OATS - Osteochondral Autograft or Allograft

Transfer System

Overview

A procedure in which a small, full-thickness cartilage defect can be patched by transferring a plug of healthy cartilage into that defect. The donor graft has immediate stability and structural properties. Using one or more smaller plugs allow the procedure to be performed arthroscopically.

Background & Rationale

This procedure involves harvesting small cylindrical cartilage plugs from low contact areas of the knee and transferring them to areas of full-thickness cartilage loss within the same knee. This is an autograft procedure. Transferring cartilage plugs from a cadaver joint to the patient’s knee is known as an allograft procedure. The term osteochondral means that there is a core of bone (osteo) that remains connected to the cartilage cap. Yamashita et al described versions of this procedure as early as 1985. Current techniques that utilize specialized instrumentation were introduced in the early 90’s by both Hangody and Bobic. The object of the procedure is to re-surface small areas of full-thickness cartilage loss with healthy plugs of cartilage that have immediate stability, material properties and viability. The procedure overcomes multiple issues with cartilage transfer techniques. The cartilage cap remains connected to the underlying boney support structure and it is technically possible to transfer this cap arthroscopically with immediate fixation. Single plugs can be used for smaller lesions while multiple plugs of various sizes can be used to fill larger defects, arranged in patterns that resemble a mosaic, hence the term mosaicplasty.

Indications

Indications are similar to that of a microfracture procedure, described above. The technique is preferably used for small to mid-sized full-thickness cartilage lesions, generally less than 4 square cm is thought to be favorable for this procedure. As with most cartilage procedures, the more extensive the damage, a less favorable outcome is to be expected. Single rather than multiple lesions are preferable. The predictability of this procedure increases in knees with normal alignment, acute rather than chronic damage, low BMI, minimal radiographic evidence of degeneration with good preservation of the joint space. Patients who are elderly with more extensive degenerative disease do less predictably well. Partial-thickness lesions are treated with debridement chondroplasty and generally not converted to full-thickness lesions. Lesion that are "contained" or “well-shouldered,” i.e., have a steep, normal surrounding cartilage rim, produce superior outcomes. Patients older than 50 years, defects deeper than 10 mm and systemic inflammatory disease are relative contraindications.

Technique

Although this procedure can be performed through a small incision, particularly if larger grafts are necessary (see section below on “Large Osteochondral Allografts”). Arthroscopic visualization of the knee is performed and the lesion is assessed for compatibility with this procedure. An MRI can be of some help in pre-identifying appropriate lesions. Very often, patients come to this procedure after they have failed to improve following a previous debridement chondroplasty.

Perpendicular access to the lesion to the lesion is necessary to the success of the procedure. The sides of the lesion are debrided to a healthy cartilaginous perpendicular rim and the base to healthy subchondral bone. The size and number of grafts are decided upon and mapped, and the depth of the defect is measured. A plug of bone is removed from the cartilage lesion and creates the recipient site. The donor plug for this site is harvested with a thin cylindrical coring tool from a low contact area of the knee. The donor plug, with its healthy cartilage cap, is then transferred and implanted into the recipient site. The plug is impacted to the correct depth, ideally flush with the surrounding native cartilage rim. This process can be repeated until the defect is filled. The areas between the grafts can undergo microfracture. The plugs taken from the lesion can be used to backfill the donor sites. The low contact donor sites will heal with cancellous bone and fibrocartilage over time.

Recovery

CPM and cold therapy are often used directly after the procedure. Crutches and toe-touch weight-bearing is generally recommended for several weeks, though this may be more or less depending on the size of the defect. Therapy is started early on to regain motion and strength. Most patients are full weight-bearing at 6 weeks, jogging progressing to running at 12 weeks and full return to sport at about 6 months.

Results

The results of osteochondral autograft transplantation reported in the literature are generally good to excellent, yet this procedure peaked soon after it was introduced and seems to be used less frequently. Though the graft appears to heal well and provide visual coverage, patients’ symptoms may persist to some degree. It should be noted that insurance companies only grudgingly approve this procedure, which may reflect on its incidence of use. Woodson and colleagues noted the following in a review of the literature. Most series are small and retrospective without long-term follow-up. Bobic initially reported on 10 patients with ACL-deficient knees undergoing ACL reconstruction and autograft transplantation concomitantly. Follow-up arthroscopy at 1 year in 9 patients demonstrated "normal cartilage appearance and color." Donor sites were not readily identifiable. Hangody's reported on 44 patients treated with mosaicplasty compared with 21 patients with "similar conditions" treated with abrasion arthroplasty. Improvement was moderately better in the OATs group (94 to 78). A number of other investigators have reported positive results with mosaicplasty. Currently, osteochondral autograft transfer is a good option for younger patients with isolated lesions (preferably less than 2 square cm) who have failed previous treatments such as microfracture. The procedure is technically demanding. Complication rates are low if performed in appropriate patients.