Definition and reliability of 3D acetabular and global offset measurements from bi-plane X-rays

Abstract

The importance of the global offset, the sum of femoral and acetabular offset, has been underlined in the literature as a key factor for the functional outcome of total hip arthroplasty (THA). However, the acetabular offset is not defined for bi-plane X-rays, a technology providing 3D measurements of the lower limb and commonly used for patients undergoing THA. The aim of this paper is to introduce a measurement method of the 3D acetabular offset with bi-plane X-rays. Our method combines the use of technical and anatomical coordinate systems. The most appropriate definition will be selected based on the best reliability and measurement error. The consequent reliability of the global offset was also assessed. Twenty-eight patients undergoing primary THA were selected retrospectively. Two operators performed three reconstructions for each patients before and after THA. Intraclass correlation (ICC) and smallest detectable change (SDC) were computed for intra-operator, inter-operator and test–retest conditions for all combinations of technical and anatomical coordinate systems. ICCs were good to excellent. One combination was more reliable than others with a moderate mean SDC of 6.3 mm (4.3–8.7 mm) for the acetabular offset and a moderate mean SDC of 6.2 mm (5.6–6.7 mm) for the global offset. This is similar to the reliability and mean SDC of the femoral offset (4.8 mm) approved for clinical use which indicates that this method of acetabular offset measurement is appropriate. This opens a research avenue to better understand the role of the acetabular offset on THA outcomes, which seems overlooked in the literature.

Sensor-to-Bone Calibration with the Fusion of IMU and Bi-Plane X-rays

“…After proper calibration of the fusion box, the orientation of the pelvis and thigh fusion boxes with respect to the bi-plane X-ray cabin coordinate system ( , with Seg the pelvis or thigh) was measured based on the radio-opaque markers visible on the sagittal and frontal images ( Figure 1 c). Then, the bone coordinate systems of the pelvis (Anat CS1 in [ 16 ]) and thigh in [ 12 ] were defined with respect to the cabin coordinate system ( ) ( Figure 1 c). The orientation of the bone coordinate system with respect to the fusion box was then computed as follows with the hypothesis that it remains constant during the whole measurement: …”

“…A previously published method was used [ 12 ]. However, the pelvis coordinate was modified to the anatomical coordinate system 1 defined in Gasparutto et al [ 16 ] to be consistent with the sensor-to-bone method described above.…”