Why hip cartilage cannot repair itself

Unlike bone, muscle, or skin, the cartilage lining the hip joint cannot heal itself — and understanding why makes clear why timing matters so much in any discussion of treatment options.

Articular cartilage has no blood supply, no nerve fibres, and no lymphatic channels. That triple absence is the biological problem. When tissue elsewhere in the body is damaged, bleeding brings a rush of repair cells and signalling proteins to the site. Cartilage gets none of that response. The chondrocytes responsible for maintaining the cartilage matrix are locked within it, unable to travel even a fraction of a millimetre to an area of damage. There is no cellular cavalry; the injury simply holds.

Because a hip joint keeps moving and loading with every step, a small area of damaged cartilage rarely stays small. The compromised surface is unable to distribute load evenly, stress concentrates at the lesion edges, and the defect tends to expand over months and years. This is the mechanism by which the window for repair quietly narrows.

Every current surgical repair technique — whether marrow-stimulation, osteochondral grafting, or cell-based implantation — is built around a focal defect: a single, well-defined area of damage with healthy cartilage around its edges to build against. Once the damage becomes diffuse and those healthy borders are gone, there is no longer a structure for any repair to anchor to.

This is why early assessment carries real clinical weight. The hip joint does not pause while a decision is made. At Lincolnshire Hip, the first step in any assessment is establishing exactly how focal or how widespread the damage is — because that determines which options remain available.

Defect size and grade: which repair technique fits where

Think of a 38-year-old runner presenting with a 2.5 cm² Grade 3 lesion on the femoral head — more than half the cartilage depth gone, but still a contained defect with healthy surrounding tissue. That scenario illustrates why technique selection is not a matter of preference: the numbers largely determine the options.

Grading the damage

Standard grading runs from Grade 2 (less than half the cartilage thickness affected) through Grade 3 (more than half the depth, approaching the subchondral bone) to Grade 4 (full-thickness loss, bone exposed). Grade and defect area together map to a fairly specific set of viable techniques.

Matching technique to defect size

Up to approximately 2 cm²: Microfracture — puncturing the subchondral bone to summon marrow repair cells — was historically the first choice for smaller full-thickness lesions. Current evidence has shifted that picture: the fibrocartilage it produces tends to break down at two to three years, and the procedure can damage the subchondral bone plate in ways that complicate future repair. It is not a modern first-line option.

Roughly 2–4 cm²: AMIC (matrix-augmented microfracture, adding a scaffold layer over the marrow-stimulation site) and OATS/mosaicplasty (osteochondral autograft plugs transferred from a lower-load area of the joint) are both single-stage options at this size. OATS carries a real donor-site consideration — harvesting plugs elsewhere introduces a secondary wound.

3 cm² and above: MACI (chondrocytes seeded on a collagen membrane) and ACI (autologous chondrocyte implantation, grown in the laboratory from the patient's own cells) are the cell-based routes. ACI is not hard-capped by defect size and can address larger lesions. Both require two stages: a biopsy harvest first, reimplantation second.

Suitable focal defects up to approximately 3 cm² (extendable to 6 cm²): ChondroFiller injection — an ultrasound-guided outpatient injectable collagen scaffold — offers a non-surgical route. The scaffold recruits the patient's own progenitor cells through matrix-induced chondrogenesis, without a theatre visit.

Back to that 38-year-old runner at 2.5 cm²: AMIC, OATS, and a ChondroFiller injection all sit within the right technical range at that size — the final choice turns on lesion depth, bone involvement, and clinical assessment.

Age, FAI, and what closes the preservation window

Two factors, more than any other, determine whether a patient is still within the preservation window: the biological advantage of younger tissue, and whether an underlying structural problem is still driving damage at the joint surface.

Why age matters — without being a hard rule

Younger patients consistently show better cartilage fill on MRI and better functional outcome scores following repair procedures. The biology is more responsive, and the rest of the joint is more likely to be structurally sound. This does not mean there is a fixed birthday at which the window closes. Evidence does not support a precise age cut-off for the hip specifically. What it supports is a straightforward principle: the earlier a focal defect is addressed — before it expands, before surrounding cartilage degrades — the more the repair has to work with.

Femoroacetabular impingement — the upstream cause many younger patients share

For many younger and middle-aged adults, significant hip cartilage damage arrives without a dramatic injury. The explanation is often femoroacetabular impingement, or FAI — a shape abnormality in either the femoral head, the acetabular rim, or both, that causes the bones to pinch against each other during normal movement. Repeated contact gradually damages the cartilage lining before any single event feels severe enough to seek help.

FAI is managed arthroscopically — keyhole surgery that reshapes the bone and removes the mechanical source of friction. Treating the cartilage damage without addressing the underlying impingement is rarely sufficient; the cause remains active and the repair is at risk of failing.

What actually closes the window

The preservation window closes when the structural conditions for repair are gone — not when a patient reaches a particular age. Diffuse Grade 4 involvement, with bone-on-bone contact across both sides of the joint, leaves no healthy cartilage borders to anchor any repair technique. At that point, the clinical pathway moves towards replacement regardless of how old the patient is.

The four-stage escalation pathway from preservation to replacement

The PRRR framework — Preserve, Repair, Regenerate, Replace — maps the four stages of the hip joint care pathway in ascending order of structural demand.

Preserve covers everything done before tissue is altered: physiotherapy, load modification, activity adjustment, and biologic injections such as PRP or hyaluronic acid used as adjuncts to reduce inflammation and slow symptom progression. This stage suits early-stage pain and lower-grade damage where the joint surface remains largely intact.

Repair and Regenerate are the heart of the preservation window — focal restoration techniques matched to what clinical assessment finds. A ChondroFiller injection (an ultrasound-guided outpatient injectable collagen scaffold) suits contained defects up to approximately 3 cm². AMIC, OATS/mosaicplasty, and the cell-based routes — ACI and MACI — extend coverage to larger and deeper lesions, as outlined in the preceding section on defect size and grade. The common thread across all these techniques is that they depend on a well-defined lesion with healthy cartilage around it; once that structure is gone, none of them has anything to build against.

Replace arrives when the joint surface is too extensively damaged for any restoration approach to remain viable. At that point, total hip replacement is the structurally correct step — not a reflection of poor earlier decisions, but the appropriate answer when the conditions for preservation no longer exist. For patients in the transition zone, hip resurfacing — capping the femoral head with a metal shell rather than removing it entirely — conserves more femoral bone stock and may suit certain younger, more active patients before committing to a full implant.

At Lincolnshire Hip, the PRRR framework guides the assessment conversation: identifying which stage reflects the current state of the joint, and what that means for the options realistically on the table.

When total hip replacement becomes the right answer

For many patients over 50 with advanced osteoarthritis, total hip replacement is not a last resort — it is the straightforwardly correct answer. The clinical indication is specific: persistent hip pain and meaningful loss of function that have not responded to non-surgical measures, combined with cartilage damage too extensive or diffuse for any focal repair strategy to address.

Hip osteoarthritis is the principal driver. As cartilage progressively wears, joint space narrows, and bone contacts bone, the structural conditions for preservation disappear. OA is most common in people aged 50 and over, though age alone is not the deciding criterion — the state of the joint surface is. A 70-year-old with a focal contained lesion may still be a preservation candidate; a 55-year-old with bone-on-bone contact across the whole hip is not.

In a standard total hip replacement, both the acetabular socket and the femoral head are replaced with prosthetic components. For certain younger or more active patients at the boundary of this decision, hip resurfacing — the bone-conserving option described in the previous section — may defer or modify the replacement conversation.

The longevity data gives the timing decision its weight. Approximately 58% of total hip replacements are estimated to last 25 years. For a patient in their late fifties or early sixties, a THR performed today may need revision before they reach their eighties. This is not an argument against replacement when it is clearly indicated — it is the structural reason why the preservation window is worth using in full before replacement becomes necessary. A well-timed repair in a suitable candidate can postpone, and in some cases avoid, the revision burden that premature replacement creates.

Lincolnshire Hip is part of the MSK Doctors group and accepts patients without referral for hip assessment at its Sleaford and Grantham sites.

What the evidence does and does not yet tell us about the hip

Published defect-size thresholds — the roughly 2 cm² boundary for microfracture and the ≥3 cm² point at which MACI outperformed microfracture in the SUMMIT trial — derive largely from studies across multiple large joints rather than the hip in isolation. Hip-specific long-term outcome data for ACI and MACI are thinner than for other joints, and no well-validated, hip-specific age cut-off for the preservation window has been established. The boundary is structural (the grade and pattern of damage, not the patient's birth year), but the evidence pinning down exactly where that boundary falls in the hip, as distinct from other joints, remains limited.

This does not undermine the techniques. It means the published figures are a starting point rather than a complete answer, and that experienced clinical judgement — from someone working specifically in hip joint care — carries real weight alongside them.

Knowing this shapes the questions worth raising at assessment:

• Has my hip been imaged with MRI? X-ray reveals bone and joint space; MRI shows cartilage grade and defect area — the two variables that actually govern technique selection.
• Does the surgeon's repair experience include the hip specifically? Because most published series span multiple joints, hip-specific caseload matters more than total repair volume alone.
• Where on the PRRR pathway does my assessment place me, and what finding would move me to the next stage?

Lincolnshire Hip is part of the MSK Doctors group and accepts patients without referral for hip assessment at its Sleaford and Grantham sites.

1. [1] Hip replacement. https://en.wikipedia.org/?curid=1125423 https://en.wikipedia.org/?curid=1125423
2. [2] Hip resurfacing. https://en.wikipedia.org/?curid=11133475 https://en.wikipedia.org/?curid=11133475