Differences in MR signal intensity of lateral collateral ligament of knee joint on fat-suppressed proton density-weighted imaging

Objective: To investigate the relationship between the increased signal intensity (SI) of proximal lateral collateral ligament (LCL) at femoral attachment site on fat-suppressed (FS) proton density-weighted (PDW) MR imaging and the corresponding histological features on cadaveric knees. Methods: MRI was obtained from 11 cadaveric knees. Two musculoskeletal radiologists evaluated SI of LCL at femoral attachment site and the remaining caudal portion on FS PDW imaging. The SI was classified into three types; I=low, II=intermediate to slightly high, III=high SI or intraligamentous discontinuity of fibre. In addition, 100 control subjects were reviewed for normal LCL SI. Results: All proximal LCLs at femoral attachment site showed increased SI (nine cases of Type II and two cases of Type III). The remaining caudal portion presented Type I in all cases. Histological examination of proximal LCL at femoral attachment site revealed loose distribution of fine collagen fibres, intervened with fat and vessels, whereas the remaining caudal portion was composed of parallel distribution of compact collagen bundles. There were no signs of degeneration or tear of the LCL in all our cadaveric knee samples, even for the two cases that presented as Type III. Clinical study identified increased SI of proximal LCL at femoral attachment site in 94% (94/100) of control subjects. Conclusion: Increased SI of proximal LCL at femoral attachment site on FS PDW imaging is due to histological characteristics, not degeneration or tear. Advances in knowledge: Increased SI of proximal LCL at femoral attachment site on FS PDW MR imaging is a common, normal finding that its clinical significance can be neglected.