The most important findings of this study were that ACL status had the greatest effect on PCLA (absolute value of standardized coefficient Beta was 0.508) and ATT had the greatest effect on PCL-PCA (absolute value of standardized coefficient Beta was 0.523). A secondary finding was that BMI, as well as a number of other bony and soft tissue structures, had an effect on both PCLA and PCL-PCA. The present findings suggest that PCLA may be a simple and easily reproducible and important supplement for the diagnosis of ACL injury; PCL-PCA is a simple and important supplement for the diagnosis of ATT.

Since the phenomenon of PCL flexion on imaging of ACL-injured individuals was first described in 1988, different methods of quantifying the PCL buckling phenomenon have emerged successively in the following decades. Simple and easily reproducible measurements include PCLA described by McCauley et al. [3]. PCLIA described by Gali et al. [4] and PCL-PCA described by Siboni et al. [5]. These three methods allow the clinical orthopedic surgeon to make simple and quick judgments about the patient. Although there are more methods to quantify the PCL buckling phenomenon, there are only a few studies on the factors influencing the PCL buckling phenomenon. A recent study by Oronowicz et al [15] found that the size of the PCL-PCA correlated with the duration of ACL injury and did not correlate with the PTS and whether the meniscus was torn. Unfortunately, in the study by Oronowicz et al., the PTS was measured on X-plain radiographs, which did not allow for differentiation between medial and lateral PTS; and only whether the meniscus was torn or not was considered, not the size of the meniscal slope.

Despite the role of the ACL in limiting anterior tibial translation, greater PTS increases the amount of anterior tibial translation has been demonstrated in studies [21, 22]. Even in populations with intact ACLs, medial and lateral PTSs have great differences. Therefore, PTS in different knee compartments (includes lateral PTS and medial PTS) should be taken into account. In the current study, only a weak correlation was found between MPTS and PCLA, and multiple linear regression analyses also showed that MPTS poorly explained PCLA (Beta absolute value of the standardized correlation coefficient was 0.17). The meniscus is the secondary important structure limiting anterior tibial translation [13] and the posterior horn of the meniscus is higher than the anterior horn, so the presence of the meniscus (especially the posterior horn of the meniscus) counteracts a portion of the PTS [23]. Therefore, in the current study, we not only measured the medial and lateral meniscal bone angles [12, 17] but also calculated the medial and lateral combined slope angles separately to quantify the extent to which the meniscus counteracted the PTS. We found that LMBA, MMBA, LCS, and MCS were all correlated with PCLA, but after multiple linear regression analysis only LCS was found to have some degree of explanatory power for PCLA (Beta absolute value of standardized correlation coefficient was 0.15). In addition to ATT, we measured the relationship between rotational laxity of the knee and the buckling angle of the PCL. Studies have shown that femoral-tibial internal rotation (FTR) can cause a change in the relative position of the femoral and tibial stops of the PCL, which can lead to a change in the flexion angle of the PCL [7, 8]. However, in our study, we did not find any correlation between FTR and PCL buckling.

Previous studies have found that BMI may affect the thickness of subcutaneous soft tissues [24]. This is because the greater the BMI, the relatively greater the body weight and the greater the load carried, which affects the thickness of subcutaneous soft tissues. This possibility was only mentioned in the study by Oronowicz et al. without a correlation analysis of whether BMI was related to PCL buckling angle. In the current study, we measured two parameters, BMI and FTD. FTD is the vertical distance from the most distal end of the femur to the most proximal end of the tibia on the most clearly sagittal MRI image shown at the PCL, and is used to reflect the magnitude of the thickness of the subcutaneous tissue. We found that BMI was significantly negatively correlated with PCLA as well as PCL-PCA, whereas FTD did not correlate with the two above mentioned PCL angles (including PCLA and PCL-PCA); this result suggests that the larger the BMI, the more severe the PCL buckling phenomenon. However, after multiple linear regression analysis, it was found that BMI had a certain degree of explanatory power only for PCLA (Beta absolute value of standardized correlation coefficient was 0.178).

In the current study, we also considered anatomical factors. The intercondylar eminence is present above the tibial endpoint of the PCL, and the alignment of the near-vertical portion of the PCL is approximately parallel to the line from the intercondylar eminence to the tibial endpoint of the PCL. We therefore measured the intercondylar eminence height (vertical distance of the intercondylar eminence from the tibial endpoint of the PCL) and width (horizontal distance of the intercondylar eminence from the tibial endpoint of the PCL) and calculated the intercondylar eminence ratio (intercondylar eminence height/ intercondylar eminence width) as a result of normalization. We found a negative correlation between IER and PCL-PCA, indicating that the higher the intercondylar eminence, the smaller the PCL-PCA angle, and the more perpendicular the tibial portion of the PCL alignment. PCL length was also considered in the current study according to Fontanella et al [14]. We found that the PCL length was greater in the ruptured ACL group compared to the healthy control group. Although the PCL length between the groups was statistically significant, the differences were very small.

Although more bony and soft tissue parameters were addressed in the current study, there are still limitations. First, for subcutaneous soft tissue thickness measurements were performed in the supine state with the knee in the straight position, and the next study should be performed in the weight-bearing position. Although changes in body position affect subcutaneous soft tissue thickness, we found a correlation between BMI and PCL buckling angle, and measurements in the weight-bearing position in subsequent studies could better complement this result. Secondly, regarding the measurement of IER, our current measurements are simple measurements based on anatomical structures, and the next step should be to incorporate more anatomical structures for measurement.