In this study, radiologic changes of BMEL and cartilage were quantitatively evaluated in ACL-reconstructed knees following acute ACL tears longitudinally over a one-year period. A one-year study period was used because we wanted to evaluate relatively acute changes in cartilage composition following ACL-injury and determine if T1ρ was capable of detecting these early changes. Local cartilage organization was assessed by evaluating T1ρ values of the full thickness, deep and superficial layers of cartilage overlying and surrounding BMEL. In this study, we have demonstrated that T1ρ is capable of detecting cartilage changes in acute ACL-injured knees as early as one-year post-reconstruction. To the best of our knowledge, this is the first imaging study that quantitatively and longitudinally evaluates cartilage matrix composition in ACL-injured knees.
Similar to previous studies(15
), we found the highest prevalence of BMEL in the lateral tibia and lateral femoral condyle in acutely injured knees. Lesions in the medial side of the knee were uncommon. While the acute quantitative characteristics of BMEL were unique to the individual knee compartments, the lesions were found to improve over 1 year in all four knee compartments, as indicated by a reduction in the lesions' volume and relative signal intensity. These observations are consistent with a recent MRI study of ACL-injured knees that reported that BMEL gradually decreased over the first year after ACL-injury(38
); although the reports in the literature regarding the natural history of BMEL have been variable(15
)). Two earlier studies suggested that these lesions are not seen in patients who had MRIs acquired six weeks(16
) or six months(39
) post-injury. More recent work by Davies et al. suggested that BMEL may persist after 12-14 weeks post-injury using Short TI Inversion Recovery images(17
). Our results in conjunction with these previous studies suggest that the initial trauma experienced by the trabeculae in each knee compartment may be different, but universally transient.
While BMEL improve over time, it is postulated that the cartilage overlying the lesions experiences irreversible damage during the initial injury(20
). In a previous study, Johnson et al.(20
) collected samples of cartilage in patients with acute ACL ruptures and observed biochemical variations and histologic changes (chondrocyte and matrix degeneration) in the cartilage overlying BMEL. In this study, we found that the full thickness of the overlying cartilage in the lateral tibia, medial tibia, and medial femoral condyle had elevated T1ρ
values relative to the surrounding cartilage at baseline and subsequently at every follow up time point, with significant differences found only in the lateral tibia. These results suggest that the overlying cartilage in the lateral tibia, medial tibia, and medial femoral condyle do have damage despite resolution of BMEL. We also found that the superficial layer of the overlying cartilage in the lateral tibia, medial tibia, and medial femoral condyle had higher T1ρ
values relative to the surrounding cartilage's superficial layer throughout the study period, and the differences were significant in the lateral tibia at baseline, 2 weeks, and 6 months. The deep layers of the overlying cartilage were also found to have greater T1ρ
values than the surrounding cartilage's deep layer for the entire study period, although none of the differences were significant. While the authors did not measure cartilage thickness over the 1-year time period to assess cartilage volume loss, a recent study showed no significant difference in cartilage thickness after 7 years following ACL reconstruction (41
), so the authors don't expect any change in morphology for this study's cohort.
In the lateral femoral condyle, the relationship between the overlying and surrounding cartilage was the reverse of the finding in the lateral tibia, medial tibia, and medial femoral condyle, as the surrounding cartilage had significantly higher T1ρ
values than the overlying cartilage. This may be due to the observation that BMEL in the lateral femoral condyle were normally seen in the weight-bearing regions (cartilage overlying the anterior horn of the meniscus and the cartilage between the anterior and posterior horns of the meniscus), which are reported to have significantly lower T1ρ
values than non-weight-bearing regions (most anterior and posterior regions of cartilage not protected by menisci in the femur)(32
). This spatial variation of T1ρ
values in weight-bearing vs. non-weight bearing regions in femoral condyles can be attributed to potential different hydration and macromolecular structure caused by mechanical compression, as well as the potential dependence of T1ρ
values on the orientation of the cartilage extracellular matrix with respect to the main magnetic field. Further experiments are needed to take spatial variation into account during comparison of T1ρ
values between overlying cartilage and surrounding cartilage in the lateral femoral condyle.
BMEL are proposed to represent the trauma experienced by the joint surfaces at the moment of ACL injury(1
). Frobell et al. report that knees that have suffered more severe injuries, i.e. with cortical depression fractures, have larger lesions(1
). Additionally, two animal studies suggested that cell death occurs in the impacted cartilage, and that it spreads in depth and width with increasing stress forces(45
). Thus, size and macromolecular characteristics of BMEL may play a role in predicting cartilage matrix injury longitudinally. By comparing each patient's overlying cartilage T1ρ
values to their surrounding cartilage values, we are able to assess the severity of each individual patient's injury more accurately. In this study, we found that the BMEL volume was correlated with the percent increase in T1ρ
values of the overlying cartilage relative to the surrounding cartilage in the lateral tibia. No correlation was found between BMEL characteristics and the T1ρ
values of the overlying cartilage in the lateral femoral condyle and statistical comparisons could not be calculated accurately for the medial tibia and medial femoral condyle given few numbers of lesions in both compartments. Therefore, these results suggest that the volume of a patient's bone marrow edema-like lesion in the lateral tibia may be used to predict the degree of damage to the overlying cartilage in the lateral tibia. The long-term goal would be to use these data to predict a patient's risk for developing secondary osteoarthritis. However, longer follow-up timepoints with arthritis measurements and evaluation of cartilage injury persistence and subsequent osteoarthritis development in all four knee compartments are warranted to accomplish this goal.
While many of the results described herein are promising, a number of limitations to this study exist. One limitation of this study is that the deep and superficial cartilage layers analyzed above do not correlate with the three physiological layers of cartilage: tangential, intermediate, and radiate. While the two layers were used instead of three because of limited resolution, it is possible that the deep and superficial layers analyzed also have limited resolution. Additional limitations of this study include the relatively short length of follow-up and the small number of patients evaluated, and no alignment data and arthroscopic evaluation were collected. One year following ACL-injury is a relatively short length of time, and it is possible that the observed aforementioned T1ρ signal changes may improve over time. Additionally, given the few number of patients and the infrequency of BMEL in the medial compartments of the knee relative to the lateral compartments, it is difficult to accurately evaluate the BMEL and associated cartilage in the medial tibia and medial femoral condyle. Another limitation of the study is that the T1ρ signal changes noted in this study were not correlated to standard MRI or to arthroscopic appearance of the cartilage. Thus, the results from this study should invite future clinical outcome studies with larger cohorts of patients and with longer follow up timepoints and include correlations between T1ρ values and arthroscopic cartilage measurements.