The results of this study show that subjects with knee abnormalities related to OA have significantly increased meniscal extrusion under loading conditions compared to normal subjects. We could also demonstrate that subjects with knee abnormalities and higher KL Scores showed significantly more changes of lesions, signal and shape in meniscus, ligaments and cartilage under loading conditions.
Our results agree with previous studies in which changes of meniscus and cartilage under loading conditions were described [4
]. However, in addition we also studied the effect of knee joint degeneration under direct loading conditions at 3 T comparing subjects with osteoarthritis based on the KL-score with healthy volunteers.
Load between the femur and tibia generates a compressive force on the meniscus which, with its wedge-shaped cross-section, results in an outwardly directed radial vector [6
]. Tibesku et al. [4
] evaluated the meniscal movement and deformation in vivo under load bearing conditions. They examined 15 healthy knees and found out, that the inner and outer distance increased with load. The inner distance increased more than the outer, resulting in a compression of the periphery. Vedi et al. [6
] also examined the meniscal movement in vivo using a vertical open magnet MR scanner in normal knees under load in 16 young footballers. These authors compared erect weight-bearing regions with sitting non-weight-bearing regions. In the current study, patients had the same, supine, knee position during loading and unloading conditions. In contrast to our results, their most significant differences between weight-bearing and non-weight-bearing were the movement and vertical height of the anterior horn of the lateral meniscus, but these investigators focused mainly on sagittal images.
Boxheimer et al. [8
] examined asymptomatic volunteers wit a 0.5 T open configuration MR-System. They obtained coronal and sagittal images with the knee supine in neutral, supine in 90° flexion with external and internal rotation, as well as in upright weight-bearing position. They showed, meniscal movement was most prominent in the anterior horn of the medial meniscus with the knee in the supine position in 90° flexion with external rotation. In accordance to our results, meniscal protrusion was more frequently present in the medial meniscus and averaged less than 3 mm in normal volunteers. In another study Boxheimer et al. [9
] examined patients with meniscal tears with this setting. Between the different knee positions, meniscal displacement of 3 mm or more was noted in 42% of menisci with tears. Displaced menisci most commonly had complex, radial, or longitudinal tear configurations. Our results also showed significantly higher meniscus extrusion when the meniscus had tears (WORMS 2–4).
Mastrokalos et al. performed an in vivo study and examined fifteen knees of healthy young volunteers [19
]. They analyzed the changes of the internal and external meniscal interhorn distance of the medial and lateral meniscus (minimum and maximum distance between anterior and posterior horn) under loading and with and without flexion. They concluded that loading increases the internal and external meniscal interhorn distance. This correlates to our findings of significant extrusion of the medial meniscus under loading.
All of these 4 investigators used low field MR scanners with a field strength varying between 0.18 and 0.5 T. Our study is the first study which used higher field strengths (3 T) under controlled loading conditions allowing enhanced anatomical visualization of the knee joint structures, in particular the cartilage. This may also in part explain the higher number of loading associated changes for cartilage and menisci shown in this study. Previous in vivo and in vitro studies have demonstrated superior visualization of cartilage, ligaments and meniscus and other small structures at 3 T vs. 1.5 T [20
]. Wong et al. demonstrated [20
] an increase in sensitivity and diagnostic performance observed at 3 T for cartilage lesion detection of the knee in vivo and Masi et al. [24
] demonstrated improved diagnostic performance at 3 T vs. 1.5 T MRI for cartilage lesions in a porcine model.
Investigators also assessed the dynamic response of cartilage thickness or volume, as well as changes of the meniscus after loading, with repeated knee bending and running [10
]. Eckstein et al. [11
] studied the in vivo deformation behavior of patellar and femorotibial cartilage for different types of physiological activities. They examined 12 volunteers after physical activity such as running (5 min), knee bends (90 s) and cycling (10 min) and 10 volunteers after knee bends on one feet, static compression (2 min static loading of one leg with 200% body weight) and high impact loading (10 jumps from a chair (40 cm) onto one leg) resulting in deformation of femoro-tibial cartilage. It was reported that patellar cartilage deformation showed a dose dependent response more intense loading leading to greater in vivo surface to surface strains. In the femoro-tibial joint little deformation was observed except during high impact activities, highly significant changes were seen in the medial and lateral tibia after jumps. and
Fig. 3 Representative MR images show the medial femoral and tibial cartilage before (a) and during loading conditions (b). There is a change of the cartilage lesion grading. A questionable superficial, partial thickness cartilage lesion at the femoral condyle (more ...)
Representative MR images show the posterior crucial ligament before (a) and during loading conditions (b). There is a change in shape of the ligament under loading with the PCL being more extended.
Kessler et al. [12
] examined 48 male athletes before and after running (51,020 km) with MRI. Tibial, patellar and meniscal volumes showed significant reductions after running. In another study [13
], they examined 20 knees of male athletes before and after a 20 km run and after a recovery period of 1 h. The subjects showed a volume reduction of the tibial cartilage of 5.1% and 8.2% of meniscus volume after the run. This study also showed that not all cartilage and meniscus volume changes induced by the 20-km run were fully restored after a 1 h rest period. It was noted that the recovery of the medial meniscus lagged behind the other structures.
In addition to assessing cartilage pathology, thickness and volume, recent studies have shown the potential of MR imaging parameters to reflect changes in the biochemical composition of cartilage under loading [10
]. T2 relaxation time mapping is currently most frequently used to study cartilage biochemical composition: it is sensitive to a wide range of water interactions in tissue and in particular depends on the content, orientation and anisotropy of collagen [26
]. Mosher et al. [10
] examined knee cartilage T2 values of seven subjects before and immediately after 30 min of running. There was a statistically significant decrease in T2 of the superficial 40% of weight-bearing femoral cartilage after exercise. These investigators assumed that cartilage compression might result in greater anisotropy of superficial collagen fibers. In our study we only examined the morphological changes under loading. Changes of the meniscus and cartilage matrix due to loading may add additional information in better understanding the evolution of osteoarthritis and need further investigation.
In conclusion our study demonstrated a relationship between OA and loading related changes of meniscus, cartilage and ligaments in individuals with OA compared with healthy subjects. Patients with radiographic and MR signs of OA had significantly increased meniscal extrusion under loading conditions compared to normal subjects and significantly more loading induced changes of signal and shape of the menisci, ligaments and cartilage as well as changes in pre-existing lesions. These findings suggest that in vivo loading may be a valuable tool to evaluate tissue degeneration in the evolution of OA.