The most important finding of the present study was that patients with PCL injuries had a larger patellar tilt angle, a lower intercondylar notch angle, and a larger femoral condyle ratio than those with PCL integrity. This was the first study to link patellofemoral alignment to PCL injuries.
Previous studies have shown that PCL injuries might be related to smaller and sharper intercondylar notch, smaller intercondylar notch width, smaller intercondylar notch volume, flatter tibial eminence, and smaller posterior tibial slope [15, 16, 28, 29]. The smaller the intercondylar notch height and the narrower the intercondylar notch width, the smaller the intercondylar volume and the greater the possible risk of PCL injuries. Van Kuijk et al. [16] concluded that patients with a smaller intercondylar size and a smaller volume of the ACL were more prone to sustain PCL rupture. In a study by Shelburne et al. [30], the authors examined the relationship between posterior tibial slope and cruciate force. They observed that a 1° increase in posterior tibial slope resulted in a decrease of 6 N in PCL force. Moreover, they also noted an increase in PCL force when the slope was decreased during squatting.
The intercondylar notch has been a topic of interest. A sharper intercondylar notch angle is often considered a risk factor for ACL injuries. Our study found that the intercondylar notch angle of the PCL injured group was lower than that of the PCL intact group, which was consistent with the conclusion of Van Kuijk et al. [13]. They believed a smaller intercondylar width with a sharper angle was associated with PCL rupture. The size of the PCL (as well as the ACL) is associated with the size of the intercondylar notch [31, 32]. It is well accepted that the volumes of the intercondylar notch, the PCL, and the ACL are all positively correlated. When the PCL is smaller, it has a lower capacity to withstand force and is more susceptible to tearing [33, 34]. An alternative interpretation is that a reduced intercondylar notch size might lead to impingement on the PCL. Triantafyllidi et al. [35] demonstrated that the PCL occupied a significant portion of the intercondylar notch during flexion, potentially resulting in impingement on the PCL in this joint position. However, it is still unclear whether the PCL injuries are due to the relatively weak tensile strength possessed by the smaller volume of the ligament or to chronic or acute injuries caused by intercondylar notch impingement of the ligament [36–39]. Due to the anatomical position of the PCL behind the knee joint, the impact of the incision may have less effect on the PCL rupture. However, biomechanical studies are required to clarify the mechanism of PCL injuries further. Liu et al. [15] believed a larger intercondylar notch width index was the most significant risk factor for PCL rupture in the female population. The reasons for his opposite conclusions may be as follows: different from the control group in the two studies, people with trauma but no knee joint structural injuries may have relatively thick ligaments and wide intercondylar notch; their study included cases of partial PCL injuries. These are all reasons that may differ from our study. Previous studies have shown that ACL injuries are associated with an increased ratio of the distal lateral femoral condyle [26, 40, 41]. In the study of Hao et al. [28], it was found that the LFCR of posterior femoral condyle depth was significantly larger in patellar instability patients, which was a risk factor for ACL injury. In our research, it was observed that an augmented depth of both the medial and lateral posterior femoral condyles exhibited a correlation with PCL injuries. It might be related to the influence of distal femur morphology on knee joint kinematics [28, 42, 43]. Altered tibiofemoral interaction due to increased posterior femoral condyle depth may lead to altered gait and loading mechanics [44]. This may increase the risk of PCL injuries. However, there are no relevant studies to demonstrate the mechanism of action of the increased length of the medial posterior femoral condyle on PCL injuries, which is a shortcoming of our article. Our study found that the larger patellar tilt angle was closely related to PCL injuries. Similar conclusions are rarely found in previous studies and are not even mentioned in the study of ACL injuries. The patellar tilt angle is often a good indicator for predicting patellar instability [45]. However, the biomechanical mechanism of PCL injuries caused by increased patellar tilt still requires to be further studied.
In our case, we found 11 patients with patellar dislocation. By comparing the two groups of patients with PCL injuries with or without patellar dislocation, we found that in patients with patellar dislocation, the patellar position was higher, the trochlear groove angle was larger than in patients with PCL injuries without patellar dislocation, and the trochlear depth was shallower than in patients with PCL injuries without patellar dislocation. Askenberger et al. [46] conducted a prospective study to evaluate the patellofemoral joint morphology of 103 cases of patellar dislocation and 69 cases of skeletal immature children without patellar dislocation for 2.5 years. The results showed significant differences in the two groups' measurement parameters related to trochlear dysplasia. Among them, the patients with patellar dislocation had a larger trochlear groove angle and lower trochlear depth (< 3 mm), and the patellar height was significantly higher than those in the control group, which was consistent with our findings.
In recent years, after studying the relationship between anterior knee pain and patellofemoral alignment, many scholars have found that abnormal patellofemoral alignment (patellar subluxation and patellar tilt) caused patellofemoral disorder, resulting in articular cartilage damage of the patella and eventually leading to advanced osteoarthritis [47, 48]. Studies found that PCL rupture can lead to abnormal movement and increased contact pressure in the medial compartment of the knee joint and the patellofemoral joint, accelerating its degenerative changes and devastating long-term impacts on the knee joint [49, 50]. This study discussed the relationship between PCL rupture and patellofemoral malalignment using the measurement techniques established in the existing literature.
A meta-analysis by Yulun et al. [8] reported that ACL injury prevention programs can significantly reduce injury rates. There appeared to be no prevention programs for PCL injury, but only as basic research progresses would these programs emerge [16]. The premise of formulating prevention programs is to determine risk factors. After confirming the influencing factors of patellofemoral malalignment and anatomical morphology of femoral condyle on PCL injuries and re-injuries after PCL graft healing and reconstruction through imaging screening, personalized prevention programs are formulated for high-risk groups. Additionally, tailored surgical plans and rehabilitation strategies can be devised for patients to rectify abnormal anatomical morphology. Through the results of this study, clinicians can comprehensively consider the risks of PCL injuries by measuring patellar tilt angle, intercondylar notch angle, and femoral condyle ratio through imaging data to evaluate the risk population of PCL injuries and give preventive suggestions and guidance for the treatment. We are convinced that this study is an important step in identifying risk factors, and we hope that we have inspired researchers to investigate the results we found further.
There are some limitations to this study. First, the trial is a retrospective case-control study with a relatively low level of evidence. Second, the study only evaluated patients with PCL lesions in Hebei Province, China. Studies that take the variability across regions and ethnicities into account were required. Third, it is difficult to achieve the complete consistency of the direction, size, and mechanism of the force when the injuries occurred. There are still confounding factors that affected the study's results, such as the site of non-contact injuries, the shoes worn, and other factors that were not controlled. There is no good distinction between the four mechanisms of PCL injuries: dashboard injuries, severe torsion with valgus or varus force, hyperextension, and hyperflexion [18, 51]. Different injury mechanisms may still interfere with the results. Fourth, the knee joint is a three-dimensional structure. This study was conducted based on imaging data to analyze two-dimensional morphological indicators for PCL injury risk factors, which was limited by the measurement means and did not analyze three-dimensional morphological indicators. Fifth, no rigorous biomechanical experiments are performed to demonstrate the injury mechanism. Therefore, further large-scale multicenter studies are needed, and the trial design should be improved to clarify the risk factors for PCL injuries.