Magnetic Resonance Imaging of Reconstructed Anterior Cruciate Ligament
Abstract & Commentary
Synopsis: This study sequentially imaged anterior cruciate ligament grafts and found increases in size and signal intensity of the grafts, presumably corresponding to graft revascularization and remodeling. It was noted that the oblique axial image was extremely helpful in assessing graft integrity.
Source: Min BH, et al. Clin Orthop. 2001;393:237-243.
Min and colleagues used MR imaging per-formed at 1, 2, 3, 6, and 12 months postoperatively to evaluate sequential changes in the cross-sectional area and signal intensity of bone-patella tendon-bone autografts in 23 patients. Of those evaluated, 8 patients had meniscal tears that were corrected at the time of surgery. All grafts were fixed with interface screws. All patients were males with an average age of 32 years (range, 16-54 years), and all were asymptomatic with good clinical and functional knee stability during the time of the investigation.
Imaging was accomplished with a 1.5 T signal MR unit with the use of fast spin-echo pulse sequences. An oblique axial image at a right angle to the reconstructed graft was obtained in proton density and T-2-weighted images in addition to the 15° oblique sagital scan.
Patients were found to have a gradual increase in graft cross sectional area and this increase was statistically significant at 1 year. Although the grafts demonstrated progressive signal intensity, this increase in intensity was not statistically significant until 1 year. Min et al felt the increase in graft cross-sectional area was indicative of ligament hypertrophy and, hence, remodeling, while the increase in signal intensity initially represented edema and later revascularization.
Although Min et al’s intent in initiating this study was to correlate MRI changes in reconstructed grafts with previously reported histologic changes, they also discovered during their imaging process 3 patients whose grafts were not well visualized on sagittal imaging. In fact, on that image alone, these patients may have been misdiagnosed as having a partial graft rupture. However, when oblique axial images were examined, the graft had a normal appearance that was verified at second look arthroscopy. Therefore, Min et al added a second message to their study, that is, to always consider the oblique axial image when evaluating graft integrity.
Comment by Letha Y Griffin, MD, PhD
Min et al attempted the difficult task of obtaining MR images at regularly scheduled time sequences in patients during the first postoperative year. Unfortunately, all patients, although examined at the first postoperative month, were not available for every sequential imaging. In fact, only 7 patients had MRIs at 2 months, 10 at 3 months, and 14 at both 6 months and 1 year postoperatively. Min et al do not comment on whether the same 14 patients were examined at 6 months and 1 year. This information would be extremely helpful in assessing the significance of their numbers for increases in cross-sectional area and graft intensity over time.
Despite this, Min et al do provide us with a helpful, historical review of current thoughts on the histology of graft remodeling as well as present the controversies over the significance of early MR changes in size and signal intensity of autografts.
Perhaps one of the greatest values of this article is its emphasis on the oblique, axial view for visualization of the ACL. Frequently, not only in the assessment of graft integrity, but also in the evaluation of the intactness of the native ligament following traumatic injury, sagittal images do not clearly reveal the intactness of the ligament. Review of the oblique axial image at right angles to the ACL typically clarifies ambiguities that exist after initial sagittal assessment.
Dr. Griffin, Adjunct and Clinical Faculty, Department of Kinesiology and Health, Georgia State University, Atlanta, GA, is Associate Editor of Sports Medicine Reports.
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