AAOS Comprehensive Orthopaedic Review

Section 1 - Basic Science

Chapter 11. Musculoskeletal Infections and Microbiology

I. Introduction

A. Infections may involve bone (osteomyelitis), joint spaces (septic arthritis), soft tissues (muscle, fascia, subcutaneous tissues, skin), or the disks and spinal column, and may present with either an acute or an indolent/chronic clinical course.


B. Infections may occur de novo or at a previous surgical site.


C. Infectious agents include bacterial, viral, fungal, or other microorganisms.

II. Clinical Presentation

A. History and physical examination


1. The most common symptom is pain localized to the site of infection; it is rare for patients to not report pain as a presenting symptom.


2. History of previous trauma in the region of the symptoms: a penetrating injury, break in the skin, or laceration.


3. Intermittent fevers, chills, sweats (night sweats with chronic infections), general malaise, reduced appetite, and weight loss (chronic infection).


4. Localized soft-tissue swelling and/or warmth, redness or skin discoloration, or fluctuance in the vicinity of the pain.


5. Symptoms of untreated infection may progress rapidly, leading to hypotension, shock, coma, or death.


B. Septic arthritis


1. In addition to the general symptoms described earlier, recent loss of and unwillingness to perform range of joint motion is suggestive of acute septic arthritis.


2. Neisseria gonorrhea is historically a common causative organism in young males with isolated monoarticular septic arthritis of the knee.


a. Migratory polyarthritis, rash, and tenosynovitis of the dorsal wrist and hand may occur.


b. The knee joint is most commonly involved.


3. Staphylococcus aureus is the second most common pathogen causing adult septic arthritis.


4. In children, involvement of the hip produces a flexed, abducted, externally rotated position to accommodate the increased joint volume.


C. Adult osteomyelitis


1. Chronic draining sinuses are commonly present in chronic osteomyelitis. A history of acute purulent drainage from the skin, preceded by fever, pain, localized swelling, tenderness, and systemic symptoms is suggestive of a draining sinus associated with osteomyelitis. Such sinuses are complicated by malignant squamous cell carcinoma transformation in 1% of cases.


2. S aureus is the most common causative organism, although other microorganisms can be involved (

Table 1).


3. The ability of the host to respond to any type of clinical infection has been classified by Cierny (

Table 2).


4. The tibia is the most common site.


5. Hematogenous osteomyelitis may occur in IV drug users.


6. Two types of pediatric osteomyelitis have been identified: acute hematogenous (AHO) and subacute hematogenous (SHO).


7. Chronic pediatric recurrent multifocal osteomyelitis is usually bilateral and symmetric. It is often associated with intermittent symptoms and a pustular rash on the palms.


D. Necrotizing infections


1. Necrotizing fasciitis



Necrotizing fasciitis is any necrotizing soft-tissue infection spreading along fascial planes, with or without overlying cellulitis. The edema


[Table 1. Microoganisms Isolated From Patients With Bacterial Osteomyelitis]

[Table 2. Cierny Classification of Host Response to Infection]


and induration extend beyond the area of erythema.


Either of the well-defined bacterial groups responsible for necrotizing fasciitis infections may be introduced into the soft tissues in contaminated environments, including IV drug use, hypodermic therapeutic injections, insect bites, or skin abrasions.


Clinical signs

i. Severe pain, hyperpyrexia, and chills


ii. The infection typically begins as a localized abscess, particularly among at-risk groups such as IV drug users, patients with diabetes mellitus, alcoholics, post-abdominal surgery patients, obese patients, or patients with either perineal infections or peripheral vascular disease.


iii. The initial findings are localized pain and minimal swelling, often with no visible trauma or discoloration of the skin.


iv. Dermal induration and erythema eventually become evident; blistering of the epidermis is a late finding. Necrotizing fasciitis may be caused by a single organism (Streptococci pyogenes orStreptococci vibrio) or a combination of organisms. Anaerobic or micro-aerophilic streptococci are believed to be the usual cause, but these microorganisms are difficult to culture.


v. The diagnosis may be missed initially in a patient presenting with mild symptoms such as pain, edema, tachycardia, and fear because signs such as skin bullae and subcutaneous gas have not yet developed.


vi. The underlying tissue destruction initially manifests itself in the skin with painful ischemic patches and overlying blisters that consolidate over the course of 3 to 5 days to form cutaneous gangrene. The tissue becomes less painful due to small vessel thrombosis, neurotoxin production, and necrosis. The anesthesia may precede obvious skin necrosis and can aid in the early differential diagnosis of necrotizing fasciitis from simple cellulitis.


vii. High elevations of body temperature can help differentiate systemic necrotizing fasciitis from anaerobic cellulitis and clostridial myonecrosis, which produce modest, if any, changes in temperature.


2. Clostridial myonecrosis (gas gangrene)


a. Triad of symptoms strongly suggests Clostridial myonecrosis:


i. Progressively severe pain out of proportion to obvious injury


ii. Tachycardia not explained by fever


iii. Crepitus


b. The buttocks, thighs, and perineum are common sites of infection.


c. Clostridium perfringens and other clostridial species are common pathogens, but the condition can also develop from gram-negative and gram-positive (streptococcal) infections.


d. Clinical presentation usually includes progressive pain; edema (distant from wound); foul-smelling, serosanguinous discharge; and feeling of impending doom. Other findings include ecchymosis, necrosis, edematous skin, dark red serous fluid, and numerous gas-filled vesicles and bullae.


e. Intense pain out of proportion to the wound is characteristic; within hours, signs of systemic toxicity appear, including confusion, tachycardia, and diaphoresis.


f. This infection may be associated with bowel cancer.


g. Radiographs typically show widespread gas in tissues.


h. Treatment is high-dose penicillin (and aminoglycoside and cephalosporin), hyperbaric oxygen (inhibits toxins), and surgical irrigation and debridement.

III. Diagnostic Evaluation

A. Radiographic findings


1. Swelling and loss of tissue planes are evident at an early stage of osteomyelitis.


2. Bone changes will not be present until 1 to 2 weeks of established infection.


3. Bone loss of 30% to 40% is required before the classic signs of osteomyelitis are seen: bone resorption, destruction, periosteal elevation.


4. New periosteal bone forms parallel to the cortex, tapering to the cortex farther from the nidus of infection. The elevated periosteum lays down new bone initially (involucrum) around the shell of old bone, and the dead medullary or cortical bone becomes a sequestrum by occlusion of the nutrient vessels caused by infection.


B. Nuclear medicine imaging


1. Three-phase technetium Tc 99m bone scan


a. Bone infection is strongly suspected when the radioactive material localizes in the bone after 3 hours.


b. Measures osteoblastic activity. Sensitive for detecting AHO and septic arthritis.


c. False-negative "cold" bone scans sometimes occur in neonates. False-positive results may be seen with other disease processes, including fractures, tumors, and sickle cell disease.


d. Technetium scans may be positive for up to 2 years after total joint arthroplasty (TJA).


2. Leukocyte-labeled indium In 111 scan


a. Delayed-imaging process obtained at 48 hours after injection.


b. Reinjected white blood cells (WBCs) accumulate at the sites of inflammation and help distinguish between an infectious and noninfectious etiology (85% sensitivity; 75% to 94% specificity).


c. Combining these two radioisotope scans (Tc 99m and In 111) produces a 90% to 93% sensitivity and 85% to 89% specificity for infection.


3. Positron-emission tomography (PET) scans have 100% sensitivity and 88% specificity in the diagnosis of chronic musculoskeletal infection.


C. CT scans


1. CT may be useful to localize a nidus of infection within an area of focal osteomyelitis.


2. Reconstructed images provide anatomic bone images helpful for localizing infection and surgical planning.


3. CT findings may include interosseous gas, decreased density of infected bone, and soft-tissue masses.




1. MRI is superior to CT for the evaluation of soft tissues and bone marrow. It will detect subtle marrow changes associated with very early stage osteomyelitis with nearly 100% sensitivity.


2. Classic findings


a. Signal change due to the increased edema and water content in bone.


b. Reduction in T1 marrow signal and an increase in T2 signal. T2 images have an increased signal because the fatty marrow has been replaced by inflammation.


3. Also useful for localizing sequestra, areas of focal infection/abscess, and sinus tracts.


E. Blood tests


1. C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) measure acute-phase response markers that are elevated in the presence of infection and/or inflammation.


2. CRP (elevated in 98% of pediatric patients with osteomyelitis) rises within a few hours of infection, reaching values up to 400 mg/L within 36 to 50 hours. It may normalize within 1 week of treatment of the infection.


3. ESR (elevated in 92% of pediatric patients with osteomyelitis) rises within 2 days of the onset of infection and continues to rise for 3 to 5 days after appropriate antibiotic treatment is instituted. The ESR returns to normal after 3 to 4 weeks. Surgical treatment prolongs both the peak time and normalization time of the ESR and CRP.


4. Normal values may vary between laboratories; however, an ESR value of 30 and a CRP of 5 are commonly considered upper limits of normal.


5. Elevated peripheral WBC count with increased number and percentage of polymorphonuclear leukocytes (PMNs) is indicative of infection, but these values are elevated in less than 50% of patients with septic arthritis. Therefore, the absence of this finding does not rule out infection, and this value alone should not be used to diagnose infection.


F. Infection site cultures


1. The use of tissue or fluid cultures from the site of infection is the gold standard for treatment and allows directed antimicrobial therapy.


2. A WBC count >50,000/mm3 is found in the synovial fluid aspirate of 50% of patients with septic arthritis.


3. Gram stain may identify the infecting organism in one third of patients with septic arthritis. Despite this low yield, initial antibiotic therapy may be directed if organisms are visualized.


4. Obtaining multiple cultures to test for anaerobic and aerobic organisms, tuberculosis, and fungal infection is mandatory.


5. Prior or concurrent administration of antibiotics, aspiration in a joint containing antibiotic-impregnated bone cement, sampling error during aspiration or biopsy, or improper handling or processing of specimens may produce inaccurate results.


G. Periprosthetic TJA infections


1. Synovial fluid WBC count >2,500/mm3 or >90% PMN is strongly indicative of infection in a total knee arthroplasty (TKA). Synovial fluid with a majority of neutrophils is suggestive of an infection in a TKA or total hip arthroplasty (THA).


2. The use of antibiotics before aspiration is a major cause of false-negative results.


3. If the TJA patient has recently received antibiotics, a 4- to 6-week delay before aspiration is recommended.


4. For periprosthetic THA infections, Gram staining has 19% sensitivity and 98% specificity.


5. Intraoperative frozen sections using >5 PMN/hpf improves the sensitivity and specificity to 80% and 94%, respectively, in THA infection.

IV. Pathophysiology of Bacterial Musculoskeletal Infections

A. Pathogenesis—Several complex interactions must take place before musculoskeletal infection can develop. This process involves the inoculation of the microorganism, interaction with the host soft tissues, virulence, release of toxins and creation of an inflammatory reaction, and local and systemic host factors that influence the infection.


1. Inoculation of the microorganism


a. S aureus is the most common organism responsible for musculoskeletal infections.


b. The ability of any organism to enter the host system depends on the mode of entry or inoculation and the host environment. The four most common forms of entry of microorganisms into the host system are:


i. Surgical procedures


ii. Trauma or injury to the musculoskeletal system (bone and soft tissues)


iii. Hematogenous spread


iv. Spread from a contiguous source


2. Possible interactions of the bacteria with the host:


a. Destroyed by the host


b. Live symbiotically with the host


c. Flourish and cause host sepsis


3. Virulence—Ability of the microorganism to overcome the host defenses and cause clinical infection.



Table 3. Classification of Periprosthetic TKA and THA Infections]

a. The virulence of an organism varies among and within species.


b. The virulence of S aureus is multifactorial, allowing this organism to survive and perpetuate infection.


c. S aureus is protected from host immune defenses by three mechanisms:


i. Excretion of protein A, which inactivates IgG.


ii. Production of a capsular polysaccharide, which reduces opsonization and phagocytosis of the organism.


iii. Formation of a biofilm (a "slime" containing an aggregation of microbial colonies embedded within a glycocalyx matrix that most commonly develops on TJA implants or a devitalized bone surface), which also secludes the organism from host defense mechanisms.


4. Local host factors that increase the likelihood of infection:


a. Reduced host vascularity


b. Neuropathy


c. Trauma


d. Presence of prosthetic implants


B. Periprosthetic infections


1. S aureus and Staphylococcus epidermidis most common


2. Classification of periprosthetic THA and TKA infections—Table 3 lists types of these infections and their treatment. Two-stage reimplantation is the gold standard for management of infected THAs and TKAs.


C. Immune response to microorganisms


1. Systemic factors such as renal or hepatic disease, malignancy, or malnutrition may reduce the ability of the host immune system to respond to the microorganism.


2. The immune response to bacterial infection comprises both cell-mediated and humoral components.


3. When PMNs attack the microorganism, damage to the bacteria releases additional chemotactic molecules, attracting larger numbers of PMNs. Bacteria in proximity to PMNs may be phagocytosed; for this to occur, opsonins or components in the serum must coat the bacteria, making them more attractive for the macrophages.


4. Nonspecific immune responses may be affected by medications such as NSAIDs, steroids, and aspirin.


5. Steroids have been implicated in reduced chemotaxis in PMNs in a tibial osteomyelitis model.


D. Infection in joints with intra-articular metaphysis


1. May result in contiguous septic arthritis.


2. Occurs most classically in the proximal femur in the child.


3. Alternatively, as the metaphyseal infection exits the bone, it may form a sinus tract to the skin.


E. Etiology of bacterial infections


1. Pathogen depends on the circumstances of the infection, patient age, and host immune response.


2. The most common pathogens and suggested empiric antibiotic therapies in musculoskeletal infections are outlined in

Table 4.


[Table 4. Most Common Pathogens and Suggested Empiric Antibiotic Therapy in Musculoskeletal Infections]

F. Cartilage injury


1. Septic arthritis with bacterial infection has the potential for irreversible cartilage destruction in an involved joint.


2. Initially, this occurs by dissolution of the glycosaminoglycan (GAG) units of cartilage. GAG units function as subunits of the proteoglycan molecule, in part giving rise to its physical properties of fluid retention or swelling pressure of the cartilage.


3. Following the destruction of the GAG units, collagen breakdown occurs and is evident by gross alteration in the cartilage. Subsequent rapid degenerative osteoarthritis may develop in association with this septic process.

V. Antibiotic Prophylaxis for Orthopaedic Surgery

A. Surgical prophylaxis is the administration of antibiotics to patients without clinical evidence of infection in the surgical field.


B. 25% to 50% of all antimicrobial usage is for the prevention rather than the treatment of infection.


C. Routine administration of prophylactic antibiotics to patients who will have a foreign body implanted, a bone graft procedure, or extensive dissection resulting in a potential residual dead space or subsequent hematoma is well accepted.


D. Routine use of prophylactic antibiotics in soft-tissue procedures or diagnostic arthroscopy is not well studied and continues to be controversial. Use of prophylaxis in these instances, especially in high-risk or immunocompromised patients, is at the discretion of the surgeon.


E. The most likely pathogens to cause infection in elective musculoskeletal procedures include S aureusS epidermidis, aerobic streptococci, and anaerobic cocci.


F. Prophylaxis is indicated when the risk of infection is low but the development of an infection would have devastating results. Procedures with a high inherent infection rate are ideally suited for prophylaxis.


G. Open fractures


1. Patients sustaining open fractures or traumatic open arthrotomy wounds should receive prophylactic antibiotics, and their tetanus status should be updated.


2. All patients with open fractures should receive a first-generation cephalosporin. A randomized trial showed a reduction in open grade II infection rates from 29% to 9% when an aminoglycoside plus cephalosporin was used, compared to a cephalosporin alone. Alternatively, a second-generation cephalosporin is effective.


3. In open fracture wounds with a high level of contamination, an additional agent may be required. Patients with open farmyard fractures should be treated with penicillin to prevent C perfringensinfection; soil injuries require anaerobic coverage with either metronidazole or clindamycin. Open fracture wounds exposed to freshwater are at risk for contamination with Pseudomonas and require additional coverage with a third-generation cephalosporin or a fluoroquinolone. Coverage should include an aminoglycoside and penicillin to cover gram-negative and anaerobic organisms, respectively, in addition to a first-generation cephalosporin.


H. Hip fracture surgery and TJA


1. Decreased incidence of postoperative surgical site infections (SSIs) has been reported when perioperative prophylactic antibiotics are used.


2. A single preoperative IV dose followed by two postoperative doses of IV antibiotics is effective for prophylactic treatment. In TJA, a single preoperative dose of cefazolin was as effective as continuation for 48 hours postoperatively. More prolonged therapy is therefore not indicated in routine surgery. More prolonged postoperative therapy may be considered in certain situations, including compromised host, immune deficiency, periprosthetic infection surgery, second-stage reimplantation joint arthroplasty surgery, and surgery on a contaminated wound.



Table 5. Patients at Increased Risk of Hematogenous TJA Infection]

I. Timing of antibiotic prophylaxis


1. Preoperative IV antibiotics should be administered such that the antibiotics are in the system within 1 hour before the time of incision.


2. When these agents are started after the surgical procedure is completed, infection rates are not significantly affected.


3. Prophylaxis given several hours before surgery has a reduced efficacy.


4. Administration 1 or more days in advance of surgery, altering the patient's normal host bacterial flora, may be detrimental.


J. Selection of antibiotic prophylaxis


1. Cephalosporins are the perioperative prophylactic antibiotic of choice in most centers. These agents provide coverage against most bacteria associated with musculoskeletal SSIs, are relatively nontoxic (hypersensitivity is rare; they have <10% cross-reactivity with penicillin allergy), and they are relatively inexpensive.


2. With the emergence of antimicrobial-resistant microorganisms, the incidence of methicillin-resistant S aureus infections (MRSAs) in association with SSIs has increased significantly.


3. The SSI profile in individual hospitals varies; communication with the infection control representative will aid in the selection of preoperative antibiotic prophylaxis.


4. Currently, in institutions where the incidence of MRSA SSIs is significant, vancomycin is used, either alone or in addition to a cephalosporin.


K. Antibiotic prophylaxis in dental patients with a TJA


1. The American Academy of Orthopaedic Surgeons and the American Dental Association have published an advisory statement regarding antibiotic prophylaxis in dental patients with a TJA.


2. Antibiotic prophylaxis is indicated in TJA patients at increased risk for hematogenous seeding (Table 5). Bacteremia may lead to hematogenous seeding even years following the TJA, with the highest risk within the first 2 years.


3. Antibiotics are not indicated for most dental patients with a TJA.


L. Antibiotic prophylaxis in urologic surgery patients with a TJA


1. Recommendations are similar to those for dental patients.


2. The only difference is that all patients with diabetes mellitus (types 1 and 2) are considered at risk and should receive antibiotic prophylaxis.

VI. Antibiotics—Mechanism of Action

A. The mechanism of action, ribosomal subunit binding, clinical use, side effect profiles, and pertinent pearls for the antibiotics most frequently prescribed to treat musculoskeletal infections are summarized in

Table 6.


B. Mechanisms of antibiotic resistance are outlined in

Table 7.

VII. Nonbacterial Infections

A. Tuberculosis


1. Tuberculosis infections are encountered in the United States. From 1985 through 1993 there was a 14% increase in morbidity in the United States. The correlated rise of HIV during that decade is the leading known risk factor for reactivation of latent infections and progression of active disease; an aging population and development of drug-resistant strains of Mycobacterium tuberculosis also have contributed to the increased rate.


2. The orthopaedic manifestation of tuberculosis infection may involve the entire skeletal system. The spine is the major site of infection, being involved in 50% of cases, half thoracic and one fourth in the cervical and lumbar spine.


3. Clinical presentation—Localized pain with fever and weight loss; when the spine is involved, rigidity, paravertebral swelling, and possible neurologic findings.


4. Radiographic evaluation


a. Radiographic changes are limited early in the disease process, with 50% bone mass loss needed for significant changes to be apparent.


b. Common findings—Cystic formation and subchondral erosions around joints, soft-tissue swelling, and mild periosteal reactions; with spine involvement, skip lesions, thinning of end plates, loss of disk height, and possible collapse and late fusion.


5. MRI is useful to identify earlier stages of the disease and also assess bone elements for abscess formation, which may differentiate from malignancies.


6. Biopsy and culture for definitive diagnosis is recommended. M tuberculosis, which stains when the Ziehl-Neelsen technique is used and therefore is considered an acid-fast bacillus, requires an egg-based medium and may take up to 2 to 4 weeks to grow. Therefore, empiric treatment is started at the initiation of biopsy.


7. Treatment


a. Management of these infections is primarily medical.


b. Extended triple drug therapy of isoniazid, rifampin, and pyrazinamide for 6 to 12 months has been shown to be effective with osseous involvement.


c. Joints—Irrigation and debridement in large joints is only necessary for abscess drainage.


d. Spine—The only absolute indications for surgical intervention of the spine are marked neurologic involvement related to a kyphotic deformity or herniation, worsening neurologic condition despite chemotherapy, an abscess with respiratory obstruction, and worsening kyphosis with instability. Anterior column involvement is most common: anterior decompression, debridement, and stabilization should be performed. A posterior approach should be reserved for specific situations involving posterior stabilization or debridement.




1. Prevalence—0.5% of the US population; as high as 10% reported in trauma centers located in high-prevalence areas. The need for orthopaedic care for HIV-positive patients is becoming increasingly common.


2. The risks of occupational transmission have been shown to be extremely low, with only 49 well-documented seroconversions among health care


[Table 6. Summary of Antimicrobial Agents and Mechanism of Action]

   workers, none of which involved needle sticks. In addition to exposure risks, patients' postoperative outcomes and complication rates have been a concern, with the potential for higher infection rates and longer healing times because of impaired cellular and humoral immunity.


3. Mechanisms that decrease host response to common pathogens and opportunistic infections include decreased absolute PMN count and function, platelet deficiency, and hypoalbuminemia.


4. The most common infecting organism is reported to be S aureus, followed by Streptococcus pneumoniae and Escherichia coli.


5. Studies comparing the infection rates of HIV-negative patients with asymptomatic HIV-positive patients have found no significant difference. Patients with CD4 counts <200, diagnostic for AIDS, were found to have a 10 times higher rate of infection than individuals without AIDS.


6. Recommendations for reducing the risk of infection during elective orthopaedic surgery include


a. Absolute PMN count >1,000.


b. Platelet count >60,000.


c. Serum albumin >25 g/L.


d. Reduction of viral loads to undetectable levels, which will in turn raise the lymphocyte count.


e. Cessation of marrow-suppressing drugs (zidovudine, didanosine, and zalcitabine) a few days before surgery and held until after the first postoperative week.


f. Drugs such as 3TC and D4T do not suppress the marrow and therefore should be maintained.


g. Evaluation of the surgical risk-to-benefit ratio and patient understanding of potential complications.


C. Lyme disease


1. Multisystem spirochetal disorder caused by Borrelia burgdorferi; transmitted by the bite of an infected tick.


2. Geographic predominance: Northeast, Midwest, and Northwest United States.


3. Disease occurs in three clinical stages


a. Early/localized disease—Pathognomonic skin lesion of erythema migrans.


b. Early disseminated disease—Neurologic and cardiac manifestations.


c. Late disease—Musculoskeletal symptoms (arthralgia, intermittent arthritis, and resultant chronic monoarthritis, most commonly in the knee) develop in 80% of untreated patients.


[Table 7. Mechanisms of Antibiotic Resistance]

4. Acute form of monoarthritis may resemble septic arthritis; however, the synovial fluid cell count (10,000 to 25,000 wbc/mm3) is typically lower than observed with bacterial septic arthritis, but still has a predominance of PMNs. Unlike chronic Lyme disease, acute monoarthritis is associated with HLA-DRB1*40, which suggests an auto-immune mechanism.


5. Diagnosis—Enzyme-linked immunosorbent assay (ELISA) testing will confirm the diagnosis.


6. Treatment—Oral amoxicillin or doxycycline for 4 weeks. IV ceftriaxone is recommended for patients with neurologic symptoms or recurrent episodes of arthritis. Prophylaxis may be administered using a single 200-mg dose of doxycycline in the event of a tick bite occurring in an endemic region.


D. Fungal infections


1. The most common location of fungal infections is in the hand.


2. Periprosthetic TJA infections may rarely be associated with fungal organisms.


a. The most common infecting organism is Candida albicans.


b. Deep periprosthetic infection with fungal organisms is rare and is associated with an immunocompromised host.


c. Less favorable results with a two-stage exchange procedure for infected TJA have been described than for bacterial organisms, requiring a more prolonged antifungal treatment between stages.


d. The use of amphotericin IV may be associated with renal toxicity and is often poorly tolerated by TJA patients.


e. Amphotericin powder is effective as a local elution antifungal agent in antibiotic-impregnated cement spacers used between stages of treatment for an infected TJA.


f. Adjuvant treatment of periprosthetic infections, in addition to IV amphotericin, may involve the use of oral fluconazole. Because of its mechanism of action, this agent requires periodic monitoring of hepatic enzymes.


3. Onychomycosis or fungal nail bed infections are commonly caused by dermatophytes such as Trichophyton rubrum.


4. Superficial skin infections are caused by dermatophytes and C albicans and may be treated successfully with topical antifungal agents, with conversion to oral agents if topical treatment is ineffective.


5. Spinal infections are commonly hematogenous bacterial or fungal infections. The widespread use of broad-spectrum antibiotics and the increasing number of immunocompromised patients have led to spinal infections with unusual organisms. Biopsies should be sent for Gram stain, acid-fast stain, and aerobic, anaerobic, fungal, and tuberculosis cultures.


6. Fungi such as Coccidioides immitis and Blastomyces dermatitidis are limited to specific geographic areas, whereas Cryptococcus, Candida, and Aspergillus are found worldwide. Candida andAspergillus are normal commensals of the body and produce disease in susceptible organisms when they gain access to the vascular system through IV lines, during implantation of prosthetic devices, or during surgery.


7. Coccidiomycosis is an infection caused by the fungus C immitis, a dimorphic fungus endemic to the southwestern United States, Central America, and parts of South America.


a. Infection with C immitis primarily causes pulmonary disease, with extrapulmonary dissemination occurring in fewer than 1% of patients. When extrapulmonary dissemination occurs, the most common sites of infection are the skin, meninges, and skeletal system.


b. Dissemination can occur in almost any organ system.


c. Septic arthritis usually occurs secondary to direct extension from infected adjacent bone, but primary synovial coccidiomycosis also may occur. The knee is the most frequently involved joint.


d. Most patients with musculoskeletal manifestations have pre-existing pulmonary coccidiomycosis or symptoms such as cough, dyspnea, and chest pain.


e. Aggressive surgical and antibiotic treatment is required to eradicate this infection.


f. Coccidiomycosis rarely should be considered in a differential diagnosis of patients with popliteal cysts without other obvious etiologies.


g. Coccidiomycosis rarely causes cardiac disease. Constrictive pericarditis in the setting of disseminated coccidiomycosis can be fatal, despite antifungal therapy and pericardiectomy.

Top Testing Facts

1. The most common general infectious symptom is pain localized to the site of infection; it is rare for patients to not report pain as a presenting symptom.


2. Septic arthritis in adults is most commonly associated with N gonorrhea in otherwise healthy patients. S aureus is the second most common pathogen causing adult septic arthritis.


3. Triad of symptoms strongly suggesting Clostridial myonecrosis: (a) progressively severe pain out of proportion to obvious injury, (b) tachycardia not explained by fever, (c) crepitus.


4. Bone loss of 30% to 40% is required before the classic signs of osteomyelitis (bone resorption, destruction, periosteal elevation) can be seen on radiographs.


5. The most sensitive imaging tool for diagnosing osteomyelitis is MRI. The classic finding includes a signal change that is due to the increased edema and water content in bone, which is manifested as a reduction in T1 and an increase in T2 marrow signal. The increased signal intensity on T2 images results because the fatty marrow has been replaced by inflammation.


6. Synovial fluid WBC count >2,500/mm3 or with >90% PMNs is strongly indicative of infection in a TKA.


7. S aureus is protected from host immune defenses by three mechanisms: (a) Excretion of protein A, which inactivates IgG; (b) production of a capsular polysaccharide, which reduces opsonization and phagocytosis of the organism; (c) formation of a biofilm (a "slime" containing an aggregation of microbial colonies embedded within a glycocalyx matrix that most commonly develops on THA/TKA implants or a devitalized bone surface), which also secludes the organism from host defense mechanisms.


8. Antibiotics have different modes of action (penicillin and cephalosporins—inhibition of cell wall synthesis; aminoglycoside—binds 30S ribosomal subunit; clindamycin—binds 50S ribosomal subunit).


9. Tuberculosis treatment—Extended triple drug therapy of isoniazid, rifampin, and pyrazinamide for 6 to 12 months has shown to be effective with osseous extrapulmonary involvement.


10. The treatment of choice for MRSA is vancomycin.


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