The aim of this study is to verify the safety and accuracy of the new robot system who can cover the femoral side during THA procedure, which maybe the first robotic arm with the function for preparing the femoral canal, as well as complete the osteotomy of femoral neck. As the classic safe range for acetabular side, Lewinnek safety zone(14) points out that the anteversion of acetabular should be 15 °±10 °. The inclination angle of acetabulum should be 40 °±10 °. The plan of this trial was also planned according to this safety zone. After operation, all of the acetabular anteversion and inclination angles were measured in the safety zone (Fig. 2). And there was no fracture in all the cadavers.
However, some new studies propose that the Lewinnek safe zone is not actually reasonable(15–17). A study by Abdel even found that many dislocations occur within the Lewinnek safety zone(18). With the development of new technology and the progress of ideas, more and more people advocate the use of combined anteversion to replace the Lewinnek safety zone(19–21). The factors of the femoral stalk were emphasized in this method, and with the development of navigation technology, the joint forward inclination will be measured more accurately(20, 22). A study by Amuwa pointed out that the safe range of combined anteversion is 25 °- 50 °, and the average value should be 35 °(19). Jolles's study suggested that the combined anteversion should be 40 °- 60 °, otherwise the risk of dislocation would be 6.9 times(23). A study of O'Connor put forward the concept of functional combined anteversion, and pointed out that standing combined anteversion should be 30° − 50° and sitting combined anteversion should be 45° to 65°(24). So far, there is no unified standard for combined anteversion. Most people think that the combined anteversion of 25 °- 50 °is more appropriate(19, 20, 25). With the help of robot arm, 23 of the 30 sets of data (76.67%) fall within this range.
The stem angulation of the femoral prosthesis is the related factor leading to the subsidence of the femoral prosthesis(26). A study of Leiss found that if the stem angulation is more than 3 °, it will increase the risk of femoral prosthesis sinking(27). The stem angulation of femur assisted by robotic arm was 1.84 ± 0.99 °, and the median was 2.00 °, all of 30 sets of data were < 3 °.
Prosthesis revision is related to prosthesis instability and aseptic loosening(28, 29), and loosening of the femoral stalk is related to poor filling rate of the femur(30). A study of Tezuka pointed out that the filling rate of proximal femur is satisfactory when the rate is ≥ 80%(31). Both Hwang and Streit have proved when the filling rate of the femur is less than 80%, it may lead to aseptic loosening or sinking of the femoral stalk(32, 33). The filling rate of all the prostheses on the median sagittal section of the osteotomy line and at the 2.5cm off the osteotomy line were more than 80%. The filling rate was > 80% in 21 cases (70%) on the coronal section of the osteotomy line 7.5cm, and > 80% in 18 cases (60%) on the coronal section of the isthmus of the femoral shaft. (Table 4)
Periprosthetic fracture is the main cause for early femoral revision (34). A recent study by Alpaugh found that the risk of periprosthetic fractures increased when the femoral medullary cavity was paired with a smaller femoral prosthesis(35). Alpaugh 's study linked the canal fill and the femoral angulation to periprosthetic fractures.
A study of 51,345 revision by Bozic et al found that 21,047 cases (41.1%) had all-component revision and 6,738 cases (13.2%) had femoral component revision(36), accounting for 54.3% of THA revision, it means that more than half of the revision operations involved femoral prosthesis. However, a study by Brown showed that revision surgery of femoral prosthesis is often complicated due to the poor bone stock or the difficulty to remove prosthesis, which affecting the effect of surgery(37).
It can be seen that the placement and size of femoral prosthesis also has an important impact on the success of THA surgery. However, at present, the auxiliary navigation system on the market can only assist the placement of the acetabular prosthesis, and it still needs to be handled manually on the femoral side.
The robot is a 7-axis manipulator, which breaks through the technical difficulties, so that the operation of the acetabular and femoral side can be completed without changing the position of the machine, and the size, angle and depth of the prosthesis can be monitored in real time (Fig. 3). And there is no need to repeat the process of rough registration and fine registration, which means that all surgical operations can be done in one registration.
The new designed robot arm was safe and accurate for both acetabular and femoral sides during total hip arthroplasty. As the first robotic assisted system that can complete the operation on femoral side, this new robotic system is a meaningful attempt. Of course, this experiment also has some limitations, such as: unable to count the amount of blood loss, the balance effect on soft tissue is not obvious, unable to obtain follow-up data under the postoperative weight-bearing state, and so on. However, this trial provides a guarantee of safety and reliability for the following clinical trials, and the limitations of the trial will be resolved at the clinical condition.