Causes of Osteomyelitis
Organisms
All types of organisms including viruses, bacteria, parasites and fungi can produce osteomyelitic infection. The most common ones are those produced by bacteria; the pyogenic bacteria and tuberculosis causing mycobacteria. Hence osteomyelitis is divided into Pyogenic and Tuberculous Osteomyelitis.
Pyogenic Osteomyelitis
Causative Bacteria
Staphylococcus aureus is responsible for 80% to 90% of the infections where the organism is recovered.
Escherichia coli, Pseudomonas and Klebsiella are frequently responsible for infections in patients who are intravenous drug abusers or in those with genito urinary infections.
Mixed infections occur in open fractures or during surgeries.
In the neonates the causative bacteria are Haemophilus influenzae and Group B Streptococci.
Individuals with sickle cell disease develop infection with Salmonella.
In 50% patients, organisms are not isolated.
Tuberculous Osteomyelitis is caused by Mycobacterium tuberculosis
From bacteria to bone - the spread of infection
The routes of spread are -
- Through the blood stream or haematogenous
- Extension from a contiguous site
- Direct implantation
In children, seemingly trivial mucosal injuries acquired during defecation or chewing or skin infections can be the trigger for the infection to enter, spread and reach the bone to cause osteomyelitis.
In adults, osteomyelitis occurs as a consequence of open fractures, surgical procedures and diabetic infections of the feet.
Healthy bone is resistant to infection.
Open wounds, fractures compromise the blood supply to the area; diabetes can cause neuropathy due to which the patient may not be aware of infection progressing due to sensory loss and poor nutrition and immunosuppression can inhibit the body defence mechanisms. Any of these factors can predispose to osteomyelitis.
Bacteria interact with the host tissues, invade the cells, and extend into the bone. Once in the bone, they proliferate and produce an inflammatory reaction leading to the production of pus, destruction of the bone and cell death. Formation of abscesses under the periosteum and eventually a draining sinus is also formed.
The presence of dead bone allows infection to persist, often indefinitely. Bacteria get enmeshed and adherent to dead bone resulting in a biofilm which protects them from the host and allows them to stay for years. Biofilm plays a significant causative role in chronic osteomyelitis.
In infants, infection can spread into the joint spaces causing septic arthritis.
In tuberculous osteomyelitis, the organisms are blood borne and originate from a focus of active tuberculosis in the body or can extend directly from the lungs to the rib or from a lymph node to the vertebra.
The spine, followed by the knees and the hips are the most common sites of involvement in tuberculous osteomyelitis.
The bony infection in tuberculous osteomyelitis is usually solitary and in some patients may be the only manifestation. In patients with AIDS - Acquired Immunodeficiency Syndrome the bone involvement is multifocal.
Pott’s disease is tuberculous disease of the spine where the infection breaks through the intervertebral disc space, infects the vertebrae and causes abscesses in the soft tissue.
