MRI Web Clinic — January 2007

ACL Graft Tear
Mark H. Awh, M.D.

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?

1a
1b
Figure 1:

Fat-suppressed proton density-weighted (1a) sagittal and (1b) coronal images

Findings

2a
2b
Figure 2:

(1a) The sagittal image reveals diffuse edema and abnormal laxity (arrows) along the course of the ACL graft, compatible with graft rupture. A displaced femoral fixation pin (arrowhead) is also apparent.  ACL graft edema (arrow) is redemonstrated on (2b) the corresponding proton density-weighted coronal image. A focal chondral defect (arrowhead) is also apparent at the lateral tibial plateau.

Diagnosis

ACL graft disruption with hardware displacement and an acute chondral defect of the lateral tibial plateau.

Discussion

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.

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Figure 3:

(3a) A T1-weighted sagittal image in a patient three years following ACL reconstruction reveals a normal low signal intensity appearance of the graft (arrow). No graft laxity is seen.

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

 

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Figure 4:

(4a) A 3-D cutaway at the intercondylar notch in the sagittal plane reveals normal positioning for a patellar tendon ACL autograft. The tibial tunnel should lie posterior to the line drawn parallel to the intercondylar notch (red) and the femoral attachment should lie posterior to a line drawn parallel to the cortex of the distal femoral diaphysis (blue). Illustration courtesy of Michael E. Stadnick, M.D.

 

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Figure 5:

(5a) A T2-weighted sagittal image in a patient with a history of ACL reconstruction redemonstrates the normal position of the tibial tunnel, parallel and posterior to the slope of the intercondylar roof.

 

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Figure 6:

(6a) A T1-weighted sagittal image in a patient with roof impingement reveals that the tibial tunnel is placed well anterior to the slope of the intercondylar roof. The ACL graft (arrow) is abnormally lax and demonstrates abnormally increased signal intensity, likely secondary to degeneration from chronic mechanical stress.

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).

 

7
Figure 7:

The femoral tunnel is normally positioned at the junction of the physeal scar and posterior intercondylar roof (asterisk) on (7a) a T2-weighted sagittal image. It lies posterior to the line drawn along the posterior cortex of the femoral shaft.

 

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Figure 8:

(8a) A T2-weighted sagittal image in another patient demonstrates abnormal placement of the femoral tunnel (asterisk), which lies significantly anterior to a line drawn along the posterior cortex of the femoral diaphysis. The graft demonstrates abnormally increased signal intensity within its substance (arrow), likely related to degeneration and stretching.

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.

 

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Figure 9:

(9a) A T2-weighted sagittal image in a patient with limited extension following ACL reconstruction demonstrates abnormal nodular soft-tissue thickening (arrow) anterior to the tibial insertion of the ACL graft, compatible with a Cyclops lesion.

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.

 

10a
10b
Figure 10:

(10a) Proton density-weighted sagittal and (10b) Inversion recovery coronal images in a patient with pain 18 months following ACL reconstruction are provided. Diffuse abnormal fluid signal intensity is seen along the course of the graft (arrows) on the sagittal image, compatible with extensive graft ganglion formation. The tibial tunnel is expanded (arrows) and surrounding bone marrow edema is present (arrowheads) on the corresponding coronal view.

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).

 

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Figure 11:

(11a) A nodular signal abnormality (arrow) is seen within the anterior joint on a proton-density weighted sagittal image from a patient with limited extension following ACL reconstruction. Is this another Cyclops lesion?

 

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Figure 12:

(12a) The axial fat-suppressed proton density-weighted axial image reveals the correct diagnosis. The migrated femoral fixation screw (arrow) is easily visualized within the anterior joint.

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Figure 13:

(13a) The ACL graft fixation screw (arrow) protrudes abnormally into the intercondylar notch on a fat-suppressed proton density-weighted coronal image. Note the abnormally increased signal intensity within the adjacent PCL (arrowhead).

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Figure 14:

(14a) A proton density-weighted sagittal image confirms the displaced femoral screw (arrow). The adjacent PCL is attenuated and irregular (arrowhead), compatible with partial tearing, perhaps caused by the migrated hardware.

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.

 

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Figure 15:

In a patient with recurrent pain and instability following ACL reconstruction, (15a) a midline sagittal inversion-recovery image demonstrates an intact and normally positioned ACL graft (arrow).

 

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Figure 16:

(16a) A more lateral proton density-weighted sagittal image reveals the MR anterior drawer sign. The tibia is displaced anteriorly by 9mm relative to a line drawn down vertically from the posterior femoral cortex.

Conclusion

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.

References

1 Finsterbush A, Frankl U, Matan Y, Mann G. Secondary damage to the knee after isolated injury of the anterior cruciate ligament. Am J Sports Med 1990; 18:475-479.

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.

3 Rak KM, Gillogly SD, Schaefer RA, Yakes WF, Liljedahl RR. Anterior cruciate ligament reconstruction: evaluation with MR imaging. Radiology 1991; 178:553-556.

4 McCauley TR. MR Imaging Evaluation of the Postoperative Knee. Radiology 2005; 234:53-61.

5 Howell SM, Clark JA, Blasier RD. Serial magnetic resonance imaging of hamstring anterior cruciate ligament autografts during the first year of implantation: a preliminary study. Am J Sports Med 1991; 19:42-47.

6 Manaster BJ, Remley K, Newman AP, et al. Knee ligament reconstruction: plain film analysis. AJR Am J Roentgenol 1988; 150:337-342.

7 Bradley DM, Bergman AG, Dillingham MF. MR imaging of cyclops lesions. Am J Roentgenol 2000; 174:719-726.

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.

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