Clinical History: A 28 year-old professional football player with a history of ACL reconstruction presents with recurrent pain and instability following reinjury. Fat-suppressed proton density-weighted (1a) sagittal and (1b) coronal images are provided. What are the findings” What is your diagnosis?
ACL graft disruption with hardware displacement and an acute chondral defect of the lateral tibial plateau.
ACL tears are a relatively common injury that if untreated can result in secondary osteoarthritis and meniscal tears1, as well as an increased risk for reinjury of the knee.2 As a result, orthopaedic surgeons recommend ACL reconstruction in most patients, particularly the young patient who desires a return to a high level of activity. ACL reconstruction is now performed between 75,000 and 100,000 times per year in the United States.
MRI is critical in evaluation of the post-operative knee, and the ACL reconstruction patient is no exception. Common indications for utilizing MRI in the post-operative ACL patient include acute reinjury, persistent instability, limitation of motion, or simply persistent pain. In cases of acute reinjury, such as the current example, MRI often directly visualizes the edema and laxity of a recurrent tear. In cases where a graft tear is poorly visualized, any of the secondary signs of ACL disruption such as pivot-shift bone bruises or PCL buckling may also be utilized in the ACL graft patient. As with native ACL injuries, MRI allows evaluation of associated meniscal, chondral, or osseous abnormalities in patients who have suffered an ACL graft tear.
In the evaluation of ACL graft patients without a history of reinjury, critical elements to consider include the appearance of the graft and its position. The most commonly utilized graft, the patellar tendon autograft, may demonstrate intermediate signal intensity within its substance for up to two years following surgery, likely due to vascular ingrowth.3 After two years, the graft should be of low signal intensity on all pulse sequences (3a). Increased signal intensity within the graft beyond this timeframe should raise suspicion for graft impingement, degeneration, ganglion formation, or partial tearing.
The position of an ACL graft is seen with high accuracy using MRI, and correct positioning is critical for the proper function and long term viability of a graft (4a). The normal tibial tunnel should be parallel and posterior to the slope of the intercondylar roof on sagittal images (5a). Placement of the tunnel too far posteriorly may lead to instability of the knee, and placement too far forward results in roof impingement.4 Roof impingement may cause pain and limitation of extension, and increases the incidence of graft degeneration and rupture. In patients with roof impingement, the position of the tibial tunnel anterior to the slope of the intercondylar notch is easily seen on MR images (6a). When detected early, roof impingement is amenable to treatment via notchplasty, and one study revealed a return to normal signal intensity of impinged grafts approximately twelve weeks following notchplasty.5
The position of the femoral tunnel is important in graft isometry, permitting constant tension of the graft throughout the range of motion of the knee. On sagittal images, the femoral tunnel should be located at the intersection of the physeal scar and posterior intercondylar roof6 (7a), and should be posterior to a line drawn along the posterior cortex of the femoral shaft. An anteriorly located femoral tunnel will elongate the graft and cause instability (8a).
An additional cause of limitation of extension in the ACL graft patient is the entity known as the Cyclops lesion.7 This abnormality is due to the presence of nodular fibrous tissue that forms anterior to the distal ACL graft. It is so named because at arthroscopy, the nodular soft tissue is composed of a reddish structure that may have a central dark region, resulting in a lesion that resembles an eye. The MR signal characteristics of Cyclops lesions are variable, which is understandable since Cyclops lesions have been found to have variable contents, including dense fibrosis, bone fragments, healthy synovium, and chronic synovitis. The nodular soft-tissue thickening anterior to the distal ACL is the key to the MR diagnosis in these patients (9a). The cure for a Cyclops lesion is arthroscopic resection.
An increasingly recognized abnormality that can cause pain and mechanical symptoms in the ACL graft patient is that of an ACL graft ganglion. Although small amounts of fluid along the course of a graft can be normal, particularly within the tibial tunnel, symptomatic graft ganglia are typically large, and may in fact cause expansion of osseous tunnels and even bone destruction (10a, 10b). ACL graft ganglia are thought to be multifactorial in etiology, with potential causes including mucinous degeneration, partial tearing, incomplete incorporation of allograft tissue, pressure necrosis, and a reaction to bioabsorbable screws.8 Graft ganglia rarely lead to graft disruption, but operative resection of these ganglia may be necessary for relief of pain and/or mechanical symptoms.
Hardware failure or migration is a rare complication of the postoperative ACL that may lead to mechanical symptoms, graft insufficiency, or damage to other knee structures due to the displaced hardware. Many of the commonly used interference screws in ACL reconstruction are difficult or impossible to visualize on plain films, and thus the correct diagnosis in cases of hardware failure is often unsuspected prior to obtaining an MRI. MRI’s high spatial resolution and multiplanar capability make identification of broken or migrated hardware much easier (11a,12a), and new injuries related to the atypically located hardware may be identified (13a,14a).
A final entity that bears mention in the evaluation of the post-operative ACL patient is one of the more subtle abnormalities to detect for a MR interpreter. This is the identification of graft insufficiency in patients who do not have a tear of their ACL graft. Indeed, in some of these cases, the ACL graft may appear entirely normal in signal and position on midline sagittal images (15a). The key to the diagnosis in this situation is the utilization of the MR anterior drawer sign. In patients without graft insufficiency, a line drawn vertically in tangent to the posterior cortex of the lateral femoral condyle will lie within 5mm of the posterior cortex of the lateral tibial plateau. With graft insufficiency, this distance is greater than 7mm, with a 5-7mm measurement being an equivocal finding9 (16a). Recognizing this finding in an otherwise normal appearing examination allows one to alert the surgeon to the possibility of graft insufficiency. Correlation with patient symptoms and a physical examination of joint laxity can then guide the need for operative revision.
MR imaging is ideally suited for the evaluation of pain or instability in the post-operative ACL patient. Graft integrity and position can be determined, and clinically challenging diagnoses such as graft ganglia or hardware failure are readily diagnosed with MRI. With the increasing prevalence of arthroscopic repair of the anterior cruciate ligament, the importance of MRI in the evaluation of this patient population will only increase.
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2 Dunn WR, Lyman S, Lincoln AE, et al. The effect of anterior cruciate ligament reconstruction on the risk of knee reinjury. Am J Sports Med 2004; 32:1906-1914.
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4 McCauley TR. MR Imaging Evaluation of the Postoperative Knee. Radiology 2005; 234:53-61.
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6 Manaster BJ, Remley K, Newman AP, et al. Knee ligament reconstruction: plain film analysis. AJR Am J Roentgenol 1988; 150:337-342.
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8 Martinek V, Friederich NF. Tibial and pretibial cyst formation after anterior cruciate ligament reconstruction with bioabsorbable interference screw fixation.
9 Gentili A, Seeger LL, Yao L, Do HM. Anterior cruciate ligament tear: indirect signs at MR imaging. Radiology 1994; 193:835-840.