Why cartilage repair alone is not enough when the leg is out of line

Repairing a cartilage defect without addressing a varus (bow-legged) deformity is treating the damage whilst leaving the cause in place. On a weight-bearing X-ray, varus malalignment shifts the mechanical axis of the leg medially, concentrating load on the very compartment that needs to heal. Cartilage repair tissue implanted into that environment faces constant excess stress that disrupts regeneration before it can mature — the repair fails not because the technique was wrong, but because the mechanical conditions were never corrected.

The problem runs deeper than mechanics. Sustained varus loading increases the catabolic activity of chondrocytes, accelerating extracellular matrix breakdown. Correcting the axis through high tibial osteotomy (HTO) does more than redistribute force: the resulting unloading appears to activate cartilage-resident mesenchymal stem cells and support endogenous repair — making realignment a biological intervention as well as a structural one.

This is why clinical consensus, reinforced by a nationwide German Cartilage Registry cohort of 736 patients, sets a clear threshold: combined HTO and cartilage repair is recommended when mechanical varus exceeds 3°, and isolated cartilage repair in the medial compartment is contraindicated above 5°. Below that upper limit, the combined procedure gives the repair its best biological environment; above it, cartilage-only surgery carries an unacceptably high risk of failure.

Which patients are suitable for the combined procedure

The key starting point is the degree of varus on a standing weight-bearing alignment X-ray. At 3° or above, most specialists recommend adding HTO to cartilage repair; isolated cartilage repair in the medial compartment becomes contraindicated once varus reaches 5°. The rationale for those thresholds was set out in the previous section — the practical question here is what else is weighed alongside them.

Defect type is as important as the angle. The combined procedure is designed for focal, well-demarcated lesions on the medial femoral condyle or tibial plateau — the kind with a healthy surrounding rim that biologic repair tissue can fill. Diffuse cartilage loss spreading across multiple compartments indicates more advanced osteoarthritis and falls outside the scope of this approach.

The state of the lateral (outer) compartment is a further prerequisite. Because osteotomy transfers load away from the medial side, it necessarily increases the demand on the outer compartment. Pre-operative diagnostic arthroscopy is performed specifically to inspect that surface before committing to surgery — significant wear on the lateral side may not tolerate the load shift.

That same registry analysis confirmed that symptom duration and opposing joint surface quality were independent predictors of which patients received a concomitant osteotomy — not the varus angle alone. Meniscal integrity, defect size, and the number of prior knee operations also differed significantly between HTO and non-HTO cohorts, reinforcing that the decision involves a constellation of factors rather than a single measurement.

The strongest candidates are typically younger patients with preserved joint space, a focal rather than pan-compartmental lesion, and adequate bone stock for the osteotomy. For this group, the combined procedure is a joint-preservation strategy — creating the mechanical and biological conditions in which cartilage repair has the best chance of succeeding.

How the procedure is planned and what happens in surgery

Once a patient is confirmed as suitable, planning moves to two practical questions: how many trips to theatre are needed, and what alignment should the bone cut aim for?

Before any osteotomy is performed, a diagnostic arthroscopy is described as mandatory. This allows the surgical team to inspect the lateral compartment directly — the side that will carry more load once the medial compartment is unloaded. Significant wear on the outer surface could make the planned correction counterproductive, so this assessment shapes the final operative decision.

For most cartilage repair methods — microdrilling, matrix-assisted marrow stimulation, and osteochondral autograft transfer (OATS) — the repair can be carried out at the same sitting as the osteotomy. Single-stage surgery is the standard approach whenever the chosen technique does not require chondrocytes to be removed, cultured in a laboratory, and reimplanted weeks later. Autologous chondrocyte implantation (ACI) is the exception: it involves harvesting a small cartilage biopsy, sending it for cell culture, and returning for a second operation once the cells are ready. Two theatre visits rather than one is a meaningful practical difference for patients.

The correction target itself is important to understand. For focal chondral defects the goal is a neutral mechanical axis — the weight-bearing line passing straight through the centre of the knee — rather than the deliberate overcorrection sometimes applied in patients with established unicompartmental osteoarthritis. Overcorrecting a focal-defect case would impose unnecessary load on the lateral compartment; restoring neutral alignment is sufficient to decompress the repair site. Most of the available clinical evidence has been generated using medial opening-wedge high tibial osteotomy (MOWHTO), which is the technique most commonly described in the literature reviewed.

Cartilage repair options used alongside tibial osteotomy

Several cartilage repair techniques can be combined with HTO, ranging from simple marrow stimulation at one end through to cell-based implantation for larger or more demanding lesions.

Marrow stimulation — microfracture or the more controlled technique of microdrilling — has historically been the most commonly used approach. Microfracture remains the historical benchmark, but current evidence points to a recognised limitation: the fibrocartilage it produces tends to degrade at two to three years, and the awl perforations can damage the subchondral bone plate in ways that compromise future repair options. For younger, active patients this makes it a less attractive modern first-line choice.

Microdrilling creates smaller, more precise channels and shows early advantages over microfracture when combined with MOWHTO. In a comparative study of 92 patients, three validated patient-reported outcome measures — the VAS pain score, Lysholm function scale, and IKDC score — favoured microdrilling at six and twelve months, as did two imaging-based cartilage quality assessments (MOCART 2.0 and ICRS-CRA grading). Most of those differences narrowed by 24 months, though the WOMAC functional score continued to favour microdrilling throughout.

Adding bone marrow aspirate concentrate (BMAC) to MOWHTO plus microfracture represents a meaningful biological refinement. In a retrospective cohort of 113 patients, BMAC augmentation produced significantly better cartilage regeneration quality at second-look arthroscopy (mean ICRS-CRA 9.50 vs 8.62; p=0.001) and superior IKDC and Lysholm scores compared with microfracture alone.

AMIC — matrix-augmented microfracture — places a collagen scaffold over the marrow stimulation site to guide repair towards more hyaline-like tissue, a single-stage option that sits between bare marrow stimulation and full cell-based repair without requiring laboratory cell culture.

For larger defects, typically above 2–4 cm², cell-based approaches such as MACI or ACI, or osteochondral autograft transfer (OATS) for smaller focal lesions where donor-site considerations are acceptable, enter the discussion. No randomised controlled trial has yet compared these techniques head-to-head in the combined HTO setting — an honest gap that deserves a place in any patient conversation about realistic expectations.

What the outcomes evidence actually shows

Reoperation rates offer the starkest headline: a 2023 PRISMA systematic review covering five studies and 2,267 patients found that 17.3% of those who had cartilage repair combined with osteotomy required further surgery, compared with 47.4% in the isolated cartilage-repair group. On re-operative burden alone, the case for the combined approach is substantial.

The more clinically important finding comes from a 2025 propensity score-matched study of 313 patients, which examined what actually drives the PRO benefit in combined cases. Patients whose cartilage had regenerated well at second-look arthroscopy — the C1 group — showed significantly better VAS pain scores (p<0.001), Lysholm functional scores (p=0.004), and several KOOS subscales (p≤0.018) compared with matched patients who had undergone isolated HTO. Patients whose cartilage had regenerated poorly — the C2 group — showed no measurable PRO advantage over the isolated-HTO controls at the same follow-up point.

This shifts the relevant clinical question away from whether the combined procedure improves outcomes — conditional on good regeneration, it does — and towards which patients will achieve that regeneration. Pre-operative predictors for this have not yet been defined, which means the limitation sits at the frontier of current research rather than being a criticism of the procedure itself. What it does mean in practice is that the regeneration step is not a guaranteed by-product of surgery; it is itself a determinant of how much additional benefit a patient receives.

The evidence underpinning these conclusions is predominantly Level 3–4 retrospective cohort data, with most studies reporting follow-up of two to three years. Long-term survivorship comparing combined versus isolated procedures has not yet been reported in adequately powered prospective studies; what those longer timelines show — particularly beyond five to ten years — remains an open question.

For patients weighing this option, realistic expectations centre on meaningful functional improvement and a markedly reduced likelihood of needing further knee surgery, rather than a guarantee of cartilage restoration. Whether a given individual's cartilage will regenerate successfully is a conversation that belongs in a clinical assessment, informed by lesion characteristics, degree of alignment correction, and biological factors that research is still working to quantify.

Getting assessed at MSK Doctors

The information below is specific to MSK Doctors clinics and is provided as a service notice separate from the clinical evidence discussed above.

Assessment for a potential combined procedure involves weight-bearing alignment X-rays, knee MRI, and a detailed clinical consultation. All three are available at MSK Doctors' sites in Sleaford, Lincolnshire and Grantham without a GP referral or NHS-style waiting list. Where biomechanical data add meaningful context — particularly the medial compartment loading pattern before any bone correction is planned — MAI Motion®, the clinic's UKCA/MHRA-registered markerless motion-capture system, can provide objective gait and force measurements that sit alongside the standing X-ray in the planning picture, rather than replacing it.

The choice between isolated cartilage repair, isolated HTO, or a combined approach is made on an individual basis by a consultant who has the full imaging, alignment data, and clinical history in front of them.

Book a consultation without a referral at mskdoctors.com.

1. [1] MESENCHYMAL STEM CELLS MAINTAIN ARTICULAR STABILIZATION AND PROMOTE ENDOGENOUS CARTILAGE REPAIR AFTER HIGH TIBIAL OSTEOTOMY: A SECOND-LOOK ARTHROSCOPY STUDY. (2024). https://doi.org/10.1302/1358-992x.2024.1.086 https://doi.org/10.1302/1358-992x.2024.1.086
2. [2] Decision making for concomitant high tibial osteotomy (HTO) in cartilage repair patients based on a nationwide cohort study of 4968 patients. (2020). https://doi.org/10.1007/s00402-020-03476-6 https://doi.org/10.1007/s00402-020-03476-6
3. [3] Clinical Benefits of Cartilage Repair in High Tibial Osteotomy Can Only Be Expected in Patients with Successfully Regenerated Cartilage. (2025). https://doi.org/10.4055/cios24409 https://doi.org/10.4055/cios24409
4. [4] Cartilage Repair of the Tibiofemoral Joint With Versus Without Concomitant Osteotomy: A Systematic Review of Clinical Outcomes. (2023). https://doi.org/10.1177/23259671231151707 https://doi.org/10.1177/23259671231151707
5. [5] Can the MRI based AMADEUS score accurately assess pre-surgery chondral defect severity according to the ICRS arthroscopic classification system?. (2022). https://doi.org/10.1186/s40634-022-00511-w https://doi.org/10.1186/s40634-022-00511-w
6. [6] Medial opening-wedge high tibial osteotomy with microfracture in treatment of varus medial compartmental knee osteoarthritis: clinical outcomes and second-look arthroscopic results. (2023). https://doi.org/10.3389/fbioe.2023.1247165 https://doi.org/10.3389/fbioe.2023.1247165
7. [7] Effect and comparison of bone marrow aspirate concentrate on cartilage regeneration and clinical outcomes following high tibial osteotomy and microfracture. (2026). https://doi.org/10.1186/s13018-026-06996-w https://doi.org/10.1186/s13018-026-06996-w