Ultrastructural and histological changes in tibial remnant of ruptured anterior cruciate ligament stumps: a transmission electron microscopy and immunochemistry-based observational study

Anterior cruciate ligament (ACL) rupture is a common injury and has a non-union rate of 40–100%. Important cellular events, such as fibroblast proliferation, angiogenesis and change in collagen fibril thickness in the ACL remnant, as described in other dense connective tissue, might have an implication in graft recovery following ACL reconstruction. Thus we conducted a study with an aim to characterize the ultrastructural and histological features of ruptured ACL tibial stump and correlate the same with the duration of injury.

This was a prospective observational study in which 60 ruptured human ACLs were evaluated for collagen fibril thickness, blood vessel density (per mm²) and fibroblast density (per mm²) with the help of transmission electron microscopy, immunohistochemistry via CD34 antibody staining and light microscopy (H&E staining). The findings were correlated with duration of injury.

Fifty-four male and six female patients with a mean duration of the injury of 23.01 weeks (SD = 26.09; range 2–108 weeks) were included for the study and were divided on the basis of duration of injury as follows: Group I (≤ 6 weeks; N = 16), Group II (7–12 weeks; N = 18), Group III (13–20 weeks; N = 7), Group IV (21–50 weeks; N = 12), Group V (> 50 weeks; N = 7). A significant correlation was seen with blood vessel density (r = 0.303, p = 0.01) and fibroblast density (r = − 0.503, p = 0.001). Thickness of collagen fibril did not correlate with the duration of injury (r = 0.15, p = 0.23). The thickness of the collagen reached its peak after 50 weeks following injury, whereas highest density of blood vessel and fibroblast was seen at 12–20 weeks. Matched pair analysis revealed a significant decrease in collagen fibril thickness and an increase in fibroblast density at 7–12 weeks.

Following injury to ACL, the ruptured tibial stump undergoes a series of changes at the cellular level vis-à-vis changes in collagen fibril thickness, vascular density and fibroblast density that possibly suggest an intrinsic healing response. This further may have implications on the functional outcome following ACL reconstruction with remnant preservation.

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