A new method of measuring acetabular cup anteversion on simulated radiographs

Abstract

Purpose: Optimal positioning of acetabular components is crucial for maintaining stability of THA. Postoperative assessment of acetabular anteversion is a vital but difficult task. Various methods have been devised with good results for measuring anteversion on plain radiographs but these methods are either too complicated or require special objects like scientific calculators, special protectors, tables, etc. A new simplified method of measuring anteversion on plain radiographs was created based on basic geometry.

Methods: Anteversion of acetabular components was estimated on computer generated images of the acetabular cup by our method and compared with two previously established methods of Liaw and Pradhan. Measurement was done at 400 different positions of acetabular cup and compared with actual values. Another analysis was done after adding the femoral head to the acetabular component, thus obscuring some of the acetabular rim.

Results: Mean and standard deviation of error for our method was 0.77° ± 0.75° as compared to 0.93° ± 0.86° and 0.72° ± 0.68° for the methods of Liaw and Pardhan, respectively, with no significant differences from actual values. Maximal errors for our method, Liaw's and Pradhan's method were 3°, 4°, and 2.91°, respectively. On analysis, after the adding femoral head, there was a significant error of measurement with Liaw's method, while our method as well as Pardhan's remained accurate. All methods showed high inter- and intraobserver reliability.

Conclusion: Our new simplified method of measuring acetabular anteversion on plain radiographs is acceptable in comparision to other established methods and requires only routinely used goniometer and calliper.

Fig. 1

Rationale of measuring anteversion on plain radiographs (see text for description of individual figures)

Fig. 2

Method. The long (AB; length a) and short (CD; length b) axes of the ellipse are drawn which intersect each other at a right angle at point O. Radius of acetabular component is measured as half of length AB (a/2; length OA) (a). The angle of anteversion is drawn after marking a point on the long axis such that its distance from the point of intersection of the short axis and acetabular ring (point D) equals the radius of acetabular component (b). The angle DXO is the angle of anteversion. Measurement of actabular anteversion in uncemented total hip replacement (c)

Fig. 3

Measurement of anteversion with femoral head of size 50 % (a) and 66 % (b) of that of the acetabular cup. A perpendicular is dropped from the visible part of the ellipse to its diameter to intersect the diameter and the outline of the acetabular cup at points O and E, respectively. Distance OE (a’) is measured and a point X is marked on the long diameter at a distance of OE from point D. The angle DXO is the angle of anteversion. Measurement of anteversion in uncemented total hip replacement using only the visible part of acetabular ring (c)

Fig. 4

Measurement of anteversion in a cemented acetabular component after drawing a circle with diameter and center same as the ellipse