This scaffold is gradually invaded by surrounding cells that proliferate and produce extracellular matrix proteins (11) and form a vascular (2) functional scar which becomes increasingly similar to the normal ligament (3).

However, in the ACL, no blood clot was observed in the gap. This defect may be due to the documented presence of fibrinolytic enzymes in synovial fluid (1, 4) which prevents intra-articular clot formation.

We hypothesize that without this clot, or provisional scaffold, there is no basis for formation of a functional scar, and thus, no foundation for the healing process (Fig. 1).

Fig. 1B: Schematic of unsuccessful connective tissue healing as exemplified by the intra-articular ACL. No clot is formed between the ligament ends, and thus no provisional scaffold is present to facilitate functional scar formation and maturation.

Fig. 2 click for larger image

Our early work focused on using collagen sponges supplemented with growth factors(5-7,9,10). While we found fairly good stimulation in vitro of important cell behaviors (see figure below), we were not confident that the changes we were observing in the petri dish would be enough to overcome the deficiencies of healing within the joint with the use of individual growth factors. This led us to the adoption of the use of platelets, which essentially function as growth factor factories in the wound site, releasing multiple growth factors that stimulate healing over days to weeks in the wound site.

Our initial study published in the Journal of Orthopaedic Research demonstrated the effectiveness of a collagen bridge in stimulating ACL cell migration into the gap between ligament fascicles in vitro (9). However, with the collagen scaffold (CS) alone, the ACL cell and tissue response was slow, and we began incorporating PRP into the CS as a source of growth factors and additional plasma proteins. Our rationale for this was that the provisional scaffold in other tissues is blood clot (which is made of PRP plus red blood cells). Thus, we reasoned that ACL healing might also be stimulated by the growth factors and extracellular matrix proteins found in platelet-rich-plasma.

We have developed in vitro assays to test the ability of ACL cells to migrate through various candidate scaffolds, as well as to proliferate and produce collagen, and have optimized the scaffold in vitro.

Our more recent work, performed in collaboration with Dr Kurt Spindler at Vanderbilt University Medical Center, has focused on the in vivo delivery of platelet-rich plasma/collagen scaffold composites to the injury site. An in vivo pilot study combined with our in vitro data, provide support for our hypothesis that it is the deficiency in provisional scaffold formation that prevents healing in the intra-articular environment.