The major discovery of our study was the correlation of the geometrical parameters of the knee and the meniscal injuries. To be specific, the decrease of NWI and medial spine height was significantly correlated with the increased risk of meniscal injury compared with the control group. Moreover, NWI and medial spine height are also significantly related to the severity of the meniscal injury. Although statistical significance was not found in grade 1 and grade 3 comparing with the control group, the result showed lower NWI in these two groups. Furthermore, we also confirmed the meniscal injury was correlated with sex and age. Ordinal regression also verified the relationship between genders, and age, which is related to the severity of the meniscal injury.
The findings of our study reveal that meniscal injury and its grade were associated with NW and NWI, which are similar to the findings in previous results on ACL injuries (12, 14). In 1938, Palmer first recognized the narrowness of intercondylar was associated with the ACL injury (28). Souryal et al. portrayed a method of measuring the intercondylar width, the NWI on plain radiograph (13), and found the correlation between the stenosis of the intercondylar notch and the injury of ACL. Previously studies have concluded that the femoral NW and NWI, as two-dimensional parameters, can evaluate the size of the femoral notch effectively (29). Previous studies demonstrated that the stenosis of intercondylar notch increasing the risk of ACL injury (12, 14). A narrow space of intercondylar notch tends to house a relatively small volume ACL, which the strength of the ligament was decreased and consequently led to the predisposition of ACL injury (19, 20). As the knee over-bent or rotated, a narrow intercondylar notch tends to lead to impact between the lateral wall of the femoral intercondylar and ACL. This phenomenon was more evident when the knee was externally rotated or at the position of flexion-valgus. As the impact of ACL harms the fiber bundles, the intensity of ACL decreased and prone to be injured (19, 20). On the other hand, the correlation between the NW or NWI and the risk of ACL injury was controversial (12, 30). Some studies showed no significant relationship between the narrow intercondylar notch and the risk of ACL injury (16, 31). A meta-analysis conducted by Li et al. showed the NW was significantly narrower in ACL injury cases, and the result was consistent with different ethnicity and sex (32). However, the NW varies a lot among patients, and the method using for measurement can generate discrepancy. NWI can be better represented the volume of the intercondylar notch by excluding the difference in height, weight, individuals, and measurements. However, as age increases, the NWI decreased, as Domzalski et al. reported (26). With different ethics and different measurements, the cut-off value for NWI reported in previous studies was different from 0.18 to 0.20 (13, 14, 26).
The tibial spine is located at the center of the tibial femoral articular surface through weight-bearing activity. The medial tibial spun has the highest contact pressure in a load-bearing knee. Previous studies described the anatomical relationship between the ACL and the tibial spine (33, 34). Oka et al. (33) found the anterior part of the medial tibial spine was attached by the medial margin of the ACL. A similar result was reported by Tensho et al. (34) using 3D-CT. They concluded that some ACL fibers were connected with the medial tibial spine, whereas no similar connection was found in the lateral tibial spine. McDonald et al. (35) observed that the intersegmental load conveyed between the medial femoral condyle and the ipsilateral tibial spine significantly increased. Levins et al. (36) also found the morphology of both medial and lateral tibial spine can influence ACL injury. The decrease in the height of tibial spines leads to the increase in anterior translation and the internal rotation of the tibia, subsequently increase the strain of ACL and finally lead to the injury of ACL (37). A similar conclusion was also found by Sturnick et al. (38), and the decreased medial spine on height males increased the risk of ACL injury, whereas a significant correlation was not observed on females. Similar to the results from studies in ACL injuries, we found the decrease of medial tibial spine height can increase the risk of meniscal injuries. And in patients with grade 2 meniscal injury, the height of the medial tibial spine is significantly decreased compared to another degree of injury and the control group.
The intercondylar angle was another parameter that described the intercondylar notch. However, the investigation of intercondylar angle was poor in ACL-related papers. Alentorn et al. (17) found the decrease of intercondylar angle would increase the risk of ACL injuries, and they suggested a 50° of cut-off value. The same cut-off value was accepted by Stein et al. (18) but they found no association between the angle and the risk of ACL injuries. Alentorn et al. found the intercondylar notch angle was significantly narrower in ACL injury patients. Therefore, they considered the intercondylar angle as a more useful parameter to describe the narrow intercondylar notch (39). A similar result was concluded by Raja et al. (40). In this study, we found intercondylar angle did not associate with meniscal injury.
Females are apt to have ACL injuries than males, and the anatomical structures were different in females compared to males (41). The results of Wolters et al. (42) showed a narrower intercondylar notch in women, whereas results from Eck et al. (43) concluded that there were no differences in NWI between genders. In addition, the risk of ACL injury increased with age, as the results from Snoeker et al. (44). The difference in age and gender can also be found in the meniscus injury. The prevalence of meniscal injury increased with the age of patients (45). On the other hand, the incidence of acute meniscal injury decreased with age (46). The difference between gender in patients with meniscal injury remains controversial. In athletes, the meniscal injury was more easily found in males than females (47). In contrast, female athletes have a higher risk for medial meniscus posterior root tears (48). In our study, we found the number of female patients is larger than male, although no significant difference was found between the groups and the severity of the injury. However, the opposite result was found after analyzed by different severity of the injury. It may occur due to the relatively small amount of patients in each grade of injury and led to potential bias.
The ACL and meniscus have inseparable correlations when it comes to the injury of the knee (2–6, 12). Meniscus tears have been reported in 40%-82% of ACL tears, and the medial meniscus was more likely to be injured compared with the lateral meniscus (45). Previous studies considered the medial meniscus also has a restraining effect on the anterior tibial translation ACL. Shybut et al. (4) found the tibial translation changed increased significantly with ACL-deficiency knee, which can lead to the injury of the meniscus. With the deficient ACL, patients were prone to have larger internal rotation, and the meniscal translation increased compared to in the intact state (5). Levy et al. showed that compared to the lateral meniscus, the medial meniscus has a significant posterior wedge effect and is firmly connected to the tibial plateau with capsular attachments (2). After performing lateral meniscectomy in an ACL-deficient knee, the anterior tibial translation insignificantly increased (3). Similar to the results of Levy et al., multiple prior studies have concluded the medial meniscus was a secondary stabilizer to ACL at the process of anterior translation (8, 9, 12). After studying cadavers with both lateral and medial meniscectomies, Musahl et al. (49) found medial meniscus played a more important role in restraining the anterior tibial translation but have no effect on pivot shift. In comparison, lateral meniscus exerted its effect on preventing rotational disability and cannot inhibit the anterior tibial translation. Arner et al. (5) found that with the decreased strength of ACL, the lateral meniscus had more mobility and are more likely to injure. The lateral meniscus tears often presented at the acute stage of ACL injury, whereas the medial meniscus tears were more likely to develop at the chronic stage. This can be explained by the greater translation in the lateral meniscus and the greater stress conducted to the medial meniscus (6). On the other hand, the injury of the meniscus can also harm the stability of the ACL-deficiency knee. Shybut et al. (4) found the meniscal posterior root tears can further decrease the stability of the knee with ACL deficiency. The underlying mechanism of that was increased pivot-shift instability in those with injury of the lateral meniscus. The position of the meniscus could be altered after ACL reconstruction, which was reported in multiple studies (50, 51) and the ACL reconstruction can also restore the abnormal biomechanics such as meniscal shift (9). These findings indicate the injury of the meniscus and the pathological extrusion were closely associated with ACL and can be influenced by the abnormality of the ACL.
Our studies have several limitations. First, our study measured the notch parameters and the tibial spine on segments of MRI, which only represent the intercondylar notch dimension at one slice. It cannot fully embody the overall volume of the intercondylar notch. Although a previous study found the two-dimensional measurement can effectively evaluate the volume of intercondylar notch (29), this measurement can lead to potential bias. However, due to the limitation of technology, the application of a three-dimensional measurement is restricted. Besides, Our study was a retrospective case-control study, and the imaging data were acquired after the injury. The cause and consequences cannot be elucidated. For example, in those ACL-injured patients accompanied with meniscal injury, whether the narrowed intercondylar notch led to ACL injury first, therefore, led to meniscal injury or the narrowed intercondylar notch influenced the ACL and meniscus separately and directly remained unknown. Moreover, Due to the characteristic of our study, the number of female and male participants was unequal. The difference in gender can lead to different results in the prevalence of ACL injuries and meniscal injuries as previously reported (41, 44, 45). Restricted by the limited time and resources, our study did not include the height and weight of subjects. Consequently, the BMI cannot be calculated. As previously reported, BMI is a risk factor of meniscal injury (44) and it may have a potential influence on our results. Future studies should take it into consideration. Only Chinese subjects were included in this study. The potential influence of ethnicity cannot be analyzed and should be considered in future studies.