In this study, the mid-term survival rate of femoral component was 94.23%, which was on the equivalent level than other similar studies. Chatelet et al  has reported that the mid-term survival rate was 96.7% of a long uncemented monobloc stem for revision total hip arthroplasty. McInnes et al  has also reported similar survival rates of two femoral components for hip revision surgery, which were 87.1% and 87.8% at 15 years follow-up. This demonstrated the applicability of short stem in patients undergoing hip revision surgery. But there were still some complications which might compromise the prognosis of patients. The main reason for re-revision was aseptic loosen, followed by bone grafting failure and recurrent-dislocation. In patients undergoing hip revision surgery, osteosclerosis and osteolysis could commonly be identified on proximal femur, especially on metaphyseal segment . Meanwhile, loss of cancellous bone would reduce the press-fit effect of the stem. These might potentially increase the incidence of aseptic loosen . In addition, allografting was performed in this study. In elderly patients with osteoporosis, bone grafting failure might occur, also causing the loosen of femoral stem [7,15].
Three risk factors for revision failure were identified in this study. The first one is aging. As it has already been well established, aging is a certain factor associated with bone strength decreasing, fragility and osteoporosis [16,17]. Furthermore, osteogenesis is also largely affected by aging, especially for hip joint. Studies have shown that femoral neck fracture could hardly achieve bony union in elderly patients [18-20]. In patients undergoing hip revision, bone grafting is commonly performed to help filling the bone deficiency of proximal femur. Meanwhile, impaction bone grafting also plays an important role which helps stabilizing the femoral stem. In elderly patients, graft bone (especially allograft) might not survive. This will cause bone resorption around femoral stem and culminate in an aseptic loosen of the stem. In this study, all patients were implanted with short uncemented stem, which means the stem stability is largely relied on the success of bone grafting. If bone grafting failed, stem loosen is prone to be occurred. Some related studies have similar findings. Lamb et al  had reported that increasing age (hazard ratio, 1.02 per year) was associated with failure of cemented stem implantation after periprosthetic femoral fracture after primary total hip arthroplasty. Cantrell et al  also found that Increasing age was significant positively associated independent risk factors for incidence of complications and 30-day readmission. Dale et al  reported that uncemented hip arthroplasties in women of age 55-75 years and over 75 years of age had higher risk of revision (mainly due to periprosthetic fracture and dislocation) compared with cemented arthroplasties. Thus, for elderly patients, hip revision with short uncemented stem is a delicate problem.
The second risk factor is osteoporosis. In this study, bone density of patient was measured by dual-energy x-ray absorptiometry. Due to the interference from metal hip prostheses, bone density of lumbar vertebrae was measured. Osteoporosis of patient was diagnosed according to the criteria from World Health Organization (T<-2.5). Comparing to those patients with normal bone mineral density, the risk of revision failure in patients with significant osteoporosis was 2.8-fold higher when short uncemented stem was implanted. As we known, the initial stability of uncemented prosthesis is depended on the press-fit between the prosthesis and the bone socket . That means the minimal anti-rupture strength of the proximal femur must exceed the pressure between the femoral stem and the medullary canal which is required for stable press-fit of the prosthesis. In patients with osteoporosis, the bone strength decreases, which might cause failure of press-fit between prosthesis and medullary canal. Thus, in patients with serious osteoporosis, cemented prosthesis should be implanted rather than uncemented prosthesis. In this study, all patients received uncemented revision, which means there might be osteolysis, osteosclerosis and bone deficiency around proximal femur. If the patient was combined with osteoporosis, incidence of press-fit failure and proximal femoral periprosthetic fracture might be increased, leading to failure of the revision surgery. Furthermore, in patients with osteoporosis, the bone grafting might not survive, which could also cause revision failure. Therefore, in patients with osteoporosis, cemented revision should be taken into account.
The strongest risk factor for revision failure in this study is intraoperative periprosthetic femoral fracture during revision surgery. Several reports [23,24] have shown that, comparting to primary hip arthroplasty, hip revision is associated with increased incidence of periprosthetic femoral fracture. Some other studies have shown that comparing with “standard” femoral stem which chartered as metaphyseal-diaphyseal fixation, these short stems are commonly associated with increased incidence of intraoperative periprosthetic femoral fractures. Moreover, periprosthetic femoral fracture is a potential cause of complications. Panula et al  reported that periprosthetic fractures were associated with increased risk of revision for dislocation after total hip arthroplasty. Devane et a  and Liu et al  also reported that intraoperative periprosthetic femoral fractures were commonly accompanied with poor clinical outcomes of patients. In this study, the femoral component was characterized as metaphyseal fixation and distal end polishing design. That means the integrity of local segment of proximal femur is crucial important for the press-fit and stability of the stem. Suppose that intraoperative periprosthetic fracture occurs and the metaphyseal segment of femur between greater trochanter and less trochanter is involved in, there might be no enough press-fit force for stable fixation of stem. In this situation, the stem is prone to be loosen regardless of internal fixation for periprosthetic fracture. We have a typical case shown on Figure 2. On the contrary, if this situation occurs when a “standard” femoral component is implanted, the distal press-fit fixation will provide stability for the prosthesis. Therefore, if intraoperative periprosthetic femoral fracture is identified during revision surgery with short femoral stem, we strongly recommend an immediate revision with long stem (metaphyseal-diaphyseal fixation stem or diaphyseal fixation stem) rather than isolated fracture fixation.
This study has several limitations. First, due to the metaphyseal fixation of short stem, only patients with Paprosky Type I, Type II and Type III-A bone deficiency were included in this study. This will make our study incomparable with other studies which might involve patients with serious bone deficiency (e.g. Paprosky Type III-B or Type IV). Second, isolated acetabular component revision failure postoperatively was excluded from this study. Hence, the overall survival rate of femoral stem might be affected. Third, the sample size was relatively small. Other potential risk factors might not be identified in this study.