OATS (osteochondral autograft transfer system) or mosaicplasty is an operation to transplant plugs of healthy cartilage and bone from a non-weight-bearing area of the knee to fill a cartilage defect in a weight-bearing area.
One or more cylindrical plugs of cartilage and underlying bone are harvested from a non-weight-bearing area of the knee (usually the edge of the trochlea) using a special coring instrument. The damaged area is prepared and the plugs are press-fit into the defect, recreating a smooth joint surface. For small defects, this can be done arthroscopically; larger defects require a small open incision. The operation takes one to one and a half hours.
OATS is recommended for localised, full-thickness cartilage defects (1–4 cm²) in the weight-bearing area of the femoral condyle, particularly in young, active patients. It provides true hyaline cartilage coverage of the defect.
Physiotherapy, activity modification, injections, microfracture (for smaller defects), and ACI/MACI (for larger defects). Osteotomy may be needed if alignment is abnormal.
MRI to assess defect size and location. Arthroscopy may be performed first to assess suitability.
Restoration of the joint surface with the patient's own hyaline cartilage. Good to excellent results in approximately 80–90% of well-selected patients at ten years.
Pain or clicking at the harvest site. Usually mild and improves.
Six to nine months for full return to sport.
Knee swelling during healing.
Plugs may not integrate or may subside.
Cartilage damage at the harvest site.
Joint infection.
Loss of knee flexion.
General or spinal anaesthesia.
Partial weight-bearing for six to eight weeks. CPM machine may be used. Physiotherapy for range of motion and strengthening. Running at four to six months. Return to sport at six to nine months.
Reviews at two weeks, six weeks, three months, and six months. MRI to assess graft incorporation.
OATS transplants true hyaline cartilage from elsewhere in the knee, providing durable joint surface restoration. Microfracture stimulates fibrocartilage formation, which is less durable. OATS is preferred for many defects between 1 and 4 cm².
For defects larger than 4 cm², ACI/MACI (laboratory-grown cartilage) or allograft transplantation may be more appropriate.