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Mid-term migration pattern of a calcar-guided short stem: A five-year EBRA-FCA-study

Open AccessPublished:February 11, 2020DOI:https://doi.org/10.1016/j.jos.2020.01.001

      Abstract

      Background

      Short-term results of several short-stem designs have indicated early axial migration. Mid- and long-term results for most designs are lacking. The objective of this study was to evaluate the mid-term migration pattern of a calcar-guided short stem five years postoperative.

      Methods

      Implant migration of 191 calcar-guided short stems was assessed by Ein-Bild-Roentgen-Analysis Femoral-Component- Analysis (EBRA-FCA) 5 years after surgery. Migration pattern of the whole group was analyzed and compared to the migration pattern of implants potentially being “at hazard” with a subsidence of more than 1.5 mm at 2 years postoperatively. Influence of preoperative Dorr types (A vs. B vs. C), age (<70 vs. >70 years), gender (female vs. male), weight (<90 kg vs. >90 kg), BMI (<30 vs. >30) and uni-vs. bilateral procedures on mid-term migration pattern was analyzed. Additionally outcome of varus- and valgus stem alignment was assessed.

      Results

      Mean axial subsidence was 1.5 mm (SD 1.48 mm) at final follow-up. Two years after surgery 73 short stems were classified “at hazard”. Of these stems, 69 cases showed secondary stabilisation in the following period, whereas 4 cases presented unstable with more than 1 mm of further subsidence. Stem revision was not required neither in the group of implants with early stabilisation nor the group with pronounced early onset migration. Male gender and heavy-weight patients had a significant higher risk for axial migration, as well as extensive valgus stem alignment, whereas for Dorr type B, compared to A, no statistical difference could be observed.

      Conclusions

      In most cases, even in the group of stems being “at hazard”, settling could be documented. While different Dorr types did not show a statistically significant impact on axial migration, particularly in male and heavy-weight patients the risk of continuous subsidence is increased. In those 4 cases with further migration, undersizing of the stem could be recognized. At present, clinical consequences are still uncertain.

      1. Introduction

      Cementless femoral short stems have gained popularity in total hip arthroplasty (THA) in recent years despite the lack of long-term results [
      • Khanuja H.S.
      • Banerjee S.
      • Jain D.
      • Pivec R.
      • Mont M.A.
      Short bone-conserving stems in cementless hip arthroplasty.
      ,
      • von Lewinski G.
      • Floerkemeier T.
      Challenges in total hip arthroplasty, 2018, p. 295–312.
      ]. A variety of different designs emerged on to the market, proposing a more physiological load transmission while allowing a tissue sparing implantation technique [
      • Bieger R.
      • Ignatius A.
      • Reichel H.
      • Dürselen L.
      Biomechanics of a short stem: in vitro primary stability and stress shielding of a conservative cementless hip stem.
      ,
      • Yan S.G.
      • Weber P.
      • Steinbrück A.
      • Hua X.
      • Jansson V.
      • Schmidutz F.
      Periprosthetic bone remodelling of short-stem total hip arthroplasty: a systematic review.
      ]. Recent studies proved equivalent mid-term clinical results compared to conventional straight-stems with decreased intraoperative complication rates such as periprosthetic fractures [
      • Molli R.G.
      • Lombardi A.V.
      • Berend K.R.
      • Adams J.B.
      • Sneller M.A.
      A short tapered stem reduces intraoperative complications in primary total hip arthroplasty.
      ,
      • van Oldenrijk J.
      • Molleman J.
      • Klaver M.
      • Poolman R.W.
      • Haverkamp D.
      Revision rate after short-stem total hip arthroplasty: a systematic review of 49 studies.
      ].
      The most common reason for implant failure is aseptic loosening [
      • Kärrholm J.
      Radiostereometric analysis of early implant migration - a valuable tool to ensure proper introduction of new implants.
      ]. A major concern in reducing diaphyseal fixation of the femoral stem is the potential reduction of implant stability which, by interfering osteointegration, might increase the risk of implant loosening [
      • Bieger R.
      • Ignatius A.
      • Decking R.
      • Claes L.
      • Reichel H.
      • Dürselen L.
      Primary stability and strain distribution of cementless hip stems as a function of implant design.
      ]. Several short-stem designs have been promoted to provide advantages without compromising implant stability [
      • Falez F.
      • Casella F.
      • Panegrossi G.
      • Favetti F.
      • Barresi C.
      Perspectives on metaphyseal conservative stems.
      ,
      • Falez F.
      • Casella F.
      • Papalia M.
      Current concepts, classification, and results in short stem hip arthroplasty.
      ]. However, short-term results of several short stems have indicated a pronounced early axial migration [
      • Kutzner K.P.
      • Freitag T.
      • Kovacevis M.P.
      • Pfeil D.
      • Reichel H.
      • Bieger R.
      One-stage bilateral versus unilateral short-stem total hip arthroplasty: comparison of migration patterns using "Ein-Bild-Roentgen-Analysis Femoral-Component-Analysis".
      ,
      • Freitag T.
      • Kappe T.
      • Fuchs M.
      • Jung S.
      • Reichel H.
      • Bieger R.
      Migration pattern of a femoral short-stem prosthesis: a 2-year EBRA-FCA-study.
      ,
      • Kaipel M.
      • Grabowiecki P.
      • Sinz K.
      • Farr S.
      • Sinz G.
      Migration characteristics and early clinical results of the NANOS® short-stem hip arthroplasty.
      ]. To date, evidence on long-term survival of these implants is still lacking.
      The amount of early postoperative implant migration has been found to be a predictor of long-term survival of conventional femoral stems [
      • Krismer M.
      • Biedermann R.
      • Stöckl B.
      • Fischer M.
      • Bauer R.
      • Haid C.
      The prediction of failure of the stem in THR by measurement of early migration using EBRA-FCA. Einzel-Bild-Roentgen-Analyse-femoral component analysis.
      ,
      • Streit M.R.
      • Haeussler D.
      • Bruckner T.
      • Proctor T.
      • Innmann M.M.
      • Merle C.
      • Gotterbarm T.
      • Weiss S.
      Early migration predicts aseptic loosening of cementless femoral stems: a long-term study.
      ]. Using EBRA (“Einzel-Bild-Roentgen-Analyse”) measurements, Krismer et al. reported that axial migration of more than 1.5 mm within the first 2 years after surgery was predictive for late aseptic loosening in conventional stems and a possible increase in risk of revision [
      • Krismer M.
      • Biedermann R.
      • Stöckl B.
      • Fischer M.
      • Bauer R.
      • Haid C.
      The prediction of failure of the stem in THR by measurement of early migration using EBRA-FCA. Einzel-Bild-Roentgen-Analyse-femoral component analysis.
      ]. However, conventional cementless and cemented implants were included in this analysis, compromising the comparability. A critical limit of axial migration of 2.7 mm within the first 2 years after surgery could be demonstrated for the CLS stem (Zimmer Biomet, Warsaw, Indiana, USA) [
      • Streit M.R.
      • Haeussler D.
      • Bruckner T.
      • Proctor T.
      • Innmann M.M.
      • Merle C.
      • Gotterbarm T.
      • Weiss S.
      Early migration predicts aseptic loosening of cementless femoral stems: a long-term study.
      ].
      Recently, Freitag et al. demonstrated for the Fitmore stem (Zimmer Biomet, Warsaw, Indiana, USA) a settlement of all implants “at risk” after 2 years with no further migration after 5 years [
      • Freitag T.
      • Fuchs M.
      • Woelfle-Roos J.V.
      • Reichel H.
      • Bieger R.
      Mid-term migration analysis of a femoral short-stem prosthesis: a five-year EBRA-FCA-study.
      ].
      The objective of the present EBRA-FCA analysis was to assess the mid-term migration pattern of a calcar guided short-stem (optimys, Mathys Ltd., Bettlach, Switzerland; 3 A* grading according to the Orthopaedic Data Evaluation Panel (ODEP) classification system) in a five-year follow-up. This report updates the recently published short-term results of a retrospective migration analysis study [
      • Kutzner K.P.
      • Freitag T.
      • Kovacevis M.P.
      • Pfeil D.
      • Reichel H.
      • Bieger R.
      One-stage bilateral versus unilateral short-stem total hip arthroplasty: comparison of migration patterns using "Ein-Bild-Roentgen-Analysis Femoral-Component-Analysis".
      ,
      • Kutzner K.P.
      • Kovacevic M.P.
      • Freitag T.
      • Fuchs A.
      • Reichel H.
      • Bieger R.
      Influence of patient-related characteristics on early migration in calcar-guided short-stem total hip arthroplasty: a 2-year migration analysis using EBRA-FCA.
      ,
      • Kutzner K.P.
      • Freitag T.
      • Donner S.
      • Kovacevic M.P.
      • Bieger R.
      Outcome of extensive varus and valgus stem alignment in short-stem THA: clinical and radiological analysis using EBRA-FCA.
      ].

      2. Materials and methods

      After institutional review board approval, 216 consecutive short stem implantations in 162 patients could be retrospectively included at a single institution [
      • Kutzner K.P.
      • Freitag T.
      • Kovacevis M.P.
      • Pfeil D.
      • Reichel H.
      • Bieger R.
      One-stage bilateral versus unilateral short-stem total hip arthroplasty: comparison of migration patterns using "Ein-Bild-Roentgen-Analysis Femoral-Component-Analysis".
      ,
      • Kutzner K.P.
      • Kovacevic M.P.
      • Freitag T.
      • Fuchs A.
      • Reichel H.
      • Bieger R.
      Influence of patient-related characteristics on early migration in calcar-guided short-stem total hip arthroplasty: a 2-year migration analysis using EBRA-FCA.
      ,
      • Kutzner K.P.
      • Freitag T.
      • Donner S.
      • Kovacevic M.P.
      • Bieger R.
      Outcome of extensive varus and valgus stem alignment in short-stem THA: clinical and radiological analysis using EBRA-FCA.
      ]. These are the first implantations of the investigated stem design at the institution, hence including the learning curve. Written consent to participate has been obtained from all patients. The following inclusion criteria were applied: a preoperative radiograph, a minimum follow-up of 5 years, a series of at least three consecutive standardized radiographs accepted by the EBRA-FCA software (University of Insbruck, Austria) and acceptance of the direct postoperative and the 5 years follow-up radiograph. Indications were as follows: primary osteoarthritis (90.8%), avascular necrosis of the femoral head (4.3%) and hip dysplasia (4.9%).
      At the final follow-up 191 hips in 142 patients satisfied the inclusion criteria. Ten hips had to be excluded because of an incomplete radiological series, in 3 cases the software rejected the radiographs. Nine patients (12 hips) deceased unrelated to the operation with prosthesis in situ. Patient characteristics are presented in Table 1. Mean follow-up time was 63.2 months (SD 4.9 months).
      Table 1Patients demographics.
      StatisticsAge (years)BMI (kg/m2)Height (cm)Weight (kg)
      Femalen60606060
      Mean (SD)62.7 (9.2)27.6 (6.0)166.6 (5.4)77.1 (18.5)
      95% CI60.3, 65.126.1, 29.2165.2, 168.072.3, 81.8
      Median63.526.0167.573.0
      Range32–8719–45152–17850–140
      Malen82828282
      Mean (SD)62.7 (10.0)28.1 (4.4)178.0 (6.5)89.1 (15.0)
      95% CI60.5, 64.927.1, 29.0176.6, 179.585.8, 92.4
      Median62.027.0178.584.5
      Range36 to 8022 to 43165 to 19266 to 153
      Totaln142142142142
      Mean (SD)62.7 (9.6)27.9 (5.1)173.2 (8.3)84.0 (17.6)
      95% CI61.1, 64.327.0, 28.7171.8, 174.681.1, 86.9
      Median63.027.0172.082.0
      Range32–8719–45152–19250–153
      Preoperatively, 135 (70.7%) of the patients were classified Dorr type A, 55 (28.8%) Dorr type B and 1 (0.5%) Dorr type C, respectively. Digital templating was performed in all cases using the MediCAD II software (Hectec, Landshut, Germany).
      All patients received a cementless, calcar-guided short-stem (optimys, Mathys Ltd Bettlach, Switzerland) (Fig. 1), which can be classified a type 2 B short-stem according to the classification of Khanuja et al. [
      • Khanuja H.S.
      • Banerjee S.
      • Jain D.
      • Pivec R.
      • Mont M.A.
      Short bone-conserving stems in cementless hip arthroplasty.
      ]. It is made of a titanium alloy with a plasma-sprayed surface and a calcium phosphate coating. The profile of the stem is tapered in three planes with a trapezoidal cross-section to provide stable press-fit fixation. The implant is aligned individually along the proximal medial cortex and the calcar. The ideal stem positioning regarding different varus- and valgus alignment is shown in Fig. 2. Additionally, the implant offers two different offset options (standard- and lateral offset). The stem was combined with a cementless press-fit cup (RM Pressfit vitamys, Mathys Ltd Bettlach, Switzerland or Fitmore cup, Zimmer, Warsaw, USA) with a 28-mm alumina-on-highly crosslinked polyethylene bearing in all hips. All surgeries were performed in supine position using a modified, minimally-invasive anterolateral approach [
      • Pfeil J.
      • Siebert W.
      Minimally invasive surgery in total hip arthroplasty.
      ,
      • Kutzner K.P.
      • Pfeil J.
      Individualized stem-positioning in calcar-guided short-stem total hip arthroplasty.
      ]. Intraoperative radiography was performed to decide on stem sizing and positioning. Full-weight-bearing using two crutches was allowed in all cases immediately after surgery.
      Fig. 1
      Fig. 1The optimys short stem (Mathys Ltd. Bettlach, Switzerland) at mid-term follow-up.
      Fig. 2
      Fig. 2Categorization regarding different CCD-angles in five groups (A–E). The stem is positioned individually, according to the hip anatomy.
      EBRA-FCA software (University of Insbruck, Austria) was used to determine axial subsidence of the stem as previously described [
      • Kutzner K.P.
      • Freitag T.
      • Kovacevis M.P.
      • Pfeil D.
      • Reichel H.
      • Bieger R.
      One-stage bilateral versus unilateral short-stem total hip arthroplasty: comparison of migration patterns using "Ein-Bild-Roentgen-Analysis Femoral-Component-Analysis".
      ,
      • Kutzner K.P.
      • Kovacevic M.P.
      • Freitag T.
      • Fuchs A.
      • Reichel H.
      • Bieger R.
      Influence of patient-related characteristics on early migration in calcar-guided short-stem total hip arthroplasty: a 2-year migration analysis using EBRA-FCA.
      ,
      • Kutzner K.P.
      • Freitag T.
      • Donner S.
      • Kovacevic M.P.
      • Bieger R.
      Outcome of extensive varus and valgus stem alignment in short-stem THA: clinical and radiological analysis using EBRA-FCA.
      ,
      • Biedermann R.
      • Krismer M.
      • Stöckl B.
      • Mayrhofer P.
      • Ornstein E.
      • Franzén H.
      Accuracy of EBRA-FCA in the measurement of migration of femoral components of total hip replacement. Einzel-Bild-Röntgen-Analyse-femoral component analysis.
      ]. Standardized radiographs were calibrated using the diameter of the prosthetic head. A total of 19 reference points were defined on the femoral head (6), the stem (3), the femoral cortex (8) and one at the greater and lesser trochanter respectively. Radiographs with significant positioning artefacts were excluded by the EBRA-FCA software.

      2.1 Implant migration

      Implant migration was assessed at 6 weeks, 6 months, 12 months, 2 years and at 5 years based on the direct postoperative condition.
      Implants with axial migration of more than 1.5 mm and more that 2.7 mm respectively at 2 years were defined as implants “at hazard” and analyzed further. Progressing migration with subsidence of more than 1 mm in the consecutive period of 3 years thereafter was defined not stable. In contrast, subsidence of less than 1 mm in the aforementioned period was defined stable [
      • Krismer M.
      • Biedermann R.
      • Stöckl B.
      • Fischer M.
      • Bauer R.
      • Haid C.
      The prediction of failure of the stem in THR by measurement of early migration using EBRA-FCA. Einzel-Bild-Roentgen-Analyse-femoral component analysis.
      ,
      • Freitag T.
      • Fuchs M.
      • Woelfle-Roos J.V.
      • Reichel H.
      • Bieger R.
      Mid-term migration analysis of a femoral short-stem prosthesis: a five-year EBRA-FCA-study.
      ].
      Influence of preoperative Dorr types of femur morphology (A vs. B vs. C), age (<70 vs. >70 years), gender (female vs. male), weight (<90 kg vs. >90 kg), BMI (<30 vs. >30) and unilateral versus bilateral procedures on mid-term migration pattern was analyzed.
      All stems were categorized regarding CCD-angles in five groups A-E according to the previous publication of 2-year data [
      • Kutzner K.P.
      • Freitag T.
      • Donner S.
      • Kovacevic M.P.
      • Bieger R.
      Outcome of extensive varus and valgus stem alignment in short-stem THA: clinical and radiological analysis using EBRA-FCA.
      ] (Fig. 2). Influence of postoperative stem alignment on mean mid-term migration was documented.
      For statistical evaluation SAS software 9.4 was used for all analyses (SAS Institute, Cary, North Carolina, USA). All analyses were performed using standard descriptive statistics. Values are reported as mean (standard deviation [SD]) and range, qualitative categorical are presented as number and percentage. Between group differences were compared using the non-parametric Wilcoxon 2- Sample test or Kruskal Wallis test, the latter in case of more than two groups. Comparisons within follow-up examinations such as differences to baseline were evaluated by means of paired t-tests. For statistical significance, a p value < 0.05 was considered.

      3. Results

      To date no stem revision in the whole collective was required at mid-term. No signs of loosening, such as radiolucent lines, were found (Fig. 1). However, a total of 4 hips (2.1%) cannot be considered stable at 5 years.
      A statistically significant axial migration could be observed over the whole period (p = 0.0001). At 6 weeks mean axial migration was 0.52 mm (SD 0.74 mm), at 6 months, 12 months, 24 months and 5 years it resulted in 0.90 mm (SD 1.02 mm), 1.13 mm (SD 1.16 mm), 1.39 mm (SD 1.41 mm) and 1.50 mm (SD 1.48 mm) respectively.
      While between follow-up time-points within the first 2 years significant mean axial migration could be observed (between 6 weeks and 6 months p < 0.0001); between 6 months and 12 months p < 0.0001 and between 12 months and 24 months p < 0.0001), there was only little further mean axial migration between 2 years and 5 years after surgery, however also being statistically significant (p = 0.0041). Mean monthly axial migration was calculated to be 0.35 mm (SD 0.495 mm) within the first 6 weeks, 0.084 mm (SD 0.105 mm) between 6 weeks and 12 months, 0.041 mm (SD 0.054 mm) between 6 months and 12 months, 0.025 mm (SD 0.038 mm) between 12 months and 2 years and 0.003 mm (SD 0.014 mm) thereafter.
      73 hips (38.2%) showed axial migration of more than 1.5 mm at 2 years and more than 2.7 mm of axial migration was found in 29 cases (15.2%) (Fig. 3). Of these implants “at hazard”, 69 cases (94.5%) showed no further axial migration between 2 and 5 years after surgery, while 4 cases (5.5%) showed progressing subsidence (Fig. 4).
      Fig. 3
      Fig. 3Mean plot of subsidence over time regarding different categories of patients being “at hazard”.
      Fig. 4
      Fig. 4Spaghetti- and Mean plot of subsidence over time for patients “at hazard” with more than 1.5 mm at 2 years. Except for 4 cases with progressing subsidence (black dashed lines), all other stems stabilized between 2 years and 5 years.
      While different Dorr types A, B and C as well as high age and high BMI showed no significant increase in subsidence (p = 0.783, p = 0.352 and p = 0.261), male patients and heavy-weight patients had a significantly higher risk for pronounced axial migration at mid-term (p = 0.0012 and p = 0.016) No differences were seen comparing unilateral and bilateral procedures (p = 0.666) (Table 2).
      Table 2Influence of Dorr type, gender, age, body weight, BMI and unilateral versus bilateral procedure on amount of axial migration at 5 years.
      Subsidence (mm)Mean (SD)Min-Maxp-value
      Dorr type
      A1.45 (1.44)7.7–1.80.783
      B1.63 (1.58)7.7–1.8
      C0.40 (n.a.)n.a.
      Gender
      Male1.74 (1.61)7.7–1.80.012
      Female1.16 (1.21)5.6–1.7
      Age
      <70 years1.44 (1.42)6.4–1.80.436
      ≥70 years1.67 (1.65)7.7–1.0
      Body weight
      <90 kg1.08 (1.26)4.3–1.10.016
      ≥90 kg1.64 (1.53)7.7–1.8
      Body Mass Index (BMI)
      <301.43 (1.43)6.4–1.80.261
      >301.71 (1.61)7.7–1.7
      Uni/bilateral
      Unilateral1.48 (1.58)7.7–1.70.666
      Bilateral1.51 (1.39)6.4–1.8
      As seen for the 2 year results, extensive valgus alignments showed markedly pronounced mean subsidence compared to varus- or neutral alignments at mid-term. However, compared to the 2 year results, no further migration could be observed. Mean axial migration in groups A-E at 5 years is shown in Table 3.
      Table 3Influence of postoperative stem alignment on mean axial migration at 5 years.
      CCD categorySubsidence (mm)
      N ObsMeanStdev.Median25% Pctl75% PctlLow CLUp. CLMinMax
      A (<124.9°)111.782.290.802.800.403.320.247.700.60
      B (125–129.9°)411.261.291.101.900.401.660.854.501.00
      C (130–134.9°)841.451.321.302.350.501.741.164.501.80
      D (135–139.9°)421.381.351.301.900.501.800.965.600.90
      E (>140°)132.722.122.104.300.803.991.446.400.40
      Total1911.501.481.302.300.501.711.297.701.80
      p = 0.268
      In all 4 cases not to be considered stable, partly severe forms of undersizing of the stem were identified (Fig. 5a/b).
      Fig. 5
      Fig. 5Severe undersizing of the stem with subsequent continuous subsidence (a: postoperative; b: 5-year follow-up).

      4. Discussion

      The present study analyzed the migration pattern of a calcar-guided short-stem in a mid-term follow-up. After an initially pronounced axial migration, in most cases “at hazard” stabilization can be observed over time. However, in some cases early subsidence resulted in further axial migration, potentially increasing the risk of revision surgery in the long-term.
      Whereas the short-term results of the present study have already been published [
      • Kutzner K.P.
      • Kovacevic M.P.
      • Freitag T.
      • Fuchs A.
      • Reichel H.
      • Bieger R.
      Influence of patient-related characteristics on early migration in calcar-guided short-stem total hip arthroplasty: a 2-year migration analysis using EBRA-FCA.
      ], for the investigated optimys short stem to date no mid-term results regarding the migration pattern, measured by either EBRA-FCA method or radiostereometric analysis (RSA), are available. At the same time, several other short femoral implants have been analyzed at short- and mid-term so far [
      • Freitag T.
      • Kappe T.
      • Fuchs M.
      • Jung S.
      • Reichel H.
      • Bieger R.
      Migration pattern of a femoral short-stem prosthesis: a 2-year EBRA-FCA-study.
      ,
      • Brinkmann V.
      • Radetzki F.
      • Delank K.S.
      • Wohlrab D.
      • Zeh A.
      A prospective randomized radiographic and dual-energy X-ray absorptiometric study of migration and bone remodeling after implantation of two modern short-stemmed femoral prostheses.
      ,
      • Schmidutz F.
      • Graf T.
      • Mazoochian F.
      • Fottner A.
      • Bauer-Melnyk A.
      • Jansson V.
      Migration analysis of a metaphyseal anchored short-stem hip prosthesis.
      ,
      • Brinkmann V.
      • Radetzki F.
      • Gutteck N.
      • Delank S.
      • Zeh A.
      Influence of varus/valgus positioning of the Nanos® and Metha® short-stemmed prostheses on stress shielding of metaphyseal bone.
      ,
      • Loppini M.
      • Grappiolo G.
      Uncemented short stems in primary total hip arthroplasty.
      ,
      • Acklin Y.P.
      • Jenni R.
      • Bereiter H.
      • Thalmann C.
      • Stoffel K.
      Prospective clinical and radiostereometric analysis of the Fitmore short-stem total hip arthroplasty.
      ,
      • Budde S.
      • Seehaus F.
      • Schwarze M.
      • Hurschler C.
      • Floerkemeier T.
      • Windhagen H.
      • Noll Y.
      • Ettinger M.
      • Thorey F.
      Analysis of migration of the Nanos® short-stem hip implant within two years after surgery.
      ].
      In EBRA studies, the Metha short stem showed mean axial migration of 1.96 mm at 1 year after surgery [
      • Brinkmann V.
      • Radetzki F.
      • Delank K.S.
      • Wohlrab D.
      • Zeh A.
      A prospective randomized radiographic and dual-energy X-ray absorptiometric study of migration and bone remodeling after implantation of two modern short-stemmed femoral prostheses.
      ]. In contrast, Schmidutz et al. showed a mean subsidence of 0.7 mm with the same stem within an observation period of 2 years [
      • Schmidutz F.
      • Graf T.
      • Mazoochian F.
      • Fottner A.
      • Bauer-Melnyk A.
      • Jansson V.
      Migration analysis of a metaphyseal anchored short-stem hip prosthesis.
      ]. Mean axial migration of the Nanos short stem (Smith & Nephew, Memphis, USA) was 2.04 mm at 1 year and the Fitmore stem (Zimmer, Warsaw, Indiana, USA) showed 1.10 mm subsidence at 2 years after surgery [
      • Freitag T.
      • Kappe T.
      • Fuchs M.
      • Jung S.
      • Reichel H.
      • Bieger R.
      Migration pattern of a femoral short-stem prosthesis: a 2-year EBRA-FCA-study.
      ,
      • Brinkmann V.
      • Radetzki F.
      • Gutteck N.
      • Delank S.
      • Zeh A.
      Influence of varus/valgus positioning of the Nanos® and Metha® short-stemmed prostheses on stress shielding of metaphyseal bone.
      ]. Stabilization after an initially pronounced settling was described in most of these studies. Influencing factors on axial migration such as gender, age, BMI, or implant size were not found in those investigations, however, Freitag et al. reported a tendency towards increased subsidence in patients with a BMI >30 kg/m2 for the Fitmore stem without statistical significance, potentially being a risk factor [
      • Freitag T.
      • Kappe T.
      • Fuchs M.
      • Jung S.
      • Reichel H.
      • Bieger R.
      Migration pattern of a femoral short-stem prosthesis: a 2-year EBRA-FCA-study.
      ,
      • Kaipel M.
      • Grabowiecki P.
      • Sinz K.
      • Farr S.
      • Sinz G.
      Migration characteristics and early clinical results of the NANOS® short-stem hip arthroplasty.
      ].
      Likewise, RSA studies demonstrated that some short stems were affected by an early slight migration within the first months after surgery, followed by a secondary stable fixation [
      • Loppini M.
      • Grappiolo G.
      Uncemented short stems in primary total hip arthroplasty.
      ,
      • Acklin Y.P.
      • Jenni R.
      • Bereiter H.
      • Thalmann C.
      • Stoffel K.
      Prospective clinical and radiostereometric analysis of the Fitmore short-stem total hip arthroplasty.
      ,
      • Budde S.
      • Seehaus F.
      • Schwarze M.
      • Hurschler C.
      • Floerkemeier T.
      • Windhagen H.
      • Noll Y.
      • Ettinger M.
      • Thorey F.
      Analysis of migration of the Nanos® short-stem hip implant within two years after surgery.
      ]. McCalden et al. performed a randomized controlled trial comparing the patterns of migration of a short modular stem with metaphyseal fixation (SMF; Smith&Nephew, London, UK) with those of a conventional stem (Synergy, Smith&Nephew, London, UK). At 24 months after surgery, there was no statistically significant difference in mean migration between the groups: total migration was 1.09 mm and 0.73 mm, respectively [
      • McCalden R.W.
      • Korczak A.
      • Somerville L.
      • Yuan X.
      • Naudie D.D.
      A randomised trial comparing a short and a standard-length metaphyseal engaging cementless femoral stem using radiostereometric analysis.
      ]. In a recent RSA study of Ferguson et al. the comparison of the MiniHip short stem (Corin, Cirencester, UK) with a conventional straight stem (MetaFix, Corin, Cirencester, UK) resulted in significantly less subsidence for the short stem (0.26 mm vs. 0.62 mm) after 2 years [
      • Ferguson R.J.
      • Broomfield J.A.
      • Malak T.T.
      • Palmer A.J.R.
      • Whitwell D.
      • Kendrick B.
      • Taylor A.
      • Glyn-Jones S.
      Primary stability of a short bone-conserving femoral stem.
      ].
      Early femoral implant migration within the first 2 years postoperative has been described to be one of the main risk factors for aseptic loosening in conventional cementless THA [
      • Krismer M.
      • Biedermann R.
      • Stöckl B.
      • Fischer M.
      • Bauer R.
      • Haid C.
      The prediction of failure of the stem in THR by measurement of early migration using EBRA-FCA. Einzel-Bild-Roentgen-Analyse-femoral component analysis.
      ]. Krimser et al. reported a predictive sensitivity of 69% for subsequent implant failure for conventional stems with early migration of more than 1.5 mm [
      • Krismer M.
      • Biedermann R.
      • Stöckl B.
      • Fischer M.
      • Bauer R.
      • Haid C.
      The prediction of failure of the stem in THR by measurement of early migration using EBRA-FCA. Einzel-Bild-Roentgen-Analyse-femoral component analysis.
      ]. In the present study, 73 stems showed an early subsidence of more than 1.5 mm at 2 years after surgery. All but 4 implants stabilized afterwards and were classified stable (migration mode B according to Krismer et al. [
      • Krismer M.
      • Biedermann R.
      • Stöckl B.
      • Fischer M.
      • Bauer R.
      • Haid C.
      The prediction of failure of the stem in THR by measurement of early migration using EBRA-FCA. Einzel-Bild-Roentgen-Analyse-femoral component analysis.
      ]). Streit et al. reported a cut-off value of 2.7 mm at 2 years after surgery with the cementless CLS with a sensitivitiy of 56% and a specificity of 99% for predicting late aseptic failure. The 18-year survival presented with 95% for implants showing less and 29% for implants showing more than 2.7 mm initial subsidence at 2 years after surgery. Most of the failed implants showed continuous migration after the second year [
      • Streit M.R.
      • Haeussler D.
      • Bruckner T.
      • Proctor T.
      • Innmann M.M.
      • Merle C.
      • Gotterbarm T.
      • Weiss S.
      Early migration predicts aseptic loosening of cementless femoral stems: a long-term study.
      ]. In the present study 29 stems presented with subsidence of more than 2.7 mm at 2 years postoperatively with, again, the above mentioned 4 cases being identified as unstable showing further subsidence. In contrast, in a recently published study using the Fitmore (Zimmer Biomet, Warsaw, Indiana, USA) after initially pronounced subsidence all stems “at risk” could be defined stable afterwards, not showing any further migration [
      • Freitag T.
      • Fuchs M.
      • Woelfle-Roos J.V.
      • Reichel H.
      • Bieger R.
      Mid-term migration analysis of a femoral short-stem prosthesis: a five-year EBRA-FCA-study.
      ]. Freitag et al. concluded that the determination of a threshold of critical early femoral short stem subsidence is necessary because of the differing migration patterns compared to conventional stems [
      • Freitag T.
      • Fuchs M.
      • Woelfle-Roos J.V.
      • Reichel H.
      • Bieger R.
      Mid-term migration analysis of a femoral short-stem prosthesis: a five-year EBRA-FCA-study.
      ].
      In previously published data of the short-term results of the present study particularly male and heavy-weight patients and hips with extensive valgus stem-alignment could be identified as being at risk regarding pronounced early migration [
      • Kutzner K.P.
      • Kovacevic M.P.
      • Freitag T.
      • Fuchs A.
      • Reichel H.
      • Bieger R.
      Influence of patient-related characteristics on early migration in calcar-guided short-stem total hip arthroplasty: a 2-year migration analysis using EBRA-FCA.
      ,
      • Kutzner K.P.
      • Freitag T.
      • Donner S.
      • Kovacevic M.P.
      • Bieger R.
      Outcome of extensive varus and valgus stem alignment in short-stem THA: clinical and radiological analysis using EBRA-FCA.
      ] The present mid-term data confirms these findings. However, regarding those stems being in extensive valgus position at mid-term no further migration could be observed. Thus, given that cortical contact to the lateral cortex has been reached, also stems in extensive valgus alignment show subsequent stabilization. Additionally, given the early subsidence of the stem, the stem tip in those cases eventually makes contact to the medial cortex, often resulting in a fit-and-fill in the proximal diaphysis (Fig. 2). Also, undersizing of the stem has been found to be the main reason for early migration [
      • Kutzner K.P.
      • Freitag T.
      • Donner S.
      • Kovacevic M.P.
      • Bieger R.
      Outcome of extensive varus and valgus stem alignment in short-stem THA: clinical and radiological analysis using EBRA-FCA.
      ,
      • Loweg L.
      • Kutzner K.P.
      • Trost M.
      • Hechtner M.
      • Drees P.
      • Pfeil J.
      • Schneider M.
      The learning curve in short-stem THA: influence of the surgeon's experience on intraoperative adjustments due to intraoperative radiography.
      ]. Again, this can be confirmed by the mid-term data. All 4 cases of progressing subsidence after the 2 year follow-up present as undersized femoral implants particularly with lack of lateral cortical contact postoperatively, which has been previously identified as common within the course of the learning curve with this implant and others [
      • Kutzner K.P.
      • Freitag T.
      • Donner S.
      • Kovacevic M.P.
      • Bieger R.
      Outcome of extensive varus and valgus stem alignment in short-stem THA: clinical and radiological analysis using EBRA-FCA.
      ,
      • Loweg L.
      • Kutzner K.P.
      • Trost M.
      • Hechtner M.
      • Drees P.
      • Pfeil J.
      • Schneider M.
      The learning curve in short-stem THA: influence of the surgeon's experience on intraoperative adjustments due to intraoperative radiography.
      ,
      • Rivera F.
      • Leonardi F.
      • Evangelista A.
      • Pierannunzii L.
      Risk of stem undersizing with direct anterior approach for total HIP arthroplasty.
      ,
      • Fottner A.
      • Woiczinski M.
      • Kistler M.
      • Schröder C.
      • Schmidutz T.F.
      • Jansson V.
      • Schmidutz F.
      Influence of undersized cementless hip stems on primary stability and strain distribution.
      ] (Fig. 5a/b). Particularly in heavy-weight patients surgeons should make sure, that contact to the lateral cortex is securely accomplished intraoperatively. In cases of undersizing, most likely the stem stabilizes as soon as it makes contact to the lateral cortex.
      However, at mid-term follow-up no clinical consequences in those cases of progressing axial migration are obvious. To date no stem revision surgery was required and radiologically no signs of loosening and osteolysis were seen. All 4 patients confirmed high rates of satisfaction with the mid-term clinical outcome.
      Long-term observational migration studies seem necessary to further correlate early implant migration with aseptic failure in short-stem designs.
      The present investigation has several limitations. First to be mentioned is the short follow-up of 5 years, not yet allowing the establishment of a predictive value of migration analysis regarding aseptic loosening and implant failure of this specific short-stem design. Secondly, no clear indications had been defined prior to the implantation of the investigated stem. However, sufficient bone quality was the main criteria, explaining the low incidence of Dorr type C in the present collective. Furthermore, given the used inclusion criteria a complete follow-up could only be achieved for almost 89% of the initially included hips. However, with 191 analyzed cases the size of the study still allows valid conclusions. Also, RSA provides higher accuracy in comparison to the EBRA-FCA method, which was used in the present study. The computer-assisted EBRA-FCA system was evaluated to be able to detect stem subsidence of ±1 mm given a specificity of 100% and sensitivity of 78% [
      • Biedermann R.
      • Krismer M.
      • Stöckl B.
      • Mayrhofer P.
      • Ornstein E.
      • Franzén H.
      Accuracy of EBRA-FCA in the measurement of migration of femoral components of total hip replacement. Einzel-Bild-Röntgen-Analyse-femoral component analysis.
      ]. However, EBRA-FCA has been established as one of the main predictive tools to estimate long-term survival. In addition, in the present study only axial subsidence was analyzed. Stem tilting and rotation were not subject to this investigation.

      5. Conclusions

      Most of the investigated stems showed delayed settlement within the first 2 years after surgery, making a different migration pattern compared to conventional stems likely. In those few cases with progressing subsidence within the first 5 years, undersizing of the stem, as part of a surgical mistake, has to be acknowledged. However, at mid-term clinically no negative consequences were obvious with 100% implant survival. Male patients and heavy-weight patients, again, are of higher risk regarding further subsidence. Stems with valgus alignment remained stable after 2 years. Long-term results will be obtained to further determine the impact of early migration on short stem survival.

      Declaration of Competing Interest

      KPK reports personal fees and non-financial support from Mathys Ltd. , Bettlach, Switzerland.
      JP reports grants, personal fees and non-financial support from Mathys Ltd. , Bettlach, Switzerland.
      All other authors declare, that they have no conflict of interest.

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