Bone and Joint Infections

There are three different routes of infection in children. The most common seems to be the hematogenesis route, which gains entry into the bone from the blood stream. Less commonly is by direct inoculation and this can be a puncture wound, such as stepping on a nail or something. This can also occur following trauma or surgery. Finally, a particular spread, which is really rare in children and seems to be more common in adults with various disabilities, especially alterations in blood flow.
What is thought to happen from the hematogenesis standpoint is that the during a course of bacteremia, as the organisms enter into the bone through the nutrient artery towards the growth plate, there are these loose capillaries that are said to have sluggish blood flow in them. It is also thought that maybe there is a fully developed reticulum within this system. There does seem to be evidence of low oxygen within the metaphysis, and we always hear about this preceding history of trauma as a possibly predisposing factor. Perhaps this is simply disruptive blood flow, but the history of trauma to children is common, and it is hard to know what really this is contributing to the pathogenesis.
The nutrient artery penetrates into the diaphysis of the bone, moving up into the metaphysis and making a hairpin turn at the epiphysis. This is why it is in a long individual, at least for the tubular long bones, that osteomyelitis is more common at the ends of the bones because of these here hairpin turns.
More recently there is some evidence in animals, specifically chickens, who actually can develop osteomyelitis spontaneously. A chicken strain of Staphylococcus aureus that appears at the endothelium within the capillaries of bones have gaps, and it looks as if the organisms can actually access the capillary system to these particular gaps. If you take a Staph aureus and inject it into the blood of the chicken, within 12 hours you can see bacteria in some of these capillaries, and subsequently a day or two later, evidence of infection at the metaphyseal epiphyseal junction. So this is sort of an interesting animal experiment, perhaps showing that these epithelial gaps, at least in chickens, play some role.
Another factor in the development of osteomyelitis, at least relating to Staphylococcus aureus, is the organism that produces this sort of slimy stuff seems to make it more adherent to various portions of the bones and thus more commonly associated with osteomyelitis than those other organs.
Microbial etiology of osteomyelitis. In the neonate, the organisms most commonly associated with osteomyelitis are typically Group B streptococcus and Staphylococcus aureus. Very small babies may involve for various gram negative bacteria and certainly cause osteomyelitis as well as some other bacteria. In the infant and older child, Staphylococcus aureus is the most common cause. Streptococcus is the second most common.
Highly encapsulated organisms are unusual causes of osteomyelitis, but 3 to 5% of patients with acute osteomyelitis will have pneumococcus as the etiology.
In the older child, the same types of organisms are seen. Salmonella is an important pathogen in patients with sickle cell anemia.
With penetrating injuries, organisms associated with the soil or the skin or on clothes can of course lead to infection. Some of these injuries, such as injuries associated with lawn mower trauma, can grind the soil-type organism into the skin and ultimately into the bone.
Now, sacroiliitis is not necessarily specifically an osteo, it is an osteo-like illness we must keep in mind, especially in dealing with certain populations, especially those who are likely to ingest under pasteurized or nonpasteurized dairy products.
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