Why varus alignment undermines cartilage repair

Imagine a car tyre that has worn almost through on its inner edge. Patching the tread will not help if the wheel alignment is never corrected — the new rubber will wear through just as quickly. The same principle applies when a surgeon considers repairing damaged cartilage in a medial knee compartment affected by varus (bow-legged) alignment.

In a varus knee, the mechanical axis — the line of force that runs from hip to ankle — sits too far inside the joint. This directs a disproportionate share of bodyweight through the medial (inner) compartment with every step. Any cartilage graft implanted into that environment faces continuous overloading: catabolic signals build up, repair tissue breaks down, and the reconstruction fails to integrate. Correcting the alignment is not optional; it is what keeps the repair alive.

High tibial osteotomy (HTO) addresses this by reshaping the upper tibia so that the mechanical axis shifts towards the centre of the joint, offloading the medial compartment. The benefit appears to extend beyond pure mechanics. Research published in 2024 found that reducing medial load activates cartilage-resident mesenchymal stem cells (MSCs) — progenitor cells that dampen the catabolic environment and help stabilise the repair site, providing a biological foundation that makes an added cartilage graft more likely to survive.

Clinical guidance has translated these principles into clear decision thresholds. When mechanical varus exceeds 3°, expert consensus recommends combining any medial-compartment cartilage repair with an unloading osteotomy. Above 5° of varus, isolated cartilage repair is widely regarded as contraindicated. These are not rough guides — they mark the point at which the mechanical environment becomes incompatible with durable repair.

HTO alone or HTO with ACI — what tips the decision

Once varus correction is agreed upon, the next question is whether osteotomy alone is sufficient. HTO is not simply a preparatory step — in several clinical scenarios it produces meaningful cartilage recovery without any additional repair procedure, and midterm results for medial osteoarthritis treated with HTO alone are well documented. The combined procedure therefore needs to justify the added complexity.

The clearest guidance comes from a 2025 propensity-matched Korean study of 313 patients. Researchers compared combined cartilage repair plus HTO against isolated HTO and found that the clinical advantage of adding cartilage repair was not universal — it appeared only in patients whose cartilage had regenerated well. That well-regenerated subgroup showed significantly better scores on VAS pain (p<0.001), Lysholm function (p=0.004), and KOOS (p≤0.018) compared with matched isolated-HTO controls. Patients whose cartilage regenerated poorly showed no benefit over HTO alone on any of those measures.

In practical terms, this makes defect characteristics the pivotal variable. Focal, well-defined lesions in a knee with correctable varus represent the sweet spot: the conditions under which reliable regeneration — and therefore a meaningful clinical return — are most achievable. Diffuse or multi-compartment disease is a different conversation entirely.

Where ACI is chosen as the cartilage partner, the evidence favours it over microfracture. A 2014 comparative series found that HTO combined with ACI produced significantly higher long-term functional scores than HTO combined with microfracture. Microfracture generates fibrocartilage rather than hyaline-like tissue, and that distinction in repair quality appears to translate into measurable differences in outcome over time. No randomised trial has yet established definitive superiority of HTO plus ACI over well-executed HTO alone, but the combination's advantage over HTO plus microfracture is supported across multiple series.

What outcomes patients can realistically expect

The numbers available at 10 years are the most concrete guide patients have. In a series of 125 patients followed to a mean of approximately 10 years, those treated with HTO plus ACI achieved a TKA-free survival rate of 94.3% — only 2 of 35 patients went on to knee replacement over that period. The overall HTO cohort in the same series reached 87.2%, suggesting the combined approach may offer meaningful additional protection, though subgroup sizes limit firm conclusions.

Reoperation data from a systematic review make the cost of skipping alignment correction particularly visible. Across five studies covering more than 2,200 patients, the reoperation rate for cartilage repair performed without osteotomy was 47.4%, compared with 17.3% when osteotomy was included. Baseline varus differed between groups (1.8° versus 5.5°), which partially explains the gap — but the protective direction of mechanical correction is consistent across those studies.

The procedure is not reserved for younger patients. In a series of 11 patients aged 60 or over with spontaneous osteonecrosis of the knee and lesions of at least 4 cm², ACI combined with opening-wedge HTO produced KOOS improvement from 38.4 to 77.8 at one year. Second-look arthroscopy graded repair tissue as normal or near-normal in 91% of cases, with no complications or reoperations observed at minimum one-year follow-up — a finding that challenges the assumption this combination is suited only to younger, active patients.

What the evidence does not yet provide is reliable data beyond the 10-year mark. Long-term follow-up at 15 or 20 years for the combined procedure remains sparse — a meaningful limitation for patients in their forties or fifties who are likely to outlive the current evidence base.

Surgical planning: alignment targets, staging, and pre-operative checks

Three practical decisions shape every combined ACI and HTO plan, and understanding them helps patients ask the right questions at their pre-operative consultation.

How much correction is needed?

For patients with a focal medial defect — as distinct from diffuse osteoarthritis — the correction goal is a straight mechanical leg axis rather than the deliberate valgus overcorrection sometimes used when treating medial OA. The reasoning is straightforward: pushing the axis into valgus shifts load towards the lateral compartment and risks creating a new problem on the other side of the joint. Neutral alignment protects the graft without overloading healthy cartilage elsewhere.

The pre-operative arthroscopy

Before committing to osteotomy, a keyhole assessment of the whole joint is considered mandatory. The lateral compartment will carry more load once varus is corrected, so any significant disease there needs to be identified in advance. If the arthroscopy reveals advanced changes across more than one compartment, ACI is no longer appropriate — the procedure is designed for focal, isolated defects, not widespread joint damage.

Single-stage or two-stage?

ACI can be combined with HTO in one operation or split across two procedures, and neither approach is universally superior. A single-stage operation avoids a second anaesthetic and a second recovery period, which appeals to many working-age patients. A two-stage approach — biopsy first, then implantation once chondrocytes have been cultured — allows more time for cell preparation and may suit cases where surgical complexity warrants it. The decision is genuinely individualised and worth discussing in detail at consultation.

Who is the right candidate — and who is not

The procedure tends to work best when several conditions align: a varus deformity that is mechanically correctable, a focal lesion confined to the medial compartment, a BMI below 35, and a reasonably active baseline. That last point is quantifiable — a higher preoperative Tegner activity score has been identified as a statistically significant protective factor against eventual TKA conversion, while a BMI of 35 or above and female sex are independent predictors of conversion, in both isolated HTO and the combined procedure. These factors should be raised openly during pre-operative counselling, not softened.

Age on its own is not a reason to decline the procedure. As the SONK series described earlier shows, patients aged 60 and over with large lesions can achieve substantial benefit in appropriately selected cases.

The factors that predict a less favourable result are largely about disease extent. When cartilage loss is diffuse rather than focal — affecting multiple areas of the joint rather than a defined patch — the biological environment is not one where implanted chondrocytes are likely to thrive. At that point, the honest pathway discussion shifts away from repair and towards joint-preservation or replacement options. Patients deserve that clarity early, not after a procedure that was unlikely to succeed.

Where the evidence stands and the next steps for patients

Honesty about evidence quality frames — rather than undermines — confidence in the combined procedure.

Most of the published data comparing HTO with and without cartilage repair sits at level 3 or 4: retrospective series, case-matched cohorts, and systematic reviews of heterogeneous studies, many of which lack an isolated-osteotomy control group. No randomised controlled trial has yet established definitive superiority of HTO combined with ACI over well-executed osteotomy alone for focal medial defects — a caveat that systematic reviewers consistently flag and that any patient researching this topic deserves to hear plainly.

What the existing evidence does support is a coherent and clinically actionable picture. When cartilage genuinely regenerates after combined surgery, outcomes are meaningfully better than osteotomy alone. A 10-year TKA-free survival of 94.3% in the HTO + ACI subgroup, alongside a reoperation rate of 17.3% versus 47.4% without osteotomy, represents real-world signal substantial enough to guide practice. The practical threshold — combined surgery when varus exceeds 3°, isolated medial-compartment cartilage repair avoided above 5° — is supported by this convergent body of evidence rather than by a single definitive study.

The open questions, principally around which patients gain meaningful benefit from adding ACI rather than relying on osteotomy alone, are likely to be resolved as machine-learning tools are applied to large orthopaedic registry datasets, where case volumes can detect subgroup effects that smaller series cannot reach.

For patients considering this pathway, the most productive questions to raise at any specialist consultation are: what is the exact mechanical varus angle; how focal, sized, and deep is the lesion; and is the lateral compartment intact enough to safely bear redistributed load? Those answers — not a procedure name — determine which route is appropriate. MSK Doctors consultants in Lincolnshire assess all three without a GP referral; appointments can be booked directly at mskdoctors.com.

1. [1] Isolated Osteotomy Versus Combined Osteotomy and Cartilage Repair for Osteoarthritis or Focal Chondral Defects of the Medial Compartment: A Systematic Review. (2023). https://doi.org/10.1177/23259671231162030 https://doi.org/10.1177/23259671231162030
2. [2] 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
3. [3] Clinical Benefits of Cartilage Repair in HTO 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] 10-Year Survival Rates After High Tibial Osteotomy: Subgroup Analysis of Outcomes After Combined ACI and HTO. (2022). https://doi.org/10.1177/23259671221078003 https://doi.org/10.1177/23259671221078003
5. [5] Autologous Chondrocyte Implantation Combined with High Tibial Osteotomy for Spontaneous Osteonecrosis of the Knee with a Relatively Large Cartilage Lesion in Elderly Patients. (2025). https://doi.org/10.1177/19476035251392531 https://doi.org/10.1177/19476035251392531
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7. [7] Mesenchymal Stem Cells Maintain Articular Stabilization and Promote Endogenous Cartilage Repair After High Tibial Osteotomy. (2024). https://doi.org/10.1302/1358-992x.2024.1.086 https://doi.org/10.1302/1358-992x.2024.1.086