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Equivalence of Anteroposterior and Frog-Leg Lateral Radiographs in Measuring Acetabular Index in Children
Atsuhiko Handa1, Pedro H Faria1, Carter R Petty1
1Boston Children's Hospital.
Insights
Frog-leg lateral radiographs are interchangeable with standard AP views for measuring the acetabular index in developmental dysplasia of the hip. This finding may reduce repeat imaging and radiation exposure in pediatric patients.
Area of Science:
- Pediatric Radiology
- Orthopedic Imaging
- Musculoskeletal Imaging
Background:
- Developmental dysplasia of the hip (DDH) is a common pediatric condition.
- Acetabular index (AI) on AP radiographs is key for DDH diagnosis.
- Poor pelvic positioning in AP views necessitates repeat imaging and increases radiation exposure.
Purpose of the Study:
- To assess the interchangeability of frog-leg lateral (FL) radiographs with anteroposterior (AP) radiographs for acetabular index (AI) measurement in pediatric DDH evaluation.
Main Methods:
- Retrospective analysis of AP and FL hip radiographs from 100 pediatric patients (6-24 months).
- Strict quality criteria applied to AP views (rotation index 0.5-2.0, tilt index 0.9-1.4).
- Four readers independently measured AI twice; interchangeability assessed using Individual Equivalence Index (IEI) with a 3° margin.
Main Results:
- The √IEI (2.07°) and its upper confidence bound (2.34°) were below the 3° interchangeability threshold.
- Intra-reader reliability (ICC) ranged from 0.845 to 0.898; FL views showed slightly higher repeatability for three readers.
- Inter-reader reliability (ICC) was 0.783 for AP and 0.807 for FL views.
Conclusions:
- Acetabular index measurements from FL radiographs are interchangeable with those from AP radiographs.
- FL views can be utilized when AP views have suboptimal positioning.
- This may reduce the need for repeat imaging and associated radiation exposure in pediatric DDH assessment.
Background:
Developmental dysplasia of the hip (DDH) is one of the most common pediatric musculoskeletal conditions, with an incidence of 1 in 1,000 live births. The acetabular index (AI), measured on anteroposterior (AP) radiographs, is the primary radiographic tool for DDH diagnosis and monitoring. However, its accuracy is critically dependent on pelvic positioning; suboptimal studies frequently require repeat imaging, resulting in additional radiation exposure for the child.
Objective:
To determine whether frog-leg lateral (FL) radiographs are interchangeable with AP radiographs for AI measurement in children being evaluated for suspected DDH.
Methods:
In this retrospective, IRB-approved study, bilateral AP and FL hip radiographs from 100 pediatric patients (aged 6-24 months) were evaluated at a single tertiary pediatric center. Only AP studies meeting strict pelvic positioning criteria (rotation index 0.5-2.0; tilt index 0.9-1.4) were included. Four readers, including three pediatric radiologists and a pediatric orthopedic surgeon, independently measured bilateral AI on each view on two separate sessions, with a 4-week washout between sessions. Interchangeability was assessed using the Individual Equivalence Index (IEI) with a 3° margin. Reliability was assessed with intraclass correlation coefficients (ICC).
Results:
The √IEI was 2.07°, with a one-sided 95% upper confidence bound of 2.34°, both below the 3° interchangeability margin. Intra-reader ICCs ranged from 0.845 to 0.898 across both views. Three of four readers showed slightly higher repeatability on the FL view. Inter-reader ICC was 0.783 (95% CI: 0.720-0.833) for AP and 0.807 (95% CI: 0.744-0.854) for FL.
Conclusions:
AI measurements from FL radiographs are interchangeable with those from AP radiographs. When the AP view is suboptimally positioned, the FL view may be used for AI measurement, potentially eliminating repeat imaging and reducing radiation exposure in this pediatric population.
