Evaluation of a computerized measurement technique for joint alignment before and during periacetabular osteotomy

Abstract

Periacetabular osteotomy (PAO) is intended to treat a painful dysplastic hip. Manual radiological angle measurements are used to diagnose dysplasia and to define regions of insufficient femoral head coverage for planning PAO. No method has yet been described that recalculates radiological angles as the acetabular bone fragment is reoriented. In this study, we propose a technique for computationally measuring the radiological angles from a joint contact surface model segmented from CT-scan data. Using oblique image slices, we selected the lateral and medial edge of the acetabulum lunate to form a closed, continuous, 3D curve. The joint surface is generated by interpolating the curve, and the radiological angles are measured directly using the 3D surface. This technique was evaluated using CT data for both normal and dysplastic hips. Manual measurements made by three independent observers showed minor discrepancies between the manual observations and the computerized technique. Inter-observer error (mean difference +/- standard deviation) was 0.04 +/- 3.53 degrees for Observer 1; -0.46 +/- 3.13 degrees for Observer 2; and 0.42 +/- 2.73 degrees for Observer 3. The measurement error for the proposed computer method was -1.30 +/- 3.30 degrees . The computerized technique demonstrates sufficient accuracy compared to manual techniques, making it suitable for planning and intraoperative evaluation of radiological metrics for periacetabular osteotomy.

Figure 1

All angles were measured from the three reformatted CT slices (coronal, sagittal, and transverse) passing through the center of the femoral head on the ipsilateral side. In the coronal view, the CE angle and the AC angle were measured. The CE angle is defined by the most lateral aspect of the acetabular rim, the center of the femoral head, and a point vertical (superior) to the center point. The AC angle measures the obliqueness of the acetabular roof and is measured based on the most lateral aspect of the acetabular rim, the most medial aspect of the sourcil, and a horizontal (lateral) line. The S-AC angle and the AcetAV are measured from image reformats in the sagittal and transverse planes, respectively. In the sagittal plane, the most anterior portion of the acetabular rim, the top of the acetabular roof, and a point directly anterior (horizontal) to the roof make the S-AC angle. The acute AcetAV angle is measured by a line parallel to the acetabular opening and one perpendicular to the centers of the femoral heads on the transverse plane. [Color version available online.]

Figure 2

The oblique slicing method for segmenting the acetabulum lunate. A) Reformatting CT slices are acquired circumferentially about the medial-lateral joint line. This allows the lunate to be traced using the two intersections (medial, lateral) on each slice. B) The creation of the parametric surface implements an oblique plane (blue) rotated about the medial-lateral axis of the joint to find and interpolate points. The acetabulum of this right hip is viewed from the posterior-lateral direction. [Color version available online.]

Figure 3

Transparent views of the acetabulum. The 3D Lunate-Trace curve is shown as a blue outline, and the region in red is the interpolated surface of the acetabulum lunate. The angles are defined using the segmented model, and are analogous to those in Figure 1. [Color version available online.]

Figure 4

Histograms of measuring error for observers 1–3 (O1-O3) and the computer assisted Lunate-Trace method (LT).

Figure 5

Vector plots of location of femoral head with respect to acetabular joint center (mm) in the superior-lateral (frontal plane) and superior-anterior (sagittal plane) directions. [Color version available online.]