Allis Test for Hip: Purpose, Procedure, and Interpretation

What is the Allis Test for Hip?

The Allis test, also known as the Galeazzi sign, is a clinical assessment primarily used to detect leg length discrepancies, particularly those caused by differences in femoral or tibial length, and can indirectly indicate hip joint issues such as developmental dysplasia of the hip (DDH) in infants and children.

Understanding Leg Length Discrepancy

Leg length discrepancy (LLD), or anisomelia, refers to a condition where one leg is shorter or longer than the other. This can be a structural difference (true LLD) due to variations in bone length (femur or tibia) or a functional difference (apparent LLD) caused by issues like pelvic obliquity, scoliosis, or muscle imbalances. Accurate assessment of LLD is crucial as significant differences can lead to gait abnormalities, compensatory movements, back pain, hip pain, and increased risk of joint degeneration over time.

Purpose of the Allis Test

The Allis test is a simple, non-invasive screening tool used to quickly assess for significant leg length discrepancies by comparing the heights of the knees when the patient is supine with flexed hips and knees. Its primary applications include:

• Screening for Developmental Dysplasia of the Hip (DDH): In pediatric populations, a positive Allis sign can suggest a dislocated or severely subluxated hip, as the femur on the affected side may appear shorter.
• Identifying Femoral or Tibial Length Discrepancies: While it doesn't quantify the difference, it provides a qualitative assessment of whether one femur or tibia is notably shorter or longer than its counterpart.
• Initial Assessment of Lower Extremity Asymmetry: It serves as a quick check during an orthopedic or physical therapy examination.

How to Perform the Allis Test

Performing the Allis test requires careful positioning and observation to ensure accurate results.

1. Patient Positioning:

• The patient lies supine (on their back) on a firm, flat surface.
• Both hips are flexed to approximately 45-60 degrees.
• Both knees are flexed to approximately 90 degrees.
• The feet are placed flat on the examination surface, with the malleoli (ankle bones) aligned and the heels touching the buttocks. Ensure the pelvis is level and not rotated.

2. Examiner Observation:

• The examiner stands at the foot of the patient, looking towards the patient's head.
• The examiner's eyes should be at the same level as the patient's knees.
• Observe the height of the knees relative to each other.

3. Interpretation:

• Negative Allis Sign: Both knees appear to be at the same height. This suggests no significant structural leg length discrepancy at the femur or tibia, or no hip dislocation.
• Positive Allis Sign: One knee appears higher than the other. This indicates a potential leg length discrepancy.

Interpreting the Results

A positive Allis sign requires further investigation to determine the underlying cause.

• Higher Knee Implies:

  • Shorter Femur on the Same Side: If the knee on one side is higher, it suggests that the femur (thigh bone) on that same side is shorter. This is a common finding in DDH where the femoral head is dislocated superiorly and posteriorly.
  • Longer Tibia on the Same Side: Alternatively, it could indicate that the tibia (shin bone) on the same side is longer.

• Differentiating Femoral vs. Tibial Discrepancy:

  • While the Allis test primarily checks knee height, to differentiate between a femoral and tibial length discrepancy, the examiner must also observe the level of the patient's tibiae from the side.
  • If the knee is higher and the tibia appears shorter (when viewed from the side, comparing the shin length), it points to a femoral shortening.
  • If the knee is higher but the tibia appears longer, it points to a tibial lengthening.
  • In the context of DDH, the primary concern is usually femoral shortening due to dislocation.

Clinical Significance and Limitations

While a valuable screening tool, the Allis test has specific applications and limitations:

• Primary Significance in Pediatrics: It is most commonly associated with and highly significant in the assessment of infants and young children for DDH. A positive Allis sign (also called Galeazzi sign in this context) warrants immediate further diagnostic imaging (ultrasound or X-ray) to confirm or rule out hip dysplasia.
• General LLD Screening: In adults and older children, it can serve as a quick initial screen for gross leg length differences, but more precise methods (e.g., tape measure, scanogram, computed tomography) are needed for accurate measurement.
• Not Diagnostic Alone: A positive Allis sign is an indicator, not a definitive diagnosis. It prompts further evaluation.
• Influenced by Soft Tissue: Obesity or significant muscle bulk can sometimes obscure accurate observation.
• Poor Sensitivity and Specificity in Isolation: While useful for gross differences, it may miss subtle discrepancies and can be influenced by factors other than bone length, such as pelvic tilt or muscle contractures. It should always be used as part of a comprehensive orthopedic examination.

Differentiating from the Galeazzi Sign

Often, the terms "Allis test" and "Galeazzi sign" are used interchangeably, particularly in the context of developmental dysplasia of the hip. The Galeazzi sign specifically refers to the finding of unequal knee heights when the hips and knees are flexed and the feet are flat, suggesting a dislocated hip on the side of the lower knee (which implies a shorter femur due to the dislocation). The Allis test is a broader term for the same maneuver used to assess any leg length discrepancy, whether due to hip dislocation or true bone length differences. In essence, the Galeazzi sign is a specific interpretation of a positive Allis test in the context of DDH.

Conclusion

The Allis test is a fundamental, quick, and non-invasive clinical maneuver used to screen for leg length discrepancies, especially in pediatric populations for conditions like developmental dysplasia of the hip. While a positive sign indicates a potential issue requiring further diagnostic investigation, its simplicity and ease of performance make it an essential component of a thorough musculoskeletal examination. Like all clinical tests, it should be interpreted in conjunction with a comprehensive patient history, physical examination, and, if necessary, advanced imaging.