then differentiated into chondrocytes for therapeutic purposes. While the cells are obtained in a painless, noninvasive fashion, the induction process to form pluripotent cells is not half as efficient; and if fibroblasts mix with the population of chondrocytes, it leads to the formation of scar tissue that interferes with the healing process.
The BMP and TGF-β family of proteins induce the differentiation into chondrocytes. Once the chondrocytes are ready for application, the periostal flap from a different bone site is resected. The cartilage to be healed is closed off with this periosteal flap and the chondrocytes are injected into the gap. The chondrocytes proliferate and mineralize the matrix to completely regenerate the cartilage. While this is an efficient mechanism, there are some cases in which the use of scaffolds are necessary. In using scaffolds, biomimetic osteoinductive biomaterials are used as substrates. The chondrocytes are then applied to the scaffold, which provides a support structure and network for the chondrocytes to proliferate. The scaffolds are then surgically inserted at the site of regeneration.
Some of the polymers that are used in bioengineering of tissue scaffolds are protein based, such as fibrin, collagen, and gelatin. Some carbohydrate-based polymers include hyaluranon, chitason, agarose, and others. Synthetic polymers are also used. A significant advantage of using such scaffolds is that they prevent immunogenic reaction and eliminate the time required for acceptance of the scaffold by the body—a significant decrease in the healing process.
Tendon Regeneration
Tendons are made of cells called tenocytes. Repair of the tendons takes place mainly due to the formation of scar tissue in the adult and it takes about a year or two to mature. The healed tissue contains fibroblast and fibrous tissue that form the scar tissue and that are not essentially the tendon cell type. Therefore, to hasten this process and to make them more robust, tendon wounds are treated through various methods.
As described above, in cartilage regeneration, stem cells that are derived from various sources are used. The cells then differentiate into tenocytes in vitro. With BMP-12, which is a subclass in the family of BMP proteins, the stem cells differentiate into tenocytes. BMP12 is transferred into the cells by using gene therapy, and this triggers the differentiation using the pluripotency network. The tenocytes are applied to the site of the wound directly where the wound environment provides the appropriate cues for further differentiation and proliferation.
These regeneration therapies have been tested in several animal models such as mouse, chicken, rabbit, sheep, and monkeys. Very few have been extended to human subjects. The clinical trial for the regeneration of tendons is the technology wherein the tenocytes are applied to the site of the wound that promotes regeneration. Similar to cartilage, scaffolds are also used to deliver these cells to the site of the wound.
Taking the treatment from animal models to human subjects requires the collaboration of cell and molecular biologists and surgeons to work together to concur on the cellular mechanics and delivery of the scaffold. There are several lobbyist groups such as the arthritis foundation that are pushing boundaries to take the therapy to the next level.
Ligament Regeneration
Ligaments are soft yet mechanical tissues that are capable of load bearing. They are of enormous mechanical strength, yet the regeneration process is not robust because of the decreased requirement of nutrients and oxygen for the ligament tissue. This results in the formation of weak tissue and so the regeneration of ligament has been gaining a fair amount of attention. One of the first therapy options for the purposes of regeneration is to apply biomolecules at the site of the wound. These biomolecules, such as TGF-beta and PDGF, promote the formation of ligament—inside the body. This process uses the human body as a bioreactor to generate the ligament. Some of the other options are very similar to those of cartilages and ligaments wherein stem cells are isolated from the body, differentiated in vitro, and then later applied to the site of the wound. Biomaterials and biomimetics are said to generate structure to generate the inserts of desirable shape.
Another important consideration in choosing scaffolding material in the case of ligaments is that, considering the function of ligaments, they need to be durable and strong. The load-bearing capacity of the scaffolding tissue is tested in vitro before it can be applied to the bone. Since ligaments are mostly tissue that are attached to bone and facilitate the easy movement of the joint, the shape and pliability of the scaffold are also very important. Natural and synthetic polymers can be used as scaffolds. The advantage of a natural polymer over synthetic polymer is that it offers a surface that is adherent for the cells. Natural polymers also do not release acidic chemicals on hydrolysis, but one of the significant disadvantages is that they are degradable, and once inside the body system, the degradation curve cannot be controlled as yet.
The regeneration of bone commonly involves the regeneration of soft tissues as well, because the bones and joints are held together with the help of cartilage, tendons, and ligaments. All the processes in concert with one another are required for efficient regeneration of the structural system. When a bone regenerates, the tendons and cartilage also need to regenerate to hold the structure in place. Combinations of scaffolds and different biomaterials for bone, cartilage, tendons, and ligaments are being theorized to facilitate a complete healing of the skeletal subunit.
Clinical Trials
Most of the clinical trials in the area of cartilage and tendon regeneration are focused on autologous regeneration. Most of the scaffold and biomaterial supplements are in the stages of animal model testing.
In autologous reconstruction of worn-out cartilage, cartilage is extracted from a different site in the patient. This extracted cartilage is carved ex vivo and then applied to the site of the wound that needs to regenerate. Clinical trials are in progress for supplementing this process with plasma platelets. These trials test individual factors such as injection mode, time of injection, volume of injection, and whether a single dose or a series of injections in smaller volumes is preferable. The scientific efficacy of the test parameters is evaluated to devise a therapeutic or surgical plan. At the end of clinical trials, the methodology is also assessed for patient benefit, affordable costs, and the risks involved in the implementation of the process. A joint patient-doctor approach is used to evaluate the aforementioned parameters.
Ligament tears are very common in athletes and sportsmen, who require a robust and quick fix to get the joints back in working condition. The anterior cruciate ligament (ACL) in the knee connects the two bones in the joint, and it is one of the most common ligament tears. In 2014, a clinical trial is in progress to test the replacement of this ligament with the polymer poly-L-Lactic acid. The polymer is inserted via the bone in both knees and it substitutes for the ACL.
Outlined above are some of the clinical trials that are devising therapeutic strategies. The cell replacement therapy and tissue engineering is yet to reach the clinical trials as far as tendons, ligaments, and cartilage are concerned.
Sharanya Kumar
Independent Scholar
See Also: Cartilage, Tendons, and Ligaments: Cell Types Composing the Tissue; Cartilage, Tendons, and Ligaments: Current Research on Isolation or Production of Therapeutic Cells; Cartilage, Tendons, and Ligaments: Development and Regeneration Potential; Cartilage, Tendons, and Ligaments: Major Pathologies; Cartilage, Tendons, and Ligaments: Stem and Progenitor Cells in Adults.
Further Readings
Kuo, C. K., J. E. Marturano, and R. S. Tuan. “Novel Strategies in Tendon and Ligament Tissue Engineering: Advanced Biomaterials and Regeneration Motifs.” Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology, v.2/20 (2010).
Paoloni, J., R. J. De Vos, B. Hamilton, G. A. Murrell, and J. Orchard. “Platelet-Rich Plasma Treatment for Ligament and Tendon Injuries.” Clinical Journal of Sport Medicine, v.21/1 (2011).
Tuan, R. S., A. F. Chen, and B. A. Klatt. “Cartilage Regeneration.” Journal of the American Academy of Orthopaedic Surgeons, v.21/5