What’s causing your young patient’s hip pain?

FIGURE 1
Ober’s test for iliotibial band syndrome

Hip pain and infection? Consider septic arthritis

Is your patient febrile or recovering from an infection? When you suspect an acute infectious cause of hip pain, there are 2 disorders to consider in the differential diagnosis: transient synovitis (TS) and septic arthritis. Both are associated with an acute onset of pain and limping or non-weight bearing, and generally affect young children: Septic arthritis is most common in children between the age of 3 and 6 years, while the typical age range for TS is 3 through 8.^1-4

There are other important differences. TS is a benign, self-limited, and common cause of hip pain in young children,⁵ which may be preceded by a viral infection.¹ Septic arthritis is a serious condition that requires rapid identification; a delay in treatment can cause significant long-term morbidity. Complications may include early arthritis, shortened limb, dislocation, and osteonecrosis.

Because it can be difficult to distinguish between septic arthritis and TS, multiple studies have looked at the best way to make that determination. ^1-7

The criteria most commonly used to identify septic arthritis are:

• fever
• refusal to bear weight
• erythrocyte sedimentation rate (ESR) >40 mm/h
• white blood cell count >12,000 mm³.

One study showed a predictive probability of 97% for septic arthritis when 3 of the 4 above criteria were present, and 99% if the patient had all 4.^2,4 Other studies added 2 additional criteria—C-reactive protein (CRP) >1 mg/dL and medial joint space widening of >2 mm—with similar predictive probabilities (93% and 99%, respectively, if 3 or 4 of the 6 criteria were positive).^2,4,6

Order lab work (ESR, CRP, complete blood count, and blood cultures) for any young patient whose hip pain is thought to be associated with an infectious process. Obtain hip anterior/posterior (AP) and frog lateral radiographs to look for joint space widening, as well.^1-4

In addition to the above criteria, suspect septic arthritis in a child who presents with acute onset of hip pain, looks ill, and has limited ROM—particularly with internal rotation. In contrast, children with TS do not look sick and, while they often have limited ROM, it is primarily just at extremes. A modified log roll test can be used to assess the rotation of the hip.

An ultrasound-guided aspiration of fluid is the gold standard for septic arthritis diagnosis. Treatment is emergent surgical drainage and parenteral antibiotics, which should be withheld until fluid is aspirated or the joint is surgically drained. The most common causative organism is Staphylococcus aureus.^3,7

It is reasonable to withhold joint aspiration for patients who don’t look sick and have normal labs and reliable follow-up, and to watch closely, with conservative management including nonsteroidal anti-inflammatory drugs (NSAIDs) and activity, as tolerated, instead.

There is some concern that TS may lead to Legg-Calves-Perthe’s disease, also known as avascular necrosis of the femoral head, although there is no strong evidence to support it. To be safe, however, obtain radiographs 2 to 18 months after TS resolves to check for further damage or complications.^1,8

Femoral head disorders: When x-rays help, when they don’t

There are 2 femoral head disorders that commonly affect children and adolescents: Avascular necrosis of the femoral head and slipped capital femoral epiphysis (SCFE). Age alone is a clue to detection; children with avascular necrosis tend to be younger than those with SCFE.

Avascular necrosis of the femoral head is most commonly seen in children between the ages of 4 and 8 years, and occurs more frequently in boys than in girls. Although there is disagreement about the etiology of avascular necrosis, the condition is thought to result from a disruption in femoral blood supply, leading to osteonecrosis and flattening of the weight-bearing surface of the femoral head. One study found that structural abnormalities in the epiphyseal cartilage may lead to collapse.⁹

The condition often begins with a painless limp that develops—typically in 1 to 3 months—into groin, thigh, or knee pain; worsens with activity; and is relieved with rest.⁵ Abduction and internal rotation are limited on exam.

In patients with avascular necrosis, hip AP and frog lateral radiographs are diagnostic, with evidence of increased density of 1 epiphysis and flattening or fragmentation of the femoral head.⁷ When you suspect avascular necrosis, advise the patient to avoid weight bearing—and provide a referral to a pediatric orthopedist.