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Septic Arthritis

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Septic arthritis\: infection and in
S t a p h y l o c o c c u s a u r e u s via direct inoculation or haematogenous spread. Consider in cases of a red, hot, painful, swollen
joint with restricted movement. An orthopaedic emergency with high morbidity and mortality.
Epidemiology\: approximately 6 cases per 100,000 per year in developed countries, peaking in those over 70. Prosthetic
joint infection has a higher incidence at 70 cases per 100,000 per year.
Aetiology\: mainly bacterial (91%), with S t a p h y l o c o c c u s a u r e u s as the most common cause. Consider N e i s s e r i a g o n o r r h o e a
in sexually active patients, gram-negative organisms and M y c o b a c t e r i u m t u b e r c u l o s i s in immunosuppressed patients and
those from high TB prevalence areas, MRSA in nursing home residents, recent hospital discharges, or those with indwelling
catheters.
Transmission\: typically haematogenous spread, but can occur via direct inoculation or contiguous spread from local
infections. Commonly a
Pathophysiology\: involves bacterial colonisation, immune response triggering in
resultant bone and cartilage necrosis due to increased intra-articular pressure and compressed blood supply.
Risk factors\: include immunosuppression (HIV, medications), diabetes, alcohol use disorder, sickle cell disease, rheumatoid
arthritis, osteoarthritis, crystal arthritides, joint prosthesis, cutaneous ulcers, chronic skin infections, IV drug use, and intra-
articular corticosteroid injections.
Clinical features\: presents as a hot, swollen, tender joint with restricted movement, often in a single joint (80% cases,
mostly knee). Symptoms include joint pain, swelling, fever, sweats, and rigors. Important history elements include past
medical, drug, social, and sexual history.
Examination\: look for swelling and erythema, feel for warmth, tenderness, joint pain, e
and function.
Di
trauma/haemarthrosis. Diagnosis con
Investigations\: joint arthrocentesis for synovial
bedside tests (urinalysis, swabs), lab tests (FBC, CRP, ESR, blood cultures, U&Es, LFTs), imaging (X-ray, ultrasound, MRI).
Management\: hospital admission, urgent joint aspiration, empirical IV antibiotics (
directed therapy, potential sepsis six protocol initiation. Follow up with orthopedic referral for inaccessible joints and
prosthetic joint cases.
Complications\: include joint destruction, osteomyelitis, sepsis, and antibiotic allergy. 11% case fatality rate for single-joint
septic arthritis, with worse outcomes in older patients, underlying joint disease, or joint prosthesis. Prompt and adequate
treatment essential to prevent irreversible damage.
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A comprehensive topic overview

Introduction

Septic arthritis is de
The most common cause of infection is S t a p h y l o c o c c u s a u r e u s , which enters the joint through direct inoculation or
haematogenous spread from a di
1
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Septic arthritis is an important di
arthritis is an orthopaedic emergency with substantial morbidity and mortality.
2
Septic arthritis can occur in adults and children and occur in native and prosthetic joints. However, to give information more
appropriate for medical students, this article has been written to provide the information needed to recognise, diagnose,
and treat cases of native joint septic arthritis in adults.

Epidemiology

Overall, septic arthritis is relatively rare, with around 6 cases per 100,000 per year in developed countries and has a peak
incidence in those over the age of 70.
3
Septic arthritis resulting from prosthetic joint infection (PJI) is an important complication of joint replacement and has an
incidence around 10 times higher than in native joints, with an incidence of around 70 cases per 100,000 per year.
4

Aetiology

Septic arthritis can rarely be caused by viral, fungal or parasitic species. However, most cases are caused by bacterial
infection. Around 91% of cases are caused by staphylococci or streptococci, with S t a p h y l o c o c c u s a u r e u s cited as the most
common causative agent, followed by S t r e p t o c o c c u s p n e u m o n i a e .
1
Despite this, it is important to consider alternative causative agents in the following groups\:
1
Sexually active (young) patients\: N e i s s e r i a g o n o r r h o e a
Immunosuppressed patients\: gram-negative organisms, M y c o b a c t e r i u m t u b e r c u l o s i s
Patients from areas of high TB prevalence\: M y c o b a c t e r i u m t u b e r c u l o s i s
Nursing home residents, patients recently discharged from hospital, patients with leg ulceration or indwelling catheters\:
methicillin-resistant S t a p h y l o c o c c u s a u r e u s (MRSA)
Less common bacterial causes include S t r e p t o c o c c u s p y o g e n e s , H a e m o p h i l u s i n

Transmission

Any joint (
several routes for the infectious agent to invade the joint.
Haematogenous spread is the most common, from distant abscesses, wounds, respiratory tract infections or sexually
transmitted infections. This method of transmission is further facilitated by neovascularisation resulting from previous joint
disease and in
Direct inoculation can be caused by joint injections, arthrocentesis, arthroscopic surgery, trauma, foreign objects, and
infected wounds.
Bacteria may also undergo contiguous spread from local infections, such as osteomyelitis, septic bursitis, and abscesses.

Pathophysiology

arthritis\:
5
Research suggests a strong pathogenic role for both the host immune response and the invading bacteria in septic
1. Bacterial colonisation\: bacteria enter and colonise the joint space, a process facilitated by the increased levels of adhesion
proteins resulting from previous joint in
2. Immune response\: synovial cells and macrophages detect pathogen-associated molecular patterns (PAMPs) and induce
an immune response, recruiting macrophages, T cells and B cells. Underlying joint disease impairs the phagocytic and
bactericidal properties of healthy synovial cells and
3. Acute in
cells, and histamine release from mast cells, which stimulates vasodilation and increases vascular permeability.
4. Bacterial factor release\: toxins, enzymes, adhesins and cell wall proteins are released into the joints space. These can
exacerbate the immune response, causing more severe in
destruction.
5. Bone and cartilage necrosis\: articular destruction resulting from the immune response, bacterial factors and the resulting
increase in intra-articular pressure which compresses the blood supply to the joint.
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Risk factors

Risk factors for septic arthritis can be categorised as those predisposing an individual to infection, those providing
opportunity for the pathological agent to gain access to the joint, and a past medical history of joint disease or
in
an impaired synovial immune response.
6,7

Predisposition to infection

Risk factors include\:
Immunosuppression (medication, HIV/AIDS)
Diabetes mellitus
Alcohol use disorder
Sickle cell disease (joint in

Past medical history

Risk factors include\:
Rheumatoid arthritis
Osteoarthritis
Crystal arthritides
Joint prosthesis (through contamination during surgery, wound infection or otherwise disruption to the normal joint
defences)

Pathogen access

Risk factors include\:
Cutaneous ulcers
Chronic skin infections
Intravenous drug use
Intra-articular corticosteroid injections

Clinical features

History

Septic arthritis classically presents as an acutely hot, swollen, and tender joint with restriction of both active and passive
movement.
In 80% of cases, this presents in a single joint, most commonly the knee (53%), followed by the hip, shoulder ankle and
wrists.
Typical symptoms of septic arthritis include\:
7
Joint pain (70-80%)
Joint swelling (71-85%)
Fever (52-62%)
Sweats (20-34%)
Rigors (15-24%)
Other important areas to cover in the history include\:
Past medical history\: identi
Drug history
Social history
Sexual history\: gonococcal arthritis is an important di

Clinical examination

A clinical examination of the a
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Typical clinical
Look\: swelling and erythema
Feel\: warmth, joint pain & tenderness (particularly on knee
e
Move\: reduced active and passive range of motion, reduced function (inability to walk)

Di

Table 1 lists alternative diagnoses for a hot, swollen, tender joint. Many of these conditions present di
septic arthritis and can be excluded with synovial . Fluid analysis in other conditions will show no micro-
organisms and reveal the presence of crystals, in the case of gout and pseudogout.
Table 1. Di
10
Diagnosis Di
Gout
Previous history of gout
Symptoms presenting in foot joints
Pseudogout Previous history of pseudogout
Osteoarthritis
Previous history of osteoarthritis
Symptoms
Bilateral symptoms
No systemic symptoms
Presence of bony deformities
Involvement of small joints
Rheumatoid
arthritis
Previous history of rheumatoid arthritis
Symptoms
Deformity (swan neck, Boutonniere’s,
ulnar deviation)
Rheumatoid nodules
Reactive
arthritis
Psoriatic
arthritis
Asymmetrical and polyarticular
HLA-B27 genetic association
Polyarticular
Cutaneous manifestations (psoriasis)
Dactylitis (DIP joints)
Trauma/haema
rthrosis
Previous history of bleeding diathesis
Traumatic event
Joint aspiration reveals blood

Investigations

The British Society of Rheumatology recommends a low threshold for suspecting septic arthritis, stating that a presentation
of a hot, swollen, tender joint with restricted movement should be considered as septic arthritis until proven otherwise.
This necessitates prompt joint arthrocentesis for synovial
8

Joint arthrocentesis

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Arthrocentesis should be performed in a sterile environment by a clinician with adequate expertise using a close-needle
approach. Hip involvement will require referral to orthopaedics for ultrasound-guided aspiration, and prosthetic
involvement will require an orthopaedic surgeon to perform the procedure in a sterile operating theatre.
Aspirate samples should be sent for crystal microscopy, gram staining, culture, and sensitivity testing. Immediate
management should involve empirical antibiotics according to local protocols.
Synovial
A positive synovial (suggestive of septic arthritis) may have the following features\:
Appearance\: yellow/green on aspiration (as opposed to clear and colourless when uninfected)
White cell count\: raised (particularly neutrophil count), though this is not 100% sensitive or speci
raised in other arthropathies
Culture\: identi
A negative culture does not exclude the diagnosis and may necessitate the use of alternative testing, depending
upon the risk factors present and the clinical impression of the causative organism.

Bedside investigations

Relevant bedside investigations include\:
Urinalysis\: raised nitrites in urinary tract infections
Swabs (oropharynx, vagina, cervix, urethra, anus/rectum)\: presence of N . g o n o r r h o e a

Laboratory investigations

Relevant laboratory investigations include\:
Full blood count\: white cell count is raised in 50% of cases, but is not 100% sensitive or speci
CRP/ESR\: may be elevated or normal, this can also be used to monitor treatment response
Blood culture\: these should always be taken, but a negative result does not exclude a diagnosis
U&Es\: may be deranged due to sepsis and can be used as a baseline prior to antibiotic therapy
LFTs\: can be used as a baseline prior to antibiotic therapy

Imaging

Relevant imaging investigations include\:
X-ray\: not diagnostic but recommended as a baseline and to reveal degenerative changes, chondrocalcinosis and
underlying joint disease
Ultrasound\: not diagnostic but can be used to guide hip aspiration
MRI\: low speci

Diagnosis

The Kocher criteria is a useful tool to determine the probability of septic arthritis.
Table 2. The Kocher criteria for diagnosing septic arthritis.
9
Finding Score
Non-weight-bearing on the a
side
Erythrocyte sedimentation rate >
40mm/hr
+1
+1
Fever > 38.5 °C +1
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White blood cell count > 12,000
cells/mm
3
+1
Interpretation of the Kocher criteria is as follows\:
Score of 1\: 3% probability of septic arthritis
Score of 2\: 40% probability of septic arthritis
Score of 3\: 93% probability of septic arthritis
Score of 4\: 99% probability of septic arthritis

Management

Patients with suspected septic arthritis are at risk of developing sepsis, irreversible joint damage and death.
These patients should be admitted to hospital in the
Society for Rheumatology suggest urgent joint aspiration to dryness (for investigation and symptoms relief) follow by
immediate empirical antibiotic therapy.
8,10
Consider initiation of the sepsis six protocol if initial assessment suggests systemic bacteraemia.

Synovial

A
If the joint is accessible, this may be performed using anatomical landmarks.
Inaccessible joints, including the hip, will require orthopaedic referral and consideration of ultrasound-guided aspiration,
or arthrocentesis in a sterile operating theatre. Prosthetic joints require urgent orthopaedic referral for surgical
arthrocentesis and washout.
10

Antibiotic therapy

Patients should be given empirical antibiotic treatment, often intravenous
immediately following synovial
Culture and sensitivity results will inform further antibiotic therapy. The speci
required for medical students, however, brief recommendations from the BNF are listed below.
Table 3. Antibiotic therapy in septic arthritis.
11
Pathogen Antibiotic
S t a p h y l o c o c c u s a u r e u s (methicillin-
Flucloxacillin
sensitive)
Clindamycin if penicillin-allergic
S t r e p t o c o c c u s s p p
N e i s s e r i a g o n o r r h o e a e Cefotaxime or ceftriaxone
Salmonella (non-typhi) Amoxicillin
Salmonella (typhi/paratyphi) Ceftriaxone
S t a p h y l o c o c c u s a u r e u s (methicillin-
resistant)
Vancomycin or teicoplanin

Complications

Septic arthritis carries signi
12
Poor outcomes have been shown to be associated with older age, underlying joint disease, and joint prosthesis.
Treatment must be adequate and prompt to avoid irreversible joint destruction and disability.
13
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Complications associated with septic arthritis and its management include\:
10
Joint destruction\: in
be required for surgical solutions.
Osteomyelitis\: the infection may spread contiguously to the surrounding bone. This may be seen on MRI and will require
advice from infectious disease and orthopaedic consultants.
Sepsis\: the colonising bacteria may enter the bloodstream, causing bacteraemia, sepsis, and septic shock. Initiation of
the sepsis six protocol should be seriously considered in septic arthritis patients.
Antibiotic allergy\: antibiotic therapy should avoid patient allergies.

References

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N o c h a n g e s i n t h e d i s t r i b u t i o n o f o r ga n i s m s r e s p o n s i b l e f o r s e p t i c a r t h r i t i s o v e r a 2 0 y e a r p e r i o d . 2002.
NICE CKS. K n e e p a i n - a s s e s s m e n t . July 2017. Available from\: [LINK]
Tarkowski, A. I n f e c t i o n a n d m u s c u l o s k e l e t a l c o n d i t i o n s \: I n f e c t i o u s a r t h r i t i s . 2006. Available from\: [LINK]
Kaandorp, Carola JE, et al. I n c i d e n c e a n d s o u r c e s o f n a t i v e a n d p r o s t h e t i c j o i n t i n f e c t i o n \: a c o m m u n i t y b a s e d p r o s p e c t i v e
s u r v e y . 1997. Available from\: [LINK]
Mathews, Catherine J., et al. B a c t e r i a l s e p t i c a r t h r i t i s i n a d u l t s . 2010. Available from\: [LINK]
Kaandorp, Carola JE, et al. R i s k f a c t o r s f o r s e p t i c a r t h r i t i s i n p a t i e n t s w i t h j o i n t d i s e a s e . 1995. Available from\: [LINK]
Margaretten, Mary E., et al. D o e s t h i s a d u l t p a t i e n t h a v e s e p t i c a r t h r i t i s ? Available from\: [LINK]
Coakley, G., et al. B S R & B H P R , B O A , R C G P a n d B S A C gu i d e l i n e s f o r m a n a ge m e n t o f t h e h o t s w o l l e n j o i n t i n a d u l t s . 2006.
Available from\: [LINK]
Kocher, Mininder S., David Zurakowski, and James R. Kasser. D i
t h e h i p i n c h i l d r e n \: a n e v i d e n c e-b a s e d c l i n i c a l p r e d i c t i o n a l g o r i t h m . Available from\: [LINK]
BMJ Best Practice. S e p t i c a r t h r i t i s . 2021. Available from\: [LINK]
BNF British National Formulary. M u s c u l o s k e l e t a l s y s t e m i n f e c t i o n s , a n t i b a c t e r i a l t h e r a p y . 2021. Available from\: [LINK]
Kaandorp, Carola JE, et al. I n c i d e n c e a n d s o u r c e s o f n a t i v e a n d p r o s t h e t i c j o i n t i n f e c t i o n \: a c o m m u n i t y b a s e d p r o s p e c t i v e
s u r v e y . 1997. Available from\: [LINK]
Kaandorp, Carola JE, et al. T h e o u t c o m e o f b a c t e r i a l a r t h r i t i s . A p r o s p e c t i v e c o m m u n i t y_‐_b a s e d s t u d y . Available from\: [LINK]

Reviewer

Dr Steve Laird
Consultant Physician in Infectious Disease

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