Although overall properties of the bundles were quite similar, several notable differences were found in intra-bundle comparisons in both mechanical and microstructural properties. Intra-bundle comparisons of mechanical properties of regional samples showed several significant linear relationships across posteromedial regions: linear modulus in the quasi-static ramp-to-failure test (p = 0.042) as well as both peak stress (p = 0.043) and equilibrium stress (p = 0.037) in the stress-relaxation test (Fig. 6 and Table II). All values decreased moving posteriorly. However, no significant intra-bundle relationships were found across anterolateral regions.
Similarly, intra-bundle comparisons of microstructural properties showed significant linear relationships across posteromedial regions (p < 0.05) but not across anterolateral regions (Fig. 7 and Table II). All measured parameters, except percent change of the mean DoLP during the stress-relaxation test, were found to have significant linear regressions across posteromedial regions (p < 0.05 for all and p < 0.001 for most; Table II), with the mean DoLP decreasing and the AoP standard deviation increasing moving posteriorly, a finding consistent with the trend seen in the mechanical properties. Notably, while no regressions reached significance across anterolateral regions, several regressions in both the mean DoLP and the AoP standard deviation trended (p < 0.20) in the same direction as the significant regressions found across posteromedial regions (Table II).
The anterolateral and posteromedial bundles of the PCL have very similar microstructural and mechanical properties under tensile loading. These findings are in contrast to previous studies of the ACL24,28,29,36, including our own28,29, which found significant regional and bundle variation in the properties of that ligament. The regional variation of these properties in the PCL appears to be much more subtle. Our findings suggest that the distinct functions of the bundles arise more from differences in size and mechanical dynamics due to geometry than from differences in underlying material and microstructural properties.
Values in the literature for mechanical properties of the PCL, based on cadaveric studies with different testing and harvesting techniques, vary greatly24-27; this variation makes drawing conclusions about the differences between bundle properties based on tissue-level values alone challenging. An advantage of the present study compared with prior work is that we were able to evaluate the behavior of the collagen microstructure of the ligaments in real time, simultaneously with mechanical testing. Previous studies were unable to simultaneously examine these variables27,30,37. The current study provides a more complete picture of ligament behavior and shows only small inter-bundle differences under dynamic loading. Additionally, the mean patient age of samples in this study was much lower than in previous studies, an important distinction since studies have demonstrated that cell proliferation, metabolism, and collagen orientation all decrease with aging38-40.
Since the anterolateral bundle was traditionally considered the dominant PCL bundle17,27,41,42, single-bundle techniques for reconstruction historically attempted to recreate this bundle alone. The findings from our study suggest that any so-called dominance of the anterolateral bundle results from greater mass and geometric orientation and location rather than any significant mechanical or microstructural differences between the 2 bundles, in contrast to conclusions of prior studies17,27. Moreover, our findings are consistent with recent studies suggesting co-dominance of the PCL bundles7,43. These findings may help to explain why smaller case studies have shown that there are similar clinical outcomes between single and double-bundle constructs for PCL reconstruction44-46 despite biomechanical differences between these constructs47-52.
The implications for operative treatment of PCL injury are not entirely clear. With only small differences in mechanical and microstructural properties between the bundles, the benefit of the double-bundle technique, if any, likely arises from the use of more tissue and from more closely reapproximating the native ligament geometry, potentially providing more natural kinematics47-50,53. In this study, the only parameters that were significantly different between bundles were related to stress-relaxation, namely less relaxation and less dispersed collagen alignment for the anterolateral bundle compared with the posteromedial bundle (Figs. 3 and 5). These results agree with those of our previous studies of ACL bundles and collagen gel tissue analogs, which showed faster relaxation and greater overall total relaxation for less aligned samples29,54. Thus, the PCL bundles exhibit some differences in time-dependent viscoelastic behavior, with a slightly more viscous response for the posteromedial bundle. While it is possible that these differences could have some relevance to injury risk, the clinical implications are not directly obvious because in vivo viscoelasticity will depend on other factors not evaluated in this study, such as physiological water content, anatomical orientation, and degree of prestress in the bundles. In addition, relatively small differences in viscoelastic behavior may not be as clinically important as other parameters measured in this study, such as elastic moduli and stresses.
There are a number of limitations to the current study. One important limitation results from anatomical variation. While our study averaged the bundle properties across all of the different anatomical variants, it is possible that material properties may be affected by the variations, especially in light of findings suggesting that the meniscofemoral ligaments, when present, may support the role of the PCL in restraining the knee55. Our sample size was not large enough to adequately compare the differences between anatomical variants, and further studies would be needed to fully characterize them. However, we believe this limitation to be of questionable clinical importance, as identifying which anatomic variant a given patient had prior to a PCL injury would likely be impossible unless the patient had had imaging prior to the injury. Also, while dissection was performed systematically to minimize differences between specimens, the anatomical variants invariably resulted in some inter-specimen differences between the sections of the posteromedial bundle, and these were subsequently identified as regions 4, 5, and 6. Finally, in order to prepare samples for testing, multiple freeze-thaw cycles were required. There is the theoretical possibility that freezing may alter the tissue, but previous research has suggested that limited freeze-thaw cycles do not cause meaningful changes in ligamentous tissue properties56.
Despite these limitations, to our knowledge, this is the first study to report both the microstructural and mechanical properties of the PCL. This ligament has relatively homogenous properties, with only small differences in stress-relaxation behavior between the anterolateral and posteromedial bundles and only subtle regional variation across each bundle. This information may help to guide graft choice and operative techniques for PCL reconstruction. Further research is warranted to assess which techniques and grafts most closely recreate the microstructure of the native PCL as well as to determine the impact of anatomical variation of the ligament, if any, on these findings.
NOTE: Samples were collected through the Mid-America Transplant Services Center (St. Louis, Missouri).
Investigation performed at Washington University in St. Louis, St. Louis, Missouri
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