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Osteomyelitis and Septic Arthritis

Osteomyelitis - Classification and Pathophysiology

Classification Systems

The classification of osteomyelitis provides the framework for both diagnostic and therapeutic decision-making. The Waldvogel classification, introduced in the 1970s, divides osteomyelitis into hematogenous, contiguous with vascular insufficiency, and contiguous without vascular insufficiency. While conceptually useful, its clinical applicability is limited. The Cierny-Mader classification offers a more surgically relevant schema, combining anatomic type (medullary, superficial, localized, or diffuse) with physiologic host class: type A hosts are immunologically normal, type B hosts are compromised by local or systemic factors, and type C hosts are those in whom the morbidity of treatment exceeds the disease burden. In everyday clinical practice, the most operationally useful distinctions are between acute versus chronic disease, hematogenous versus contiguous acquisition, and native bone versus prosthetic material involvement, as these categories directly guide both the diagnostic workup and the surgical and antimicrobial approach.

Pathophysiology

The pathogenesis of osteomyelitis differs fundamentally according to the route of infection. Hematogenous osteomyelitis reflects the bacteremic seeding of bone, with a characteristic predilection for the metaphysis in children, where the sluggish blood flow within metaphyseal sinusoids creates a permissive environment for bacterial lodgment and proliferation. In adults, hematogenous osteomyelitis preferentially involves the vertebral body, reflecting the rich arterial supply of the vertebral endplates. Contiguous osteomyelitis, in contrast, arises from direct extension of infection into bone from adjacent soft tissue, occurring most commonly in the post-surgical, post-traumatic, and diabetic foot settings.

Biofilm formation represents the critical transition from treatable acute infection to refractory chronic disease. Bacteria begin to adhere to bone matrix and form biofilm within hours of inoculation, and by two to three weeks, a mature biofilm has developed that renders eradication without surgical debridement nearly impossible. The hallmark pathologic features of chronic osteomyelitis are the sequestrum, a segment of devascularized necrotic bone that harbors bacteria within a biofilm sanctuary inaccessible to systemic antibiotics, and the involucrum, a sleeve of reactive new bone that forms around the sequestrum as the periosteum attempts to wall off the infection.

Microbiology

The microbiology of osteomyelitis varies predictably with the route of infection, the patient's age, and the clinical context. In adults with hematogenous osteomyelitis, Staphylococcus aureus predominates at 60 to 70 percent of cases, followed by streptococci, gram-negative organisms in the vertebral setting, and coagulase-negative staphylococci when prosthetic material is involved. In the pediatric population, S. aureus remains the most common pathogen, but Kingella kingae is increasingly recognized as a major cause of bone and joint infections in children under five years of age, particularly with the improved detection afforded by PCR-based diagnostics. Group A Streptococcus also contributes significantly in pediatric cases.

Contiguous and post-surgical osteomyelitis frequently involves S. aureus and coagulase-negative staphylococci, with polymicrobial infection being common, particularly when gram-negatives and anaerobes are introduced through surgical wounds or traumatic injuries. Diabetic foot osteomyelitis exemplifies the polymicrobial end of the spectrum, characteristically involving S. aureus, streptococci, Enterobacterales, Pseudomonas aeruginosa, and anaerobic organisms including Bacteroides and Peptostreptococcus. The classic teaching that Salmonella is the predominant cause of osteomyelitis in sickle cell disease, while an important association to recall, should not obscure the fact that S. aureus remains the most common overall pathogen even in this population. In injection drug users, S. aureus and Pseudomonas are the predominant organisms, with a predilection for axial skeleton involvement including the sternoclavicular joint.

Osteomyelitis - Diagnosis

Clinical Features

The clinical presentation of osteomyelitis depends on the acuity and chronicity of the disease. Acute osteomyelitis presents with pain, swelling, erythema, and warmth over the affected bone, accompanied by systemic signs such as fever and malaise. Chronic osteomyelitis, in contrast, may present more insidiously, with a draining sinus tract representing the pathognomonic finding, particularly when the tract can be probed to bone. Persistent pain and recurrent soft tissue breakdown over a previous surgical site or fracture are characteristic features of chronic disease.

Vertebral osteomyelitis deserves special clinical attention. Back pain is the most common presenting symptom, often preceding the diagnosis by weeks to months. Fever is present in only approximately 50 percent of cases, and neurologic deficits from epidural abscess, which complicates 10 to 20 percent of vertebral osteomyelitis cases, may be the presenting finding that prompts urgent evaluation.

Laboratory Studies

Inflammatory markers are elevated in more than 90 percent of osteomyelitis cases, with CRP being the more useful marker for monitoring treatment response given its short half-life of approximately 19 hours compared to the much slower kinetics of the ESR, which responds over days to weeks. Procalcitonin is less useful in osteomyelitis than in systemic infections, as it may remain normal in the setting of localized bone infection without significant bacteremia. The white blood cell count is normal in up to 50 percent of patients with chronic osteomyelitis, underscoring its limited sensitivity as a diagnostic marker for this disease.

Imaging

Plain radiography should serve as the initial imaging study, though clinicians must recognize that radiographic changes do not appear until 10 to 14 days after disease onset, with sensitivity in the early phase being only 43 to 75 percent. When radiographic changes do appear, they include periosteal reaction and lytic lesions. MRI is the gold standard imaging modality for osteomyelitis, achieving sensitivity of 90 to 100 percent and specificity of 80 to 90 percent. The characteristic MRI findings include low signal on T1-weighted images reflecting marrow edema, high signal on T2-weighted and STIR sequences, and gadolinium enhancement of involved bone and surrounding soft tissue. CT provides better cortical detail than MRI and is superior for identifying sequestra and guiding surgical planning, though it is inferior to MRI for assessment of marrow involvement. Nuclear medicine studies, including triple-phase bone scan, white blood cell-labeled scans combined with bone marrow scanning, and 18F-FDG PET/CT, serve roles when MRI cannot be obtained or when hardware artifact limits MRI interpretation, with PET/CT having an emerging role particularly in prosthetic joint infections.

Microbiologic Diagnosis

Bone biopsy is the gold standard for microbiologic diagnosis and should ideally be obtained before initiating antibiotics whenever clinical circumstances permit. Percutaneous CT-guided biopsy achieves a sensitivity of 50 to 70 percent, with higher yields when no prior antibiotics have been administered, and specimens should be sent for aerobic, anaerobic, mycobacterial, and fungal cultures along with histopathology. Blood cultures are positive in 50 to 75 percent of hematogenous osteomyelitis cases and should always be obtained. In the diabetic foot, the probe-to-bone test carries a positive predictive value of 89 percent in high-risk populations with a pre-test probability exceeding 60 percent. Clinicians should avoid treating based on superficial wound or sinus tract cultures, as the polymicrobial flora of the wound surface is often discordant with the true bone pathogen, with the notable exception that S. aureus isolated from superficial culture demonstrates approximately 75 percent concordance with bone culture results.

<image>A diagnostic imaging comparison for osteomyelitis showing four panels. Panel 1: "Plain radiograph" of a tibia showing periosteal elevation and lytic destruction in chronic osteomyelitis with a sequestrum visible. Panel 2: "MRI T1-weighted" showing low signal intensity in the vertebral body (vertebral osteomyelitis) with disc space involvement. Panel 3: "MRI STIR sequence" of the same vertebra showing high signal intensity in the vertebral body and disc with paravertebral soft tissue enhancement. Panel 4: "CT scan" showing cortical destruction and sequestrum formation in a long bone with surrounding involucrum. Each panel should be labeled with key findings and arrows pointing to pathology. Use grayscale medical imaging conventions.</image>

Osteomyelitis - Treatment

Surgical Principles

Surgical debridement of necrotic bone is a fundamental principle of osteomyelitis management, rooted in the recognition that dead bone does not receive antibiotic delivery and serves as a sanctuary for bacterial persistence within biofilm. Source control requires complete removal of sequestra and excision of sinus tracts. Vertebral osteomyelitis is predominantly managed medically, with surgery reserved for specific indications including neurologic compromise from epidural abscess with deficit, spinal instability, and failure of medical therapy. Diabetic foot osteomyelitis may require amputation, with the level of amputation guided by the vascular status and the extent of infection.

Antibiotic Therapy

The traditional duration of antibiotic therapy for osteomyelitis is six weeks of intravenous administration. However, this paradigm has been fundamentally challenged by the OVIVA trial published in 2019, which demonstrated that oral step-down after at least one week of intravenous therapy was non-inferior to completion of the full intravenous course for bone and joint infections. This trial has revolutionized practice by establishing that high-bioavailability oral agents can replace prolonged intravenous therapy and the attendant risks of peripherally inserted central catheter lines. The oral regimens employed require agents with reliably high bioavailability, including fluoroquinolones, trimethoprim-sulfamethoxazole, linezolid, rifampin combinations, and doxycycline.

For MSSA osteomyelitis, nafcillin or cefazolin is administered intravenously during the initial phase, with oral step-down to high-dose dicloxacillin or cephalexin at one gram four times daily, with or without adjunctive rifampin. MRSA osteomyelitis is treated with vancomycin or daptomycin intravenously, followed by oral trimethoprim-sulfamethoxazole double-strength twice daily or doxycycline in combination with rifampin, with linezolid serving as an alternative though requiring monitoring for toxicity when used beyond two weeks. Gram-negative osteomyelitis is typically managed with ceftriaxone intravenously followed by an oral fluoroquinolone if susceptibility has been confirmed. Rifampin serves as a critical adjunctive agent for staphylococcal infections involving prosthetic material or hardware but must always be used in combination, as monotherapy leads to the rapid emergence of resistance.

Vertebral Osteomyelitis Specifics

The IDSA 2015 guidelines for native vertebral osteomyelitis recommend a total antibiotic duration of six weeks, delivered either intravenously or via highly bioavailable oral agents. Biopsy-guided therapy is preferred over empiric treatment, with empiric therapy reserved for patients who are septic or hemodynamically unstable and cannot await biopsy results. Epidural abscess complicating vertebral osteomyelitis represents a surgical emergency when neurologic deficits are present or progressing, though medical management may be appropriate for small abscesses without neurologic deficits when the causative organism is known and susceptible. Follow-up MRI should be obtained at four to six weeks only if the clinical response is suboptimal; routine end-of-treatment imaging often shows persistent enhancement that does not indicate treatment failure, and therapy should not be extended solely on the basis of imaging abnormalities.

Septic Arthritis

Epidemiology and Pathophysiology

Septic arthritis occurs at an incidence of 2 to 10 per 100,000 per year, with substantially higher rates among patients with rheumatoid arthritis, prosthetic joints, and immunosuppression. Hematogenous seeding is the most common route of joint infection, followed by direct inoculation through trauma or surgery, and contiguous spread from adjacent osteomyelitis or soft tissue infection. The synovial membrane's lack of a basement membrane and its rich vascular supply render joints inherently susceptible to hematogenous seeding. Once bacteria enter the joint space, cartilage destruction begins within hours, driven by proteolytic enzymes released from both infiltrating neutrophils and the bacteria themselves, which degrade glycosaminoglycans and collagen. Because this cartilage destruction is irreversible, septic arthritis represents both a medical and surgical emergency that demands rapid diagnosis and intervention.

Microbiology

Non-gonococcal septic arthritis is dominated by S. aureus, which accounts for 60 to 70 percent of cases, followed by streptococci at 15 to 20 percent and gram-negative organisms at 10 to 15 percent, with the latter being more common in elderly and immunocompromised patients. Neisseria gonorrhoeae is the most common cause of septic arthritis in sexually active young adults, presenting as disseminated gonococcal infection with a dermatitis-arthritis syndrome. Prosthetic joint infection has a distinct microbiology dominated by coagulase-negative staphylococci at 30 to 40 percent, followed by S. aureus at 20 percent, streptococci, gram-negatives, and Cutibacterium acnes, the latter being particularly common in shoulder prostheses.

Diagnosis

Synovial fluid analysis is the essential diagnostic procedure for suspected septic arthritis. A white blood cell count exceeding 50,000 per cubic millimeter is highly suggestive, though this threshold has a sensitivity of only approximately 60 percent and a specificity of 80 percent, while counts exceeding 100,000 are strongly indicative of infection. The differential should show greater than 90 percent polymorphonuclear cells. Gram stain sensitivity is 50 to 75 percent for staphylococci but less than 25 percent for gram-negatives. Synovial fluid culture is positive in 70 to 90 percent of non-gonococcal cases but is notably insensitive for gonococcal arthritis, with only 10 to 25 percent positivity from joint fluid, necessitating supplementation with nucleic acid amplification testing from urogenital, pharyngeal, and rectal sites. A critical teaching point is that crystal analysis showing gout crystals does not exclude concurrent septic arthritis, as these two conditions can and do coexist, and the presence of crystals should never be used as the sole basis for deferring treatment of a potentially infected joint. Blood cultures are positive in 50 to 70 percent of cases. Imaging includes plain radiography for baseline assessment of joint effusion and soft tissue swelling, ultrasound for effusion detection and aspiration guidance, and MRI for evaluating adjacent osteomyelitis or soft tissue abscess.

<image>A clinical algorithm for the evaluation and management of acute monoarthritis suspected of being septic arthritis. Start with "Acute hot, swollen joint" at the top. First step: "Arthrocentesis (BEFORE antibiotics if possible)." Branch into synovial fluid analysis results showing three columns: "WBC >50,000, >90% PMN" (high probability septic arthritis), "WBC 20,000-50,000" (possible septic arthritis -- consider crystal arthropathy overlap), and "WBC <20,000" (less likely bacterial -- consider crystal, reactive, viral). For high probability path: "Start empiric antibiotics + Orthopedic consultation for drainage." Show antibiotic selection: "Native joint: vancomycin + ceftriaxone (empiric)" with branches for MSSA, MRSA, GC, and gram-negative definitive therapy. Include drainage options: "Repeated needle aspiration vs. Arthroscopic lavage vs. Open arthrotomy" with indications for each. Use a clinical decision tree format with color-coded risk levels.</image>

Treatment of Septic Arthritis

Drainage of the infected joint is mandatory, and the method of drainage should be tailored to the clinical situation. Repeated needle aspiration is appropriate for most accessible joints, particularly the knee, with daily aspiration performed until the effusion resolves and the synovial white blood cell count normalizes. Arthroscopic lavage is preferred for large joints, loculated effusions, and cases where needle aspiration has failed. Open arthrotomy is indicated for the hip joint, which is difficult to aspirate percutaneously, for prosthetic joints, and for cases where arthroscopic drainage has been unsuccessful.

Empiric antibiotic therapy with vancomycin plus ceftriaxone provides coverage against MRSA, streptococci, gram-negatives, and gonococcus, representing a rational initial regimen. Definitive therapy is then guided by culture results: MSSA is treated with nafcillin or cefazolin for three to four weeks; MRSA with vancomycin for three to four weeks; streptococci with penicillin or ceftriaxone for three to four weeks; gram-negatives with ceftriaxone or a fluoroquinolone for three to four weeks; and gonococcal arthritis with ceftriaxone one gram intravenously daily for seven to fourteen days, with concurrent treatment of presumed chlamydia co-infection. The standard treatment duration for native joint septic arthritis is three to four weeks, with shorter courses appropriate for gonococcal infection at approximately two weeks, and oral step-down supported by the OVIVA trial data.

Gonococcal Arthritis (DGI)

Disseminated gonococcal infection presents in two distinct clinical phases. The bacteremic phase is characterized by migratory polyarthralgia, tenosynovitis, and dermatitis with vesiculopustular skin lesions, while the septic joint phase presents as purulent monoarthritis. The diagnostic approach requires nucleic acid amplification testing from all mucosal sites including urogenital, pharyngeal, and rectal specimens, in addition to synovial fluid culture and blood cultures. Treatment consists of ceftriaxone one gram intravenously or intramuscularly daily until clinical improvement occurs, typically within 24 to 48 hours, followed by oral step-down with cefixime or a fluoroquinolone if susceptible to complete a total duration of seven to fourteen days. Concurrent treatment for presumed chlamydia co-infection is always indicated, with doxycycline 100 milligrams twice daily for seven days or azithromycin one gram as a single dose.

Key Clinical Pearls

• Bone biopsy before antibiotics is the ideal diagnostic approach for osteomyelitis -- superficial cultures (especially wound swabs) are unreliable except for S. aureus
• The OVIVA trial supports early oral step-down for bone and joint infections -- prolonged IV therapy via PICC lines carries its own risks (line complications, OPAT burden)
• Septic arthritis is a surgical emergency -- cartilage destruction begins within hours; drainage cannot be delayed
• Crystal arthropathy and septic arthritis can coexist -- finding crystals does not exclude infection
• Probe-to-bone test in diabetic foot ulcers has high PPV when pre-test probability is high (infected, deep ulcer)
• For vertebral osteomyelitis, always image the entire spine (contiguous or skip lesions) and assess for epidural abscess
• Rifampin should never be used as monotherapy -- always combine; its primary role is in prosthetic/hardware-associated staphylococcal infections

References

1. Berbari EF, Kanj SS, Kowalski TJ, et al. 2015 IDSA clinical practice guidelines for the diagnosis and treatment of native vertebral osteomyelitis. Clin Infect Dis. 2015;61(6):e26-e46.
2. Li HK, Rombach I, Zamber R, et al. Oral versus intravenous antibiotics for bone and joint infection (OVIVA). N Engl J Med. 2019;380(5):425-436.
3. Lipsky BA, Berendt AR, Cornia PB, et al. 2012 IDSA clinical practice guideline for the diagnosis and treatment of diabetic foot infections. Clin Infect Dis. 2012;54(12):e132-e173.
4. Sharff KA, Richards EP, Townes JM. Clinical management of septic arthritis. Curr Rheumatol Rep. 2013;15(6):332.
5. Zimmerli W. Vertebral osteomyelitis. N Engl J Med. 2010;362(11):1022-1029.

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