Why hip cartilage size matters more than pain alone

Yes — the measured area of a hip cartilage defect directly determines which repair option is technically possible, and in some cases whether repair is possible at all. Pain alone is a poor guide: a small but deep defect on the femoral head can cause severe discomfort, while a larger lesion may remain surprisingly quiet for months before accelerating towards osteoarthritis.

Once hip cartilage is lost to a full-thickness defect, it cannot regenerate on its own — making early, size-informed triage essential.

Focal chondral and osteochondral lesions in the hip joint occur most often at the anterosuperior acetabulum and the central femoral head, the surfaces that take the greatest load during walking and rotation. Clinically, the lesion is characterised by two dimensions: depth and area. Depth is graded on the ICRS scale; Grade III (greater than 50% cartilage thickness) and Grade IV (through to subchondral bone) mark the threshold at which active repair, rather than watchful waiting, is typically indicated. Left untreated, full-thickness defects carry a meaningful risk of progression to hip osteoarthritis and eventual hip replacement.

Area — measured in square centimetres on MRI — is the single most practical triage factor. Defects under roughly 2 cm² suit different techniques from those in the 2–4 cm² band, which in turn differ from larger lesions exceeding 4 cm²; each band has its own realistic repair options, and understanding that boundary is the purpose of this article.

The three size bands and the techniques they map to

Three approximate size bands emerge from hip-specific procedure series, and each points toward a different repair strategy.

Under 2 cm² — the small, contained defect. Osteochondral autograft transfer (OATS, also called mosaicplasty) performs best in this territory: the surgeon harvests a plug of the patient's own bone and cartilage from a low-load area of the femoral head and presses it into the defect. Hip outcome data show a clear performance drop when defect area exceeds this threshold, which is why the roughly 2 cm² mark has become the practical OATS ceiling for hip lesions. ChondroFiller injection — a CE-marked acellular collagen scaffold placed under ultrasound guidance as an outpatient procedure — is also applicable here, and in its injectable form carries no strict upper size limit.

Roughly 2–4+ cm² — the mid-range defect that has outgrown autograft but may still respond to augmented marrow-stimulation techniques. ChondroFiller injection remains an option across this band as an additive, top-down cushioning mechanism.

Larger lesions, typically averaging 4–5 cm² — the territory of osteochondral allograft transplantation (OCA), where donor tissue replaces both the cartilage and the underlying bone in a single surgical procedure.

These boundaries are triangulated from individual procedure series rather than a direct head-to-head comparison trial, so they function as clinical guides rather than absolute rules. The detail sections below work through the evidence behind each technique in turn.

ChondroFiller injection: the outpatient option for smaller and diffuse defects

Delivered as a clinic-based appointment rather than a theatre procedure, ChondroFiller injection is distinct from the surgical options described alongside it. Where OATS and OCA require hip dislocation or arthroscopic exposure in a dry operative field, ChondroFiller is placed under ultrasound or fluoroscopic guidance during an outpatient session — no general anaesthetic, no incision, no post-operative ward stay.

The active material is an acellular Type I collagen gel. Once injected into the hip joint, it polymerises in situ and forms a three-dimensional matrix that the patient's own progenitor cells migrate into — a process termed matrix-induced chondrogenesis. No cartilage cells are harvested or transplanted; the scaffold provides the structural environment and the body supplies the regenerative biology.

That delivery mechanism is also what removes the size ceiling that constrains autograft and allograft procedures. OATS is bounded by the area of graft harvestable from the femoral head; OCA requires a defect large enough to justify open surgical exposure. ChondroFiller injection, applied across the articular surface in a fluid joint environment, is feasible for isolated focal Grade III/IV defects with healthy surrounding borders and for more diffuse cartilage wear where no single contained lesion exists — a range that neither OATS nor OCA covers. It is CE-marked as a Class III medical device.

Hip-specific published evidence sits at case-series and case-report level. A 2025 report documented complete pain resolution and full range of motion following injection to a 15 × 5 mm femoral head lesion in a 32-year-old; published series report a modified Harris Hip Score improvement of approximately 30 points, with patient numbers remaining small. In practice, the injectable route tends to be considered when a focal defect or diffuse wear has not responded adequately to conservative management, and theatre-based reconstruction is not yet indicated or not suitable for the patient — a circumstance where it provides a clinically meaningful step between watchful waiting and surgery.

OATS: autograft transfer for contained defects up to around 2 cm²

Published hip-specific outcome data pin OATS to a practical ceiling close to 2 cm², and the numbers behind that threshold are among the sharpest size-stratified findings available for any hip cartilage technique. In a 24-hip mosaicplasty series followed to a mean of 9.5 years, patients whose femoral head lesions measured less than 2 cm² had a 12% rate of conversion to total hip arthroplasty and a modified Harris Hip Score of approximately 89. Those with lesions at or above 2 cm² converted at 57% — nearly five times the rate — and function scores fell accordingly. Mean lesion size in the group whose repair held was 1.4 cm²; in those who ultimately needed a hip replacement it was 2.3 cm². The implication is direct: OATS in the hip performs well within its size range and poorly outside it.

The reason the ceiling sits lower in the hip than in the knee comes down to graft availability. Plugs are harvested from the non-weight-bearing zone of the ipsilateral femoral head — a limited surface area that constrains how much tissue can safely be taken. Knee procedures can draw from larger non-load-bearing donor sites; the hip cannot match that supply.

A further prerequisite is the integrity of the underlying bone. OATS transfers both cartilage and a bone base, so a defect in which the subchondral plate is already compromised is not a straightforward candidate. Where bone stock is intact and the lesion falls within the size range, the graft beds in and integrates; where it does not, outcomes fall sharply — as the conversion data above illustrate.

OCA: fresh allograft transplantation for larger hip cartilage lesions

Once a femoral head defect exceeds what autograft tissue can cover, osteochondral allograft transplantation (OCA) becomes the principal joint-preservation option for younger patients not yet ready for hip replacement.

OCA uses donor-sourced osteochondral tissue to resurface the defect, restoring both articular cartilage and the subchondral bone beneath it in a single operation. Because the hip cannot be accessed arthroscopically at this scale, OCA requires surgical hip dislocation — an open procedure under general anaesthetic, not an outpatient or minimally invasive option. Two 2025 case series, comprising 24 and 29 patients respectively (mean age 21–22 years), report mean defect sizes of 418–488 mm² (approximately 4.2–4.9 cm²), with individual lesions ranging up to 972 mm². Candidacy in those cohorts spanned post-traumatic injury, developmental dysplasia, femoroacetabular impingement, Legg-Calvé-Perthes disease, and avascular necrosis — confirming that lesion size, rather than aetiology, is the primary selection criterion.

Functional improvement was clinically meaningful: modified Harris Hip Score rose from approximately 58–62 before surgery to around 84 afterwards, and 84% of patients achieved the minimum clinically important difference. However, 14–25% required conversion to total hip arthroplasty within 3–5 years in published hip series. That figure should not be softened: OCA is most accurately understood as a joint-preservation bridge that can restore function and delay replacement in young patients with large defects, not a definitive surgical solution. Long-term hip-specific survivorship data beyond five years remain limited. Graft preservation methodology may also influence results — MOPS-preserved tissue achieved markedly better success rates than standard preservation in one series, though patient numbers were small — and this warrants discussion during pre-operative planning.

What else shapes the decision: bone, labrum, imaging, and your age

Size provides the framework, but several clinical factors combine to determine which repair technique is technically feasible and likely to hold.

Bone stock. OATS transfers both cartilage and a bone plug together, so the recipient site requires an intact subchondral plate. Where the underlying bone is compromised — through cyst formation, avascular necrosis, or deep osteochondral loss — autograft integration becomes unreliable, shifting the decision toward OCA or, for smaller lesions, a ChondroFiller injection rather than an autograft procedure.

Labral integrity and joint stability. Hip stability research suggests labral preservation combined with capsule repair significantly improves cartilage healing, with measurably greater Type II collagen and aggrecan expression than is seen in unstable joints. A concurrent labral tear will typically need addressing as part of the same episode of care, regardless of which cartilage repair technique is used.

Pre-operative imaging. MRI characterises defect depth, area, and surrounding cartilage health before any technique decision is made. Plain radiographs can also identify anterolateral femoral head lesions — the Windshield Wiper Sign flags instability and may indicate that cartilage repair needs to be paired with bony realignment such as a periacetabular osteotomy.

Age, activity, and aetiology. Younger, more active patients generally tolerate the recovery demands of OATS or OCA better, and the long-term benefit of delaying joint replacement carries proportionally greater weight. The lesion's underlying cause — whether post-traumatic injury, FAI, developmental dysplasia, or avascular necrosis — can also determine which procedure is technically appropriate and whether alignment surgery should accompany cartilage repair.

These variables are assessed together at consultation, not from a size measurement alone. Lincolnshire Hip accepts patients without referral for hip assessment, with clinics in Sleaford and Grantham.

1. [1] Hip mosaicplasty provides good-to-excellent clinical scores at 10 years in patients with small femoral osteochondral lesions. (2025). https://doi.org/10.1093/jhps/hnaf069.277 https://doi.org/10.1093/jhps/hnaf069.277
2. [2] Osteochondral Allograft Transplantation of the Femoral Head via Surgical Hip Dislocation: Survivorship and PROMs at Minimum 2-Year Follow-Up. (2025). https://doi.org/10.1177/23259671251385115 https://doi.org/10.1177/23259671251385115
3. [3] Clinical Outcomes following Osteochondral Allograft Transplantation of the Femoral Head via Surgical Hip Dislocation. (2025). https://doi.org/10.1093/jhps/hnaf011.045 https://doi.org/10.1093/jhps/hnaf011.045
4. [4] Clinical and radiological outcomes of OATS for avascular necrosis of the femoral head. Preliminary report. (2013). https://doi.org/10.1007/s00264-013-1893-6 https://doi.org/10.1007/s00264-013-1893-6
5. [5] Rates of Achieving Meaningful Outcomes 2-Year After Microfracture Augmented with Allograft Cartilage and Autologous PRP for Hip Chondral Defects. (2025). https://doi.org/10.1093/jhps/hnaf011.072 https://doi.org/10.1093/jhps/hnaf011.072
6. [6] The Hip 'Windshield Wiper Sign': An Osteochondral Defect of the Anterolateral Femoral Head, Predicts Instability and Allows Planning of Combined Arthroscopic Cartilage Therapy. (2024). https://doi.org/10.1016/j.arthro.2024.06.003 https://doi.org/10.1016/j.arthro.2024.06.003
7. [7] Preserving Hip Stability Yields Better Cartilage Repair With Microfracture Treatment: A Rabbit Study. (2025). https://doi.org/10.1016/j.asmr.2025.101284 https://doi.org/10.1016/j.asmr.2025.101284
8. [8] Femoral Head Fracture with Hip Dislocation Treated by Autologous Osteochondral Transfer (Mosaicplasty): A Case Report. (2020). https://doi.org/10.12671/jkfs.2020.33.2.96 https://doi.org/10.12671/jkfs.2020.33.2.96