What high tibial osteotomy does to a varus knee

High tibial osteotomy is a realignment procedure, not a repair. Rather than treating damaged cartilage directly, it changes the way load travels through the knee — shifting weight away from the worn medial compartment and redistributing it towards the healthier lateral side. In a varus ('bow-legged') knee, the mechanical axis runs too far medially, concentrating force on an already arthritic surface; HTO corrects that by reshaping the proximal tibia itself.

The medial open-wedge technique, now the most widely used approach, involves cutting partially through the inner side of the tibia and gently opening the gap to the calculated angle. A locking plate holds the correction in place while bone heals. The surgical target is the Fujisawa point — a position 62% across the tibial plateau from the medial joint line — which evidence suggests achieves optimal load redistribution without over-correcting into the lateral compartment.

The goal throughout is joint preservation: buying years of comfortable function and, for younger patients, delaying or avoiding knee replacement altogether. Where focal cartilage defects coexist with the malalignment, HTO can be performed alongside cartilage restoration procedures, addressing both the mechanical cause and the local tissue damage in a single episode of care.

Who is the right candidate

If you are an active adult in your 40s or early 50s with wear on one side of the knee — the medial, or inner, compartment — and your leg has a bow-legged alignment, HTO is likely to be worth a serious conversation. That profile maps closely onto the patients who show the strongest long-term results: varus-correcting HTO achieves roughly 87% survival at ten years, with the best outcomes consistently seen in those who undergo surgery before age 55. Some centres extend eligibility to 65, though evidence beyond that threshold is limited.

The cartilage damage must be confined to the medial compartment and should not have progressed beyond Ahlback grade II — moderate narrowing rather than bone-on-bone collapse. Where the lateral compartment or patellofemoral joint is also significantly arthritic, or where damage is tricompartmental, realigning load across the tibia no longer produces a meaningful improvement and HTO is not appropriate.

Varus alignment needs to be confirmed on a standing, full-length radiograph rather than a seated or partial-weight-bearing view — static imaging can miss the deformity that only becomes apparent under load. Range of motion matters too: flexion should reach at least 90°, ideally 120°, and any fixed flexion contracture should not exceed 5–15° depending on clinical assessment.

Body weight is a practical factor. Most clinical guidelines set a BMI above 35 kg/m² as a contraindication; some authorities flag concern from BMI 30 upwards, but 35 is the threshold most commonly applied. The same applies to smoking, which is associated with impaired bone healing and poor fixation, and to osteoporosis, which raises non-union risk — both factors that a consultant assessment will screen for.

None of these criteria work as a simple checklist in isolation. A full assessment, including standing radiographs and a clinical review of activity level, bone quality, and overall health, is what translates this general profile into an individual decision.

When HTO is not appropriate

Several clinical features constitute absolute bars to HTO — not a barrier to any intervention, but to this specific procedure, because the mechanical logic breaks down when they are present.

Inflammatory joint disease, including rheumatoid arthritis, is one firm line: the underlying disease process continues to destroy joint tissue regardless of how load is redistributed. A fixed valgus deformity exceeding 20° is another — HTO is a varus-correcting procedure, and a substantial valgus alignment requires a different surgical approach entirely. Large areas of full-thickness cartilage loss, defined in most guidelines as exposed bone covering more than 15 × 15 mm, exceed what realignment alone can remedy. Tricompartmental arthritis and significant patellofemoral OA fall into the same category: where multiple joint surfaces are involved, offloading one compartment does not produce a net gain.

Ligamentous instability occupies a more nuanced position. Historically it was listed as an absolute contraindication, but combined HTO and ACL reconstruction is now an established approach for specific instability patterns — so instability is better understood today as a case-by-case consideration rather than a blanket bar, and guidelines in this area continue to evolve.

When HTO is not appropriate, the clinical pathway does not stop — it reorients. For a patient in the 55–65 range whose medial OA has progressed beyond what realignment can reliably address, unicompartmental knee arthroplasty tends to offer better-matched outcomes: it replaces the worn surface directly rather than asking the joint to respond to a positional correction. At earlier stages, or where surgery is not yet indicated, biologic support and joint-preservation options remain on the table. Matching the right procedure to the right person is exactly what a consultant assessment is designed to establish.

The rule of 57 and how correction is calculated

Deciding how much to correct a varus knee is one thing; translating that decision into an exact measurement on the bone during surgery is another. The rule of 57 is the tool that bridges the two — turning degrees of angular correction on a pre-operative X-ray into millimetres of wedge width at the operating table.

The rule rests on a geometric identity. A circle with a radius of 57 mm has a circumference of exactly 360 mm, which means each degree of arc corresponds to precisely 1 mm of length. At a bone width of 57 mm, therefore, opening or closing the osteotomy by 1 mm produces 1° of angular change — a clean one-to-one relationship. For bones wider or narrower than 57 mm, the same relationship holds proportionally, giving the working formula:

Wedge Size (mm) = Bone Width (mm) × Desired Correction (°) ÷ 57

A straightforward example: a bone measured at 60 mm across and a target correction of 5° gives 60 × 5 ÷ 57 = 5.3 mm. That is the wedge the surgeon needs to open or close to achieve the planned alignment.

The formula remains highly accurate for corrections up to approximately 30°, which covers the vast majority of HTO cases. Beyond that range, the linear approximation begins to drift from true trigonometric values, though this ceiling is rarely reached in routine clinical practice.

Beyond HTO, the same rule applies to guided growth procedures and certain spinal deformity corrections — wherever a wedge-shaped bone change needs to be sized quickly and reliably without a calculator in the operating theatre.

What outcomes and recovery to expect

Recovery from medial open-wedge HTO follows a predictable arc. Most patients progress from protected weight bearing to full weight bearing at around 8 to 9 weeks — a timeline that reflects the biology of bone consolidation across the osteotomy gap. Return to recreational activity typically extends beyond that point, and the willingness to commit to a structured rehabilitation programme is, in practice, part of what makes someone a realistic candidate for the procedure.

The ten-year survival figure already cited holds most reliably when the correction actually lands where it was planned — at the Fujisawa point, approximately 62% across the tibial plateau from the medial joint line. This is where load redistribution has its greatest mechanical effect; under- or over-correction shifts the long-term outlook meaningfully, which is why intraoperative precision in achieving the target angle matters as much as the pre-operative selection decision.

Complications are infrequent but worth understanding plainly. Lateral hinge fracture — a crack at the preserved cortex on the far side of the wedge — is the most structurally significant, as it can jeopardise correction and slow healing. Loss of correction over time remains a concern, particularly where fixation is inadequate. Superficial wound infection and peroneal nerve palsy are reported less commonly, but both are documented risks in published series.

For patients in the 55-to-65 bracket where HTO and unicompartmental knee arthroplasty both sit on the table, no high-quality randomised trial yet resolves which approach offers superior long-term outcomes. Individual bone quality, cartilage extent, activity demands, and anatomy all shift the balance — and this is precisely where consultant-led assessment, rather than population-level statistics, provides the most reliable guide.

Getting assessed without a referral

Establishing candidacy for HTO begins with a standing full-length weight-bearing radiograph — the only view that captures the knee's true mechanical axis under load — combined with a clinical assessment of compartment involvement, range of motion, and the patient's activity demands and goals. These are the inputs the surgeon needs before any conversation about correction angle or timing can be meaningful.

For patients who want that assessment without joining a waiting list, MSK Doctors consultants can see you directly. Where a more detailed picture of how load moves through the knee during walking or stair use would inform the decision, objective biomechanical assessment using markerless motion capture can complement the radiological findings.

Consultation is available at clinics in Sleaford and Grantham in Lincolnshire; patients based in London are seen through the London Cartilage Clinic. A first appointment can be booked at mskdoctors.com without a GP referral.

1. [1] High Tibial Osteotomy: An Update for Radiologists. (2021). https://doi.org/10.2214/AJR.21.26659 https://doi.org/10.2214/AJR.21.26659
2. [2] Implantable Shock Absorber vs. HTO in Medial Knee OA – 2-Year Report. (2023). https://doi.org/10.1177/19476035231157335 https://doi.org/10.1177/19476035231157335
3. [3] The Rule of 57: Orthopaedic Trigonometry Made Easy. (2019). https://doi.org/10.5435/JAAOS-D-18-00677 https://doi.org/10.5435/JAAOS-D-18-00677