Why alignment can make or break cartilage repair

For many patients, the natural question is whether the cartilage repair can simply go ahead — and the bone surgery be left for another day, or skipped entirely. In cases where the knee is measurably malaligned, that instinct is understandable but problematic. Load distribution, not surgical preference, is what determines whether a cartilage graft survives.

A varus (bow-legged) or valgus (knock-kneed) knee does not share body weight evenly across the joint. Instead, the mechanical axis shifts into one compartment, concentrating force on a narrow strip of tissue. Placing a cartilage repair in that compartment without correcting the axis is, in mechanical terms, similar to patching a tyre on a buckled wheel — the patch is sound, but the forces acting on it guarantee failure. Expert consensus reflects this: above a certain degree of malalignment, isolated cartilage repair is not merely suboptimal; it is widely regarded as contraindicated.

Correcting the alignment does more than remove a destructive force, however. Research published in 2024 found that reducing medial compartment load activates cartilage-resident mesenchymal stem cells — progenitor cells that actively dampen the catabolic, inflammatory environment and support integration of the implanted graft. Unloading the joint is therefore both a protective and a regenerative act.

The clinical question is never 'osteotomy or cartilage repair'. It is whether alignment correction must accompany repair for the graft to have a realistic chance of maturing and holding.

The varus thresholds that trigger osteotomy

Three degrees of varus is the clinical inflection point. Below that threshold, a medial-compartment cartilage repair may proceed without alignment correction — the mechanical load, though imperfect, is unlikely to overwhelm a well-placed graft. Above 3°, expert consensus requires an accompanying osteotomy. Above 5°, that same consensus establishes isolated cartilage repair as contraindicated.

The 2026 PMC cohort study (n=96) translated these angular measures into a radiological marker: a mechanical axis (MA) passing through less than 40% of the tibial plateau width signals medial deviation sufficient to compromise repair. Of those 96 patients, 44 met this threshold and underwent corrective osteotomy alongside MACI.

The outcome data from that cohort make the strongest possible case for correction. At arthroscopic second look, graft-site ICRS scores were 10.2 in the osteotomy group and 10.0 in the naturally aligned group — a difference of 0.2 points that was statistically insignificant (P=.70). The correction, in other words, restored repair-site quality to what a surgeon would expect in a knee that was never malaligned. Thirteen patients in the same cohort had varus alignment but declined osteotomy; their Lysholm and KOOS ADL scores at one year were significantly inferior. The comparative data show that osteotomy is not a precautionary add-on — it is what protects the outcome.

Durability extends well beyond one year. A 10-year case series of HTO combined with ACI for focal medial defects recorded 94.3% TKA-free survival, compared with 87.2% in the broader HTO cohort — which included patients with established osteoarthritis. Only two of 35 patients in the combined group required conversion to total knee arthroplasty over that decade.

Patellofemoral lesions: when patellar tracking drives the decision

The patellofemoral (PF) compartment operates by different alignment rules. Rather than axial varus or valgus, what matters here is how the kneecap tracks through its groove — and whether it sits at the right height. Three measurements guide the decision: the TTTG distance (how far laterally the patellar tendon pulls relative to the trochlear groove), the AP-TTTG (the same tendon's front-to-back offset, which indicates how much compressive load the cartilage surface on the front of the kneecap must absorb), and patella height, which reflects how high or low the kneecap rides at rest. When any of these fall outside the normal range, the repaired surface faces loading patterns that no graft can reliably tolerate.

The corrective procedure — a tibial tubercle osteotomy (TTO) — must be matched to the specific abnormality rather than applied as a standard add-on. Medialisation corrects lateral tracking; anteriorisation reduces the compressive load on the cartilage surface; distalisation corrects a high-riding patella. Applying the wrong vector risks trading one mechanical problem for another.

The outcome evidence for getting this right is striking. A 2025 database study of more than 61,000 patients found that isolated ACI for PF lesions carried an odds ratio of 10.13 for requiring revision surgery. When a concomitant TTO was added, that figure fell to 1.75 — roughly six times the revision risk when patellar tracking was left uncorrected. That pairing produced the lowest revision odds of any cartilage-restorative technique in the dataset, which frames TTO not as a routine accompaniment to PF cartilage repair but as a necessary correction whenever the measured parameters are abnormal.

Who benefits: age, lesion size, and the SONK indication

Many patients over 55 arrive at consultation having already ruled themselves out — assuming that combined cartilage repair and bone realignment belongs to a younger cohort. A study published in Cartilage offers a direct challenge to that assumption.

Eleven knees affected by spontaneous osteonecrosis of the knee (SONK) — a condition more prevalent in older adults — underwent ACI combined with opening-wedge HTO. All patients were aged 60 or over, and all had lesions of 4 cm² or more, a size traditionally associated with higher repair complexity. At one year, KOOS scores improved from 38.4 to 77.8; at arthroscopic review, 91% of repair sites were rated normal or near-normal. No postoperative complications or additional surgical interventions were required.

The evidence base here is still limited — eleven knees in a single case series — and larger cohorts are needed before these results can be treated as established guidance rather than an encouraging signal. What the data do shift, however, is the framing of the decision. Chronological age appears less relevant than biological fitness for surgery and bone quality. Osteoporosis is a recognised contraindication because it compromises osteotomy healing; a patient in their sixties with good bone stock and preserved joint condition may be a stronger candidate than that criterion alone suggests.

What the combined procedure involves in practice

Planning the combined procedure begins at the initial consultation, where MSK Doctors' consultants map out the full sequence before any operation is booked. Understanding that sequence helps patients set realistic expectations — and appreciate why it is structured as it is.

The two-stage logic of ACI

ACI's biology cannot be compressed into a single visit to theatre. At the first operation, a small sample of cartilage cells is taken from a low-load area of the knee — a biopsy. Those cells are then sent to a specialist laboratory, where they are cultured and expanded over several weeks before being ready for implantation. The surgical plan turns this interval to advantage: the osteotomy — the bone realignment — is performed at the same first operation as the biopsy. Two procedures are folded into one anaesthetic, reducing the total number of operations the patient undergoes.

The second operation, implanting the cultured cells, follows once the culture period is complete. By that point, the corrected alignment is already healing, which means the new cartilage is placed into a joint that is already mechanically protected.

Why single-stage repair is reserved for other techniques

For cartilage repairs that do not rely on cell culture — microfracture or OATS, for example — the osteotomy and the cartilage repair can genuinely happen simultaneously in a single operation. ACI does not offer that option; the biology of cell culture is fixed, not a planning inconvenience.

The contralateral compartment assessment

Before any osteotomy proceeds, an arthroscopic inspection of the opposite compartment — the one that will take on additional load after alignment is corrected — is essential. If that compartment is too degenerated to tolerate the transferred stress, the surgery plan must be reconsidered. This check is built into the pre-operative pathway, not treated as optional.

The overall timeline is longer than a standalone cartilage procedure, but the total number of operations is fewer than it would be if the two procedures were staged entirely separately.

Contraindications and what the evidence cannot yet answer

Not every patient with a malaligned knee and a cartilage defect is a candidate for the combined approach. Five findings represent absolute contraindications: rheumatoid arthritis, significant joint instability, fixed valgus deformity greater than 20°, active nicotine dependence, and osteoporosis. Each compromises either osteotomy healing or the biological environment required for graft integration. It is worth noting that some patients with one of these findings may still be candidates for osteotomy or cartilage repair considered individually — the contraindication applies to the combination, not necessarily to either procedure in isolation.

One clinical question remains genuinely unresolved. Where the established thresholds — varus beyond 3°, mechanical axis below 40% — clearly require osteotomy alongside cartilage repair, debate persists around a narrower group: patients whose cartilage surface, once inspected at arthroscopy, proves more intact than imaging suggested. Surgeons who favour osteotomy alone in such cases argue that correcting the mechanical environment is sufficient — that the joint's remaining cartilage biology can stabilise once catabolic loading is removed, a position sometimes described as 'morphotype correction'. Those who advocate adding ACI counter that any full-thickness defect above a modest size carries a structural deficit too large for endogenous repair, regardless of how well the axis is corrected. The answer in practice often turns on what is found in the joint at the time of the first operation. That intraoperative judgement — informed by pre-operative imaging, biomechanical assessment, and the patient's activity demands — is precisely why the combined decision cannot be reduced to a single radiological measurement alone.

1. [1] Effect of Lower Limb Alignment and Corrective Osteotomy on Clinical Outcomes After MACI for Extensive Cartilage Defects of the Knee — PMC. (2026). https://pmc.ncbi.nlm.nih.gov/articles/PMC12799972/ https://pmc.ncbi.nlm.nih.gov/articles/PMC12799972/
2. [2] ACI Combined with HTO for Spontaneous Osteonecrosis of the Knee with Large Cartilage Lesions in Elderly Patients. (2025). https://doi.org/10.1177/19476035251392531 https://doi.org/10.1177/19476035251392531
3. [3] 10-Year Survival Rates After HTO With Subgroup Analysis of Combined ACI and HTO. (2022). https://doi.org/10.1177/23259671221078003 https://doi.org/10.1177/23259671221078003
4. [4] Concomitant Tibial Tubercle Osteotomy Decreases Odds of Revision Patellofemoral Cartilage Surgery After ACI. (2025). https://doi.org/10.1016/j.asmr.2025.101286 https://doi.org/10.1016/j.asmr.2025.101286
5. [5] OCA and ACI Are Both Valuable Treatment Options for Patellofemoral Joint Cartilage Defects. (2024). https://doi.org/10.1016/j.arthro.2024.07.022 https://doi.org/10.1016/j.arthro.2024.07.022
6. [6] Offloading Patellofemoral Chondral Defects Requires Tailored Tibial Tuberosity Osteotomy Procedures. (2024). https://doi.org/10.1016/j.arthro.2024.10.012 https://doi.org/10.1016/j.arthro.2024.10.012