The proximal femur is one of the most frequent region for primary and metastatic bone tumors[14, 15]. The invasion of the tumor and the concomitant pathological fracture have caused great suffering to the patients and seriously affected their quality of life. In the past, amputation is the dominating choice of treatment for bone tumors, which caused severe physical and psychological trauma [16, 17].
With the continuous improvement of surgical technique, imaging technique and pathological diagnosis, prosthetic replacement after tumor resection has become the preferred treatment for proximal femoral tumor on the basis of chemotherapy, radiotherapy and immunotherapy[18, 19]. The prosthetic replacement after tumor resection can preserve the patient's limb, relieve pain, prompt early recovery of limb function, and improve the patient's life quality significantly [20–22]. However, due to tumor invasion and severe damage of soft tissue around the bone and joint caused by the surgery, the postoperative joint stability is affected, and joint dislocation is more likely to occur than common hip replacement [23–25]. In previous studies, hip joint instability (hip dislocation) resulted in most cases of failure of proximal femur prosthetic replacement, and limb function after surgery was unsatisfactory. PUCHNER S E et al reported that the general rate of dislocation following proximal femoral prosthetic replacement was 13% after an average period of 7 ± 8 months (range from 0.3 to 33 months) after surgery. From 1982 to 1986 the dislocation rate was about 33% and slowly dropped to 9% in the following two decades (1987–2008) . Postoperative dislocation of the hip joint causes pain to patients, increases treatment cost, and brings difficulties for the doctors. Therefore, it is of great importance to determine the accurate implantation angle of the prosthesis stem during the procedure [27, 28].
The main factors related to the joint stability after replacement include the accurate implantation angle of the prosthesis stem and the reconstruction of the soft tissue around the joint . In order to remove tumor tissue as thoroughly as possible, the prosthetic replacement after tumor resection will cause more serious damage to the soft tissue around the joint than common joint replacement. Moreover, the soft tissue around the joint might not be completely and effectively reconstructed during surgery, which leads to postoperative joint dislocation. Under these circumstances, accurate implantation angle of the prosthesis stem appears to be particularly important, which requires the surgeon to ensure that the implantation angles of the acetabular cup and prosthesis stem are appropriate and accurate. To ascertain the implantation angle of the acetabular cup, the surgeons could refer to the normal acetabular angle, because typically the proximal tumor of the femur does not invade the acetabulum. There have been many studies exploring methods for positioning the acetabular cup, including the use of anatomical markers, pre-evaluation with CT scans, and three-dimensional directional monitoring. However, there are few reports about the concrete procedure to determine the correct implantation angle of the prosthesis stem.
Because of the tumor invasion of the proximal femur, it is often necessary to cut off the greater trochanter and lesser trochanter during the osteotomy, thus losing the anatomical markers as the reference for implanting the prosthesis stem. Surgeons usually make marks on the junction before resecting the involved bone, and make marks on the corresponding part of the prosthesis stem to offer reference when implanting the prosthesis stem [31, 32]. After implanting the prosthesis stem, it will be fixed with bone cement. And once the prosthesis has been fixed, even if the implantation angle of the prosthesis stem is not satisfactory, the surgeon cannot readjust it, resulting in the prosthetic head and acetabular cup not being optimally matched, leading to a greater tendency towards hip joint dislocation[33, 34].
In this study, we used a novel embracing fixator made of the Ni-Ti shape memory alloy to help us determine the implantation angle of prosthesis stem. Ni-Ti alloy is a shape memory alloy with high strength, flexible plasticity, and good biocompatibility that can automatically restore its original shape at a specific temperature . Thus, the embracing fixator can be artificially spread to permit surgeon to adjust prosthesis stem as required. After heating to 40 °C ~ 50 °C(caused by lavaging warmed normal saline), the embracing fixator can recover the original memorized shape and contract to grasp tightly the prosthesis and the distal bone together. During the operation, we did not use bone cement to fix it after implanting the prosthesis stem immediately. Instead, we temporarily used the Ni-Ti shape memory alloy embracing fixator to fix between the junction of the prosthesis stem and the femur to keep the prosthesis stem stable. Then the hip joint was reduced by the temporary fix and moved in all directions to assess the stability of the joint. If the surgeon felt that the implantation angle of the prosthesis stem was not ideal, he can remove the embracing fixator easily and readjust the implantation angle of the prosthesis stem. When the prosthesis stem was at the optimal implantation angle, the surgeon marked the angle of the prosthesis stem relative to the femur and then fixed the prosthesis stem with bone cement. By this way, the implantation angle of the prosthesis stem can be repeatedly tested to determine the best implantation angle of the prosthesis stem, which greatly reduces the incidence of postoperative dislocation.
In our study, we found that patients who received prosthetic replacements with the use of embracing fixators had lower rate of hip dislocation, better limb function, and greater range of active hip movement on flexion and abduction than those without the use of embracing fixators. The most significant complication is dislocation. Puchner SE et al reported in his study that 8%-33% of patients treated without the embracing fixators suffered dislocation, which is similar to our study find that 6 (27.3%) patients experienced in control group. A lot of postoperative dislocations are caused by poor placement of prosthesis. In our research, we inserted the prosthesis stem according to our experience and then used the embracing fixator to fix between the junctional part of the prosthesis stem and the femur to keep the prosthesis stem stable. Then the joint was reduced and moved in multiple directions to test the stability of the joint. If dislocation occurred at a certain angle, we could remove the embracing fixator and readjust the implantation angle of the prosthesis stem until dislocation didn’t occur at any angle. The use of embracing fixator allows surgeons to adjust the implantation angle of prosthesis stem repeatedly until the satisfactory angle is reached, so that the prosthesis can be optimally matched. This method could avoid the disadvantage of traditional methods in determining the implantation angle of prosthesis stem only one time, which could not be adjusted even if it is not satisfied. This is very helpful for preventing post-operational dislocation. In addition, the best matching of prosthesis can effectively accelerate the recovery of limb function and ROM of hip joint after operation[36, 37]. In this study, the MSTS and HHS scores for the patients with the use of embracing fixators were better than those without the use of embracing fixators (P < 0.05). This may be attributed to the accurate placement of the prosthesis and the absence of post-operational dislocation, which were very important for the rehabilitation of the limb function and hip joint movement.
Our study had some limitations because it was a retrospective study. Firstly, low morbidity resulted in small sample size, although we collected data covering an 8-year span. Secondly, during the 8 years of the study, the progress in surgical technique and prosthesis design might have also played a role in lowering dislocation rate and improving limb functions. These biases might affect our interpretation of the results.