Acute osteomyelitis ia a commonly tested topic in orthopedic examinations. So I would now summarise the key points in the pathogenesis, clinical diagnosis and management of this common orthopedic condition- Acute osteomyelitis.
Definition
Osteomyelitis is an inflammation of the bone caused by an infecting organism.
Classification of osteomyelitis
1. Duration--
- acute (less than 2 weeks),
- subacute ( 2-6weeks)
- chronic (>6 weeks)
2. Mechanism--
- exogenous
- hematogenous.
Exogenous osteomyelitis is caused by open fractures, surgery (iatrogenic), or contiguous spread from infected local tissue. The hematogenous form results from bacteremia.
3. Host response to the disease --
- pyogenic
- nonpyogenic.
4. Cierny and Mader classification system for chronic osteomyelitis based on host factors and anatomical criteria.
Acute Hematogenous Osteomyelitis
· the most common type of bone infection
· usually is seen in children.
· M > F.
. age distribution- bimodal,< 2 years & 8 to 12 years.
Pathogenesis of acute osteomyelitis
Acute hematogenous osteomyelitis is caused by a bacteremia, which is a common occurrence in childhood. Bacteriological seeding of bone generally is associated with other factors such as localized trauma, chronic illness, malnutrition, or an inadequate immune system.
In children the infection generally involves the metaphyses of rapidly growing long bones.
Bacteremia-->Sluggish flow of blood in hair pin like loops of vessels in highly vascular metaphysis -->Bacteria settle in metaphysis--> inflammatory reaction -->à local ischemic necrosis of bone --> abscess formation -->abscess enlarges --> intramedullary pressure increases -->cortical ischemia --> allow purulent material to escape through the thin cortex into the subperiosteal space --> subperiosteal abscess.
If left untreated -->extensive sequestra formation and chronic osteomyelitis.
The metaphysis is commonest site of osteomyelitis, because-
· Is highly vascular
· Has a hair pin like arrangement of capillaries
· Has sluggish blood flow
· has relatively fewer phagocytic cells than the physis or diaphysis, allowing infection to occur more easily in this area
· thin cortex
In children younger than 2 years, some blood vessels cross the physis and may allow the spread of infection into the epiphysis.
A resulting abscess will break through the thin metaphyseal cortex, forming a subperiosteal abscess.
The diaphysis rarely is involved, and extensive sequestration occurs infrequently except in the most severe cases.
In children older than 2 years of age, the physis effectively acts as a barrier to the spread of a metaphyseal abscess to epiphysis.
because the metaphyseal cortex in older children is thicker, the infection spreads into the diaphysis --> endosteal blood supply may be jeopardized.
With a concurrent subperiosteal abscess, the periosteal blood supply is damaged and can result in extensive sequestration and chronic osteomyelitis if not properly treated.
Hematogenous seeding of bone in adults usually is seen in a compromised host. generally the vertebral bodies are affected. In these patients, abscesses spread slowly, and large sequestra rarely form.
Spread of infection to a contiguous joint
In children younger than 2 years of age,
the common blood supply of the metaphysis and epiphysis crosses the physis .
metaphyseal abscess --> epiphysis --> joint.
The hip joint is the most commonly affected in young patients;
physes of the proximal humerus, radial neck, and distal fibula also are intraarticular, and infection in these areas can lead to septic arthritis as well.
In severe infection, epiphyseal separation can occur in children younger than 2 years of age.
In older children this common circulation is no longer present and septic arthritis is rare.
After the physes are closed, infection can extend directly from the metaphysis into the epiphysis and involve the joint. Therefore septic arthritis due to acute hematogenous osteomyelitis generally is seen only in infants and adults.
Causative microbes in acute osteomyelitis
· Staphylococcus aureus is the most common infecting organism found in older children and adults with osteomyelitis.
· Gram-negative bacteria -vertebral body infections in adults.
· Pseudomonas -- intravenous drug abusers.
· Fungal osteomyelitis-- chronically ill patients receiving long-term intravenous therapy or parenteral nutrition.
· Salmonella osteomyelitis- SS or SC hemoglobinopathies. tends to be diaphyseal
· In infants -S. aureus (mc),group B streptococcus & gram-negative coliforms . S. aureus or gram-negative organisms - orthopaedic infections found in premature infants
· Group B streptococcus - otherwise healthy infants 2 to 4 weeks of age.
· Haemophilus influenzae-6 months and 4 years.
DIAGNOSIS of acute osteomyelitis
In infants, the elderly, or immunocompromised patients, clinical findings may be minimal. Fever and malaise/ pain and local tenderness / Swelling/ compartment syndrome has been reported in children.
· WBC count often normal,
· ESR and CRP level elevated.
· The CRP is a measurement of the acute phase response and is especially useful in monitoring the course of treatment of acute osteomyelitis because it normalizes much sooner than the ESR.
· Skeletal changes, such as periosteal reaction or bony destruction, generally are not seen on plain films until 10 to 12 days into the infection.
· Technetium 99m bone scans can confirm the diagnosis as early as 24 to 48 hours after onset in 90% to 95% of patients.
· Gallium scans and indium 111–labeled leukocyte scans also can aid in diagnosis when used in conjunction with technetium scanning.
· MRI can show early inflammatory changes in bone marrow and soft tissue.
· The causative organism can be identified in approximately 50% of patients through blood cultures.
· Bone aspiration usually gives an accurate bacteriological diagnosis and should be performed with a 16- or 18-gauge needle in the area of maximal swelling and tenderness, usually the long bone metaphysis. The subperiosteal space should be aspirated first by inserting the needle to the level of the outer cortex. If no purulent material or fluid is encountered, the needle is placed through the cortex to obtain a marrow aspirate.
· CT or ultrasound-assisted aspiration in suspected hip or vertebra OM
TREATMENT of acute osteomyelitis
It has been well established that sequestered abscesses demand surgical drainage. However, areas of simple inflammation without abscess formation can be treated with antibiotics alone.
Nade's principles for the treatment of acute hematogenous osteomyelitis:
1. an appropriate antibiotic will be effective before pus formation;
2. antibiotics will not sterilize avascular tissues or abscesses and such areas require surgical removal;
3. if such removal is effective, antibiotics should prevent their reformation and therefore primary wound closure should be safe;
4. surgery should not further damage already ischemic bone and soft tissue;
5. antibiotics should be continued after surgery.
general supportive care--- IV fluids, analgesics, and comfortable positioning of the affected limb.
If no abscess (by subperiosteal or bone marrow aspirate) then intravenous antibiotics based on the gram stain should be started.
Empirical antibiotic coverage for the most likely infecting organism should be started if gram stain is negative, and the patient then should be carefully monitored. The CRP should be checked every 2 to 3 days. If no appreciable clinical response within 24 to 48 hours, then occult abscesses must be sought and surgical drainage considered.
Surgery for acute osteomyeilitis
Indications
· an abscess requiring drainage
· failure to improve despite appropriate IV antibiotic treatment for 48 hours.
Objective
· to drain any abscess cavity and
· remove all nonviable or necrotic tissue.
Surgical Procedure for acute osteomyelitis
· Do not exsanguinate the limb with an elastic bandage if infection is present
· When a subperiosteal abscess is found in an infant, several small 4 mm holes should be drilled through the cortex into the medullary canal.
· If intramedullary pus is found, then a small window of bone is removed.
· The skin is then closed loosely over drains, and the limb is splinted.
· The limb is protected for several weeks to prevent pathological fracture.
· 2 weeks IV antibiotics,à4 weeks oral antibiotics
I hope this simplifies your study of acute osteomyelitis.
