Prosthetic Joint Infections

Microbiology and Clinical presentation – depends on the stage of infection (based on timing after surgery). 
1) Stage I (Early Onset) - < 3 months after surgery - due to intraop contamination.  Caused by virulent organisms - Staph aureus and Gram negative rods.
2) Stage II (Delayed Onset) – 3-24 months after surgery - also due to intraop contamination, caused by less virulent organisms (hence the delayed presentation) - due to less virulent organisms, namely Coag negative staph (and Propionibacterium acnes for shoulder joint infections).
3) Stage III (Late Onset) - > 2 years after surgery - due to hematogenous seeding of the joint.  Usually caused by Staph aureus and Coag negative staph.

Diagnosis – no uniform diagnostic criteria exist!  The below criteria are quite specific, but not great in terms of sensitivity

• 2 or more positive joint cultures – basically this is diagnostic!  1 culture is enough if it is a virulent organism like Staph aureus, but for an organism like S. epidermidis, it is difficult to distinguish from contaminant.
• Purulent synovial fluid
• Acute inflammation on pathology of periprosthetic tissue
• Sinus tract communicating with prosthesis

Labs: Sometimes have elevated ESR/CRP but obviously not specific.

Imaging:

• Plain X-rays – often normal, but can show loosening of the joint – should always raise suspicion for infection!  (although can signify simple prosthesis failure as well).
• Nuclear imaging (e.g. WBC tagged scan) – can be useful, but nonspecific, especially after surgery (can light up due to periprosthetic bone remodeling
• CT/MRI – often limited due to imaging artifacts from the prosthesis

Joint Aspiration – often requires IR-guidance for aspiration.  Like for native joint septic arthritis, WBC is usually 50k – 150k with PMN predominance, but often less than this in delayed onset infections with less virulent organisms.  Gram stain is insensitive but highly specific; cultures are a bit more sensitive.

Treatment:
1)  Surgery Options:

• “One Stage” – Remove prosthesis, debride underlying bone, and place new prosthesis during the same operation.  This is done more commonly in Europe.
• “Two-stage” – Remove prosthesis, debride bone, stabilize joint with Abx-impregnated spacer.  Then treat with IV abxs for > 6 weeks before reimplanting prosthesis.  This is more commonly done in the US and likely has better efficacy in reducing reinfection/relapse, although better studies are needed.
•  Arthrodesis – after removing prosthesis, the joint can be fused – this is sometimes done if infection is due to highly resistant organisms or recurrent infections.
•  Amputation – obviously, a last resort

2)  Antibiotics:

• Long courses of abxs are needed, as mentioned above, with > 6 weeks before reimplantation of prosthesis.
• Of note is Rifampin which has a special role in treating Staphylococcal prosthetic joint infections, due to its ability to penetrate biofilms, which is used in conjunction with Vancomycin or Beta-lactams.
• Long-term suppressive Abx therapy is often needed for elderly patients who are poor surgical candidates, but without surgery this is usually unsuccessful.

(Chanu Rhee MD, 1/10/11)