The cup placement accuracy using our method was 2.4° ± 2.1° for inclination and 2.8° ± 2.6° for anteversion. The method using the alignment guide reportedly deviates from the safe zone of Lewinnek in many cases [16]. Some previous reports have investigated accuracy of image-free navigation systems and CT-based navigation systems. Accuracy of CT-based navigation systems is reported as 1.2°–3.2° for inclination and 1.0 °–3.3° for anteversion [8, 17–19]. Accuracy of image-free navigation systems is reported as 2.9°–3.6° for inclination and 4.2°–6.7° for anteversion [17, 20–22] (Table 2). Although the result obtained using our method was inferior to that obtained using the CT-based navigation system, it was comparable to the result obtained using image-free navigation. These results indicate that this method has practicality that can demonstrate clinically reliable accuracy. In 2012, Peters reported an intra-operative angle measurement method using the accelerometer and camera function of the iPhone for improving the accuracy without using the navigation system [12]. In that method, inclination is measured using an application that uses an accelerometer, whereas anteversion is measured using an application that displays a protractor with a camera function. This method does not consider the movement of the pelvis during surgery and assumes that the pelvic plane is always in the ideal position.
Table 2
Accuracy of imageless and CT-based navigation systems reported in the literature
| Inclination (degree) Absolute value | Anteversion (degree) Absolute value | Type | Navigation system |
Kalteis (30 hips) | 3.0° ± 2.6° | 3.3° ± 2.3° | CT-based | The Vector Vision hip 3.0 |
Iwana (117 hips) | 1.8° ± 1.6° | 1.2° ± 1.1° | CT-based | Stryker CT-Hip System V1.0-29 |
Nakahara (49 hips) | 1.2° ± 1.3° | 1.0° ± 0.5° | CT-based | Stryker Navigation System2 |
Tetsunaga (30 hips) | 3.2° ± 2.4° | 3.0° ± 2.5° | CT-based | The Vector Vision Hip CT-based version 3.5.2 |
Kalteis (30 hips) | 2.9° ± 2.2° | 4.2° ± 3.3° | Image-free | The Vector Vision hip 3.0 |
Ybinger (37 hips) | 3.5° ± 4.4° | 6.5° ± 7.3° | Image-free | The PiGalileo THR, Plus |
Lass (62 hips) | 3.2° ± 2.4° | 6.5° ± 3.7° | Image-free | The Navitrack |
Takeda (118 hips) | 3.6° ± 2.6° | 6.7° ± 3.6° | Image-free | The Orthopilot THA Pro |
Current study (30 hips) | 2.4° ± 2.1° | 2.8° ± 2.6° | | |
The alignment guide method, which is based on the floor plane and the longitudinal axis plane of the body, is susceptible to intra-operative pelvic movement [23]. Kanazawa reports that the pelvis tilts in each of the sagittal, axial and coronal planes during surgery [24]. Compared with Peters’ method, our method can correct intra-operative pelvic movement with the help of the pelvic positioner.
Since the THA cup protractor is a simple digital angle measuring application, it cannot follow intra-operative pelvic movement like the navigation system. Therefore, when measuring the placement angle, it is necessary to confirm that the ASIS is in the centre of the pelvic positioner fixture. If there is a deviation, it is necessary to return the positional relationship between ASIS and pelvic positioner to the state at the time of set-up.
Navigation systems generally require pins to be inserted into the pelvic to fix the navigation tracker. Therefore, it requires invasion of the patient and additional operative time. In comparison, our method uses the pelvic positioner as a reference plane; thereby making patient invasion unnecessary which is also an advantage of our method.
It has been reported that the surgical approach affects the cup placement accuracy. The minimally invasive surgery (MIS) approach has the disadvantage that anatomical recognition is difficult because of the small field of view. Also, the cup placement accuracy is inferior because it is easily affected by intra-operative pelvic movement. Therefore, it is recommended that the navigation system be used in this approach [25]. The modified Watson–Jones approach, which is a MIS approach, has the advantage of maintaining hip abduction muscle strength and posterior stability. However, there are many variations in the cup placement position, such as a significantly larger inclination than the posterior approach [26]. In this study, there was no significant difference in the cup placement accuracy between the modified Watson–Jones approach and the Dall approach. This result suggests that our method ensures high cup placement accuracy irrespective of the approach used.
This study has limitation. It was not randomised, however, the patients’ demographic factors were unlikely to have affected the results because the two groups were comparable in terms of gender, BMI and underlying disease.