MRI to Predict Joint Laxity in ACL Reconstruction Patients
MRI to Predict Joint Laxity in ACL Reconstruction Patients
MRI is the best known method to assess the structural integrity of the ACL, and physical examination is the most commonly used method to assess the degree of knee joint laxity caused by ACL injury. However, an association between the results of the two methods has not been well established. In the present study, we examined whether MRI findings are of value in predicting the degree of knee joint laxity as measured using two typical physical examinations, i.e., the Lachman and pivot shift tests. A few MRI findings were associated with high-grade laxity determined by the physical examinations, suggesting that MRI may be of limited usefulness in predicting the grade of knee laxity. With a shorter time between injury and image acquisition, MRI was more likely to be helpful for predicting high-grade knee laxity caused by ACL injury.
Our analysis of the associations between MRI findings and the grade on physical examinations in patients with symptomatic ACL injury suggests that MRI has limited value for predicting the grade of laxity. Among the four primary findings evaluated, only nonvisualization was associated with a high grade on the Lachman test. As nonvisualization of the ACL on MRI represents a complete absence of the ACL on images, this result is reasonable. However, it is curious that the three other primary findings, particularly discontinuity, had no associations with the grade of clinically assessed laxity given that these MRI findings directly reflect significant abnormalities of the ACL bundles per se. Our findings were in disagreement with the results of a previous study in which the accuracies of discontinuity and abnormal orientation of the ACL on MRI for diagnosis of unstable ACL injury were 79% and 87%, respectively. In this previous study, all of the surgically confirmed complete ACL tears were considered to be unstable ACL injuries, even though clinical examinations were also used to define unstable ACL injuries. In contrast, we used only the results of physical examinations to determine high-grade laxity, regardless of the arthroscopic findings, for comparison with the MRI findings. Several additional factors such as scarring of the torn ACL remnant, concomitant injuries, and bone geometry may influence the degree of stability after ACL injury. Furthermore, MRI findings were shown to introduce bias in the evaluation of cases of chronic ACL tears because a previously torn ACL can heal with scar tissue. Therefore, our findings suggest that with regard to the degree of knee laxity, predictions based only on the primary MRI findings may not be accurate.
Anterior translation of the tibia relative to the femur was the only secondary finding associated with high-grade laxity on physical examinations in the present study. It is possible that signs of anterior translation of the tibia relative to the femur are associated with high-grade laxity after ACL injury; however, even in cases with high-grade ACL injury, anterior translation of the tibia would not be easy to discern because standard MRI is performed with the patient in the supine position. Therefore, the observation of anterior translation of the tibia on MRI would reflect considerable laxity due to ACL injury. Interestingly, anterior translation of the tibia relative to the femur on MRI was significantly associated with the grade on the pivot shift test and not with the grade on the Lachman test. It should be noted that anterior translation of the tibia was measured in the lateral compartment. Several studies have indicated that anterior translation is coupled with internal rotation of the tibia, and ACL deficiency would lead to increased anterolateral subluxation of the tibia on the femoral condyle, especially in the lateral compartment. In other studies, anterolateral translation of the tibia was related to the pivot shift test results. Thus, anterior translation of the tibia relative to the femur measured in the lateral compartment on MRI has a reasonable chance of reflecting the injury grade determined on the pivot shift test.
We hypothesized that the value of MRI for predicting the grade of clinically assessed knee laxity is affected by the MRI acquisition time in patients with symptomatic ACL injury. Even though the predictive value of MRI for laxity was modest in both the early and late MRI groups, it was more significant in the early MRI group than in the late MRI group. We speculate that this finding stems from the biological response to ACL injury, which would lead to a change in the condition of the ACL over time. This is supported by previous studies showing that discontinuity and abnormal signal intensity on MRI can be restored with time and that typical MRI findings can change unpredictably over time.
The present study had some limitations. First, all subjects included in the study were patients requiring ACL reconstruction. Therefore, our findings may not be applicable to patients over the whole spectrum of ACL injuries. However, information on the use of MRI findings for predicting the grade of knee laxity would be of more practical value for patients warranting ACL reconstruction, i.e., those with significant subjective instability caused by trauma as the mechanism of ACL injury. Moreover, several factors other than physical examination results and MRI findings, including the patient's age, activity level, and willingness to undergo surgery, can contribute to the decision regarding ACL reconstruction. Thus, by limiting the study subjects to those requiring ACL reconstruction, it is possible that more practical information was achieved and the influence of uncontrolled factors was reduced. Second, the Lachman and pivot shift tests are not free from interexaminer variability. To overcome this problem, all physical examinations of the patients included in this study were carried out by experienced knee specialists. In addition, dichotomization of the physical examination results might have reduced the variability.
Discussion
MRI is the best known method to assess the structural integrity of the ACL, and physical examination is the most commonly used method to assess the degree of knee joint laxity caused by ACL injury. However, an association between the results of the two methods has not been well established. In the present study, we examined whether MRI findings are of value in predicting the degree of knee joint laxity as measured using two typical physical examinations, i.e., the Lachman and pivot shift tests. A few MRI findings were associated with high-grade laxity determined by the physical examinations, suggesting that MRI may be of limited usefulness in predicting the grade of knee laxity. With a shorter time between injury and image acquisition, MRI was more likely to be helpful for predicting high-grade knee laxity caused by ACL injury.
Our analysis of the associations between MRI findings and the grade on physical examinations in patients with symptomatic ACL injury suggests that MRI has limited value for predicting the grade of laxity. Among the four primary findings evaluated, only nonvisualization was associated with a high grade on the Lachman test. As nonvisualization of the ACL on MRI represents a complete absence of the ACL on images, this result is reasonable. However, it is curious that the three other primary findings, particularly discontinuity, had no associations with the grade of clinically assessed laxity given that these MRI findings directly reflect significant abnormalities of the ACL bundles per se. Our findings were in disagreement with the results of a previous study in which the accuracies of discontinuity and abnormal orientation of the ACL on MRI for diagnosis of unstable ACL injury were 79% and 87%, respectively. In this previous study, all of the surgically confirmed complete ACL tears were considered to be unstable ACL injuries, even though clinical examinations were also used to define unstable ACL injuries. In contrast, we used only the results of physical examinations to determine high-grade laxity, regardless of the arthroscopic findings, for comparison with the MRI findings. Several additional factors such as scarring of the torn ACL remnant, concomitant injuries, and bone geometry may influence the degree of stability after ACL injury. Furthermore, MRI findings were shown to introduce bias in the evaluation of cases of chronic ACL tears because a previously torn ACL can heal with scar tissue. Therefore, our findings suggest that with regard to the degree of knee laxity, predictions based only on the primary MRI findings may not be accurate.
Anterior translation of the tibia relative to the femur was the only secondary finding associated with high-grade laxity on physical examinations in the present study. It is possible that signs of anterior translation of the tibia relative to the femur are associated with high-grade laxity after ACL injury; however, even in cases with high-grade ACL injury, anterior translation of the tibia would not be easy to discern because standard MRI is performed with the patient in the supine position. Therefore, the observation of anterior translation of the tibia on MRI would reflect considerable laxity due to ACL injury. Interestingly, anterior translation of the tibia relative to the femur on MRI was significantly associated with the grade on the pivot shift test and not with the grade on the Lachman test. It should be noted that anterior translation of the tibia was measured in the lateral compartment. Several studies have indicated that anterior translation is coupled with internal rotation of the tibia, and ACL deficiency would lead to increased anterolateral subluxation of the tibia on the femoral condyle, especially in the lateral compartment. In other studies, anterolateral translation of the tibia was related to the pivot shift test results. Thus, anterior translation of the tibia relative to the femur measured in the lateral compartment on MRI has a reasonable chance of reflecting the injury grade determined on the pivot shift test.
We hypothesized that the value of MRI for predicting the grade of clinically assessed knee laxity is affected by the MRI acquisition time in patients with symptomatic ACL injury. Even though the predictive value of MRI for laxity was modest in both the early and late MRI groups, it was more significant in the early MRI group than in the late MRI group. We speculate that this finding stems from the biological response to ACL injury, which would lead to a change in the condition of the ACL over time. This is supported by previous studies showing that discontinuity and abnormal signal intensity on MRI can be restored with time and that typical MRI findings can change unpredictably over time.
The present study had some limitations. First, all subjects included in the study were patients requiring ACL reconstruction. Therefore, our findings may not be applicable to patients over the whole spectrum of ACL injuries. However, information on the use of MRI findings for predicting the grade of knee laxity would be of more practical value for patients warranting ACL reconstruction, i.e., those with significant subjective instability caused by trauma as the mechanism of ACL injury. Moreover, several factors other than physical examination results and MRI findings, including the patient's age, activity level, and willingness to undergo surgery, can contribute to the decision regarding ACL reconstruction. Thus, by limiting the study subjects to those requiring ACL reconstruction, it is possible that more practical information was achieved and the influence of uncontrolled factors was reduced. Second, the Lachman and pivot shift tests are not free from interexaminer variability. To overcome this problem, all physical examinations of the patients included in this study were carried out by experienced knee specialists. In addition, dichotomization of the physical examination results might have reduced the variability.
