We found the enhanced femoral workflow provides more accurate assessments on HL change when compare with preoperative plan, compared to the express femoral workflow. The COs are similar using the two registrations. The operating time for both workflows is also similar.
Robot-assisted and computer navigation in THA has known to restore the hip offset and LLD successfully to a very high degree of accuracy. Robot-assisted hip replacements use intraoperative references and measurements to improve the accuracy of LLD and COs. Measurements were recorded during the operation using arrays on the femur and pelvis. In traditional THA, Ranawat et al [13] found that the incidences of LLD after THA are between 1% and 27%, and the variances are between 3 mm and 70 mm. However, in recent years, applications of robotic technology have integrated as a potential answer to these concerns [14]. Since the core of the operation involves positioning of the acetabular component, most of the published studies with robotic systems have focused on the position of the acetabular cup with and without robotic assistance[15, 16]. However, some studies have determined the accuracy of this technology in restoring LLD and CO. Nawabi et al.[15, 16] in a cadaveric study of robot-assisted THA, found that the mean difference between the postoperative changes compared on CT was 1.0 ± 0.7 mm for LLD and 1.2 ± 1.1 mm for CO. Recently, Nodzo et al.[17] utilized a CT study to evaluate the accuracy of implant placement when using robotic assistance during THA, and found that the postoperatively measured mean change in overall leg length and overall hip CO was 1.6 mm ± 2.9 and 0.5 mm ± 3.0, respectively.
However, Enhance formal require the insertion of one large screw in the trochanter to hold the femoral array. Thus, the robotic system is mainly reliant on the stability of this screw and array for accuracy of LLD and CO measurement. Any disruption or loosening of arrays is likely to compromise the accuracy. This is a rather usual event since the mounting screw of the femoral array has a single point of fixation.
During the procedure, the femoral array is repetitively mounted and dismounted, making the screw more susceptible to loosening throughout the procedure, particularly in osteoporotic bone. Although the loosening of the screw does not affect the positioning of the acetabular component, it does affect intraoperative feedback on leg length and offset. This has been demonstrated to occur in 5% of robotic THAs[18]. Once the screw loosened, any alternative options are possible, and LLD and CO measurement had to be performed manually.
For the express registration, the distance between the pelvic frame and the marking points of the femur is recorded. However, it is less reliable, because the landmarks of the proximal trochanter are changed with the intraoperative movement of the femur. Before and after hip dislocation, the proximal landmarks are changes, and the distal landmarks are changed with the femur movement. The electrocardiographic lead also moves with the skin. All those factors decrease the accuracy of LLD measurements. For the enhanced registration, both the pelvic and femur are registered altogether, which increases the accuracy of LLD, resulting in a smaller variance between the two legs.
Compared with the express workflow, the enhance workflow increased the registration of the femoral side, so the average registration time increased by 2.9 minutes. The overall operation time was extended by 4.5 minutes. Although the difference of operation time between the two groups was statistically significant. But we think with the learning curve progress and experience improved, the operating time is possibly decreased.
To our knowledge, this is the first study in the difference which the LLD and CO were measured with the two femoral workflows. enhanced femoral workflow provides more accurate assessments on LLD, compared to the express femoral workflow.
This study has limitations. First, the sample size is relatively small, and outcomes from larger cohorts may vary. Second, postoperative X-rays should add in the future study, because the overall position of prosthesis is well demonstrated on X-ray.