Posterior cruciate ligament is twisted and flat structure: new prospective on anatomical morphology, by Kato et al. KSSTA (2018) 26(1):31–39
This cadaveric study aimed to elucidate PCL morphology by observing the anatomical relationship with other structures and the fibre layers of the PCL in cross section for remnant preserving PCL reconstruction.
Seventeen fresh-frozen cadaveric knees were studied, using the clock-face method to analyse the anatomical relationship between the PCL and Humphrey’s ligament. The width and thickness of the PCL, Humphrey’s and Wrisberg’s ligaments were measured. The PCL was cut sharply perpendicular to the tibia shaft, and the fibre layers were observed in cross section.
The PCL was located between 12 and 4 o’clock in the right knee (8 and 12 o’clock in the left), while Humphrey’s ligament was located between 2 and 4 o’clock in the right knee (8 and 10 o’clock in the left). Humphrey’s ligament at femoral insertion, midsubstance and lateral meniscus insertion averaged 8.7 ± 2.3, 5.9 ± 2.1 and 6.1 ± 2.0 mm, respectively, while the thickness at each level averaged 2.0 ± 1.2, 1.6 ± 0.6 and 1.9 ± 0.6 mm. The width of the PCL at midsubstance and at medial meniscus level averaged 13.3 ± 2.0 and 11.0 ± 1.6 mm, respectively, while the thickness of the PCL averaged 5.4 ± 0.8 and 5.5 ± 1.4 mm. In cross section, multiple, interconnected layers were observed which could not be divided. The main layers at each level were aligned from the posterolateral to the anteromedial aspect and formed a C-shape at the medial meniscus level.
The PCL at midsubstance is flat. PCL appears as a twisted ribbon composed of many small fibres without clearly separate bundles. When remnant preserving PCL reconstruction is performed, it is necessary to take account of not only PCL morphology but also the ligaments of Humphrey and Wrisberg. These findings may affect the PCL footprint and the graft shape in the future remnant preserving PCL reconstruction.
Frequency of the ligaments of Humphrey and Wrisberg
Humphrey’s ligament alone was found in 6 out of 17 specimens (35.3%), while Wrisberg’s ligament alone was found in 3 out of 17 specimens (17.6%). Both were observed in 8 out of 17 specimens (47.1%).
The clock‑face method and the angle measurement for the ligaments of Humphrey and Wrisberg
All right PCLs were located between 12 and 4 o’clock; meanwhile, all right Humphrey’s ligaments were located between 2 and 4 o’clock. All left PCLs were located between 8 and 12 o’clock; meanwhile, all left Humphrey’s ligaments were located between 8 and 10 o’clock. The angle of the PCL averaged 86.4 ± 4.4°, whereas the angle of Humphrey’s ligament averaged 45.2 ± 8.7°. The angle of Wrisberg’s ligament averaged 34.1 ± 4.4°.
The variability of Wrisberg’s ligament
The “tibio-femoral” ligament, a posterior medial oblique ligament, covered the PCL like Wrisberg’s ligament in four specimens (Fig. 3a). This posterior ligament was superficial to the PCL and attached to the tibia more laterally than the PCL in spite of the same femoral attachment of Wrisberg’s ligament. Because the layer was obviously different from the PCL, it was recognized that this ligament did not form part of the PCL. This ligament was observed in the knees that did not have the typical Wrisberg’s ligament. It was named the “tibio-femoral” ligament, distinct from the PCL.
Morphology of the PCL and the fibre layers in cross section
The PCL is composed of many small fibres (Fig. 1). A relatively flat structure could be observed at the midsubstance (Fig. 4a), while multiple layers were observed in cross section. The layers connected with each other and could not be divided. The main layers observed in the cross section were aligned from the posterolateral to the anteromedial direction at the midsubstance, but formed a C-shape at the level of the medial meniscus (Fig. 4b). The axis of the arc was aligned in a similar direction at midsubstance.