Figure 3.11 shows sketches of the fascicles of the anterior (Fig. 3.11(a)) and posterior cruciate ligament (Fig. 3.11(b)) drawn by Friederich, Muller and O’Brien (1992) and augmented by Feikes (1999) from her own investigations. The diagrams show clear relations between the position on the femur of the origins of individual fascicles and the position on the tibia of their insertions, a relationship we have called fibre mapping (Lu & O’Connor 1996b; Zavatsky & O’Connor 1992a). The sketches by Mommersteeg et al. (1995) confirmed the pattern of fibre mapping in the ACL (Fig. 3.11(c)). Feikes (1999) also confirmed these observations and found similar patterns of fibre mapping for the collateral ligaments.
Figure 3.11 Fibre mapping of the ACL (a) and PCL (b) according to Friederich et al. (1992) and mapping of the ACL (c) according to Mommersteeg et al. (1995). The sketches demonstrate clear relationships between the position of the origin of a fascicle on the femur and the position of its insertion on the tibia. (Reproduced with permission from Friederich NF, Müller W, O’Brien WR. [Clinical application of biomechanical and functional anatomical data of the knee joint.] Orthopäde 1992; 21:41-50 and Mommersteeg TJ, Kooloos JG, Blankevoort L, Kauer JM, Huiskes R, Roeling FQ. The fibre bundle anatomy of human cruciate ligaments, J Anatomy 1995;187 Pt II: 461-71.)
Figure 3.12 reproduces sketches of fascicles of the ACL and the PCL with the knee at extension, 60° and 120° published by Friederich et al. (1992). These figures confirm the patterns of fibre mapping of Figure 3.11 and demonstrate how the apparent shapes of the ligaments in the sagittal plane change as their attachment areas rotate relative to each other and the points of origin and insertion of different fascicles move relative to each other. Figure 3.12 suggests that the most anterior fibre of the ACL (shown as a straight line) remains isometric during passive flexion. Other fibres are tight in extension but slacken (and are shown buckled) and then retighten again during further flexion. Friederich’s sketch of the PCL suggests that a fibre within the body of the ligament, closer to the posterior edge, remains isometric during passive flexion. More anterior fibres are slack in extension (shown as buckled) but tighten as the joint is flexed. More posterior fibres are tight in extension and slacken and retighten as the joint is flexed.
Figure 3.12. Sketches of the fascicles of the ACL and PCL in extension and flexion. (Reproduced with permission from Friederich NF, Müller W, O’Brien WR. [Clinical application of biomechanical and functional anatomical data of the knee joint]. Orthopäde 1992;21:41-50.)
This pattern of isometry within the ACL is well supported by independent evidence (Feikes, 1999; Wang & Walker, 1973; Sapega et al., 1990; Sidles et al., 1988). The concept of isometry of fibres within the interior of the PCL has also been supported by independent evidence from Covey et al. (1996) and Feikes (1999). Feikes used a fibre mapping of the PCL and calculated the distances apart of the points of origin and insertion of various ligament fascicles over the range of flexion. Her analysis shows that a band of fascicles within the ligament remain isometric while the distance between the points of origin and insertion of the most anterior fascicle of the PCL increases by as much as 30% as the joint flexes to 90°. Brantigan and Voshell (1941) stated that “some portion” of the posterior cruciate ligament is “taut in all positions of [passive] extension and flexion”.
Using the same technique, Feikes showed that anterior superficial fibres of the MCL also remain isometric during passive flexion.
