What Grade 4 knee cartilage loss actually means

Bone exposed at the joint surface — that is what Grade 4 means in practice. Under the ICRS classification, a Grade 4 chondral defect represents full-thickness cartilage loss that extends through the subchondral bone, the hard plate directly beneath the cartilage. Nothing protective remains between the opposing joint surfaces, which is why the term 'bone-on-bone' is used clinically.

This matters because articular cartilage has no blood supply and cannot repair itself. Left without intervention, a Grade 4 lesion does not stabilise — progressive deterioration is the default course.

Not all Grade 4 damage follows the same pattern, and that distinction shapes every treatment decision. A focal defect is a contained area of full-thickness loss — a pothole within an otherwise intact surface. Diffuse or pan-articular loss, by contrast, affects the compartment or joint broadly, more like a road that needs resurfacing than a single repair. The treatment ceiling for each is very different: biologic repair techniques are designed for focal lesions and are explicitly not appropriate for widespread arthritis.

Grade 4 is too advanced for symptom management alone. Lesion size, location, lower-limb alignment, age, activity level, and BMI all bear on whether repair, realignment, or replacement offers the most durable outcome — and each of these variables is addressed in the sections that follow.

Cartilage repair options for focal Grade 4 defects

Choosing between repair techniques depends primarily on how large the focal defect is — a size-anchored hierarchy has emerged from both clinical experience and randomised trial data.

For the smallest focal lesions, microfracture was for many years the default first step: perforating the subchondral bone to stimulate a marrow-based healing response. Its limitation is fundamental. The tissue it produces is fibrocartilage — mechanically inferior to native hyaline cartilage — and published series document a predictable deterioration at two to three years. Critically, repeated microfracture damages the subchondral bone plate and can compromise future repair attempts. Current use is declining as better options have become available.

AMIC (autologous matrix-induced chondrogenesis) takes the same marrow-stimulation base and adds a collagen scaffold, held in place over the defect to guide cell ingrowth toward more organised, hyaline-like repair tissue. It is single-stage, NICE-approved, and applicable to focal defects broadly comparable in size to traditional microfracture indications — a practical bridge when chondrocyte culture is not available or the patient cannot commit to a two-stage procedure.

For defects in the 1–4 cm² range, osteochondral autograft transfer (mosaicplasty or OATS) is well-established. Plugs of healthy bone-and-cartilage are harvested from a lower-load area of the same knee and press-fitted into the defect, delivering structurally intact hyaline cartilage. Donor-site morbidity — the consequence of harvesting those plugs — is a meaningful consideration and limits how much tissue can be taken.

Once defect size reaches roughly 3 cm² or above, MACI (matrix-induced autologous chondrocyte implantation) has the strongest evidence base. The SUMMIT randomised controlled trial demonstrated superior KOOS pain and function scores versus microfracture at both two and five years — currently the best Level 1 data in this size range.

For the largest or most complex focal lesions, particularly those arising after trauma, fresh osteochondral allograft (OCA) is the established option. At a mean six-year follow-up, approximately 71% of patients achieved very good or excellent knee function, and around 75% returned to sport or recreational activity.

When lower-limb alignment is loading the damaged compartment

Think of the knee as a set of scales. In a well-aligned leg, roughly 60–70% of load passes through the medial compartment during walking; varus deformity tips that balance further inward, concentrating force on cartilage that is already damaged. Valgus deformity does the opposite, overloading the lateral compartment. In either case, malalignment is not merely a background finding — it is an active mechanical amplifier of Grade 4 loss, and no repair procedure can survive if the force driving that damage remains uncorrected.

High tibial osteotomy (HTO) is the established correction for varus malalignment with medial compartment overload. The tibia is cut and realigned — in most cases using a medial open-wedge technique with a biplanar cut and angle-stable implants — to shift the weight-bearing axis toward the intact lateral compartment. It is most commonly chosen in younger, active patients where the goal is to protect the joint and delay arthroplasty.

Distal femoral osteotomy (DFO) performs the valgus analogue: correcting the femoral alignment to unload the lateral compartment in patients with valgus deformity.

Neither procedure regenerates cartilage on its own — their role is to remove the mechanical cause of ongoing damage and create an environment where the joint, or a repair graft placed alongside, has a realistic chance of surviving. The TKA-delaying benefit of osteotomy is well-supported in the literature, and for patients where diffuse damage or other factors exclude biologic repair, standalone osteotomy remains a legitimate joint-preservation strategy in its own right.

Combining realignment with cartilage repair

Some surgeons have historically staged these interventions years apart — correcting alignment first, then revisiting the joint for cartilage work once recovery was complete. The problem with that gap is real: a misloaded defect continues to deteriorate during the waiting period, and repair tissue implanted into a joint that was only recently realigned has had no time to benefit from the improved mechanical environment. Where both malalignment and a repairable focal defect are present, addressing both together is increasingly the preferred approach.

Before any osteotomy, arthroscopy of the same knee is mandatory. The compartment that will bear greater load after correction — the lateral side after HTO, the medial side after DFO — must be assessed directly; significant damage there changes the risk-benefit calculation for the whole strategy.

Single-stage or two-stage?

The answer depends on which cartilage procedure is planned. Marrow-stimulation techniques and OATS can be performed at the same sitting as the osteotomy, making a single operation feasible. MACI and first-generation ACI require chondrocyte harvesting and laboratory culture, so a two-stage approach is unavoidable — the biopsy is taken at arthroscopy, the osteotomy is performed once cells are ready.

The longest available data for this combined strategy comes from Ferruzzi et al, whose cohort of HTO combined with cartilage repair showed sustained clinical results at eleven-year follow-up. That is a cohort study, not a randomised controlled trial, so the findings inform rather than settle the question. What remains openly unresolved is the optimal correction angle in HTO or DFO for maximising graft survival — an area of active research, and an honest reason why individual surgical planning still requires experienced judgement.

When partial or total replacement becomes the right answer

Biologic cartilage repair — ACI, MACI, and AMIC — has a hard contraindication that the evidence is clear about: it cannot substitute for replacement when damage is widespread across two or three compartments. Cartilage grafting works only when there is a focal target to fill; diffuse, pan-articular loss leaves no viable scaffold environment for regeneration.

For patients who fall between focal repair and total replacement — typically those with compartmentally isolated Grade 4 loss where the biological window for repair has closed — unicompartmental knee arthroplasty (UKA) offers a meaningfully different proposition from TKA. UKA resurfaces only the damaged compartment, preserving the cruciate ligaments and leaving the native cartilage of the unaffected side untouched. The clinical advantages follow from that: less tissue disruption, a smaller incision, and faster recovery compared with total replacement. It is better understood as a partial resurfacing than as a scaled-down version of a full replacement.

Total knee arthroplasty becomes the appropriate pathway when disease is pan-articular, when previous preservation attempts have failed, or when the patient's biological profile makes repair implausible regardless of defect pattern.

The implant lifespan — approximately 20 years — creates a practical incentive to exhaust joint-preservation options in patients under roughly 55. Revision surgery within a working lifetime is a genuine clinical risk, not a hypothetical one. That said, when preservation is no longer viable, TKA is not a failure of the pathway — it is the right answer for that stage of disease.

The five variables that decide the pathway

Defect size is the first branch point. Focal lesions below roughly 4 cm² sit within the repair window — AMIC and OATS for smaller areas, MACI where the SUMMIT trial demonstrated superiority over microfracture for defects of 3 cm² or more. Once involvement extends beyond a focal target, restoration gives way to resurfacing.

Location shapes technical difficulty independently of size. The medial femoral condyle is the most studied repair site; trochlear defects introduce patellofemoral contact complexity; tibial lesions require careful graft architecture matching. Each carries a different depth of supporting evidence.

Malalignment — measurable varus or valgus on a standing full-length radiograph — shifts planning toward osteotomy. A corrected mechanical axis is a precondition for any repair graft to survive; implanting cartilage into a persistently overloaded compartment is not a durable strategy.

Patient age and biological potential are linked but distinct. The MACI and OCA series with the strongest long-term data predominantly enrolled patients in their 30s and early 40s; above roughly 65 with diffuse change, the biological environment is unlikely to support durable regeneration and replacement becomes appropriate. The decade between those anchors requires individual judgement — an honest reflection of the published evidence, not a gap in the framework.

BMI and activity demands complete the picture. Higher BMI concentrates load through any repair graft and raises complication risk; whether the patient's goal is return to competitive sport or pain-free daily walking shapes both technique selection and realistic recovery expectations.

These five variables rarely point cleanly in one direction. A specialist assessment works through each systematically — cross-referencing imaging findings, mechanical axis measurement, and a structured activity history — before any pathway decision is reached.