A focal defect versus hip arthritis — why the distinction matters

Does an MRI scan actually show how bad your hip cartilage damage is? The honest answer is: it shows a great deal — but only if you know what you are looking at and why the type of damage matters as much as its severity.

A focal chondral defect is a discrete, bounded area of cartilage loss — a pothole rather than a worn road surface. That distinction is clinically significant. Diffuse osteoarthritis (OA) involves deterioration spread across much or all of the joint; a focal defect is localised, which means the surrounding cartilage may still be intact and the joint as a whole may still be worth preserving. Articular cartilage has a low intrinsic capacity for self-repair, so an untreated focal defect can progress over time to the kind of generalised OA that eventually requires joint replacement — but that progression is not inevitable if the defect is identified and managed early.

The hip adds a specific imaging challenge. Acetabular cartilage is typically only 1–2 mm thick and covers a complex curved surface, so even subtle damage is easy to miss on a standard scan. Both the acetabular socket and the femoral head can be affected, and the location influences both how the lesion is imaged and which repair options are feasible.

One further point is worth setting early: symptoms are an unreliable guide to severity. Groin pain, clicking, and restricted internal rotation frequently accompany focal defects, but the intensity of symptoms does not map neatly onto imaging grade. Some patients with significant cartilage loss report modest discomfort; others with early changes are markedly limited. This is precisely why a structured imaging assessment — rather than symptom severity alone — drives the treatment decision.

Why the pathway starts with X-ray before MRI

For most patients, the imaging journey begins not with MRI but with a set of plain X-rays — often taken at the GP surgery or an urgent-access clinic before any specialist is involved. There is good reason for this sequence.

Weight-bearing X-rays, taken with the patient standing, can reveal joint space narrowing, subchondral sclerosis (increased density in the bone just below the cartilage), and osteophyte formation. These are proxy markers of cartilage loss rather than direct pictures of the cartilage itself, since X-ray does not visualise soft tissue. Crucially, a normal-appearing joint space on X-ray does not rule out a focal defect — the cartilage surface can be damaged in a discrete area while the overall gap looks preserved. This is one of the most common reasons referral is delayed.

What X-ray does show well is bony morphology. A cam lesion — a bony prominence at the femoral head-neck junction that reduces the normal spherical shape of the ball — and pincer-type overcoverage, where the acetabular rim extends too far over the femoral head, are both identifiable on plain film. So is acetabular dysplasia (undercoverage) and joint malalignment. These structural findings matter because they drive repetitive abnormal contact that damages cartilage over time, and correcting them is often necessary alongside any cartilage repair for a durable outcome. Loose bodies — calcified fragments within the joint — are also visible and relevant to surgical planning.

In short, X-ray findings determine whether subsequent MRI needs to be targeted at FAI, dysplasia, or an isolated chondral lesion — making it a necessary first step rather than a redundant one.

MRI and MR arthrography — the current diagnostic standard

Once the X-ray has mapped the bony landscape, MRI takes over as the primary tool for visualising cartilage itself. Standard MRI uses fluid-sensitive sequences — pulse techniques that make joint fluid appear bright — to detect areas of cartilage thinning, signal change within the cartilage layers, and oedema in the underlying bone. It is non-invasive, requires no injection into the joint, and gives a useful overview of the hip's soft tissues, including the labrum, tendons, and adjacent bursae.

Where standard MRI has limits, MR arthrography (MRA) adds precision. The procedure involves injecting a dilute contrast agent — a gadolinium-based dye — directly into the hip joint under image guidance before the MRI scan is performed. The contrast fluid distends the joint capsule and seeps into any gaps, clefts, or tears, making them visible against the surrounding tissue. Chondral flaps, delamination zones where cartilage has separated from its base, and labral detachments that would be compressed and invisible on a standard scan become clearly outlined. The 2015 AJR review by Link (PMC7433778) identifies MRA as the preferred imaging technique for characterising focal acetabular chondral lesions and labral pathology — because the injected contrast illuminates surface-level defects that standard MRI misses.

One important limitation is that MRA is fundamentally a surface technique. It maps what the contrast fluid can reach: breaks, clefts, and detachments at the cartilage surface. It does not reliably distinguish between partial-thickness damage at different depths within the cartilage, and it cannot detect early biochemical deterioration when the surface remains intact. That is a meaningful gap — cartilage biochemistry begins to change well before any visible surface break appears — and it is the reason more advanced compositional MRI techniques have a role in preoperative planning.

The co-existence of labral tears and chondral defects is common in hip pathology, so MRA's ability to assess both structures in a single study is a practical advantage when mapping the full extent of joint damage before treatment decisions are made.

Biochemical MRI — detecting early damage before it shows on standard scans

Cartilage is not simply a layer of material with a measurable thickness; its load-bearing capacity depends on two internal components — proteoglycans (glycosaminoglycans, or GAGs) that hold water and resist compression, and a collagen fibre network that gives the tissue its structural shape. Both of these can deteriorate significantly before any thinning appears on a standard scan.

Delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) exploits the negative electrical charge carried by GAGs. After an intravenous gadolinium injection, the contrast agent distributes inversely to GAG density: areas where proteoglycan has already been depleted attract more gadolinium and appear brighter on the scan. Bittersohl's 2011 technical review (PMC3206513) establishes dGEMRIC as a validated method for detecting early cartilage damage before irreversible morphological loss — a capability standard MRI and MR arthrography cannot match. At 3T field strength, the technique has been used in preoperative assessment of femoroacetabular impingement and to predict which patients with hip dysplasia are at risk of early failure following periacetabular osteotomy.

T2 mapping and T1rho relaxometry do not require additional contrast injection. T2 values reflect the organisation of water and collagen within the cartilage matrix; T1rho is sensitive to proteoglycan content. Both serve as quantitative imaging biomarkers for early matrix change and for monitoring the response to treatment over time.

When standard MRI appears acceptable yet symptoms persist, biochemical MRI may uncover subclinical compositional damage that shifts the management plan. These sequences are not available at every imaging centre, but specialist assessment units with research-grade 3T scanners can incorporate them into a preoperative workup where clinical suspicion warrants it.

Cartilage grading scales and what each grade means in practice

Two grading systems dominate clinical reporting of hip cartilage damage: the Outerbridge scale, originally devised for knee arthroscopy and subsequently adapted for MRI, and the ICRS (International Cartilage Repair Society) classification. They run in parallel and are broadly aligned in their depth thresholds; ICRS is now more widely used in research and surgical planning, partly because its sub-grades carry more decision-relevant detail.

Both systems divide damage into four main grades:

• Grade I — surface intact, matrix softened. The cartilage surface has not broken down, but the underlying matrix has begun to swell or soften. On standard MRI this is often invisible; it may only be confirmed by arthroscopic probing. A normal-looking scan at this grade does not rule out early biochemical change.
• Grade II — partial-thickness loss, shallower than 50% of cartilage depth. A definite defect is present but less than half the full thickness of cartilage is gone. Optimised MRI sequences can usually detect this, particularly on a 3T scanner with dedicated hip protocols.
• Grade III — deep partial-thickness loss, exceeding 50% of depth. The defect is substantial but the subchondral bone beneath is not yet exposed. ICRS adds clinically important sub-grades (A–D) that note whether the calcified cartilage layer at the base of the defect is involved and whether surface blistering is present — distinctions that influence which repair technique is most appropriate.
• Grade IV — full-thickness loss with exposed subchondral bone. The most severe focal stage. Bone is directly visible at the base of the defect, and the risk of progression to diffuse osteoarthritis rises sharply without intervention.

Depth grade alone, however, does not determine treatment. Surface area carries equal weight: a Grade III lesion of 1.5 cm² and a Grade III lesion of 5 cm² sit in entirely different management categories. The two measures — grade and area — are read together when a treatment plan is being formed.

How grade and defect size shape the treatment pathway

Grade and defect size together form the two axes of the treatment decision — and addressing one without the other leaves the plan incomplete.

Conservative management first

For most focal defects without clear red flags — severe mechanical catching, rapidly progressive imaging change, or major functional loss — a structured period of non-operative care is the appropriate first step. Targeted physiotherapy to strengthen the gluteal and core musculature, activity modification, NSAIDs, and intra-articular corticosteroid or hyaluronic acid injections can meaningfully reduce symptoms and help clarify whether the defect is truly driving the pain. A supervised trial of three to six months is typical before surgical options are formally revisited.

Structural cause correction is non-negotiable

Where FAI or hip dysplasia is the underlying mechanism, correcting the bony morphology — arthroscopically for FAI, by periacetabular osteotomy for dysplasia — is a prerequisite for any cartilage repair to hold. An uncorrected mechanical environment will re-damage any repair over time, regardless of technique.

Repair options matched to grade and size

Two practical thresholds divide the surgical pathway:

• Smaller defects (<2–4 cm²): Microfracture and mosaicplasty (osteochondral autograft) are both established options; published long-term follow-up data favour mosaicplasty on functional outcome scores.
• Larger defects (≥3 cm²): The SUMMIT trial demonstrated that matrix-induced autologous chondrocyte implantation (MACI) produced superior KOOS pain and function scores versus microfracture at both two and five years.

For patients with a contained focal defect who prefer to avoid or defer open surgical repair, specialist assessment may identify suitability for a ChondroFiller injection — an acellular collagen scaffold delivered as an ultrasound-guided outpatient procedure. This sits in a different treatment category from the surgical options listed above; individual defect characteristics, including size and depth grade, determine eligibility.

The imaging grade does not dictate a single answer — it narrows the field. Depth, surface area, and structural context read together are what allow a specialist to sequence a plan that protects the cartilage that remains rather than simply reacting to what has already been lost.

Specialist assessment of defect grade and treatment suitability, including ChondroFiller injection, is available at the London Cartilage Clinic on Harley Street — appointments via londoncartilage.com.

1. [1] Femoroacetabular impingement — Wikipedia. https://en.wikipedia.org/?curid=20754811 https://en.wikipedia.org/?curid=20754811