Medical School · Year 2 · Msk Dermatology · includes a quiz and discussion video

Lecture 06: Musculoskeletal Infections

Unit 2.10: Musculoskeletal and Dermatology

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the pathophysiology and classification of osteomyelitis
2. Explain the clinical features and diagnosis of osteomyelitis
3. Describe the management of acute and chronic osteomyelitis
4. Explain the clinical features and diagnosis of septic arthritis
5. Describe the treatment of septic arthritis
6. Explain special considerations in prosthetic joint infections

Lecture Outline

I. Osteomyelitis Overview and Classification

Osteomyelitis represents infection of bone tissue that may be caused by bacterial, fungal, or mycobacterial organisms, with bacteria being the most common etiology in clinical practice. The infection can follow three distinct routes of acquisition: hematogenous spread involves blood-borne seeding of bacteria to bone tissue and is particularly common in children and affects the vertebral column in adults; contiguous spread occurs when infection extends from adjacent soft tissue directly into underlying bone; and direct inoculation introduces pathogens through penetrating trauma, surgical procedures, or open fractures. The clinical course may be acute, subacute, or chronic depending on the duration and host response to infection.

Classification by duration provides important therapeutic and prognostic implications for managing osteomyelitis effectively. Acute osteomyelitis is defined as infection present for less than two weeks and is characterized by active inflammation without significant bone necrosis, offering the best opportunity for cure with antibiotic therapy alone. Subacute osteomyelitis spans from two weeks to three months and may present as a Brodie abscess, which is a walled-off collection within the bone metaphysis with surrounding sclerosis. Chronic osteomyelitis exceeds three months in duration and is characterized by bone necrosis with formation of sequestrum (dead bone), involucrum (new bone formed around infected areas), and cloaca (openings in the involucrum that allow drainage).

The route of infection acquisition significantly influences the typical patient population, anatomic site, and microbiology of osteomyelitis. Hematogenous osteomyelitis predominates in children affecting the metaphysis of long bones where sluggish blood flow facilitates bacterial seeding, while adults with hematogenous osteomyelitis most commonly develop vertebral infections. Contiguous spread osteomyelitis occurs in patients with overlying soft tissue infections, pressure ulcers, or post-surgical wound infections that extend to involve underlying bone. Vascular insufficiency osteomyelitis affects patients with diabetes mellitus or peripheral vascular disease, particularly involving the feet where minor trauma and poor healing create portals of entry for bacteria.

Risk factors for osteomyelitis encompass patient-related, local, and bacteremia-related categories that clinicians must recognize for appropriate prevention and treatment. Patient factors include diabetes mellitus, peripheral vascular disease, immunocompromising conditions such as HIV infection or immunosuppressive medications, and malnutrition. Local factors include recent trauma, surgical procedures, presence of orthopedic hardware or prosthetic material, and skin breakdown or ulceration. Sources of bacteremia that may seed bone include intravenous drug use, infective endocarditis, central venous catheters, and urinary tract infections. Specific clinical scenarios predispose to particular organisms, such as Salmonella osteomyelitis in patients with sickle cell disease and polymicrobial diabetic foot osteomyelitis.

<image>Panel A: Cross-sectional diagram showing the three routes of osteomyelitis infection including hematogenous spread with bacteria traveling through blood vessels to metaphyseal bone, contiguous spread from overlying soft tissue infection, and direct inoculation from penetrating trauma with arrow indicating bacterial entry. Panel B: Comparison illustration of acute versus chronic osteomyelitis showing acute inflammation with hyperemia and edema contrasted with chronic changes including sequestrum (dead bone fragment), involucrum (new bone shell), and cloaca (drainage sinus). Panel C: Anatomical diagram highlighting risk factor sites including diabetic foot with ulcer, vertebral body with adjacent disc involvement, and long bone metaphysis in pediatric patient. Panel D: Flowchart depicting osteomyelitis classification by route (hematogenous, contiguous, direct) and duration (acute less than 2 weeks, subacute 2 weeks to 3 months, chronic greater than 3 months).</image>

II. Osteomyelitis Microbiology

Staphylococcus aureus stands as the most common pathogen causing osteomyelitis across all age groups, anatomic sites, and routes of infection due to its multiple virulence factors that facilitate bone adherence and survival. Coagulase-negative staphylococci, particularly Staphylococcus epidermidis, are commonly associated with prosthetic joint and orthopedic hardware infections where they form protective biofilms on foreign material surfaces. Streptococcal species represent the second most common bacterial cause of osteomyelitis overall and are particularly associated with hematogenous spread and soft tissue extension. Gram-negative organisms including Escherichia coli, Pseudomonas aeruginosa, and other Enterobacteriaceae occur more frequently in elderly patients, immunocompromised hosts, and those with urinary tract sources of bacteremia.

Special populations demonstrate distinct microbiology patterns that guide empiric antibiotic selection in osteomyelitis. Neonates are susceptible to Group B Streptococcus, S. aureus, and Escherichia coli reflecting their immature immune systems and exposure to maternal flora during delivery. Children beyond the neonatal period most commonly develop S. aureus osteomyelitis, though Kingella kingae has emerged as an important pathogen in children under five years, particularly in daycare settings. Patients with sickle cell disease have a unique predisposition to Salmonella osteomyelitis, although S. aureus remains common in this population. Intravenous drug users develop osteomyelitis from S. aureus and Pseudomonas aeruginosa, with unusual sites including the axial skeleton, sternoclavicular joint, and pubic symphysis.

Site-specific microbiology reflects the local flora, blood supply, and typical routes of infection at particular anatomic locations. Vertebral osteomyelitis is caused by S. aureus, gram-negative organisms (particularly with urinary tract sources), and Mycobacterium tuberculosis (Pott disease) in endemic regions. Diabetic foot osteomyelitis is typically polymicrobial involving S. aureus, streptococci, gram-negative organisms, and anaerobes, reflecting the complex wound environment and chronic nature of these infections. Post-traumatic osteomyelitis involves S. aureus and gram-negative organisms introduced at the time of injury or subsequent surgical intervention. Prosthetic joint infections feature coagulase-negative staphylococci, S. aureus, enterococci, and gram-negative organisms, with Cutibacterium acnes being particularly associated with shoulder prosthesis infections.

Bacterial virulence factors contribute significantly to the pathogenesis, persistence, and treatment resistance of osteomyelitis. Biofilm formation represents a critical survival mechanism where bacteria adhere to bone or foreign material surfaces and produce an extracellular polysaccharide matrix that protects against host immune responses and impairs antibiotic penetration by factors of 100-1000 fold. Staphylococcus aureus possesses specific adhesins including fibronectin-binding proteins and collagen-binding proteins that mediate attachment to bone matrix components. Bacterial toxins including alpha-toxin and Panton-Valentine leukocidin cause tissue destruction and contribute to the necrosis characteristic of osteomyelitis. Intracellular survival within osteocytes and other bone cells allows bacteria to evade extracellular host defenses and explains the persistence of infection despite apparent adequate antibiotic therapy.

<image>Panel A: Pie chart showing relative frequency of osteomyelitis pathogens with Staphylococcus aureus as largest segment, followed by coagulase-negative staphylococci, streptococci, gram-negative organisms, and anaerobes, with specific percentages for each organism category. Panel B: Microscopic illustration of biofilm formation showing bacteria adhering to bone surface, producing extracellular matrix, and forming mature biofilm community with antibiotic molecules being blocked from penetrating the protective layer. Panel C: Table-style diagram comparing special population microbiology including neonates (GBS, S. aureus, E. coli), children (S. aureus, Kingella), sickle cell (Salmonella, S. aureus), and IVDU (S. aureus, Pseudomonas). Panel D: Anatomical diagram showing site-specific pathogens with vertebral (S. aureus, gram-negatives, TB), diabetic foot (polymicrobial), prosthetic (CoNS, S. aureus), and post-traumatic (S. aureus, gram-negatives) labeled at respective locations.</image>

III. Osteomyelitis Clinical Features

Acute hematogenous osteomyelitis in children classically presents with localization to the metaphysis of long bones where the unique vascular anatomy with sluggish blood flow in sinusoidal capillaries facilitates bacterial seeding and establishes infection. Children typically present with fever, localized bone pain, and refusal to bear weight or use the affected extremity, with symptom onset often following minor trauma that draws attention to the affected area. Physical examination reveals point tenderness over the metaphysis, warmth, soft tissue swelling, and decreased range of motion of adjacent joints. Blood cultures are positive in approximately 50% of cases, while white blood cell count may be normal or elevated and inflammatory markers (ESR, CRP) are typically elevated with CRP responding more rapidly to treatment than ESR.

Vertebral osteomyelitis presents a distinct clinical syndrome characterized by insidious onset of back pain that may evolve over weeks to months before diagnosis. The lumbar spine is most commonly affected followed by the thoracic and cervical regions, with pain typically worse at night, with movement, and with percussion over the affected vertebrae. Fever is present in only about half of patients, contributing to diagnostic delay that averages six to twelve weeks in many series. Infection characteristically involves the disc space (discitis) and spreads to involve adjacent vertebral bodies, and complications include epidural abscess formation with potential for spinal cord compression requiring emergent surgical intervention. Common sources of bacteremia include urinary tract infections, dental procedures, and intravenous catheter infections.

Contiguous osteomyelitis and diabetic foot osteomyelitis develop from overlying soft tissue infections or chronic wounds that extend to involve underlying bone. Patients with diabetes mellitus are particularly susceptible due to peripheral neuropathy that allows unrecognized repetitive trauma, peripheral vascular disease that impairs wound healing and antibiotic delivery, and hyperglycemia that impairs neutrophil function. The probe-to-bone test, in which a sterile metal probe can be passed through a wound to contact underlying bone, is highly suggestive of osteomyelitis with positive predictive values exceeding 90% when the pre-test probability is high. Clinical signs include chronic drainage, failure of wounds to heal despite appropriate local care, exposed bone, and surrounding soft tissue infection.

Chronic osteomyelitis follows an indolent course with periodic exacerbations characterized by sinus tract drainage, localized pain, and occasionally systemic symptoms. The hallmark pathologic features include sequestrum (fragments of necrotic bone that serve as a nidus for persistent infection), involucrum (new periosteal bone that forms a shell around the infected area), and cloaca (openings in the involucrum through which purulent material drains). Sinus tracts connecting the infected bone to the skin surface allow continuous drainage and serve as a clinical marker of chronic disease. The persistence of infection despite antibiotic therapy reflects the presence of biofilm, sequestered bacteria within necrotic bone that antibiotics cannot penetrate, and the relative avascularity of chronically infected bone.

<image>Panel A: Pediatric long bone diagram showing acute hematogenous osteomyelitis localized to metaphysis with inflammation, bone destruction, and periosteal elevation, with inset showing blood vessels and bacterial seeding in the metaphyseal sinusoids. Panel B: Sagittal view of spine showing vertebral osteomyelitis with disc space involvement, adjacent vertebral body infection, and epidural abscess causing spinal cord compression with clinical signs of back pain and neurologic deficits. Panel C: Diabetic foot illustration demonstrating contiguous osteomyelitis with overlying ulcer, exposed metatarsal head, positive probe-to-bone test with sterile probe reaching bone, and surrounding cellulitis. Panel D: Cross-section of chronic osteomyelitis showing sequestrum (dead bone fragment), involucrum (surrounding new bone), cloaca (drainage opening), and sinus tract extending to skin surface with purulent discharge.</image>

IV. Osteomyelitis Diagnosis

Laboratory studies provide supportive but non-specific evidence for osteomyelitis diagnosis and are most valuable for monitoring treatment response. White blood cell count may be normal or elevated and is neither sensitive nor specific for osteomyelitis diagnosis. Erythrocyte sedimentation rate (ESR) is elevated in most cases and is particularly useful for monitoring treatment response, though it rises and falls slowly over weeks. C-reactive protein (CRP) is also elevated and responds more rapidly to changes in infection status, typically beginning to decline within 24-48 hours of effective treatment initiation. Blood cultures should be obtained before antibiotic initiation and are positive in approximately 50% of hematogenous osteomyelitis cases, potentially providing microbiologic diagnosis without the need for bone biopsy.

Imaging modalities play crucial roles in osteomyelitis diagnosis, with the choice of study depending on clinical suspicion, anatomic location, and timing of presentation. Plain radiographs are the initial imaging study but are insensitive early in the disease course, with changes including soft tissue swelling visible within days but bone changes delayed 10-14 days for periosteal elevation and 2-3 weeks for lytic lesions. Magnetic resonance imaging (MRI) is the most sensitive imaging modality, demonstrating bone marrow edema as increased T2 signal and decreased T1 signal within days of infection onset. Computed tomography (CT) provides superior bone detail useful for identifying sequestrum and planning surgical debridement but is less sensitive than MRI for early infection. Nuclear medicine studies including bone scintigraphy and labeled white blood cell scans are useful when MRI is contraindicated or in the setting of metallic hardware artifact.

Radiographic findings evolve predictably over the course of osteomyelitis and provide clues to disease duration and chronicity. Soft tissue swelling represents the earliest radiographic finding, visible within the first few days of infection before any bony changes occur. Periosteal elevation appears at 7-10 days as infection spreads from the metaphysis through the cortex to elevate the periosteum, stimulating new bone formation. Lytic lesions with areas of bone destruction become apparent at 10-21 days when sufficient bone mineral has been lost to be detectable on plain radiographs. Chronic osteomyelitis demonstrates sclerosis, sequestra (dense fragments of dead bone), and involucrum (new bone formation surrounding the infected area).

Tissue diagnosis through bone biopsy or intraoperative cultures represents the gold standard for establishing the microbiologic etiology of osteomyelitis. Image-guided percutaneous bone biopsy using CT or fluoroscopic guidance allows sampling of deep infections without open surgery and should be performed before antibiotic initiation when possible. Surgical specimens obtained during debridement provide optimal tissue for both culture and histopathologic examination demonstrating acute or chronic osteomyelitis. Superficial wound swabs and sinus tract cultures are unreliable and frequently grow colonizing organisms rather than the true pathogen causing bone infection. If the clinical situation allows, antibiotics should be held for at least two weeks before biopsy to maximize culture yield, though this must be balanced against the risk of untreated infection.

<image>Panel A: Comparison of plain radiograph findings at different stages showing normal bone at presentation, periosteal elevation at 10 days, and lytic destruction at 3 weeks with labeled timeline and corresponding clinical stages. Panel B: MRI image demonstrating vertebral osteomyelitis with T2 hyperintensity in adjacent vertebral bodies, disc space involvement, and contrast-enhancing epidural collection with labeled anatomic structures. Panel C: CT scan showing chronic osteomyelitis features including cortical destruction, dense sequestrum surrounded by lucency, involucrum with periosteal new bone formation, and cloaca with soft tissue extension. Panel D: Flowchart depicting diagnostic algorithm starting with clinical suspicion, proceeding through laboratory studies, imaging selection (XR to MRI to nuclear medicine), and tissue diagnosis with bone biopsy providing cultures and histopathology.</image>

V. Osteomyelitis Treatment

General treatment principles for osteomyelitis center on pathogen identification through cultures before antibiotic initiation, selection of antibiotics with appropriate bone penetration and activity against identified organisms, and surgical intervention when indicated for chronic infection, necrotic bone, or abscess formation. The duration of antibiotic therapy is prolonged, typically a minimum of four to six weeks of intravenous or highly bioavailable oral antibiotics, reflecting the slow turnover of bone tissue and need to eradicate bacteria from the bone matrix. Source control through surgical debridement of necrotic bone is essential for chronic osteomyelitis where antibiotics alone cannot penetrate devascularized tissue and biofilm-protected bacteria. Treatment success is measured by clinical improvement, normalization of inflammatory markers, and radiographic stability or improvement.

Antibiotic selection should be guided by culture results when available, with empiric therapy based on the most likely pathogens given the clinical scenario and patient population. Methicillin-sensitive Staphylococcus aureus (MSSA) is optimally treated with nafcillin, oxacillin, or cefazolin intravenously, with oral step-down options including dicloxacillin and cephalexin for highly bioavailable coverage. Methicillin-resistant Staphylococcus aureus (MRSA) requires vancomycin as first-line therapy, with daptomycin as an alternative, and oral options including trimethoprim-sulfamethoxazole, doxycycline, or linezolid for step-down therapy. Gram-negative organisms are treated with ceftriaxone, fluoroquinolones, or other agents based on susceptibility testing, while polymicrobial diabetic foot infections require broad-spectrum coverage with subsequent narrowing based on culture results.

Duration of therapy and route of administration depend on the specific clinical scenario, adequacy of surgical debridement, and patient factors affecting oral bioavailability. Acute hematogenous osteomyelitis without surgical debridement typically requires four to six weeks of intravenous antibiotic therapy. Following complete surgical debridement of chronic osteomyelitis, antibiotic duration of four to six weeks is standard, though some experts advocate for longer courses in complex cases. Vertebral osteomyelitis is generally treated for six weeks, with longer courses if complicated by epidural abscess or if surgical debridement is incomplete. Recent evidence supports oral step-down therapy with highly bioavailable antibiotics such as fluoroquinolones combined with rifampin for staphylococcal infections, achieving outcomes comparable to intravenous therapy in selected patients.

Surgical management is integral to osteomyelitis treatment, particularly for chronic infections, and encompasses debridement, hardware management, and dead space management strategies. Debridement involves aggressive removal of all necrotic bone (sequestrum) and infected soft tissue until bleeding, viable bone margins are reached. When orthopedic hardware is present, the decision to remove or retain the implant depends on the stability of fixation, duration of infection, and whether the bone has healed. Dead space management following debridement may employ antibiotic-impregnated polymethylmethacrylate beads, antibiotic-loaded calcium sulfate, or vascularized tissue flaps to fill defects and deliver local antibiotics. Diabetic foot osteomyelitis may require limited amputation when vascular supply is inadequate for healing or when infection threatens life or limb.

<image>Panel A: Antibiotic selection flowchart showing empiric therapy by clinical scenario, culture-directed therapy by organism (MSSA, MRSA, gram-negative), and oral step-down options with specific drug names and dosing considerations. Panel B: Timeline diagram showing treatment duration by scenario including acute osteomyelitis (4-6 weeks), post-debridement chronic osteomyelitis (4-6 weeks), and vertebral osteomyelitis (6 weeks) with inflammatory marker monitoring curves. Panel C: Surgical debridement illustration showing removal of sequestrum and necrotic tissue, bleeding bone margins indicating viable tissue, and dead space management with antibiotic beads or cement spacer. Panel D: Decision algorithm for hardware-associated osteomyelitis showing pathways for hardware removal versus retention with suppressive antibiotics based on bone healing status, infection duration, and implant stability.</image>

VI. Septic Arthritis Overview

Septic arthritis constitutes a medical emergency requiring prompt diagnosis and treatment to prevent irreversible joint destruction from bacterial proteases and the inflammatory response. Bacteria reach the joint space through hematogenous seeding (most common), direct inoculation from trauma or procedures, or contiguous spread from adjacent osteomyelitis or soft tissue infection. The joint's poor ability to clear infection relates to the avascular nature of articular cartilage, which depends on synovial fluid diffusion for nutrient delivery and cannot mount an effective local immune response. Without treatment, bacterial enzymes and host inflammatory mediators destroy articular cartilage within days, leading to permanent joint dysfunction, ankylosis, or the need for joint replacement.

Risk factors for septic arthritis encompass joint-related, systemic, and procedural categories that clinicians must recognize when evaluating patients with acute monoarthritis. Pre-existing joint disease, particularly rheumatoid arthritis, creates damaged synovium that is more susceptible to bacterial seeding during bacteremia and may also mask the early symptoms of infection due to chronic joint pain. Prosthetic joints represent foreign material that bacteria can colonize and form biofilms upon, creating a unique management challenge discussed later in this lecture. Systemic risk factors include diabetes mellitus, immunocompromising conditions (HIV, immunosuppressive medications), chronic kidney disease, and intravenous drug use. Procedural risks include intra-articular injections, arthroscopy, and joint replacement surgery, with infection rates for joint injections approximately 1 in 10,000 to 1 in 50,000.

Staphylococcus aureus dominates the microbiology of septic arthritis, causing 50-70% of non-gonococcal cases across all age groups and clinical settings. Streptococcal species are the second most common cause of septic arthritis, with Group A and Group B streptococci being particularly important pathogens. Gram-negative organisms cause approximately 10-15% of cases, with increased frequency in elderly patients, immunocompromised hosts, and those with urinary tract sources of bacteremia. Neisseria gonorrhoeae is an important cause of septic arthritis in young, sexually active adults and presents with distinctive clinical features discussed separately. Polymicrobial infections occur in the setting of penetrating trauma, and certain pathogens such as Salmonella are associated with specific conditions including sickle cell disease.

The knee is the most commonly affected joint in septic arthritis, involved in approximately 50% of cases due to its large size and superficial location. The hip is the second most commonly affected joint and presents particular diagnostic challenges due to its deep location making effusion difficult to detect on physical examination; hip septic arthritis in children requires emergent surgical drainage. Other commonly affected joints include the shoulder, ankle, wrist, and small joints of the hands and feet. Polyarticular septic arthritis occurs in 10-20% of cases and is associated with rheumatoid arthritis, overwhelming sepsis, and certain organisms including gonococcus and Staphylococcus aureus. Intravenous drug users may develop septic arthritis in unusual locations including the sternoclavicular, sacroiliac, and pubic symphysis joints.

<image>Panel A: Joint anatomy diagram showing routes of bacterial entry including hematogenous spread through synovial vessels, direct inoculation through needle or trauma, and contiguous spread from adjacent bone with arrows indicating bacterial pathways. Panel B: Illustration of cartilage destruction progression showing normal cartilage at baseline, bacterial colonization at day 1, inflammatory infiltrate at day 3, and severe cartilage erosion at day 7 demonstrating the urgency of treatment. Panel C: Pie chart showing septic arthritis microbiology with S. aureus (50-70%), streptococci (20%), gram-negatives (10-15%), N. gonorrhoeae (variable by population), and other organisms with percentages labeled. Panel D: Human figure showing joint distribution with frequency percentages including knee (50%), hip (20%), shoulder (10%), ankle (8%), wrist (5%), and small joints (7%) with each joint highlighted.</image>

VII. Septic Arthritis Clinical Features and Diagnosis

The clinical presentation of septic arthritis typically involves an acutely painful, swollen joint with severe limitation of motion, though presentations may be subtle in immunocompromised patients or those with pre-existing joint disease. Approximately 90% of cases present as monoarticular arthritis, with polyarticular involvement suggesting specific pathogens (gonococcus, S. aureus) or underlying conditions (rheumatoid arthritis). Pain is characteristically severe, constant, and exacerbated by any joint motion regardless of direction, distinguishing it from the pain of crystal arthropathy that may be more tolerable. Fever is present in 50-70% of patients but is not universally present, particularly in elderly or immunocompromised hosts. Hip septic arthritis in children typically presents with the hip held in flexion and external rotation to maximize joint capsule volume and minimize pain.

Physical examination reveals findings consistent with joint inflammation and effusion that must prompt arthrocentesis for definitive diagnosis. The affected joint demonstrates warmth, swelling with visible effusion in superficial joints, and diffuse tenderness over the joint capsule. Erythema of overlying skin may be present but is variable and is more prominent in superficial joints such as the knee, ankle, and small joints of the hands. Range of motion is severely limited, with pain occurring throughout the arc of motion in all directions, helping distinguish septic arthritis from mechanical conditions. Examination should also assess for findings suggesting the source of bacteremia, including heart murmurs suggesting endocarditis, injection sites in drug users, and skin or soft tissue infections.

Synovial fluid analysis provides the critical diagnostic information for septic arthritis and should be performed emergently when the diagnosis is suspected. Fluid appearance is typically purulent, cloudy, or opaque, though less purulent fluid does not exclude infection, particularly early in the disease course or with certain organisms. White blood cell count in septic arthritis typically exceeds 50,000 cells per microliter with greater than 90% polymorphonuclear predominance, though lower counts occur early in infection or in immunocompromised patients. Gram stain is positive in 50-75% of cases and provides immediate information to guide empiric therapy. Synovial fluid culture represents the gold standard for diagnosis and should be inoculated into blood culture bottles to enhance sensitivity; glucose is characteristically low though this finding is neither sensitive nor specific.

Additional diagnostic studies complement synovial fluid analysis in the evaluation of suspected septic arthritis. Blood cultures should be obtained in all patients and are positive in approximately 50% of cases, potentially identifying the organism when synovial fluid cultures are negative. Complete blood count typically shows leukocytosis, and inflammatory markers (ESR, CRP) are elevated though nonspecific. Plain radiographs establish a baseline, may reveal underlying joint disease, and can exclude osteomyelitis, though findings specific to septic arthritis (joint space widening from effusion) develop late. Ultrasound is highly sensitive for detecting joint effusion and can guide arthrocentesis, particularly for deep joints such as the hip. MRI is reserved for cases where osteomyelitis is suspected, the diagnosis is uncertain, or surgical planning requires detailed anatomic information.

<image>Panel A: Clinical photograph representation showing acutely swollen knee with visible effusion, overlying erythema, and position of comfort in slight flexion, with inset showing pediatric hip held in flexion and external rotation. Panel B: Synovial fluid analysis comparison showing test tubes with normal fluid (clear, yellow, low WBC), septic fluid (cloudy, purulent, WBC greater than 50,000), and inflammatory fluid (cloudy, WBC 2,000-50,000) with characteristic values labeled. Panel C: Microscopic view of Gram stain showing gram-positive cocci in clusters (S. aureus), gram-positive cocci in chains (streptococci), and gram-negative diplococci (N. gonorrhoeae) with organism identifications. Panel D: Ultrasound image showing hip joint with effusion, demonstrating needle trajectory for guided arthrocentesis, femoral head, acetabulum, and anterior joint capsule distension.</image>

VIII. Septic Arthritis Treatment

Treatment principles for septic arthritis combine joint drainage to remove purulent material and reduce intra-articular pressure with systemic antibiotics to eradicate the causative organism. Joint drainage is essential and must be performed urgently, as studies demonstrate that delay in drainage correlates with worse functional outcomes and increased likelihood of joint destruction. Empiric antibiotics should be initiated immediately after arthrocentesis is performed, without waiting for culture results, given the rapid progression of cartilage destruction. Antibiotic therapy is subsequently narrowed based on culture results and continued for a total of two to four weeks depending on the organism and clinical response.

Empiric antibiotic selection should provide coverage for the most likely pathogens based on patient risk factors, Gram stain results, and clinical presentation. Native joint septic arthritis in immunocompetent adults without specific risk factors should receive vancomycin (for MRSA coverage) plus ceftriaxone (for gram-negative and streptococcal coverage) pending culture results. When Gram stain reveals gram-positive cocci, vancomycin monotherapy provides coverage for both MSSA and MRSA pending susceptibility testing. Gram-negative rods on Gram stain suggest treatment with ceftriaxone or a fluoroquinolone based on local resistance patterns. Suspected gonococcal arthritis should be treated with ceftriaxone, and prosthetic joint infection requires broader coverage with vancomycin plus an antipseudomonal beta-lactam.

Joint drainage may be accomplished through repeated needle aspiration or surgical intervention, with the choice depending on the joint involved, response to initial drainage, and presence of loculations. Daily aspiration with drainage until the effusion is minimal or fluid becomes sterile is effective for most peripheral joints accessible to needle drainage, including the knee, ankle, shoulder, wrist, and small joints. Arthroscopic or open surgical drainage is preferred for the hip joint (particularly in children where emergent surgical drainage is standard of care), the shoulder, when repeated aspirations fail to control infection, and when loculations prevent adequate needle drainage. Urgent surgical drainage is mandatory for prosthetic joint infections, which cannot be cured with needle aspiration due to biofilm on the prosthesis.

Duration of antibiotic therapy depends on the organism, clinical response, and whether adequate source control has been achieved. Gram-positive infections typically require three to four weeks of antibiotic therapy, with transition from intravenous to oral therapy after clinical improvement and conversion of repeat synovial fluid cultures to negative. Gram-negative septic arthritis similarly requires three to four weeks of therapy with agents selected based on susceptibility testing. Gonococcal arthritis responds rapidly to ceftriaxone and requires only seven to fourteen days of total therapy, with oral step-down to cefixime if the organism is susceptible. Concurrent treatment for Chlamydia trachomatis with azithromycin is recommended for gonococcal infections, and sexual partners must be notified and treated.

<image>Panel A: Treatment algorithm flowchart showing septic arthritis suspected leading to arthrocentesis, then empiric antibiotics (vancomycin plus ceftriaxone), followed by culture-directed narrowing, and drainage method selection based on joint and response. Panel B: Comparison diagram of drainage methods showing serial needle aspiration technique with daily aspirations, arthroscopic lavage with camera and instruments, and open arthrotomy with surgical exposure and debridement for each approach. Panel C: Antibiotic selection table showing empiric therapy by clinical scenario (native joint, gram-positive, gram-negative, gonococcal, prosthetic) with specific drug recommendations and dosing for each category. Panel D: Timeline showing treatment duration by organism including gram-positive (3-4 weeks), gram-negative (3-4 weeks), and gonococcal (7-14 days) with markers for IV-to-oral transition and clinical milestones for switching.</image>

IX. Gonococcal Arthritis

Gonococcal arthritis represents a unique form of septic arthritis occurring in young, sexually active individuals as a manifestation of disseminated gonococcal infection (DGI). The typical patient is an adolescent or young adult presenting with the classic triad of dermatitis, tenosynovitis, and arthritis following genitourinary, rectal, or pharyngeal gonococcal infection. Risk factors include female sex (particularly during menstruation or pregnancy when cervical mucus changes facilitate bacterial invasion), complement deficiencies that predispose to Neisseria infections, and multiple sexual partners or unprotected sexual activity. DGI occurs in approximately 1-3% of patients with mucosal gonococcal infection and represents hematogenous dissemination from the primary site.

The clinical features of disseminated gonococcal infection include characteristic skin lesions, tenosynovitis, and arthritis that may progress through distinct phases. Skin lesions begin as small erythematous macules that evolve into pustular or hemorrhagic papules, typically few in number (less than 30) and distributed peripherally on the extremities. Tenosynovitis causes pain, swelling, and erythema along tendon sheaths, most commonly affecting the wrists, fingers, ankles, and toes, and may be the predominant finding early in the course. The arthritis may be migratory initially, affecting multiple joints sequentially before localizing to one or a few joints, or may present as established septic arthritis. Some patients present with the "arthritis-dermatitis syndrome" with prominent tenosynovitis and skin findings, while others have purulent monoarticular arthritis without skin lesions.

Diagnosis of gonococcal arthritis requires a high index of suspicion and appropriate specimen collection from multiple sites, as joint cultures are often negative. Synovial fluid analysis reveals elevated white blood cell count, typically in the range of 30,000-50,000 cells per microliter, which may be lower than classic bacterial septic arthritis. Gram stain of synovial fluid is often negative due to the low organism burden in gonococcal arthritis, making it less helpful than in other forms of septic arthritis. Synovial fluid culture has low sensitivity (25-50%) and requires special handling with chocolate agar or Thayer-Martin medium. Nucleic acid amplification testing (NAAT) of synovial fluid, urine, and genital/rectal/pharyngeal swabs substantially improves diagnostic yield and should be performed in all suspected cases.

Treatment of gonococcal arthritis achieves excellent outcomes with appropriate antibiotic therapy and does not routinely require surgical drainage. Initial therapy consists of ceftriaxone 1 gram intravenously or intramuscularly daily, which rapidly sterilizes the joint and prevents complications. Improvement should occur within 24-48 hours of initiating therapy, and failure to improve should prompt reconsideration of the diagnosis. Oral step-down to cefixime 400 mg twice daily may be considered after clinical improvement if susceptibility testing confirms sensitivity. Total treatment duration is seven to fourteen days, substantially shorter than non-gonococcal septic arthritis. Concurrent treatment with azithromycin 1 gram as a single dose addresses possible Chlamydia trachomatis coinfection, and sexual partners from the preceding 60 days must be notified and treated.

<image>Panel A: Patient silhouette showing distribution of DGI findings including skin lesions on distal extremities (pustular papules with hemorrhagic centers), tenosynovitis at wrists and ankles (shown with swelling along tendon sheaths), and knee arthritis with effusion. Panel B: Close-up illustration of characteristic DGI skin lesions showing progression from erythematous macule to pustule to hemorrhagic papule with necrotic center, typically on acral surfaces. Panel C: Diagnostic approach flowchart showing clinical suspicion leading to synovial fluid analysis, NAAT from joint/urine/genital/pharyngeal sites, culture on appropriate media, and blood cultures with sensitivity of each modality noted. Panel D: Treatment timeline showing ceftriaxone initiation, expected improvement at 24-48 hours, oral step-down criteria, total duration 7-14 days, and partner notification requirements with azithromycin co-treatment.</image>

X. Prosthetic Joint Infection

Prosthetic joint infection (PJI) represents a serious complication of joint replacement surgery, occurring in 1-2% of primary arthroplasties and at higher rates in revision surgery, with devastating consequences for patient function and quality of life. Classification of PJI by timing provides crucial information about pathogenesis and treatment approach. Early infection occurring within three months of surgery results from perioperative inoculation during implantation, involving organisms introduced at the time of surgery. Delayed infection occurring between three and twenty-four months typically involves low-virulence organisms such as coagulase-negative staphylococci that were inoculated at surgery but manifest later. Late infection occurring beyond twenty-four months results from hematogenous seeding of the prosthesis during bacteremia from another source.

The microbiology of prosthetic joint infection reflects the biofilm-forming capabilities of causative organisms and their ability to adhere to prosthetic materials. Coagulase-negative staphylococci, particularly Staphylococcus epidermidis, are the most common pathogens due to their ubiquitous presence on skin and exceptional biofilm-forming ability. Staphylococcus aureus causes more virulent infections with more prominent clinical findings and may occur through both perioperative inoculation and hematogenous seeding. Streptococcal species and enterococci are common and often associated with hematogenous seeding from dental or genitourinary sources. Gram-negative organisms are associated with urinary tract and gastrointestinal sources of bacteremia. Cutibacterium acnes (formerly Propionibacterium acnes) is particularly associated with shoulder prosthesis infection due to its abundance in the sebaceous glands of the shoulder region.

Diagnosis of prosthetic joint infection relies on clinical presentation, laboratory studies, imaging, and synovial fluid analysis interpreted through established criteria. Clinical presentation includes joint pain (the most common symptom), swelling, wound drainage, erythema, and occasionally systemic symptoms such as fever. Laboratory studies reveal elevated ESR and CRP, which are useful screening tests with the combination having sensitivity exceeding 90% for chronic PJI. Synovial fluid analysis using lower thresholds than native joint septic arthritis (greater than 1,100 cells per microliter or greater than 64% polymorphonuclear cells for knee PJI) provides critical diagnostic information. The Musculoskeletal Infection Society (MSIS) criteria provide a standardized approach to PJI diagnosis, combining major criteria (sinus tract communicating with joint, or same organism isolated from two separate cultures) with minor criteria (elevated inflammatory markers, synovial fluid findings, positive histology).

Treatment strategies for prosthetic joint infection vary based on infection timing, host factors, organism virulence, and implant stability. Debridement, antibiotics, and implant retention (DAIR) is appropriate for early or acute hematogenous infections with stable implants, short symptom duration, and susceptible organisms, combined with exchange of modular components (polyethylene liner, femoral head). One-stage exchange involves removal of the infected prosthesis, thorough debridement, and immediate reimplantation of a new prosthesis in a single surgery, appropriate for healthy hosts with low-virulence organisms and minimal bone loss. Two-stage exchange represents the most reliable treatment for chronic PJI, involving prosthesis removal, placement of an antibiotic-impregnated cement spacer, a course of antibiotics, and reimplantation after infection control is confirmed. Chronic suppressive antibiotic therapy without prosthesis removal is reserved for patients who cannot tolerate surgery or refuse further intervention.

<image>Panel A: Timeline diagram showing PJI classification with early (less than 3 months, perioperative inoculation), delayed (3-24 months, low-virulence organisms), and late (greater than 24 months, hematogenous) infections mapped against time since surgery with pathogenic mechanisms for each. Panel B: Illustration of biofilm on prosthetic surface showing bacteria adhering to metal/polyethylene, mature biofilm matrix protecting organisms from antibiotics and immune cells, and persister cells within the biofilm explaining treatment resistance. Panel C: MSIS criteria diagnostic diagram showing major criteria (sinus tract, two positive cultures with same organism) and minor criteria (elevated ESR/CRP, synovial WBC greater than 1,100, positive histology, positive single culture) with scoring for definite versus probable PJI. Panel D: Treatment algorithm showing pathways for DAIR (early, stable implant), one-stage exchange (healthy host, low-virulence organism), two-stage exchange (chronic infection), and suppression (non-surgical candidate) with specific indications for each approach.</image>

Summary

• Osteomyelitis is a bone infection occurring via hematogenous, contiguous, or direct inoculation routes with acute, subacute, or chronic duration
• Staphylococcus aureus is the most common osteomyelitis pathogen across all patient populations and clinical scenarios
• MRI is the most sensitive imaging modality for osteomyelitis; bone biopsy provides definitive microbiologic diagnosis
• Osteomyelitis treatment requires targeted antibiotics for 4-6 weeks minimum, with surgical debridement for chronic infection
• Septic arthritis is a joint infection constituting a medical emergency requiring urgent drainage to prevent cartilage destruction
• Synovial fluid with WBC greater than 50,000 per microliter and greater than 90% PMN strongly suggests septic arthritis
• Septic arthritis treatment combines immediate joint drainage with empiric antibiotics (vancomycin plus ceftriaxone)
• Gonococcal arthritis presents with the DGI triad of dermatitis, tenosynovitis, and arthritis; treatment is ceftriaxone for 7-14 days
• Prosthetic joint infection is classified by timing as early, delayed, or late, with CoNS being the most common pathogen
• Two-stage exchange arthroplasty provides the most reliable cure for chronic prosthetic joint infection

Key Terms

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