Introduction

In total hip arthroplasty, a high radiographic inclination angle (RI) of the acetabular component has been linked to short- and long-term complications. There are several factors that lead to RI outliers including cup version, pelvic orientation and angle of the cup introducer relative to the floor. The primary aim of this study was to analyse what increases the risk of having a cup with an RI outside the target zone when controlling cup orientation with a digital inclinometer.

The orientation of the acetabular component is influenced by the orientation at which the surgeon implants the component and the orientation of the pelvis at the time of implantation. When operating with the patient in the lateral decubitus position, pelvic orientation can be highly variable. The goal of this study was to examine the effect of two different pelvic supports on cup orientation.

In this prospective study, 200 consecutive patients undergoing uncemented primary THA in the lateral decubitus position were included. In the control group a single support over the pubic symphysis (PS) was used. In the study group, a single support over the ipsilateral anterior superior iliac spine (ASIS) was used. In every patient, the cup was inserted and the angle of the cup introducer relative to the floor (apparent operative inclination; OIa) was measured with the aid of a digital inclinometer. The radiographic inclination (RI) was measured on anteroposterior pelvic radiographs at 6 weeks postoperatively. The target zone for cup inclination was 35–45°.

In both cohorts the cups were implanted close to the target OIa with an absolute difference with the OIa of 0.86° SD 0.82 in the PS cohort and 1.03° SD 0.99 in the ASIS cohort (p=0.18). The difference between the RI and OIa was higher in the PS cohort 12.2° SD 4.1 compared with 7.5° SD 3.7 in the ASIS cohort (p<0.0001) with also a bigger variance (p=0.04) in the PS cohort. The mean RI was 38.5° SD 4.4 compared with 39.2° SD 4.1 (p=0.26) respectively. There were more cups outside the RI target zone in the PS cohort compared with the ASIS cohort (respectively 26 versus 15; p<0.05).

In this study the mean difference between the RI and OIa (the angle of the cup introducer during surgery) was significantly less when using a support over the ASIS compared with a support over the pubic symphysis. Apparently using a support over the ASIS causes less pelvic motion during surgery compared with a support over the pubic symphysis. This resulted in less variance and inclination outliers when using a tight target zone of 35–45°.

Introduction

In total hip arthroplasty (THA), a high radiographic inclination angle (RI) of the acetabular component has been linked to an increased dislocation rate, liner fracture, and increased wear. In contrast to version, we have more proven boundaries when it comes to a safe zone for angles of RI. Although intuitively it seems easier to achieve a target RI, most studies demonstrate a lack of accuracy and the trend towards a high RI with all surgical approaches when using a freehand technique or a mechanical guide. This is due to pelvic motion during surgery, which can be highly variable.

The current study had two primary aims, each with a different primary outcome. The first aim was to determine how accurate a surgeon could obtain the target operative inclination (OI) during THA when using a cementless cup using a digital protractor. The second aim was to determine how accurate a surgeon can estimate the target OI to obtain a RI of 40° based on the patient's hip circumference as demonstrated in a previous study.

Introduction

A high inclination angle has been linked to an increased dislocation rate, liner fracture, and increased wear. The aim of this study was to compare the operative (OI) with the radiological inclination (RI) angle and determine the influence of patient morphology on pelvic tilt and cup inclination angle.