Why defect size and OA grade are the first questions

Two questions come before any treatment name is mentioned when a patient presents with hip cartilage damage: how large is the defect, and how advanced is the underlying arthritis?

Hip articular cartilage cannot heal itself — which is why getting that sorting right from the outset determines everything that follows. The goal of every approach discussed here, from a clinic-based injection to open femoral head surgery, is the same: meaningful pain relief and the delay or avoidance of total hip replacement.

The sorting works on two axes. The first is defect size, measured in cm². Lesions under 2 cm² generally suit marrow-stimulation techniques or autograft transfer (OATS/mosaicplasty). At 2–3 cm² and above, matrix-based reconstruction — in particular AMIC — shows substantially better durability. Larger, complex femoral head defects that cannot be addressed with autograft are the territory of osteochondral allograft (OCA).

The second axis is osteoarthritis stage, graded on the Tönnis or Kellgren-Lawrence scale. This is the overriding gate. When Tönnis grade reaches 2 or above, surgical cartilage repair becomes ineffective — the joint environment can no longer support tissue regeneration. At that point, treatment shifts to injection-based support aimed at symptom control and cartilage protection rather than structural restoration.

One practical caution: plain X-ray, even when rated Tönnis 0–1, underestimates intraoperative chondral damage in more than 85% of cases in published series. Pre-operative MRI and direct intraoperative inspection are both essential before any treatment decision is confirmed.

OATS for small focal hip defects

OATS — osteochondral autograft transfer, sometimes called mosaicplasty — fills a focal cartilage hole using the patient's own bone-and-cartilage tissue taken from a non-weight-bearing region of the same femoral head. Because graft and recipient are from the same person, there is no rejection risk and no waiting for a donor, and the whole procedure is completed in a single stage.

The size boundary matters here. In a multicenter series comparing OATS and allograft transplantation for femoral head defects, autograft cases averaged 1.6 × 1.0 cm — notably smaller than the 2.8 × 2.1 cm mean seen in the allograft group. Within that smaller-defect range, the autograft cohort reached a mean modified Harris Hip Score of approximately 87 with no conversions to total hip replacement, which is a clinically meaningful result.

OATS has also been reported as a joint-preserving option in avascular necrosis (AVN) of the femoral head, though timing is critical. At pre-collapse stages (ARCO IIA and IIB), published series describe an 85.7% four-year survival and a Harris Hip Score rise from 42 to nearly 88 at 46 months. Outcomes fall away sharply once collapse has occurred — making OATS, in the AVN setting, principally a time-buying procedure for younger patients rather than a definitive solution.

The practical ceiling on OATS is the volume of harvestable autograft. Removing too large a plug risks morbidity at the donor site within the same hip, so defects above roughly 2 cm² — or those too deep for single-plug coverage — are better suited to AMIC or, where the femoral head is involved at scale, to osteochondral allograft.

AMIC for medium-to-large focal lesions

For defects that sit above the autograft size ceiling — roughly 2 cm² and beyond — AMIC (autologous matrix-induced chondrogenesis) is currently the best-evidenced single-stage surgical option for focal hip cartilage damage.

The technique builds on microfracture: the surgeon perforates the subchondral bone to release a blood clot rich in marrow-derived progenitor cells, then seals it beneath a bi-layer Type I/III collagen membrane. That membrane gives the clot a structured scaffold in which to consolidate and mature, rather than leaving it exposed to joint fluid. No donor harvest is required and everything is completed under one anaesthetic — a practical advantage over two-stage approaches such as ACI or MACI, which demand a biopsy visit, cell culture, and a second theatre episode.

The headline evidence comes from a 2025 systematic review and meta-analysis of 628 hip cases across 12 studies. Pooled success reached 99.6%, with a mean improvement of 35.8 points on the modified Harris Hip Score. The weighted mean lesion size was 3.3 cm² and weighted mean follow-up approximately 47 months — placing these results squarely in the mid-term range.

The THA conversion comparison is perhaps the most clinically meaningful figure for any patient weighing their options. Across four comparative cohorts totalling 209 hips, AMIC groups recorded 0% conversion to total hip replacement, against 2%–32.6% for microfracture-alone controls at similar follow-up. Microfracture functions here as a declining historical baseline rather than a current clinical equal.

The typical AMIC candidate is around 35 years of age, with a focal ICRS grade III or IV acetabular lesion and a preserved joint space (Tönnis ≤1). Mid-term data from a surgical hip dislocation cohort treating large ICRS III–IV defects confirmed satisfactory outcomes at five years — Oxford Hip Score rising from 38.1 to 43.4, with acetabular defects performing slightly better than femoral ones.

One limitation is worth naming: robust data beyond five years in the hip remain sparse. The mid-term picture is strong, but long-term durability has not yet been established to the same degree as in some knee series.

OCA when the defect outgrows autograft supply

Occasionally a femoral head defect is simply too large for autograft tissue to cover — and this is where fresh osteochondral allograft (OCA) becomes the only realistic joint-preserving route. Donor cartilage-and-bone is shaped to match the lesion and transplanted in a single procedure via surgical hip dislocation, replacing the damaged surface without needing to harvest tissue from elsewhere in the patient's own hip.

The typical candidate is considerably younger than those suited to AMIC — mean age around 22 years across published series — and the lesions are substantially larger, averaging 418–488 mm² (roughly the size of a small coin). Aetiologies vary: post-traumatic damage, developmental dysplasia, Legg-Calvé-Perthes disease, and avascular necrosis all figure. What they share is a large focal defect in a joint that still has enough structural integrity to be preserved.

Functional gains are meaningful. Two published cohorts report mHHS improving from approximately 62 preoperatively to 84 at follow-up, with iHOT-12 rising from around 35 to 77. These are real improvements in pain and activity for patients who would otherwise face replacement at a very young age.

The THA conversion rate — up to 25% at mean 3.8 years in one series, 13.7% in another — reflects the biological challenge of large allograft integration rather than a failure of surgical intent. It is a meaningful risk that should be discussed openly at assessment, not minimised.

One practical detail has an outsized effect on outcome: graft preservation method. Published data show MOPS-preserved OCA achieving 100% success against approximately 50% for standard preservation — an odds ratio of 47. Patients and referrers are reasonable to ask which preservation protocol will be used before proceeding.

ChondroFiller injection when structural repair is offthe table

Some patients arriving at this decision point have already been told the same thing: the damage is too widespread, or the joint space too reduced, for a focal repair procedure. ChondroFiller injection exists for exactly this group — those with Kellgren-Lawrence or Tönnis grade III–IV osteoarthritis whose hip is beyond what OATS, AMIC, or OCA can address.

The delivery is deliberately different from any technique described above. Rather than a theatre procedure under general anaesthesia, ChondroFiller injection is administered as an outpatient treatment under ultrasound or fluoroscopic guidance. The product — an acellular, CE-marked Class III Type I collagen scaffold — self-gels once placed inside the joint, rather like a sponge settling into position and drawing in the body's own progenitor cells. That scaffold creates a structured matrix environment in which matrix-induced chondrogenesis may occur, while simultaneously providing a viscoelastic cushioning layer over the degraded cartilage surface to reduce mechanical load.

This is not a substitute for OATS, AMIC, or OCA. Those procedures address focal, full-thickness defects in a joint with preserved space; ChondroFiller injection at this stage targets diffuse articular wear in patients for whom focal surgical repair is no longer a viable pathway. The distinction matters clinically: different pathology, different mechanism, different setting — and critically, no need for general anaesthesia, which makes it accessible for patients with comorbidities that would complicate theatre-based care.

Published hip data indicate a mean improvement of approximately 30 points on the modified Harris Hip Score following ChondroFiller injection, with a reported complaint rate of around 0.06%.

Getting assessed at Lincolnshire Hip

The decision framework across these four techniques reduces to the same two variables: defect size and OA grade. In broad terms, a focal lesion under roughly 2 cm² with preserved joint space points toward OATS; a focal defect of 2 cm² or more toward AMIC; a very large femoral head lesion in a patient in their twenties toward OCA via surgical hip dislocation; and diffuse advanced-grade wear — Tönnis or Kellgren-Lawrence grade III–IV — toward ChondroFiller injection rather than structural repair. Tönnis grade 2 or above remains a contraindication to the surgical options, redirecting those patients to the injection pathway.

Applying that framework to a specific hip requires thorough imaging. MRI at 3.0T or above provides the lesion mapping — size, depth, ICRS grade, surrounding cartilage quality — that X-ray cannot reliably supply. Clinical history, age, mechanical demands, and any concurrent bony abnormality such as FAI sit alongside it to determine which branch of the framework fits.

Lincolnshire Hip is part of the MSK Doctors group and accepts patients without GP referral for hip assessment, with access points in Sleaford and Grantham. Prof Paul Y. F. Lee has specific expertise in hip cartilage repair and preservation and is available through the service.

1. [1] Osteochondral Allograft and Autograft Transplant for Femoral Head Defects: A Multicenter Study. (2025). https://doi.org/10.1177/03635465251338062 https://doi.org/10.1177/03635465251338062
2. [2] FP3.7 Hip articular cartilage repair with autologous mesenchymal stem cells (MSCs): 4-year results in prospective controlled study. (2025). https://doi.org/10.1093/jhps/hnaf011.025 https://doi.org/10.1093/jhps/hnaf011.025
3. [3] Arthroscopic treatment of chondral defects in the hip: AMIC, MACI, microfragmented adipose tissue transplantation (MATT) and other options. (2017). https://doi.org/10.1051/sicotj/2017029 https://doi.org/10.1051/sicotj/2017029
4. [4] Autologous Matrix-Induced Chondrogenesis for the Treatment of Hip Acetabular Chondral Lesions Demonstrates Improved Outcomes: A Systematic Review. (2024). https://doi.org/10.1016/j.arthro.2024.04.028 https://doi.org/10.1016/j.arthro.2024.04.028
5. [5] MID-TERM RESULTS OF AUTOLOGOUS MATRIX-INDUCED CHONDROGENESIS (AMIC) USED FOR LARGE CHONDRAL DEFECTS IN FEMOROACETABULAR IMPINGEMENT HIPS. (2023). https://doi.org/10.1302/1358-992x.2023.12.029 https://doi.org/10.1302/1358-992x.2023.12.029
6. [6] Clinical and radiological outcomes of treatment of avascular necrosis of the femoral head using autologous osteochondral transfer (mosaicplasty). Preliminary report. (2013). https://doi.org/10.1007/s00264-013-1893-6 https://doi.org/10.1007/s00264-013-1893-6
7. [7] Outcomes of autologous chondrocyte transplantation (ACT) and autologous matrix-induced chondrogenesis (AMIC) in the hip: a systematic review and meta-analysis. (2025). https://doi.org/10.1186/s13018-025-05862-5 https://doi.org/10.1186/s13018-025-05862-5
8. [8] Trends in Cartilage Repair Techniques for Chondral Defects in the Hip in Germany: An Epidemiological Analysis from 2006 to 2022. (2024). https://doi.org/10.3390/life14101262 https://doi.org/10.3390/life14101262
9. [9] EP4.1 Chondral Damage Severity in Older Patients with Low Tonnis Grades X Rays Undergoing Hip Arthroscopy. (2025). https://doi.org/10.1093/jhps/hnaf011.254 https://doi.org/10.1093/jhps/hnaf011.254
10. [10] Osteochondral Allograft Transplantation of the Femoral Head via Surgical Hip Dislocation: Survivorship and Patient Reported Outcome Measures at Minimum 2-Year Follow-Up. (2025). https://doi.org/10.1177/23259671251385115 https://doi.org/10.1177/23259671251385115