What AMIC does that microfracture alone cannot

Is AMIC simply a fancier version of microfracture? The short answer is no — and the distinction is worth unpacking.

Microfracture is the starting point for both procedures. The surgeon uses a fine angled awl to create small holes in the bone beneath the damaged cartilage (the subchondral plate), releasing blood and bone-marrow progenitor cells that pool into a clot over the defect. In an ideal world, that clot matures into repair tissue. In practice, the clot is mechanically fragile: joint movement during early recovery can disperse it before it consolidates, which explains why microfracture survivorship falls below 60% at three years (Solheim et al.) and why clinical scores often decline between 18 and 36 months post-operatively (Kreuz et al.).

AMIC addresses this directly. Once microfracture is complete, a thin, dissolvable collagen patch — Chondro-Gide®, a bilayer membrane derived from porcine collagen I/III — is placed over the defect and held in place with a biological glue (fibrin glue). The membrane physically shields the stem-cell-rich clot during the critical early regeneration window, keeping progenitor cells in contact with the bone and providing a stable structure on which they can progressively develop into repair tissue. The membrane then gradually resorbs as new tissue forms in its place.

Crucially, everything is completed in a single operation. Unlike ACI — which requires a separate cartilage biopsy, laboratory cell culture, and a second surgical procedure — AMIC needs no prior biopsy and no cell-culture stage. That one-stage design positions AMIC between standard microfracture and full cell-based repair: more durable than the former, considerably simpler than the latter.

The hip lesions AMIC is designed for

The structural picture matters as much as the technique. AMIC is suited to a specific category of hip joint damage: a focal, full-thickness cartilage lesion on the acetabulum (the hip socket) or femoral head (the ball), graded ICRS III or IV — meaning cartilage loss that reaches or penetrates the underlying bone surface. At these grades, the joint surface can no longer self-repair, yet the underlying subchondral bone remains structurally sound enough to respond to microfracture. Irreparable subchondral damage, where the bone architecture itself has collapsed, removes that foundation and rules the technique out.

Defect size is a central criterion. The evidence supports AMIC for lesions roughly between 2 cm² and 8 cm² — the range at which microfracture alone is inadequate yet a single-stage scaffold repair remains technically feasible. A 2024 Bone & Joint Journal study confirmed encouraging outcomes at mean 6.2-year follow-up for lesions of at least 2 cm² in patients with femoroacetabular impingement (FAI).

FAI is the most frequently cited cause of this pattern of hip damage, producing focal cartilage injury through abnormal bony contact at the rim of the hip socket. Trauma, labral tears, osteonecrosis, and repetitive loading from sporting activity are further recognised triggers.

What changes the picture is diffuseness. When cartilage loss is widespread rather than focal — reflecting established hip osteoarthritis rather than a contained lesion — AMIC is no longer appropriate, and the pathway typically moves towards joint replacement instead.

Patient criteria: who tends to be offered AMIC

The typical candidate is a physically active adult aged between 18 and 50. Most published series — including the NHS REPAIR pilot RCT, which specifically targets the 18–40 age bracket — reflect a clear clinical gap: the patient who is too young for hip replacement yet has a lesion that microfracture alone is unlikely to sustain over the long term. Activity level is central to this; patients who need to maintain sport or manual labour have the most to gain from repair tissue that holds.

Good residual joint space on pre-operative imaging is essential, confirming that damage is focal and the wider joint architecture remains intact. When femoroacetabular impingement is present — as it frequently is — AMIC is almost always combined with surgical correction of the impinging bony morphology at the same operation. Without that step, the mechanical cause of the original lesion remains active, and the repair tissue faces the same contact stress that produced the damage in the first place.

Certain factors shift the appropriate pathway elsewhere. Generalised hip osteoarthritis, inflammatory arthropathy such as rheumatoid arthritis, uncorrectable bony deformity, and morbid obesity each make focal cartilage repair unsuitable. In those situations the joint's wider condition means the repair would lack the mechanical environment it needs to succeed; depending on severity and age, joint-preservation surgery or hip replacement is generally the more appropriate direction to explore at assessment.

What the procedure and recovery involve

For most patients, the operation is performed arthroscopically — through small portal incisions rather than a formal open approach — though certain lesion locations may require wider surgical access, which a specialist surgeon will determine from pre-operative imaging. The repair itself builds on the two steps described earlier: once the subchondral bone has been stimulated and the collagen membrane secured, the joint is closed and the work is done in a single theatre visit.

Where femoroacetabular impingement is present, bone reshaping (osteoplasty) is carried out in the same sitting before the cartilage repair step. Addressing the structural cause of the original lesion at the same time avoids a second anaesthetic and a second recovery, though it does extend the overall surgical time.

Recovery and rehabilitation

The first priority after surgery is protecting the repair while the membrane integrates and early tissue formation begins. Most patients remain on crutches for around three to four weeks to limit load through the hip during this window. Gentle physiotherapy typically begins within days of the operation — early movement of the joint is encouraged; it is full weight-bearing that is restricted, not mobility.

Return to light sport for most patients falls around the three-month mark, though this varies with lesion size, whether FAI correction was performed in the same sitting, and individual progress through rehabilitation. Specific timelines are best discussed with the treating surgeon at assessment, since the evidence supports ranges rather than fixed promises.

The clinical evidence: what outcomes studies show

At a mean follow-up of 6.2 years, none of the 24 patients in a 2024 Bone & Joint Journal study had required conversion to total hip arthroplasty — a meaningful marker for a procedure whose core aim is to protect the joint long enough to keep replacement off the table. All patients in that cohort had chondral lesions of 2 cm² or larger alongside FAI, and both Oxford Hip Score and COMI scores improved significantly over the follow-up period.

Registry data from Gille et al. (57 patients, mean age 37.3 years, mean defect 3.4 cm²) showed statistically significant reductions in VAS pain scores and functional gains maintained at two years post-operatively (p<0.001). Systematic review evidence suggests favourable outcomes extending to five to nine years, indicating that the repair tissue generated through the AMIC scaffold can remain clinically durable over the medium term.

For comparison with cell-based alternatives, the Fossum et al. RCT (41 patients) found outcomes comparable between AMIC and autologous chondrocyte implantation at one and two years, with both groups showing significant improvement from baseline. Since ACI requires a separate biopsy and laboratory culture stage, AMIC achieves similar results with considerably less procedural complexity.

The hip-specific evidence base, though encouraging, remains smaller and less mature than the body of evidence for AMIC in the knee. Long-term randomised trial data for the hip are still accumulating — which is precisely the gap the REPAIR pilot RCT is designed to address.

NHS access in the UK and the REPAIR trial

Unlike ACI for the knee — which has held NHS approval since 2017 — hip AMIC does not yet sit on a defined NHS commissioning pathway. This is not a reflection of the procedure's clinical rationale; it reflects the evidence stage. Commissioning decisions for novel surgical procedures typically require robust randomised trial data, and for hip AMIC that data is still being generated.

The primary UK vehicle for generating it is the REPAIR pilot RCT (IRAS 326268), sponsored by Northumbria Healthcare NHS Foundation Trust and running over 3.5 years. This is the first UK randomised trial to compare AMIC directly with microfracture alone for full-thickness acetabular cartilage defects in adults aged 18–40 — the patient group most likely to benefit and for whom hip replacement is not yet appropriate. Its results will form part of the evidence base commissioners need before establishing a routine NHS pathway for hip AMIC.

Outside the trial, hip AMIC may be available through specialist cartilage centres on a private or self-pay basis. Patients in Lincolnshire and nearby areas who want to establish whether they meet the clinical criteria — lesion grade, joint health, and underlying cause — can request a specialist hip assessment without a GP referral through Lincolnshire Hip, part of the MSK Doctors group, at its Sleaford and Grantham locations. What that assessment finds will determine which pathway, investigational or private, is appropriate for that individual.

The REPAIR trial represents where the NHS evidence base for hip AMIC currently stands: encouraging, in progress, and not yet complete.