Hip dislocation is one of the most common complications following total hip arthroplasty (THA). Registry-based studies have reported that dislocation is among the leading causes of revision after primary THA [19, 20]. Factors influencing the risk of dislocation are many and complex. Malposition of the acetabular component is also a risk factor for post-operative dislocation after THA. Various safe ranges are proposed by many authors [2, 4, 17]. However, several investigators have questioned whether the historic concept of a safe zone is clinically relevant [21, 22].
There is an opinion that combined anteversion is more crucial, that incorporates femoral stem and acetabular orientation, may be a better indication, but is considerably more difficult to measure and this may require further investigation. Many combined anteversion patterns by many authors have been reported [23, 24]. However, there are also various theories and consensus not yet reached. Combined anteversion should be decided by each patient’s state [25]. Thus, it is very difficult to determine the safe zone by considering only the acetabular component.
Due to the multifactorial nature of THA dislocation, safe zone for cup positioning in THA could not be justified alone. Several patient and surgery-related risk factors for dislocation have been identified, including spinal fusion and stiff spine. According to the nationwide database, a history of spinal fusion was the most significant independent risk factor for early dislocation within six months [26]. This is because the pelvic tilt directly affects the acetabular version [18, 27, 28]. To take adequate AP hip X-rays with proper pelvic tilt, uniform AP hip radiographs were taken in a cross-table fashion centered on the superior aspect of the pubic symphysis and perpendicular to the patient. The pubic symphysis to sacrococcygeal distance was within the known normal range in this study. However, the results could be inaccurate when the pelvis is tilted or when the contralateral hip joint or the lumbar spine is stiff [11].
Dislocation after THA is more common in patients with a lumbar spinal fusion [29]. A fused spine stiffens the lumbar segments and directly affects mobility of the pelvis. Spinal fusions to the pelvis may result in a reduction in pelvic tilt in sitting, this may explain why patients with a lumbar spinal fusion have a higher rate of posterior dislocation. In this study, one case of dislocation occurred even though the acetabular component was in the target range in the patient with a lumbar fusion.
Since a modified Gibson’s posterolateral approach was used in this study, the target cup range was set up more anteverted than Lewinnek’s proposal. A posterolateral approach may influence soft tissue and muscular weakness at the surgical site, predisposing to posterior dislocation [2, 17]. Though there are various opinions on the scope of safe zone, and there is no consensus as mentioned above, it is encouraging to set the target range, put the acetabular component in the desired zone, and present the possibility of reducing the rate of dislocations.
Measuring the anteversion of acetabular component is more difficult to measure than inclination. There are several methods and also, other Area and Orthogonal methods have been introduced [30]. However, these methods used to focus on measuring the version after surgery. In this study, Widmer’s method was applied intraoperatively and the results showed that there was no significant difference from the measured value in CT scans as reference.
Assuming the safe zone and whatever its range, there has been minimal research on the proper way to put components into the safe zone intraoperatively. According to the literature, many surgeons tried to set up the safe zone and put the component inside it using X-rays, but without specific methods, it was difficult to put it stably [16]. With an application of Widmer’s method, the acetabular component could be reliably placed in the target range in this study. Whether other methods can be applied during surgery may need more investigation.
The Widmer’s method assumed that radiographic cup orientation is measured on a plain radiograph centered on the symphysis and showing both hips. However, because of the narrow field of view of C-arm used, it is difficult to conduct research in this ideal situation. Although the range of the C-arm image is limited, the intraoperative radiograph centered on the symphysis including both hips. X-rays focused on the acetabular component have also been investigated, in which this situation can affect the outcome. Further research is needed to determine wherein the results of measurements can vary depending on the focus.
Although controversial, there are reports showing that the reliability of the validity of the Widmer method is not satisfactory. However, this formula was adopted since it was considered the most appropriate method available intraoperatively. There is a desire to be cautious in that we have identified the possibility of changing the acetabular component properly during surgery.
Since the CT scan is known as a more accurate imaging tool to measure the acetabular component, we want to use the postoperative CT scans measurement as reference standard. The resulting costs are not negligible, and the authors’ country may compensate the cost to some extent with the expansion of medical insurance.
The method of Widmer was accurate using intraoperative imaging intensifier for the measurement of the anteversion of the acetabular component during THA, when compared to the anteversion obtained from the 3D computed tomography. Using the intraoperative C-arm imaging was very valuable because it allowed for correction of the position of the acetabular cup intraoperatively. Ultimately, with an application of Widmer’s method frequency of subsequent early dislocations can be approximately 1% which was not above than known figures. The mean Harris hip score after THA in one year was 94.2 (82–98).