A 6-month-old infant is being evaluated for developmental dysplasia of the hip (DDH). Which assessment finding would be most indicative of DDH in this infant?

Asymmetrical gluteal and thigh skin folds are a classic sign of DDH in infants, indicating unequal leg lengths due to hip displacement. Other options (Babinski reflex, Moro reflex, increased muscle tone) are normal findings or unrelated to DDH.

Understanding the Clinical Presentation of DDH in a 6-Month-Old

When evaluating a 6-month-old infant for developmental dysplasia of the hip (DDH), the physical examination shifts from relying on dynamic instability maneuvers to identifying static anatomical changes that result from a persistently dislocated or subluxated femoral head. By this age, the pathognomonic findings of the neonatal period, such as the Barlow and Ortolani signs, often become less apparent or disappear entirely as soft tissue contractures develop. Therefore, the assessment must focus on secondary signs of asymmetry caused by the proximal migration of the femur.

Why Asymmetrical Skin Folds Are the Most Indicative Finding

The correct answer is 2. Asymmetrical gluteal and thigh skin folds. In a 6-month-old infant, the most reliable static indicator of a unilateral hip dislocation is a visible and palpable asymmetry in the skin folds of the gluteal and thigh regions. The pathophysiological basis for this finding is directly related to the biomechanical disruption caused by the dislocated femoral head. When the femoral head loses its concentric reduction within the acetabulum and migrates proximally and posteriorly, the limb becomes functionally shortened. This shortening does not simply make the leg look shorter; it causes the soft tissues of the thigh to bunch up or "shrink" over the now-shortened bony framework. This tissue redundancy manifests as an increased number of skin folds, or folds that appear deeper and more posteriorly positioned, on the affected side compared to the contralateral healthy hip ^[1]. While it is crucial to note that asymmetric skin folds (ASFs) can exist in infants without DDH, in the context of a targeted DDH evaluation, their presence is a sensitive clinical clue that necessitates further diagnostic workup, such as an ultrasound or radiograph, according to screening guidelines [1,2].

Analysis of Incorrect Options

A detailed understanding of normal neurodevelopment and the specific pathophysiology of DDH explains why the other options are incorrect.

- 1. Positive Babinski reflex: The Babinski reflex, characterized by dorsiflexion of the great toe and fanning of the other toes upon stroking the lateral sole of the foot, is a normal upper motor neuron response in infants. Its presence is expected until approximately 12 to 24 months of age, after which it is integrated by the maturing corticospinal tract. A positive Babinski reflex in a 6-month-old is a normal neurological finding and has no association with the structural orthopedic pathology of DDH .
- 3. Absence of the Moro reflex: The Moro reflex is a primitive startle reflex that typically integrates and disappears by 4 to 6 months of age. Its absence in a 6-month-old infant is an expected and normal neurodevelopmental milestone, signifying the maturation of the central nervous system. It is not a clinical marker for hip joint stability or anatomy .
- 4. Increased muscle tone in lower extremities: DDH is a structural disorder of the hip joint's formation and stability; it is not a neurological condition. Therefore, it does not cause a primary increase in lower extremity muscle tone (hypertonia). Hypertonia is a classic sign of an upper motor neuron lesion, such as in cerebral palsy. While chronic, untreated DDH can lead to contractures and a functional limitation of hip abduction over time, this is not the same as a true spastic increase in muscle tone. Furthermore, the initial physical exam for DDH in a 6-month-old would more likely reveal a limitation of passive hip abduction due to adductor tightness, not a generalized increase in tone [3,4].

Integrating Physical Exam with Clinical Guidelines

The physical examination of the pediatric hip is a critical component of DDH screening, but its components and reliability change with the patient's age. European national guidelines, summarized in recent reviews, consistently recommend a careful physical exam for all infants, with specific attention to signs of asymmetry, including limb length discrepancy and asymmetric skin folds, particularly after the neonatal period . The finding of ASFs or limited hip abduction in an older infant should trigger a definitive imaging study. The pediatrician's or nurse practitioner's role is to recognize these subtle anatomical clues during routine well-child visits, as early diagnosis is the primary factor that allows for less invasive treatments like harness therapy and prevents long-term disability .