The results of this study suggest that IACFM increases blood flow and alters microvascular morphology in the vicinity of healing knee ligaments. IACFM treatment of surgically-induced knee MCL injuries increased regional tissue perfusion, as assessed using LDI. The increase was not observed immediately following intervention suggesting IACFM does not lead to a direct increase in tissue perfusion due to vasodilation. In contrast, IACFM increased tissue perfusion days following treatment, an effect that persisted for 1 wk following the final IACFM intervention. Subsequent investigation found IACFM to have no effect on global microvascular morphology within the injured ligament and its surrounding connective tissue, as assessed by quantitative micro-CT imaging of vessels filled with a contrast agent. However, further analyses revealed one portion of the assessed tissue region to have a greater proportion of smaller diameter blood vessels (5.9 to <41.2 μm), suggesting IACFM treatment had a subtle, yet measurable influence on localized microvascular morphology. Overall, our findings suggest that IACFM altered regional vascular properties following ligament injury. Whether the detected vascular changes contribute to the beneficial effect of IACFM on the recovery of knee MCL biomechanical properties observed in a previous study  requires further exploration.
The importance of blood supply to knee ligament healing is well established. Each stage of ligament healing depends upon adequate vascularity and blood flow for the delivery and removal of cells and metabolic substrates at the injury site . Numerous studies have associated variations in blood supply with alterations in knee MCL healing [25–28], and differences in blood supply are thought to contribute to the differential healing of MCL and anterior cruciate ligament injuries [29, 30]. Massage therapy is often introduced with the intent of altering tissue blood flow; however, studies have generally been limited to the exploration of immediate, short-term effects [31–34]. The immediate effects of massage on blood flow are hypothesized to be as a result of a vasodilatory effect of the therapy on vessels within the treated region. To our knowledge, no study has reported an effect of massage on regional vascular properties, particularly at delayed time points (>24 hours) following massage completion.
The delayed effect of IACFM therapy in the current study on tissue perfusion is a novel finding and suggests this form of massage therapy had a morphological effect on the vascular system in the vicinity of the healing MCL, as opposed to an immediate, more temporary vasodilatory effect. This hypothesis was partially supported by quantitative micro-CT imaging of vessels filled with a contrast agent, which showed the tibial portion of the healing MCL and its adjacent connective tissue to have a larger proportion of blood vessels in the diameter range of arterioles. The finding of alterations in the proportion of arteriole-sized vessels is potentially important as these vessels regulate the flow of blood through the capillary beds they supply.
It is uncertain why expansion of arteriole-sized vessels was isolated to the distal portion of the ligament. More dense distributions of epiligamentous vessels are located near the bony insertions of the MCL . It may be that neovascularization within the injured MCL originates from these vessels. Alternatively, it is possible that expansion of arteriole-sized vessels in the distal portion of the MCL resulted from intensifying of treatment pressures at this site, with the ligament being compressed during intervention between the rigid massage tool and underlying tibia. An earlier study demonstrated elevated IACFM pressures were associated with enhanced cellular responses .
The current study extends knowledge regarding IACFM effects during ligament healing; however, the data are not without limitations. Rats were studied for handling, housing and cost considerations; however, they have small and difficult to study ligaments relative to larger species. The injury model of surgically transecting the MCL produced highly reproducible injuries for comparative research purposes, but it does not replicate the strain-related injury mechanism that occurs clinically. LDI provided interesting data regarding delayed effects of IACFM, yet it possesses a limited depth of penetration with only 37% of incident energy remaining after 1 mm of penetration in skin . Thus, it remains unanswered whether IACFM actually influenced perfusion within the ligament proper in the current study. Other investigators have opened the skin to enable LDI to provide a more specific assessment of MCL perfusion [36, 37]. This approach was not implemented in the current study as we wanted to obtain non-invasive measures in the same animals over time. There are also limitations with the technique used to image vascular morphology, including potential incomplete filling and/or overdistension of vessels, and the inability of being able to distinguish between the arterial and venous vascular systems.