For patients with displaced femoral neck fracture of the hip and ischemic necrosis of the femoral head, total hip arthroplasty(THA) or hemiarthroplasty(HA) is often chosen to improve the quality of life and reduce bed-related complications, and its good clinical efficacy helps patients return to work and life soon . At present, cemented fixation and cementless fixation are commonly used in femoral prosthesis fixation. The choice of cemented fixation for hip arthroplasty in elderly patients is easier to achieve initial stability and reduce the probability of intraoperative fracture, but it is also accompanied by some serious complications related to bone cement . The cementless prosthesis is initially applied to young patients, because the biotype prosthesis takes advantage of bone growing into the pore of the prosthesis, and the hydroxyapatite coating on the prosthesis surface can be more closely combined with the femoral cortex of the patient, so it has a lower prosthesis revision rate . Some researchers have begun to explore whether cementless prostheses can provide the same advantage in elderly patients. The use of cementless prosthesis can significantly reduce the operation time and avoid BCIS, and improve the perioperative survival rate of elderly patients.
Koettnitz et al.  proved that, compared with patients < 60 years, cementless femoral prosthesis implantation in a patient more than 80 years is a safe procedure with no increased risk of surgery-related complications. Huang et al.  also showed that the use of cementless THA can bring good clinical results for patients older than 80 years, but the deficiency is that fewer patients are included, and the evaluation results are only Harris score and postoperative complications, without imaging measurement results. But some scholars put forward the opposite view. Because elderly patients are often complicated with hip osteoporosis, they are considered as the preferred choice for femoral cemented fixation . The use of cementless fixation may increase the risk of reoperation due to periprosthetic fractures . Through a meta-analysis, Raja et al.  proved that cemented prosthesis group had better results and fewer complications, such as lower revision rate of prosthesis, and elderly patients should be given priority to use cemented prosthesis. Fernandez et al.  also demonstrated that in patients over 60 years with hip fractures, cemented hemiarthroplasty resulted in a significantly improved quality of life and a lower risk of periprosthetic fractures compared with cementless hemiarthroplasty. Therefore, it is still inconclusive whether cementless prosthesis should be used. Especially with the increasing elderly population, it is necessary to determine the exact efficacy of cementless prostheses. Current studies have focused on complications and functional outcomes without imaging evaluation. There was also a lack of results compared with younger patients. Therefore, we investigated whether the use of cementless femoral stem prostheses in older patients could provide similar clinical outcomes to those obtained in younger patients.
In this study, we found that in elderly patients with hip arthroplasty, the use of cementless prostheses on the femoral side can achieve similar results compared with younger patients. At 1-year follow-up, the Harris score in the elder group increased from 38.35 ± 18.21 to 86.91 ± 12.55 (P < 0.01). In addition, the recovery trend of the elder group was very similar to that of the young group, and the functional score increased significantly in the first three months, indicating that the cementless prosthesis can also provide stable fixation for elderly patients to get out of bed early, and enable patients to participate in functional training and return to life early. At the same time, this study found that within one year after surgery, the prosthesis in both groups had a certain degree of subsidence with the prolongation of time (see Table 1). At the same time, the distance of stem sinking in the elder group was larger than that of the young group at each follow-up time point, which may be related to the fact that most patients in the elder group had osteoporosis, resulting in slower binding between bone and prosthesis. The distance of stem sinking of the elder group in 6 months was more significant than that in 3 months (P < 0.05), but there was no significant difference in the distance of stem sinking between 12 and 6 months (P = 0.40), indicating that the cementless prosthesis began to become stable in 6 months. There was no significant difference in complication rates, and periprosthetic fractures were no more common in the older group than in the younger group.
As the first study to compare cementless femoral stem prostheses in older and younger patients. Short-term follow-up of functional outcomes, imaging findings, and complications was performed. It is found that elderly patients with cementless femoral stem prosthesis can quickly acquire good function outcome. Even though it takes longer for the implant to firmly bind to the cortical bone than in younger patients, the implant interface tends to stabilize at 6 months. There were no significant increases in incidence rate of periprosthetic fractures, infections, and deep vein thrombosis compared with younger patients. This study is similar to the results of Huang et al.  and Koettnitz et al. , and provides valuable imaging evidence on this basis.
The cementless femoral stem is mechanically pressed to achieve initial stability, and then achieve long-term bone ingrowth at the metal interface. Imaging results are very important indicator to estimate the initial stability of femoral stem prosthesis. The most intuitive expression of early stability on X-ray film is the relative displacement of prosthesis and medullary cavity. After comparing various measurement methods, Walker et al.  believed that it was highly reliable and repeatable to measure and evaluate the subsidence displacement of the prosthesis by taking the vertical distance from the vertex of the greater trochanter to the lateral collar of the prosthesis neck. Our results showed that the distance of stem sinking in the elder group was 2.10 mm in 3 months, which was similar to the 2 mm of Floerkemeier et al. . This distance is similar to the sinking distance in fracture of Vancouver B2 with cementless hip arthroplasty  and less than the 4 mm of Seral et al. . This indicates that the sinking distance achieved osseointegration within a safe range. However, compared with the 1.96 mm of young patients, the sinking distance (2.53 mm) in the elder group is still large. Considering that elderly patients are more complicated with osteoporosis, the bone metabolism rate is slowed down, and the osteogenic effect is weakened, so the osseointegration is longer and the sinking distance is relatively larger than that of young patients. To increase the contact area to achieve better compression matching, the medullary cavity file was used to expand the medullary cavity. The destruction of local intramedullary bone and blood supply by the medullary reaming itself may also be one of the factors affecting the early subsidence of the prosthesis.
However, osseointegration of femoral stem prosthesis was also achieved in the elder group within one year after operation. However, the initial stability of different types of prostheses may be determined by one or more factors, which is a comprehensive and complex process. The geometric shape, length, surface roughness and surface coating properties of femoral stem prosthesis, proximal shape of femoral bone marrow cavity and bone quality are all important factors affecting the initial stability of the prosthesis . This study also suggests that osteoporosis may not be an absolute contraindication to the use of cementless femoral stems. With the progress and development of surgical technique, material science, biomechanics and other aspects, cementless femoral stem can also achieve good results in elderly patients. However, the cementless prosthesis may cause uneven pressure, lead to stress shieding, and cause thigh pain, which should also attract scholars' attention . At present, there is no clear value of the safe distance of stem sinking, and we hope that our data and results can provide a new reference.
Meantime, the study still had some shortcomings. First, the follow-up period was relatively short, long-term survival rate and clinical function of the prosthesis still need further follow-up. Secondly, this study was a retrospective study, there was a high rate of loss to follow-up in the process of data collection, which may affect the study results. Thirdly, imaging data of the acetabular cup in total hip arthroplasty were ignored. Finally, subgroup comparisons of different bone qualities were not performed due to the limited number of patients.