📄 genetic and genomic medicine

Dissecting the genetic architecture of knee alignment reveals its contribution to osteoarthritis risk

This study integrates population-scale imaging and genetic analyses to reveal that knee alignment is partially genetically determined by pathways related to skeletal development and cartilage biology, with evidence suggesting that osteoarthritis susceptibility influences alignment rather than alignment being a primary causal driver of osteoarthritis risk.

Original authors: Faber, B. G., Alomar, F., Coveney, C. R., Chen, S., Orr, S. E., Mimpen, J. Y., Nikolic, M., Flynn, K. A., Zhang, Y., Ebsim, R., Saunders, F. R., Gregory, J. S., Aspden, R. M., Harvey, N. C., Lindner

Published 2026-06-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Faber, B. G., Alomar, F., Coveney, C. R., Chen, S., Orr, S. E., Mimpen, J. Y., Nikolic, M., Flynn, K. A., Zhang, Y., Ebsim, R., Saunders, F. R., Gregory, J. S., Aspden, R. M., Harvey, N. C., Lindner, C., Abram, S. G., Hammond, C., Davey Smith, G., Zeggini, E., Snelling, S., Capellini, T. D., Rice, S. J., Kemp, J. P., Tobias, J. H., Cootes, T. F.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

The Big Picture: The "Bow-Legged" Mystery

Imagine your legs are like the pillars of a house. If the pillars are perfectly straight, the weight of the roof (your body) is shared evenly. But if the pillars lean inward (like "bow legs" or varus alignment) or outward (like "knock knees" or valgus alignment), the weight shifts to one side.

For a long time, doctors and scientists have suspected that when your knees lean inward (bow legs), it wears down the cartilage on the inside of the knee, leading to osteoarthritis (painful, stiff, worn-out joints).

The big question this study tried to answer is: Did the bad alignment cause the arthritis, or did the arthritis cause the bad alignment? It's like asking: "Did the car crash cause the bent bumper, or did the bent bumper cause the crash?"

How They Solved the Puzzle

The researchers used a massive digital library called the UK Biobank, which contains health data and DNA from over 500,000 people.

The "Robot" Measurement: Instead of asking people to stand on X-ray machines (which is hard to do for 50,000 people), they used a computer program (machine learning) to look at whole-body scans (DXA) and automatically measure the angle of everyone's legs.
The "Family Tree" Check: They looked at people's DNA to find specific genetic "switches" that make knees naturally lean inward or outward.
The Time Travel Test: They used a method called Mendelian Randomization. Think of this as a "genetic time machine." Since your DNA is set at birth (before you ever get arthritis), if people with "bow-leg genes" get arthritis, it proves the alignment caused the disease. If the link goes the other way, it means the disease changed the alignment.

What They Found

1. The Observation: "Bow Legs" and Pain Go Hand-in-Hand

When they just looked at the data, the link was obvious. People with knees that leaned inward (varus) had much higher rates of knee pain, hospital-diagnosed arthritis, and knee replacements.

The Analogy: It's like seeing a tire that is worn down on one side. The more the tire is tilted, the more worn it gets.
The Catch: This didn't prove why. The wear could have caused the tilt, or the tilt could have caused the wear.

2. The Genetic Discovery: 20 New "Blueprints"

The study found 20 specific locations in our DNA that determine how our knees are aligned.

The Analogy: Imagine your body is a construction site. These 20 DNA locations are like the foremen who decide how the leg bones are built during childhood.
The Blueprint: These genes aren't just about bones; they are heavily involved in cartilage development and how bones grow. This suggests that the way your legs are built is deeply connected to how your joints are built from the start.

3. The "Time Machine" Result: Who Caused What?

This is the most surprising part. When they used the genetic "time machine" to test causality:

Does bad alignment cause arthritis? The evidence was weak. The study suggests that, for the general population, having naturally bow legs doesn't automatically guarantee you will get arthritis.
Does arthritis cause bad alignment? The evidence was strong. The study found that having a genetic tendency toward knee arthritis actually changes your knee alignment, pushing it into a "bow-legged" position.
The Analogy: Think of a tree. If the tree gets sick (arthritis), it might start to lean (alignment change). The study suggests the sickness is often the cause of the lean, not the other way around. The "leaning" is often a symptom of the damage already happening inside the joint.

4. The "Special Case" Exception

While the general rule is that arthritis changes the alignment, the researchers found a small group of people where specific genetic switches linked the alignment directly to the disease risk.

The Analogy: For most people, the leaning is a result of the house falling apart. But for a few specific people, the blueprint itself was slightly flawed, causing the house to lean and fall apart at the same time.

The Bottom Line

Knee alignment is a "Canary in the Coal Mine": If your knees are bowing inward, it's a very strong sign that you have or are getting arthritis. It tells doctors how severe the problem is.
It's likely a symptom, not the main villain: For most people, the bad alignment isn't the primary thing starting the disease; rather, the disease process is reshaping the leg into that position.
It's written in our DNA: How our legs align is partly decided by our genes, specifically the genes that build our bones and cartilage when we are growing up.

In short: Your knee alignment is a mix of how you were built as a child and how your joints have changed as you've aged. While "bow legs" are a huge warning sign for arthritis, the study suggests that for the average person, the arthritis is often the one pulling the strings, making the knee lean, rather than the lean causing the arthritis from the start.

Problem Statement
Knee osteoarthritis (OA) is a leading cause of chronic pain and disability globally. Lower limb malalignment, particularly varus ("bow-legged") and valgus ("knock-kneed") deformities, is widely recognized as a key determinant of load distribution across the knee joint and a risk factor for OA development. However, the causal direction of this relationship remains uncertain: it is unclear whether malalignment drives the onset of OA, or if OA progression (through cartilage loss and bone remodeling) causes the alignment to shift. Furthermore, while the heritability of femorotibial alignment is estimated at approximately 50%, the specific genetic architecture, biological pathways, and life-course mechanisms underlying these traits are poorly understood. Previous studies have been limited by small sample sizes, reliance on weight-bearing radiographs in surgical cohorts (where cause and effect are difficult to disentangle), and a lack of functional genomic integration.

Methodology
This study leverages the UK Biobank resource, integrating population-scale imaging, genome-wide association studies (GWAS), and functional genetic analyses to dissect the genetic architecture of knee alignment and its relationship with OA.

Phenotype Derivation: The femorotibial angle (FTA) was derived from whole-body dual-energy X-ray absorptiometry (DXA) scans of 67,613 participants using a machine-learning algorithm (BoneFinder) based on Random Forest regression. The algorithm automatically placed 73 outline points to define anatomical axes, calculating the angle between the femoral and tibial axes.
Observational Analysis: Associations between FTA (categorized into varus, neutral, and valgus) and clinical outcomes (knee/hip pain, hospital-diagnosed OA, and total joint replacements) were assessed using clustered logistic regression and Cox proportional hazard models, accounting for within-individual correlations.
Genome-Wide Association Study (GWAS): A GWAS of mean FTA was performed on 58,203 individuals of European ancestry using BOLT-LMM, adjusted for age, sex, genotyping array, and principal components.
Post-GWAS Functional Analysis:
- Fine-mapping: Effector genes were prioritized using colocalization with expression quantitative trait loci (eQTL) from adult joint tissues (cartilage, synovium, fat pad) and methylation QTLs (mQTL) from adult and fetal cartilage.
- Chromatin Accessibility: ATAC-seq data from fetal skeletal tissues and adult chondrocytes were used to identify regulatory regions.
- Gene-Set Analysis: MAGMA was used to map SNP statistics to genes and perform pathway enrichment.
- Disease Mapping: Identified genes were cross-referenced with curated gene sets for OA, limb development, and skeletal dysplasia.
Causal Inference: Bidirectional Mendelian Randomization (MR) was employed to estimate causal effects between FTA and knee OA, using lead variants as instruments. Sensitivity analyses (MR-Egger, weighted median, MR-Clust) were used to assess pleiotropy and heterogeneity.

Key Contributions and Results

Observational Associations: Varus alignment showed a strong, progressive association with knee pain, hospital-diagnosed knee OA, and total knee replacement (HR 3.42 for severe varus). No significant associations were found between knee alignment and hip OA. Valgus alignment showed weaker associations with knee outcomes, and severe valgus was associated with outcomes but with smaller effect sizes than severe varus.
Genetic Architecture: The GWAS identified 20 independent genome-wide significant loci associated with FTA. The trait exhibited modest SNP heritability ( $h^2 = 0.19$ ).
Genetic Correlation: There was a significant negative genetic correlation between FTA and knee OA ( $r_g = -0.16$ ) and total knee replacement ( $r_g = -0.25$ ), indicating that genetic variants predisposing to lower FTA (varus) are associated with higher OA risk. No such correlation was found with hip OA.
Functional Insights: Fine-mapping prioritized 59 plausible effector genes. Pathway enrichment highlighted biological processes related to chondrocyte differentiation, cartilage development, and endochondral ossification. Regulatory evidence was found across fetal and adult joint tissues, suggesting life-course influences. Notable genes included PTHLH, CHST3, TBX5, and RPUSD4, which overlap with known OA, developmental, and skeletal dysplasia gene sets.
Causal Direction (Mendelian Randomization):
- FTA $\rightarrow$ OA: The primary analysis provided limited evidence for a causal effect of FTA on knee OA risk (OR 0.93, 95% CI 0.79–1.10). However, MR-Clust identified a specific cluster of variants (including those near RPUSD4, PTHLH, and CHST3) that did show a negative causal estimate, suggesting a subset of genetic mechanisms where varus alignment contributes to OA susceptibility.
- OA $\rightarrow$ FTA: In the reverse direction, genetic liability to knee OA showed a strong causal effect on reducing FTA (shifting towards varus) ( $\beta = -0.11$ , 95% CI -0.16 to -0.06). This supports the hypothesis that OA progression drives malalignment.

Significance and Claims
The authors conclude that knee alignment is a biologically regulated trait influenced by both skeletal development and disease-related remodeling. The study challenges the view of malalignment as a universal primary cause of knee OA onset. Instead, the findings suggest that at the population level, the observed association is largely driven by OA causing a shift in alignment (structural remodeling), rather than alignment causing the disease.

However, the study identifies a distinct subset of genetic variants that link varus alignment directly to increased OA susceptibility via developmental and cartilage biology pathways. This implies that while general alignment modification may not prevent OA in the general population, specific biological pathways connecting skeletal patterning to joint degeneration could be targets for patient stratification.

Clinically, the authors argue that knee alignment should be viewed primarily as a marker of disease severity and progression rather than a primary causal target for prevention. Consequently, alignment-modifying interventions (e.g., osteotomy, bracing) are supported for slowing progression in established OA but are not recommended as a means of preventing disease onset in the general population. The identification of novel genetic loci and pathways offers potential for future targeted treatments and improved phenotypic stratification.