Title : Rare cause of septic arthritis: Knee infection by streptococcus sanguinis post intra-articular injection
Abstract:
Objectives: Septic arthritis is a potentially devastating complication of intra-articular injections, typically caused by Staphylococcus aureus or β-hemolytic streptococci. Infections due to viridans streptococci are exceedingly rare. Among them, Streptococcus sanguinis, an oral commensal frequently associated with endocarditis, has only rarely been described as a pathogen in septic arthritis. We present such a case to raise awareness of this rare cause of septic arthritis.
Methods: A 68-year-old female developed acute knee septic arthritis 10 days after an intra-articular injection (hyaluronic acid to treat chronic knee arthritis). On admission (18/07/2025), she presented with fever, severe pain, joint swelling and restricted range of motion. Arthroscopic lavage was performed, and empiric therapy with vancomycin, clindamycin, and ciprofloxacin was initiated. Synovial fluid cultures yielded Streptococcus sanguinis. No oral or dental pathology was identified as a possible source of bacteremia. Based on susceptibility testing, treatment was switched to tigecycline and benzylpenicillin. The patient showed progressive clinical improvement and was discharged on 30/07/2025 with oral amoxicillin/clavulanate and minocycline for two more weeks. The patient was regularly reevaluated through clinical examinations and laboratory tests.
Results: On the last follow-up examination, her knee showed no effusion, maintained good joint function and CRP levels were within the normal range. She reported occasional pain and difficulty in walking, attributed to her underlying knee osteoarthritis. Streptococcus sanguinis septic arthritis is very uncommon, with only sporadic cases described in the literature. The temporal association with intra-articular injection suggests either direct inoculation or hematogenous spread from an unrecognized oral source. This case underscores the importance of prompt surgical intervention, broad empiric coverage, and subsequent adjustment according to culture results.
Conclusion: Streptococcus sanguinis has only exceptionally been described as a pathogen in septic arthritis. Its occurrence after intra-articular injection highlights the need for strict aseptic technique. Clinicians should consider unusual organisms such as S. sanguinis in post-procedural septic arthritis. Early microbiological confirmation and targeted therapy are critical for favorable outcomes.
