- Design
- Retrospective multicentre cohort study
- Population
- 297 patients undergoing primary robotic-assisted TKA
- Primary outcome
- Oxford Knee Score at 3 months, 6 months and 1 year by change in CPAK phenotype
- Effect
- Two-phenotype shift (n = 19): smaller OKS gain at 3 months (p = 0.008) and 6 months (p = 0.036)
A retrospective multicentre cohort of 297 patients had primary robotic-assisted knee replacement with long-leg radiographs before and after surgery. The authors classified knees using the Coronal Plane Alignment of the Knee (CPAK) system, which combines arithmetic hip-knee-ankle angle and joint line obliquity, and tracked Oxford Knee Scores to one year.
About 30.6% of knees stayed in their original CPAK type, 62.3% moved one type and 6.4% (19 patients) moved two. Those who moved two types gained less on Oxford Knee Score at three months (p = 0.008) and six months (p = 0.036) than those who stayed in the same type. Femoral coronal changes of more than 4 degrees from native were linked to the smallest early gains.
The key group is only 19 patients, the design is retrospective and the follow-up for a difference was early. The authors call it exploratory. It adds weight to the idea that the femoral component's position matters, but it does not set a target.
- Most knees (62.3%) shifted one CPAK type after robotic TKA.
- Only 19 patients shifted two types, and they had smaller early Oxford Knee Score gains.
- Femoral coronal change over 4 degrees was linked to the smallest early gains.
- Treat as exploratory when planning alignment strategy.
- Audit your own alignment shifts and early scores.
Why it matters
It supports the idea that restoring native alignment may matter, which is currently debated in knee replacement.
Don't overread it
A retrospective exploratory subgroup of 19 patients cannot establish an alignment target.
The statistics, in plain English
A p value of 0.008 in a group of 19 patients is a signal worth testing, not proof; small subgroups can produce striking differences by chance and by who happened to be in them. Differences at 3 and 6 months do not tell us about the 1-year or long-term result.
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