Early stem subsidence following a cementless THA is correlated with aseptic loosening of the femoral component [9, 12,13,14]. Despite the excellent short- to midterm outcomes of several short stem designs [7, 8], it remains unclear whether all cementless short femoral stems could achieve an adequate stem fixation. The aim of the current study was to evaluate the 5-year subsidence of a metaphyseal-anchored short femoral stem in cementless THA and to correlate stem subsidence with patient demographics, implant characteristics, and clinical outcomes. The results of this study showed that the highest subsidence occurred during the first 3 months, the implant began to stabilize at about 24 months, but continued to slowly subside with an average total subsidence of 2.04 mm at 5 years following the THA. The stem subsidence was not significantly correlated with patient’s age, gender, weight or bone quality. The only implant factor that affected stem subsidence was stem size of 6 or more in females. The amount of stem subsidence was not associated with worse clinical outcomes.
The short femoral stems in cementless THA exhibit the highest subsidence during the first 3 months following THA, but subsidence might occur up to 5 years. Specifically, Freitag et al.  reported an average subsidence of 1.1 mm (range: -5 mm to 1.5 mm) in a different short stem design (Fitmore®, Zimmer Inc., Warsaw, Indiana, USA) up to the 5 year follow-up with the maximum subsidence occurring during the first 3 months following THA. However, Acklin et al.  observed a slightly lower average subsidence with the Fitmore short stem of 0.39 mm at 3 months which was stable until the 2-year follow-up. Brinkmann et al.  comparing the Metha stem (Aesculap AG, Tuttlingen, Germany) with the Nanos stem (Smith & Nephew plc, London, UK) reported subsidence of 1.96+/− 2.37 mm and 2.04 +/− 2.65 mm, at 1 year following THA, respectively, with most migration occurring also during the first 3 months. In accordance with the literature, we observed that the optimys metaphyseal-anchored stem subsidence was 0.96 +/− 0.76 mm at 3 months, 1.43 +/− 1.07 mm at 12 months, 1.71 +/− 1.26 mm at 24 months, and 2.04 +/− 1.42 mm at the final follow-up, 60 months postoperative. It showed the highest subsidence during the first 3 months with an average subsidence of 0.96 mm, began to stabilize at about 24 months but continued to slowly subside until 5 years following the THA. The data suggest that the optimys metaphyseal-anchored short stem in cementless THA exhibit similar subsidence pattern as other contemporary short stem designs.
Among all the different factors influencing subsidence, press-fit is believed to be one of the key factors [18, 19]. Even though the surgeon is guided by visual, sensory and auditory clues when inserting the femoral stem, finding a good balance between a perfect press-fit level and not fracturing the femur by the stem remains challenging. Even though, in our study femoral stems did not seem undersized when evaluating the postoperative x-rays, a discrete undersizing which may not be visible on x-rays could be responsible for the postoperative migration and settle-in of the femoral prosthesis. The stems migrate until they reach a firm press-fit level and therefore the rate of migration decreases over time. This assumption is supported by the fact, that in our study almost all stems showed some degree of initially pronounced migration but only one of 68 stems (1.5%) showed signs of aseptic loosening at the 5 year follow-up. Similar results were reported by Kutzner et al. , who showed initial migration but no aseptic loosening up to 2 years postoperative. Further studies are necessary to confirm that even though these uncemented short stems migrate postoperatively, they remain stable in the long term.
The uncemented short femoral stems might show similar subsidence with conventional straight femoral stems. Specifically, Ferguson et al.  in a randomized controlled trial comparing the subsidence of the Meta Fix conventional stem (Corin Group, Cirencester Gloucestershire, UK) with the short stem (MiniHip, Corin Group) reported an average subsidence of 0.62 +/− 0.56 mm and 0.26 +/− 0.38 mm, respectively, at the 2-year follow-up. McCalden et al.  reported a slightly higher, but statistically not significant subsidence in the SMF short stem (Smith & Nephew plc) compared to the Synergy conventional stem (Smith & Nephew plc) using RSA analysis (0.94 +/− 1.71 mm versus 0.32 +/− 0.45 mm, p = 0.66), 2 years following the THA. The result of the present study suggests that despite the metaphyseal-anchoring of the short femoral stem in cementless THA it might show a similar subsidence rate as the conventional straight femoral stem.
According to the literature, stem subsidence in several short stem design is affected by patient characteristics such as weight, BMI and age, whereas other stem designs stay unaffected from patient demographics. Kutzner et al.  investigating the same metaphyseal-anchoring stem as in our study (optimys, Mathys Ltd.) reported a significant influence of weight above 75 kg on mean axial migration at the 2-year follow-up. However, weight did not appear to influence stem subsidence when adjusted for age and gender. Stihlsen et al.  investigating the Vision-2000 stem (Depuy Orthopaedics Inc., Warsaw, Indiana, USA) found that body weight over 75 kg has a significant impact on stem subsidence at 2-year follow-up. In Fitmore (Zimmer Inc.) and Nanos (Smith & Nephew plc) short stem, BMI ≥30 kg/m2, age and weight did not influence the stem subsidence [15, 24]. Similarly, in our cohort body weight, BMI, age and gender did not have a statistically significant impact on the amount of stem subsidence.
The present study is the first to report a correlation between stem size and subsidence. Stem size ≥6 showed higher subsidence up to 5 years postoperative compared to stems < 6 in women only. One possible explanation might be that the surgeon did not chose a larger stem due to the fear of intraoperative periprosthetic fractures. Since women tend to need smaller implants than men, surgeons might be especially cautious when inserting bigger stem sizes in women and therefore do not achieve press-fit fixation in this cohort. In other implants such as the Nanos stem (Smith & Nephew plc), implant size did not influence the amount of migration .
In our cohort, postoperative stem migration did not have a negative impact on the clinical outcome or revision rate up to 5 years postoperative. This suggests that stem migration up to 2.04 +/− 1.42 mm at 5 years following THA is not clinically relevant in an uncemented metaphyseal-anchored short femoral stem.
The present study should be interpreted in light of its potential limitations. Although the RSA method is considered the gold standard for measuring the stem subsidence, it is nowadays rarely used due to the need for marker implantation and potential complications. The computer-assisted EBRA-FCA system is however an established and accurate method in measuring femoral component subsidence with a specificity of 100% and a sensitivity of 78% compared with the RSA for the detection of migration of over 1 mm . Additionally, the current study investigated only the optimys short femoral stem. Although this design is similar to other short femoral implants available, our findings might not apply to other stem designs.