Surgeons are used to performing hip replacements in the dominant side and their preference caused by handedness had further adverse impact on cup positioning in the non-dominant side. Right-handed surgeons were more likely to place the left cup in larger anteversion (24.77±10.44 vs 22.44±8.67, p=0.043). The patients in manual group were significantly more likely to have different cup positioning between bilateral hips than those in robotic group (77% vs 45%, p=0.000). More manual THA were located out of the target zone than robot-assisted THA (70% vs 48%, p=0.001). Robot-assisted THA was more stable in cup positioning than manual THA and robot might help surgeon eliminate the adverse impact of personal innate handedness. The above results were the important findings of this study.
Handedness is also called the lateralization of chirality, which is defined as the property of using one hand more than the other . The effect of handedness, which can be reflected in most of surgery, isn’t generally considered to have a significant effect on their surgical outcome, because surgeons can adjust their positioning and perspective to optimize intracorporeal maneuverability . However, the impact of handedness on orthopedic surgery is far greater than other surgeries. Bones are symmetrical anatomical structures and different from the internal organs in a constant position of the body [6-7]. The surgeon’s standing position during operation will directly affect the accuracy of spatial positioning and an unaccustomed perspective will further lead to visual errors. Furthermore, the division of labor between the right and left hands is radically distinct during the bilateral orthopedic surgeries. When the non-dominant hand dominates one surgical procedure, it could compromise the surgical performance and clinical outcomes.
In 1994, Moloney et al first reported the impact of handedness on the surgical outcome in orthopaedics . They concluded that malpositioning of the failures occurred significantly more frequently on the left than on the right, in a Unit where all the surgeons were right-handed.
In 2014, Pennington et al first reported that surgeon handedness appeared to influence acetabular component position during THA after analyzing 160 patients who were operated by 4 surgeons . However, their study had several obvious drawbacks. The sample size of single surgeon was relatively small. No demographic comparisons between the patients who underwent different side of THA. The type and fixation of the prosthesis were also not controlled. There was only one observers and no repetitive measurement to perform consistency analysis. Furthermore, the outcome didn’t include the anterversion, functional score and complications.
Current debates regarding optimal position of the acetabular cup remained unsolved. Several surgeons have put forward the safe zones for inclination and anteversion respectively [22-23]. The most commonly used safe zone was established by Lewinnek et al (anteversion: 5-25°; inclination: 30-50°) in 1978 . However, recent studies have reported that an inclination of 45° or greater was associated with a significant increase in linear wear per year compared with an inclination less than 45° [24-25]. Thus, Callanan et al redefined the safe zone (anteversion: 5-25°; inclination: 30-45°) in 2011 .
In 2019, Crawford et al compared the acetabular component position differences between right and left hips for a right-hand dominant surgeon . In their study, right hips had a significantly lower abduction and less combined Lewinnek outliers through direct anterior approach, and right hips had significantly higher anteversion and Lewinnek anteversion outliers through posterolateral approach. Significant superiority of cup positioning were found in both approaches based on the surgeons dominant and non-dominant side. However, they also ignored an important influence factor, which were the comparability between the two sides of THA has not been fully established.
In this study, the target zone referred the above literature and was further reduced (anteversion: 15-25°; inclination: 35-45°). If the acetabular cup in one side was accurate (anteversion: 20°; inclination: 40°), the other side would reach the outlier with the difference of 5°. An inclination of 45° or greater would increase stress concentrations to degrade component durability [24-25]. And that the robotic system’s acceptable error for cup anteversion and inclination was 5°. That’s why we used a difference of 5° as the cut off to define the different cup positioning. We enrolled the patients who underwent the simultaneous bilateral THA to avoid the difference of the acetabular bone mass and demographic differences between the patients who underwent unilateral THA. Another strength of this study was that we included the anteversion, functional score and complications. Finally, we compared the robot-assisted with manual THA to verify the robot's advantage in eliminating handedness.
In manual THA, cup positioning comes from first the angular proprioception and then manual implantation. The previous study in our institute showed that the placement of cup performed by dominant hands is more accurate than that performed by non-dominant sides .This study also confirmed this result. The surgeons’ handedness had significant influence on cup positioning and right-handed surgeons were more likely to place the left cup in larger anteversion in manual THA. As Kanawade et al reported, they performed robot-assisted THA in 38 patients (43 hips) and measured the cup positioning by postoperative CT scans. There were 12% and 16% outlier of 5° in inclination and anteversion respectively . However, whether the more accurate cup positioning by robot could improve the patients’ clinical outcomes remained to be seen. In this study, no significant difference of postoperative HHS between bilateral THA was found. Even so, we should be aware of the potential side-effect that may be introduced by the surgeon’s handedness and laterality of operated extremities. Each surgeon should consider taking extra precautions to diminish or eliminate the adverse results when operating on the non-dominant side . Because the scoring system of Harris has two inherent disadvantages, namely ceiling effect and low sensitivity, the postoperative hip function and dislocation rate between bilateral cups may be significant with the refinement of scoring system, enlarging of sample size and extension of follow-up period.
In the recent years, sophisticated tools have been emerging to reduce the differences between surgeons and sides. In addition to these tools, robot might be one alternative to eliminate the adverse influence of handedness [4,21,27]. However, as the surgeon has to use the non-dominant hand to complete critical procedures, such as acetabular registration, reaming and cup implantation during robot-assisted THA of the non-dominant side, the surgeon’s handedness could still influence the cup positioning theoretically.
The results of this study found that the surgeons’ handedness had no significant influence on robotic cup positioning. While the bilateral cup positioning existed some deviation in robot-assisted THA, it had no inclination to either side. Robot was capable of eliminating the innate handedness in early practise of robot-assisted THA, regardless of the surgeon's experience.
Intraoperative feedback mechanisms was the important factor which contributed to the consistent clinical outcome between bilateral robot-assisted THA. The robotic real-time feedback mechanism had powerful ability to help surgeons get rid of the limitations of visual spatial positioning.
In the future, medical training may be one of the promising directions of robot application in orthopedics [7,29]. The undifferentiated performance of robot among different surgeons and different sides demonstrates its great potential role in the surgical training and education. In joint replacement, the perception of component positioning requires a lot of practice and immediate feedback to reach a steady state, especially on the non-dominant side. The adoption of robot could allow novice surgeons to form the correct sense of spatial orientation and reduce the risk of prosthetic malposition. The accumulation of experience and the progress of learning can be accelerated with the haptic feedback of semi-active robot.
This study has several limitations. Firstly, the measuring bias couldn’t be ignored. Although the application of Orthoview systems in measuring anteversion and inclination was reported in several studies, the postoperative measurements basing on the X-rays was inferior than the computerized tomography (CT) and magnetic resonance imaging (MRI). Postoperative CT scan would have enabled more accurate assessment of radiographic outcomes. Secondly, both of the surgeons enrolled in this retrospective study were right-handed and had rich experience in joint replacement. The prospective studies including the left-handed and young surgeons should be conducted in future. Thirdly, the small sample size and short follow-up period might mask the possible differences. The significant difference of cup positioning didn’t bring out the significant change of clinical outcomes. Although the power of comparison of different cup positioning (0.96) and target zone ratio (0.93) was convincing, the power of comparison of anteversion was relative low (0.53), which affected the persuasion of the conclusion to some extent. Fourthly, dislocation is the result of various factors and combined femoral-acetabular components’ position is crucial important. In this study, the different proportion of DDH cases with presumably higher femoral anteversion between two groups may have influenced the dislocation rate.