Why defect size is the starting point

When a hip cartilage lesion is found on MRI or arthroscopy, two measurements determine what happens next: how wide the damaged area is (recorded in cm²) and how deeply it extends through the cartilage layer. Together, these two values place the patient into a treatment band — and each band points toward a different repair strategy.

Depth is scored using the ICRS (International Cartilage Repair Society) grading scale. Grades 1 and 2 represent partial-thickness loss and are often managed conservatively. Grade 3 — where more than half the cartilage depth is gone — and Grade 4, where damage has broken through to the subchondral bone, are the thresholds at which active treatment is typically warranted.

Area, measured in cm², creates three broad clinical bands. Defects under roughly 2 cm² sit in one category; those between 2 cm² and 4 cm² in a second; and those above 4 cm² in a third. Each band corresponds to a different set of options — injectable scaffold, surgical scaffold, autograft, or allograft — and both measurements are needed to land in the right one: a small but bone-deep defect calls for a different approach than a wide but superficial lesion.

One important boundary sits at the other end of the spectrum. Diffuse or advanced hip osteoarthritis — where cartilage loss is widespread rather than focal — falls outside the focal-repair pathway entirely. At that stage, the conversation shifts towards joint preservation or replacement rather than targeted cartilage reconstruction.

ChondroFiller injection: focal defects managed as an outpatient

For patients whose hip cartilage lesion falls within the smaller focal band — typically up to approximately 3 cm², though clinical evaluation may extend this to around 6 cm² — the ChondroFiller injection offers a regenerative pathway that does not require an operating theatre or a general anaesthetic. The procedure is delivered under ultrasound guidance as an outpatient appointment at Grantham or Sleaford, placing it at the least invasive end of the active treatment spectrum.

The injectable material is a type-I collagen scaffold — a CE-marked Class III medical device (Meidrix Biomedicals GmbH) that forms a gel within the joint once placed. Its mechanism is matrix-induced chondrogenesis: the scaffold acts as a temporary framework that recruits the patient's own progenitor cells into the defect site, supporting biologic repair tissue to build over a period of roughly 3 to 6 months. This is distinct from cushioning or symptom-masking injections; ChondroFiller injection is a regenerative scaffold pathway, not a viscosupplement or a filler. Patients sometimes ask how it differs from Arthrosamid — the two work differently: Arthrosamid is a hydrogel used for osteoarthritis symptom management, whereas ChondroFiller injection targets focal cartilage repair in a joint where the surrounding cartilage remains reasonably intact.

Hip-specific published evidence reports a mean Harris Hip Score improvement of approximately +33 points following ChondroFiller injection, with MOCART MRI cartilage regeneration scores in the range of 70–87. More than 19,000 cases have been performed globally across knee, hip, and other joints. A protected-loading period of around 4 to 6 weeks follows the injection, after which the repair tissue continues to mature.

ChondroFiller injection is not appropriate for diffuse or end-stage hip arthritis — as noted in section one, widespread cartilage loss sits outside the focal-repair pathway. Where the hip joint still has a defined lesion with healthy cartilage borders, and where bone involvement is limited, the ultrasound-guided injection route avoids surgical admission while still addressing the underlying defect.

AMIC: scaffold-augmented repair for medium full-thickness defects

AMIC sits one step up from the ChondroFiller injection pathway in the defect-size spectrum, and one step below the structurally demanding osteochondral autograft techniques described in the following section. Its typical territory is full-thickness focal defects roughly exceeding 2 cm² — lesions where an injectable scaffold alone is unlikely to provide sufficient mechanical support, but where the defect still falls within a size that autologous tissue can address in a single procedure.

The technique combines two elements in one operation. Microfracture perforates the subchondral bone to draw marrow-derived stem cells and growth factors into the defect. On its own, microfracture produces fibrocartilage that tends to break down within two to three years and leaves the subchondral bone plate vulnerable to damage that can compromise any future repair — which is why stand-alone microfracture is no longer considered the default first-line option for symptomatic focal defects. AMIC adds a bi-layer collagen I/III membrane over the microfractured surface. The membrane acts like a lid: it holds the marrow clot in place and structures the repair environment while new tissue matures, producing a more organised result than marrow stimulation alone.

The supporting evidence is substantive. A retrospective analysis of 21 patients at seven-year follow-up found AMIC effective for full-thickness defects larger than 2 cm², and a randomised controlled study recorded stable clinical and functional improvements out to nine years. For defects at or above 3 cm², the SUMMIT trial showed meaningfully better pain and function scores over microfracture alone at both two and five years.

Most of this controlled data comes from knee cartilage studies. Hip-specific randomised evidence is limited, and consultants apply the size-based thresholds by analogy to hip defects, supported by retrospective hip series. That gap does not make AMIC experimental — it reflects a research landscape where knee cartilage trials have simply run longer and at greater scale than hip-specific programmes.

OATS: osteochondral autograft when bone is also involved

Bone involvement changes the calculation. When a focal hip lesion penetrates through the full cartilage depth and disrupts the subchondral plate beneath, a scaffold applied only to the cartilage layer — whether injectable or membrane-based — lacks the structural foundation to support lasting repair. This is where OATS (Osteochondral Autograft Transfer System, also called mosaicplasty) becomes the appropriate option for smaller defects.

The principle is straightforward: a small cylinder of healthy cartilage and bone is taken from a quieter, non-weight-bearing part of the same hip joint and pressed into the prepared defect site. One or more of these plugs reconstruct both the cartilage surface and the underlying bone scaffold in a single procedure. Crucially, the transferred tissue is true hyaline cartilage rather than the fibrocartilage produced by microfracture-based techniques, and this structural quality is expected to confer greater durability over time.

The typical indication is a focal osteochondral defect in the region of 1–2 cm², with a mosaic configuration of multiple smaller plugs extending the treatable area to approximately 4 cm². Beyond that, plug geometry and the curved surface anatomy of the femoral head begin to limit how well the transferred cylinders can be seated flush — a constraint that is more pronounced in the hip than in flatter joints.

Donor-site morbidity is a genuine consideration. Harvesting plugs from within the hip joint creates a secondary lesion, and surgeons weigh this carefully against the benefit. For larger or irregularly shaped osteochondral defects, where donor supply becomes insufficient, fresh osteochondral allograft — described in the next section — provides the alternative.

OCA: fresh allograft for large or structurally complex hip defects

Fresh osteochondral allograft (OCA) extends the OATS principle beyond the boundaries autograft can reach. Where defect size exceeds roughly 4 cm², or where the geometry of bone loss makes a single matched structural replacement more practical than multiple harvested plugs, donor tissue becomes the logical source.

In an OCA procedure, a graft of cartilage and subchondral bone taken from a matched donor is shaped to fit the prepared defect site, reconstructing both surface and underlying bone scaffold in a single stage. No second lesion is created within the patient's own hip — the donor-site morbidity that constrains OATS is removed, though it is replaced by the logistical demands of tissue matching and donor availability.

Published data report approximately 87.9% graft survival at five years and around 77.2% at ten years, with meaningful improvements in functional scores — IKDC rising from a baseline of 41.8 to 68.1 post-operatively. Much of this long-term evidence derives from studies of other joints; hip-specific series are less numerous than the wider literature, and outcomes may vary with the curved anatomy and loading patterns particular to the femoral head.

OCA is therefore the option a consultant considers when focal hip cartilage damage is too extensive for autograft, yet the surrounding joint remains healthy enough to make preservation — rather than replacement — a realistic goal.

What else shapes the decision beyond size

Defect area provides the framework, but several other factors refine the final decision in clinic.

Patient age and activity level both matter. Younger, more active patients generally tolerate the donor-site harvesting that OATS requires and have the biological capacity to benefit from hyaline cartilage restoration; those who are older or less physically active may reach a better risk-benefit balance with less invasive approaches.

Concurrent hip pathology — particularly femoroacetabular impingement (FAI) or a labral tear — must be identified and addressed at the same time as any cartilage repair procedure. Leaving an unresolved mechanical cause in the joint will progressively undermine whichever technique is chosen.

Prior marrow-stimulation procedures narrow the picture further. Microfracture damages the subchondral bone plate, complicating any subsequent attempt at formal cartilage reconstruction; the procedural history a patient brings to assessment carries real clinical weight.

The cm² thresholds described in earlier sections are clinical guidelines rather than fixed rules. Intraoperative findings — cartilage quality at the defect margin, the integrity of the bone bed, and the precise geometry of the lesion — can shift the decision even when preoperative MRI pointed clearly in one direction.

Lincolnshire Hip is part of the MSK Doctors group and accepts patients without referral for hip assessment. It is worth stating plainly that direct, head-to-head trials comparing all four techniques specifically in the hip do not yet exist; the size-based framework described in this article draws partly on knee cartilage research, and that is a genuine limitation of the current evidence base.

1. [1] Articular cartilage repair. https://en.wikipedia.org/?curid=19042351 https://en.wikipedia.org/?curid=19042351
2. [2] Autologous matrix-induced chondrogenesis (AMIC). https://en.wikipedia.org/?curid=29760859 https://en.wikipedia.org/?curid=29760859