Small Animal
Skeletally mature
Sequestrum

Signs and Symptoms:

History:  The sequestrum is normally a result of trauma.  The sequestrum becomes avascular and introduction of bacteria may result in osteomyelitis.  The introduction of bacteria may occur at the same time as the trauma, which is usually the case when surgery is the cause.  It is for this reason that sterile technique is very important.  In small animals, it is likely that this form of osteomyelitis is the result of surgery.  If the infection was introduced after the sequestrum was formed, the infection could have been introduced through a direct would, hematogenously, or iatrogenically.  

Clinically Signs:  Intermittent or persist lameness associated with a draining tract.  
Atrophy, local pain, fluctuant swellings, and period febrile episodes are also common.
Radiographs will show bone destruction, and bone formation according to the activity of the organism.
Radiographs will also show a sequestrum, which is an avascular piece of bone.

Management Plan:

The sequestrum should be approached surgically, as the lack of blood supply precludes the antibiotics from reaching the source of infection.  
Any dead or avascular bone should be removed and the remaining bone should be curetted until bleeding to assure vascular supply.
Systemic antibiotics should also be given systemically.  It is important to run a culture and sensitivity test and to choose a broad spectrum antibiotic.  This treatment should be continued for at least 4 to 6 weeks, and the site should be recultured following the treatment to assure that the infection has cleared.
The prognosis is guarded until the dead bone has been removed, and then it is fair.

Difference from Case 3:

Case 3 was an acute infection and didn't involve a sequestrum.
Case 3 did not need to be treated with surgery, but systemic antibiotics would be beneficial in both cases.
The acute infection in case 3 has a better prognosis.
The probable cause of osteomyelitis in the large animal is trauma and in small animals it is surgery.