This week my son Isaac had surgery on his knee for a dislocated patella. Fortunately it all went well, but he will be in plaster for the next 5 weeks. Shouldn’t he have anticoagulant prophylaxis to prevent a blood clot? In Adults knee surgery is associated with a high risk of thrombosis and many international guidelines suggest giving prophylaxis for several weeks after knee surgery especially knee replacement. However the risk in children is much lower and prophylaxis is not recommended unless there is some other risk factor.
Cause of clots
In fact the cause of blood clots in children differ from those seen in adults. In adults we are aware that surgery, obesity, immobility and cancer are all risk factors, but in children one of the major causes of a blood clot is the use of central venous catheters; these are lines into a vein used for various types of treatment. 
Also children with nephrotic syndrome (a kidney condition causing protein loss), and congenital heart disease have a higher risk of thrombosis, along with all the known inherited thrombophilias.
How common are clots in children?
The incidence in children is very low. It is highest during the first year of life at around 5 per 100,000 births then drops throughout childhood and starts to rise in adolescence.
It is not well understood why the incidence is lower in children. There are probably many factors relating to the chemistry of the blood, the state of the blood vessels and the activity of the individual.
My son had surgery on Wednesday, within 2 days he was walking around. If it was me I would be moaning about my leg for days but kids just seem to get up and get on with life.
It’s good to hear that your son is recovering from the surgery. Hoping he will get well soon. Thanks for the post!