 1 hour after injury; average time, minutes), 6, 12, and 30 hours after trauma. ecause of cost and research scanner access, we were not able to obtain scans at hourly intervals. Our initial animal was scanned out to 12 hours with subsequent animals scanned out to 30 hours. Two animals died after the 12-hour scan; another died after the 6-hour scan; only the right knee in one animal was imaged at the 6-hour time frame, with the left knee not imaged after the initial 1-hour scan. Two musculoskeletal radiologists independently reviewed each MR examination documenting patellar and femoral signal intensity changes and then resolved differences in interpretation by consensus reading. Each patella and the trochlear portions of the distal femoral condyles were subjectively evaluated for changes in the MR appearance of the marrow from the normal appearance of adolescent swine knees on the T1-weighted, fatsaturated T2-weighted, and STIR images. The T1- weighted and fluid-sensitive sequences were read independently of each other. The readers were not blinded to the timing of the images. Histologic Studies fter the final MRI, the knee joints were removed after the animals were sacrificed. fter fixation with 10% formalin, the femurs were cut longitudinally with a band saw slightly anterior to the midline. The femurs and patellae were subsequently decalcified, and two coronal sections from the distal femur and two transverse sections from the midportion of the patellae were submitted for paraffin embedding. Histologic sections were cut at 4 mm in thickness, stained with H and E, and evaluated for the presence of trabecular microfractures,, and edema. The location of the sections used for histologic examination Percent bnormal (No. Positive/ll Cases) 6 Hours 12 Hours 30 Hours T1-weighted images 93 (13/14) 100 (12/12) 100 (12/12) 86 (6/7) Fluid-weighted images 87 (13/15) 100 (13/13) 100 (13/13) 88 (7/8) Femoral condyles T1-weighted images 7 (1/14) 0 (0/12) 8 (1/12) 29 (2/7) Fluid-weighted images 33 (5/15) 77 (10/13) 85 (11/13) 100 (6/6) was chosen by estimating the site of the patellar impact on the distal femur. The edema in the histologic specimens was measured by estimating the fraction of edema in the marrow space. This was done with low-power examination of the slide by estimating the acellular area on the slide because edema histologically appears as a separation of cells. The edema on each slide was estimated twice by the same reviewer at different times to ensure internal consistency. Histologically, edema is represented by a separation of, or space between, the hematopoietic cells of the bone marrow space. We attempted to exclude artifact separation of cells by identifying edema that was accompanied by fibrin, proteinaceous debris, and extravasated RCs. Results The one control knee had normal signal within the patellae and femurs on the initial and serial MR scans. The normal adolescent swine knee has prominent patellar and distal femoral ossification centers that have high signal intensity on T1-weighted images and low signal intensity on fat-saturated T2- weighted or STIR images (Fig. 4). Impact Load plot of eight impacts on the traumatized knees (Fig. 5) shows the reproducibility of the impact loading. Peak loads observed were around 15,000 N and occurred at around 115 milliseconds after the first measured contact. verage peak compressive load was calculated to be 14,900 N, average impact energy was 35.6 J, and final velocity was 16.8 m/s. Loads are quite high but very short in duration, keeping total impact energy below the mean value reported by tkinson and Haut [7] for bone fracture producing impacts to human knees (60 ± 25 J). MRI Findings In all MR scans in which both a fat-saturated T2-weighted and STIR series were obtained, when a high-signal-intensity abnormality was identified, it was always seen on both sequences except in the femur of one knee on the 1-hour scan in which the STIR images were positive but the T2-weighted images were negative. Overall, the findings were slightly more conspicuous (higher signal and more obvious) on the STIR images. In view of their similar edema sensitivity, the following data consider the findings on either the fat-saturated T2-weighted or STIR sequence as representing an abnormality on a fluid-weighted MR sequence. JR:190, January 2008 W3

4 lankenbaker et al. Patella oth the T1-weighted and fluidweighted sequences were sensitive in identifying findings of a patellar bone bruise soon after the injury (Fig. 6). Low-signal abnormality on T1-weighted images was seen in 93% (13/14) of the patellae and high-signal abnormality in 87% (13/15) of the patellae on fluid-weighted MR sequences obtained within 1 hour of the injury (Fig. 7). ll patellae had both T1-weighted and fluid-weighted MR abnormalities at 6 hours after the injury. These changes persisted at 12 and 30 hours after the injury except for one patella at 30 hours in which the signal abnormality was no longer seen on either T1-weighted or fluid-sensitive sequences. n increase in low signal intensity over time was seen in six of the patellae and an increase in high signal on the fluid-sensitive sequences in 10 patellae. n overview of the results is provided in Table 1. TLE 2: Histologic nalysis of 15 Traumatized Knees in Eight Swine Swine Right Patella Right Femur Left Patella Left Femur 1 30% edema 5 10% edema 50% edema, 20% 2 90% edema < 5% edema or 3 10% edema 10% focal edema, < 5% None a 1% edema and None a 95% edema 5 10% edema 4 100% edema 50% focal edema > 95% edema 50 60% focal edema 5 > 90% edema < 5% edema 80% edema and 5% edema % edema 1 5% edema 80 90% edema 5 10% edema 7 100% edema 5 10% edema 100% edema 20% edema 8 100% edema < 5% edema 100% edema 5% edema a The left patella and femur in swine 2 were the control nontraumatized knee. Fig. 4 Control nontraumatized swine knee. C, Patellar and femoral ossification centers show high signal intensity on sagittal T1-weighted image () and low signal intensity on T2-weighted fat-saturated () and STIR (C) images. Femoral condyles Unlike the situation in the patellae, a low-signal abnormality in the femoral condyles was noted on T1-weighted images in only one knee at the 1-, 6-, and 12-hour intervals after the injury and in 29% (2/7) of the knees imaged at 30 hours after the injury. In contrast, the fluid-weighted MR sequences had a high-signal abnormality in 33% (5/15) on the examination performed within 1 hour of the injury (Fig. 8). The frequency of MR abnormality then steadily increased until the 30-hour examination when 100% (6/6) of the femurs showed a high-signal abnormality. n increase in high signal intensity over time was seen in nine femurs on the fluid-sensitive sequences. No change in T1 low signal intensity was detected within the femurs. n overview of the results is provided in Table 1. Histologic Findings Table 2 summarizes our histologic analysis of 16 knees. The histology was normal only in the control left knee of swine 2. ll 15 traumatized patellae and femurs had varying degrees of edema, with the greatest amounts of edema within the patellae. Hemorrhage was seen in two patellae and three femurs (Fig. 9). There were no trabecular fractures. C W4 JR:190, January 2008

5 MRI of cute one ruises Fig. 5 Graph of impacts of traumatized swine knees. Time (ms) ,000 Load (N) 4,000 6,000 8,000 10,000 12,000 14,000 16,000 Discussion MRI is sensitive in the detection of occult stress and posttraumatic fractures in and around the knee joint [4, 5]. one bruises were first described in association with major knee injuries [8, 9]. The bone bruise may be the only obvious sign of injury and prompts a closer inspection for more subtle lesions, such as ligamentous and meniscal tears [5]. Detection of bone bruises on MRI depends on imaging the marrow and its evolution in response to an acute injury. The MRI appearance of a bone bruise likely represents the underlying pathology thought to be trabecular disruption,, and edema [4, 10]. Microfractures of the subchondral trabeculae may lead to local cancellous increased fluid content from [10]. Previous studies have described the natural history of bone bruises [11, 12]; however they did not describe when the bone bruise first occurs at MRI. In the study by Miller et al. [11], follow-up MRI after initial MR was completed at various time intervals ranging from 1.5 to 12 months in 24 (83%) of 29 patients with isolated medial collateral ligament injuries. Complete resolution of bone bruises was observed in all cases. In the study by Escalas and Curell [12] using the rabbit model, an experimental occult bone injury was produced in the rabbit knee and MRI was performed after killing the animals at 1, 3, and 9 weeks after impact. t 1 and 3 weeks similar changes were noted on MRI, and at 9 weeks MRI showed less intense image patterns and one knee did not show any abnormal imaging findings. The clinical significance of our study goes back to the question in the evaluation of our patient who had a traumatic injury to the hip. We had originally assumed that a bone bruise should be seen on an MR image obtained 2 hours after the injury. Thus we thought that the signal abnormality noted on the MR image had to represent osteonecrosis. However, the location of the MR abnormality on the anterosuperior aspect of the femoral head was not a typical appearance or location for osteonecrosis. In fact, this location suggested a bone injury suffered during reduction of a presumed posterior hip subluxation similar to the humeral head Hill-Sachs injury in an anterior shoulder dislocation. On the basis of our animal study showing that a bone bruise may not be visible on MRI for more than 12 hours after injury, we now think that our patient had a slowly resolving bone bruise rather than osteonecrosis. Our results indicate that after a high-impact direct bone injury such as the patellae in our Fig. 6 Sagittal T1-weighted MR image of traumatized swine patella at 1 hour after injury shows low signal intensity (arrow) within patella. model, a bone bruise is present approximately 90% of the time on either T1-weighted or fluid-sensitive MR sequences done within 1 hour after the injury and at 100% at 6 hours after injury. However, with lesser-impact-force injuries across an osteochondral surface, such Fig. 7 Traumatized swine patella. and, Sagittal STIR MR images at initial 1-hour scan () show edemalike signal intensity (arrow) within patella and at 30-hour scan () show increased signal intensity (arrow) within patella. JR:190, January 2008 W5

6 lankenbaker et al. Fig. 8 Traumatized swine patella. and, Sagittal STIR MR image at 1 hour after injury shows subtle subchondral edema (arrow). This area of abnormality increases in signal intensity by 12-hour sagittal image () (arrow). In, there is also extensive highsignal abnormality in patella. as the femoral condyle in our model, a bone bruise may take more than 12 hours to appear on a fluid sequence. In our study, all six swine femoral condyles that were imaged at 30 hours showed a bone bruise pattern on a fluid-weighted MR sequence. We also found that both T1-weighted imaging and fluid-sensitive imaging (fat-saturated T2-weighted or STIR) were nearly equally sensitive in the detection of high-impact-injury bone bruises in the patellae. However, fluid-sensitive MR sequences were more sensitive than T1-weighted images in the detection of bone bruises within the femoral condyles. We hypothesize that the MRI findings were more apparent in the patella than in the femoral condyles because the patellar injuries were more severe due to the direct bone impact on to the patella. In contrast, the injuries of the condyles were likely of a lower impact force because the chondral surfaces of the patella and femur diffused the forces impacting the femoral condyles. The greater sensitivity of fluid-weighted sequences for marrow abnormality when compared with T1-weighted images is well documented [1, 3 5, 13, 14]. To our knowledge, there has been one previous study on an animal model of bone bruises [15] in swine. In this study, a small metal hammer was used to create a bone bruise of the medial aspect of the proximal tibia. The authors did not quantify the force on the tibia. Each tibia was struck with a hammer until the cortex of the tibia was mildly depressed. The animals were imaged immediately after surgery and biweekly until they were sacrificed. They did not determine when bone bruises first appeared. We found that fat-saturated T2-weighted and STIR imaging were comparable in detection of bone marrow edema. In only one femoral condyle did we find edema on the STIR images that was not detected on the fatsaturated T2-weighted images. This finding agrees with a previous study evaluating bone contusions of the knee comparing fat-saturated T2-weighted and STIR images in which a comparable sensitivity and specificity were found for the two pulse sequences [1]. In that study, STIR imaging had slightly increased lesion conspicuity, whereas T2-weighted fatsaturated images showed slightly improved overall subjective image quality. We agree with their assessment that when rapid detection of bone contusions is desired or when a lesion is suspected, STIR imaging is preferable over fat-saturated T2-weighted imaging. However, we did not find that fat-saturated T2-weighted imaging had improved subjective image quality. This difference between the findings of the previous study and ours likely reflects improvement in MR image quality in the 10 years since their study. Histologic analysis of the knees showed marrow edema within all of the traumatized patellae and adjacent femurs but no marrow abnormalities in the nontraumatized knee. We were surprised to find that was noted histologically in only two of the traumatized patellae and three of the traumatized femurs. In addition, no trabecular microfractures were found. Our histologic findings support the findings reported in the literature of the histology of a bone bruise. t histopathologic examination in MR-documented bone bruises, microfractures of cancellous bone, edema, and were also identified [6, 15]. Fig. 9 Photomicrograph of left patella of pig 7 shows replacement of approximately 80% of marrow space by edema, extravasated RCs, and fibrin. (H and E, 200) W6 JR:190, January 2008

7 MRI of cute one ruises Our study has a number of limitations. First, we were not able to image all of the animals at all intervals because of the death of several animals during the study. We kept the animals under analgesic sedation after the impact injury per the requirement of our animal care committee. This continuous sedation likely contributed to the deaths, which occurred before the scheduled delayed scanning at 30 hours. Second, because of the practical aspects of research scanner access and the need for anesthesia during MRI, we were not able to obtain scans at hourly intervals. Thus when an MR abnormality was not noted at 1 hour but was seen at 6 hours, we cannot be certain when the MR abnormality appeared during the 1- to 6-hour interval. Third, we did not evaluate whether there might be additional MR abnormalities that occurred later than the 30-hour MR image. Fourth, we did not obtain a pretrauma MR image in every knee; however, it is unlikely that the juvenile swine would have preexisting bone contusions. In conclusion, our study showed that in an animal model, MRI can depict early changes in bone marrow resulting from direct and indirect injury to bone. oth direct and indirect bone bruises are usually present on MRI performed at 12 hours after trauma but may be delayed up to 30 hours. Clinically, if an MR examination is performed too soon after trauma and is negative for injury, with the patient having continued pain or limitations, a follow-up MRI may be useful 5 7 days after trauma. cknowledgments The authors gratefully thank Kimberly Mauer, Experimental Surgery, University of Wisconsin School of Medicine and Public Health for the expert assistance in sedation and care of our swine during this project. The authors also thank Kelli Hellenbrand, MR research technologist, Department of Radiology, University of Wisconsin School of Medicine and Public Health for assistance in scanning our subjects. We also thank en Graf, Professor of Orthopedics and Rehabilitation, University of Wisconsin School of Medicine and Public Health, for his advice and input before beginning this project. References 1. Yao L, Lee JK. Occult intraosseous fracture: detection with MR imaging. Radiology 1988; 167: rndt WF 3rd, Truax L, arnett FM, Simmons GE, rown DC. MR diagnosis of bone contusions of the knee: comparison of coronal T2-weighted fast spin-echo with fat saturation and fast spin-echo STIR images with conventional STIR images. JR 1996; 166: Rosen M, Jackson DW, erger PE. Occult osseous lesions documented by magnetic resonance imaging associated with anterior cruciate ligament ruptures. rthroscopy 1991; 7: Mink JH, Deutsch L. Occult cartilage and bone injuries of the knee: detection, classification, and assessment with MR imaging. Radiology 1989; 170: Newberg H, Wetzner SM. one bruises: their patterns and significance. Semin Ultrasound CT MR 1994; 15: Rangger C, Kathrein, Freund MC, Klestil T, Kreczy. one bruise of the knee: histology and cryosections in 5 cases. cta Orthop Scand 1998; 69: tkinson PJ, Haut RC. Subfracture insult to the human cadaver patellofemoral joint produces occult injury. J Orthop Res 1995; 13: Kaplan P, Gehl RH, Dussault RG, nderson MW, Diduch DR. one contusions of the posterior lip of the medial tibial plateau (contrecoup injury) and associated internal derangements of the knee at MR imaging. Radiology 1999; 211: Schweitzer ME, Tran D, Deely DM, Hume EL. Medial collateral ligament injuries: evaluation of multiple signs, prevalence and location of associated bone bruises, and assessment with MR imaging. Radiology 1995; 194: Lynch TCP, Cruses JV III, Morgan FW, et al. one abnormalities of the knee: prevalence and significance at MR imaging. Radiology 1989; 171: Miller M, Osborne JR, Gordon WT, Hinkin DT, rinker MR. The natural history of bone bruises: a prospective study of magnetic resonance imaging-detected trabecular microfractures in patients with isolated medial collateral ligament injuries. m J Sports Med 1998; 26: Escalas F, Curell R. Occult posttraumatic bone injury. Knee Surg Sports Traumatol rthrosc 1994; 2: Kapelov SR, Teresi LM, radley WG, et al. one contusions of the knee: increased lesion detection with fast spin-echo MR imaging with spectroscopic fat saturation. Radiology 1993; 189: Stafford S, Rosenthal DI, Gebhardt MC, rady TJ, Scott J. MRI in stress fracture. JR 1986; 147: Ryu KN, Jin W, Ko YT, et al. one bruises: MR characteristics and histological correlation in the young pig. Clin Imaging 2000; 24: JR:190, January 2008 W7