Start with the diagnosis, not the operation

The first decision is not whether ACI, MACI, HTO or a partial knee replacement is “best”; it is which problem the knee actually has. ACI and MACI sit in the cartilage-restoration pathway for focal cartilage defects, with MACI representing the modern matrix-associated form of ACI used in the knee. In plain terms, that is a localised area of damage rather than widespread arthritis. A 29-year-old with a single defect after a football twist is in a very different category from someone whose whole inner knee compartment is worn.

HTO and partial knee replacement belong to that other category: medial compartment osteoarthritis. Comparative evidence is clear that HTO and unicompartmental knee arthroplasty (UKA, or partial knee replacement) are both used for medial compartment osteoarthritis, but they are not interchangeable operations. In practice, the choice depends on whether the diagnosis and goals point more toward preservation or replacement. By contrast, if arthritis is diffuse rather than localised, cell-based cartilage repair usually falls outside the usual ACI/MACI discussion, and joint-preservation or replacement becomes the more realistic pathway.

Who suits ACI or MACI

For most patients in 2024, the practical question is less “ACI or MACI?” than “does this knee defect suit cell-based repair at all?” Classic ACI is the older first-generation method, with cells implanted under a sutured membrane, whereas MACI is the matrix-associated, third-generation form that most modern knee units are more likely to discuss. In day-to-day terms, they sit in the same family, but MACI was developed to make implantation simpler and, in some settings, less invasive.

The group most likely to enter that conversation is patients with a symptomatic focal cartilage lesion who are willing to go through a cell-based repair pathway followed by structured rehabilitation. Size matters here. For smaller defects — roughly under 2 to 4 cm² in the available evidence — marrow stimulation or osteochondral grafting may still be considered. Once the defect moves beyond that small-defect range, especially from about 3 cm² upwards, the case for MACI or related third-generation ACI techniques becomes stronger.

That shift is not just theoretical. A randomised-trial synthesis found third-generation ACI had lower failure rates and better short-term patient-reported outcomes than microfracture for focal knee defects, and 10- to 17-year follow-up for MACI has shown durable improvement, with reoperation reported in 9.0% and progression to total knee arthroplasty in 7.4%. The evidence is thinner on a strict modern split between “who should have classic ACI” and “who should have MACI”, so MACI is usually the contemporary form encountered when cell-based repair is appropriate.

Who suits high tibial osteotomy

High tibial osteotomy makes most sense when the main goal is to keep the native knee working for longer rather than move straight to replacement. In practical terms, HTO is used as a joint-preservation strategy for medial compartment osteoarthritis, especially when the treatment plan favours preserving the native knee. The strongest long-term candidate profile comes from a 20-year study: patients aged under 55, with BMI under 30, and without severe symptomatic disability had the best survivorship. That is why HTO is usually framed as a joint-preservation operation for younger, relatively active people whose arthritis pattern still fits a preservation strategy.

There are also reasons it remains relevant beyond very early wear. A 2024 systematic review covering 1,296 knees with radiologically advanced medial osteoarthritis reported average 10-year survivorship of 74.6%, suggesting HTO may still be considered in some more advanced medial-compartment cases when the overall picture still favours preservation. Comparative reviews also report a trade-off that suits some patients: HTO tends to preserve range of motion better and may have lower revision rates than UKA. In other words, the best fit is not simply “young with knee pain”, but someone whose diagnosis and priorities all point towards preservation rather than resurfacing.

Who suits a partial knee replacement

The turn towards a partial knee replacement usually comes when the diagnosis has narrowed to isolated medial-compartment osteoarthritis — wear on the inner side of the knee — rather than a focal cartilage defect. In that setting, the goal changes: instead of trying to preserve or unload the worn area, surgeons may replace only the diseased compartment, provided the rest of the knee is still suitable for a unicompartmental procedure. Age on its own does not settle that decision; the published comparative evidence focuses more on disease distribution and the intended treatment goal than on a simple age cut-off.

The practical cues are therefore fairly specific. This is more likely to be a partial-replacement knee when the arthritis pattern is genuinely confined to the medial compartment and the main priority is stronger pain relief and day-to-day function, rather than preserving the native joint at all costs. That is why UKA and HTO are not interchangeable. In a 2023 meta-analysis of 38 studies, UKA was associated with less postoperative pain, fewer complications and better WOMAC scores, while HTO kept advantages in range of motion and revision profile. So the switch from preservation to replacement is driven less by birthday or label, and more by where the arthritis sits and what the operation is trying to achieve.

What can rule an option in or out

In practice, the quickest way to narrow the list is to look for the common rule-outs rather than chase one perfect age or one MRI measurement. The recent literature points to a small number of recurring filters.

• For ACI/MACI, the option becomes less attractive when the knee is no longer a clean focal cartilage problem. Published evidence also suggests lesion size matters: for smaller defects — roughly under 2 to 4 cm² in the available studies — marrow stimulation or osteochondral grafting may still be considered, whereas larger lesions are more likely to bring MACI or related third-generation ACI techniques into the discussion.
• For HTO versus UKA, the memorable filters are whether the arthritis is truly isolated to one compartment, whether the overall aim is joint preservation or replacement, and whether factors such as BMI and symptom burden make preservation more or less realistic. In the 20-year HTO study, the best survivorship was in patients under 55 with BMI under 30.
• Across comparative studies, the choice between preservation and replacement is therefore usually based on the whole pattern of disease and priorities, not one number alone.

How the final decision is made

By the end of a knee consultation, the decision can usually be held in one simple 3-part rule: repair, realign or replace. That choice is built from the history, examination, imaging, and alignment assessment, then checked against activity goals and the pattern of damage across the knee.

• Repair fits when there is a symptomatic focal cartilage defect and the rest of the joint is still preserved enough for restoration to make sense. The decisive questions are size, depth and location. Evidence around MACI becomes more persuasive once a lesion moves beyond the smaller-defect range where marrow stimulation or osteochondral grafting may still be considered.
• Realign fits when the main problem is medial compartment osteoarthritis in a patient whose overall profile still favours joint preservation. In that setting, HTO is the preservation route most often discussed.
• Replace fits when the wear is genuinely isolated to the medial compartment and the stronger priority is pain relief and day-to-day function. In the 2023 meta-analysis, UKA tended to favour pain and WOMAC outcomes, while HTO favoured range of movement and revision profile.

Additional imaging or biomechanical analysis can sharpen the picture, but they do not overrule the diagnosis or the patient’s goals. Patients who want a consultant opinion on which pathway fits their knee can book online without referral at mskdoctors.com.

1. [1] High survivorship rate and good clinical outcomes after high tibial osteotomy in patients with radiological advanced medial knee osteoarthritis: a systematic review. (2024). https://doi.org/10.1007/s00402-024-05254-0 https://doi.org/10.1007/s00402-024-05254-0