Orthopaedic Surgery - Hip Transient Synovitis

Basics

Transient synovitis of the hip is a self-limited inflammatory condition characterized by acute unilateral hip pain, limp, and temporary restriction of hip motion.

It is one of the most common causes of acute hip pain in children.

The most important diagnostic challenge is distinguishing it from septic arthritis, which requires urgent treatment.

Symptoms usually improve progressively over several days and resolve completely within a few weeks.

Synonyms

Transient synovitis is also known as:

Toxic synovitis or irritable hip.

Epidemiology

Transient synovitis is generally considered the most common cause of acute hip pain in children.

However, it remains a diagnosis of exclusion, meaning that infection and other important structural disorders must be ruled out before the diagnosis is accepted.

Age

The condition may occur from infancy through adolescence, but most cases occur between approximately 3 and 8 years of age.

Cases have been reported as early as about 9 months.

Lifetime Risk

The estimated probability that a child will experience at least one episode is approximately 1–3%.

Laterality

The right and left hips are affected with approximately equal frequency.

Symptoms are typically unilateral.

Sex

Boys are affected more frequently than girls, with a male-to-female ratio of approximately 2:1.

Incidence

Transient synovitis has historically accounted for approximately 0.5% of annual pediatric orthopaedic admissions in some series.

Reported incidence varies among populations.

Risk Factors

Recognized associations include:

Male sex and a recent upper respiratory or other minor infection.

Genetics

Transient synovitis is not considered an inherited disorder.

No consistent genetic predisposition has been established.

Etiology

The exact cause remains uncertain.

The condition is thought to represent a temporary, noninfectious inflammatory response of the hip synovium, possibly mediated by the immune system.

Recent Infection

A preceding or concurrent illness is commonly reported.

Associated conditions have included:

Upper respiratory infection, pharyngitis, and otitis media.

Older studies reported a recent nonspecific infectious illness in as many as 70% of affected children.

Importantly, the hip itself is not infected in uncomplicated transient synovitis.

Minor Trauma

Minor trauma has been reported in up to approximately 30% of cases.

It is uncertain whether trauma directly causes the synovitis or merely draws attention to an already-irritable hip.

Allergic Predisposition

A history of allergic or hypersensitivity tendencies has been described in a minority of patients, historically up to approximately 25%.

Associated Conditions

The most commonly associated clinical history is a recent upper respiratory tract infection.

Diagnosis

Signs and Symptoms

The classic presentation is the acute onset of unilateral hip pain in an otherwise well child.

Pain Location

Pain is usually felt in the:

Groin or hip, but may be referred to the anterior thigh or knee.

Because pediatric hip pathology commonly presents as knee pain, the hip should always be examined in a child with unexplained knee discomfort or limp.

Limp

A limp or antalgic gait is common.

Some children refuse to bear weight on the affected side because of pain.

Resting Position

The child may hold the hip in slight:

Flexion and external rotation, which reduces capsular tension and may be more comfortable.

Range of Motion

Hip motion is painful and restricted, particularly:

Internal rotation, abduction, and extension.

Despite discomfort, the hip often retains a substantial portion of its normal passive range when examined slowly and gently.

Fever

A low-grade temperature may occasionally occur.

High fever should increase concern for septic arthritis or another infection.

Physical Examination

The child usually appears less toxic and less distressed than a patient with septic arthritis.

Nevertheless, substantial overlap may occur, and clinical appearance alone cannot reliably exclude infection.

Kocher Criteria

The Kocher criteria were developed to estimate the likelihood of septic arthritis rather than transient synovitis in a child with an irritable hip.

The classic criteria are:

Refusal to bear weight

Temperature greater than 38.5°C

Peripheral white blood cell count greater than 12,000/mm³

ESR greater than 40 mm/hr

A subsequently described modification also incorporates C-reactive protein, particularly a substantially elevated CRP.

The greater the number of concerning findings, the greater the suspicion for septic arthritis.

These criteria support clinical judgment but should not be used as an absolute rule.

Hip Examination

The child usually localizes discomfort to the:

Groin, anterior thigh, hip, or knee.

Passive range of motion is typically painful and somewhat restricted.

Compared with septic arthritis, however, the child with transient synovitis generally has less severe pain with gentle passive motion.

Preserved Motion

When examined slowly, many patients retain at least approximately 50% of normal hip motion.

Severe pain with nearly all passive movement should raise concern for infection.

Gait

When the child can walk, an antalgic limp is common.

Some children refuse to walk entirely during the acute phase.

Muscle Atrophy

Ipsilateral thigh or gluteal muscle atrophy is unusual.

If present, it suggests that symptoms have existed longer than expected and should prompt consideration of an alternative diagnosis such as:

Legg–Calvé–Perthes disease, chronic infection, neuromuscular disease, or other longstanding pathology.

Laboratory Tests

Laboratory results in transient synovitis are generally normal or only mildly abnormal.

Their principal value is helping to exclude infection and other diagnoses.

White Blood Cell Count

The peripheral leukocyte count is usually normal or only mildly elevated.

ESR

The ESR may be normal or mildly elevated.

Older series reported average values around 20 mm/hr.

A marked elevation raises greater concern for infection or inflammatory disease.

C-Reactive Protein

CRP is typically normal or only minimally elevated.

A substantial elevation should prompt careful reconsideration of septic arthritis or osteomyelitis.

Additional Tests

Other studies such as:

Urinalysis, blood cultures, rheumatoid testing, Lyme serology, or tuberculosis testing

are generally normal in transient synovitis and should be ordered only when the clinical context suggests another diagnosis.

Joint Aspiration

Routine aspiration is not required when the clinical picture strongly favors transient synovitis and infection is unlikely.

However, aspiration should be considered when septic arthritis cannot be excluded.

Synovial Fluid

When aspirated, the joint usually contains a small sterile effusion, historically around 1–5 mL, with negative bacterial cultures.

Synovial-fluid testing is nonspecific except that it helps exclude infection.

Imaging

Plain Radiographs

AP and lateral radiographs of the hip or pelvis may be obtained.

In uncomplicated transient synovitis, radiographs are typically normal or nonspecific.

Their principal purpose is to exclude other pathology such as:

Legg–Calvé–Perthes disease, fracture, or SCFE in older children.

Ultrasound

Ultrasound is useful for detecting a hip joint effusion.

It can also guide diagnostic aspiration when infection remains a concern.

However, the presence of an effusion alone does not distinguish transient synovitis from septic arthritis.

MRI

MRI is not routinely required.

It may be useful when:

Pain persists longer than expected, symptoms recur, radiographs are unrevealing, or infection and other serious diagnoses have been excluded but uncertainty remains.

MRI can assess the femoral head, bone marrow, joint, and surrounding soft tissues.

Pathological Findings

Synovial biopsy, when historically performed, demonstrates nonspecific nonpyogenic synovial hypertrophy and inflammation.

No characteristic bacterial process is present.

Differential Diagnosis

Transient synovitis should only be diagnosed after clinically important alternative disorders have been considered.

Septic Arthritis

This is the most important diagnosis to exclude.

Septic arthritis often presents with:

More severe pain, fever, refusal to bear weight, marked restriction of passive motion, and elevated inflammatory markers.

Urgent aspiration and surgical management may be necessary.

Osteomyelitis

Infection of the proximal femur or pelvis may mimic an irritable hip and should be considered in children with persistent pain or systemic features.

Tuberculous Arthritis

Chronic tuberculous infection may cause prolonged hip pain, stiffness, and constitutional symptoms.

Psoas Abscess

An iliopsoas abscess can cause hip flexion posture, pain with extension, fever, and limp.

Pyomyositis

Infection of muscles around the hip may produce pain, swelling, fever, and restricted movement.

Juvenile Idiopathic Arthritis

Inflammatory arthritis should be considered when symptoms are prolonged, recurrent, bilateral, or associated with other involved joints.

Acute Rheumatic Fever

Migratory joint symptoms in the appropriate systemic context may suggest rheumatic fever.

Legg–Calvé–Perthes Disease

Perthes disease can initially resemble transient synovitis, especially during early stages when radiographs may still be subtle or normal.

Persistent or recurrent symptoms should raise suspicion.

Tumor

Bone or soft-tissue tumors should be considered when pain is persistent, progressive, nocturnal, or associated with systemic symptoms.

Slipped Capital Femoral Epiphysis

SCFE should be considered especially in an older, overweight child or adolescent with hip, thigh, or knee pain and obligatory external rotation during hip flexion.

Hip Dislocation

Traumatic dislocation is generally apparent from the mechanism, deformity, and imaging.

Sacroiliac Joint Infection

Septic sacroiliitis may cause buttock or hip-region pain and gait disturbance.

Treatment

General Principles

Transient synovitis is usually self-limited and requires primarily rest, symptom control, and close observation until infection is confidently excluded.

Natural History

Symptoms commonly improve substantially within several days.

The average symptomatic period is often less than 1 week, although mild discomfort or stiffness may occasionally persist longer.

Most children recover completely without clinical or radiographic sequelae.

Rest

During the painful phase, the child should reduce activity.

Short-term bed rest or relative rest may be appropriate until:

Pain decreases and comfortable hip motion returns.

Weight Bearing

Weight bearing should be limited while significant pain is present.

Normal walking can gradually resume once the child is comfortable and has regained near-full range of motion.

Strenuous activity should be delayed somewhat longer.

Traction

Routine traction is not necessary.

Historically, traction has been used for comfort in particularly painful cases.

If used, the hip may be positioned in approximately 30° of flexion to reduce intra-articular pressure and improve comfort.

Joint Aspiration

Routine aspiration is unnecessary in a typical low-risk presentation.

It becomes appropriate when the possibility of septic arthritis remains significant.

NSAIDs

NSAIDs may be used to reduce pain and inflammation.

Some clinicians historically avoided early anti-inflammatory medication out of concern that symptom improvement might mask infection.

In practice, NSAID response alone should never be used to distinguish transient synovitis from septic arthritis.

Physical Therapy

Formal physical therapy is usually unnecessary.

Parents can generally supervise gradual return to normal activity once symptoms resolve.

Persistent stiffness or weakness should prompt reevaluation rather than routine prolonged therapy.

Follow-Up

Prognosis

The prognosis is excellent.

Transient synovitis is usually self-limiting and resolves without clinically important long-term consequences.

Relationship to Perthes Disease

Some older studies reported subsequent diagnosis of Legg–Calvé–Perthes disease or femoral-head osteonecrosis after an apparent episode of transient synovitis.

This probably reflects, at least in some cases, early Perthes disease initially being mistaken for transient synovitis rather than transient synovitis causing osteonecrosis.

Persistent or recurrent symptoms therefore warrant repeat evaluation.

Complications

True complications are uncommon.

The principal clinical risk is misdiagnosing septic arthritis, osteomyelitis, Perthes disease, or another significant disorder as transient synovitis.

Patient Monitoring

The child should remain under observation until infection and other urgent causes are considered sufficiently unlikely.

Re-Examination

Follow-up examination within approximately 1–2 weeks is appropriate to confirm:

Resolution of pain, return of normal hip motion, and normalization of gait.

Return to Activity

Full activity should resume only after:

Pain has resolved, gait is normal, and hip range of motion has returned.

Return Precautions

Parents should seek prompt reassessment if the child develops:

Increasing pain, high fever, persistent refusal to bear weight, worsening limp, recurrent symptoms, new systemic illness, or failure to improve as expected.

These findings should prompt reconsideration of diagnoses such as septic arthritis, osteomyelitis, or Legg–Calvé–Perthes disease.