Mosaicplasty or autologous osteochondral autograft/allograft transfer system or OATS is a single stage resurfacing procedure for focal defects of the femoral condyles, trochlea, patella, proximal and distal tibia, and talus. It may be used for both focal chondral defects as well as cases of osteochondritis dissecans.
Indications of Mosaicplasty
Ideal lesions are small, focal, full-thickness chondral or osteochondral defects measuring 10 to 20 mm square in diameter. Some have extended the indications to include defects of up to 30 or 40 mm square. The size is limited by donor site morbidity beyond this size. The primary limiting factor when using autologous tissue is the availability of donor tissue.
Allograft tissue may be used for more extensive lesions, including larger and deeper defects of cartilage and subchondral bone. The depth of the lesion is also a limiting factor and ideally should be no greater than 6 to 8 mm. Lesions deeper than this make it difficult to reconstruct subchondral bone while achieving coverage and maintaining congruency of the transplanted cartilage surface.
Traumatic, unipolar, grade IV lesions are the best candidates for mosaicplasty.
Best candidates are between age 15 to 50 years and preferably the body mass index is less than 30.
The procedure should not be done in patients with generalized osteoarthritis, rheumatoid arthritis, or other inflammatory arthritis and noncompliant patients. Tumors, infection, and the lack of an appropriate donor area for the osteochondral grafts, bipolar lesions are also contraindications.
Associated injuries must be addressed simultaneously or in staged fashion in order for cartilage repair to be successful.
Meniscal transplantation must be done concurrently with mosaicplasty as the two procedures are inherently interdependent for a good outcome to be achieved. Ligament reconstruction and joint realignment may be safely performed at the same time as mosaicplasty, although a staged approach may be undertaken if desired. In case of staged procedures, mosaicplasty is done after ligament reconstruction or osteotomy
Technique of Mosaicplasty
Mosaicplasty may be performed entirely arthroscopically, as a combined arthroscopic and mini-arthrotomy procedure, or as an open technique. Tourniquet control is recommended.
For knee lesions, the patient is put in supine position for arthroscopy. A standard lateral portal is made for scope insertion and diagnostic arthroscopy is undertaken to evaluate all cartilage damage as well as concomitant knee or other joint pathology.
Once identified the edges of the defect are sharply debrided with curettes, and an arthroscopic resection is used to abrade the lesion down to a viable subchondral bone. It is important to prepare the shoulder of the lesion such that sharply defined vertical walls of intact, normal, hyaline cartilage surround the defect.
Lesion size is estimated using sizers on prepared subchondral bone. At this point planning of the number and size of grafts is completed to give optimal lesion filling with available donor sites.
Donor sites are chosen from areas that have the least contact pressure in the joint like superior lateral aspect of the lateral femoral condyle followed by the superior medial aspect of the intercondylar notch. When performing core harvest, it is essential that the tube harvester be oriented perpendicular to the donor articular cartilage.
The principle of recipient bed preparation is to create a socket in the subchondral bone that enables a secure press-fit osteochondral graft to be placed. No additional fixation is used, thus graft size and socket size are created to achieve a secure press-fit fixation upon seating of each core.
Larger diameter grafts are more stable than smaller ones and that reinsertion after pullout significantly reduced primary fixation strength. When multiple grafts are to be transplanted it is vital that each one is completed before the creation of additional recipient sockets to avoid fracture of the recipient tunnel walls.
The donor harvester tubes are 1 mm larger than the recipient harvester tubes, allowing for press-fit insertion of grafts.
Graft transfers are continued until the defect is filled adequately and congruency is achieved with the surrounding articular surface.
After the closure of the wound, drains are routinely placed and left for 24 hours or until the output is minimal. Cold therapy is used in all patients for up to 7 to 10 days for control of swelling and pain relief.
Immediate continuous passive motion is started in the hospital and continued upon discharge.
The technique of mosaicplasty outside the knee is identical to that already described, although the approach to the lesion is joint specific. The ipsilateral knee is used for donor grafts.
Allograft Transplantation with Mosaicplasty Technique
The operative technique is no different with allograft. Lesion preparation is performed followed by sequential allograft core harvest. Allograft tissue is expensive, and there is a limited availability of age-appropriate donor tissue. Other concerns include the risk of disease transmission, immune reaction, and infection.
Rehabilitation should always be tailored to the individual lesion and patient. The patient is put on immediate mobilization but kept nonweight and progressed to weight bearing over the next 2 to 3 weeks, followed by full weight bearing.
When mosaicplasty is combined with ligament reconstruction or osteotomy, brace use is required. Return to athletics should be delayed in order for adequate graft integration and healing to occur. Once quadriceps strength has been regained and symptoms are absent, return to play is allowed around 6 to 9 months.