The diagnosis of pediatric septic arthritis of the hip can be elusive and can create significant anxiety for the treating orthopaedic surgeon. Septic arthritis must be considered when treating a child with an atraumatic history who refuses to bear weight on the affected extremity. The differential diagnosis is extensive, and differentiation between septic arthritis and transient synovitis in a child with an acutely irritable hip can be very challenging. Furthermore, delay in diagnosis of a septic hip can be detrimental to long-term outcomes.^1,2,3 Treatment options for septic arthritis of the hip range from simple joint aspiration under ultrasonography guidance to formal anterior arthrotomy and more recently to arthroscopic lavage and decompression.⁴

Septic arthritis of the hip most commonly affects children younger than 4 years. In this age group, a thorough patient history is helpful in making the diagnosis. Parents and patients will often describe a history of progressive hip pain and increasing reluctance to bear weight or use the affected leg. In addition, parents may describe a recent illness or note that the child may be having flulike symptoms, such as general malaise and fever. There may also be a remote history of trauma, which is often noncontributory. The neonatal patient will be most challenging to identify because often the presentation is subtle, with no increase in fever or inflammatory markers. A high index of suspicion should be held if a neonate demonstrates pseudoparalysis or irritability of a particular limb.

If an effusion is present, the hip will be held in a position of flexion and obligate external rotation, to reduce intracapsular pressure. Rotation of the hip is poorly tolerated, but the degree of discomfort is related to the duration of the infection. Palpation of an effusion is not possible in most cases.

A standard AP pelvic radiograph is useful to rule out other causes of limp, such as slipped capital femoral epiphysis, bone neoplasm, chronic osteomyelitis, and Legg-Calvé-Perthes disease. Greater than 2 mm of side-to-side difference from the medial part of the femoral head to the medial part of the acetabulum is diagnostic of an effusion.¹ Ultrasonography of the hip is now a common modality for diagnosing the presence of an effusion and, in some centers, a diagnostic aspiration can be completed simultaneously. Important diagnostic blood work includes a complete blood cell count, including differential, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP) level, Lyme titer (depending on geographic region), blood culture, and antistreptolysin O titer.

Kocher et al⁵ have described a clinical prediction rule for the differentiation between septic arthritis and transient synovitis of the hip that is based on four independent, multivariate predictors of septic arthritis of the hip: a history of fever (temperature >39.5°C), non-weight bearing, ESR greater than 40 mm/hr, and a serum white blood cell count greater than 12,000 cells/mm³ (>12.0 × 10⁹/L). More recently, Caird et al⁶ demonstrated the utility of an elevated CRP level (>20 mg/L) as a fifth independent predictor in diagnosing septic arthritis. At the authors’ institution, a combination of Kocher’s criteria and CRP level is used to help make the diagnosis of septic arthritis. In addition to these predictors, an aspiration of the hip joint (either in the radiology suite or the operating room) can be diagnostic. A well-accepted definition of septic arthritis is a joint aspirate with a white blood cell count greater than 50,000 cells/mm³ (50.0 × 10⁹/L). Conversely, a joint aspiration with a white blood cell count under 25,000 cells/mm³ (25.0 × 10⁹/L) has been universally accepted to reflect an inflammatory process, such as transient synovitis. Aspirations that reveal a white blood cell count between 25,000 and 75,000 cells/mm³ are more challenging for the orthopaedic surgeon; in these situations, a combination of clinical examination, inflammatory markers, and aspiration results help guide the surgeon in decision making.⁷ At our institution, when the diagnosis is still uncertain, MRI of the affected
joint is performed to help differentiate between septic arthritis and osteomyelitis of the proximal femur or the acetabulum.

Kocher’s criteria for differentiating a septic hip have been applied at other institutions with variable results.^2,3,4 This has led authors to conclude that using a clinical prediction algorithm to differentiate between septic arthritis and transient synovitis has improved overall care for children. Despite these excellent prediction rules, the ultimate decision for surgical incision and drainage should be based on clinical judgment.