Abstract
Objectives:
Anterior cruciate ligament (ACL) injury is a frequent knee injury that commonly results in post-traumatic osteoarthritis (PTOA). ACL injury has been shown to increase the risk of osteoarthritis (OA) over eightfold within 11 years following injury. While the development of OA is well documented, the mechanism by which this accelerated cartilage degradation occurs is not well understood. The ACL is crucial in providing anterior-posterior and rotational stability of the tibia relative to the femur. Our previous work has demonstrated that ACLR does not restore pathologic anterior tibial translation (ATT) in the lateral or medial tibiofemoral compartment to levels present in uninjured knee. Increased static ATT, specifically the lateral tibial plateau on static magnetic resonance imaging (MRI) in ACL-deficient knees is associated with rotary instability. However’ how ATT relates to clinical outcomes, specifically the development of PTOA, has not been well studied.
The objective of this study was two-fold. (1) Compare T2 relaxation times of cartilage in injured knees [a measure of water content and collagen organization of articular cartilage that indicates early cartilage breakdown] 6 months after ACL reconstruction (ACLR); (2) investigate whether greater ATT associates with longer (worse) T2 relaxation times in the lateral and medial compartment. We hypothesized that T2 relaxation times would increase (worsen) from before to 6-months post-operatively, and would be positively correlated with greater amounts of ATT.
Methods:
Twenty-nine participants between the ages of 15 and 35 were enrolled within one month of ACL injury but prior to ACLR. Individuals older than 35 were excluded due to possible baseline cartilage degeneration. Other exclusion criteria included previous injury or surgery to either knee, acute chondral lesions, concomitate Grade III tear to other ligaments of the knee, or open growth plates requiring altered ACLR technique.
Bilateral knee quantitative MRI data were acquired on a 3-Tesla Phillips Ingenia MRI scanner T2 relaxation maps were generated by using multi-echo MRI data at each signal to fit the signal equation using Levenberg-Marquardt nonlinear least squares algorithm (Si = the signal at echo time TEi, and S0 = signal at TE = 0) within Interactive Data Language (Harris Geospatial Solutions Inc.). Cartilage segmentation was manually segmented using ITK-SNAP software (Figure 1)
The medial and lateral tibial (MTC, LTC) and femoral (MFC, LFC) condyles were divided into three weightbearing regions of interest (ROI) related to the meniscal horns in the sagittal plane (figure 1). Accuracy of segmentation and ROI boundaries were confirmed by a bord-certified, fellowship-Trained musculoskeletal radiologist.
ATT was measured on MRI using the validated method described by Iwaki et al. A midsagittal point was identified halfway between the posterior cruciate ligament (PCL) insertion and the lateral edge of the LTC was identified on sagittal MRI. At the midsagittal point, a vertical line was drawn off the posterior cortex of the tibia, perpendicular to the plateau. A best-fit circle was then drawn over the posterior LFC and a second line perpendicular to the LTC was drawn off the posterior margin of the best fit circle. The distance between the two vertical lines was used to calculate ATT.
Paired t tests were used to determine if T2 relaxation times differed in the injured limb from before to 6 months after ACLR. Pearson correlations were used to determine the relationship in each weightbearing ROI between 1) change in ATT and percent change in T2 relaxation times from before to 6 months after ACLR, and 2) between-limb difference in ATT before ACLR and percent change in T2 relaxation time.
Results:
Demographic data including age, sex, race, and pre-injury cutting and pivoting sport participation are presented in Table 1. Twenty-six (89.7%) individuals participated in level one activities (i.e. basketball, soccer) prior to injury. T2 relaxation times increased (worsened) in all weightbearing ROIs from before to 6 months after ACLR except the posterior weightbearing region in the LFC (p=0.296) and the central weightbearing region in the LTC (0.470). Neither the postoperative change in ATT nor the difference in ATT between injured and uninjured limbs before ACLR were significantly correlated with increase in T2 relaxation times (Table 3).
Conclusions:
The results of this study support our hypothesis that T2 relaxation times would be significantly longer, representing cartilage degradation, after ACLR, even at an early time point of 6 months post-operatively. However, greater ATT in ACL-injured knees remaining at 6 months after ACLR as well as relative to the uninjured limb before ACLR was not correlated with changes in T2 relaxation times in the weightbearing cartilage of the femur and tibia. Future work is needed to identify possible dynamic changes in cartilage loading related to early OA development following ACL injury and reconstruction.
