Infections
Musculoskeletal infection is usually taught as a list of organisms and regimens, which makes it feel arbitrary.
It becomes systematic once you hold two ideas.
The first is that bone infection is a race between bacteria and blood supply. Antibiotics arrive through blood, and so do neutrophils. Wherever perfusion fails, treatment fails with it, and infection sets up permanently in exactly those places where blood does not reach.
The second is that bacteria change their behaviour once they attach to a surface. Free-floating organisms are killed by ordinary doses. Organisms embedded in biofilm on dead bone, a sequestrum or an implant tolerate concentrations hundreds of times higher.
Together these explain the central clinical rule of the chapter. Acute infection with living, perfused bone is a medical problem. Infection involving dead bone or metal is a surgical one, and no antibiotic course will substitute for removing the surface.
1. Why Children Get Osteomyelitis in the Metaphysis
Acute haematogenous osteomyelitis is largely a disease of children, and it has a fixed favourite site: the metaphysis of a rapidly growing long bone, most often around the knee.
The reason is vascular architecture. Nutrient artery branches reach the physis, turn sharply in hairpin loops and drain into wide venous sinusoids. Flow slows abruptly at the turn, and the lining lacks the phagocytic cells found in the sinusoids of liver and spleen.
A transient bacteraemia therefore delivers organisms into a low-flow, poorly policed region. Minor trauma, which is universal in children, provides a small haematoma for them to grow in.
Age changes the anatomy and therefore the disease
Under about 18 months, transphyseal vessels still cross the growth plate, so metaphyseal infection can spread directly into the epiphysis and the joint. After those vessels involute, the physis acts as a barrier and infection tends to spread laterally under the periosteum instead.
There is a second route into a joint that persists at all ages. Four metaphyses lie inside their joint capsules: the proximal femur, the proximal humerus, the radial neck and the distal lateral tibia at the ankle. At those sites, pus breaking through the metaphyseal cortex enters the joint directly and produces septic arthritis.
The proximal femur is the dangerous one, because pus inside the hip capsule raises intracapsular pressure and can tamponade the retinacular vessels that supply the femoral head.
2. Acute Osteomyelitis
Staphylococcus aureus is the commonest organism at every age. Reported paediatric series find methicillin-sensitive strains in roughly 45 per cent and methicillin-resistant strains in around 31 per cent, so empirical cover must reflect local resistance rather than a textbook default.
| Setting | Organism to add |
|---|---|
| Neonate | Group B streptococcus, Gram-negative bacilli |
| Child under 4 | Kingella kingae, often culture-negative on standard media |
| Sickle cell disease | Salmonella species |
| Puncture wound through a shoe | Pseudomonas aeruginosa |
| Immunocompromised, chronic, India | Tuberculosis, fungal infection |
The imaging trap
Plain radiographs are normal for the first 10 to 14 days, because visible lysis requires loss of roughly 30 to 40 per cent of bone mineral. A normal film in the first week excludes nothing.
MRI shows marrow oedema within days and is the investigation of choice. It also shows subperiosteal collections and adjacent joint effusion, which change the operation.
Blood cultures are positive in a substantial minority and should always be taken. Aspiration or bone biopsy before antibiotics gives the highest yield, and in a stable child it is worth the short delay.
Treatment
Antibiotics are started intravenously after cultures. Where there is no abscess and the child improves, an early switch to oral therapy after roughly three to seven days is now standard, with total duration usually four to six weeks.
The evidence supporting the early switch is practical: cure rates are similar, and catheter-related complications are fewer. Prolonged intravenous therapy is not a marker of thoroughness.
Surgery is indicated for a subperiosteal or intraosseous abscess, for failure to improve within about 48 hours, and for any coexisting septic arthritis.
3. Biofilm Changes Every Rule
Within hours of contacting a surface, bacteria attach and secrete an extracellular polymeric matrix. Inside it they shift from a planktonic to a sessile state: slow-growing, metabolically quiet and physically shielded.
The consequences are all clinically visible.
Antibiotics that kill dividing organisms work poorly on slow-growing ones. The matrix impedes penetration. Host neutrophils cannot phagocytose an adherent film. Sessile bacteria tolerate concentrations that may be hundreds of times the ordinary minimum inhibitory concentration, so the sensitivity report on the plate does not predict behaviour on the implant.
Rifampicin is the exception that proves the principle. It penetrates staphylococcal biofilm well, which is why it is central to implant-related infection, but resistance emerges rapidly if it is used alone. It is always given in combination.
The general rule follows: a biofilm on a surface that can be removed is cured by removing the surface. A biofilm on a surface that must stay is at best suppressed.
4. Chronic Osteomyelitis
Chronic osteomyelitis is defined by dead bone. Once a segment of cortex loses its blood supply it cannot be sterilised, and it becomes a permanent scaffold for biofilm.
The vocabulary describes what the body does with it.
Sequestrum is the dead bone fragment, separated and avascular. Radiographically it is dense, because it cannot be resorbed while living bone around it demineralises. Involucrum is the sleeve of new bone laid down by the lifted periosteum around the sequestrum. Cloaca is the opening through the involucrum through which pus drains, and a sinus carries it to the skin.
Brodie abscess is the walled-off subacute form: a lucent metaphyseal cavity with a sclerotic rim, often with insidious pain and few systemic features.
Cierny-Mader staging
The classification is useful because it combines the anatomy of the infection with the biology of the patient, and both determine what surgery is reasonable.
| Anatomical type | Description |
|---|---|
| I | Medullary |
| II | Superficial, cortical surface |
| III | Localised, full thickness but stable after debridement |
| IV | Diffuse, requires segmental resection, unstable |
Hosts are graded A for normal, B for compromised, subdivided into systemic and local factors, and C for the patient in whom the treatment would be worse than the disease.
A type IV infection in a class B host with peripheral vascular disease, diabetes and smoking is a different proposition from a type I infection in a healthy adolescent, even though both are labelled chronic osteomyelitis.
Treatment is surgical: debride to bleeding bone, manage the dead space, achieve soft-tissue cover, stabilise the skeleton, then give targeted antibiotics.
A long-standing discharging sinus carries a risk of squamous cell carcinoma, the Marjolin ulcer. A change in the character of discharge or the appearance of heaped-up edges warrants biopsy.
5. Septic Arthritis Is a Surgical Emergency
The urgency in a septic joint comes from cartilage, not from sepsis.
Bacterial enzymes, neutrophil proteases and chondrocyte death degrade proteoglycan within a day or two, and collagen loss follows. Cartilage does not regenerate, so the damage done while the diagnosis is being considered is permanent.
Synovium has no basement membrane, which is why organisms reach the joint so readily from the blood and why the inflammatory response is so intense once they arrive.
Making the diagnosis
Aspiration is the diagnostic test and should precede antibiotics wherever possible. Septic fluid is turbid, with white cell counts typically above 50,000 per cubic millimetre, more than 75 to 90 per cent neutrophils, and glucose around 30 per cent of the serum value.
Send Gram stain, culture, and crystals, because gout and pseudogout mimic sepsis and can also coexist with it. A negative Gram stain does not exclude infection.
Treatment
Drainage plus antibiotics. Arthrotomy or arthroscopic washout, or repeated aspiration in selected accessible joints, but the joint must be decompressed. Antibiotics alone treat the bacteraemia and leave the enzymes in the joint.
A septic hip is drained without delay for the same reason a fractured neck of femur threatens the head: intracapsular pressure obstructs the retinacular vessels.
Gonococcal arthritis is worth separating: a young sexually active adult with migratory polyarthralgia, tenosynovitis and pustular skin lesions, often with a sterile joint aspirate, responding rapidly to ceftriaxone.
6. Transient Synovitis and the Kocher Criteria
The commonest real question in a limping febrile child is whether this is a septic hip or transient synovitis, and the two overlap clinically.
Kocher's four predictors are non-weight-bearing, fever above 38.5 degrees Celsius, erythrocyte sedimentation rate above 40 mm per hour, and white cell count above 12,000 per cubic millimetre.
| Criteria present | Probability of septic arthritis |
|---|---|
| 0 | 0.2 per cent |
| 1 | 3 per cent |
| 2 | 40 per cent |
| 3 | 93 per cent |
| 4 | 99.6 per cent |
The value of the tool is that it is explicitly probabilistic. It does not decide; it tells you how much doubt is left, and the two-criteria row at 40 per cent is precisely the situation in which aspiration settles the question.
Transient synovitis follows a viral illness, the child is systemically well, and it settles over days with rest and analgesia. It is a diagnosis reached after septic arthritis has been excluded, not instead of considering it.
7. Tuberculosis of the Spine
India carries about a quarter of the world's tuberculosis. Incidence fell from 237 per lakh in 2015 to 187 per lakh in 2024, a 21 per cent decline running at roughly twice the global pace, but the absolute burden remains the largest of any country and India accounts for more than 32 per cent of global multidrug-resistant and rifampicin-resistant disease.
Extrapulmonary disease is 15 to 24 per cent of Indian cases, and the spine is the commonest skeletal site.
Why the disc is spared early
Spinal tuberculosis usually begins paradiscally, in the vertebral body adjacent to the endplate, and spreads under the anterior longitudinal ligament to the next vertebra.
The intervertebral disc is avascular and mycobacteria produce few proteolytic enzymes, so disc height is preserved relatively late. Pyogenic spondylodiscitis, by contrast, destroys the disc early.
On MRI, two adjacent vertebral bodies destroyed with a relatively preserved disc and a large paraspinal collection is tuberculosis until proved otherwise. The same picture with early disc destruction and a small collection suggests pyogenic infection.
Cold abscess, gibbus and paraplegia
The abscess of spinal tuberculosis lacks the heat and redness of pyogenic pus, hence "cold". It tracks along fascial planes and presents at a distance, in the psoas sheath, the groin or the retropharynx.
Anterior vertebral body collapse produces the sharp angular kyphosis called a gibbus, which is structural and does not correct with treatment.
Pott paraplegia divides usefully by timing. Early-onset paraplegia occurs during active disease and results from abscess, granulation tissue or caseous material pressing on the cord, all of which can resolve with treatment, so the prognosis is good. Late-onset paraplegia appears years after apparently healed disease, from a bony ridge, fibrosis or progressive deformity, and does far less well.
Treatment
Antitubercular chemotherapy is the treatment; surgery is an adjunct. Under India's national programme the intensive phase is eight weeks of isoniazid, rifampicin, pyrazinamide and ethambutol, and the continuation phase for skeletal disease is extended, commonly to a total of at least nine to twelve months in practice.
The important principle, and one that examiners like, is that treatment is continued until healing is demonstrated rather than until a calendar date is reached, with contrast MRI used to judge resolution.
Surgery is indicated for neurological deficit that does not improve on chemotherapy or that worsens, for spinal instability or severe progressive deformity, for a large abscess needing drainage, and for diagnostic uncertainty requiring tissue.
8. Tuberculosis Elsewhere in the Skeleton
The hip and knee are the commonest peripheral joints. Tubercular arthritis destroys cartilage slowly and diffusely, so the joint space narrows uniformly with marked periarticular osteopenia and little reactive sclerosis, the triad of Phemister.
Contrast this with pyogenic arthritis, which is faster and more destructive, and with osteoarthritis, which narrows the joint asymmetrically and builds sclerosis and osteophytes.
Tuberculous dactylitis, or spina ventosa, is the expanded, spindle-shaped short tubular bone of a child's hand or foot.
9. Infection Around Implants
Prosthetic joint infection is the clearest clinical demonstration of biofilm.
Acute infection, whether early after implantation or late and haematogenous with a short symptom duration, may be treated with debridement, antibiotics and implant retention. The biofilm is immature, and the construct is stable and well fixed.
Chronic infection with a mature biofilm requires implant removal, most often as a two-stage revision with an antibiotic-loaded cement spacer.
For staphylococcal infection managed with implant retention, guidance combines targeted intravenous therapy with rifampicin, followed by rifampicin plus an oral companion drug, for a total of about three months for a hip and six months for a knee. The companion drug exists to prevent rifampicin resistance, not because it is needed for potency.
Open fractures and internal fixation follow the same logic. An infected fracture with a stable implant may be treated with debridement, suppression and retention until union, because stability itself favours healing; once united, the implant is removed and the infection resolves.
10. Worked Examples
Example 1. A 6-year-old has 3 days of fever and refuses to bear weight, with tenderness over the distal femoral metaphysis. Radiographs are normal. What does the normal film mean?
Nothing reassuring. Plain radiographs stay normal for the first 10 to 14 days of acute osteomyelitis because visible change requires loss of roughly 30 to 40 per cent of bone mineral. MRI is the investigation of choice and will show marrow oedema within days, along with any subperiosteal collection. Take blood cultures and aspirate before starting antibiotics.
Example 2. A 14-month-old with proximal femoral osteomyelitis develops a septic hip. Explain the two anatomical reasons this happened at this site and this age.
First, under about 18 months transphyseal vessels still cross the growth plate, so metaphyseal infection can spread directly into the epiphysis rather than being contained. Second, the proximal femoral metaphysis lies inside the hip capsule, so pus breaking through the metaphyseal cortex enters the joint directly. The proximal humerus, radial neck and distal lateral tibia share this intracapsular arrangement.
The hip is the most dangerous of the four because intracapsular pressure can tamponade the retinacular vessels and infarct the femoral head.
Example 3. A patient with a total knee replacement develops infection at 4 years with 3 weeks of pain and swelling. Culture grows Staphylococcus aureus, fully sensitive on the plate. Why will six weeks of that sensitive antibiotic alone probably fail?
Because sensitivity is measured on planktonic bacteria in broth, and the organism on the implant is in a mature biofilm. Sessile bacteria grow slowly, so agents that kill dividing cells work poorly, the matrix impedes penetration, and neutrophils cannot phagocytose an adherent film. Tolerance can be hundreds of times the reported minimum inhibitory concentration. Three weeks of symptoms at four years indicates a mature biofilm, so treatment requires implant removal, usually two-stage revision, rather than antibiotics alone.
Example 4. A 40-year-old has 4 months of back pain, evening fever and weight loss. MRI shows destruction of two adjacent vertebral bodies with a relatively preserved intervening disc and a large paraspinal collection. What is the diagnosis and why does the disc appearance help?
Spinal tuberculosis. The disc is avascular and mycobacteria produce few proteolytic enzymes, so disc height is preserved relatively late while the vertebral bodies collapse. Pyogenic spondylodiscitis destroys the disc early and produces smaller collections. The large paraspinal collection is the cold abscess, which tracks along fascial planes and may present in the psoas sheath or groin.
Example 5. A 5-year-old refuses to walk. Temperature is 39 degrees Celsius, ESR is 55 mm per hour, white cell count is 15,000. How should this be managed?
All four Kocher predictors are present, giving a probability of septic arthritis of about 99.6 per cent. This is treated as a septic hip: urgent aspiration under imaging guidance for Gram stain, cell count, glucose and culture, followed by surgical drainage and empirical antibiotics covering Staphylococcus aureus with attention to local methicillin resistance. Waiting for culture before drainage would allow enzymatic destruction of cartilage that cannot be reversed.
Summary
Bone infection is a race between bacteria and blood supply; treatment fails wherever perfusion fails.
Children get metaphyseal osteomyelitis because hairpin capillary loops slow flow in a region without phagocytic sinusoidal lining.
Under 18 months, transphyseal vessels let infection reach the epiphysis and joint.
Four metaphyses are intracapsular: proximal femur, proximal humerus, radial neck, distal lateral tibia.
Staphylococcus aureus leads at all ages; add Salmonella in sickle cell, Pseudomonas after a shoe puncture, Kingella under 4.
Radiographs stay normal for 10 to 14 days; MRI is the early test.
Early intravenous to oral switch at three to seven days, four to six weeks in total, is standard for uncomplicated disease.
Biofilm makes sensitivity reports unreliable; rifampicin penetrates it but is never used alone.
Chronic osteomyelitis means dead bone: sequestrum, involucrum, cloaca, sinus, and a Marjolin risk in long-standing sinuses.
Cierny-Mader combines anatomical type with host class, because both decide what surgery is sensible.
Septic arthritis destroys cartilage within days, so drainage plus antibiotics, never antibiotics alone.
Kocher's four predictors run from 0.2 per cent to 99.6 per cent; two criteria at 40 per cent means aspirate.
Spinal tuberculosis spares the disc early, produces cold abscesses and a gibbus, and is treated medically with surgery as an adjunct.
Early-onset Pott paraplegia does well; late-onset does not.
Around implants, an immature biofilm may be retained and a mature one must be removed.
