The posterior cruciate ligament angle in the setting of anterior cruciate ligament deficient knees: the effect of gender, age, time from injury and tibial slope

All MRI scans were performed on a high-field MRI scanner (≥ 1.5 T) using a standard knee protocol including at least four sequences (slice thickness ≤ 3 mm): sagittal, coronal, and axial fat-suppressed proton-density weighted, and sagittal T1-weighted [15].

Every single image were evaluated separately by two musculoskeletal radiologists. A combined, second-phase MRI evaluation was conducted to achieve consensus in cases of discrepancies after 4–6 weeks. All appropriate MRI scans were imported into a DICOM medical imaging viewer (Horos v3.0, Horos project, Annapolis, MD) to conduct measurements with digital tools with an accuracy of 1.0 mm [16].

For each patient, the following parameters were evaluated:

The angle of the posterior cruciate ligament was delineated, as per McCauley et al., as the angle formed between a line extending through the central region of the tibial insertion of the posterior cruciate ligament and a line extending through the central region of the femoral insertion of the posterior cruciate ligament (Fig. 1a) [17].

The medial and lateral tibial slopes were measured via MRI, as per Hudek et al. [14]. The initial step entailed choosing the center sagittal picture (Fig. 1a), wherein (1) the tibial attachment of the PCL, (2) the intercondylar eminence, and (3) the anterior and posterior tibial cortices manifested a concave configuration. Step two entailed placing one cranial and one caudal circle in the tibial head. The cranial circle was required to contact the anterior, posterior, and cranial tibial cortices, whereas the caudal circle needed to engage the anterior and posterior cortical borders. In instances with indistinct boundaries between the cortex and the medullary canal, the midpoint of the transition zone between a distinct black cortex and a pale gray medullary canal was used. The center of the caudal circle was located on the perimeter of the cranial circle to establish a uniform relative distance between the two circles. The MRI longitudinal axis was established by a line linking the centers of the two circles, which was overlaid and remained stationary during the whole picture series (Fig. 1a). Step three entailed locating the MRI depicting the mediolateral centroid of the medial plateau. A tangent to the medial plateau, linking the superior-anterior and posterior cortical boundaries, was illustrated in this illustration. The slope of the medial plateau was determined by the perpendicular line to the MRI-longitudinal axis and the tangent to the medial plateau (Fig. 1b). The lateral plateau PTS was measured at the mediolateral center of the lateral plateau by a tangent to the highest flat region between the superior-anterior and posterior cortices (Fig. 1c) [18].

For anterior tibial translation, standardized reference points were utilized on sagittal MRI scans to assure measurement consistency. Medial ATT was assessed on the initial MRI picture depicting the origin of the medial gastrocnemius tendon on the femur; lateral ATT was evaluated on the image revealing the most medial portion of the fibula at the tibiofibular joint. A circle was constructed to optimally match the posterior femoral condyle using the sagittal scans. A line orthogonal to the tibial plateau was delineated along the posterior edge of the circle. A supplementary line, orthogonal to the tibial plateau, was delineated in the posterior region of the tibia. The interval between these lines signified the ATT (Fig. 2) [19].

We determined that at least 189 subjects undergoing primary ACL reconstruction were needed to estimate a 40% ± 7% proportion of subjects with a PCL angle of < 105°, a positive indicator of ACL injury, with a confidence level of 95% [17]. Sample characteristics were presented with absolute frequencies and percentages or mean ± standard deviation. Continuous variables were tested for normality using the Shapiro–Wilk test. Radiological scores (PCL angle, medial tibial slope angle, lateral tibial slope angle, medial anterior tibial translation, and lateral anterior tibial translation) were presented overall and categorized by sex, age, and time interval between injury and MRI. Age groups were predefined by dichotomizing age at its mean value, while a cut-off of 90 days was selected to differentiate between chronic and acute lesions. Differences between groups were assessed using t-tests or Wilcoxon-Mann Whitney tests, depending on score distribution. Spearman rank correlations were also estimated to explore correlations among the collected variables.

A two-tailed p-value of < 0.05 was considered to indicate statistical significance. All statistical tests were performed using R version 4.3.0 (R Foundation for Statistical Computing, Vienna, Austria. URL https://​www.​R-project.​org/​).

A total of 193 patients were included in the study, comprising 91 (47.2%) females and 102 (52.8%) males, with a mean age of 30.27 ± 12.54 years. In 109 (56.5%) cases, the left knee was involved, while the right knee was affected in 84 (43.5%) cases. The mean time from injury to MRI was 14.18 ± 55.77 days. Demographic details are shown in Table 1.

In the overall population, the mean PCL angle was 128.72 ± 10.33°, the mean medial tibial slope was 3.57 ± 2.33°, and the mean lateral tibial slope was 6.07 ± 3.52°. The mean medial and lateral anterior tibial translations were 4.76 ± 2.02 mm and 7.01 ± 2.48 mm, respectively. Details are shown in Table 2.

In 190 cases (98.4%), the PCL angle was ≥ 105°, while only three cases (1.6%) were below this threshold.

The PCL angle negatively correlated with medial and lateral anterior tibial translation (p < 0.05). Other statistically significant correlations are shown in Table 3.

No differences were noted between patients younger and older than 30 years for all radiological scores (p > 0.05). Details are shown in Appendix 1.

Patients with chronic ACL injuries showed a lower PCL angle compared to those with acute injuries (p = 0.032) and a superior grade of lateral anterior tibial translation (p = 0.015). Details are shown in Table 5.

This study has limitations. Initially, we did not link the MRI measurement data with clinical or arthroscopic evaluations of the individuals studied, as the diagnosis was solely based on MRI analysis. Nonetheless, the radiologists conducting the assessments evaluated MRI scans of instances with a definitive diagnosis of either damaged or intact ACL. The data included only cases of unequivocally injured ACLs, so removing those that could be classified as partial injuries. Nonetheless, we can investigate the mechanical properties of the residual ACL fibers after partial injuries. Future research is essential to elucidate the characteristics and biomechanical properties of partial ACL injuries, as well as the implications of partial ACL insufficiency.