Future Treatments

Researchers have begun to investigate how to improve the slow, poor healing process of injured tendons and ligaments. Many of the experiments have been done on acute surgically-induced injuries, but the research should help develop techniques for chronic injuries as well.

I've given some examples of studies done on the various treatments listed below. My discussion is not meant to be an exhaustive review of the literature, but this sampling should give you an idea of the possibilities for future tendinosis treatments. Email me if you have research you think I should add to this section (be sure to give me references to the relevant journal articles published in the medical literature).

Note: The numbers in brackets after some sentences on this page are references found on the References page.

Local Injection of Stem Cells

Stem cells are progenitor cells found in embryos and also in some tissues of adults; these special cells can differentiate into cells for many different kinds of tissue such as bone, fat, cartilage, tendon, nerve, blood, brain, or muscle. Embryonic stem cells can differentiate into more tissue types than adult stem cells, but adult cells are more available and avoid the ethical and political issues associated with the use of embryonic cells. Adult stem cells have been found in many parts of the body, for example in fat, bone marrow, and skin.

One kind of stem cell is the mesenchymal stem cell or MSC; this type of cell can differentiate into various kinds of connective tissue. Adult bone marrow is one source for MSCs. Researchers are exploring how to use MSCs to repair tissues such as bone, tendon, ligament, and cartilage.[47]

Osiris Therapeutics Inc. is a small, private company in Baltimore, Maryland that is developing methods to use MSCs from adult bone marrow to treat injuries and diseases. Osiris can take a small number of MSCs and culture them to grow into large numbers of cells; then the cells can be directed to differentiate into cells for tendon, cartilage, bone, bone stroma, or muscle. Osiris is working on products to treat bone fractures, to treat patients who need to regrow bone marrow, and to create new heart tissue after heart attacks. Osiris is also investigating using the MSCs to treat osteoarthritis, cartilage injuries, and tendon/ligament injuries.

In several tendon studies, Osiris researchers surgically created one-centimeter-long gap defects in rabbit tendons and then implanted composites of stem cells suspended in Type I collagen gel into the injuries.[2,3,15] In one study, the MSC treated tendons were twice as strong as the untreated tendons after 4, 8, and 12 weeks.[15] The treated tendons also had larger cross-sectional area and better aligned collagen fibers. The authors concluded, "The results indicate that delivering mesenchymal stem cell-contracted, organized collagen implants to large tendon defects can significantly improve the biomechanics, structure and probably the function of tendon after injury."[15]

The researchers used autologous cells for this study (cells from the rabbits in the experiment), but Osiris has discovered that MSCs don't seem to provoke an immune response. Therefore, donors of MSCs do not need to be matched to recipients. Many researchers hope that we can develop a storage bank of MSCs that could be used to treat various injuries and diseases.

Although the Osiris studies looked at acute surgically-created tendon injuries, this research could be extended to look at chronic tendon injuries. Instead of surgically implanting MSCs into tendon gaps, MSCs could be injected directly into the area of chronic injury. The MSCs would be healthy cells uninjured by repetitive motion, and they could go to work creating new healthy collagen to slowly repair the area of failed healing.

One of the beauties of stem cell treatment for tendinosis is that it could be helpful under many of the scenarios postulated for the failed healing response (see Possible Reasons for the Failed Healing of Tendinosis). If the failed healing was caused by fibroblasts that were damaged by growth factors (or something else) associated with the repetitive motion, the new healthy cells would not have this damage and could heal the injury. For example, if the repetitive motion caused the fibroblasts to react abnormally to growth factors or to produce abnormal levels of proteolytic enzymes, the injected MSCs would provide some new cells with normal healing behavior. If the failed healing was caused by an abnormality in the genes of the fibroblasts, the MSCs could be taken from a donor who does not have that abnormality. In cases where the patient had genetically weaker tendons to start with, perhaps from a high Type III/Type I collagen ratio, stem cells taken from a donor without tendinosis would produce normal tendon collagen that might be stronger than the patient had originally.

Stem cell treatments would not make people immune to tendinosis, nor would they provide instant healing. They might, however, provide a way to get the tendon to heal with better collagen so that people could return to the way they were before the injury. Stem cells might be able to get the tendon out of its cycle of failed healing. Even though the healing would still take time, the end result might be tendon that would be much closer to normal uninjured tendon.

Stem cell treatments look very promising for acute and chronic tendon injuries; companies like Osiris need more funding to complete this research more quickly. Osiris put its tendon research on hold and is now concentrating on other products that are closer to being marketable. Osiris did have a $750,000 grant from the NIH for the tendon/ligament studies, but that grant was made several years ago and has been depleted.

If you're interested in trying to help this research move forward, please email me . I'm hoping we can speed up the process by finding funding that is specifically targeted for researching stem cell treatments for chronic tendon injuries.

Manipulating Growth Factors

Growth factors are proteins that stimulate cell proliferation and differentiation. Some growth factors can cause normal uninjured tendon fibroblasts to proliferate and synthesize more collagen and proteoglycans. Since growth factors play an important role in tissue healing, researchers have wondered if they could be used to improve the healing of tendons and ligaments.

Research into growth factor treatments is difficult because the effects of growth factors can be very different in vivo than in vitro and because fibroblast cells injured by repetitive motion can react differently to growth factors than normal cells. [1] In a study of carpal tunnel syndrome, wrist ligament cells from injured and uninjured people were exposed to four growth factors, including transforming growth factor beta (TGF-beta).[1] The cells from the injured patients produced abnormally high amounts of Type III collagen and low amounts of Type I collagen when exposed to the growth factors, as compared to the controls. The cells in the injured patients seemed to have been altered by the injury so that their response to growth factors was different. Therefore, studies that use growth factors to improve healing of acute tendon injuries might not apply to healing of tendinosis injuries.

Nevertheless, growth factors are worth studying to determine their potential for treating acute tendon and ligament injuries and to see if any of the growth factors have positive effects on repetitive motion injuries as well. If growth factor treatments don't seem to produce a good response from cells injured by repetitive motion, stem cell treatment could be combined with growth factor treatment; the stem cells would provide normal uninjured cells for the growth factors to stimulate, and the growth factors could stimulate them to produce healthy tendon/ligament collagen. See the previous section "Local Injection of Stem Cells" for more information about stem cells.

Another obstacle with growth factor therapy is that a fine line could exist between too little and too much of the growth factor; too little could cause inability to heal and too much could cause abnormal healing, scar formation, or other negative effects. When wounds and acute injuries heal normally, the body provides the correct balance of growth factors at the correct time in sequence as healing progresses from one stage to the next. More research is needed to investigate whether we can control the timing and the amount of added growth factors well enough to optimize healing. Researchers will need to investigate how the effects of various growth factors depend on the dose, the injury site, the stage in the healing process, and the interactions with other growth factors.

Various delivery methods for growth factors have been tried. Growth factors can be injected directly into the site of injury, but they tend to break down quickly. Researchers have had difficulty maintaining constant enough levels with the injection method. Other researchers have tried implanting controlled-release polymer matrices or microspheres into the injury site to slowly release growth factors into the tissue; these methods could be appropriate for some acute injuries, but a non-surgical method is better for tendinosis. Many researchers are now looking toward gene therapy delivery methods as being the most promising way to use growth factors to improve healing of injuries. See the section below on "Gene Therapy."

The following list of growth factors describes some of the studies that have been done to determine whether these substances can be used to help improve the healing of tendon and ligament injuries.

• IGF-1
  Insulin-like growth factor 1, or IGF-1, is a growth factor that is important for tissue healing. It can stimulate an increase in Type I collagen when added to normal fibroblasts.
  
  One study showed that tenocytes from healthy equine tendon made more Type I collagen relative to Type III collagen when treated with IFG-1 in vitro.[31] The tendon samples had "greater numbers of larger and more metabolically active fibroblasts," and IGF-1 enhanced collagen synthesis in a dose dependant manner. The authors suggest that IGF-1 might help treat horses with tendinosis. A growth factor that helps promote Type I collagen relative to Type III collagen in tendon is certainly worth more study for its potential use in treating tendinosis.
  
  Several other studies showed that a combination of IGF-1 and platlet-derived growth factor increased the rupture force, stiffness, and breaking energy in rat medial collateral ligaments.[32,33] Also, one study showed that treating injured rat Achilles tendons with IGF-1 reduced the "maximal functional deficit" and the "time to functional recovery."[34] Another study showed that IGF-1 and IGF-II stimulated collagen, proteoglycan, and DNA synthesis in a dose-dependent manner in rabbit flexor tendon in vitro.[35]
  
  IGF-1 was not one of the growth factors tried in the previously mentioned carpal tunnel syndrome study[1], so it would be interesting to discover its effect on cells from tendinosis patients.

• GDF-5
  Growth and differentiation factor 5, or GDF-5, has been linked to tendon healing in several studies. One study showed that the tensile strength of healing rat tendons increased in a dose-dependent manner when treated with GDF-5.[36] Another study showed that GDF-5 deficiency caused mouse tail tendon to have a 17% increase in the proportion of medium diameter collagen fibrils at the expense of larger diameter fibrils, as well as a 33% increase in irregularly-shaped polymorphic fibrils.[37] These structural differences did not cause major differences in biomechanical properties of the tendon, but did cause the fibers to relax 11% more slowly than controls during time-dependent stress/relaxation tests. More research would be needed to see if GDF-5 could play a role in the treatment of tendinosis.

• CDMP-2
  One research group has investigated the potential for treating tendon injuries with cartilage derived morphogenetic protein, or CDMP-2.[25] This protein is a member of the TGF-beta super family. The researchers treated injured rat Achilles tendons with injections of CDMP-2 and found that the treated tendons were 39% stronger than controls after 8 days. The tendons were also mechanically loaded during healing because the researchers suspected that loading would help the CDMP-2 induce tendon-like tissue instead of bone or cartilage tissue. (The abstract didn't say if the control tendons were also mechanically loaded; if not, the improved healing could be from the loading rather than from the CDMP-2. Presumably, they loaded both the controls and the treated injuries.)

• TGF-beta1
  Transforming growth factor beta1, or TGF-beta1, is a growth factor important in wound and tissue healing. It has been associated with excessive scar tissue formation in some cases. A group of researchers studied the effect of reducing TGF-beta1 because they were looking for a way to reduce the adhesions and scar tissue that commonly form between the site of injured hand flexor tendon and the surrounding tissues.[26,27] These adhesions reduce normal range of motion. Injured rabbit flexor tendons treated with neutralizing antibody to TGF-beta1 had approximately twice as much range of motion as the controls after 8 weeks of healing. This research might not have direct implications for treating tendinosis, but it does show that sometimes lowering growth factors can lead to better healing; more is not always better when it comes to growth factors.

• BMP-12
  Bone morphogenic protein 12, or BMP-12, has been shown to improve tendon healing; researchers found that in vivo gene therapy delivery of BMP-12 caused a two-fold increase in tissue strength and stiffness of healing chicken tendons.[38] See the section below "Gene Therapy."

Gene Therapy

The science of gene therapy is in its early stages, but it holds promise for treating all sorts of diseases and injuries. Gene therapy involves delivering a desired gene into cells and tissues in the patient's body to achieve therapeutic results. It can mean replacing a defective gene, or adding a gene that will cause cells to make beneficial proteins, or adding a gene that will cause cells to make proteins that will block harmful proteins. When applied to the healing of injuries, gene therapy could deliver a gene that encodes for a protein that would enhance the healing process, such as a growth factor. This method is better than simply injecting the growth factor directly into the injury because delivery via gene therapy allows the level of the growth factor to be maintained for the long periods of time required for tissue healing.

One of the biggest challenges facing gene therapy researchers is finding a good way to carry the desired gene into the targeted cell. Various gene carriers, or "vectors," have been tried, including liposomes and disabled viruses. Liposomes or viruses can carry the gene into the targeted cells of the body, and then those cells use that DNA to make the protein the DNA encodes. Nonviral vectors such as liposomes are less efficient at transferring genes, so more research has been done with disabled viruses than with nonviral vectors.

The vectors can be injected systemically into the patient's bloodstream, or they can be delivered locally to the desired site. Local delivery can be accomplished in several ways. The vector can be injected directly into the site in vivo, or the vector can be introduced into cells in vitro and then the modified cells can be injected into the desired site. The cells that carry the gene can be the patient's own cells that are modified and then reinjected or they can be stem cells.[19] The in vitro vector method is safer than the in vivo method because the virus is not introduced directly into the patient's body.

Gene therapy has successfully delivered genes to tendon and ligament tissue.[20,21,22,23,24,38] Gene therapy could help people who are especially prone to tendinosis because of genetic differences; if we could identify the genetic abnormalities that were making some patients prone to tendinosis, we could fix the abnormalities. This kind of gene therapy that involves treating a genetic problem is a long-term ambitious goal, and it only addresses genetic causes of tendinosis. Another approach is to use gene therapy to modify the failed healing of tendinosis rather than some genetic defect in the individual.

One example of using gene therapy to improve tendon healing is a study in which researchers used the gene for a growth factor called bone morphogenic protein 12, or BMP-12, to improve healing of injured chicken tendon.[38] First, the researchers used a virus to transfer the BMP-12 gene into chicken tendon cells in vitro; following treatment, the tendon cells increased their synthesis of Type I collagen. Then the researchers transferred the gene into lacerated tendons of chickens in vivo and observed "a two-fold increase of tensile strength and stiffness of repaired tendons, indicating improved tendon healing in vivo." The gene therapy succeeded in transferring the gene to the site of injured tendon and succeeded in improving the healing process. To see whether this success could translate into a treatment for tendinosis, we'd need to investigate whether BMP-12 could help stimulate Type I collagen synthesis even in tendon injured by repetitive motion (possibly in combination with stem cell therapy).

Another study used gene therapy to improve the scar tissue that forms when injured ligaments heal.[24] The researchers injected antisense decorin oligodeoxynucleotides into injured rabbit ligament to down-regulate the proteoglycan decorin and improve the mechanical properties of the area after it healed. After 6 weeks, the treated ligaments showed an 83-85% improvement in strength compared to the untreated controls. However, the authors state in a supplementary article that "mRNA for multiple genes were affected by the decorin-specific antisense treatment and therefore all of the observed improvements in scar tissue cannot be directly ascribed to depressing decorin levels."[39] More research will be needed, but the gene therapy did have a positive effect on healing.

The authors of the decorin study stressed that they used a method of gene therapy that did not permanently alter the tissue; it only temporarily modulated some substances during the early stages of healing. Researchers are working on various ways for gene therapy to target specific sites of injury and be activated only for limited periods of time. For local tissue healing applications, gene therapy should not have permanent or systemic effects.

Gene therapy could be used to regulate various substances, such as growth factors and proteolytic enzymes, that affect the healing process in tendon. We need more research to determine which substances are most helpful and harmful to the tendinosis injury. After identifying the substances, gene therapy offers a method of delivering the substances or their inhibitors to the site of injury.

For more information about gene therapy, see "Gene Therapy and Tissue Engineering in Sports Medicine".[43]

Nitric Oxide Synthase

Nitric oxide synthase, or NOS, is an enzyme that reacts with L-arginine (an amino acid) to produce nitric oxide. Researchers found that the three NOS isoforms are up-regulated following tendon injury and that inhibiting NOS activity with oral drugs reduces the cross-sectional area and failure load of healing Achilles tendon in rats.[28,29,30] Further study showed that the three isoforms are expressed by fibroblasts "in a coordinated temporal sequence during tendon healing." [30] The authors of the third study suggest that each NOS isoform might play a different role in healing and that these substances might be able to be manipulated to achieve therapeutic effects.[30] Another question would be whether any of the NOS isoforms are somehow inhibited in the failed healing process of tendinosis. We don't know much about NOS's role in tendon healing yet, so its potential as a therapeutic agent is unknown.