Open-face plane orientation of a BioMedtrix biologic fixation acetabular cup can be accurately determined from a lateral radiograph
Abstract
Objective
To develop and validate a method to calculate biologic fixation (BFX) acetabular cup open-face plane orientation based on measurements from a lateral radiograph.
Methods
The formulas describing the open-face plane orientation were derived using geometric arguments and trigonometry. To assess the method, an in vitro plastic model of a pelvis was implanted with a BFX acetabular cup. A lateral radiograph of the implanted model was obtained and used to calculate the cup open-face plane orientation using the derived method. Direct measurement of the open-face plane orientation was performed, and the calculated values were compared to the directly measured values. The procedure was repeated with the cup in a different orientation for each trial.
Results
52 trials were performed. The derived calculation method demonstrated high repeatability and reproducibility, with intra- and interobserver intraclass correlation coefficients > 0.94. The direct measurement method demonstrated high reproducibility, with interobserver intraclass correlation coefficients > 0.98. The mean absolute angular difference between the calculated and directly measured orientation vectors was 2.8 ± 1.3°, with 95% of trials within 4.7°.
Conclusions
The open-face plane orientation determined by the derived method agreed closely with the directly measured orientation.
Clinical Relevance
Knowledge of the BFX open-face plane orientation can inform decisions regarding cup revision, help identify complications associated with cup malpositioning, and allow the surgeon to improve their cup placement performance.
Keywords: total hip replacement; BFX; orientation; ALO; version
Accurate positioning of the acetabular cup is central to successful total hip replacement (THR). Knowledge of cup orientation informs decisions regarding cup revision, helps identify whether cup malpositioning contributed to postoperative complications, and allows the surgeon to improve their performance. The orientation of the open face of the cup is particularly critical in determining how the cup interacts with the femoral component. This orientation is traditionally described by the spherical coordinates angle of lateral opening (ALO) and open-face version (OFV).1 Published recommendations for cup orientation include an ALO of 45° and an OFV of 15° to 25° retroversion.1
Intraoperatively, cup orientation is assessed via several methods. The acetabular reamer handle and cup impactor handle can be fitted with orientation bars that guide orientation during reaming and implantation. Cup orientation is also evaluated in relation to the local anatomy of the cranial and caudal acetabular pillars. Finally, the cup orientation relative to the femoral component can be assessed by comparing the plane of the open face of the cup to the flat surface of the back of the femoral head with the hip in reduction and the leg in a neutral position. According to the manufacturer, these planes should be parallel.2
Postoperatively, assessment of implant positioning is typically performed using orthogonal radiographs of the pelvis. A cemented fixation (CFX) cup is radiolucent and contains an embedded circular metallic wire marker whose plane is parallel to the open face. On a ventrodorsal (VD) radiograph, the shape and orientation of this wire marker can be used to readily determine ALO and OFV.1,3 For a biologic fixation (BFX) cup, it is challenging to determine the open-face plane orientation from radiographs. The shell of the cup is titanium and forms a relatively homogenous metal-opacity radiographic silhouette, making it difficult to discriminate the geometry that lies within this silhouette.
Several methods have been proposed for estimating aspects of BFX cup orientation from a lateral or VD radiograph.4–7 Most estimate ALO alone rather than complete open-face plane orientation and tend to become less accurate when larger values of OFV combine with ALO to produce foreshortening of the cup silhouette. One study6 assessed both ALO and version using an assistive device placed in the cup at the time of imaging. This device may improve ease of measurement when it is in place but has the disadvantages that it can only be used intraoperatively and only by using intraoperative radiography. Determining complete open-face plane orientation postoperatively from the cup radiographic silhouette alone remains challenging.
The objectives of this study were to develop a method to calculate BFX acetabular cup open-face plane orientation based on measurements from a lateral radiograph, derive the mathematical basis of the method, and assess its agreement with direct measurement. The proposed method uses the portion of the open face that is visible as the ventral margin of the cup silhouette on the lateral radiograph. It fits an ellipse to this profile, measures the shape and orientation of this ellipse, and then uses trigonometry to calculate the acetabular axis vector. This ellipse-based radiographic method bears similarities to methods for describing a CFX cup on a VD radiograph,1,3 which in turn traces its origins to a method first described by Lewinnek et al8 in human hip arthroplasty in 1978 that has been built upon in the decades since.9
Using this radiograph-based method and direct measurement of a plastic model implanted with a cup, we assessed agreement between the calculated orientation and the directly measured orientation.
Methods
Geometric derivation of the method to calculate ALO and OFV
The dorsal plane was defined as the plane containing the most dorsal points of the cranial iliac spines and the ischiatic tuberosities. On a lateral radiograph, this plane forms the ilioischial axis. The x-axis is the cranial-caudal axis that lies in the dorsal plane. The y-axis is the ventral-dorsal axis normal to the dorsal plane. The z-axis is the medial-lateral axis that lies in the dorsal plane.
The BFX acetabular cup approximates a hemisphere with a portion removed for the dorsolateral truncation. Thus, the ventrolateral open face of the BFX acetabular cup is a circle with a portion removed by the truncation. The acetabular axis was defined as the unit vector perpendicular to the open face.3 The open-face plane orientation can be described by 2 angles: ALO and OFV. The ALO is the angle between the open-face plane and the dorsal plane.1 The OFV is the angle formed between the z-axis and the projection onto the dorsal plane of the acetabular axis. In this study, OFV was 0° when this vector projection was parallel to the z-axis and positive when the vector angled caudally (retroversion). Because the projected acetabular axis is perpendicular to the ellipse major axis and the z-axis is perpendicular to the x-axis, this angle is equivalent to the angle that would be measured on a VD radiograph between the ellipse major axis and the x-axis as could be performed using the wire marker on a CFX cup.1 The open-face plane orientation is therefore described by a spherical coordinate system, with ALO and OFV the spherical coordinates of the acetabular axis vector.
The derivation assumes that the lateral radiographic projection, with the beam centered on the acetabula, approximates an orthographic projection at the level of the cup, the laterality (left or right) of the implanted acetabulum is known, and the cup is implanted with the acetabular axis vector pointing in a generally ventrolateral direction.
On a lateral pelvis radiograph, the circle of the cup open face is seen obliquely, and its projection forms an ellipse minus the truncation. This ellipse has a shape, which can be described by the aspect ratio (R), the ratio of major axis (MA) to minor axis (mA), and an orientation, which can be described by the angle between the ellipse major axis and the ilioischial axis. We define this angle as major axis inclination (MAI). The 2 parameters, R and MAI, define the shape and orientation of the ellipse. Under these assumptions, the ellipse uniquely determines the 3-D open-face plane orientation.
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