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Musculoskeletal Imaging |
1 From the Departments of Radiology (G.S.H., C.Y.C.), Orthopedic Surgery (C.H.L.), and Pathology (H.S.L.), Tri-Service General Hospital, National Defense Medical Center, No. 325, Section 2, Cheng-Kung Rd, Neihu, Taipei 114, Taiwan, R.O.C.; Department of Radiology, Taipei Medical University-Municipal Wan Fang Hospital, Taipei, Taiwan, R.O.C. (W.P.C.); and Department of Radiology, Ohio State University Medical Center, Columbus (J.S.Y.). Received November 9, 2001; revision requested December 12; revision received March 1, 2002; accepted April 3. Address correspondence to G.S.H. (e-mail: gsh5@seed.net.tw).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: MR images in 15 patients who had presented with a block to terminal knee extension after sustaining an ACL tear were retrospectively reviewed. The MR imaging appearances of entrapped ACL stumps were defined and correlated with arthroscopic and histologic findings.
RESULTS: The entrapped ACL stumps had two distinct appearances. Type 1 stumps (n = 11) were characterized by a nodular mass located at the anterior aspect of the intercondylar notch, interposed between the lateral femoral condyle and tibia. Type 2 stumps (n = 4) were characterized by a tonguelike free end and angulation of the stump. Histologic evaluation of the resected specimens showed distorted ACL fibers intermixed with variable fibrosis and inflammation.
CONCLUSION: Entrapment of an ACL stump can limit knee extension. The two observed appearances of entrapped ACL stumps likely represent two points along a spectrum of appearances, which possibly includes the cyclops lesion.
© RSNA, 2002
Index terms: Knee, injuries, 452.4191, 452.4852, 452.4857 • Knee, ligaments, menisci, and cartilage, 452.4191, 452.4852, 452.4857 • Knee, MR, 452.121411, 452.121412, 452.121416
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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We identified 15 patients (15 male patients; age range, 17–26 years; mean age, 22 years) who fulfilled our selection criteria. All patients presented with anterior knee pain and restricted knee extension. In every instance, the knee injury was a result of a twisting injury caused by an athletic activity. Twelve patients sustained injuries playing basketball, and three patients sustained injuries playing tennis. At physical examination, the loss of terminal knee extension ranged from 10° to 30°, in comparison with the normal expected range and the range of motion of the contralateral knee. Examination after anesthetic was administered revealed that passive knee extension was diminished from 5° to 15°.
Imaging
All MR examinations were performed by using a 1.5-T magnet (Vista; Picker, Cleveland, Ohio) and a dedicated extremity coil. The images were obtained in the sagittal and coronal planes by using spin-echo intermediate-weighted (1,800–2,200/20–30 [repetition time msec/echo time msec]) and T2-weighted (1,800–2,200/80–90) sequences, and transverse images were obtained by using a gradient-recalled-echo sequence (650/13; flip angle, 20°). The field of view was 14–16 cm, the section thickness was 3–4 mm with a 0.5-mm gap, the matrix was 192–256 x 256, and an average of one to two signals were acquired.
Evaluation
Fourteen patients underwent imaging within 1 week after the injury, but one patient underwent imaging 6 months after the injury. Arthroscopy was performed within 2 weeks after the MR examination in each patient. Arthroscopic findings were evaluated by one experienced arthroscopist (C.H.L.) for the ACL tear location; position and appearance of the distal stump of the disrupted ACL; fibrosis of the infrapatellar fat pad; adherence of the ACL stump to the fat pad; and integrity of the menisci, cartilage, and bones.
All MR images were reviewed simultaneously by two experienced musculoskeletal radiologists (G.S.H., W.P.C.), and a consensus was reached for every reading. MR images were evaluated for the ACL tear location; position, appearance, and signal intensity of the distal ACL stump; integrity of the menisci, bones, cartilage, and collateral ligaments; fibrosis of the infrapatellar fat pad; and joint effusion. Ten patients also had resected specimens for histologic evaluation. The specimens were evaluated by an experienced pathologist (H.S.L.) for the architecture of the ACL fibers, inflammation, fibrosis or fibrinoid degeneration, angiogenesis, neural or fatty components, and mitotic activity.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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The appearance of the entrapped ACL stump was characterized as being of one of two types. A type 1 stump appeared as a nodular mass located in the anterior recess of the joint, in the anterior aspect of the intercondylar notch, interposed between the lateral femoral condyle and tibia. This appearance was identified in 11 patients (Fig 1). A type 2 stump had a tonguelike free end, and the stump itself appeared folded anteriorly, displacing out of the intercondylar notch into the anterior joint recess. This appearance was identified in four patients (Fig 2).
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All affected knees had joint effusions. No patient had fibrosis of the infrapatellar fat pad, and the ACL stumps did not adhere to the fat pad. Nine patients had a medial meniscal tear, and four patients had a lateral meniscal tear, all of which were confirmed at surgery. Osteochondral impaction with marrow edema in the lateral femoral condyle was identified on MR images in seven patients, and marrow edema in the posterolateral aspect of the tibial plateau was identified in four patients. The medial collateral ligament appeared thickened in every patient.
Histologic evaluation of 10 resected specimens revealed that the nodular masses were composed of distorted ACL collagen fibers with loss of their recognizable architecture. Intermixed within the fibers were a variable degree of fibrotic tissue and regions of inflammatory reaction. In four specimens, focal angiogenesis with fibrinoid degeneration was noted. There was no neural or fatty element in any of the specimens or evidence of mitotic activity.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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When a patient presents with locking, treatment should be expeditious, since functional disability due to arthrofibrosis may occur when treatment is delayed (2). The most common cause of knee locking in the setting of acute trauma is a displaced fragment from a torn meniscus (16). Intraarticular entrapment of an ACL stump between the femoral condyle and tibial plateau in a patient with a ruptured ACL is rare (1,2,14). Results from a recent article (14), however, indicate that at arthroscopy 3% of patients with an acute ACL tear may present with a mechanical block caused by an ACL stump.
To our knowledge, there is no specific study in the current literature in which the appearance of a displaced and entrapped ACL stump in an acutely ruptured ACL has been described. In our study, two distinct appearances were evident.
A type 1 stump had a free end that was bulbous, nodular, or masslike in appearance and interposed between the lateral femoral condyle and tibia. In 10 of the 11 patients who had this appearance, resection of the mass was required for restoration of the range of motion. The inflammatory response caused by the impingement produced thickening of the ACL fibers, as well as a fibroproliferative process characterized by the deposition of fibrous tissue between the ACL fibers. The signal intensity of the nodular mass may in part have been related to the length of time that the ACL stump was impinged, thus allowing for the spectrum of signal intensities seen on the T2-weighted images that ranged from heterogeneous low signal intensity reflecting fibrosis to focal areas of high signal intensity indicative of an inflammatory response.
A type 2 stump had a characteristic fold in the anteriorly displaced stump and a tonguelike free end without nodularity. This appearance was less common and was seen only in patients who had a long distal stump (ie, those who had proximal tears).
We hypothesize two possible mechanisms for the development of this lesion. When an ACL rupture occurs in the proximal third of the ACL, the redundant distal stump sags in the intercondylar notch and assumes a more horizontal orientation. As fibrosis develops at the free end, it forms an enlarged bulbous configuration. When the unstable knee translates anteriorly once again, the thickened end can be displaced anteriorly and become entrapped in the anterior recess because of its mass effect when the knee recoils into anatomic alignment. This mechanism is the most likely for the development of type 1 lesions.
When tears occur at the femoral attachment and produce a long distal stump, it is possible for the stump to fold on itself with the initial injury, creating a type 2 appearance. It may transform into a type 1 lesion if fibrosis develops in the free end. Once entrapped, the stump may be difficult to reduce, causing patients to present with restricted terminal knee extension.
The differential diagnosis is a cyclops lesion, or localized anterior arthrofibrosis, which produces a mass in the anterior recess of the knee in patients with ACL reconstructions (7–10). Recently, cyclopslike lesions were identified arthroscopically in patients who had disrupted ACLs who had not undergone ligament reconstruction (1). The main difference between the cyclops lesion and the anteriorly entrapped ACL stump is discontinuity with the tibial attachment of the ACL. We believe, however, that a number of these lesions actually represent the end stage of a displaced ACL stump.
Arthroscopic observations reported by Jomha et al (14) indicate that 3% of patients with ruptured ACLs present with a mechanical block caused by a displaced stump. The histologic observations in our study also support this hypothesis, since there was a spectrum of fibrotic changes surrounding the displaced ACL fibers in the stump, some of which had characteristics similar to those reported in the cyclops lesion.
Our study had several limitations. First, the population in our study was chosen on the basis of arthroscopic findings in patients who presented with restricted terminal knee extension. We do not know the prevalence of ruptured ACLs in patients who may have an asymptomatic displaced stump, since only those with restricted extension were selected for this study. It is clear, however, in our experience, that the observation of an anteriorly recessed mass, in general, is rare in patients who have not undergone surgery.
Second, we cannot estimate, on the basis of our observations, what other factors predispose a proximal tear to displacement of its distal stump except for the location of the tear. There may be a correlation with the size of the bulbous free end, as has been suggested by some investigators (1), but we did not identify a specific size that predisposed to entrapment. Patient activity may be a contributing factor, since all of the patients in our study were high-caliber athletes.
Third, we recognize the value of sequential imaging for evaluating the evolution of these lesions, but in reality, once the observation is made prospectively, it is an indication for immediate surgical exploration, so it is not feasible to perform a follow-up examination.
In conclusion, entrapment of an ACL stump in a patient who has an acute rupture of the ACL can restrict knee extension in a fashion similar to that seen in the cyclops lesion in patients who have undergone ACL reconstruction. There are two distinct appearances of the entrapped ACL stumps, which may represent two points along a spectrum of appearance, and a cyclops lesion may represent the end point of this spectrum. Resection of the nodular mass is essential prior to reconstruction of the ACL to restore proper range of motion.
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, G.S.H.; study concepts, G.S.H., C.H.L., J.S.Y.; study design, G.S.H.; literature research, G.S.H., C.H.L.; clinical studies, G.S.H., C.H.L.; data acquisition, G.S.H., C.H.L., H.S.L.; data analysis/interpretation, G.S.H., C.H.L., W.P.C., H.S.L.; manuscript preparation, G.S.H.; manuscript definition of intellectual content and editing, G.S.H., J.S.Y.; manuscript revision/review, W.P.C., C.Y.C., J.S.Y.; manuscript final version approval, G.S.H.
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REFERENCES |
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