Introduction

Meniscal root tears are increasingly recognized as the ‘root cause’ of osteoarthritis (OA) and considered a previously unrecognized ‘silent epidemic’ ¹. Medial meniscus posterior root tears (MMPRTs) in particular, are one of the strongest risk factors for the development and progression of knee OA². Meniscal tears cause meniscal extrusion³. This is thought to lead to accelerated cartilage breakdown². Previous studies of T2 and T2* relaxation time mapping demonstrated a strong correlation with collagen architecture and histologic zones of articular cartilage^{4, 5}. The aim of this study was to utilize the signal-to-noise ratio (SNR) and resolution gains at 7T for the evaluation of compositional changes in the cartilage of patients with arthroscopically verified MMPRTs using 3D T2* mapping. We hypothesize that the T2* values will increase in the cartilage of the patients, suggestive of degenerative changes in the tissue due to the MMPRTs.

Methods

This is an IRB approved and HIPPA compliant study. Ten patients with unilateral MMPRTs (mean age: 52 years; age range: 36-62 years; mean body mass index (BMI): 28 kg/m2, 8 females) and five asymptomatic controls (mean age: 48 years; age range: 34-60 years; mean BMI: 28 kg/m2, 3 females) were included in the study. MRI was conducted on a 7T system using a birdcage transmit and 28-channel receive phased-array knee coil. The MRI protocol included morphological sequences and a quantitative 3D multi-echo gradient recalled echo (GRE) sequence with seven echo times (TE) between 3.1 and 21.4 ms (Fig 1). The T2* maps of the articular cartilage were calculated by fitting a mono-exponential signal decay of the multi-echo T2* data with a two-parametric least-square fitting routine in Matlab. The fitting accuracy was evaluated by calculating the root mean square error (RMSE) normalized to the estimated signal intensity at the echo time of 0 ms. The 3D segmentation of the medial and the lateral femoral and tibial cartilage was manually performed on 120 high resolution T2*-weighted images using ITK-SNAP. Each compartment of the femoral cartilage was subdivided into lateral and medial regions and then into four subregions: anterior weight bearing (anterior WB), central WB, and posterior WB and posterior regions⁶ (Fig. 2). A linear mixed effect model was used to compare the T2* values of the cartilage regions between the controls, and the corresponding regions in the MMPRT patients. Extrusion of the medial and lateral menisci in patients and controls was measured by three musculoskeletal radiologists, blinded to T2* results, as described previously⁷. Arthroscopic evaluation at the time of meniscal repair confirmed the presence of MMPRT in all patients and cartilage was intraoperatively assessed.

Results

Quantitative T2* evaluation: Longer T2* relaxation times were found in the distal femoral articular cartilage in the central WB (P < 0.05) and posterior WB (P < 0.01) regions in both the medial and lateral femoral compartments. In addition, the posterior region of the lateral femoral condyle revealed significantly higher T2* values (P < 0.05) when compared to the same regions in the controls (Figure 3). The highest differences were found in the posterior WB region of the medial femoral condyle. No statistical differences were found in the medial and lateral tibial plateau. The mean normalised RMSE was below 5% for all the analysed regions, which demonstrates a good reliability of the fitting results. Meniscus extrusion: Positive extrusion criteria were met in 10 patients medial meniscus, criteria were not met in in lateral menisci and control group. Arthroscopic evaluation: Arthroscopy evaluation revealed grades I to III cartilage lesions on the femoral, tibial and patellar cartilage. MMPRT was confirmed in all patients.

Discussion

In the medial and lateral knee joint compartment, elevated T2* values were found in the weight bearing areas of articular cartilage in patients with MMPRT’s. Previous studies of T2 and T2* relaxation time mapping demonstrated a strong correlation with alteration of collagen architecture and association with osteoarthritis^{8, 9 10.} It has been shown that biomechanical consequences of a posterior root tear in the medial meniscus are similar to total meniscectomy, which significantly increases articular cartilage contact pressure in the medial as well as the lateral compartment¹¹. Our findings of prolonged T2* relaxation times in both, the medial and lateral compartments in patients emphasise the significant impact of the medial meniscal posterior root tear on the entire knee joint health.

Conclusion

The results support the hypothesis that MMPHRT’s are the 'root cause' of OA as evidenced by prolonged cartilage T2* relaxation times in patients. Thus, identifying early cartilage degeneration could inform timely treatments, potentially preventing the onset of OA.

Acknowledgements

National Institute of Biomedical Imaging and Bioengineering (P41 EB027061: Technology to Realize the Full Potential of UHF MRI), S10 OD025256 (7 Tesla Terra Instrument) and R01EB034575.

References

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