Two different answers to the same painful joint

Both procedures address pain from arthritis affecting one side of the knee — but they take fundamentally different approaches to the problem, and understanding that difference is the starting point for every conversation about which is right.

A high tibial osteotomy (HTO) does not touch the joint surface at all. Instead, the surgeon cuts and reshapes the shin bone (tibia) to shift the body's weight line away from the worn compartment, reducing load and slowing further damage. The natural joint is preserved.

A partial knee replacement — more precisely a unicompartmental knee arthroplasty (UKA) — resurfaces the damaged compartment with a metal-and-plastic implant, replacing the worn surface rather than realigning away from it.

Neither option suits widespread arthritis across the whole knee; both are designed for disease confined to one compartment. So the question is rarely which procedure is superior in the abstract. It is which strategy fits this patient — their age, their anatomy, their activity goals, and how far the arthritis has progressed. The sections below explain the specific criteria surgeons weigh when making that decision.

How age, arthritis severity, and activity goals shape the choice

Three overlapping filters — age and physical demand, arthritis stage, and what the patient most wants from surgery — form the practical framework your surgeon works through. No single factor is decisive in isolation.

Age and demand. HTO tends to be favoured for patients under 50–55, where preserving the natural joint makes most clinical sense; UKA is more commonly recommended above 60. Between 55 and 65, there is genuine clinical disagreement, and two patients of the same age can reasonably receive different advice. A 52-year-old who does heavy manual work or plays competitive sport sits in a very different category from a 52-year-old with a sedentary lifestyle — your surgeon will weigh physiological demand alongside the number on the birth certificate.

Arthritis stage. HTO depends on there being meaningful cartilage still present — it works by unloading a joint that can still respond to reduced pressure, making it appropriate for early-to-moderate disease. UKA, by contrast, is designed for end-stage bone-on-bone arthritis confined to a single compartment; it replaces the worn surface rather than working around it.

Activity goals. A 2026 study (Bertha) confirmed what surgeons generally observe in practice: patients wishing to return to high-impact activities — running, skiing, construction work — are guided towards HTO, while those primarily seeking pain relief who are prepared to moderate their activity level are better suited to UKA.

These thresholds are consensus-based clinical guidelines rather than hard evidence-derived cut-offs, and individual assessment will always take precedence over any rule of thumb.

Why leg alignment and the ACL are often decisive

Symptoms alone do not reveal everything a surgeon needs to know. Two anatomical factors — leg alignment and the state of the anterior cruciate ligament (ACL) — can independently determine which procedure is safe or even feasible, regardless of where the pain is felt.

Alignment. Varus ('bow-legged') or valgus ('knock-kneed') deformity is quantified on full-length standing hip-to-ankle X-rays, which map the mechanical axis — the load line running from the hip centre to the ankle. HTO is built around correcting that deformity; the realignment is central to how it works. UKA cannot correct meaningful angular deviation and should not be used when the deformity exceeds roughly 10° from neutral, because an implant placed into a malaligned knee will wear unevenly and fail early. Accurate measurement matters: alongside radiographic planning, objective biomechanical analysis — including AI-driven MRI assessment and markerless motion capture — can add dynamic joint-load data that static films alone do not capture, giving the surgeon a more complete picture of how the limb actually moves under load.

The ACL. A unicompartmental implant depends on an intact anterior cruciate ligament for its stability. Without it, the implant risks early failure or dislocation — making a deficient ACL a hard contraindication for UKA, not merely a caution. When a patient presents with both unicompartmental arthritis and ACL deficiency, the pathway typically moves toward total knee replacement rather than UKA. HTO, by contrast, can be performed alongside ACL reconstruction; the realignment simultaneously unloads the repaired ligament during healing, making this combination an established and expanding indication.

What the outcome evidence actually shows

The published evidence points in different directions depending on which outcome is measured — and those differences are clinically useful precisely because they reflect what each procedure is designed to do.

A 2018 meta-analysis by Cao and colleagues — cited over 195 times — found that UKA patients experienced lower revision rates, fewer post-operative complications, and less pain than those who had undergone HTO. On range of motion, however, the picture reversed: HTO patients achieved superior joint movement, a finding that carries real weight for manual workers, athletes, and younger patients for whom functional range matters as much as pain scores. It would be a misreading of this data to conclude that UKA is simply the better operation; these populations differ meaningfully in age, disease stage, and physical demand, and much of the measured difference in revision and complication rates reflects careful patient selection rather than technique superiority alone.

A 2023 observational study of 42 patients with isolated medial compartment arthritis found no statistically significant difference between the two groups in Oxford Knee Score, KOOS, or EQ-5D-5L at one year post-operatively — both procedures produced meaningful improvements from baseline.

The honest answer is that high-quality head-to-head trial data are limited. Most comparative studies are retrospective and modestly sized; no large randomised controlled trial has directly compared HTO and UKA in matched populations. Both procedures produce comparable patient-reported outcomes when selection criteria are respected. Where the evidence is weakest is in the patient who does not fit neatly into either profile — and that is precisely the group requiring the most careful shared decision-making.

Planning decades ahead: which pathway to knee replacement is simpler

For patients under 55, the question is not just 'which procedure works better now?' but 'what does this choice mean in 20 years?'

Knee replacements last approximately 20 years in patients over 65 — but in those under 50, longevity drops to around 10 years. A younger patient who has a partial or total replacement today is therefore likely to need a revision operation before they reach old age. Revisions are technically more demanding, carry higher complication rates, and produce less predictable results than primary surgery. Shifting that first replacement even a decade later — to an age where implant longevity better matches remaining life expectancy — is a meaningful clinical gain, not merely a delay.

A successful HTO can achieve precisely that. By preserving the natural joint and reducing compartment loading, it may buy 10–15 years of continued function before arthroplasty becomes necessary.

There is a further consideration. A 2025 meta-analysis by Li and colleagues, drawing on 11 retrospective studies and 10,045 patients, found that revision total knee replacement after a prior osteotomy required significantly fewer complex implant components than revision after a prior partial replacement. In plain terms: if a full knee replacement eventually becomes necessary, the operation is likely to be simpler — and the recovery less complicated — in patients who first had an osteotomy.

Li (2025) is a well-powered analysis, but the studies feeding into it were retrospective; it should be read as informative rather than definitive. The long-term pathway argument also matters less for older patients, for whom UKA's lower short-term revision rate and faster recovery carry considerably more weight than theoretical future revision complexity.

The grey zone, contraindications, and making the decision

There is no clean algorithm for the patient who is mid-fifties, moderately arthritic, and still reasonably active. Experienced surgeons assess the same profile and reach different conclusions — not from inconsistency, but because this cohort sits where the selection criteria for both procedures genuinely overlap, and shared decision-making carries as much weight as the clinical findings.

Several findings close off one option before the discussion starts. HTO is ruled out by a BMI above 35, a fixed flexion contracture greater than 15°, knee flexion below 90°, arthritis extending to both compartments or the patellofemoral joint, inflammatory joint disease, a required correction exceeding 20°, or heavy smoking — which raises non-union risk substantially in open-wedge techniques. Diabetes increases complication risk for either procedure and warrants pre-operative optimisation. UKA, for its part, is inappropriate where arthritis is diffuse rather than single-compartment, or where inflammatory joint disease or significant patellofemoral involvement is present.

Patients navigating this grey zone most productively arrive at any consultation with a clear account of their priorities — return to physical work or sport, reliable pain control, or long-term simplicity if further surgery becomes necessary — alongside a frank assessment of their own comorbidities. Those specifics allow a surgeon to translate general criteria into an individual recommendation. MSK Doctors consultants assess these questions directly, without referral or NHS-style waiting lists, at mskdoctors.com. Neither procedure is universally superior; the evidence ultimately converges on match between individual profile and procedure design.