The most important finding of the present study was strong evidence for an association displacement to yield and failure after enhanced ACL repair with VEGF receptor expression. No evidence was found for a direct influence of VEGF receptor density on the maximum load for the healing ligaments. Finally, strong evidence emerged that the association between VEGF receptor expression and load was affected by age, suggesting that age masks at least some of the association between these two factors. This interpretation is supported by findings from prior studies [23
VEGF is a well-known factor in tissue healing, which is primarily associated with endothelial cells and angiogenesis. Recently, the role of VEGF in ligament and tendon healing has been studied more closely. Such analyses showed that fibroblasts, especially during wound healing, release VEGF [11
] and express VEGF-receptors [12
], both in vitro and in a graft used for ACL reconstruction [30
]. Other recent evidence shows that VEGF addition to grafts for ACL reconstruction results in increased AP laxity and reduced linear stiffness [40
]. However, the exact function of VEGF and, especially, VEGF-receptors in fibroblasts, is still elusive.
This study showed significant associations between displacement and VEGF-receptor 1 expression, but none with VEGF-receptor 2. A greater expression of VEGF-receptor at the time of surgery resulted in greater displacements to yield and failure. The finding that this association exists only for receptor 1 may be due to the fact that the activity of receptor 2, which is the main receptor in VEGF transduction, is modified by receptor 1 [31
]. These findings suggest that a higher VEGF receptor expression results in a more extensible repair tissue.
There was no association of load, either a t yield or at maximum, with either VEGF receptor expression. More interestingly significant associations were seen between age and load, which were slightly attenuated after adjusting for receptor expression levels. These findings suggest that VEGF receptors play only a small role in the association between age and load that has been described earlier [23
]. Load is a function of fibroblastic synthetic activity, which has been shown to be enhanced by PRP [3
]. The independence of load from VEGF receptors, despite the proven stimulation of fibroblast bioactivity by PRP [23
], might be interpreted as an indicator of the higher importance of other PRP-related growth factors, such as TGF-beta or PDGF [7
]. Such an interpretation is in accordance with findings from earlier studies, showing that TGF-beta might counteract any VEGF-mediated detrimental action in scar formation [14
The third studied parameter was linear stiffness. Stiffness, as a compound parameter of load over displacement, and thus offers a more comprehensive evaluation of ACL healing. As seen in earlier studies, there was a significant, negative association between serum VEGF levels and stiffness [40
]. However, there was no association with VEGF receptors. Interestingly, there was, again, strong evidence for residual confounding by age.
Also, strong evidence was found that the association between yield load and maximum load and VEGF receptor expression was influenced by age, in such a way as that accounting for age reduced the strength of the association between load and VEGF receptors. Recent studies have shown repeatedly and consistently that age is negatively associated with functional outcome after PRP-augmented primary ACL repair, as well as tissue maturation and ACL fibroblast migration, proliferation and collagen production [23
] suggesting growth factor receptor expression as a reason for the observed age-dependence.
These findings have potential for important clinical implications. Since they describe the interaction of two important determinants of functional outcome of enhanced ACL repair, the identification of these determinants allows using a tailored therapeutic approach based on individual patient characteristics. Second, a number of drugs have been developed that influence the VEGF pathway and might be added to such a surgical technique. Finally, these findings suggest the overstimulation of VEGF receptors might lead to a poorer functional result, implying that higher concentrations of platelets in PRP might not necessarily produce better results in sports medicine [22
This study has potential limitations. Only one growth factor was studied, thus potential interaction between different growth factors could not be assessed. Secondly, the data build on a snapshot of receptor expression, which might change in response to PRP. Lastly, no post-translational control on the peptide level to confirm the mRNA changes, but earlier research confirmed PRP-initiated up-regulation of VEGFR-1 and R2 mRNA expression by histology and immunohistochemistry [17