The role of nickel-titanium shape memory alloy embracing xator in determination of the implantation angle of hip tumor prosthesis stem following tumor resection and the analysis of its ecacy (cid:0) a retrospective study

Background: To explore the role of nickel-titanium(NiTi) shape memory alloy embracing xator in determination of implantation angle of hip tumor prosthesis stem and analyze its ecacy. Methods: 36 patients with proximal femur tumor were treated with extended tumor resection and prosthetic replacement. 14 patients received prosthetic replacements with the embracing xators xing between the junction of the prosthesis stem and the femur temporarily, while the other 22 patients received the same replacements but without the xators. Following aspects were compared: occurrence of complications, limb function and active hip ROM. Results: There are fewer cases of hip dislocation in the group received prosthetic replacements with the use of embracing xators. Occurrence of deep infection presented no difference between the two groups. Better limb function and higher active range of motion (ROM) on abduction or exion were also found in the group using embracing xators. Conclusion: Ni-Ti shape memory alloy embracing xator plays a key part in assisting the accurate implantation angle of the prosthesis stem in the prosthetic replacement. The prosthesis stem can be adjusted to the optimal angle with the help of the embracing xator. Patients have lower chance of dislocation, better limb function, and higher active hip ROM.


Background
The proximal femur is the most frequent region for primary and metastatic bone tumors. Currently, surgical removal of the tumor is an important part of its comprehensive treatment. Surgical treatments include amputation and limb preserving surgery. Prosthetic replacement following tumor resection is one of the most widely used and effective methods in limb salvage surgery [1]. Because the prosthetic replacement after tumor resection can preserve limb function and improve life quality, it has gained increasing preference in treating proximal femoral tumors. The key point of the prosthetic replacement following tumor resection is seeking balance between limb function, the stability of the prosthesis and the complete clearance of the tumor. However, in order to ensure the complete clearance of involved site, surgeons are compelled to cut off the femur at a lower plane, resulting in the loss of the marker for determining the implantation angle of the prosthesis stem, which causes great di culties in determining the accurate implantation angle of the prosthesis stem after osteotomy [2]. In general, the implantation of the prosthesis stem mainly depends on the experience of the surgeon, and the angle must be adjusted before the bone cement was completely shaped and solidi ed. Therefore, there is an urgent need to temporarily x the prosthesis stem so that the surgeon can adjust the angle after implantation.
Ni-Ti shape-memory alloys rst appeared in the 1960s, which are characterized by smart mechanical properties, have been gaining increasing importance in diagnostic applications and minimal invasive surgery, such as the neurosurgical [3], cardiovascular [4], orthodontic elds [5] and so on. With its shapememory effect, corrosion resistance, super-elasticity, and favorable biocompatibility, nitinol has been used for internal xation since the 1990s [6][7][8][9][10]. Our group designed a novel Ni-Ti shape memory alloy embracing xator for temporary xation in the implantation of the prosthesis stem, which has achieved favorable outcome in clinical practice. This study intends to explore the role of Ni-Ti shape memory alloy embracing xator in determine the implantation angle of hip tumor prosthesis stem and its e cacy by comparing with traditional technique.

Study Design and Patients Retrieval
This was a retrospective observational cohort study, and the present study was approved by the The 36 patients were classi ed into two groups: 14 had received prosthetic replacements with embracing xators xing at the junction of the prosthesis stem and the femur temporarily after proximal femur tumor resection and 22 had received the same replacements but without the use of embracing xator. Results were compared between the groups that had received prosthetic replacements with or without the use of embracing xators to assess whether the application of the embracing xator would reduce the incidence of hip dislocation and provide better post-operational outcomes.
Structure and working principles of the Ni-Ti shape memory alloy embracing xator The Ni-Ti shape memory alloy embracing xator device (Huzhou Swan Biological Memory Medical Devices Co., Ltd., Zhejiang, China) is designed based on the anatomical structure of femoral shaft and manufactured with 2 mm-thick Ti-Ni shape memory alloy. The embracing xator comprises o 50%-53% nickel, with the remainder comprising titanium. The device has six pairs compression arms that are connected to the waist. The embracing xator is malleable at lower temperatures (martensite phase), thus, it is placed in 0-4 °C ice water for cooling before implantation. When the xation is completed, 40 °C-50 °C water was used to warm the device to stimulate its memory mechanical memory functions (austenite phase), providing a continuous lateral compressive force for temporary xation. Brief illustrations of the embracing xator application process are shown in Fig. 1. The shape change process of the Ni-Ti shape memory alloy embracing xator is shown in video 1 (supplement material 1)

Surgical Methods
Preoperative MRI and PET-CT were performed to assess the extent of tumor invasion and provide basis for determining the scope of resection. All operations were performed using posterolateral hip incision.
The piriform muscle and the externally rotating muscle were cut off. The capsule of the hip joint was then cut open with an H-shaped incision (It can be resected if it is tumor-invaded). The surgical margin of osteotomy was 2-3 cm distal to the invasion extent of the tumor, in accordance with preoperative MRI imaging. The resection extent of soft tissue was acme distal to the reactive zone, and the extent of resection was determined again according to the result of intraoperative frozen section. The acetabular cup was biologically xed. According to the preoperative design, we selected the appropriate femoral stem and femoral head. After implanting the prosthesis stem, a Ni-Ti memory alloy embracing xator was used temporarily to x between the junction of the prosthesis stem and the femur so that the xator could keep the prosthesis stem stable. Then the prosthesis joint was reduced and moved in all directions to assess its stability. If the prosthesis joint was easily dislocated, the implantation angle of the prosthesis stem would be readjusted. When the prosthesis stem was adjusted to an optimal angle at which the prosthesis joint was the most stable and exible, the implantation angle of the prosthesis stem was marked on the femur. Then the embracing xator and prosthesis stem were removed. Bone cement was injected into the pulp cavity, and the prosthesis stem was inserted according to the marked angle. The prosthesis joint was reduced after coagulation of the bone cement. The prosthesis joint was moved in all directions again to observe prosthesis stability. Then the muscles around the hip joint was reconstructed in sequence before closing the incision. The external rotation muscles and the piriform muscle were xed on the greater trochanter ring of the prosthesis, and the iliopsoas muscle was xed on the lesser trochanter ring (Fig. 2). Detailed surgical procedure please refers to the sketch map of the application of embracing xator in the replacement of proximal femur tumor prosthesis (Fig. 3) and the dynamic simulation of the overall process is shown in video 2 (supplement material 2). Preoperative MRI of proximal femur tumor, the resected tumor part and postoperative imaging are displayed (Fig. 4).

Postoperative Management
The patients were placed in supine position after surgery and triangular pillows were placed between the legs. The patients were told to practice muscle contraction and relaxation, as well as joint exion and extension, turn over in bed and take deep breaths. Low-molecular-weight heparin was subcutaneous injected in all patients for 35 days to prevent thrombosis. The drainage tubes were removed in 2 to 3 days and the sutures were removed in 10 to 12 days after the operation. The patients began to practice sitting up, sitting at the bedside and standing on the oor 3 weeks postoperatively, and began to practice walking with walk aid instruments 4 weeks postoperatively. Radiotherapy and chemotherapy were also administered when needed. Professional physical therapy was suggested for all patients after being discharged. Post-operational follow-ups were demanded every three months for every patient, including clinical examination and imaging assessment.
Clinical outcomes evaluation Follow-up and radiographic assessments were routinely performed at 1,3 and 6 months after prosthesis stem implantation and long-term postoperative review was conducted for 12-18 months once, the average follow-up was 14.0 (range, 12-24) months according to our team experience. Complications like hip dislocation, infection, aseptic loosening, tumor progression and procedure-related minor complications were counted between two groups in medical records. Regarding functional evaluation, the Musculoskeletal Tumor Society Scores (MSTS), Harris hip scores (HHS) and ROM were also achieved in the previous records. The MSTS score [11,12](Musculoskeletal Tumour Society Score) is the doctorbased questionnaire which evaluates the functional condition after completed tumor treatment. This examination assesses six criteria, for the lower limb the components are pain (evaluated by VAS), function, emotional acceptance of the treatment outcome, walking, gait and need for walking aids. Among all criteria, the assessment is made from bad to very good with parallel awarding of points (0 to 5). Harris Hip Score is also a common functional evaluation, which has been used in the proximal femoral replacement surgery post-operative assessment [13]. Range of motion (ROM) included exion, abduction and external rotation is evaluated after the replacement.

Statistical Analysis
Statistica squared test was used for comparison of the rate. The independent-sample t-test was used to determine the statistical signi cance of differences in measurement data between the two groups. We used the Chi-square test for analysis of enumeration data. P < 0.05 was considered to be statistically signi cant.

Results
All 36 patients received prosthetic replacements with or without the use of embracing xators survived the perioperative period. The two patient groups presented no signi cant difference in age (P=0.399), length of femur resection (P=0.949), operation time (P =0.664), blood loss (P=0.765) and chemotherapy (P=0.810). (Table 1) Table 1 Comparative features, diagnoses, and complications between groups with and without the use of embracing fixator. Page

Complication Occurrence
The group that used embracing xators during prosthetic replacements had lower incidence of hip dislocation than the group that didn't use embracing xators. The incidence of hip dislocation in the group with the use of embracing xators was 0% (0/14), compared to 27.3% (6/22) in the group without the use of embracing xators (P=0.032). The incidence of infection exhibited no signi cant difference between two groups: 0% (0/14) and 4.5% (1/22) (P=0.429). The incidence of aseptic loosening, and tumor progression showed no difference between groups (Table 2). Procedure-related minor complications also included 1 case of leg-length discrepancy in the group with the use of embracing xator compared to 3 cases in the group that didn't use embracing xator (no difference), and 1 case of deep vein thrombosis in the no embracing xator group. (Table 2) Table 2 Mode of failure of the prosthetic replacements with or without the use of embracing fixator.

Discussion
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 signi cantly [20][21][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][24][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 [26]. 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) [23]. Postoperative dislocation of the hip joint causes pain to patients, increases treatment cost, and brings di culties 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 [29]. 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 [29]. 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 [30]. 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 xed with bone cement. And once the prosthesis has been xed, 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 xator 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, exible plasticity, and good biocompatibility that can automatically restore its original shape at a speci c temperature [35]. Thus, the embracing xator can be arti cially 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 xator 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 x it after implanting the prosthesis stem immediately. Instead, we temporarily used the Ni-Ti shape memory alloy embracing xator to x 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 x 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 xator 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 xed 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 xators had lower rate of hip dislocation, better limb function, and greater range of active hip movement on exion and abduction than those without the use of embracing xators. The most signi cant complication is dislocation. Puchner SE et al reported in his study that 8%-33% of patients treated without the embracing xators suffered dislocation [23], which is similar to our study nd 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 xator to x 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 xator and readjust the implantation angle of the prosthesis stem until dislocation didn't occur at any angle. The use of embracing xator 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 satis ed. 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 xators were better than those without the use of embracing xators (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.

Conclusion
Ni-Ti shape memory alloy embracing xator plays a key part in determining the accurate implantation angle of the prosthesis stem in the prosthetic replacement following the proximal femur bone tumor resection. The implantation angle of the prosthesis stem can be adjusted repeatedly with the aid of the Ni-Ti embracing xator, so that the prosthesis stem can be implanted at an optimal angle and the prosthetic head and the acetabular cup can be optimally matched. This technique can prevent hip dislocation effectively, with better limb function recovery and fewer complications.

Declarations
Ethics approval and consent to participate: The Ethics Committee of Changhai Hospital approved the study. The informed consent was waived because this is a retrospective study.