Which hip defect calls for injection and which for surgery

Two quite different clinical problems can hide behind the same presenting complaint of hip pain and cartilage damage, and the distinction between them is what determines whether an injectable scaffold or surgical repair is the appropriate next step.

The first problem is diffuse, advanced joint wear — cartilage thinning spread across the femoral head or acetabulum, typically graded Kellgren-Lawrence III or IV. Here the damage is too widespread for a surgeon to debride back to a clean, stable border and reconstruct the surface from below. A ChondroFiller injection is designed for exactly this scenario: it lays a viscoelastic collagen cushion over the worn surface, preserving what remains rather than removing it.

The second problem is a focal cartilage defect — a localised, relatively well-contained lesion with healthy surrounding tissue. When a defect has defined borders and sufficient healthy cartilage around it, surgical techniques such as AMIC, MACI, or OATS can work from the bottom up: debriding to a clean bone bed, then implanting a scaffold or graft that is held in place by the intact rim.

Neither treatment is universally superior. The defect profile — its distribution, depth, surface area, and the condition of the surrounding cartilage — is the primary clinical filter. Hip imaging and grading by a specialist are the essential first step before either pathway can be sensibly recommended.

Hip cartilage grading and what it tells a clinician

Grading systems give clinicians a shared language for describing how far hip cartilage damage has progressed — but two separate scales are in routine use, and they measure different things.

The ICRS (International Cartilage Repair Society) system classifies individual lesions by depth. Grade 1 is superficial softening; Grade 2 extends less than halfway through the cartilage; Grade 3 (subtypes A–D) extends more than 50% through the cartilage thickness; Grade 4 reaches subchondral bone. ICRS grading is used when a clinician needs to characterise a specific focal lesion — its depth, its borders, and the condition of the tissue immediately around it.

The Kellgren-Lawrence (KL) scale works differently. It grades the overall hip joint on plain X-ray, from Grade I (minor osteophytes) through to Grade IV (severe joint-space loss with sclerosis). KL grading captures the joint as a whole rather than one lesion in isolation.

Defect surface area adds a third dimension. Focal lesions smaller than roughly 2 cm² have historically been managed with microfracture, whilst defects of 2–10 cm² fall within the range addressed by MACI or ACI; ChondroFiller's CE evidence supports defects up to 3 cm², extendable to approximately 6 cm². Lesion containment — whether healthy cartilage borders surround the defect — matters as much as area alone, because surgical scaffold techniques depend on an intact rim to hold implanted material in place.

Microfracture's declining role in the treatment hierarchy stems directly from what it produces: fibrocartilage, which is mechanically inferior to native hyaline cartilage. Kreuz et al. documented significant score deterioration between 18 and 36 months post-operatively, and Solheim et al. reported survivorship below 60% at three years, with a mean time to failure of approximately four years. Crucially, the microfracture process itself damages the subchondral bone plate, which can narrow the options available if a second intervention is later needed.

How ChondroFiller injection works in the hip

Delivered as an outpatient ultrasound-guided injection, ChondroFiller is a CE-marked Class III medical device composed of acellular, murine-derived Type I collagen. Under local anaesthetic — no operating theatre, no general or spinal anaesthesia required — the liquid collagen is placed directly into the hip joint under image guidance and begins to self-polymerise within minutes of contact with the joint environment.

The 'top-down additive' character of the injection is the key mechanical distinction from surgery. Rather than clearing away damaged tissue to build from the bone bed upwards, the gel coats the worn articular surface as it stands, forming a viscoelastic cushioning barrier that absorbs mechanical stress and shields the joint surface from further bone-on-bone contact. Existing tissue is preserved throughout — nothing is debrided to gain access.

The scaffold then supports a secondary biological process: matrix-induced chondrogenesis. Progenitor cells from the surrounding synovial fluid and subchondral bone migrate into the collagen matrix, mature towards chondrocyte-like cells, and progressively lay down new matrix. As this process unfolds over weeks to months, the scaffold gradually biodegrades and is resorbed, leaving the body's own repair tissue behind.

In the hip, the injection is indicated for diffuse Kellgren-Lawrence Grade III/IV osteoarthritis — the pattern of wear too widespread for focal surgical reconstruction. The ability to perform the procedure in a fluid joint environment, without drying the field or requiring arthroscopic access, is precisely what makes this route practicable for diffuse hip OA.

Published clinical data reference a mean Modified Harris Hip Score improvement of approximately 30 points following ChondroFiller injection in the hip; cross-joint evidence from the knee shows comparable IKDC gains of around 30 points at 12 months. Hip-specific long-term randomised trial data remain limited, which means that individual suitability and expected benefit warrant discussion at consultant assessment.

What surgical hip cartilage repair involves

Surgical cartilage repair in the hip works from the bone bed upwards. The surgeon debrides damaged tissue to a clean margin first, then implants a scaffold or graft into the prepared site. This bottom-up sequence demands a focal, well-defined lesion enclosed by healthy cartilage — the surrounding rim is what contains and stabilises whatever is placed inside.

Three techniques cover most of the focal-defect surgical landscape:

• AMIC (autologous matrix-induced chondrogenesis) augments microfracture with a collagen matrix membrane to stabilise the repair clot and guide cell differentiation. It is single-stage. Registry data from 57 patients (mean age 37.3 years, mean defect 3.4 cm²) showed significant VAS pain reduction sustained at one and two years post-operatively.
• MACI (matrix-induced autologous chondrocyte implantation) is two-stage: cartilage cells are harvested at a first procedure, cultured on a collagen membrane, then re-implanted at a second operation several weeks later. The SUMMIT trial provided RCT evidence supporting MACI over microfracture at two and five years for defects of 3 cm² or larger.
• OATS (osteochondral autograft transfer) uses cylindrical bone-and-cartilage plugs taken from a less-loaded part of the joint and press-fitted into the defect — single-stage, but with donor-site considerations at the harvest point.

All three require an operating theatre, general or spinal anaesthesia, a dry joint field, and structured post-operative rehabilitation that may extend over several months. That logistical profile — theatre access, anaesthesia, and a substantive recovery period — is materially different from an outpatient injection pathway for anyone weighing time off work and travel to a surgical centre.

None of these techniques transfers to diffuse, advanced hip OA. Without a healthy surrounding tissue border, the joint cannot provide the mechanical containment that bottom-up scaffold repair depends on — the precise condition that makes widespread degenerative wear unsuitable for surgical cartilage restoration.

Matching treatment to defect: the practical decision criteria

Three clinical questions bring the grading and mechanism differences to a practical decision point.

First: how widespread is the damage? Diffuse Kellgren-Lawrence Grade III/IV wear — cartilage loss spread across the femoral head or acetabulum rather than confined to a single zone — excludes surgical cartilage reconstruction. Without a healthy surrounding border to contain and stabilise a bottom-up scaffold or graft, no surgical repair technique can hold. ChondroFiller injection is the appropriate scaffold pathway in this setting: the collagen gel coats the worn surface as it stands, requiring neither containment tissue nor a dry joint field.

Second: is there a focal, well-defined lesion with clear healthy margins? Where assessment confirms an isolated ICRS Grade III or IV defect enclosed by sound surrounding cartilage, bottom-up surgical repair becomes viable. Defect size then guides technique selection: ChondroFiller's CE evidence supports lesions up to 3 cm² (extendable to 6 cm²); AMIC and MACI are suited to focal defects broadly in the 2–10 cm² range; OATS is typically used for lesions of 1–4 cm².

Third: has end-stage hip OA already removed the option of preservation? If joint surfaces are exhausted beyond the threshold for cartilage restoration, neither injection nor surgical repair is the indicated pathway — hip replacement becomes the appropriate clinical discussion at that point.

Beyond anatomy and grading, the outpatient profile of the injection pathway carries its own clinical weight. For patients who cannot tolerate general or spinal anaesthesia, or who need to minimise time away from work, an ultrasound-guided injection under local anaesthetic represents a materially different burden from theatre-based surgery with a structured rehabilitation period.

Borderline cases — moderate focal damage within a diffusely worn joint, or intermediate defect sizes — are genuinely complex and do not resolve neatly into either column. A consultant assessment is needed to determine which pathway fits a specific hip presentation.

Getting assessed at Lincolnshire Hip

Deciding between ChondroFiller injection and surgical cartilage repair cannot be done on symptom severity alone — the two pathways serve structurally different hip presentations, and confirming which applies requires imaging-supported clinical assessment.

Lincolnshire Hip is part of the MSK Doctors group and accepts patients without a GP referral. Clinics are available in Sleaford and Grantham, offering direct access to hip-specialist assessment for patients across Lincolnshire and the wider region. A typical first appointment covers clinical history, a targeted hip examination, and review of any existing imaging — or arrangement of further imaging such as MRI — to confirm defect grade, surface area, and the presence or absence of containment tissue. These are the variables that determine pathway eligibility, not symptoms alone.

Prof Paul Y. F. Lee's clinical focus includes hip cartilage preservation and injectable scaffold pathways, and he leads the evaluation process for patients weighing ChondroFiller injection or surgical options.

Patients uncertain whether they are candidates for an injection or a surgical procedure should not try to resolve that question independently. The distinction is made on clinical and imaging findings — a consultant assessment is the appropriate starting point.

Lincolnshire Hip accepts patients without referral for hip assessment.