Articular cartilage is the smooth, white glistening tissue that covers the ends of bones where they come together to form joints. Healthy cartilage in our joints makes it easier to move as it allows the bones to glide over each other with very little friction.
Articular cartilage can be damaged by injury or normal wear and tear. Damaged cartilage does not heal itself well, and a number of surgical techniques have been developed to stimulate the growth of new cartilage. Restoring articular cartilage can relieve pain and allow better function. Most importantly, it may delay or prevent the onset of arthritis.
Hyaline Cartilage
The main component of the joint surface is a special tissue called hyaline cartilage. When it is damaged, the joint surface may no longer be smooth. Moving bones along a tough, damaged joint surface is difficult, due to the increased friction, and causes pain. Damaged cartilage can also lead to arthritis in the joint.
The goal of cartilage restoration procedures is to stimulate new hyaline cartilage growth.
Patient Eligibility
Most candidates for articular cartilage restoration are young adults with a single injury, or focal lesion. For the autologous chondrocyte procedures, the age cut-off is currently 55 years. Older patients, or those with many lesions in one joint, are less likely to benefit from the surgery. The knee is the most common area for cartilage restoration. Ankle, shoulder and occasionally hip problems may also be treated.
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Surgical Procedures
Many procedures to restore articular cartilage are done arthroscopically. In general, recovery from an arthroscopic procedure is quicker and less painful than a traditional, open surgery.
The most common procedures for cartilage restoration are:
- Microfracture
- Drilling
- Abrasion Arthroplasty
- Autologous Chondrocyte Implantation
- Osteochondral Autograft Transplantation
- Osteochondral Allograft Transplantation
Microfracture
The goal of microfracture is to stimulate the growth of new articular cartilage by creating a new blood supply to the focal defect/lesion. A sharp tool called an awl is used to make multiple holes in the joint surface. The holes are made in the bone beneath the cartilage deficiency, called subchondral bone. This action creates a healing response. New blood supply can reach the joint surface, bringing with it new cells that may stimulate the formation of the new cartilage. Microfracture can be done with an arthroscope. The best candidates are young patients with single lesions and healthy subchondral bone.
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(Left) Damaged cartilage is removed. (Centre) Awl is used to make holes in the subchondral bone. (Right) Healing response stimulates new, healthy cartilage cells.
Drilling
Drilling, like microfracture, stimulates the production of healthy cartilage. Multiple holes are made through the injured area in the subchondral bone with a surgical drill or wire. The subchondral bone is penetrated to allow for the passage of a new blood supply to create a healing response. Drilling can be done with an arthroscope. It is less precise than microfracture and the heat of the drill may cause injury to some of the tissues.
Abrasion Arthroplasty
Abrasion arthroplasty is similar to drilling, but Instead of drills or wires, a high-speed burr is used to remove the damaged cartilage and reach the subchondral bone. Abrasion arthroplasty can be done with an arthroscope.
Autologous Chondrocyte Implantation (ACI) and MACI
ACI is a two-stage procedure. New cartilage cells are grown and then implanted in the cartilage defect.
At stage 1, which is an arthroscopic procedure, healthy cartilage tissue is harvested from a non-weight bearing area of the joint. The tissue that contains the healthy cartilage cells, or chondrocytes, is then sent to the laboratory. The cells are cultured and increase in number over a 3- to 6-week period.
At stage 2, an open surgical procedure, or arthrotomy, is then done to implant the newly grown cells. The cartilage defect area is cleaned and prepared. A layer of bone-lining tissue, called periosteum, is sewn over the area (this periosteum is most easily obtained from an adjacent area on the exposed non-articular part of the lower femur inside the joint space). This cover is sealed with fibrin glue. The newly grown cells are then injected into the space under the periosteal cover. A restriction on weight-bearing will continue for at least 6-8 weeks post ACI graft to allow the cells to adhere to the underlying bone. Further, range of motion may need to be limited in a brace for a period.
One of the commonest complications is periosteal hypertrophy, due to the formation of excessive scar tissue at the edges of the periosteal patch, and then may need a subsequent arthroscopic trimming. Other complications include failure of the chondrocyte graft to proliferate and adhere (implant failure), infection, and stiffness.
ACI is most useful for younger patients (under 55 years) who have single defects larger than 2 cm in diameter. ACI has the advantage of using the patient's own cells, so there is no danger of a patient rejecting the tissue. It does have the disadvantage of being a two-stage procedure that requires an open incision. It also takes several weeks to complete, and this is often requires serial MRI scans to confirm incorporation.
A modification of the ACI technique exists, called MACI (matrix autologous chondrocyte implantation), where the harvested healthy cartilage cells are grown in the laboratory on a collagen matrix sheet. The advantage of this in that the matrix sheet is already impregnated with the cartilage cells, the sheet itself may be shaped to size and directly glued (with fibrin glue) and sutured to the defect area, and no periosteal flap is required, which takes away the risk of periosteal hypertrophy. Also, as there is no injection, as the cells are on/in the matrix sheet, the cells cannot leak away from the area.
Osteochondral Autograft Transplantation
In osteochondral autograft transplantation, cartilage is transferred from one part of the joint to another. Healthy cartilage tissue -- a graft -- is taken from an area of the bone that does not carry weight (non-weight bearing). The graft is taken as a cylindrical plug of cartilage and subchondral bone. It is then matched to the surface area of the defect, a hole is created, and the combined cartilage and bone plug is impacted into place. This leaves a smooth healthy cartilage surface in the joint. A single plug of cartilage may be taken, or a procedure using multiple plugs, called mosaicplasty, may be performed. Osteochondral autograft is used for smaller cartilage defects. This is because the healthy graft tissue can only be taken from a limited area of the same joint. It can be done with arthroscopic assistance.
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Mosaicplasty type osteochondral autograft transplantation procedure.
Osteochondral Allograft Transplantation
If a cartilage defect is too large for an autograft, an allograft may be considered. An allograft is a tissue graft taken from a cadaver donor. Like an autograft, it is a block of cartilage and bone. In the laboratory it is sterilized and prepared, as well as being tested for any possible disease transmission. The best results have been obtained using fresh allograft, which has not been irradiated (as this may damage any surviving cartilage cells). An allograft is typically larger than an autograft. It can be shaped to fit the exact contour of the defect and then press fit into place, often supplemented with some form of internal fixation (such as screws). In the knee, a combined realignment osteotomy is often performed t the same time to unload the new graft area. Allografts are typically done through an open larger incision.
This technique has been in use for well over 20 years. Professor Allan Gross, from Toronto, Canada, has been one of the doyens of this procedure. A/Prof Woodgate previously has been a fellow of Prof Gross, and has been instructed and performed the procedure.
Stem Cells and Tissue Engineering
Current research focuses on new ways to make the body grow healthy cartilage tissue. This is called tissue engineering. Growth factors that stimulate new tissue may be isolated and used to induce new cartilage formation.
The use of mesenchymal stem cells is also being investigated. Mesenchymal stem cells are basic human cells obtained from living human tissue, such as bone marrow, although they can be found in much more limited numbers in fat (abdominal wall). When stem cells are placed in a specific environment, they may revert from their precursor form and differentiate into cells that are similar to the host tissue. The hope is that stem cells placed near a damaged joint surface will stimulate hyaline cartilage growth.
Unfortunately, despite numerous market driven forces, stem cell transplantation/injection into knees, harvested from abdominal wall fat via a liposuction procedure, has failed to produce any significant cartilage regrowth. Whilst some patients have anecdotally reported an improvement in symptoms for up to 6-12 months, this cannot be explained fully as there has been no new cartilage formation.
In Australia, this procedure has been deemed to be ineffective by both the Australian Orthopaedic Association and the Royal Australasian College of Surgeons. Sports physicians most commonly offer the procedure, but given the lack of cartilage regeneration documented, it cannot currently be recommended as an option.