Good inter- and intraobserver reliability for assessment of the slip angle in 77 hip radiographs of children with a slipped capital femoral epiphysis

Abstract

Background and purpose - The decision on and the outcome of treatment for a slipped capital femoral epiphysis (SCFE) depend on the severity of the slip. In 2015, web-based registration was introduced into the Swedish Pediatric Orthopedic Quality (SPOQ) register. To determine whether the inclusion of commonly used methods in Sweden for radiographic measurement of SCFE (the calcar femorale [CF] method and the Billing method) is justified, we measured the inter- and intraobserver reliability of these 2 measurements. We also evaluated the internationally more commonly used head-shaft angle (HSA) method. Material and methods - 4 observers with different levels of experience with radiographic measurements analyzed 77 routine preoperative hip radiographs of children with SCFE. Inter- and intraobserver reliability was evaluated. Results - The interobserver reliability analysis for the 4 observers showed for CF an ICC of 0.99 (CI 0.97-0.99) and for Billing an ICC of 0.99 (CI 0.98-0.99). The interobserver reliability analysis for 2 observers showed for HSA an ICC of 0.98 (CI 0.97-0.99). Intraobserver reliability (2 observers) showed a mean difference below 1° for all 3 methods and with a 95% limit of agreement not exceeding ±6.8°. Interpretation - We found good reliability for both intra- and interobserver measurements of all 3 methods used for the assessment of the slip angle on routine preoperative lateral hip radiographs.

Figure 1.

Slip angle measured using the calcar femorale method in a Lauenstein view. 1. Identify the calcar femorale (cf) and the lesser trochanter (1). 2. From the level of the lesser trochanter draw a line (2) three cm in a proximal direction parallell to the calcar femorale. 3. Extend a line (3) parallell to line (2) up through the femoral neck. 4. Define a line (4) through the physeal anterior and posterior margins. 5. Draw a line (5) perpendicular to line (4). 6. Slip angle (6).

Figure 2.

Slip angle measured using the Billing method in Billing lateral view with the patient positioned according to the figure to the right. 1. Draw a line (1) along the anterior cortex of the proximal femur. Extend the line up through the femoral head and neck. 2. Draw a line (2) along the anterior border of the femoral neck. 3. Draw the bisector (3) to lines (1) and (2). 4. Define a line (4) through the physeal anterior and posterior margins. 5. Draw a line (5) perpendicular to line (4). 6. The slip angle (6) is the angle between lines (4) and (5).

Figure 3.

Slip angle measured using the lateral head–shaft angle method in a Lauenstein view. 1. Draw a line (1), parallel with the proximal femoral shaft, further up into the femoral neck. 2. Define a line (2) through the physeal anterior and posterior margins. 3. Draw a line (3) perpendicular to line (2). 4. Lateral head–shaft angle (4).

Figure 4.

Study material.

Figure 5.

Intraobserver variation (°) for HSA – Observer 1 (left panel). The solid line represents the mean value and the dotted lines show the limits for 2 standard deviations above and below the mean value.

Figure 6.

Intraobserver variation (°) for CF – Observer 2 (right panel). The solid line represents the mean value and the dotted lines show the limits for 2 standard deviations above and below the mean value.