The same operation — with one key difference

If you've been told you may need 'OATS or mosaicplasty' and wondered whether your surgeon is describing one thing or two, the short answer is: largely one thing, with a size-driven variation.

Both techniques transfer cylindrical plugs of a patient's own bone and cartilage — harvested from a low-load area of the femoral condyle — and press-fit them directly into the damaged surface of the knee. The material transplanted, the harvest site, and the single-operation format are identical. What differs is the number and diameter of those plugs, which reflects the size of the defect being treated.

OATS transfers one larger cylinder, typically 8–12 mm in diameter, suited to compact focal defects of roughly 2 cm² or less. Mosaicplasty — a technique that entered clinical use in 1992, and whose name refers to the pattern the repair creates — tiles several smaller plugs, each 2.7–8 mm wide, across a wider repair area of 2–4 cm². Think of the difference between laying a single floor tile and piecing together a mosaic from multiple smaller ones: the underlying craft is the same; the scale changes the approach.

In clinic, the two terms are sometimes used interchangeably, which can cause confusion. The practical distinction is defect size, not a fundamentally different surgical philosophy.

Why both procedures use your own cartilage

The plugs described above are not simple bone cylinders — each carries a surface layer of genuine hyaline cartilage, the smooth, load-bearing material that lines a healthy knee joint. Transplanting this tissue directly into the repair site is what gives both procedures their clinical rationale.

Harvest comes from the peripheral rim of the femoral condyle, where the joint surface is under considerably less mechanical stress than the central weight-bearing zone. Taking small cylinders from this region causes minimal functional disruption to the donor area while delivering tissue that is structurally identical to what the patient has lost.

The distinction from marrow-stimulation surgery — such as microfracture — matters here. Microfracture does not transfer cartilage; instead, it perforates the subchondral bone to draw in stem cells, which then form fibrocartilage at the repair site. Fibrocartilage is a scar-like tissue with lower stiffness and reduced tolerance for repeated loading. Some studies report that it begins to break down within two to three years in physically active patients, a finding that has contributed to a notable decline in microfracture as a primary option for this group.

Because the graft comes from the patient's own body, there is no rejection risk and no need to source or process donor tissue. Both techniques also complete in a single operating session — a practical simplicity relative to procedures that separate an initial biopsy from a later re-implantation.

Defect size drives the choice between the two

Defect area, measured at the time of arthroscopic assessment, is the primary variable guiding the decision. The size thresholds map to each technique fairly cleanly:

• Under approximately 2 cm² — OATS is the typical choice: a single larger plug fills the zone without gaps or mismatched surfaces.
• 2–4 cm² — Mosaicplasty was specifically developed for this range, where one plug provides insufficient coverage but the tiled pattern of multiple smaller cylinders can fill the repair area effectively.
• Above approximately 4 cm² — neither technique provides adequate coverage. At this scale, cell-based options such as MACI or ACI — which culture the patient's own cartilage cells before re-implantation — or fresh osteochondral allograft (OCA) sourced from a donor tissue bank become the more appropriate routes.

Anatomical location adds a second constraint. OATS, in particular, may not be technically feasible at certain sites: the trochlea, the undersurface of the patella, and the tibial surface present geometrical challenges that can make precise plug placement impractical. Mosaicplasty, with its smaller and more configurable grafts, may offer slightly greater flexibility on curved or irregular surfaces.

Surgical access also affects the practical ceiling. An open approach can accommodate defects roughly three times larger than an arthroscopic one, meaning the planned route into the joint — not just the defect size — influences which size threshold is achievable in practice.

These thresholds give a useful framework, but exact defect dimensions, depth, and location can only be confirmed by direct consultant assessment.

Who is a suitable candidate

Three questions tend to determine suitability: how active is the patient, how localised is the damage, and what has already been tried?

Age and activity level

Both procedures are designed for durability — the transplanted cartilage must withstand sustained mechanical loading over many years. The typical candidate is a younger, physically active patient for whom long-term joint function is a clear priority and for whom a repair that holds up under sport or regular strenuous activity is the meaningful measure of success.

Lesion type and grade

The target is a focal, full-thickness defect graded ICRS III or IV — meaning cartilage worn through to or into the underlying bone in a clearly defined zone, rather than thinning diffusely across the joint. ICRS (International Cartilage Repair Society) grades run from I (surface softening) to IV (bone exposed); grades III and IV represent the depth and severity where active cartilage restoration is most strongly supported by evidence.

Prior treatment

A period of conservative management — physiotherapy, activity modification, and appropriate injection support — is the usual prerequisite before surgery is considered. Patients who have not yet worked through those options are not typically at the referral stage for these procedures.

Diffuse or advanced osteoarthritis falls outside the scope of OATS and mosaicplasty. Where cartilage loss affects the whole joint compartment rather than a contained focal patch, neither technique can address the wider deterioration.

Underlying alignment also matters: significant varus or valgus malalignment places uneven load on the repaired area and may need to be corrected — sometimes through an osteotomy carried out at the same time — to give the new graft a realistic chance of long-term survival.

Outcomes and the donor site trade-off

Published series suggest favourable results for OATS in appropriately selected patients. Gudas et al. reported good clinical outcomes in around 86–90% of cases at three-year follow-up, with an overall failure rate of approximately 13% at a mean of 3.6 years. Longer follow-up data — notably a ten-year comparative study (Gudas 2012) — reinforces the durability advantage of osteochondral autograft transfer over marrow-stimulation techniques in physically active patients.

The trade-off both techniques carry is donor site morbidity. Harvesting plugs from the low-load peripheral rim of the femoral condyle creates a secondary defect in the same knee, and for some patients this produces mild aching or stiffness at the harvest site in the period following surgery. The low-demand nature of the harvest zone is chosen specifically to minimise functional impact, and symptoms typically resolve over months — but it is a genuine clinical consideration rather than a theoretical one, and worth discussing with a surgeon before a decision is reached.

Where the evidence is less settled is in the direct comparison between OATS and mosaicplasty. Most comparative studies measure each technique against microfracture or allograft rather than against each other, and detailed long-term outcome data for mosaicplasty beyond ten years remains limited in the published literature. In the absence of head-to-head trial data, the available registry evidence and prevailing surgical consensus point to broadly comparable results within each technique's appropriate size range. The choice between them rests primarily on defect dimensions and anatomy — not on a demonstrated outcome difference between the two approaches.

Where OATS and mosaicplasty sit in the repair pathway

Cartilage repair follows a rough clinical sequence: conservative measures — physiotherapy, load management, and appropriate injection support — come first. Where those approaches have been exhausted and imaging confirms a focal full-thickness defect of the right grade and size, restorative surgery enters the picture.

OATS and mosaicplasty occupy the middle of that ladder. Beyond approximately 4 cm², the options shift: defects too large for autograft techniques point towards cell-based approaches such as MACI or ACI — two-stage procedures capable of addressing wider lesions. For the largest or most structurally compromised cases, fresh osteochondral allograft (OCA) extends the reach of restorative surgery further still. Where cartilage loss has become diffuse rather than focal, joint replacement becomes the relevant conversation.

Identifying where a patient sits within that sequence takes a specialist consultation. Defect size and grade are only part of the picture: location, joint alignment, and the patient's functional goals all shape what is technically appropriate and what a surgical team can realistically achieve. Search MSK lists cartilage repair specialists across the UK who offer OATS, mosaicplasty, and related procedures — filtering by region and subspecialty is a practical way to identify a surgeon with the relevant experience.