The present study aimed to analyze the migration pattern of a calcar-guided short stem using EBRA-FCA in patients with ONFH at mid-term. To date, no stem-related complications could be observed and none of the investigated implants required revision surgery. At mid-term, the mean axial migration resulted in 1.56 mm. In most cases, after 2 years a stabilization was observed. Subsidence of more than 1.5 mm at last follow-up was detected in 20 hips (44.4%). While no influence of age, BMI and ARCO classification was found, a tendency of increased axial migration was observed in male and heavy-weight patients.
The treatment of ONFH is challenging. It remains controversial, whether ONFH represents a risk factor for failure after the implantation of short stems. The main concern of using short-stem THA in patients with ONFH arises due to the potentially reduced bone quality and the osteonecrotic area beyond the femoral head also affecting the femoral neck and the metaphyseal bone. According to the critics, a THA designed for metaphyseal anchoring may be associated with poor primary stability, impaired osteointegration and thus an increased risk of loosening. Previous histological studies suggested that ONFH includes structional alterations not only of the femoral head but also the femoral neck [9, 28]. Tingart et al. analyzed the bone matrix composition and trabecular microarchitecture of the femoral metaphysis in patients with ONFH . They concluded that alterations in bone metabolism and architecture might contribute to the higher rates of stem loosening after THA in patients with ONFH [28, 29]. Thus, to date, conventional THA with pronounced diaphyseal anchorage is considered the gold standard in patients with ONFH [2, 4, 5, 16]. Kim et al.  investigated the outcome of a conventional THA with a modular femoral component in patients with ONFH and younger than 50 years. The survival rate with the endpoint of stem revision for any reason was 93.8% and 100% for aseptic loosening at 16.8 years . Garino et al.  reported a 96% survival rate in 123 cemented and hybrid THAs in patients with ONFH after 55 months.
There are only a few previous studies investigating short-stem THA in patients with ONFH. The newest generation of short stems aims at a physiological metaphyseal fixation and load transmission to reduce stress-shielding and to preserve the proximal femoral bone . One of the most popular short stems, solely allowing metaphyseal anchorage, is the Metha stem (B. Braun, Tuttlingen, Germany), for which controversial outcomes in patients with ONFH have been published [16, 33]. Floerkemeier et al.  reported encouraging results using the Metha stem in a total of 73 hips in a mean follow-up of 34 months with only two revisions needed. Recently, Suksathien et al.  reported a Kaplan–Meier survivorship, with the endpoint being any stem revision, of 98.7% at 7 years. However, Schnurr et al.  compared 231 implantations of the Metha stem in patients with ONFH to 1455 operations in patients with primary osteoarthritis using data over a 10-year period. Whereas the total revision rate turned out not to be significantly increased in patients with ONFH compared to patients with primary osteoarthritis, however, they found that the aseptic loosening rate of the short stems was significantly elevated in those patients with ONFH. Particularly male patients and patients providing risk factors such as alcohol abuse, cortisol intake and radiation were prone to early revision surgery.
Very little data is available on primary stability and migration regarding short stems in patients with ONFH. Zeh et al.  concluded in a study using the Mayo stem (Zimmer Inc., Warsaw, USA) that no significant migration and tilt occurred in patients with ONFH after 7.9 years. However, they found a mean axial migration of over 3 mm and the method used has not been validated before.
Impaired stability and pronounced migration are considered an indicator for subsequent aseptic loosening and mechanical failure . Krismer et al.  reported that axial migration of more than 1.5 mm after 2 years in conventional cementless THA was predictive for late aseptic loosening and a potential increase in the risk of revision. But it is still unknown if this prediction can be transferred to short-stem THA as well. Previous studies have performed migration analyses in short-stem THA following the indication of primary osteoarthritis using EBRA-FCA [7, 21, 34]. Kutzner et al.  investigated the optimys stem in patients with primary and secondary osteoarthritis. Axial migration of 1.43 mm at 2 years was reported. 39.6% of the stems showed subsidence of 1.5 mm or more . However, at mid-term, no significant further migration was observed. In only four hips, due to undersizing as part of a surgical mistake, stems did not stabilize after 2 years. At mid-term, however, no stem revision was needed . Another study, analyzing the Fitmore stem (Zimmer Inc., Warsaw, USA) reported a mean axial migration of 1 mm after 2 years. A potential critical migration of more than 1.5 mm was detected in 25% of the investigated hips . Again, at mid-term, all stems stabilized. No implant failure was observed, neither in the group of implants with early stabilization, nor the group with extensive early-onset migration . These findings are in line with previous publications regarding different stem designs. Floerkemeier et al.  in a prospective radiostereometric analysis (RSA) study using the Metha stem, found increased early migration, but again not being associated with a higher risk of subsequent implant failure. A migration analysis of the Nanos stem (Smith and Nephew GmbH, Marl, Germany) also confirmed slight initial migration within three months after surgery, followed by secondary stabilisation, suggesting a low risk of aseptic loosening . Only one stem revision due to postoperative periprosthetic fracture was observed.
To date, to our best knowledge, no analysis of the migration pattern of a new-generation short-stem design in patients with ONFH has been published, using a validated method like EBRA-FCA. The results of the present study showed similar outcomes compared to the previously published data in patients with osteoarthritis [7, 10, 21, 34]. As it was found in patients with osteoarthritis, at mid-term no stem failure and revision occurred. These findings strongly support that the optimys stem is a safe option in the treatment of patients with ONFH. They indicate a sufficient primary stability and successful osteointegration also for this group of patients. Both, the results in patients with osteoarthritis as well as the outcomes of the present study suggest that the 1.5 mm threshold of axial migration may not be valid for predicting aseptic loosening and implant failure in calcar-guided short-stem THA. However, further monitoring of those stems with pronounced initial subsidence is obligatory to detect potential signs of loosening and failure.
In accordance with the previously published data on patient-related factors, which influence the rate of stem migration of the investigated stem design, again, male and heavy-weight patients are to be considered at risk for pronounced early migration . The present mid-term results in patients with ONFH confirm these findings. This is in line with previously published data using different stem designs [21, 37]. However, in a retrospective analysis of migration data from two different short stem studies using the Metha stem and the Nanos stem, factors, including age, height, weight and gender, did not affect the migration pattern . It seems obvious that migration patterns of different stem designs, providing different concepts of anchorage, may be affected differently by patient-related influencing factors.
Besides patient-related factors, surgical technique highly influences stabilization into the femoral bone, especially in heavy-weight patients. Stems providing a poor fit-and-fill into the bone and lack of cortical contact have been reported to show reduced primary stability . Surgeons, therefore, are highly recommended to use intraoperative radiography to confirm correct positioning and sizing intraoperatively .
The design of calcar-guided stems, such as the optimys stem, differs to that of the early short stem designs with solely metaphyseal anchorage, such as the Metha stem. Whereas most varus hips achieve stabilization by three-point fixation in the metaphyseal bone, in calcar-guided short-stem THA, due to the design properties, some neutral and most valgus hips may also be stabilized by supplementing an additional fit-and-fill fixation in the proximal diaphysis  (Fig. 4). Already in 2012, Floerkemeier et al.  found in a review of short- to mid-term results of short stems in patients with ONFH predominantly good outcomes. However, marked differences in the desgin of short stems and their type of anchorage had to be acknowledged. They concluded, that those short stems with primary or additional diaphyseal fixation do not reveal an increased risk of failed osseointegration or loosening. For designs with a primary metaphyseal anchorage, and the osteonecrosis exceeding the femoral neck, an implantation could not be recommended. Regarding the successful achievement of sufficient primary stability, especially in hips with ONFH, the design properties of calcar-guided short stems, given an individualized meta-diaphyseal anchorage, may therefore account for significant advantages compared with earlier short-stem designs. This can be confirmed by previously published data. Jerosch et al. published mid-term results of the calcar-guided MiniHip stem (Corin Medical, Cirencester, UK) with 100% stem survival and encouraging clinical outcome . Furthermore, Capone et al. found excellent clinical results of the calcar-guided Nanos stem (Smith and Nephew, Marl, Germany) and successful osteointegration at mid-term without any revision needed for any reason .
For the safe usage of calcar-guided short stems, a preoperative MRI may be helpful. Depending on the spread of the area of ONFH, the stem alignment can be done individually. If only affecting the femoral head, metaphyseal anchoring based on three-point-anchoring can be aimed for. If also affecting the femoral neck and large parts of the metaphysis, an additional diaphyseal anchorage should be pursued (Fig. 5).
The present study has several limitations. First, the mid-term follow-up does not allow definite conclusions about the long-term outcome of short-stem THA in patients with ONFH. However, early migration analysis may allow a prediction of implant survival and may indicate undesirable results. Second, MRI has not been carried out on every patient to identify the precise amount of osteonecrosis. In those hips with radiological documented already fractured subchondral femoral bone, an ARCO stage IV was assumed. Thus, a proof of metaphyseal involvement has not been supplied. Third, the fact that a control group is missing in the study design does not allow for a direct comparison of patients with ONFH and primary osteoarthritis. However, data on patients with osteoarthritis has previously been published by the same study group, using the same implants and the identical standardized postoperative care. Another limitation results in the EBRA software failing to evaluate all radiographs. The requirements for EBRA measurement are quite challenging, leading to a some of the radiographs not being accepted by the EBRA software. Furthermore, radiostereometric analysis (RSA) provides higher accuracy in comparison to the EBRA method. The accuracy of EBRA-FCA has been reported to be ± 1 mm for subsidence, with a specificity of 100% and a sensitivity of 78% to detect migration . RSA, however, requires the implantation of markers intraoperatively and would have caused intense cost and effort.