Normal Hip Teardrop Distance Size on Radiography
The teardrop distance is the horizontal measurement from the medial wall of the acetabulum (the teardrop figure) to the medial cortex of the femoral head on an anteroposterior (AP) pelvis radiograph. This measurement serves as a reliable, reproducible indicator of the femoral head's position within the acetabulum. Accurate assessment is clinically important for identifying joint effusion, early hip subluxation, and post-arthroplasty component positioning.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Anteroposterior | 6-11 mm |
Clinical Significance
A teardrop distance within the normal range of 6–11 mm indicates appropriate femoral head centering in the acetabulum. Asymmetric or bilaterally increased distances may signal pathological lateral migration of the femoral head or joint space widening secondary to effusion. A distance exceeding the upper limit of normal warrants further evaluation.
Conversely, a decreased or absent teardrop distance may indicate medial migration of the femoral head, as seen in conditions causing protrusio acetabuli. Comparison with the contralateral hip is essential, as subtle asymmetry (typically >2 mm side-to-side difference) can be the earliest radiographic sign of disease.
Common clinical scenarios in which teardrop distance assessment is critical include:
- Septic arthritis — joint effusion causing lateral head displacement
- Legg-Calvé-Perthes disease — early lateral migration of the femoral head
- Developmental dysplasia of the hip (DDH) — abnormal acetabular coverage and head positioning
- Protrusio acetabuli — medial migration reducing or obliterating the distance
- Total hip arthroplasty follow-up — assessing component positioning and loosening
Reference: Yochum TR, Rowe LJ. Essentials of Skeletal Radiology. (2005).
Imaging Notes
The teardrop distance is measured on a standard anteroposterior (AP) pelvis or hip radiograph with the patient supine, lower limbs in approximately 15° of internal rotation to profile the femoral neck. The measurement is taken as the shortest horizontal distance between the lateral margin of the pelvic teardrop (os acetabuli mediale) and the medial cortex of the femoral head. Consistent patient positioning is critical — external rotation or pelvic tilt can artifactually increase or decrease the apparent distance.
When comparing serial radiographs, standardized positioning and exposure technique should be maintained to ensure reliable longitudinal assessment. On weight-bearing views, slight physiological reduction in joint space is expected; supine non-weight-bearing projections are preferred for this measurement.