The preservation window — and why timing matters

Am I still a candidate for repair, or has replacement become the right conversation? That question sits at the heart of every hip cartilage consultation — and the honest answer depends on timing as much as on the damage itself.

The hip's articular cartilage — the smooth lining that allows the femoral head to glide within the acetabulum — does not regenerate on its own once it is meaningfully damaged. What matters clinically is whether that damage remains focal and contained, or whether it has spread into the broader joint. When it is still contained, a preservation window exists: a period during which surgical repair can restore the joint surface and defer, or in some cases avoid, replacement entirely. Once that window closes — through progressive osteoarthritis, structural problems left uncorrected, or simply delayed diagnosis — joint-preserving repair becomes unsafe or ineffective, and hip replacement planning is the appropriate next step.

Four clinical variables determine where a patient sits on this pathway: the stage of osteoarthritis, the size and depth of the cartilage defect, the status of the cartilage and surrounding structures (which matters more than age alone), and whether any underlying mechanical cause — such as impingement or dysplasia — has been addressed.

NHS waiting times add a practical dimension. A patient who is borderline eligible for preservation today may cross into replacement territory before a surgical slot becomes available if the joint continues to deteriorate during the queue. This article maps those variables clearly, so readers can approach a consultation with a grounded sense of where they are on the pathway.

OA stage: the clearest gate on the pathway

Osteoarthritis stage is the most decisive of those four variables, and the Tönnis grading system is the standard radiographic tool surgeons use to assess it before any hip preservation procedure.

The grades translate into practical terms:

• Grade 0–1: Minimal or early radiographic change, joint space largely preserved. This is the preservation window — cartilage repair and arthroscopic procedures are appropriate for suitable patients.
• Grade 2: Definite joint space narrowing, moderate femoral head deformity, possibly small cysts. Hip arthroscopy at this stage carries a high early failure rate and a significant risk of rapid conversion to replacement, making grade 2 a relative to strict contraindication.
• Grade 3–4: Advanced narrowing and bone-on-bone contact. Arthroscopic cartilage repair is an absolute contraindication here; hip replacement is the clinically appropriate path.

Plain radiograph measurement adds a further, independent signal: joint space width below 2 mm is on its own a strong indicator that preservation is unlikely to succeed.

One caveat worth understanding: the Tönnis system shows only fair inter-observer reliability (kappa 0.30), meaning two experienced surgeons reviewing the same X-ray may sometimes arrive at different grades. This is why a surgeon will want to review imaging in person — and assess symptoms and functional status alongside it — rather than offer a definitive verdict remotely. The NHS England hip OA decision support tool and the National Joint Registry patient decision tool are publicly available, UK-validated resources that can help patients frame their own position before that conversation takes place.

Defect size and which repair technique fits

Depth before size — that is the first clinical question. Surgical cartilage repair is indicated only where cartilage loss is full-thickness (ICRS grade 3–4); superficial fraying or surface irregularity at grade 1–2 does not meet the threshold for formal biological reconstruction. Once full-thickness loss is confirmed, defect area guides technique selection.

The size ranges below are clinically informed benchmarks, not rigid categories. Within any overlapping zone, the surgeon weighs lesion depth, shape, bone stock, concurrent structural pathology, and the patient's activity demands before settling on a technique — size alone does not determine the answer.

• Minimally invasive option (focal defects, outpatient setting): ChondroFiller injectable collagen scaffold, delivered as an ultrasound-guided outpatient injection, works through matrix-induced chondrogenesis — an acellular scaffold that recruits the patient's own progenitor cells to lay down new tissue. This pathway requires no general anaesthesia or theatre admission. In published hip series, improvements of approximately 30 points in the modified Harris Hip Score have been reported.
• Small to mid-range defects (~1–3 cm²), single-stage surgical escalation: AMIC (autologous matrix-induced chondrogenesis) augments marrow stimulation with a biological matrix, bridging the gap between basic marrow-stimulation techniques and full cell-based repair. Published series in recreational athletes support sustained benefit in this size range.
• Larger focal defects (up to approximately 3 cm² on the acetabulum), surgical: MACI, ACI, and osteochondral allograft (OCA) transplantation are appropriate where defect area exceeds what AMIC can reliably address. A Cartilage Journal review (Husen, 2022) identified 3 cm² as the practical upper limit most clinicians accept for biological acetabular restoration; above this threshold, joint-preserving options narrow considerably and replacement planning typically becomes the next discussion.

A note on microfracture: historically the default for smaller defects below 2 cm², microfracture produces fibrocartilage rather than true hyaline cartilage — tissue that tends to break down within two to three years — and damages the subchondral bone plate in ways that can limit the success of any subsequent repair attempt. It is not a recommended modern first-line choice, though its historical role in establishing hip cartilage repair as a field remains relevant context.

Age and cartilage status: what actually decides eligibility

Many patients assume hip replacement becomes inevitable once they reach a certain age — but the evidence points to cartilage condition as the more decisive factor.

Registry data from Germany (116,179 hip cartilage procedures, 2006–2022) show that regenerative repair procedures cluster around a mean patient age of 27.73 years, compared with 43.42 years for debridement. That gradient is real — biological repair capacity does diminish over time — but it describes a population distribution, not a clinical cut-off.

A 2024 study of hip arthroscopy and periacetabular osteotomy in patients aged 45 and older makes this distinction concrete. Where articular cartilage was absent or only minimally affected, outcomes in the older cohort were comparable to those seen in younger patients. The authors concluded that cartilage status — not chronological age — was the operative variable in determining who benefited from preservation surgery.

Femoral head cartilage grade gives surgeons a measurable expression of this principle. Higher Outerbridge grade at hip arthroscopy is an independent predictor of eventual conversion to total hip replacement (odds ratio 6.75), meaning the surface condition of the femoral head carries more prognostic weight than the patient's birth year. Five-year follow-up data from a registry of 281 patients with femoroacetabular impingement syndrome reinforce this: small cartilage injury (under 1 cm²) combined with preserved joint space consistently predicted better functional outcomes than larger defects with reduced joint space width.

The practical implication runs both ways. An older patient with a focal lesion and intact joint space may still be a sound preservation candidate; a younger patient with diffuse cartilage loss across the joint may already be beyond the repair window. Age shapes the risk–benefit calculation — it does not determine the answer.

When the preservation window has closed

Three clinical findings typically close the preservation window: Tönnis grade 3 or 4 on radiograph, joint space width below 2 mm, or a prior repair that has failed without sufficient joint space remaining to support further intervention. At that point, cartilage restoration is no longer a realistic goal — the articular surface and underlying bone are too compromised for regenerative techniques — and total hip arthroplasty (THA) becomes the appropriate clinical answer.

THA replaces both sides of the damaged joint: the acetabular cup is fitted with a new socket component, and the femoral head is replaced by a metal or ceramic ball on a stem fixed into the femur. For patients with advanced osteoarthritis, this resurfaced, mechanically stable joint reliably relieves pain and restores function in ways that cartilage repair techniques are not designed to achieve at this stage of disease.

The longevity figure most relevant for patients planning ahead is that approximately 58% of total hip replacements are estimated to last 25 years. That number carries two messages at once: the majority of primary replacements remain functional well into the third decade, but revision surgery is a genuine possibility within a younger patient's lifetime. For someone in their late forties or early fifties who has reached this point, implant longevity is worth discussing in full with a consultant — it is a planning consideration, not a reason to avoid the procedure.

For patients with advanced disease, hip replacement is the evidence-based endpoint; outcomes are well-established and the surgical pathway is clearly defined. Lincolnshire Hip is part of the MSK Doctors group and accepts patients without referral for hip assessment, including those considering whether they have reached the replacement stage, with post-operative physiotherapy support available locally in Grantham and Sleaford.

Structural causes, surgical prerequisites, and getting assessed

Cartilage repair does not happen in isolation. Whether a patient is a candidate for microfracture, AMIC, or a cell-based procedure, the outcome is shaped as much by the mechanical environment of the joint as by the repair technique itself.

Femoroacetabular impingement (FAI) and hip dysplasia are the two most common structural contributors to focal cartilage damage. Left uncorrected at the time of repair, the abnormal loading or contact stress that caused the original lesion continues to act — and early repair failure follows. Correcting the underlying structural pathology concurrently is a clinical prerequisite, not a supplementary decision.

Surgical technique carries weight at a finer level too. A 2025 animal model study found that preserving the labrum and capsule during microfracture produced significantly better cartilage healing — measurable in type II collagen and aggrecan expression — compared with microfracture alone. How the joint is handled during the procedure influences how well the repair consolidates.

Five-year registry data from 281 patients with femoroacetabular impingement syndrome illustrate the cost of delay. Normal joint space width was associated with a 21-point greater functional improvement versus severely reduced joint space; cartilage injury under 1 cm² consistently outperformed larger defects at five years. Each additional degree of structural deterioration narrows what remains technically possible.

Full assessment — OA grade, defect size, structural contributors — requires weight-bearing radiographs, clinical examination, and in many cases MRI. These establish not just a diagnosis but a pathway position. The difference between Tönnis grade 1 and grade 2, or between a 1.5 cm² and a 3.5 cm² defect, determines whether the preservation conversation remains open — or whether replacement planning has become the more honest clinical priority.

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