Researchers studied children with bent knees to figure out why it happens. They found that kids with bent knees were more likely to have low vitamin D levels than kids with straight knees. But here’s the interesting part: just having low vitamin D wasn’t enough to cause bent knees. The problem happened when low vitamin D caused the body to make too much of another chemical called parathyroid hormone. When both problems happened together, kids were much more likely to have bent knees. This discovery might help doctors understand why some children develop this condition.

The Quick Take

  • What they studied: Whether low vitamin D causes bent knees in children when there are no other obvious reasons for the problem
  • Who participated: 67 children between ages 7 and 14: 38 kids with bent knees (either bowing inward or outward) and 29 kids with straight knees as a comparison group
  • Key finding: Kids with bent knees were more likely to have low vitamin D (79% vs. 48%). More importantly, when low vitamin D caused the body to produce extra parathyroid hormone, kids were about 10 times more likely to have bent knees compared to kids without these problems
  • What it means for you: If your child has bent knees without an obvious cause, a doctor might want to check vitamin D and parathyroid hormone levels. However, this is just one possible explanation, and more research is needed before making treatment decisions

The Research Details

This was a cross-sectional study, which means researchers looked at a group of children at one point in time and compared those with bent knees to those with straight knees. They measured vitamin D levels and other blood chemicals in both groups to see if there were differences. The children with bent knees had no other medical conditions or X-ray findings that would normally explain why their knees were bent. This design is useful for finding connections between things, but it can’t prove that one thing causes another.

The researchers carefully selected their participants to make sure the comparison was fair. Kids in the bent-knee group had no diseases, injuries, or X-ray evidence of rickets (a bone disease from severe vitamin D deficiency). This helped them focus on whether vitamin D deficiency alone could cause the problem. They measured specific blood markers including vitamin D levels and parathyroid hormone levels in all participants.

This study design is important because it lets researchers look at many children at once and find patterns. By comparing children with bent knees to a control group without bent knees, they could see which blood markers were different. This approach is good for generating new ideas about what might cause a problem, even though it can’t prove cause and effect like a treatment experiment could.

This study has some strengths: it compared two similar groups of children, measured specific blood markers carefully, and used proper statistical methods. However, because it’s a cross-sectional study, it shows associations (things that happen together) rather than proving that one thing causes another. The sample size is relatively small (67 children total), which means the results might not apply to all children. The study was published in a respected pediatric orthopedics journal, which suggests it met scientific standards for publication.

What the Results Show

The most surprising finding was that average vitamin D levels were similar between the two groups (16.7 in the bent-knee group vs. 19.9 in the straight-knee group), but this difference wasn’t statistically significant. However, when researchers looked at how many children had low vitamin D, they found a clear difference: 79% of children with bent knees had low vitamin D compared to only 48% of children with straight knees.

The really important discovery came when researchers looked at children who had both low vitamin D AND high parathyroid hormone levels. In this group, 85% had bent knees, compared to only 28% of children without these problems. This means children with both conditions were about 10 times more likely to have bent knees than children without these problems.

When researchers looked at children with only low vitamin D (but normal parathyroid hormone), bent knees were not more common than in the control group. This suggests that low vitamin D alone doesn’t cause bent knees—something else has to happen too.

The study found that the combination of low vitamin D and high parathyroid hormone was the key factor. Children with just one of these problems didn’t show increased bent knees. This suggests that the body’s response to low vitamin D (making more parathyroid hormone) might be what actually causes the knee bending, rather than the low vitamin D itself.

Previous research has shown that severe vitamin D deficiency causes rickets, a disease that affects bone development and can cause bent knees. However, this study suggests that milder vitamin D deficiency might cause bent knees through a different mechanism—by triggering the body to produce extra parathyroid hormone. This adds a new piece to our understanding of why some children develop bent knees without having rickets.

This study has several important limitations. First, it only looked at children at one point in time, so researchers couldn’t follow what happened to them over time. Second, the sample size was relatively small (only 67 children), which means the results might not apply to all children everywhere. Third, the study couldn’t prove that vitamin D deficiency and high parathyroid hormone actually cause bent knees—it only shows they happen together. Fourth, the study didn’t look at other factors that might affect knee shape, like genetics or physical activity levels. Finally, the results need to be confirmed by other researchers before doctors should change how they treat children with bent knees.

The Bottom Line

If your child has bent knees without an obvious cause, it may be worth asking a doctor about checking vitamin D and parathyroid hormone levels. However, this is just one possible explanation among many. Treatment decisions should be made with a pediatric orthopedic specialist who can evaluate your child’s specific situation. The evidence is moderate—this study suggests a possible connection, but more research is needed before this becomes standard practice.

Parents of children with unexplained bent knees should be aware of this research. Pediatricians and orthopedic doctors treating children with bent knees might consider checking vitamin D levels. Children living in areas with limited sunlight or those who don’t get much sun exposure might be at higher risk. This research is less relevant for children whose bent knees have an obvious cause (like injury or known disease) or for adults.

If low vitamin D and high parathyroid hormone are contributing to bent knees, correcting vitamin D levels would likely take several weeks to months to show effects. Bone changes happen slowly, so improvements in knee alignment would probably take months to years to become noticeable. Any treatment should be monitored by a doctor over time.

Want to Apply This Research?

  • Track your child’s vitamin D intake (through food and supplements) and sun exposure time weekly. Record any changes in knee appearance or walking pattern monthly with photos taken from the same angle and distance.
  • If your child has low vitamin D, work with a doctor to increase vitamin D through diet (fortified milk, fatty fish, egg yolks), safe sun exposure, or supplements. Track compliance with any recommended vitamin D supplementation daily in the app.
  • Set reminders for monthly progress photos and quarterly blood work appointments if recommended by your doctor. Track vitamin D supplement doses and dietary sources daily. Monitor for any changes in knee alignment, walking pattern, or activity level over 3-6 month periods.

This research suggests a possible connection between vitamin D deficiency, parathyroid hormone levels, and bent knees in children, but it does not prove cause and effect. This information is for educational purposes only and should not replace professional medical advice. If your child has bent knees or you’re concerned about their vitamin D levels, please consult with a pediatrician or pediatric orthopedic specialist. Do not start or stop any treatments or supplements without medical guidance. Individual results may vary, and this research applies specifically to children aged 7-14 without other medical conditions.