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Musculoskeletal Imaging |
1 From the Departments of Radiology (P.A.K., R.H.G., R.G.D., M.W.A.) and Orthopedics (P.A.K., R.G.D., D.R.D., M.W.A.), University of Virginia Health System, Lee St, Charlottesville, VA 22908. Received September 9, 1998; revision requested October 23; revision received January 18, 1999; accepted January 21. Address reprint requests to P.A.K.
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Abstract |
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MATERIALS AND METHODS: A retrospective review of 215 consecutive MR examinations for knee trauma was conducted to identify contusions of the posterior lip of the medial tibial plateau. Any additional contusions and internal derangements were documented in the cases with these contusions. Medical charts and arthroscopic results, when available, were reviewed for mechanisms of injury.
RESULTS: The specific medial tibial contusion was demonstrated in 25 of 215 (12%) knee MR examinations. Associated anterior cruciate ligament (ACL) tears were found in 25 of the 25 (100%) examinations. Injury to the meniscocapsular junction (14 of 25) or a peripheral tear of the posterior horn of the medial meniscus (10 of 25) occurred in a combined 96% of the cases. Lateral compartment contusions were noted in 24 (96%) cases. Pivot, twisting, or valgus forces were reported mechanisms of injury.
CONCLUSION: Contusions involving the posterior lip of the medial tibial plateau may result from a contrecoup impaction injury directly following an ACL tear, as the knee reduces. These contusions are almost always associated with a far peripheral meniscal tear or with a meniscocapsular junction injury affecting the posterior horn of the medial meniscus.
Index terms: Knee, injuries, 452.40, 454.40 • Knee, ligaments, menisci, and cartilage, 452.4852, 452.4853, 452.4857 • Knee, MR, 452.12141, 452.121412, 452.121413
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Introduction |
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Most bone contusions occur in the lateral joint compartment and are seen in association with anterior cruciate ligament (ACL) tears, lateral patellar dislocations, and medial collateral ligament tears (1–13). Contusions in the medial compartment of the knee are far less common, and their importance has not been well established.
The purpose of this investigation was to determine if there are any predictable magnetic resonance (MR) imaging patterns of internal derangement associated with a bone contusion of the posterior lip of the medial tibial plateau and to offer a biomechanical explanation for the findings.
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MATERIALS AND METHODS |
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A coronal turbo short inversion time inversion-recovery (STIR) (5,500/29/150 [repetition time msec/echo time msec/inversion time msec]) sequence was used in place of the WE DESS sequence in the last 26 cases evaluated. The strength of the STIR sequence is its contrast resolution and, therefore, its ability to demonstrate fluid.
All knees were imaged with an extremity coil by using a 4-mm section thickness with a 10% gap, a 14-cm field of view, and one signal acquired. The matrix was 384 x 512.
MR studies in patients with a contusion of the posterior lip of the medial tibial plateau were then evaluated by consensus by three musculoskeletal radiologists (P.A.K., R.G.D., M.W.A.) for any additional bone contusions, meniscal tears, abnormal signal intensity at the meniscocapsular junction, cruciate and collateral ligament injuries, or other anatomic disruptions of the knee, using accepted criteria (14). The time elapsed from the time of injury to the time of MR examination was documented in patients with the medial tibial contusion.
Clinical data from patient charts were reviewed by an orthopedic surgeon (D.R.D.) in those who had the medial tibial contusion to document the forces and the position of the knee at the time of injury to help determine the mechanism of injury. These data included information such as the athletic activity, the position of the knee at the time of injury, and the direction of the blow to the knee. Arthroscopic results available in patients with contusion of the posterior lip of the medial tibial plateau were reviewed (D.R.D., P.A.K.).
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RESULTS |
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MR Findings in Patients with a Medial Tibial Plateau Contusion
Bone contusions.—Each of the 25 contusions of the medial tibial plateau was located on the posterior margin or lip of the tibia in the subchondral portion of bone (Fig 1). Bone contusions were defined as geographic regions of abnormal signal intensity—low signal intensity on T1-weighted images and high signal intensity on STIR, WE DESS, or gradient-echo images—abutting the subchondral cortical bone. The margins of the contusion protruded into the marrow with a convex configuration. In the axial plane, if the tibial plateau is viewed as the face of a clock, this contusion was predictably located in approximately the 5- or 7-o'clock position for the right and left knees, respectively (Fig 2a). No osteochondral or impaction fracture was seen in relation to these medial contusions.
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Ligaments and tendons.—All patients with a medial tibial plateau bone contusion had tears of the ACL. There was high signal intensity along the path of the disrupted ACL from hemorrhage and/or edema, which indicated these were acute injuries. Two knees showed partial tears of the medial collateral ligament. Two other knees had tears of the posterior portion of the medial patellar retinaculum immediately anterior to the medial collateral ligament. High signal intensity within the semimembranous tendon compatible with a partial tear was evident on MR images for four of 25 knees.
Menisci.—Medial meniscal tears were identified in 16 of 25 (64%) patients with a medial tibial contusion involving the posterior horn; 10 (62%) of these patients had medial meniscal tears that involved the far peripheral 20% of the posterior horn near the attachment of the meniscus to the joint capsule (Fig 4). Two of the 16 medial meniscal tears were bucket-handle tears, with meniscal tissue displaced into the intercondylar notch.
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Chart Review
All but two patients were injured during athletic activities that included basketball (n = 11), soccer (n = 4), football (n = 3), skiing (n = 2), lacrosse (n = 2), and softball (n = 1). The nonathletic injuries involved considerable trauma to the lateral side of the knee caused by a car's hitting a pedestrian in one case and by someone sliding down a bannister into the victim's knee in the other instance. The mechanisms of injury in all cases were pivotal, twisting, or valgus forces that predisposed the knee to ACL tears. Marked flexion of the knee was not present in any of the patients at the time of injury.
Arthroscopic Findings
Twelve of 25 patients with a contusion of the posterior lip of the medial tibial plateau had undergone arthroscopy. Three arthroscopic procedures were performed within 3 weeks of the injury; nine procedures were performed between 8 and 12 weeks after the injury.
Complete ACL tears were found in all of these patients. MR images were interpreted as showing far peripheral tears of the posterior horn of the medial meniscus near the meniscocapsular junction in five patients who underwent arthroscopy, and the finding was corroborated in four (80%) patients. Abnormal signal intensity at the meniscocapsular junction, believed to indicate either contusion or meniscocapsular separation, was seen in seven of 12 (58%) patients who underwent arthroscopy, and the injury was confirmed in three of the seven (43%) individuals.
Other data regarding arthroscopic findings will not be delineated since the correlation between MR imaging and arthroscopy has been well proved in the literature, and other abnormalities found in this study are not specifically relevant to the medial tibial contusion.
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DISCUSSION |
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Bone contusions in the knee have often been used as secondary signs for detecting other associated abnormalities and as a means of increasing confidence in the diagnosis (1–11). The classic example is the lateral compartment contusion of the posterior aspect of the lateral tibial plateau and of the midlateral femoral condyle just above the anterior horn of the lateral meniscus, which is associated with ACL tears (Fig 2). However, relying on secondary signs such as lateral compartment bone contusions to diagnose an ACL tear is generally unnecessary and could lead to an inaccurate diagnosis (15). The reasons for the last statement are that the ACL tear is easily, accurately, and directly identified at MR imaging, and ligamentous laxity allows for the contusions to occur without disruption of the ACL in 28% of patients 20 years old or younger (15).
The real value in recognizing lateral compartment contusions is they may be a source of pain; they may lead to osteochondral defects and early degenerative joint disease, if severe; and they are a reflection of the biomechanics of the injury. These same points apply to bone contusions elsewhere in the knee or in other joints in the body. Recognizing the mechanism of injury and analyzing the biomechanics can help lead to a more systematic and focused evaluation of an injured joint. Bone contusions reveal part of what took place at the time of the injury and may lead to more careful scrutiny of other structures that are possibly injured. Bone contusions may be more easily recognized than are the abnormalities with which they are associated. This is the case with medial compartment bone contusions in the knee, which are far more conspicuous than most of the associated meniscal or meniscocapsular abnormalities.
We propose that the bone contusion of the posterior lip of the medial tibial plateau and the occasionally evident associated contusion of the medial femoral condyle most likely occur as a contrecoup injury as the tibia reduces following an ACL rupture (Fig 6). While there are valgus stresses at the knee, the tibia typically displaces anterior to the femur, and the femur externally rotates relative to the tibia, which places stresses on the lateral compartment and on the ACL that result in its rupture. As the ACL ruptures, the posterior aspect of the lateral tibial plateau impacts the midportion of the articular surface of the lateral femoral condyle (this is the initial, or coup, portion of the injury), creating the familiar lateral compartment contusions.
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Dissipation of forces occurs after the initial impact of the bones in the lateral compartment, which explains why the medial contusions are considerably smaller and less commonly encountered than are those on the lateral side. Only injuries with large forces could result in medial bone contusions, which was the case with the young patients and with the sports-related trauma that led to the injuries in all but two of the cases in this investigation. The contusion of the medial femoral condyle is more posterior than is the lateral femoral condyle contusion, and this can also be explained by dissipation of forces since there would be less internal than initial external rotation of the femur, and because the amount of anterior tibial subluxation would be decreased.
The knee with medial, but not lateral, compartment contusions can probably be explained by a slightly different mechanism of injury, with major varus rather than valgus forces applied at the time of injury. This was one of only two knees with a bucket-handle tear of the medial meniscus, which indicates sizable compressive forces directed medially rather than laterally.
The subchondral contusion involving the peripheral margin of the medial femoral condyle is almost certainly more common than these study findings suggest. All but one of the medial femoral condyle contusions were seen in the few cases in which we used the STIR sequence. Although the WE DESS sequence used for the coronal images in most patients was designed to show fluid, it was clear that the STIR sequence was far more sensitive to the presence of fluid in marrow. Our protocol for the sagittal imaging plane consists of T1-weighted and gradient-echo sequences, neither of which would be expected to produce images that depict marrow edema with as great a sensitivity as that of STIR, WE DESS, or turbo T2-weighted fat-saturated sequences (16).
Still, it is reasonable to assume that the medial femoral condyle contusions will be present less often and will be less severe than will be the medial tibial contusions. This is the case in the lateral compartment, in which there is almost always a tibial contusion present, but not always a femoral contusion, in association with an ACL tear; presumably this occurs because the tibia is weaker and more easily fractured than is the femur (5).
There is a reference (17) to medial bone contusions in the orthopedic literature in which MR imaging examinations of the knees in recreational skiers with ACL tears were evaluated to try to determine if the mechanism of injury for ACL tears in skiers was different than the mechanism of injury in others with ACL tears. Speer et al (17) found a lower incidence of lateral compartment bone contusions (especially contusions of the lateral femoral condyle) and found several instances of medial compartment contusions that were different than the contusions found in patients with ACL tears from non-skiing injuries in which a shoe-surface interface was involved. The distribution of the bone contusions was attributed to a mechanism of injury in which the knee was flexed markedly and resulted in more posterior contusions.
The bone contusions described in our investigation are identical to those described by Speer et al (17), except the contusions in our study included a much higher incidence of lateral compartment contusions. Although knee flexion may play a role in the location of the contusions in skiers, it is unlikely to be the sole mechanism that produces the medial compartment contusions. Only two patients in our investigation were injured during skiing, and none of the patients could relate marked knee flexion at the time of injury; the other 23 patients had their feet firmly placed on the ground (at the shoe-surface interface) at the time of injury. Unfortunately, any proposal for the mechanism of injury is based only on informed speculation, since a model to prove a hypothesis is not available.
The injuries to the periphery of the posterior horn of the medial meniscus and to the posteromedial meniscocapsular junction probably occur from a shearing mechanism because tension is placed on these structures during the subluxation of the tibia on the femur, with simultaneous rotation of the femur on the tibia. It is also possible that this tissue is trapped and crushed between the bones as they impact against one another.
The medial compartment bone contusions were far more conspicuous than were the meniscocapsular injuries and/or separations and many of the far peripheral meniscal tears. Meniscocapsular separations or very peripheral meniscal tears are known to be subtle and difficult to detect on MR images, particularly in the presence of an ACL tear (18). Once the medial bone contusions were detected in our study patients, a careful search for these subtle meniscocapsular abnormalities on the sagittal and coronal images showed them to be present in all but one patient.
Of the 215 knee MR imaging studies reviewed, only one meniscocapsular separation was seen without the medial tibial plateau contusion also being present. This particular knee was imaged 17 months after the initial injury because of recurrent injury during athletic activities, with the knee "giving way." Thus, there may have initially been bone contusions at the time of injury that healed by the time the MR examination was performed, but the meniscocapsular separation did not heal; the patient with this injury also had a chronic ACL tear.
Meniscocapsular injuries are important in many instances. The periphery of the meniscus and the meniscocapsular junction is well vascularized. Thus, injuries to these regions have a good chance of healing either spontaneously or with surgical repair at arthroscopy (19,20). If these lesions are unrecognized and patients continue unrestricted activity, the tears or the separation may propagate, leading to a free-floating, dysfunctional meniscus and to knee pain, locking, and instability. It must also be recognized that identifying changes of meniscocapsular injury at MR imaging does not mean these abnormalities will necessarily be detectable at arthroscopy owing to either spontaneous healing or the far peripheral location of the lesions, which can be a difficult area for arthroscopists to evaluate well (21–23).
We purposely refer to the abnormal high signal intensity identified between the posterior horn of the medial meniscus and the joint capsule on gradient-echo, WE DESS, or STIR images as either meniscocapsular injury or meniscocapsular separation because of our inability to prove at arthroscopy what is occurring in that region. The abnormal signal intensity may simply indicate a contusion of the area, with hemorrhage and edema being present, rather than a meniscocapsular separation. It is probably most accurate to indicate that the abnormal signal intensity indicates an injury that may represent either a contusion or a separation at the meniscocapsular junction.
Of those 12 individuals who had medial tibial contusions and who underwent arthroscopy, four of five (80%) individuals who had findings interpreted as far peripheral meniscal tears and three of seven (43%) individuals who had findings interpreted as meniscocapsular injuries at MR imaging had the findings proved arthroscopically. That a higher percentage of abnormalities were not confirmed at arthroscopy does not necessarily indicate the absence of lesions. Not only is this area difficult for arthroscopists to identify, but the majority of the patients (nine of 12) did not undergo arthroscopy until 2–3 months after their injuries, which allowed time for healing in this vascularized region. One arthroscopy report indicated the identification of a partially healed meniscocapsular separation 8 weeks after the injury occurred.
Meniscal tears are found in 40%–70% of patients with acute ACL tears (14). In our study, of the 25 patients who had the medial tibial plateau contusion, 16 (64%) had a tear of the posterior horn of the medial meniscus, and 10 of the 25 (40%) knees had injuries that involved the peripheral 20% of the posterior horn. In addition, 14 of the 25 (56%) patients with the medial tibial contusion had evidence of a meniscocapsular junction injury.
Yao and Lee (24) previously reported two cases of fracture of the posteromedial corner of the medial tibial plateau. Both cases had associated ACL tears. These fractures were true linear fractures and were believed to be avulsion injuries at the site of attachment of the semimembranous tendon. In our investigation, four of the 25 patients with medial tibial plateau contusions showed increased signal intensity in the semimembranous tendon compatible with injury. We believe our cases are different from those published by Yao and Lee (24) because the tibial lesions in our study had no linear fracture component or separate fragments; they all were geographic areas of abnormal signal intensity in the subchondral bone. Undoubtedly, there must be a spectrum of findings possible that depends on the severity of the injury.
In conclusion, we propose that medial compartment bone contusions involving the posterior lip of the medial tibial plateau and, occasionally, the medial femoral condyle occur as contrecoup impaction injuries following ACL tears. These bone contusions are almost always associated with far peripheral meniscal tears, meniscocapsular contusions, or separations affecting the posterior horn of the medial meniscus. The meniscal or meniscocapsular injuries generally are much less conspicuous than are the bone contusions and should be carefully searched for when the contusion is seen.
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Footnotes |
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Author contributions: Guarantor of integrity of entire study, P.A.K.; study concepts, P.A.K., R.G.D.; study design, P.A.K., R.H.G., R.G.D.; definition of intellectual content, P.A.K.; literature research, P.A.K., R.H.G., R.G.D.; clinical studies, P.A.K., R.G.D., M.W.A., D.R.D.; data acquisition and analysis, P.A.K., R.H.G., R.G.D., M.W.A., D.R.D.; manuscript preparation, P.A.K., R.H.G.; manuscript editing and review, P.A.K., R.H.G., R.G.D., M.W.A., D.R.D.
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References |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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