Many researchers have discussed whether long-distance running causes injury to the knee joint[15–17]. Research on the impact of long-distance running on the knee joint has been carried out using conventional MRI analysis describing the structural changes of the knee joints before and after marathons[18, 19]. It has been found that there is no significant change in the ligament MRI signals after a marathon. Some non-professional marathon athletes have increased joint effusion, meniscus, and signs of bone marrow damage. These changes returned to the initial level by 6–8 weeks. No apparent evidence suggests that long-distance running causes significant damage to the knee or knee arthritis. However, conventional MRI can only evaluate cartilage morphology. T2 mapping imaging can quantitatively analyze the biochemical components of cartilage by measuring the T2 value of cartilage and provide imaging basis for early diagnosis of cartilage injury at the molecular level [20]. In addition, in most studies, only experienced professional marathon athletes have been included, and the impact of running full marathons on the knee joints of non-professional marathon enthusiasts has not been clearly established[21–23]. The T2 value is related to the content of free water in the cartilage and to the arrangement of cartilage collagen fibers. If collagen fibers disintegrate, irreversible cartilage damage occurs, resulting in an increase in the T2 value[24–26]. Long-distance running can cause biochemical changes in the articular cartilage. In this study, the T2 value of the knee joint cartilage of non-professional marathon runners was determined, which indirectly reflected the changes in the biochemical components of the articular cartilage before and after running. It was found that there biochemical changes in the articular cartilage to a certain extent, which may be related to the changes in the collagen network structure and water content, but there was morphological abnormality of the knee joint after the marathon.
As an interstitial fluid, water provides certain hardness to the articular cartilage. During weight-bearing, water is squeezed into the articular cavity from the cartilage matrix to lubricate the articular surface. With the decrease in water content in the matrix, the T2 time of the cartilage decreases. The T2 time of the cartilage is related to the distribution of water content[27]. The T2 relaxation time map confirms that the T2 time of cartilage decreases with the decrease in water content [28]. The results of our study showed that the T2 values of the articular cartilage of the knee joint, including those of the medial and lateral tibial plateau of the left knee joint and the lateral tibial plateau and the lateral femoral condyle of the right knee were significantly decreased after the marathon (P < 0.05). The lateral femoral condyle and medial tibial plateau also had a decreasing trend (P > 0.05). Many studies have pointed out that the T2 time of the load-bearing part of the knee joint cartilage in healthy people decreases to varying degrees before and after exercise [29, 30]. Mosher et al. showed that the cartilage of subjects of different ages have different degrees of thinning after running, which may be related to the decrease of water content in the cartilage due to its compression[29]. From the T2 relaxation time diagram we obtained, the orange red pixels representing lower T2 times increased, while the green pixels representing higher T2 times decreased.
In addition, we found that the T2 value of the right knee medial femoral condyle cartilage after exercise was significantly higher than before exercise, while that of the left knee was also higher than before exercise, but there was no statistical significance. This may be related to the form of exercise. Some studies have shown that among the three forms of activity, walking, running, and climbing stairs, running has the greatest impact on the femoral cartilage [25]. Hinterwimmer et al. studied the effect of six-month marathon training on the volume and thickness of the knee joint cartilage of marathon beginners and found that the volume and thickness of the femoral condyle cartilage decreased significantly, while the volume and thickness of other cartilage subsets did not [31]. Therefore, in the process of running, the cartilage of the femoral condyle of both knees bears greater stress, which may damage the collagen network structure, change the arrangement of collagen fibers, and increase water permeability. At the same time, collagen fracture causes the accumulated proteoglycan to expand, dispersing and exposing more anions, thus further increasing the content of water in the cartilage. Because of the increase in cartilage water content, T2 time distribution shows an opposite change[32].
The T2 time was also sensitive to the changes in collagen fiber structure. Shinar et al. considered that the change in collagen fiber direction was the main factor that leads to the decrease in the T2 time[33]. At present, there are few reports on MRI detection of changes in the cartilage collagen fiber network after exercise. Generally, the cartilage is divided into different regions according to the loading area and non-loading areas of the joint, but there is no specific standard for this division. In our study, according to whether the articular cartilage is covered by a meniscus or not, and based on the load, the knee joint cartilage was divided into eight different points for measurement. The results showed that the T2 values of loci L2, L4, L5, L6, L8, R2, R5, and R8 decreases after the marathon; the T2 values of R3 increased; while the T2 values of loci L1, L3, L7, R1, R4, R6, and R7 did not significantly differ before and after the competition. This may be due to the complex stress-strain process of the cartilage. Cartilage microdeformation is not only related to matrix fiber arrangement and cartilage water content but also stress structure changes to the subchondral bone and to the flow of the cartilage matrix[32]. In addition, it has been shown that short-time jogging, in opposition to marathon running, is not enough to cause obvious changes in the collagen fiber structure[34]; thus, this may explain why there are only slight morphological changes in normal cartilage before and after loading. Moreover, there may be a more complex correlation between the changes of collagen fiber structure and the changes in the T2 time, which need further research and demonstration[34].
We believe that marathon running, as an extreme endurance sport, may damage the local cartilage of the knee joint of non-professional marathon runners in the short term. We have shown, for the first time, that these changes can be detected in amateur marathon runners by T2 MRI mapping. However, the mechanism of articular cartilage change after exercise is very complex. T2 relaxation time maps only measure the change in T2 time at two-time nodes before and after exercise to evaluate the change in cartilage volume. However, the mechanism of water leakage, the influence of the collagen fiber type, and the process of gradual recovery after exercise are not clear. Whether marathon exercise can cause permanent changes in the biochemical properties of the cartilage remain to be observed.
There are some limitations to this study: the sample size was relatively small; the follow-up time after the marathon was relatively short; and some cartilage subregions were not recovered to the precompetition level; thus, in future studies, the follow-up time should be extended to evaluate the changes in the knee joint soft bone more accurately.