Osteochondral Autograft Transfer (OATS)

Osteochondral autograft transfer (OATS) — also termed mosaicplasty when multiple plugs are used — is a joint-preserving cartilage restoration technique in which cylindrical osteochondral plugs harvested from a low-load-bearing donor area of the knee are transferred to fill a full-thickness chondral or osteochondral defect. The technique transplants hyaline cartilage (not fibrocartilage) and provides durable, biomechanically superior cartilage at the repair site compared to marrow-stimulation techniques (microfracture). It is the preferred single-stage cartilage restoration procedure for small to medium full-thickness defects in the active patient.

• Indications: full-thickness chondral or osteochondral defect of the femoral condyle (medial or lateral), trochlea, or patella; defect size 1–4 cm² (ideal 1–2.5 cm²); young active patients (typically <45–50 years); failed or unsuitable for microfracture; symptomatic lesion causing pain, swelling, mechanical symptoms; stable knee (address ligamentous instability and malalignment concurrently)
• The key advantage of OATS over microfracture is the quality of cartilage: hyaline cartilage is transplanted rather than fibrocartilage produced by marrow stimulation; hyaline cartilage has superior biomechanical properties (stiffness, durability) and is the normal articular surface; fibrocartilage degrades faster and provides inferior long-term results particularly in large defects and high-demand patients
• Donor sites: the low-load-bearing periphery of the femoral condyles — the far lateral and medial femoral condyle margins, the lateral trochlear margin (intercondylar notch region); these areas have the least contact stress; the donor plug cross-sectional area must not exceed the safe harvest area or donor site morbidity becomes clinically significant

• Outerbridge classification (historically used at arthroscopy): Grade 0 normal; Grade I softening/swelling; Grade II fragmentation <0.5 inch; Grade III fragmentation >0.5 inch; Grade IV full-thickness loss down to bone; the ICRS classification is now preferred for research and reporting

• Can be performed arthroscopically or via mini-arthrotomy depending on defect location and surgeon preference; arthroscopic technique requires careful portal placement to achieve perpendicular access to the defect
• Perpendicular plug insertion is essential: the osteochondral plug must be inserted perpendicular to the articular surface to ensure flush seating of the cartilage; an angled plug creates a step-off at the articular surface which causes shear stress, cartilage damage to adjacent tissue, and inferior outcomes; access to certain defects (particularly the posterior femoral condyle) requires knee flexion >90° or a mini-arthrotomy to achieve perpendicular orientation
• Steps: (1) defect preparation — debride to stable cartilage margins and measure defect dimensions; (2) recipient tunnel creation — a tubular reamer creates a cylindrical tunnel at the defect site; the depth must match the plug depth; (3) donor site harvest — same size reamer harvests the plug from the donor site; (4) plug insertion — press-fit insertion into the recipient tunnel; plug cartilage surface must be flush with the surrounding articular surface (neither proud nor recessed)
• Mosaicplasty: for larger defects (>1.5–2 cm²) multiple smaller plugs (typically 4.5–8.5 mm diameter) are harvested and inserted in a mosaic pattern to fill the defect; the inter-plug gaps fill with fibrocartilage over time; a mosaic of 2–7 plugs can cover defects up to 4 cm²; the percentage of the defect covered by hyaline cartilage (the plugs) vs fibrocartilage (the gaps) influences long-term outcomes
• Donor site: typically the far periphery of the lateral or medial femoral condyle at the junction with the trochlea; donor plugs have slightly different cartilage thickness and mechanical properties compared to weight-bearing condylar cartilage — this "cartilage matching" is imperfect but clinically acceptable for small plug transfers

• OATS outcomes: good to excellent results in 70–90% of patients at 5–10 years for appropriately selected defects (1–2.5 cm²) in active patients; results are superior to microfracture in high-demand patients and in defects >2 cm²; sport return rates of 70–80%; best results for medial femoral condyle defects; inferior results for trochlear and patellar defects (more technically demanding access and different mechanical environment)
• Donor site morbidity: pain at the harvest site in approximately 5–15% of cases; rarely limits activity; the donor site fills with fibrocartilage over 6–12 months; limiting the total harvest area to <5–6 cm² reduces donor site complications; harvest from the intercondylar notch region has lower symptomatic morbidity than peripheral condyle harvest
• Plug proud or recessed: the most common technical complication; a proud plug (cartilage surface elevated above the surrounding articular surface) causes shear damage to adjacent cartilage and increased contact stress; a recessed plug leads to fibrocartilage fill of the gap and step-off; intraoperative assessment of plug flush-ness is critical
• Graft failure: loss of plug fixation or cartilage death; uncommon with correct technique; re-operation may be required
• Concurrent procedures: OATS should not be performed in isolation if underlying pathology is not addressed — concurrent ACL reconstruction, HTO (for varus malalignment), or meniscal repair are frequently required; failure to address malalignment is the most common reason for cartilage procedure failure

• OATS vs microfracture: the landmark RCT by Gudas et al. (2005) comparing OATS to microfracture in athletes demonstrated significantly superior results for OATS at 3-year follow-up — 96% good/excellent vs 52% for microfracture in an athletic population; multiple systematic reviews confirm superiority of OATS over microfracture for larger defects (>2 cm²) and high-demand athletes; microfracture remains appropriate for smaller defects (<2 cm²) in lower-demand patients or as a first-line procedure before escalating to OATS/ACI
• ACI vs OATS for medium defects (2–4 cm²): the SUMMIT trial and other comparative studies show broadly equivalent functional outcomes between ACI and OATS at 2 years; ACI has the advantage of no donor site morbidity and can treat larger defects; the two-stage nature of ACI (biopsy at arthroscopy then implantation 6–8 weeks later) is a disadvantage; ACI is preferred for very large defects (>4 cm²), multiple defects, and salvage after failed OATS; OATS is preferred when a single-stage procedure is desirable
• Osteochondritis dissecans (OCD) and OATS: OCD of the femoral condyle is one of the most common indications for OATS in young patients; stable OCD (fragment in situ) in skeletally immature patients often heals with activity restriction; unstable or displaced OCD fragments require: fixation if the fragment is viable and large enough, or OATS if the fragment is unsuitable for fixation or has already been debrided; the OCD crater often has associated subchondral bone loss requiring plugs with adequate bone depth
• Rehabilitation after OATS: protected weight-bearing for 6–8 weeks (toe-touch or partial weight-bearing) to allow plug osseous integration; early range of motion exercises are encouraged to prevent stiffness and promote cartilage nutrition; return to full activity at 4–6 months; return to sport typically at 6–9 months; compliance with the rehabilitation protocol is critical for optimal integration and outcome

• OATS: hyaline cartilage transplanted (not fibrocartilage); single-stage; ideal defect size 1–4 cm²; young active patients; donor from low-load-bearing femoral condyle periphery
• Microfracture: fibrocartilage fill; <2 cm²; results deteriorate >2 years in high-demand patients; inferior to OATS for larger defects and athletes
• OATS vs microfracture RCT (Gudas 2005): 96% vs 52% good/excellent at 3 years in athletes; OATS clearly superior for active patients and defects >2 cm²
• ACI: two-stage; hyaline-like cartilage; larger defects (2–10 cm²); no donor site morbidity; equivalent outcomes to OATS at 2 years for medium defects
• Fresh osteochondral allograft: >4 cm² or large bony defects; no donor site morbidity; fresh graft within 28 days required; limited availability
• Perpendicular plug insertion is critical — angled insertion causes cartilage step-off, shear stress, and poor outcomes; key technical principle of OATS
• Mosaicplasty: multiple plugs (4.5–8.5 mm) for larger defects; inter-plug gaps fill with fibrocartilage; up to 4 cm² coverage
• Address concurrent pathology: malalignment (HTO), ligament instability (ACL), meniscal deficiency — failure to do so is the most common cause of cartilage procedure failure
• Donor site morbidity: 5–15% symptomatic; fills with fibrocartilage over 6–12 months; limit harvest to <5–6 cm² total
• Rehabilitation: protected weight-bearing 6–8 weeks; return to sport 6–9 months; early ROM encouraged