Although the Thessaly test confronts some challenges, we were compelled to investigate direct morphological variation taking place at the medial meniscus under various loading phases of the Thessaly test and to better understand various degrees of MME. The Thessaly test at 20° of flexion showed highest diagnostic accuracy level of 94% in the diagnosis of medial meniscal tears 9 and our hypothesis was that the greatest degree of MME would be observed at 20° knee flexion.
Our data also showed that the mean MME in healthy normal subjects was 2.3 ± 0.5 mm for the supine and 2.8 ± 0.8 mm for the standing positions. This result conflicts with prior work, where MME of > 3mm3,4 or > 2mm5 has shown association with osteoarthrosis, cartilage loss, and medial meniscal tear. Despite prior conclusions of strong associations between MME to various meniscal pathologies, Achtnich and colleagues proposed that the current cut-off value of 3mm for meniscal pathologies be reconsidered 11. Although they demonstrated normal physiological degrees of MME, there are some measurement differences compared to our current study (Supine: 2.3mm ± 0.5 vs. Achtnich et al. 1.1mm ± 0.5; 20° flexion: 1.9mm ± 0.8 (IR), 2.3mm ± 0.7 (ER) vs. Achtnich et al. 1.9mm ± 0.9). The difference in the measurement is likely due to subtle differences in the measurement reference points and interexaminer variability. In our study, a line was drawn connecting the free edges of the medial tibial cortex to the medial femoral cortex, whereas in Achtnich and colleagues, a tangential line was drawn on the medial tibia without connecting to the femur. Another difference is the addition of internal and external rotation to the 20° flexion position in our study.
The most noteworthy findings from our study were observed during supine to standing where largest degree of increase in mean MME was noted (refer to Fig. 2), while going from standing to 20° of flexion IR resulted in largest degree of decrease (refer to Fig. 3). We found that the mean MME at 20° flexion IR demonstrated the smallest degree of MME; initially, least amount at first glance, least amount of stress applied across the meniscus was assumed. However, upon review of the literature 17–21, a reasonable explanation for this paradoxical reduction of the mean MME at 20° of flexion was found. Both medial and lateral menisci are roughly wedge-shaped in their short axis and with a semilunar longitudinal morphology. It is this wedge-shaped meniscal morphology that converts the vertical load to circumferential tensile loads as the shear forces develop within the menisci, deforming it radially 17. While the knee undergoes flexion, the menisci conforms to the geometry of the femoral condyle18. In flexion, the posterior femoral condyle is in contact with the tibial plateau, which structurally has lesser radii of curvature compared to the contact point of the femoral condyle at extension (i.e. standing), resulting in a decreased contact area18. Understanding that the stresses are inversely proportional to the contact area, flexion distributes larger stress across the meniscus during flexion. As a result of the different parts of the femoral condyle contacting the tibial plateau during extension and flexion, the medial meniscus withstands posterior displacement while the femoral condyle rolls on the tibial plateau during flexion18,21 The degree of MME could be an indirect measure of assessing the contact area/force relationship applied at the meniscus. These findings reinforce the reason for significant decrease in the mean MME during 20° of flexion which initially was hypothesized to demonstrate the largest degree of MME from greatest applied stress across the meniscus.
Our study demonstrated physiological variations of the medial meniscus during the Thessaly test. Although it does not assume any further diagnostic ability of the Thessaly test, the largest degree of reduction in MME occurred at 20° flexion with IR, which may implicate that this loading phase places most degree of stress among all loading phases. The degree of the MME varies depending on the dynamic phases of the Thessaly test; therefore, the absolute degrees of the extrusion during different loading phases should be investigated in pathologic knees. Future investigation should consider correlating MME variability between age, weight, and gender differences. Further, natural history of physiologically ‘larger’ MME and its progression to osteoarthrosis or tear is still unknown and future investigation is needed.
One of the limitations of the study is the measurement site for the MME. MCL was used as the sole landmark for measurements, thus the posteriorly displaced meniscus could not have been observed during knee flexion. Another limitation is considerable experience difference between the examiners (10 years vs. 1 year) measuring the MME. Although interexaminer reliability was poor to moderate, the actual difference between the measurement points were less than 1/10mm, which likely has little clinical impact.