LIMITATIONS OF SURGICAL REPAIR
Traditional surgical treatments for meniscal injury have been meniscectomy, meniscal repair, and meniscal allograft— each having shortcomings and minimal long-term benefits (see Appendix A). Although there is some short-term improvement in aspects such as pain control, the long term effects of meniscectomy, meniscal repair, and meniscal allograft transplantation reveal that symptoms, such as pain and instability, will persist for years afterward. The main reason that these and other treatments are ineffective in healing the meniscus can simply be attributed to the fact that, regardless of what is done to structurally repair the meniscus, it is still primarily an avascular cartilaginous structure which cannot heal without a sufficient supply of nutrition. The poor healing potential of meniscal tears has led to the investigation of methods to provide blood supply to the injured area. The methods include vascular access channels and synovial pedicale flaps. Unfortunately, no surgical treatment to date has been shown to stimulate healing of the meniscus. On the contrary, surgeries often have the opposite effect. They initiate additional damage to the joint, further decreasing the probability of healing (see Table 3).
The bottom line is that surgical procedures do not initiate the regenerative process needed in these traumatized knee joints. Left alone or treated by the surgery, the degenerative process initiated by the initial trauma continues, unless something is done to initiate regeneration. The reverse of degeneration is simply regeneration. In other words, a degenerative process can only be reversed when stimulated to repair itself. Degeneration of the meniscus is initiated by a damaged meniscus’ inability to repair itself, and the surgical procedures themselves accelerate the degenerative process. Therefore, the ideal treatment for a damaged meniscus is one that can stimulate regeneration of the degenerated or torn meniscus.
| Table 3. Effects of treatments for meniscal tears. Only Prolotherapy stimulates the repair of injured meniscal tissue. |
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PLATELET RICH PLASMA FOR MENISCAL PATHOLOGY
In order to understand how growth factors affect the treatment of meniscus injuries, it is first important to understand the role that they play in the natural process of healing. The preliminary steps of healing begin with the attraction of blood cells to the site of an injured tissue. When a tissue is injured, bleeding will naturally occur in that area. A specialized blood component called platelets rapidly migrate into the area to initiate coagulation, or the clotting of blood cells, to prevent excessive bleeding from an injury. In addition, platelets also release growth factors which are an integral part of the healing process. Each platelet is made up of an alpha granule and a dense granule which contain a number of proteins and growth factors. The growth factors contained in the alpha-granule are an especially important component to healing. When activated by an injury, the platelets will change shape and develop branches to spread over injured tissue to help stop the bleeding in a process called aggregation, and then release growth factors, primarily from the alpha granules.
At this point, the healing process then proceeds in three basic stages: inflammatory, fibroblastic, and maturation. After growth factors are released from the platelets, they stimulate the inflammatory stage with each growth factor playing a key role (see Table 4). This stage is marked by the appearance of monocytes which are white blood cells that respond quickly to inflammatory signals and elicit an immune response. Growth factor production is at its highest level immediately following the inflammatory stage. Fibroblasts begin to enter the site within the first 48 hours after an injury and become the most abundant cells in that area by the seventh day. The fibroblasts deposit collagen, the main building block of tissues such as the meniscus, for up to many weeks afterward. The maturation of collagen may then continue for up to one to two years after the initial inflammatory event.
| Table 4. Various growth factors found in platelets and their actions. |
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It is important to understand that each of these stages stimulates the next. If the inflammatory stage does not occur, neither will the fibroblastic stage, and so on. If there is not a significant enough immune response to completely regenerate the damaged tissue in any of these stages, the injury will be unable to heal completely, leaving the person with a chronic degenerated knee.
In the case of the injured meniscus, it is clear that the damaged tissue can not repair itself. Healing in the meniscus depends on having enough of a blood supply and or/growth factors at the site of the injury. Since less than 20% of the meniscus is vascularized by the time a person reaches the age of 40 years, meniscal healing is generally incomplete.91 Once torn, the menisci, because of its low cellularity and incomplete healing response, is unable to fully repair itself.92
IN VIVO AND IN VITRO GROWTH FACTOR STUDIES TO STIMULATE MENISCAL REPAIR
Because growth factors are known to be a basic component of healing, the adjunct use of growth factors to stimulate connective tissue repair has been studied as a potential for the treatment of injured soft tissues, including the meniscus. Direct exposure of connective tissues to fibroblastic growth factors can indeed cause new cell growth and formation of collagen. Therefore, injecting growth factors at the site of a soft tissue injury allows the damaged tissue to heal itself.
Before any treatment is tested on humans, it is common practice to investigate the effect of that treatment (in this case growth factors) on cells, as well as on animal models with similar pathology to humans. The primary objective of these studies is to determine if and how a poorly vascularized tissue such as the meniscus can be stimulated for reliable cellular and tissue repair. In such studies, growth factors, such as the ones extracted and secreted from the platelets are incubated with meniscal cells and then injected into injured meniscal tissue to see if cellular repair and regeneration occurs. Many studies demonstrate that injection of various growth factors can increase meniscal cells activity and stimulate repair in this tissue and other connective tissues.93 The ideal mode of treatment for meniscal tears and degeneration would stimulate the production of meniscal fibrochondrocytes and its synthesis of extracellular matrix (ECM). Increased ECM synthesis would render the generated meniscal tissue more able to withstand the forces placed on the knee since it is the collagen, proteoglycans and glycoproteins in the ECM, that give the meniscus its compressive properties to withstand tensile loads.94
Platelet-derived growth factor (PDGF) is one growth factor commonly used in animal meniscus studies. One recent study measured both cell proliferation and extracellular collagen matrix formation in each of the inner, middle, and outer regions of sheep menisci in the presence of PDGF-AB. After one week, meniscal cell proliferation was apparent in all three meniscal zones, reaching an 800% increase in the inner vascular zone compared to control. The formation of the collagen matrix had increased by 450% in the middle zone and by 300% in the outer zone (see Figure 9). An increase in the production of glycosaminoglycans, a main component of synovial fluid, in each of the three zones was observed.95 Meniscal cell migration was also stimulated.
| Figure 9. Effect of platelet-derived growth factor-AB on DNA synthesis in cells from the three zones of the meniscus. Results are mean + SEM (N = 12). Platelet-derived growth factor at 1 and 10 ng/ml increased DNA synthesis (in both the middle and inner zones) by over 400% compared with control. Source: Bhargava MM, et al. The effect of cytokines on the proliferation and migration of bovine meniscal cells. Am J Sports Med. 1999;27:636-643. |
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A similar in vitro study found that cell production of sheep menisci increased in proportion to the increased concentration of PDGF-AB used. This study observed a 2.5-fold increase in cell production.96 Another in vitro study placed bovine meniscal cells in different solutions containing cytokines and measured the effect of each on the synthesis of new cells in each of the three meniscal zones. The authors reported that significant DNA synthesis occurred in meniscal cells treated with PDGF-AB, hepatocye growth factor, and bone morphogenic protein-2 in all three regions.97
Similar results were found when analyzing the effect of basic fibroblastic growth factor (bFGF) on meniscal cells from sheep. When cultured in the bFGF, the formation of DNA increased by as much as seven-fold and protein synthesis increased by as much as 15-fold in the inner (avascular) zone of the meniscus. The results of the outer and middle zones likewise yielded statistically significant cell growth.98,99 The synthesis of proteoglycans, the principle component of the extracellular collagen matrix, was specifically measured in another study on sheep menisci. In all meniscal zones, transforming growth factor beta (TGF-â) stimulated proteoglycan production by up to 100% and the proteoglycans were larger than controls. TGF-â also stimulated cell division in the fibrochondrocyte cultures.100 Other authors have also confirmed that meniscal fibrochondrocytes from all three zones, including the avascular zone, can proliferate and generate new extracellular matrix given the proper stimuli.101-104 Such findings have been the basis of the integration of growth factors in the treatment of meniscal pathology.
One study involved the use of growth factors TGF-â1 and insulin-like growth factor (IGF-1) as an aid in the insertion of meniscal plugs into the avascular portion of the meniscus. This study found that TGF-â1 was effective in forming an attachment between the actual meniscus and the plugs, and IGF-1 was effective in cell proliferation. Both growth factors also significantly increased the cell density of the plugs.105 Canine menisci with a defect in the avascular portion documented a 10- fold increase in healing by the addition of a fibrin sealant and endothelial cell growth factor.100 In this study, the ingrowth of new blood vessels (neovascularization) and granulation tissue (connective tissue) to the avascular portion of the meniscus was noted. Growth factors have even been introduced into surgical treatments, particularly meniscal transplantation, to preserve and enhance joint tissue.106,107
The evidence that avascular cells are capable of regeneration when properly stimulated to do so, serves as the basis and rationale for Platelet Rich Plasma Prolotherapy in the treatment of meniscal pathology as described in the following case reports.
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