Musculoskeletal infections caused by fungal and atypical organisms

1. High-Yield Summary

Musculoskeletal infections caused by fungal pathogens and nontuberculous mycobacteria (NTM) represent insidious, diagnostically challenging clinical entities characterized by chronic granulomatous inflammation, absence of acute systemic toxicity, and frequent misdiagnosis as noninfectious inflammatory arthropathies or repetitive strain injuries. Diagnostic delays averaging 3 to 12 months are standard, frequently resulting in extensive articular cartilage destruction, tendon necrosis, or permanent functional disability prior to microbiological isolation.

• Dual Pathogen Spectrum: Causative organisms divide into Nontuberculous Mycobacteria (NTM: photochromogens such as Mycobacterium marinum and M. kansasii; nonphotochromogens including Mycobacterium avium complex [MAC]; and rapidly growing mycobacteria [RGM] such as M. fortuitum, M. chelonae, and M. abscessus) and Fungi (opportunistic yeasts like Candida albicans and C. parapsilosis; opportunistic angioinvasive molds like Aspergillus species; endemic dimorphic fungi including Blastomyces, Coccidioides, and Histoplasma; and subcutaneous dimorphic fungi like Sporothrix schenckii).
• The Diagnostic Gold Standard: Superficial swab cultures and routine joint aspirates have low diagnostic yields (<20% to 30% for acid-fast smears; <50% for fungal aspirate cultures). Definitive diagnosis mandates deep intraoperative synovial and tissue biopsies submitted for specialized histological stains (Grocott-Gomori methenamine silver [GMS], Periodic acid-Schiff [PAS], Ziehl-Neelsen, and auramine-rhodamine fluorochrome), prolonged culture on selective media (Sabouraud dextrose agar, Lowenstein-Jensen slants, Middlebrook 7H10/7H11), and molecular identification (16S/18S rRNA PCR, targeted sequencing, or metagenomic next-generation sequencing [mNGS]).
• Mandatory Dual-Temperature Culture: Routine microbiological incubation at 35°C to 37°C suppresses or kills Mycobacterium marinum and severely impairs Mycobacterium chelonae. Extremity and cutaneous tissue specimens must be incubated at 28°C to 30°C in addition to standard 35°C to 37°C and held for a minimum of 6 to 8 weeks before declaring cultures negative.
• The Corticosteroid Trap: Inadvertent local corticosteroid injection into undiagnosed chronic tenosynovitis or monoarthritis constitutes a severe clinical error. Local glucocorticoids suppress local cell-mediated immunity, dismantle protective granulomas, accelerate fungal hyphal penetration and mycobacterial replication, and frequently precipitate tendon rupture or rapid chondrolysis.
• Bimodal Management Paradigm: Successful eradication requires radical, meticulous surgical debridement (complete flexor tenosynovectomy preserving critical A2/A4 pulleys, radical joint synovectomy, cold abscess evacuation, or two-stage hardware explantation in prosthetic joint infection) combined with prolonged, susceptibility-guided multidrug antimicrobial therapy (minimum 6 to 12 months) tailored to the specific isolated organism.

2. Anatomy

Surgical intervention for atypical mycobacterial and fungal musculoskeletal infections demands detailed familiarity with synovial sheath topography, deep fascial spaces, regional neurovascular corridors, and vascular seeding channels.

Upper Extremity Synovial Sheaths & Deep Fascial Spaces

The flexor tendon sheaths and deep fascial compartments of the hand and wrist represent the most frequent anatomic targets of direct inoculation by atypical mycobacteria and fungi:

• Digital Flexor Sheaths & Pulley Architecture: The flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) tendons are enclosed within a continuous double-walled synovial sheath spanning from the metacarpal neck to the base of the distal phalanx. The biomechanically critical annular pulleys (A2 pulley over the proximal phalanx; A4 pulley over the middle phalanx) prevent tendon bowstringing and must be preserved during radical debridement. The intervening cruciate (C1, C2, C3) and minor annular (A1, A3, A5) pulleys can be vented or excised when heavily infiltrated by granulomatous synovium.
• Radial & Ulnar Bursae: In up to 80% of individuals, the flexor tenosynovial sheath of the small finger communicates directly with the ulnar bursa (common flexor sheath), which encompasses all eight digital flexor tendons as they transit beneath the transverse carpal ligament. The sheath of the flexor pollicis longus (FPL) tendon continues proximally into the radial bursa. In approximately 50% of hands, the radial and ulnar bursae communicate proximally in the distal forearm, creating a horseshoe-shaped conduit for the unchecked proximal spread of granulomatous tenosynovitis.
• Parona’s Space: Located in the distal volar forearm deep to the flexor digitorum profundus tendons and superficial to the pronator quadratus and interosseous membrane, Parona’s space communicates with both the radial and ulnar bursae. Chronic fungal or NTM tenosynovitis routinely dissects into Parona’s space, forming large, painless cold abscesses that compress the anterior interosseous nerve or cause extensive proximal forearm swelling.
• Extensor Retinacular Compartments: On the dorsum of the wrist, the extensor retinaculum forms six distinct fibro-osseous compartments. Granulomatous tenosynovitis most commonly affects the first compartment (abductor pollicis longus and extensor pollicis brevis), mimicking de Quervain’s stenosing tenosynovitis, or the fourth compartment (extensor digitorum communis and extensor indicis proprius), presenting as a painless dorsal soft tissue mass.

Neurovascular Danger Zones During Surgical Debridement

Extensive tenosynovectomy and soft tissue clearance place adjacent peripheral nerves and vascular structures at significant risk due to inflammatory fibrosis and distorted tissue planes:

• Median Nerve & Recurrent Motor Branch: In the carpal tunnel, the median nerve lies directly volar to the swollen digital flexor tendons. Chronic granulomatous proliferation elevates intracarpal pressure, effacing normal perineural fat planes. The recurrent motor branch of the median nerve typically arises from the radial and volar aspect of the main nerve trunk approximately 4 to 6 mm distal to the distal border of the transverse carpal ligament, though transligamentous (23%) and subligamentous (31%) variants are common. The branch must be visualized and preserved before debriding the tenosynovium surrounding the FDS and FDP tendons to the index digit.
• Palmar Cutaneous Branch of the Median Nerve (PCBMN): Originating from the radial aspect of the median nerve approximately 4 to 7 cm proximal to the distal wrist flexion crease, the PCBMN courses within a distinct fascial sheath between the flexor carpi radialis (FCR) and palmaris longus (PL) tendons before crossing superficial to the transverse carpal ligament. Extended surgical exposures of the volar wrist must place incisions along the ulnar border of the fourth ray, parallel to the thenar crease, to avoid transecting or tethering this sensory branch.
• Digital Neurovascular Bundles: Paired digital nerves and arteries course anterolateral to the flexor tendon sheath in the digit, located superficial and lateral to the annular pulleys. Hypertrophic synovial fronds and cold abscesses expand laterally through cruciate pulley windows, displacing digital bundles into the surgical field. Meticulous mobilization of neurovascular bundles under optical magnification is mandatory prior to incising the sheath.
• Popliteal Neurovascular Corridor: In the posterior knee, synovectomy for fungal septic arthritis or debridement of infected Baker’s cysts requires precise identification of the popliteal artery, popliteal vein, and tibial nerve. The popliteal artery rests directly on the posterior joint capsule and oblique popliteal ligament. Arthroscopic posterior trans-septal instrumentation or open capsular debridement must be performed with the knee flexed to 90 degrees, allowing the gastrocnemius muscle bellies to fall posteriorly and drop the neurovascular structures away from the posterior capsule.

Osseous Microvascular Architecture & Batson’s Plexus

The skeletal distribution of fungal and atypical mycobacterial osteomyelitis correlates directly with regional vascular anatomy:

• Metaphyseal Capillary Loops: In the pediatric and adolescent skeleton, terminal nutrient arterioles enter long bone metaphyses and form acute loops before emptying into large sinusoidal veins. Sluggish, laminar blood flow within these non-anastomotic capillary loops promotes endothelial adherence and extravasation of fungal blastoconidia (e.g., Candida, Blastomyces) and slow-growing mycobacterial bacilli, explaining the predilection for metaphyseal osteomyelitis.
• Batson’s Vertebral Venous Plexus: Batson’s plexus is a valveless, low-pressure longitudinal venous network that extends from the sacrum to the skull base, interconnecting the pelvic, abdominal, and intercostal veins with the vertebral bodies, epidural venous spaces, and basivertebral veins. Increases in intra-abdominal or intrathoracic pressure produce retrograde flow, allowing direct hematogenous seeding of fungal elements (particularly Candida and Aspergillus) and atypical mycobacteria directly into vertebral subchondral bone and intervertebral discs without prior filtration through the pulmonary capillary bed.

3. Pathogenesis

The pathogenesis of atypical mycobacterial and fungal musculoskeletal infections reflects a balance between specific microbial virulence traits, routes of inoculation, and host cell-mediated immune responses.

Microbiological Spectrum & Virulence Profiles

Causative organisms encompass distinct microbiological classes with specialized adaptations:

• Nontuberculous Mycobacteria (NTM): Acid-fast, aerobic, non-motile bacilli categorized according to the Runyon classification based on growth rate and carotenoid pigmentation. Cell walls contain an exceptionally high concentration of long-chain mycolic acids (60 to 90 carbons) and complex glycolipids (lipoarabinomannan, trehalose 6,6′-dimycolate), rendering them resistant to standard hydrophilic beta-lactam antibiotics, drying, and intracellular macrophage degradation. Mycobacterium marinum exhibits optimal growth between 28°C and 30°C, limiting its clinical manifestation primarily to the cooler distal extremities. Rapidly growing mycobacteria (M. fortuitum, M. chelonae, M. abscessus) possess robust biofilm capability on synthetic polymers and titanium implants; M. abscessus frequently expresses the inducible erythromycin methylase gene (erm(41)), conferring intrinsic resistance to macrolides.
• Opportunistic Fungal Yeasts (Candida species): Commensal organisms of the gastrointestinal and genitourinary tracts. Transition from budding blastoconidia to pseudohyphal and true hyphal morphotypes enhances tissue invasiveness and secretion of extracellular aspartyl proteinases, phospholipases, and lipases. Candida albicans and particularly Candida parapsilosis produce copious extracellular polymeric substances, forming resilient biofilms on polymethylmethacrylate (PMMA) bone cement, polyethylene liners, and metal arthroplasty components that are impervious to standard physiological host defenses.
• Opportunistic Filamentous Molds (Aspergillus species): Monomorphic saprophytic molds producing narrow (3 to 6 μm), septate hyphae with dichotomous 45-degree branching. Characterized by aggressive angioinvasion: hyphae penetrate vascular endothelial basements and lumen walls, inducing local arterial thrombosis, acute infarction of trabecular and cortical bone, and dense osseous sequestration.
• Endemic Dimorphic Fungi: Thermally dimorphic organisms that exist as mycelial molds with infectious microconidia in environmental soil and convert to pathogenic yeast or spherule forms at mammalian body temperature (37°C). Blastomyces dermatitidis converts into large (8 to 15 μm), thick-walled yeasts with broad-based unipolar budding and high osteotropism (involving bone in 20% to 30% of disseminated cases). Coccidioides immitis and C. posadasii form massive (20 to 100 μm) thick-walled spherules filled with hundreds of endospores; rupture releases endospores that incite granulomatous bone destruction. Histoplasma capsulatum transforms into small (2 to 4 μm) oval yeasts that survive and replicate intracellularly within reticuloendothelial macrophages.
• Subcutaneous & Opportunistic Encapsulated Yeasts: Sporothrix schenckii is a dimorphic fungus found in vegetative matter; in tissue, it appears as pleomorphic cigar-shaped budding yeasts (3 to 5 μm) and induces Splendore-Hoeppli phenomenon (asteroid bodies with radiating eosinophilic proteinaceous deposits). Cryptococcus neoformans and C. gattii are encapsulated budding yeasts surrounded by a copious glucuronoxylomannan polysaccharide capsule that inhibits phagocytosis and produces clean, lytic “soap-bubble” osseous lesions with minimal surrounding host sclerosis.

Inoculation versus Hematogenous Seeding Corridors

Musculoskeletal localization occurs through two primary mechanisms:

• Direct Traumatic Inoculation & Contiguous Extension: The predominant mechanism for upper extremity NTM and subcutaneous fungal infections. Minor penetrating trauma (fish fin punctures, oyster shell lacerations, rose thorns, splinter wounds, contaminated nail salon instruments, or surgical incisions) bypasses the cutaneous stratum corneum, depositing organisms directly into hypovascular flexor sheaths, olecranon bursae, or prepatellar spaces. Because synovial fluid contains nutrition with minimal baseline cellular defense, organisms replicate unhindered within the synovial lining.
• Hematogenous Dissemination: Primary mechanism for Candida osteomyelitis, Aspergillus spondylodiscitis, and endemic dimorphic fungal infections. Inoculation originates from central venous line colonization, total parenteral nutrition, long-term indwelling urinary catheters, intravenous drug abuse (IVDA), or pulmonary conidial inhalation. Circulating blastoconidia or fungal yeasts lodge in vascular beds with slow laminar flow, notably the vertebral subchondral bone via Batson’s plexus, long bone metaphyses, or synovial capillary plexuses of large weight-bearing joints.

Granulomatous Host Response & Diagnostic Failure Mechanisms

The host response against fungal and atypical mycobacterial pathogens is mediated predominantly by CD4+ T helper 1 (Th1) cells and activated macrophages. Macrophages secrete interleukin-12 (IL-12), inducing Th1 cells to produce interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α), which activate macrophages into epithelioid histiocytes and multinucleated Langhans giant cells, coalescing into organized granulomas.

Unlike classic tuberculous granulomas that exhibit uniform caseous necrosis, fungal and NTM infections characteristically produce suppurative granulomas—organized histiocytic cuffs surrounding central microabscesses packed with degenerate neutrophils. Standard microbiological tests fail systematically in this setting:

• Gram Stain Failure: The thick lipid- and mycolic acid-rich cell walls of NTM repel water-soluble crystal violet and safranin stains, leaving mycobacteria invisible or appearing as faint, unstained “ghosts.” Fungal cell walls, composed of chitin and beta-glucan, absorb Gram stain inconsistently, presenting as faint Gram-variable or poorly stained granular artifacts that are routinely misinterpreted as amorphous debris.
• Smear Insensitivity: Standard acid-fast bacilli (AFB) Ziehl-Neelsen staining requires an organism density exceeding 10,000 bacilli per milliliter of tissue for reliable visual detection. Musculoskeletal NTM infections are characteristically paucibacillary, resulting in AFB smear sensitivity rates below 15% to 30%.
• Premature Culture Discard & Thermal Mismatch: Routine automated bacterial blood culture bottles and standard 48-hour agar plates are discarded after 3 to 5 days, long before slow-growing NTM (2 to 6 weeks) or dimorphic fungi (4 to 8 weeks) form macroscopic colonies. Furthermore, standard hospital incubators operate at 35°C to 37°C, an environment that inhibits or terminates Mycobacterium marinum.

4. Natural History

The natural history of atypical mycobacterial and fungal musculoskeletal infections is defined by insidious progression, prolonged diagnostic delay, catastrophic exacerbation following corticosteroid exposure, and progressive irreversible tissue destruction.

Insidious Progression & Diagnostic Latency

Because atypical organisms lack aggressive pyogenic exotoxins and endotoxins, they elicit minimal acute systemic inflammatory response. Patients rarely develop high fevers, rigors, erythema, or marked leukocytosis. Symptoms progress over months, presenting as indolent swelling, dull aching discomfort, joint stiffness, or mild functional limitation. The median interval from symptom onset to definitive microbiological diagnosis ranges from 3 to 12 months, with documented delays exceeding 24 months in endemic fungal presentations.

Corticosteroid-Induced Exacerbation

Owing to the indolent, boggy swelling and joint stiffness, up to 40% of patients are initially misdiagnosed with noninfectious inflammatory conditions, including rheumatoid arthritis, de Quervain’s tenosynovitis, carpal tunnel syndrome, trigger finger, or gouty arthritis. Consequently, many patients receive one or more empiric local corticosteroid injections into the tendon sheath or joint:

• Immunological Collapse: Exogenous glucocorticoids suppress macrophage transcription factors, inhibit TNF-α, IL-12, and IFN-γ release, and dismantle the architectural integrity of protective granulomas.
• Accelerated Tissue Destruction & Tendon Rupture: Without cell-mediated containment, fungal hyphae and mycobacterial bacilli proliferate exponentially. The tenosynovial sheath becomes engorged with necrotic debris, extensive rice bodies form, and intratendinous invasion triggers collagen breakdown. Spontaneous tendon rupture—most commonly the flexor pollicis longus or extensor pollicis longus—occurs in 15% to 25% of patients exposed to local steroids, significantly degrading functional reconstructive potential.

Host Vulnerability & Long-Term Destructive Sequelae

While atypical mycobacteria (such as M. marinum) and subcutaneous fungi (Sporothrix) frequently infect immunocompetent individuals via direct inoculation, severe, disseminated, or opportunistic fungal infections occur predominantly in hosts with compromised cell-mediated immunity. Key risk profiles include:

• Biologic Immunosuppression: Patients receiving anti-TNF-α monoclonal antibodies (infliximab, adalimumab) or receptor fusion proteins (etanercept) exhibit a dramatically increased susceptibility to NTM and endemic mycoses due to targeted disruption of granuloma maintenance.
• Organ Transplantation & Hematologic Malignancy: Solid organ transplant recipients on calcineurin inhibitors and patients with prolonged post-chemotherapy neutropenia face high rates of invasive Aspergillus, Candida, and rapid-growing NTM infections with high mortality rates (20% to 40%).
• End-Stage Medical Disease & Comorbidities: Hemodialysis-dependent end-stage renal disease, poorly controlled diabetes mellitus, and advanced HIV infection (CD4 count < 200 cells/μL) are associated with hematogenous seeding to bone and joints.
• End-Stage Musculoskeletal Destruction: Left untreated, chronic infection progresses to full-thickness chondrolysis, joint space obliteration, subchondral bone collapse, osteolytic cavitary lesions, pathological fractures, chronic draining cutaneous sinuses, and rigid joint ankylosis.

5. Clinical Presentation

Accurate clinical diagnosis requires a high index of suspicion, combining targeted occupational and environmental exposure histories with meticulous physical examination of the musculoskeletal system.

History

Surgeons must systematically query for specific environmental, recreational, travel, and medical exposures:

• Aquatic & Marine Exposures: Cleaning home freshwater or saltwater fish aquariums, commercial fish handling, handling shellfish, oyster shucking, or minor abrasions incurred in coastal or freshwater environments strongly suggest Mycobacterium marinum (“fish tank granuloma”).
• Botanical & Agricultural Trauma: Rose gardening, handling thorny vegetation, sphagnum moss exposure, forestry work, or wooden splinter punctures point to Sporothrix schenckii (“rose gardener’s disease”).
• Cosmetic, Surgical & Puncture Exposures: Recent salon pedicure whirlpool footbaths, cosmetic dermal filler or botulinum toxin injections, acupuncture, prior elective hand or orthopaedic surgery, or deep puncture wounds through rubber-soled footwear are classic historical triggers for rapidly growing mycobacteria (M. fortuitum, M. chelonae, M. abscessus).
• Geographic Travel & Endemic Residence: Residence in or travel to the desert Southwest United States (Arizona, San Joaquin Valley of California), Mexico, or Central/South America suggests Coccidioides (“Valley Fever”). Travel within the Ohio, Mississippi, and St. Lawrence River valleys points to Histoplasma capsulatum or Blastomyces dermatitidis.
• Medical History & Refractory Course: Indwelling central venous catheters, total parenteral nutrition, history of intravenous drug abuse (classically associated with Candida sternoclavicular or sacroiliac osteomyelitis), active immunosuppressive or biologic therapy, and multiple prior failed courses of standard oral antibiotics (cephalexin, amoxicillin-clavulanate, ciprofloxacin) without clinical resolution.

Physical Exam

Physical findings reflect chronic proliferative synovitis, cold abscess formation, and specialized lymphatic tracking patterns:

• Inspection (Boggy Synovial Thickening & Cold Abscesses): Joints, tendon sheaths, or bursae exhibit soft, “doughy”, non-tender or mildly tender fluid distention out of proportion to erythema or warmth (“cold joint”). Chronic draining cutaneous sinuses discharging serous, gelatinous, or granular fluid may be visible overlying tendon sheaths or bone prominences.
• Modified Kanavel Signs (Chronic Tenosynovitis): Unlike the acute toxic presentation of pyogenic flexor tenosynovitis, chronic fungal and NTM infections display a muted or modified presentation with characteristic diagnostic parameters.
• Kanavel Sign 1 (Flexed Digital Posture): Digit is held in slight resting flexion, but passive movement is flexible and lacks the rigid guarding seen in acute suppurative tenosynovitis.
• Kanavel Sign 2 (Fusiform Sausage Digit): Prominent, boggy, symmetric cylindrical enlargement extending the entire length of the flexor sheath without intense cutaneous calor or bright erythema.
• Kanavel Sign 3 (Dull Palpatory Tenderness): Diffuse, dull aching along the anatomical borders of the flexor sheath rather than exquisite focal tenderness.
• Kanavel Sign 4 (Pain on Passive Extension): Mild to moderate pain or stiffness at terminal passive extension, limited by mechanical volume and fibrous thickening rather than extreme agony.
• Rice Body Crepitus (Snowball Sign): Palpation of the volar wrist, flexor digital sheaths, or subdeltoid/prepatellar bursae elicits soft, palpable, fine-to-coarse crepitus reminiscent of rolling a soft snowball. This sensation reflects hundreds of condensed fibrin-collagen rice bodies (corpora oryzoidea) moving within the distended synovial cavity.
• Sporotrichoid Nodular Lymphangitis: A characteristic diagnostic physical sign consisting of a primary crusted or ulcerated nodule at the distal inoculation site, accompanied by a linear chain of firm, painless or mildly tender subcutaneous erythematous nodules that track proximally along regional lymphatic channels toward regional lymph nodes. Pathognomonic for Sporothrix schenckii, Mycobacterium marinum, and occasionally M. chelonae or Nocardia.
• Compressive Median Neuropathy: Palpation of the volar wrist demonstrates fullness in the carpal canal with positive Tinel’s sign and positive Phalen’s maneuver, reflecting median nerve compression secondary to chronic proliferative flexor tenosynovitis.
• Physical Examination Diagnostic Benchmarks: Modified Kanavel signs demonstrate sensitivity of 80% to 85% and specificity of 45% to 55% for chronic tenosynovitis. Sporotrichoid lymphatic nodule chains demonstrate high diagnostic specificity of 90% to 95% for Sporothrix and M. marinum.

6. Imaging and Diagnostic Studies

Establishing the diagnosis of fungal and atypical mycobacterial musculoskeletal infections requires an integrated diagnostic pathway progressing from standard imaging to invasive deep-tissue biopsy and molecular testing.

Radiographic & Ultrasonographic Manifestations

Initial imaging studies establish anatomical baseline and distinguish indolent granulomatous infection from pyogenic osteomyelitis and bone neoplasia:

• Plain Radiographs: Early stages show non-specific soft tissue shadow distention, obliteration of adjacent fascial fat stripes, and pronounced regional juxta-articular osteopenia. A critical distinguishing feature is the conspicuous absence of early reactive sclerosis or aggressive periosteal new bone formation, which contrasts sharply with acute pyogenic bacterial osteomyelitis. Late-stage joint involvement exhibits the classic Phemister-like triad: severe juxta-articular osteopenia, peripheral marginal osseous erosions at capsular reflections, and prolonged relative preservation of the joint space. Fungal osteomyelitis (notably Blastomyces and Cryptococcus) produces well-circumscribed, punched-out, lytic lesions without reactive sclerotic margins.
• High-Resolution Musculoskeletal Ultrasound: Highly sensitive modality for evaluating upper and lower extremity tenosynovitis and bursitis. Demonstrates marked hypoechoic to heterogeneous tenosynovial sheath thickening, complex anechoic-to-hypoechoic fluid distention, and multiple floating, non-shadowing, hyperechoic punctate or ovoid foci corresponding to rice bodies. Color and Power Doppler demonstrate mild to moderate low-resistance hypervascularity within thickened synovium, contrasting with the intense hypervascularity seen in acute bacterial phlegmon. Real-time dynamic imaging assesses flexor tendon excursion, tendon delamination, and partial or complete tendon ruptures.

Magnetic Resonance Imaging (MRI)

MRI represents the imaging modality of choice to define the exact anatomical extent, soft tissue compartment involvement, tendon structural integrity, and osseous invasion:

• T1-Weighted Sequences: Demonstrate intermediate-to-low signal intensity synovial proliferation distending tendon sheaths and bursae, effacement of surrounding normal fat planes, and ill-defined low signal marrow edema in adjacent bone.
• T2-Weighted & STIR Sequences: Show marked hyperintensity of complex, loculated fluid collections tracking along anatomical sheaths and into deep fascial spaces (such as Parona’s space). High-signal bone marrow edema highlights underlying osseous extension or subchondral osteolysis.
• Rice Body “Bag of Rice” Appearance: Innumerable well-demarcated, uniform ovoid or rice-grain-shaped bodies displaying intermediate to low signal intensity on both T1- and T2-weighted images (reflecting compact fibrin, fibronectin, and dense collagen core) surrounded by high-signal synovial fluid. This appearance is characteristic of chronic granulomatous tenosynovitis (NTM, fungal, tuberculosis, and rheumatoid arthritis).
• T1 Post-Contrast (Gadolinium) Sequences: Reveals thick, irregular, nodular, frond-like peripheral synovial enhancement surrounding central non-enhancing fluid or caseous necrosis. Intramuscular and peritendinous cold abscesses show smooth, uniform peripheral rim enhancement without diffuse infiltrating muscular edema.

Deep Tissue Biopsy, Histopathology & Special Stains

Definitive diagnosis cannot be established on clinical or radiological grounds alone. Superficial swabs of open wounds or sinus tracts are strictly non-diagnostic, yielding skin colonizers (such as Staphylococcus epidermidis or Corynebacterium) that mislead clinical management. Similarly, arthrocentesis fluid yields low sensitivity:

• Deep Operative Tissue Biopsy Mandate: The absolute diagnostic gold standard. During operative debridement, the surgeon must harvest multiple (minimum 3 to 5) large, distinct tissue specimens from abnormal synovium, tenosynovium, capsule, and bone margins. Specimens must be divided equally: one portion sent immediately in sterile saline (never formalin) for aerobic, anaerobic, mycobacterial, and fungal cultures; the second portion submitted in formalin for comprehensive histological evaluation.
• Histopathology & Granuloma Evaluation: Demonstrates necrotizing (caseating) or non-necrotizing epithelioid granulomas, Langhans multinucleated giant cells, and chronic mononuclear inflammatory infiltrates. The presence of suppurative granulomas (granulomas centered on microabscesses of degenerate polymorphonuclear neutrophils) is strongly suggestive of fungal or atypical mycobacterial etiology. Rice bodies microscopically demonstrate an amorphous central core of eosinophilic fibrin surrounded by a thin layer of collagenous and fibronectin-rich connective tissue.
• Grocott-Gomori Methenamine Silver (GMS) Stain: Stains fungal cell wall carbohydrates black/dark brown against a pale green light background. Excellent for visualizing yeasts, pseudohyphae, branching hyphae, and spherules.
• Periodic Acid-Schiff (PAS) Stain: Stains fungal cell walls bright magenta/pink, facilitating identification of fungal morphology within granulomatous tissue.
• Acid-Fast Bacilli (AFB) Stains: Ziehl-Neelsen (heat-fixed) and Kinyoun (cold) carbolfuchsin stains highlight mycobacterial bacilli bright red against a methylene blue background. Auramine-rhodamine fluorochrome staining provides superior sensitivity under ultraviolet fluorescent microscopy and is the preferred screening stain.
• Mucicarmine & Fontana-Masson Stains: Specific for Cryptococcus neoformans, staining the mucopolysaccharide capsule rose-red and cell wall melanin black.

Specialized Culture Requirements & Incubation Protocols

The microbiological isolation of atypical mycobacteria and fungi requires explicit communication between the orthopaedic surgeon and the clinical microbiology laboratory:

• Obligate Dual-Temperature Incubation: Cutaneous, extremity, and superficial synovial specimens must be inoculated and incubated at both 28°C to 30°C and 35°C to 37°C. Mycobacterium marinum has a strict upper thermal limit, growing luxuriantly at 30°C but failing to grow or growing very poorly at standard incubator temperatures of 37°C. Mycobacterium chelonae similarly exhibits enhanced growth kinetics at 30°C.
• Mycobacterial Culture Media & Extended Incubation: Inoculation onto solid egg-based media (Lowenstein-Jensen slants), solid agar-based media (Middlebrook 7H10 or 7H11 agar plates), and automated continuous-monitoring liquid broth systems (Mycobacteria Growth Indicator Tube [MGIT] / BACTEC). Cultures must be incubated in 5% to 10% CO2 and maintained for a minimum of 6 to 8 weeks before final negative reporting.
• Fungal Culture Media & Extended Incubation: Inoculation onto Sabouraud Dextrose Agar (SDA) supplemented with chloramphenicol and gentamicin (to suppress bacterial overgrowth) and Brain-Heart Infusion (BHI) agar supplemented with 5% sheep blood. Media sets should be run both with and without cycloheximide, as cycloheximide inhibits pathogenic non-dermatophyte opportunistic molds such as Aspergillus and Fusarium. Fungal cultures must be maintained at 25°C to 30°C and held for 4 to 8 weeks.

Molecular Testing: PCR & Metagenomic Next-Generation Sequencing (mNGS)

Given the slow turnaround and potential insensitivity of conventional cultures, molecular diagnostics provide rapid, definitive organism identification:

• Targeted Polymerase Chain Reaction (PCR) & Sequencing: Direct PCR amplification of the 16S ribosomal RNA (rRNA) gene, the 65-kDa heat shock protein (hsp65) gene, and the RNA polymerase beta subunit (rpoB) gene provides definitive species-level identification of NTM within 48 to 72 hours directly from fresh tissue or formalin-fixed paraffin-embedded (FFPE) blocks. For fungal pathogens, PCR amplification targeting the internal transcribed spacer (ITS1/ITS2) regions and 18S/28S rRNA genes enables rapid fungal classification.
• Metagenomic Next-Generation Sequencing (mNGS): Unbiased, high-throughput sequencing of all cell-free and cell-associated microbial DNA/RNA directly extracted from synovial fluid or deep tissue biopsies. Demonstrates superior sensitivity (>85% to 90%) in culture-negative musculoskeletal infections, polymicrobial atypical infections, and in patients pre-treated with empiric antibacterial or antifungal agents. Results are typically available within 24 to 48 hours, enabling early targeted therapy before culture colonies appear.

7. Classification

Classification frameworks for atypical mycobacteria and fungi establish microbiological taxonomy and directly dictate targeted antimicrobial selection and surgical urgency.

Runyon Classification of Nontuberculous Mycobacteria (NTM)

Developed by Ernest Runyon, this system categorizes NTM into four distinct phenotypic groups based on in vitro growth rate and carotenoid pigment production under light exposure:

Classification of Fungal Musculoskeletal Pathogens

Fungal musculoskeletal infections are classified by organism morphology, environmental source, and host immune status:

8. Treatment

Eradication of fungal and atypical mycobacterial musculoskeletal infections requires an integrated multimodal approach combining prolonged, susceptibility-guided systemic antimicrobial therapy with aggressive, complete surgical debridement.

Medical / Nonoperative

Antimicrobial selection must be guided by formal minimum inhibitory concentration (MIC) susceptibility testing, as empiric regimens frequently fail due to intrinsic or acquired resistance patterns:

• Mycobacterium marinum Multidrug Regimen: Monotherapy is strictly contraindicated to prevent acquired macrolide resistance. Dual therapy is standard for superficial disease: Clarithromycin (500 mg PO BID) or Azithromycin (500 mg PO daily) PLUS Rifampin (600 mg PO daily) or Ethambutol (15 mg/kg PO daily). For deep flexor tenosynovitis or osteoarticular involvement, a triple-drug regimen (Clarithromycin + Rifampin + Ethambutol) is mandatory. Treatment is maintained for a minimum of 3 to 6 months, continuing for at least 1 to 2 months beyond complete resolution of all clinical symptoms.
• Mycobacterium fortuitum Regimen: Generally susceptible to oral fluoroquinolones (Moxifloxacin 400 mg daily or Levofloxacin 750 mg daily), Trimethoprim-Sulfamethoxazole (160/800 mg PO BID), and Doxycycline (100 mg PO BID). Severe disease requires initial IV induction with Amikacin (15 mg/kg daily) plus Cefoxitin (2g IV q6-8h) for 2 to 4 weeks before transitioning to oral dual therapy for 6 to 12 months.
• Mycobacterium chelonae Regimen: Typically susceptible to Clarithromycin, Tobramycin, Tigecycline, and Linezolid; resistant to cefoxitin. Induction with IV Tobramycin or Tigecycline followed by oral Clarithromycin combined with Linezolid or Clofazimine for 6 to 12 months.
• Mycobacterium abscessus Multidrug Regimen: The most drug-resistant NTM. Isolates must be tested for the inducible erythromycin methylase gene (erm(41)); if functional, macrolides cannot be relied upon. Regimen consists of an intensive induction phase (4 to 12 weeks) with IV Amikacin (10 to 15 mg/kg daily) PLUS IV Imipenem (1g IV q8h) or IV Tigecycline (50 mg IV q12h), followed by an oral continuation phase (macrolide [if erm-negative] plus Clofazimine 100 mg daily plus oral Omadacycline 300 mg daily or Linezolid 600 mg daily) for a total of 12 months.
• Mycobacterium avium complex (MAC) Regimen: Clarithromycin (500 mg PO BID) or Azithromycin (500 mg PO daily) PLUS Rifampin (600 mg PO daily) PLUS Ethambutol (15 mg/kg PO daily) for a minimum duration of 12 months.
• Candida Osteomyelitis & Septic Arthritis Regimen: First-line for susceptible isolates (C. albicans, C. parapsilosis): Fluconazole 400 mg (6 mg/kg) PO daily for 6 to 12 months. For severe systemic toxicity, refractory cases, or fluconazole-resistant species (C. glabrata, C. krusei): Liposomal Amphotericin B (3 to 5 mg/kg IV daily) or an Echinocandin (Caspofungin 70 mg loading, then 50 mg IV daily; Micafungin 100 mg IV daily) for an initial 2 to 3 weeks, followed by step-down to oral Voriconazole or Posaconazole for a total duration of 6 to 12 months.
• Aspergillus Osteoarticular Regimen: Voriconazole is the primary drug of choice (loading dose 6 mg/kg IV q12h for two doses, followed by 4 mg/kg IV q12h or 200 to 300 mg PO BID). Therapeutic drug monitoring (TDM) is mandatory: maintain voriconazole trough serum levels between 1.5 and 5.0 mcg/mL to ensure efficacy and avoid neurotoxicity. Alternatives include Isavuconazole (200 mg PO/IV TID x 6 doses, then 200 mg daily) or Liposomal Amphotericin B (3 to 5 mg/kg daily). Total duration: minimum 6 to 12 months.
• Blastomycosis Regimen: Itraconazole 200 mg PO TID for 3 days, followed by 200 mg PO BID for 6 to 12 months. Severe disease, vertebral collapse, or CNS involvement warrants initial Liposomal Amphotericin B (3 to 5 mg/kg daily) for 1 to 2 weeks prior to oral itraconazole.
• Coccidioidomycosis Regimen: Fluconazole 400 to 800 mg PO daily or Itraconazole 200 mg PO BID for a minimum of 12 months. Patients with vertebral destruction or ongoing immunosuppression frequently require lifelong azole suppression. Refractory or rapidly destructive cases warrant Liposomal Amphotericin B.
• Sporotrichosis Regimen: Itraconazole 200 mg PO BID for 6 to 12 months is standard for osteoarticular and tenosynovial disease. Disseminated or refractory joint infection requires Liposomal Amphotericin B induction.

Operative

The core goals of surgery are complete macroscopic source control, mechanical decompression of neurovascular structures, harvesting of abundant diagnostic tissue, and stabilization of structural skeletal defects:

• Radical Flexor Tenosynovectomy Exposure: Extended Bruner zigzag incisions or mid-axial digital incisions combined with an extended carpal tunnel release. The incision crosses the wrist flexion crease obliquely across the ulnar border of the fourth ray to protect the palmar cutaneous branch of the median nerve, extending into the distal volar forearm if Parona’s space is involved.
• Systematic Pulley Preservation (A2 and A4): Meticulous, circumferential synovectomy of both FDS and FDP tendons from their musculotendinous junctions to their digital insertions. The A2 and A4 annular pulleys must be preserved intact to prevent crippling tendon bowstringing. The A1, A3, and A5 pulleys can be vented or excised. Granulomatous synovium and rice bodies trapped beneath intact A2/A4 pulleys are cleared by flexing the digit, delivering the tendons into adjacent windows, and performing pulsed irrigation using a soft 5-French pediatric catheter.
• Tendon Management & Debridement: Debride intratendinous granulomatous nodules. If the FDS tendon is frayed >50% or necrotic, it is sharply excised while preserving the intact FDP tendon. Tendon ruptures are debrided; primary tendon repair or tendon transfers (e.g., ring FDS to thumb FPL, or EIP to EPL) are strictly contraindicated in the infected field and must be delayed until microbiological cure is established.
• Extended Carpal Tunnel Decompression & Parona’s Space Evacuation: Complete division of the transverse carpal ligament, external neurolysis of the median nerve and its recurrent motor branch, complete clearance of the ulnar and radial bursae, and drainage of Parona’s space by elevating the flexor tendons over the pronator quadratus.
• Radical Joint Synovectomy & Osseous Debridement: Arthroscopic or open complete synovectomy for monoarthritis, evacuating all rice bodies and fibrinous exudate. For osteomyelitis, unroof cortical bone over subchondral cold abscesses, perform thorough curettage of cavitary lesions, and excise all devitalized bone sequestra until uniform bleeding margins (“paprika sign”) are reached. Large cavitary bone voids are managed with antibiotic-loaded calcium sulfate or PMMA beads.
• Prosthetic Joint Infection (Two-Stage Revision Standard): Debridement, Antibiotics, and Implant Retention (DAIR) is contraindicated in fungal or atypical NTM PJI, resulting in failure rates exceeding 70% to 80% due to dense biomaterial biofilm formation. Management requires a planned two-stage protocol: complete component and cement explantation, radical debridement, and placement of a high-dose antimicrobial-loaded PMMA articulating or static spacer.
• Antifungal & Antimicrobial Cement Spacers: Amphotericin B deoxycholate (200 to 500 mg per 40g batch of PMMA cement) or Voriconazole (500 to 1000 mg per 40g batch), typically combined with heat-stable antibacterial agents (Vancomycin 2 to 3g plus Tobramycin 2.4 to 3.6g per batch). Ensure hand mixing to preserve drug elution porosity.
• Stage 2 Reimplantation Criteria: Delayed reimplantation is undertaken only after complete clinical wound healing, normalization of inflammatory markers (ESR, CRP), a minimum 2- to 4-week antimicrobial-free holiday, sterile repeat joint aspiration, and intraoperative frozen sections demonstrating <5 neutrophils per high-power field across multiple samples.

Indications & Contraindications

Operative intervention is guided by precise clinical and anatomical parameters:

• Operative Indication (Acute/Subacute Neurological Compression): Objective sensorimotor median neuropathy with static two-point discrimination > 6 mm, thenar motor weakness, or measured carpal tunnel compartment pressure > 30 mmHg.
• Operative Indication (Impending Tendon Rupture): Proliferative tenosynovitis encasing >50% of the tendon circumference, invasive intratendinous granulomas, or persistent pain and swelling despite 4 to 6 weeks of targeted medical therapy.
• Operative Indication (Loculated Cold Abscesses & Rice Body Accumulation): Subcutaneous, subfascial, or intramuscular fluid collections > 2 cm in diameter, or massive rice body collections producing mechanical block to joint motion.
• Operative Indication (Structural Osseous Instability & Sequestra): Cortical bone destruction compromising >50% of shaft diameter, pathological fracture, or large avascular bone sequestra.
• Operative Indication (Prosthetic Joint / Foreign Body Infection): Presence of confirmed fungal or atypical mycobacterial colonization on implants or arthroplasty components.
• Operative Indication (Diagnostic Failure in Chronic Granulomatous Synovitis): Chronic synovitis, bursitis, or monoarthritis persisting > 4 weeks without microbiological isolation despite non-invasive testing.
• Contraindication (Local Corticosteroid Injections): Absolute contraindication in any active, suspected, or unconfirmed fungal or atypical mycobacterial infection.
• Contraindication (DAIR in Chronic Fungal or NTM PJI): Relative to absolute contraindication; retaining implants in the presence of fungal or NTM biofilm leads to persistent sepsis and massive bone loss.
• Contraindication (Single-Stage Revision Arthroplasty): Contraindicated in fungal and atypical mycobacterial PJI due to unacceptable rates of recurrent deep infection.
• Contraindication (Immediate Primary Tendon Reconstruction): Contraindicated in active tenosynovitis; tendon transfers, intercalary tendon grafts, or silicon rod placement must be staged until the surgical field is sterile.
• Contraindication (Isolated Conservative Therapy in Compartment Compression): Relying solely on medical therapy in the presence of elevated compartment pressures, acute nerve compression, or large sequestra is contraindicated.

9. Outcome

Outcomes for musculoskeletal fungal and atypical mycobacterial infections depend heavily on the timeliness of diagnosis, the thoroughness of initial surgical debridement, and host immunological status.

Eradication Rates & Functional Recovery

Clinical success rates vary substantially between clinical entities:

• Nontuberculous Mycobacterial Tenosynovitis: Combined radical surgical tenosynovectomy and multidrug antimycobacterial therapy achieves clinical infection eradication in 85% to 92% of cases. The single most important determinant of functional outcome is the duration of pre-treatment diagnostic delay. In patients diagnosed within 3 months of symptom onset, average digit Total Active Motion (TAM) exceeds 220 degrees. Conversely, in patients experiencing diagnostic delays > 6 to 12 months, permanent digital stiffness (TAM < 180 degrees), flexor lag, and grip strength deficits occur in up to 40% of cases due to peritendinous scarring, tenosynovial fibrosis, and pulley contracture. Recurrence rates range from 10% to 15%, primarily driven by premature discontinuation of antimicrobials (<3 months), monotherapy, or incomplete tenosynovial excision.
• Candida Osteomyelitis & Native Septic Arthritis: Clinical cure is achieved in 80% to 88% of patients receiving surgical debridement and 6 to 12 months of systemic azole therapy. Residual articular stiffness and joint space narrowing are common, especially in weight-bearing joints, with secondary post-infectious osteoarthritis developing in 30% to 45% of patients over 5 years.
• Invasive Mold & Aspergillus Osteomyelitis: Carries a guarded prognosis, achieving clinical eradication in only 60% to 75% of immunocompromised or transplant patients. Recurrence rates range from 20% to 35%, frequently requiring indefinite oral azole suppression.
• Sporothrix Flexor Tenosynovitis: Excellent prognosis, with eradication rates exceeding 90% following radical tenosynovectomy and 6 to 12 months of oral itraconazole. However, permanent interphalangeal flexion contractures persist in 25% to 35% of affected digits due to extensive cicatricial scarring.

Arthroplasty Salvage & Failure Predictors

In prosthetic joint infections caused by fungi or atypical mycobacteria:

• Two-Stage Revision Success: Overall joint salvage and infection eradication is achieved in 70% to 80% of patients utilizing aggressive two-stage revision arthroplasty with high-dose antifungal/antimicrobial PMMA spacers. Mortality at 1 year ranges from 10% to 20%, reflecting severe underlying host medical comorbidities.
• DAIR Failure Rate: Retention strategies (DAIR) in fungal or atypical PJI fail in >75% to 85% of cases, frequently necessitating emergency resection arthroplasty (Girdlestone procedure), chronic antimicrobial suppression, or amputation.
• Predictors of Treatment Failure: Independent risk factors for treatment failure and recurrent infection include prior local corticosteroid injection, retained foreign hardware or cement fragments, immunocompromised status (active chemotherapy, solid organ transplant, anti-TNF therapy), polymicrobial co-infection, and infections caused by rapidly growing mycobacteria with inducible macrolide resistance (M. abscessus).

10. Complications

Complications associated with fungal and atypical mycobacterial musculoskeletal infections are categorized by their temporal relationship to presentation and surgical intervention:

11. Pearls and Pitfalls

Mastery of atypical mycobacterial and fungal infections requires strict adherence to diagnostic principles and avoidance of common surgical judgment traps.

Clinical & Surgical Pearls

• The Dual-Temperature Incubation Rule: Whenever an indolent extremity infection, cold tenosynovitis, or atypical bursitis is biopsied, the surgeon must personally write on the laboratory requisition: “Hold for 8 weeks; incubate at both 28°C–30°C and 35°C–37°C for atypical mycobacteria and fungi.” Standard automated microbiology systems will otherwise discard specimens after 5 days or incubate exclusively at 37°C, entirely missing Mycobacterium marinum and M. chelonae.
• Multiple Large Deep Tissue Biopsies: Superficial swabs of draining sinus tracts grow colonizing skin flora (such as coagulase-negative staphylococci) that deceive the clinician. Always obtain 3 to 5 distinct, pea-sized deep synovial, capsular, and bone biopsies from viable margins during operative exploration.
• Pulley Preservation in Radical Tenosynovectomy: Never sacrifice the A2 or A4 pulleys to gain exposure. Flex the metacarpophalangeal and interphalangeal joints to deliver the FDS and FDP tendons into the A1–A2, C1–A3, and C2–A4 intervals. Strip the visceral epitenon meticulously under direct visualization, and flush the sub-pulley tunnels using a flexible 5-French catheter.
• Intraoperative Frozen Section as a Diagnostic Anchor: Submit fresh tissue for intraoperative frozen section analysis. The demonstration of necrotizing or non-necrotizing granulomas with multinucleated giant cells confirms a granulomatous etiology immediately, providing the surgeon with objective justification to proceed with radical tenosynovectomy or hardware explantation rather than a limited incision and drainage.
• Therapeutic Drug Monitoring (TDM) for Azoles: For all patients initiated on voriconazole or posaconazole for mold infections, obtain trough serum concentrations within 5 to 7 days. Voriconazole displays non-linear pharmacokinetics and wide CYP2C19 genetic polymorphism; maintaining trough levels between 1.5 and 5.0 mcg/mL avoids therapeutic failure while preventing severe neurotoxicity.

Common Diagnostic & Therapeutic Pitfalls

• The “Trigger Finger / Carpal Tunnel” Steroid Trap: Never inject corticosteroids into an atypically swollen digit, boggy wrist, or indolent monoarthritis without prior MRI or microbiological evaluation. A single corticosteroid injection suppresses local cell-mediated immunity, leading to rapid disease explosion, extensive tendon necrosis, and spontaneous tendon rupture.
• Relying on a Negative AFB Smear: Never assume a negative acid-fast smear rules out mycobacterial infection. Musculoskeletal NTM infections are paucibacillary; smear sensitivity is <20% to 30%. A negative smear has virtually zero negative predictive value.
• Monotherapy for NTM Infections: Never initiate single-agent oral antimicrobial therapy (such as clarithromycin monotherapy) for suspected or confirmed NTM infections. Rapid emergence of high-level macrolide resistance develops within 4 to 8 weeks, rendering future curative medical regimens largely ineffective.
• Attempting DAIR in Fungal or Atypical PJI: Retaining prosthetic implants in the presence of fungal or NTM biofilm results in clinical failure rates exceeding 75% to 85%. Attempting DAIR delays definitive two-stage explantation and leads to catastrophic bone loss and soft tissue destruction.
• Immediate Primary Tendon Reconstruction: Never perform primary tendon grafting or tendon transfers in the presence of active granulomatous tenosynovitis. Reconstruction in a contaminated or inflamed bed results in immediate graft necrosis and recurrent infection. Staged reconstruction must wait until the patient completes antimicrobial therapy and remains infection-free for 3 to 6 months.

12. Controversy

Several critical diagnostic and therapeutic management strategies in fungal and atypical mycobacterial musculoskeletal infections remain subjects of active debate in orthopaedic surgery and infectious disease literature.

Prolonged versus Lifelong Antifungal Suppression in Immunocompromised Hosts

In patients with permanent immunosuppression (such as solid organ transplant recipients, individuals with advanced HIV, or those requiring lifelong anti-TNF biologic therapy for severe rheumatoid arthritis), the required duration of targeted antifungal or antimycobacterial therapy remains contentious. Proponents of stopping therapy after 12 months argue that prolonged azole exposure carries substantial risks of progressive hepatotoxicity, cardiac QTc prolongation, cumulative nephrotoxicity, and drug-drug interactions via CYP3A4 inhibition. Conversely, advocates of lifelong secondary oral suppression point to clinical relapse rates exceeding 30% to 40% when immunosuppression is resumed, particularly in coccidioidal osteomyelitis, blastomycosis, and invasive aspergillosis. Current clinical consensus increasingly favors individualized therapy: discontinuation may be trialed only if immunosuppressive medications can be substantially reduced, inflammatory markers normalize, and repeat high-resolution imaging demonstrates complete resolution of all soft tissue cold abscesses and osseous margins.

Single-Stage versus Two-Stage Revision for Fungal and Atypical PJI

While American and international consensus guidelines (IDSA, MSIS) universally endorse two-stage revision arthroplasty as the mandatory standard of care for fungal and atypical mycobacterial prosthetic joint infections, selected European specialized centres advocate for single-stage revision combined with radical debridement, targeted high-dose antifungal PMMA cement fixation, and extended systemic therapy. Proponents of single-stage revision cite reduced operative morbidity, preservation of joint bone stock, avoidance of interval joint contracture, and lower total hospitalization costs. However, critics point out that larger multi-center cohorts show recurrent infection rates exceeding 30% to 50% following single-stage revision, emphasizing that fungal biofilms (particularly Candida parapsilosis) and rapidly growing mycobacteria (M. abscessus) possess formidable adherence characteristics that rarely permit complete eradication without an interim antimicrobial-loaded spacer period.

Metagenomic Next-Generation Sequencing (mNGS) as a First-Line Modality

The clinical role and timing of metagenomic next-generation sequencing (mNGS) for diagnosing atypical musculoskeletal infections remains actively debated. Proponents emphasize its rapid 24- to 48-hour turnaround, unbiased microbial detection, and superior sensitivity (>85% to 90%) in culture-negative presentations, prior antimicrobial exposure, and paucibacillary infections. Opponents caution against routine first-line use, citing substantial financial costs, risk of false-positive reads from environmental and reagent contamination (particularly common with ubiquitous environmental NTM and fungal conidia), and lack of phenotypic antimicrobial susceptibility testing. At present, most major centers position mNGS as a high-yield second-line rescue modality in suspected granulomatous infections where conventional stains and 14-day cultures remain negative, or as an intraoperative adjunct in severely immunocompromised hosts.

13. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

14. References

• Bariteau JT, Waryasz GR, McDonnell M et al. Fungal osteomyelitis and septic arthritis. J Am Acad Orthop Surg. 2014. doi:10.5435/JAAOS-22-06-390 [Level IV: Clinical Review and Treatment Algorithm]
• Pappas PG, Kauffman CA, Andes DR et al. Clinical Practice Guideline for the Management of Candidiasis: 2016 Update by the Infectious Diseases Society of America. Clin Infect Dis. 2016. doi:10.1093/cid/civ933 [Level IV: Clinical Practice Guideline]
• Bi S, Hu FS, Yu HY et al. Nontuberculous mycobacterial osteomyelitis. Infect Dis (Lond). 2015. doi:10.3109/23744235.2015.1040445 [Level IV: Multicenter Cohort Analysis & Systematic Review]
• Bilolikar VK, Ilyas AM. Mycobacterial Infections of the Hand. Hand (N Y). 2022. doi:10.1177/1558944720940064 [Level IV: Clinical Review & Surgical Algorithm]
• Akuri MC, Bencardino JT, Peixoto JB et al. Fungal Musculoskeletal Infections: Comprehensive Approach to Proper Diagnosis. Radiographics. 2024. doi:10.1148/rg.230176 [Level IV: Multimodality Diagnostic & Imaging Review]

15. Master Class Pro-Tip

When encountering an indolent, boggy flexor tenosynovitis with rice bodies, the single most critical intraoperative decision is the systematic preservation of the A2 and A4 annular pulleys during radical synovectomy. Never sacrifice the A2 or A4 pulleys to gain exposure; doing so creates crippling flexor tendon bowstringing that cannot be reconstructed in a scarred, infected bed. Instead, perform wide unroofing through the cruciate intervals and A1, A3, and A5 windows, flex the metacarpophalangeal and interphalangeal joints to deliver the superficialis and profundus tendons into the adjacent windows, and meticulously peel the visceral epitenon under optical magnification. Flush the sub-pulley canals using a flexible 5-French pediatric catheter with pulsed saline. Prior to wound closure, ensure that fresh tissue is hand-delivered to the microbiology laboratory with explicit instructions for dual-temperature culture incubation at 28°C to 30°C and 35°C to 37°C on Lowenstein-Jensen and Sabouraud media held for a minimum of 8 weeks.

Master Class Surgical Pearl: Atypical & Fungal Musculoskeletal Infections