Combined and hybrid marker models for radiostereometry assessment of polyethylene liner motion in dual mobility hip prosthesis: a proof-of-concept study

Abstract

Background: Investigation of polyethylene liner movement in total hip arthroplasty requires bead-marking for radiographic visibility of the liner. However, occlusion of markers poses a challenge for marker registration in radiographs.

Methods: The polyethylene of a dual mobility acetabular system was marked with twelve 1-mm tantalum markers (four groups of three markers) using a custom-made drill guide. Liner motion in a phantom and a patient was investigated with dynamic radiostereometry analysis (dRSA) at 1-year follow-up and static radiostereometry analysis (sRSA) postoperatively and at 1- and 2-year follow-up. A combined marker configuration (CMC) model was calculated from the registered positions of the liner markers and the femoral head in several images. Furthermore, the CMC model and the theoretic marker positions from computer-assisted models of the drill guide were combined in a hybrid model.

Results: The CMC model included eleven markers in the phantom and nine markers in the patient, which was sufficient for dRSA. Liner movement in the phantom followed liner contact with the femoral neck, while liner movement in the patient was independent. The hybrid model was necessary to determine liner orientation in sRSA recordings, which clearly changed from postoperative to 1- and 2-year follow-up even though the patient was positioned similarly.

Conclusion: Polyethylene liner motion in dual mobility hip prosthesis can be assessed with CMC models in dRSA recordings. In sRSA, the liner position between follow-ups is unpredictable and analysis requires inclusion of all markers in the model, accomplished with a hybrid marker model.

Trial registration: ClinicalTrials.gov [ NCT02301182 ], 25 October 2015.

Keywords: Hip prosthesis; Phantoms (imaging); Polyethylene; Radiography; Radiostereometric analysis.

Fig. 1

The occlusion of liner markers by the shell and the head/neck/stem (only the left radiostereometric analysis image frame is shown). Despite a high number of markers in the liner (n = 12) (a), they tend to be occluded by the head/neck (b) and cup (c) in RSA recordings. Marker information can be used as a simple marker model (green markers) (d), a combined marker configuration model (e) that merges marker information from several recordings (blue markers) or a hybrid model (f) that adds the theoretic marker positions (red markers) from the computer-assisted drawings of the drill guide used to insert the tantalum markers

Fig. 2

A customised tool with a drill guide for inserting markers in the individual liner sizes of the system was developed (left) and machined in stainless steel (right). Three markers with increased depth ensured distinctive marker groups

Fig. 3

Radiostereometric analysis set-up with a direct anteroposterior angle for static recordings (a) and a 45-degree recording angle (b) for dynamic recordings in order to obtain an optimal view of the polyethylene liner

Fig. 4

Liner rotation and stem rotation of the phantom (a) and patient (b). Dashed lines show stem movements as indicated by the pictogram. In the phantom, liner movement (solid lines) occurs in the end range of modified FABER motion (at 4 and 12 s) and of modified FADIF motion (at 8 s) when the liner/neck angle (black) approaches 36.6° (solid red). In the patient, liner movement occurs in the end range of modified FABER motion (at 4 s) without the liner/neck angle approaching 36.6°

Fig. 5

Example of anatomic dual mobility cup with a combined marker configuration model. The y-axis is shown for the femoral neck (blue), the cup (red) and the liner (green). ∠A indicates the liner/neck angle. The red circles indicate the detected marker projections in the image, as well as the centre of the femoral head

Fig. 6

Radiographic anteversion and inclination of the cup. A Acetabular axis. A’ Projected acetabular axis. ∠RI indicates the radiographic inclination, defined as the angle between the longitudinal axis (Y) and the acetabular axis (norm of cup) projected perpendicular on the coronal plane (A’). ∠RA indicates the radiographic anteversion, defined as the angle between the acetabular axis (A) and the coronal plane. X, Y, and Z represent the coordinate system of the radiostereometric analysis recording