Orthopedics

Feature Article 

MRI Measurement of the 2 Bundles of the Normal Anterior Cruciate Ligament

Steven B. Cohen, MD; Corinne VanBeek, MD; James S. Starman, MD; Derek Armfield, MD; James J. Irrgang, PhD, PT, ATC; Freddie H. Fu, MD, DSc(Hon), DPs(Hon)

Abstract

Anatomical studies have shown that the normal anterior cruciate ligament (ACL) consists of 2 distinct functional bundles: the anteromedial and posterolateral bundles. To date, no study has assessed the magnetic resonance imaging (MRI) appearance of the anteromedial and posterolateral bundles. The purpose of this study was to measure the anteromedial and posterolateral bundles using high-field digital MRI.

Fifty MRIs of the knees of 50 patients were prospectively collected using a 1.5-T magnet. The length and width of each ACL bundle was measured on sagittal and coronal digital MRIs, independently performed by 2 observers blinded to each other’s measurements. The average length and width of the anteromedial and posterolateral bundles were determined for all patients. Intraclass correlation coefficients were calculated to determine intertester test–retest reliability. In the sagittal plane, the anteromedial bundle averaged 36.9±2.8 mm in length and 5.1±0.7 mm in width. The posterolateral bundle, by contrast, averaged 20.5±2.4 mm in length and 4.4±0.8 mm in width. In the coronal plane, the width of the anteromedial bundle averaged 4.2±0.8 mm and of the posterolateral bundle averaged 3.7±0.8 mm. Interobserver reliability for length of the ACL in the sagittal plane was 0.85, with a 95% CI of 0.75 to 0.91 for the anteromedial bundle and 0.75 with a 95% CI of 0.60 to 0.85 for the posterolateral bundle.

Providing precise measurement of the ACL anteromedial and posterolateral bundles on MRI may improve the ability to detect damage to 1 or both of the bundles following injury.

Anatomical studies reveal that the normal anterior cruciate ligament (ACL) consists of 2 distinct functional bundles, termed the anteromedial and posterolateral bundles (Figure 1) based on their tibial insertions.1-4 Measurements in cadaveric specimens suggest that the anteromedial bundle is approximately 38 mm in length, while the posterolateral bundle is approximately 18 mm in length.2,5-6 With respect to midsubstance diameter, the anteromedial and posterolateral bundles are similar, with an average size of 7.1 mm and 6.7 mm, respectively.5,6 Arthroscopic observations of the ACL provide similar conclusions regarding the size of each bundle. Furthermore, both the anteromedial and posterolateral bundles are present in all individuals with an intact ACL.1-3 The 2 bundles of the ACL have also been visualized in the developing fetus, confirming their early presence (Figure 2).

As we have become more aware of the true anatomy of the ACL, including its 2 bundles, we have been able to observe, particularly on high-field magnetic resonance imaging (MRI) (>1.5 T), the appearance of both bundles on routine MRI scans of the knee. Several other authors have also characterized the size and appearance of the ACL on standard MRI viewing planes.7 Steckel et al8 described the MRI appearance of the anteromedial and posterolateral bundles in a cadaveric model. However, to date no published study has described the MRI appearance of the anteromedial and posterolateral bundles using an in vivo model. A recent pilot study at the senior author’s (F.H.F.) institution suggests that the individual anteromedial and posterolateral bundles may be reliably detected using a 1.5-T MRI in standard sagittal and coronal viewing planes.

Figure 1: Arthroscopic view of the anteromedial and posterolateral bundles of the ACL. Figure 2: Fetal knee specimen demonstrating evidence of the 2 ACL bundles. Abbreviations: AM, anteromedial; PL, posterolateral.

This study sought to quantify the precise length and width of each bundle in a series of patients with an intact ACL using 1.5-T sagittal and coronal digital MRI images. We hypothesized that both anteromedial and posterolateral bundles would be present in all patients with no previous ACL pathology undergoing an MRI scan and would allow accurate measurement on digital radiography. In addition, a small…

Abstract

Anatomical studies have shown that the normal anterior cruciate ligament (ACL) consists of 2 distinct functional bundles: the anteromedial and posterolateral bundles. To date, no study has assessed the magnetic resonance imaging (MRI) appearance of the anteromedial and posterolateral bundles. The purpose of this study was to measure the anteromedial and posterolateral bundles using high-field digital MRI.

Fifty MRIs of the knees of 50 patients were prospectively collected using a 1.5-T magnet. The length and width of each ACL bundle was measured on sagittal and coronal digital MRIs, independently performed by 2 observers blinded to each other’s measurements. The average length and width of the anteromedial and posterolateral bundles were determined for all patients. Intraclass correlation coefficients were calculated to determine intertester test–retest reliability. In the sagittal plane, the anteromedial bundle averaged 36.9±2.8 mm in length and 5.1±0.7 mm in width. The posterolateral bundle, by contrast, averaged 20.5±2.4 mm in length and 4.4±0.8 mm in width. In the coronal plane, the width of the anteromedial bundle averaged 4.2±0.8 mm and of the posterolateral bundle averaged 3.7±0.8 mm. Interobserver reliability for length of the ACL in the sagittal plane was 0.85, with a 95% CI of 0.75 to 0.91 for the anteromedial bundle and 0.75 with a 95% CI of 0.60 to 0.85 for the posterolateral bundle.

Providing precise measurement of the ACL anteromedial and posterolateral bundles on MRI may improve the ability to detect damage to 1 or both of the bundles following injury.

Anatomical studies reveal that the normal anterior cruciate ligament (ACL) consists of 2 distinct functional bundles, termed the anteromedial and posterolateral bundles (Figure 1) based on their tibial insertions.1-4 Measurements in cadaveric specimens suggest that the anteromedial bundle is approximately 38 mm in length, while the posterolateral bundle is approximately 18 mm in length.2,5-6 With respect to midsubstance diameter, the anteromedial and posterolateral bundles are similar, with an average size of 7.1 mm and 6.7 mm, respectively.5,6 Arthroscopic observations of the ACL provide similar conclusions regarding the size of each bundle. Furthermore, both the anteromedial and posterolateral bundles are present in all individuals with an intact ACL.1-3 The 2 bundles of the ACL have also been visualized in the developing fetus, confirming their early presence (Figure 2).

As we have become more aware of the true anatomy of the ACL, including its 2 bundles, we have been able to observe, particularly on high-field magnetic resonance imaging (MRI) (>1.5 T), the appearance of both bundles on routine MRI scans of the knee. Several other authors have also characterized the size and appearance of the ACL on standard MRI viewing planes.7 Steckel et al8 described the MRI appearance of the anteromedial and posterolateral bundles in a cadaveric model. However, to date no published study has described the MRI appearance of the anteromedial and posterolateral bundles using an in vivo model. A recent pilot study at the senior author’s (F.H.F.) institution suggests that the individual anteromedial and posterolateral bundles may be reliably detected using a 1.5-T MRI in standard sagittal and coronal viewing planes.

Figure 1: Arthroscopic view of the anteromedial and posterolateral bundles of the ACL Figure 2: Fetal knee specimen demonstrating evidence of the 2 ACL bundles

Figure 1: Arthroscopic view of the anteromedial and posterolateral bundles of the ACL. Figure 2: Fetal knee specimen demonstrating evidence of the 2 ACL bundles. Abbreviations: AM, anteromedial; PL, posterolateral.

This study sought to quantify the precise length and width of each bundle in a series of patients with an intact ACL using 1.5-T sagittal and coronal digital MRI images. We hypothesized that both anteromedial and posterolateral bundles would be present in all patients with no previous ACL pathology undergoing an MRI scan and would allow accurate measurement on digital radiography. In addition, a small pilot study was performed to correlate the MRI measurements with the actual structural measurement during arthroscopy. This information allowed a comparison to existing cadaveric and arthroscopic data on the length and diameter of the anteromedial and posterolateral bundles. It also evaluated the usefulness of MRI for imaging the anteromedial and posterolateral bundles’ anatomy in light of emerging surgical techniques for anatomic ACL reconstruction.

Materials and Methods

Study Design

After approval was obtained from our institution’s ethics review committee, MRIs of the knee were prospectively collected as part of routine diagnostic workups for 81 consecutive patients during 3 weeks in March 2006, using a closed 1.5-T magnet (GE Signa; GE Healthcare, Waukesha, Wisconsin) with the knee positioned in full extension. Standard sagittal and coronal images were used without specific orientation in the plane of the ACL. Standard T1- and T2-weighted images were performed and analyzed using 3-mm section thickness. Of the initial cohort, 31 patients were excluded due to new or previous ACL pathology after further questioning and clinical examination, leaving a total of 50 images for review. None of the remaining patients had a history of instability, giving way, or pivoting injury, and none were unstable based on physical examination. Basic demographic data was collected for each patient, including age, sex, height, and weight.

Anatomically, the nomenclature of the 2 bundles corresponds to their tibial insertion sites. On the femoral side, the anteromedial bundle originates more proximally and the posterolateral bundle originates more distally. On the tibial side, the anteromedial bundle inserts anteromedially, while the posterolateral bundle inserts posterolaterally. The relative position of the 2 bundles varies with the flexion angle of the knee. In extension, the 2 bundles are parallel. In flexion, the femoral insertion site of the posterolateral bundle moves anteriorly, and the 2 bundles are crossed. In flexion, the anteromedial bundle tightens as the posterolateral bundle loosens. In extension, the posterolateral bundle tightens and the anteromedial bundle loosens. The posterolateral bundle tightens during internal and external rotation of the knee.

The intact ACL was measured on T1- and T2-weighted sagittal and coronal views for the anteromedial bundle and the posterolateral bundle. In addition, the difference between male and female measurements was assessed. Based on the documented literature and for the purposes of this study, on MRI in the sagittal plane the anteromedial bundle was defined as the oblique fibers inserting at the anterior border of the ACL on the tibia and the proximal aspect of the femoral insertion on the lateral femoral condyle. This was readily visible and corresponded to the anteromedial bundle appearance seen arthroscopically. Similar to the anteromedial bundle, the posterolateral bundle was defined as the oblique fibers inserting posteriorly on the tibial insertion and on the distal aspect of the femoral insertion on the lateral femoral condyle. There is a septum or dividing line on MRI where the bundles can be identified for measurement. In the coronal plane, the anteromedial and posterolateral bundle widths were measured by delineating the 2 bundles and measuring the transverse measurement or width of each bundle.

The images were independently measured in a blinded fashion by 2 observers: 1 fellowship-trained musculoskeletal radiologist (D.A.) and 1 orthopedic sports medicine fellow (S.B.C.). The physicians were blinded to each other’s measurements and to previous measurements performed in the literature to prevent measurement bias. The measurements were obtained by using the ruler function on a digital radiology viewing program (Stentor; Philips Healthcare, Andover, Massachusetts). The accuracy of the digital MRI system adjusts to physical data points, so there is no inherent system error to the measurements; however, the error lies in the subjective measurements made by the observers.

For the pilot study, an intraoperative measurement of the anteromedial and posterolateral bundles was performed using a measuring device on 10 knees that had undergone both MRI and knee arthroscopy. The bundles were measured on MRI by the musculoskeletal radiologist digitally and during surgery by an experienced sports medicine surgeon (S.B.C.). The measurements were then averaged and compared accordingly. The actual measurement of the bundles was performed using a 2-dimensional calibrated ruler (OBI Plugs; Smith & Nephew Endoscopy, Andover, Massachusetts).

Data Analysis

Data were analyzed using SPSS version 14 software (SPSS Inc, Chicago, Illinois). Descriptive statistics, including frequency counts for categorical variables such as sex and measures of central tendency (means, medians, standard deviations, range) for continuous variables such as age were calculated for all variables. The average length of the anteromedial and posterolateral bundles in the sagittal plane and width of the anteromedial and posterolateral bundles in the sagittal and coronal planes were calculated for each examiner and for both examiners combined. Differences between examiners were evaluated with a paired t test. In addition, differences between sexes were also evaluated and analyzed statistically using a Student t test. Intertester reliability was calculated with an intraclass correlation coefficient using the two-way random effects model for absolute agreement for single measures. To calculate measurement precision, the standard error of measurement was calculated using the intraclass correlation coefficient and standard deviation of the measurements using the formula.

Figure 3: 1.5-T sagittal T1-weighted MRI showing AM and PL bundle length and width Figure 4: AM and PL bundle width measurements

Figure 3: 1.5-T sagittal T1-weighted MRI showing AM and PL bundle length and width. The AM bundle is approximately twice as long as the PL bundle and has a similar cross-sectional diameter. The vertical lines are examples of the measurements (length) taken for the AM and PL bundles. Abbreviations: AM, anteromedial; PL, posterolateral. Figure 4: Coronal T1-weighted MRI showing AM and PL bundle width measurements. The horizontal lines are examples of the measurements (width) taken for the AM and PL bundles. Abbreviations: AM, anteromedial; PL, posterolateral.


Results

Thirty men and 20 women had an average age of 33.5 years (range, 13-62 years). The anteromedial and posterolateral bundles were visualized in all 50 patients by both observers. The average length and width of the anteromedial and posterolateral bundles in the sagittal plane (Figure 3) and width in the coronal plane (Figure 4) are summarized in Table 1. Overall, the anteromedial bundle averaged 36.9±2.8 mm in length, 5.1±0.7 mm in sagittal width, and 4.2±0.8 mm in coronal width. The posterolateral bundle, by contrast, averaged 20.5±2.4 mm in length, 4.4±0.8 mm in sagittal width, and 3.7±0.8 mm in coronal width. The average length of the anteromedial bundle was 36.9±2.9 mm and of the posterolateral bundle was 20.5±2.5 mm for men compared to 36.9±2.7 mm and 20.5±2.2 mm, respectively, for women (Table 2). The average width of the anteromedial bundle was 5.0±0.7 mm in the sagittal plane and 4.1±0.8 mm in the coronal plane for men and 5.3±0.7 mm and 4.3±0.9 mm, respectively, for women. The average width of the posterolateral bundle was 4.4±0.8 mm in the sagittal plane and 3.7±0.8 mm in the coronal plane for men and 4.4±0.8 mm and 3.6±0.8 mm, respectively, for women. There was no significant difference between measurements for men and women; however, no power analysis was performed to determine the number of patients requirement to detect differences.

Table 1: Observer Measurements of the AM and PL Bundles

Table 2: AM and PL Bundle Measurements in Men and Women

Table 3: Intertester Reliability for Measuring AM and PL Bundles

Table 4: Intraoperative Arthroscopic Measurements of the AM and PL Bundles

There were no differences in anteromedial and posterolateral bundle sizes as measured by the observers. Intertester reliability is summarized in Table 3. In general, intertester reliability was higher for measurement of the length of the bundles in the sagittal plane (Figure 5) compared to measurement of the width of the bundles in the sagittal or coronal planes. However, the measurement error, defined in terms of the standard error of measurement, was on the order of 0.5 to 0.6 mm for measurement of the width of the anteromedial and posterolateral bundles. Thus, although the intraclass correlation coefficients are only fair to moderate (0.40 to 0.66), the width of the anteromedial and posterolateral bundles was measured with a relatively high level of precision.

Figure 5: Sagittal T2-weighted MRI showing the AM and PL bundles
Figure 5: Sagittal T2-weighted MRI showing the AM and PL bundles. Abbreviations: AM, anteromedial; PL, posterolateral.

The results of the pilot study (Table 4) revealed an average intraoperative measurement of the length and width of the anteromedial and posterolateral bundles of 35.3 and 6.8 mm and 19.1 and 5.4 mm, respectively (Figure 6), compared to the MRI of 37.9 and 5.1 mm and 20.5 and 4.4 mm, respectively. The differences between these measurements were not statistically significant.

Discussion

While initially described by Palmer,4 numerous studies have confirmed the presence of the anteromedial and posterolateral bundles of the ACL.2,3,9 Amis and Dawkins1 and Gabriel et al10 described the significant contribution of the anteromedial and posterolateral bundles to knee stability during flexion and extension. In recent years, several studies have evaluated the size and length of the anteromedial and posterolateral bundles using cadaveric and arthroscopic observations. Hollis et al11 measured the individual bundle lengths in dissected cadavers to be, on average, 34 mm for the anteromedial bundle and 22.5 mm for the posterolateral bundle with the knee in extension. Similar results were found by Steckel et al12 in a cadaveric model, in which the average length and width of the anteromedial and posterolateral bundles were 37.7 and 8.5 mm and 20.7 and 7.7 mm, respectively. Our data obtained by MRI demonstrate the same trend, in which the anteromedial bundle was larger than the posterolateral bundle. Furthermore, little discrepancy exists between the measurements observed in the cadaveric studies and our radiographic study, which is the first study to assess bundle appearance and size using MRI.

Our data offer additional supporting evidence that the anteromedial bundle is approximately twice the length of the posterolateral bundle in the intact ACL, while the midsubstance diameter of both bundles is similar (Figures 3, 4). In addition, the pilot study confirms the ability to measure the 2 intact bundles at the time of arthroscopy. Agreement between cadaveric and arthroscopic studies and our study’s digital high-field MRI-based observations of anteromedial and posterolateral bundle dimensions suggests that MRI holds potential for accurate evaluation of these structures in the clinical setting. As demonstrated in the results, intertester reliability was better for length than width, which may be more clinically pertinent when assessing for a possible partial ACL tear.

Figure 6A: Intraoperative measurement of the ACL using a calibrated measuring device Figure 6B: Intraoperative measurement of the ACL using a calibrated measuring device
Figure 6C: Intraoperative measurement of the ACL using a calibrated measuring device Figure 6D: Intraoperative measurement of the ACL using a calibrated measuring device

Figure 6: Intraoperative measurement of the ACL using a calibrated measuring device. Measuring the length (A) and width (B) of the PL bundle. Measuring the length (C) and width (D) of the AM bundle. Abbreviations: AM, anteromedial; PL, posterolateral.

Magnetic resonance imaging has previously been established as a valuable tool for evaluating ACL pathology, used in combination with clinical examination tests such as the Lachman test, pivot shift test, and KT-2000 measurement.13-18 Standard sagittal and coronal imaging planes have a high sensitivity and specificity for the detection of complete ACL tears.19-22 Diagnosis of partial ACL tears on MRI has proven more challenging, however. Several authors have identified the MRI features that are most helpful in this situation, including the presence of some intact fibers, a thin ligament, and focally increased signal intensity in the acute setting.23,24 However, to date there is no published MRI study that attempts to identify the individual anteromedial and posterolateral bundles in cases where either a partial or complete ACL tear is diagnosed. A literature review identified 1 article on MRI of the ACL in which the individual bundle anatomy was described, but this was an anecdotal observation in 1 image.19

The clinical relevance of evaluating the anteromedial and posterolateral bundles individually closely relates to recent trends in the surgical approach to ACL reconstruction. In light of increasing evidence of the importance of both bundles to knee stability and kinematics, traditional surgical approaches to ACL reconstruction have been challenged.25 The current gold standard for ACL reconstruction consists of a single-bundle approach that replaces the ACL with a single-bundle graft placed in the middle of the anteromedial and posterolateral bundles with more of a reapproximation of the anteromedial bundle but placed more on the lateral wall of the intercondylar notch (10:30 or 1:30 position). However, some surgeons advocate a double-bundle ACL reconstruction approach that recreates the individual anteromedial and posterolateral bundles and their anatomical footprints.26-28 This approach has also been applied to a small subset of partial ACL injuries that leave the anteromedial or posterolateral bundle intact. Using the anatomic double-bundle reconstruction approach, the intact bundle is preserved and the injured bundle replaced with graft tissue.29,30 A more accurate preoperative evaluation in these cases may allow for improvements in preoperative planning and surgical approach.

Our study is an important first step in reaching the goal of improved preoperative assessment of ACL-injured patients. We have demonstrated that 1.5-T (high-field) magnet MRI images in standard viewing planes may offer good visualization of both bundles of the ACL. However, an important limitation of our study is the fact that it was conducted exclusively on ACL-intact patients; therefore, we can make no conclusions regarding the use of MRI in detecting isolated bundle pathology as confirmed by arthroscopic visualization. Such a study is an important next step in further validating the clinical applicability of MRI as it relates to single-bundle or partial ACL tears. Furthermore, inherent inaccuracies exist in measuring a 3-dimensional structure in a 2-dimensional fashion, which occurs both on MRI and at the time of arthroscopy. To have the highest degree of accuracy, the bundles would require significant dissection, which would injure the intact ACL. In addition, some authors have advocated the use of additional viewing planes for imaging of the ACL, including the oblique-sagittal (Figure 7) and paracoronal (Figure 8) planes, which are based on the natural course of the ACL bundles and allow optimal visualization.19,31,32 Future work using these imaging planes may lead to even better visualization of the anteromedial and posterolateral bundles, and may play a potential role in detecting single-bundle pathology with high sensitivity and specificity.

Figure 7: Oblique-sagittal T1-weighted MRI showing the AM and PL bundles Figure 8: Paracoronal T1-weighted MRI showing the AM and PL bundles

Figure 7: Oblique-sagittal T1-weighted MRI showing the AM and PL bundles. Abbreviations: AM, anteromedial; PL, posterolateral. Figure 8: Paracoronal T1-weighted MRI showing the AM and PL bundles. Abbreviations: AM, anteromedial; PL, posterolateral.

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Authors

Dr Cohen is from Rothman Institute Orthopaedics and the Department of Orthopedic Surgery, Thomas Jefferson University, Philadelphia, and Drs Starman, Armfield, Irrgang, and Fu are from University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania; and Dr VanBeek is from Columbia University, New York, New York.

Drs Cohen, VanBeek, Starman, Armfield, Irrgang, and Fu have no relevant financial relationships to disclose.

Correspondence should be addressed to: Steven B. Cohen, MD, Department of Orthopedic Surgery, Thomas Jefferson University, 925 Chestnut St, Philadelphia, PA 19107.

DOI: 10.3928/01477447-20090728-35

10.3928/01477447-20090728-35

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