Acute septic arthritis of the knee can lead to joint damage or sepsis, if early diagnosis and treatment fail to occur, which includes drainage of the joint, adequate antibiotic coverage and resting of the knee. Classically, drainage of the knee was performed either with multiple aspirations or open arhtrotomy. The arthroscopic approach has becoming widely accepted, as it allows adequate drainage of the pus and debridement with partial or total sinovectomy of the joint.

The aim of this study was to evaluate the differences between arthroscopy and open arthrotomy in the clinical outcomes and rate of recurrence in patients with septic arthritis of the knee joint.

We reviewed patients with acute septic arthritis of the knee admitted in our center between January 2010 and December 2014. The criteria for diagnosis was report of purulent material when arhtrotomy or arthroscopy was performed or a positive culture of the joint fluid. Patients with recent surgery or documented osteomyelitis of the femur or tíbia were excluded.

We used the Oxford Knee Score (OKS) to classify the clinical outcomes in the end of follow-up, and registered the rate of recurrence in each group. The statistical evaluation of the results was performed using Student's t-test.

65 patients were treated during this period, 37 by an open arthrotomy through a lateral supra-patellar aproach, and 28 by arthroscopy through 2 standard anterior portals. All the patients were imobilized with a cast or orthosis in the immediate post-operation period for a mean period of 13 days in the arthrotomy group (8–15) and 9 days in the arthroscopy group (6–12) and received endovenous antibiotics for at least 10 days, followed by oral antibiotics for a mean total of 36 days in the the arthrotomy group (30–48) and 32 days in the arthroscopy group (22–36). The mean follow-up was 22 months in the arthrotomy group (8–28 months) and 18 months in the arthroscopy group (14–24). The mean OKS was 31 in the the arthrotomy group (21–39) and 35 in the arthroscopy group (25–44). There was 1 recurrence in the arthrotomy group and 1 recurrence in the arthroscopic group, both managed by knee arthrotomy.

Drainage is a key step in treatment of knee pyoarthrosis, either through an open or an arthroscopic approach. Both seem to be equally effective, with no significant statistical difference in terms of recurrence. The functional results tend to favour the arthroscopic approach, but with no statistical significance.

Acute septic arthritis of the knee may be a challenging diagnosis in the emergency department and must always be excluded in any patient with knee pain and local or systemic signs of infection. Arthrocentesis of the suspected knee is mandatory, since the analysis of the synovial fluid gives useful information like the white blood cell count (WBC)/mm3 or the polymorphonuclear cell percentage (PMP). These parameters will help the clinician to make the decision to drain the joint in the operation room, without having to wait for the culture or Gram stain, which may take several days to be available. The classical cutoff of 50,000 WBC/mm3 with more than 90% of PMP may fail to include all the septic arthritis of the knee, since significant variation have been described in recent years. The aim of this study was to evaluate the accuracy of WBC/mm3 and PMP in the synovial fluid in the diagnosis of acute septic arthritis of the knee.

We reviewed the clinical data of patients diagnosed with acute septic arthritis of the knee admitted in our center between January 2010 and December 2014, specifically the WBC/mm3 and the PMP of the synovial joint fluid. The criteria for diagnosis of an acute septic arthritis of the knee was report of purulent material when arhtrotomy or arthroscopy was performed or a positive culture of the joint fluid. The statistical evaluation of the results was performed using Student's t-test.

48 patients matched the inclusion criteria. The mean WBC/mm3 was 44.333 (14.610–182.640) and the mean PMP was 91,89% (86,4%–98,1%). 28 patients (58,33%) had a WBC/mm3 below 50.000 and 44 patients (91,67%) had a PMP above 90%, both with no statistical significance.

Knee arthrocentesis is mandatory in every patient suspected to have an acute knee pyoarthrosis, since the joint fluid analysis may show several abnormal findings. Our results show that a considerable number of patients may show a relatively low WBC/mm3 in the joint fluid in the presence of a knee pyoarthrosis. The PMP may be a better criteria, but again failed to achieve statistical significance, probably because of the low number of patients. The synovial fluid analysis alone is probably misleading in the diagnosis of an acute septic arthritis of the knee if the clinician is guided by the classical guidelines. The physical examination, medical history, laboratory and imagiologic tests are all key elements in this challenging diagnosis.