Introduction: Hip dislocation remains a relevant complication of total hip arthroplasty.The implants position plays a major role, especially cup anteversion.It has been demonstrated that anteversion measured on CTscan depends on the pelvic position in a lying patient. This prospective study evaluates the influence of pelvic tilt according to standing and sitting positions.

Material and Methods: The radiological records of 328 consecutive asymptomatic patients with THP were analyzed. These were routine radiological controls of non cemented THP with metal back acetabular implants. All patients had AP and lateral radiographs in standing and sitting position and a “low-dose” CT scan of the pelvis in lying position.Patients were checked for the absence lower limb length discrepancy and lumbosacral junction abnormality.

All the measurements were done by two independent observers and averaged. From the standard radiographs, the sacral slope (SS), the acetabular frontal inclination (AFI), and the acetabular sagittal inclination (ASI) were measured in standing, sitting, and lying positions.

From the CT scan sections, the anatomical ante-version (AA) was measured in lying position on axial images according to Murray. The results were compared to a previously described protocol replicating standing and sitting positions: CTscan sections were oriented according to sacral slope.

Results: We confirmed that the anatomical anteversion (AA), the frontal inclination (FI), and the sagittal inclination (SI) were functional parameter which significantly varied between standing, sitting, and lying positions according to sacral slope variations.The acetabular parameters in lying position highly correlated to the one in standing position, while poorly correlated with sitting position. The difference between the lying and the sitting positions was about 10°, 25°, and 15° for the AA, the AFI, and the ASI respectively.Mean lying anteversion angle was 24.2° (SD6,9°).Posterior pelvic tilt in sitting position, (sacral slope decrease) was linked to anteversion increase (mean value 38,8° - SD 5,4°). Anterior pelvic tilt in standing position (sacral slope increase) was linked to lower anteversion (mean value 31,7° - SD5,6°).

Discussion and Conclusions: Our study confirms the interest CTscan sections oriented according to sacral slope.The strong correlation between lying and standing measurements suggests that classical CTscan protocol is relevant for standing anteversion. According to the poor correlation between lying and sitting positions, it is less contributive for the investigation of dislocations in sitting position.

Purpose: Uncertain position of the acetabular implant has been the cause of dysfunction in certain cases of total hip arthroplasty (THA). Classical computed tomographic analysis of anteversion has certain limitations. Integrated reconstruction of positions at risk allows a better diagnostic approach.

Material and methods: We studied 46 THA because of posterior malposition (n=17, anterior subluxation in the standing position in twelve, and true dislocation in five) and anterior malposition (n=29, posterior subluxation in sixteen and true dislocation in thirteen). Two groups of 70 naïve hips and a group of 56 THA with no functional problem served as controls. The position of the acetabulum was studied on optimised computed tomography slices reconstructing the planes of analysis for the standing, sitting and reclining positions. The reference planes for the slices was given by the sacral tilt angle measured on the lateral views of the patient in the corresponding positions. The optimised computed tomographic measurements of anteversion were compared with the classical measures. None of the patients had abnormal femoral anteversion and/or an oblique pelvis and/or leg length discrepancy greater than 10 mm. The frontal inclination of the acetabular implants was 40°–50°.

Results: In the naïve hips, acetabular anteversion varied: 19.2 with the conventional method, 15.7 in the standing position and 31 in the sitting position. In the THA controls, anteversion measurements differed: 21.3 with the conventional method, 21.4 in the standing position and 35.8 in the sitting position. In the THA with a posterior malposition, 18/29 could not be explained by the conventional measurement, but the optimised measurement enabled an understanding in 17 hips (defective anteversion in the sitting position).

Discussion: Changes in pelvis orientation between the sitting and standing positions modifies real anteversion of the cup. In particular, subjects with THA tend to have a spontaneous posterior tilt of the pelvis related to trunk ageing. This element should be taken into account for the analysis of both major and minor THA dysfunction.