Who benefits from combined ACI and HTO

Combining autologous chondrocyte implantation (ACI) with a high tibial osteotomy (HTO) is a joint-preservation option for a specific group of patients — not a general solution for knee pain. The typical candidate is a younger, active adult, usually under 50, who has a focal area of cartilage damage (a chondral lesion) in the medial — inner — compartment of the knee, alongside a measurable bow-legged alignment (genu varum). The damage is contained: one area of the joint surface, rather than widespread wear across multiple compartments.

Before surgery is considered, patients should have completed a properly supervised neuromuscular rehabilitation programme without sufficient improvement. Where there is also ligament laxity or a deficient meniscus, those problems are typically addressed within the same surgical episode to give the repair the best possible mechanical environment.

The combined procedure is not appropriate for advanced, multi-compartment osteoarthritis. Some earlier series that reported disappointing results after ACI with HTO included patients who already had significant OA across more than one compartment — outcomes in those cases do not reflect what is achievable in true focal-lesion candidates.

Absolute contraindications include rheumatoid arthritis, osteoporosis, active smoking, significant knee instability, and insufficient range of motion. A consultant assessment is needed to determine whether an individual's pattern of damage and overall joint health make them a realistic candidate for this approach.

How varus malalignment undermines cartilage repair

Think of a car tyre worn unevenly on its inner edge — replacing the tyre solves nothing if the wheel alignment stays out of true. The same principle applies to the varus knee. Bow-legged alignment shifts the body's weight-bearing axis towards the medial compartment, concentrating load on an area that, in a neutral limb, would be more evenly distributed. Any cartilage repair placed into that mechanical environment will carry disproportionate stress from the moment weight is put through the joint, slowing biological integration and raising the risk of mechanical failure over time.

Specialist evidence translates this into clear decision thresholds. When mechanical varus exceeds 3°, medial cartilage repair should be accompanied by an unloading osteotomy; addressing the cartilage alone at this degree of malalignment is not considered sufficient. When varus exceeds 5°, isolated medial cartilage repair is relatively contraindicated — realignment becomes near-mandatory before or alongside any restorative procedure.

Why the correction target differs from OA surgery

Surgeons performing HTO for unicompartmental osteoarthritis deliberately overcorrect the alignment to produce a slightly valgus — outward-tilting — axis. The reasoning is straightforward: shifting load well away from a broadly worn medial compartment provides maximum mechanical relief across an already-damaged joint surface.

For focal-lesion patients, the target is different. Because the aim is to protect a defined area of cartilage repair rather than offload a compartment already lost to widespread OA, most specialists plan for a neutral straight-leg mechanical axis rather than overcorrection. Pushing a young patient's alignment into valgus serves no purpose in this setting and risks transferring excess load to a lateral compartment that is otherwise healthy — trading one problem for another.

Achieving the right correction relies on precise preoperative planning using full-length, weight-bearing (long-leg) standing radiographs, which allow the clinical team to calculate and template the exact angle of correction needed for that individual's limb geometry. This is not a standard OA protocol applied by default — it is a bespoke calculation based on where that patient's axis actually sits.

Why ACI is performed in two separate stages

Two operations are nearly always needed when ACI is combined with HTO — and the reason sits in the laboratory, not the operating theatre. Once a small sample of healthy cartilage cells is harvested from a non-weight-bearing area of the knee at the first procedure, those cells must be cultured and expanded in a specialist lab before they can be re-implanted. That cultivation step typically takes several weeks and physically prevents a single combined operation: the cells simply do not exist at implantation-ready volume on the day of the osteotomy. The standard sequence is therefore osteotomy first (with the biopsy taken at the same visit), then ACI implantation at a second operation once the cell preparation is ready.

Not every cartilage technique carries this delay. Procedures that work with the patient's own marrow or donor tissue directly — microfracture, AMIC, and OATS/mosaicplasty — require no laboratory cultivation period and can be combined with HTO in a single sitting.

Whatever the chosen repair technique, an arthroscopic assessment of the lateral compartment is performed immediately before the osteotomy. Because realigning the axis loads the lateral side more heavily post-operatively, the condition of that compartment must be confirmed as sound before proceeding — it is a deliberate safety check rather than an additional complication.

For larger defects (≥3 cm²), the cultivation wait is well justified by the evidence. The SUMMIT trial found that MACI — in which cultured chondrocytes are seeded onto a collagen membrane — produced superior KOOS pain and function scores compared with microfracture at both two and five years, supporting ACI-based approaches for the sizeable lesions that most commonly accompany significant malalignment.

How the osteotomy is planned and performed

The procedure centres on a wedge-shaped correction made in the upper shinbone. For genu varum — the inward-bowing alignment seen in most medial-compartment cartilage patients — the preferred approach is a medial open-wedge HTO: the inner aspect of the tibia is cut to a pre-planned angle and carefully opened to shift the weight-bearing axis. Where the deformity runs in the opposite direction, a lateral closing-wedge distal femoral osteotomy corrects the alignment from the femoral side instead.

Biplanar bone cuts — two intersecting planes rather than a single flat slice — give the correction site greater rotational stability. An angle-stable plate, most commonly the Tomofix, is secured across the gap. It is this rigid fixation that makes early weight-bearing possible: most patients can load the limb at around two weeks, considerably sooner than older implant designs allowed.

Patellar height is one of several individual factors the surgeon accounts for. A lateral closing-wedge technique tends to lower the kneecap and is therefore avoided when the patella already sits high (patella alta); the open-wedge route sidesteps that risk.

Surgical templating relies on full-length standing radiographs to calculate the precise correction angle for that individual limb. Objective gait and biomechanical data can complement this at both the planning and recovery stages — at MSK Doctors, MAI Motion® markerless motion capture provides repeatable, measurable data points that support the clinical team's pre- and post-operative assessment alongside the imaging.

What the evidence shows and where gaps remain

The evidence base for this combined procedure is well-supported in specialist literature but lacks large randomised controlled trials — a distinction worth holding clearly when patients are weighing their options.

The most direct head-to-head data come from a three-arm comparative study: HTO alone (n=20), HTO combined with ACI (n=18), and HTO combined with microfracture (n=18). Groups were comparable at baseline except for BMI. Studies at this scale are clinically meaningful, though they cannot yet settle all outstanding questions about comparative benefit across technique combinations.

A concern sometimes raised is published series — including Bauer et al. — that reported disappointing outcomes after combined ACI and HTO. The explanation is consistent across those analyses: the series included patients with advanced multi-compartment osteoarthritis, who fall outside the appropriate indication for ACI. In focal-lesion patients without diffuse joint disease, the outcomes profile is materially different, and attributing those published poor results to the technique itself misreads the evidence.

The 3–5° varus threshold for mandating an osteotomy reflects specialist consensus drawn from biomechanical reasoning and accumulated clinical experience rather than definitive trial data. That the precise cutpoint remains incompletely resolved by comparative RCTs is a genuine limitation — but specialist consensus thresholds of this kind, built across the orthopaedic literature, carry real weight in guiding practice.

For a young, active patient with a confirmed focal medial lesion and measurable varus, the combined approach addresses both the tissue damage and the mechanical environment that is driving it. The remaining evidence gaps are a reason for careful specialist assessment and patient selection, not a reason to attempt cartilage repair in an uncorrected malaligned knee.