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Functional Femoral Anteversion: Axial Rotation of the Femur and its Implications for Stem Version Targets in Total Hip Arthroplasty

Open AccessPublished:October 11, 2022DOI:https://doi.org/10.1016/j.artd.2022.09.002

      Abstract

      Background

      Acetabular and femoral component positioning are important considerations in reducing adverse outcomes after total hip arthroplasty (THA). Previous assessments of femoral anteversion examined anatomic femoral anteversion (AFA) referenced to anatomic landmarks. However, this does not provide a functional understanding of the femur’s relationship to the hip. We investigate a new measurement, functional femoral anteversion (FFA), and sought to measure its variability across a large sample of patients undergoing THA.

      Methods

      A total of 1008 consecutive patients underwent THA surgery between September 2019 and July 2021. All patients were measured for supine and standing functional femoral rotation (FFR), AFA, and FFA.

      Results

      The mean standing FFA was 13.2° ± 12.2° (−27.8° to 52.3°). The mean change in FFR from supine to standing was −2.2° ± 11.8° (−43.0° to 41.9°). Of all, 161 (16%) patients had standing FFA version greater than 25°. Four hundred sixty (46%) patients had standing FFR (internal or external) greater than 10°. One hundred twenty-three (12%) patients exhibited an increase in external rotation from supine to standing of greater than 10°. A moderate, negative linear relationship was observed between AFA and standing external femoral rotation (P <<.001, R = −0.46), indicating people may externally rotate their femur as AFA decreases with age.

      Conclusions

      Functional alignment of the femur in patients requiring THA is understudied. It is now understood that the femur, like the pelvis, can rotate substantially between functional positions. Enhancing our understanding of FFA and FFR may improve both acetabular and femoral component positioning.

      Keywords

      Introduction

      Acetabular cup positioning has long been recognized as an important consideration in total hip arthroplasty (THA) [
      • Biedermann R.
      • Tonin A.
      • Krismer M.
      • Rachbauer F.
      • Eibl G.
      • Stockl B.
      Reducing the risk of dislocation after total hip arthroplasty: the effect of orientation of the acetabular component.
      ,
      • Brown T.D.
      • Callaghan J.J.
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      ,
      • Malik A.
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      • Dorr L.D.
      Impingement with total hip replacement.
      ,
      • Callanan M.C.
      • Jarrett B.
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      • Rubash H.E.
      • Freiberg A.A.
      • et al.
      The John Charnley Award: risk factors for cup malpositioning: quality improvement through a joint registry at a tertiary hospital.
      ,
      • Esposito C.I.
      • Walter W.L.
      • Roques A.
      • Tuke M.A.
      • Zicat B.A.
      • Walsh W.R.
      • et al.
      Wear in alumina-on-alumina ceramic total hip replacements: a retrieval analysis of edge loading.
      ,
      • Patel A.B.
      • Wagle R.R.
      • Usrey M.M.
      • Thompson M.T.
      • Incavo S.J.
      • Noble P.C.
      Guidelines for implant placement to minimize impingement during activities of daily living after total hip arthroplasty.
      ]. Malpositioning of the acetabular cup has been shown to lead to increased rates of wear, edge loading, and impingement which may lead to instability, osteolysis, or dislocation [
      • Biedermann R.
      • Tonin A.
      • Krismer M.
      • Rachbauer F.
      • Eibl G.
      • Stockl B.
      Reducing the risk of dislocation after total hip arthroplasty: the effect of orientation of the acetabular component.
      ,
      • Brown T.D.
      • Callaghan J.J.
      Impingement in total hip replacement: mechanisms and consequences.
      ,
      • Malik A.
      • Maheshwari A.
      • Dorr L.D.
      Impingement with total hip replacement.
      ,
      • Callanan M.C.
      • Jarrett B.
      • Bragdon C.R.
      • Zurakowski D.
      • Rubash H.E.
      • Freiberg A.A.
      • et al.
      The John Charnley Award: risk factors for cup malpositioning: quality improvement through a joint registry at a tertiary hospital.
      ,
      • Esposito C.I.
      • Walter W.L.
      • Roques A.
      • Tuke M.A.
      • Zicat B.A.
      • Walsh W.R.
      • et al.
      Wear in alumina-on-alumina ceramic total hip replacements: a retrieval analysis of edge loading.
      ,
      • Patel A.B.
      • Wagle R.R.
      • Usrey M.M.
      • Thompson M.T.
      • Incavo S.J.
      • Noble P.C.
      Guidelines for implant placement to minimize impingement during activities of daily living after total hip arthroplasty.
      ].
      Lewinnek et al. were among the first authors to recognize the relationship between acetabular cup positioning and dislocation rates by proposing a widely cited “safe zone” [
      • Lewinnek G.E.
      • Lewis J.L.
      • Tarr R.
      • Compere C.L.
      • Zimmerman J.R.
      Dislocations after total hip-replacement and arthroplasties.
      ] that was believed to reduce the incidence of dislocation. However, further examination in larger samples of THA patients revealed that it does not preclude dislocation [
      • Esposito C.I.
      • Gladnick B.P.
      • Lee Y.Y.
      • Lyman S.
      • Wright T.M.
      • Mayman D.J.
      • et al.
      Cup position alone does not predict risk of dislocation after total hip arthroplasty.
      ,
      • Abdel M.P.
      • von Roth P.
      • Jennings M.T.
      • Hanssen A.D.
      • Pagnano M.W.
      What safe zone? The vast majority of dislocated THAs are within the Lewinnek safe zone for acetabular component position.
      ] and is insufficient for surgical guidance.
      Assessments of pelvic tilt in functional positions, when edge-loading and impingement are most likely [
      • Patel A.B.
      • Wagle R.R.
      • Usrey M.M.
      • Thompson M.T.
      • Incavo S.J.
      • Noble P.C.
      Guidelines for implant placement to minimize impingement during activities of daily living after total hip arthroplasty.
      ,
      • Pierrepont J.
      • Yang L.
      • Arulampalam J.
      • Stambouzou C.
      • Miles B.
      • Li Q.
      The effect of seated pelvic tilt on posterior edge-loading in total hip arthroplasty: a finite element investigation.
      ], later revealed large interindividual variability between positions [
      • DiGioia A.M.
      • Hafez M.A.
      • Jaramaz B.
      • Levison T.J.
      • Moody J.E.
      Functional pelvic orientation measured from lateral standing and sitting radiographs.
      ,
      • Lazennec J.Y.
      • Charlot N.
      • Gorin M.
      • Roger B.
      • Arafati N.
      • Bissery A.
      • et al.
      Hip-spine relationship: a radio-anatomical study for optimization in acetabular positioning.
      ,
      • Pierrepont J.
      • Hawdon G.
      • Miles B.P.
      • O’Connor B.
      • Baré J.
      • Walter L.R.
      • et al.
      Variation in functional pelvic tilt in patients undergoing total hip arthroplasty.
      ], with pelvic rotation between positions as low as 5° and as high as 70°. These changes in pelvic tilt would correspond to changes in acetabular anteversion of 4° and 50°, respectively, [
      • Lembeck B.
      • Mueller O.
      • Reize P.
      • Wuelker N.
      Pelvic tilt makes acetabular cup navigation inaccurate.
      ] and help explain why some individuals with a given cup orientation dislocate while others with the same cup orientation do not.
      Similar to the variability of pelvic tilt, investigations into anatomic femoral anteversion (AFA) revealed large variability [
      • Koerner J.D.
      • Patel N.M.
      • Yoon R.S.
      • Sirkin M.S.
      • Reilly M.C.
      • Liporace F.A.
      Femoral version of the general population: does “normal” vary by gender or ethnicity?.
      ,
      • Bargar W.L.
      • Jamali A.A.
      • Nejad A.H.
      Femoral anteversion in THA and its lack of correlation with native acetabular anteversion.
      ,
      • Hartel M.J.
      • Petersik A.
      • Schmidt A.
      • Kendoff D.
      • Nuchtern J.
      • Rueger J.M.
      • et al.
      Determination of femoral neck angle and torsion angle utilizing a novel three-dimensional modeling and analytical technology based on CT datasets.
      ,
      • Pierrepont J.W.
      • Marel E.
      • Baré J.v.
      • Walter L.R.
      • Stambouzou C.Z.
      • Solomon M.I.
      • et al.
      Variation in femoral anteversion in patients requiring total hip replacement.
      ], ranging up to 80° [
      • Pierrepont J.W.
      • Marel E.
      • Baré J.v.
      • Walter L.R.
      • Stambouzou C.Z.
      • Solomon M.I.
      • et al.
      Variation in femoral anteversion in patients requiring total hip replacement.
      ]. Gender and age differences were also observed, with AFA decreasing for males as age increases [
      • Pierrepont J.W.
      • Marel E.
      • Baré J.v.
      • Walter L.R.
      • Stambouzou C.Z.
      • Solomon M.I.
      • et al.
      Variation in femoral anteversion in patients requiring total hip replacement.
      ]. These studies measured AFA as the angle between the femoral neck and the posterior condylar line (PCL) projected onto the axial plane [
      • Koerner J.D.
      • Patel N.M.
      • Yoon R.S.
      • Sirkin M.S.
      • Reilly M.C.
      • Liporace F.A.
      Femoral version of the general population: does “normal” vary by gender or ethnicity?.
      ,
      • Hartel M.J.
      • Petersik A.
      • Schmidt A.
      • Kendoff D.
      • Nuchtern J.
      • Rueger J.M.
      • et al.
      Determination of femoral neck angle and torsion angle utilizing a novel three-dimensional modeling and analytical technology based on CT datasets.
      ,
      • Bargar W.L.
      • Jamali A.A.
      • Nejad A.H.
      ] or onto a plane perpendicular to the long femoral axis [
      • Pierrepont J.W.
      • Marel E.
      • Baré J.v.
      • Walter L.R.
      • Stambouzou C.Z.
      • Solomon M.I.
      • et al.
      Variation in femoral anteversion in patients requiring total hip replacement.
      ]. This measurement references only anatomic landmarks and is constant between different patient positions as it does not account for functional femoral rotation (FFR). However, supine and standing FFR have been shown to vary significantly between individuals [
      • Tezuka T.
      • Inaba Y.
      • Kobayashi N.
      • Choe H.
      • Higashihira S.
      • Saito T.
      The influence of patient factors on femoral rotation after total hip arthroplasty.
      ,
      • Uemura K.
      • Takao M.
      • Sakai T.
      • Nishii T.
      • Sugano N.
      • Uemura Keisuke M.D.
      • et al.
      The validity of using the posterior condylar line as a rotational reference for the femur.
      ] and change from presurgery to postsurgery following THA [
      • Tezuka T.
      • Inaba Y.
      • Kobayashi N.
      • Choe H.
      • Higashihira S.
      • Saito T.
      The influence of patient factors on femoral rotation after total hip arthroplasty.
      ]. These rotational changes would alter the functional stem anteversion (FSA) of an implanted femoral component and thus have implications for the optimal femoral stem target. Specifically, individuals who exhibit a high internal FFR may be at risk of anterior impingement when anatomic stem anteversion (ASA) is low or negative, and individuals who exhibit a high external FFR may be at risk of posterior impingement when ASA is high.
      Therefore, the aims of this study were three-fold: first, to further investigate the variability of a new measurement proposed by Uemura et al. [
      • Uemura K.
      • Takao M.
      • Otake Y.
      • Koyama K.
      • Yokota F.
      • Hamada H.
      • et al.
      Can anatomic measurements of stem anteversion angle be considered as the functional anteversion angle?.
      ] called functional femoral anteversion (FFA) across a larger sample of patients undergoing THA; second, to assess its implications for femoral stem version targets; third, to investigate the number of individuals whose FFR would transition them out of suggested combined anteversion (CA) zones.

      Material and methods

      A total of 1008 consecutive patients underwent THA surgery between September 2019 and July 2021. Of the total, 627 were female (62%), and the average age was 66 years (16-95). Inclusion criteria included patients that had a long-limb computed tomography (CT) scan and standing lateral X-ray of the distal femur (Fig. 1) as a part of the 360 Med Care (Sydney, Australia) THA preoperative planning protocol. The lateral knee radiograph taken was previously not standardized and, as far as the authors are aware, not routinely taken for any other preoperative THA planning protocol. However, our imaging protocol contains detailed instructions for radiographers to position the patient and acquire the imaging, and all radiology centers must be approved via an in-person explanation of the protocol. We believe these steps ensure repeatable lateral knee radiographs of patients in standing and permit accurate measurement of their FFR. From these imaging data, assessments were made for supine and standing FFR, defined as the angle between the PCL and the coronal plane and for AFA, defined as the angle between the femoral neck and the PCL, projected onto the femoral anatomic axis.
      AFA was measured using the approach described by Pierrepont et al. [
      • Pierrepont J.W.
      • Marel E.
      • Baré J.v.
      • Walter L.R.
      • Stambouzou C.Z.
      • Solomon M.I.
      • et al.
      Variation in femoral anteversion in patients requiring total hip replacement.
      ]. The CT scan was segmented in ScanIP Medical (Simpleware; Synopsys, Exeter, United Kingdom) to recreate 3D models of the patient’s bony anatomy. Landmarks on the operative side were measured to establish the PCL and the femoral neck axis. Supine FFR was measured in RadiAnt DICOM Viewer v2.2.5.10715 (Medixant, Poznan, Poland) on CT scans with a line that was tangential to the posterior femoral condyles and a horizontal baseline. Standing FFR was measured by first taking the sagittal distance between the posterior condyles on the lateral knee radiograph. A computational algorithm then recreated this distance in 3D using a segmented model of the patient’s femur and modified the femoral rotation, measured against the coronal plane, until the length of the PCL converged to the value previously measured. This is equivalent to calculating the FFR required to make the PCL and coronal plane parallel. Positive FFR values were interpreted as external rotation. Therefore, adding the FFR to the AFA resulted in FFA. A visual representation of the relationship among AFA, FFR, and FFA can be found in Figure 2, which also shows the FSA by using 3D templating to recreate AFA.
      Figure thumbnail gr1
      Figure 1(Left) The stance taken by a patient while undergoing their preoperative functional knee radiology. (Right) Functional knee radiograph acquired preoperatively to enable calculation of the patient’s standing functional femoral rotation.
      Figure thumbnail gr2
      Figure 2Left: Illustration of the relationship between anatomic femoral anteversion, functional femoral rotation, and functional femoral anteversion. Right: Illustration of the relationship between anatomic stem anteversion, functional femoral rotation, and functional stem anteversion using 3D templating to recreate the anatomic femoral anteversion.
      A positive change in FFA from the supine to standing position was interpreted as external rotation of the femur between these positions. All imaging and measurements were evaluated twice by qualified surgical planning engineers.

      Statistical analysis

      Statistical analysis was performed in RStudio v1.3.1903 (RStudio, Boston, MA). An alpha value of 0.05 was used to determine clinical significance. Two-way t-tests were used to determine significant differences in continuous variables. Pearson’s correlations were used to assess the linear relationship between continuous variables.

      Ethics

      This retrospective analysis was approved by the Bellberry Human Research Ethics Committee (study number 201203710).

      Results

      Results for the supine and standing FFR, AFA, standing FFA, and the change in FFA from supine to standing are given in Table 1 and Figure 4, Figure 5, Figure 6, Figure 3. The mean supine FFR was 0.4 ± 10.6° (−34.0° to 31.4°). There was a significant gender difference (P << 0.001) with a mean supine FFR of 3.3° (−31.4° to 31.4°) for females and −4.5° (−34.0° to 24.5°) for males. The mean AFA was 15.6° ± 9.8° (−24.4° to 68.4°), and significant gender differences were observed (P << 0.001). The mean AFA was 16.8° (−24.4° to 63.6°) for females and 13.8° (−9.4° to 68.4°) for males. The mean standing FFR was −2.7° ± 13.1° (−37.0° to 46.0°), and significant gender differences were observed (P << 0.001). The mean standing FFR was −5.2° (−37.0° to 46.0°) for females and 1.7° (−34.0° to 35.7°) for males, indicating that, while standing, females tend to be internally rotated and males tend to be externally rotated. The mean standing FFA was 13.2° ± 12.2° (−27.8° to 52.3°), and significant gender differences were observed (P << 0.001). The mean standing FFA was 11.7° (−27.8° to 52.3°) for females and 15.6° (−8.5° to 49.9°) for males. The mean change in FFR from supine to standing was −2.2° ± 11.8° (−43.0° to 41.9°), and no significant gender differences were observed (P = .18). The absolute mean change in FFR from supine to standing was 8.7° ± 7.2° (−0° to 43.0°), and no significant gender differences were observed (P = .09).
      Table 1Tabulated results for all 1008 consecutive patients across supine femoral rotation, anatomic femoral anteversion, standing functional femoral rotation, standing functional anteversion, and the change in functional femoral rotation from supine to standing.
      Mean (absolute mean)RangeStandard deviationP value (difference between genders)
      Supine femoral rotation0.4°−34.0° to 31.4°10.6°<<.001
      Indicates statistical significance.
      Anatomic femoral anteversion15.6°−24.4° to 68.4°9.8°<<.001
      Indicates statistical significance.
      Standing functional femoral Rotation−2.6°−46.0° to 37.0°13.1°<<.001
      Indicates statistical significance.
      Standing functional femoral anteversion13.1°−27.8° to 52.3°12.2°<<.001
      Indicates statistical significance.
      Change in functional femoral rotation (supine to standing)−2.2° (8.7°)−43.0° to 41.9°11.8°.18
      a Indicates statistical significance.
      One hundred sixty-one (16%) patients had standing FFA version of greater than 25° (Figure 4, Figure 6). Considering a CA zone of 25°-45° and a conventional standing cup anteversion of 20°, 72% of patients would fall within the CA zone when anatomic femoral landmarks are considered (AFA), but only 59% fall within the zone when the functional position of the femur (FFA) is considered. Therefore, considering FFA would place an additional 13% of patients at risk of posterior impingement in a widely targeted CA zone. Four hundred sixty (46%) patients had a standing FFR (internal or external) greater than 10° (Fig. 6). One hundred twenty-three (12%) patients exhibited an increase in external rotation from supine to standing of greater than 10° (Figure 5, Figure 6). Three hundred thirty-five (33%) patients exhibited an absolute change in FFR (internal or external rotation) of greater than 10° (Fig. 6). These patients’ femoral components would be functionally oriented in an alignment that is considerably different to the alignment when the prosthesis is implanted on the operating table, or when only AFA is considered, and may place them at risk of functional malorientation.
      Figure thumbnail gr4
      Figure 4Histogram of the standing functional femoral anteversion results. Sixteen percent of patients had functional femoral anteversion greater than 25°, which may place them at risk of functional malorientation when considered in the context of combined anteversion.
      Figure thumbnail gr5
      Figure 5Histogram of the change in functional femoral rotation results. Twelve percent of patients exhibited external femoral rotation of greater than 10° from supine to standing position, which may place them at risk of functional malorientation.
      Figure thumbnail gr6
      Figure 6Summary of the key findings for FFR and FFA. ER, external rotation; IR, internal rotation.
      A weak but significant relationship was found with older patients exhibiting less AFA (P << 0.001, R = −0.17); greater standing FFR (P << 0.001, R = 0.18); and greater change in FFA from supine to standing (P << 0.001, R = 0.17). A moderate, negative linear relationship was also observed between AFA and standing FFR (P << 0.001, R = −0.46), indicating people may externally rotate their femur as AFA decreases with age (Fig. 7).
      Figure thumbnail gr7
      Figure 7Scatter plot of standing functional femoral rotation and anatomic femoral anteversion and the correlation between these. The statistically significant negative relationship indicates that people may externally rotate their femur as their anatomic femoral anteversion decreases as a compensatory mechanism to maintain soft-tissue tensioning.

      Discussion

      Functional alignment of the femur in patients requiring THA is understudied. Several studies have previously investigated AFA [
      • Koerner J.D.
      • Patel N.M.
      • Yoon R.S.
      • Sirkin M.S.
      • Reilly M.C.
      • Liporace F.A.
      Femoral version of the general population: does “normal” vary by gender or ethnicity?.
      ,
      • Bargar W.L.
      • Jamali A.A.
      • Nejad A.H.
      Femoral anteversion in THA and its lack of correlation with native acetabular anteversion.
      ,
      • Hartel M.J.
      • Petersik A.
      • Schmidt A.
      • Kendoff D.
      • Nuchtern J.
      • Rueger J.M.
      • et al.
      Determination of femoral neck angle and torsion angle utilizing a novel three-dimensional modeling and analytical technology based on CT datasets.
      ,
      • Pierrepont J.W.
      • Marel E.
      • Baré J.v.
      • Walter L.R.
      • Stambouzou C.Z.
      • Solomon M.I.
      • et al.
      Variation in femoral anteversion in patients requiring total hip replacement.
      ] and how this presents across samples of patients undergoing THA. However, it is known that there is large variation in FFR among individuals and from presurgery to postsurgery [
      • Tezuka T.
      • Inaba Y.
      • Kobayashi N.
      • Choe H.
      • Higashihira S.
      • Saito T.
      The influence of patient factors on femoral rotation after total hip arthroplasty.
      ,
      • Uemura K.
      • Takao M.
      • Sakai T.
      • Nishii T.
      • Sugano N.
      • Uemura Keisuke M.D.
      • et al.
      The validity of using the posterior condylar line as a rotational reference for the femur.
      ], which AFA does not capture. In this study, we sought to further investigate FFA, a new measurement explored by Uemura et al. [
      • Uemura K.
      • Takao M.
      • Otake Y.
      • Koyama K.
      • Yokota F.
      • Hamada H.
      • et al.
      Can anatomic measurements of stem anteversion angle be considered as the functional anteversion angle?.
      ], across a larger sample of patients undergoing THA, assess its implications for femoral stem version targets, and investigate the number of individuals whose femoral rotation may place them at risk of malorientation.
      Our results of a mean supine femoral rotation of 0.4° ± 10.6° are similar to those of studies by Uemura et al. [
      • Uemura K.
      • Takao M.
      • Sakai T.
      • Nishii T.
      • Sugano N.
      • Uemura Keisuke M.D.
      • et al.
      The validity of using the posterior condylar line as a rotational reference for the femur.
      ,
      • Uemura K.
      • Takao M.
      • Otake Y.
      • Koyama K.
      • Yokota F.
      • Hamada H.
      • et al.
      Can anatomic measurements of stem anteversion angle be considered as the functional anteversion angle?.
      ] who observed median supine femoral rotations of −0.4° ± 10.9° (n = 324) and 0.3° ± 8.3° (n = 191). Both studies by Uemura et al. [
      • Uemura K.
      • Takao M.
      • Sakai T.
      • Nishii T.
      • Sugano N.
      • Uemura Keisuke M.D.
      • et al.
      The validity of using the posterior condylar line as a rotational reference for the femur.
      ,
      • Uemura K.
      • Takao M.
      • Otake Y.
      • Koyama K.
      • Yokota F.
      • Hamada H.
      • et al.
      Can anatomic measurements of stem anteversion angle be considered as the functional anteversion angle?.
      ] and our study demonstrate that, despite having a mean supine rotation value near neutral, the PCL cannot be used as a reference for measuring FFA due to significant interpatient variability. Additionally, we have shown that supine FFR can be over 40° different from standing FFR, 46% of patients have significant standing FFR (>10° internal or external), and 33% of patients undergo significant changes in FFR (>10° of rotation) between positions. Therefore, standing FFR and FFA should not be assumed to be similar to supine FFR.
      Our results of a mean AFA of 15.6° (−24.4° to 68.4°) with significant gender differences and the property of decreasing with age are comparable to previous assessments of AFA. In a study of 1215 patients requiring THA, Pierrepont et al. found a median AFA of 14.4° (−27.1° to 54.5°) with significant gender differences and the property of decreasing with increasing age for males [
      • Pierrepont J.W.
      • Marel E.
      • Baré J.v.
      • Walter L.R.
      • Stambouzou C.Z.
      • Solomon M.I.
      • et al.
      Variation in femoral anteversion in patients requiring total hip replacement.
      ]. Similarly, Hartel et al. found a median AFA of 14.2° (−23.6° to 48.7°) and significant gender differences across 1070 patients [
      • Hartel M.J.
      • Petersik A.
      • Schmidt A.
      • Kendoff D.
      • Nuchtern J.
      • Rueger J.M.
      • et al.
      Determination of femoral neck angle and torsion angle utilizing a novel three-dimensional modeling and analytical technology based on CT datasets.
      ].
      As far as the authors are aware, previous investigations of standing FFR and the change in FFR from supine to standing are limited to 1 study by Uemura et al. [
      • Uemura K.
      • Takao M.
      • Otake Y.
      • Koyama K.
      • Yokota F.
      • Hamada H.
      • et al.
      Can anatomic measurements of stem anteversion angle be considered as the functional anteversion angle?.
      ]. This paper, which assessed preoperative and postoperative AFA and FFR, found a mean preoperative supine FFR of 0.3° ± 8.3°, mean preoperative standing FFR of −4.5° ± 8.8°, and significant changes between preoperative and postoperative states. However, this study did not explore the implications of femoral rotation regarding soft-tissue tensioning around the joint or stem version targets, and its findings may be limited to a Japanese population, with the mean AFA of 25.6° ± 10.6° being significantly higher than that reported in previously published studies [
      • Koerner J.D.
      • Patel N.M.
      • Yoon R.S.
      • Sirkin M.S.
      • Reilly M.C.
      • Liporace F.A.
      Femoral version of the general population: does “normal” vary by gender or ethnicity?.
      ,
      • Bargar W.L.
      • Jamali A.A.
      • Nejad A.H.
      Femoral anteversion in THA and its lack of correlation with native acetabular anteversion.
      ,
      • Hartel M.J.
      • Petersik A.
      • Schmidt A.
      • Kendoff D.
      • Nuchtern J.
      • Rueger J.M.
      • et al.
      Determination of femoral neck angle and torsion angle utilizing a novel three-dimensional modeling and analytical technology based on CT datasets.
      ,
      • Pierrepont J.W.
      • Marel E.
      • Baré J.v.
      • Walter L.R.
      • Stambouzou C.Z.
      • Solomon M.I.
      • et al.
      Variation in femoral anteversion in patients requiring total hip replacement.
      ].
      Regarding the soft-tissue consequences of femoral rotation, we found a moderate correlation whereby decreasing AFA (which is correlated with increasing age) was associated with increasing external FFR in standing. This indicates the possibility of a compensatory mechanism occurring around the joint where the functional positioning of the femur corrects for anatomical variations, which could be occurring to maintain tension in the soft tissue around the capsule. Specifically, an individual’s FFA may remain roughly constant over time, but their FFR may increase to compensate for a decrease in their AFA. Consequently, large intraoperative changes to ASA may result in subsequent FFR changes, altering the patient’s postoperative functional CA and/or soft-tissue tensioning. Similar ideas have been described by Rivière et al. [
      • Rivière C.
      • Lazic S.
      • Villet L.
      • Wiart Y.
      • Allwood S.M.
      • Cobb J.
      Kinematic alignment technique for total hip and knee arthroplasty: the personalized implant positioning surgery.
      ,
      • Rivière C.
      • Maillot C.
      • Harman C.
      • Cobb J.
      Kinematic alignment technique for total hip arthroplasty.
      ] in studies investigating kinematic alignment of the hip and by Akiyama et al. [
      • Akiyama K.
      • Nakata K.
      • Kitada M.
      • Yamamura M.
      • Ohori T.
      • Owaki H.
      • et al.
      Changes in axial alignment of the ipsilateral hip and knee after total hip arthroplasty.
      ]. Follow-up studies might aim to show clinical differences with large changes to AFA or investigate why FFR changes between preoperative and postoperative states.
      Further exploring this idea of compensatory mechanisms occurring at the hip, the authors have noted that major adjustments to acetabular version from native, although protective for instability and edge-loading, can create gait disturbances despite no changes to the AFA. The belief is that the significant body of evidence that supports preoperative analysis of the hip-spine relationship [
      • Heckmann N.
      • McKnight B.
      • Stefl M.
      • Trasolini N.A.
      • Ike H.
      • Dorr L.D.
      Late dislocation following total hip arthroplasty: spinopelvic imbalance as a causative factor.
      ,
      • Tezuka T.
      • Heckmann N.D.
      • Bodner R.J.
      • Dorr L.D.
      Functional safe zone is superior to the Lewinnek safe zone for total hip arthroplasty: why the Lewinnek safe zone is not always predictive of stability.
      ,
      • Vigdorchik J.M.
      • Sharma A.K.
      • Buckland A.J.
      • Elbuluk A.M.
      • Eftekhary N.
      • Mayman D.J.
      • et al.
      2021 Otto Aufranc Award: a simple Hip-Spine Classification for total hip arthroplasty: validation and a large multicentre series.
      ] has led in some instances to dramatic increases in cup anteversion, leading to an uncoupling of the native combined version. Such a patient with significantly increased cup version, but restored anatomic AFA, may have altered gait biomechanics, walk with significant internal rotation due to kinematic disharmony, and present with resultant iliopsoas pain. There is evidence to support this in the study by Uemura et al. [
      • Uemura K.
      • Takao M.
      • Otake Y.
      • Koyama K.
      • Yokota F.
      • Hamada H.
      • et al.
      Can anatomic measurements of stem anteversion angle be considered as the functional anteversion angle?.
      ] who observed a mean change in standing FFR from preoperative to postoperative configurations of −9.8° (internal rotation). Further studies of preoperative to postoperative changes in acetabular anteversion, AFA, and FFR are needed to substantiate these beliefs. However, we believe that the concepts of FFR and FFA may help improve surgical prescriptions on both the acetabular and femoral sides.
      It has long been recognized that, to achieve optimal alignment and impingement-free range of motion in THA, both the femoral and acetabular component orientations should be considered. As such, CA targets have been proposed [
      • Ranawat C.S.
      • Maynard M.J.
      Modern techniques of cemented total hip arthroplasty.
      ,
      • Widmer K.H.
      • Zurfluh B.
      Compliant positioning of total hip components for optimal range of motion.
      ,
      • Yoshimine F.
      The safe-zones for combined cup and neck anteversions that fulfill the essential range of motion and their optimum combination in total hip replacements.
      ,
      • Dorr L.D.
      • Malik A.
      • Dastane M.
      • Wan Z.
      Combined anteversion technique for total hip arthroplasty.
      ]. Despite their utility, these zones rarely incorporate pelvic tilt and its impact on functional cup anteversion and, to the authors knowledge, have not been previously formulated to consider FFR, referencing only AFA. However, considering femoral anteversion only in relation to anatomic landmarks, as AFA does, could be seen as analogous to only considering the cup orientation in a supine AP radiograph, as it does not provide an understanding of the functional position of the prosthesis. It is now understood that both the pelvis and femur can rotate substantially between functional positions, altering the orientation of the components. Given the degree of variation observed in our study, CA targets may need to consider FFA instead of AFA, as we have noted that an additional 13% of patients would fall outside a widely cited CA zone when considering the functional alignment of the femur. Therefore, like the adoption of preoperative functional pelvic radiography to understand the patient’s pelvic mobility [
      • Vigdorchik J.M.
      • Sharma A.K.
      • Buckland A.J.
      • Elbuluk A.M.
      • Eftekhary N.
      • Mayman D.J.
      • et al.
      2021 Otto Aufranc Award: a simple Hip-Spine Classification for total hip arthroplasty: validation and a large multicentre series.
      ,
      • Feng J.E.
      • Anoushiravani A.A.
      • Eftekhary N.
      • Wiznia D.
      • Schwarzkopf R.
      • Vigdorchik J.M.
      Techniques for optimizing acetabular component positioning in total hip arthroplasty.
      ], functional knee imaging should be considered to understand if a patient’s functional CA is significantly different to their anatomic CA.
      Our study has several limitations. First, we did not incorporate pelvic tilt values, and as such, the patients defined as being “at risk” due to their FFR from supine to standing may have had a favorable pelvic tilt change that would not place them at risk of impingement. However, this was not seen as consequential as the aims of the paper were to define FFA, demonstrate its high variability across a large sample of patients requiring THA, and discuss its implications for femoral stem anteversion targets in THA. Second, we did not stratify or exclude patients based on the pathology. This meant our sample of patients included patients with primary and secondary osteoarthritis, where secondary osteoarthritis may have been associated with dysplasia, slipped capital femoral epiphysis, Perthes-Legg-Calves disease, or other conditions that can dramatically increase a patient’s AFA. However, this improves the generalizability of the study as the purpose of the paper was to define the full range of FFA of all patients requiring THA, not specifically for patients with primary osteoarthritis. Third, it should be emphasized that our study contains only preoperative imaging data of patients with pathological hips. It has been shown that pelvic tilt changes from preoperative to postoperative states [
      • Ishida T.
      • Inaba Y.
      • Kobayashi N.
      • Iwamoto N.
      • Yukizawa Y.
      • Choe H.
      • et al.
      Changes in pelvic tilt following total hip arthroplasty.
      ], with greater changes occurring in patients with a more anteriorly rotated pelvis. These larger changes are likely due to preoperative hip contractures resolving, which may be a mechanism to reduce pain that is secondary to the patient’s arthritis. Therefore, similar phenomena may occur on the femoral side whereby patients excessively rotate their femurs externally or internally to reduce pain, and therefore, their postoperative FFR may naturally differ from their preoperative FFR. Further research into this is needed to understand which patients may undergo natural changes to their FFR from preoperative to postoperative states, and our results should be interpreted accordingly. Fourth, there is a degree of inapplicability to surgeons and patients who do not have access to the imaging requirements for the preoperative analysis discussed in our study. However, EOS (EOS Imaging, Paris, France) scans have been shown to accurately determine FFR [
      • Folinais D.
      • Thelen P.
      • Delin C.
      • Radier C.
      • Catonne Y.
      • Lazennec J.Y.
      Measuring femoral and rotational alignment: EOS system versus computed tomography.
      ] and present another option of measuring this parameter preoperatively, increasing its accessibility. Further research could also investigate how well FFR can be assessed from preoperative anteroposterior radiographs by using the lesser trochanters as a reference. Finally, our study did not address the extent to which AFA can be changed intraoperatively to achieve a desirable ASA and FSA. Previous studies have shown that a patient’s unique femoral morphology dictates the achievable AFA when using metaphyseal-filling, press-fit components; however, this is more controllable in cemented or modular stems [
      • Bargar W.L.
      • Jamali A.A.
      • Nejad A.H.
      Femoral anteversion in THA and its lack of correlation with native acetabular anteversion.
      ,
      • Khanuja H.S.
      • Vakil J.J.
      • Goddard M.S.
      • Mont M.A.
      Cementless femoral fixation in total hip arthroplasty.
      ]. Therefore, when using an uncemented femoral component, it may not be feasible to alter FFA significantly. However, preoperative knowledge of a patient’s unique femoral morphology using 3D templating can provide surgeons with the knowledge of the achievable femoral anteversion with different implant types.

      Conclusions

      In sum, functional alignment of the femur is understudied. We have demonstrated that the femur, like the pelvis, can rotate substantially between supine and standing positions, altering the functional orientation of the femur. These changes in FFR between positions may escalate the risk of prosthetic or bony impingement or may have downstream consequences on soft-tissue tensioning when dramatic alterations to femoral or acetabular anteversion are made intraoperatively. Therefore, like the hip-spine relationship, we believe the pelvis-femur relationship plays a significant role in patient outcomes, and further research into FFA/FFR may improve both acetabular and femoral component positioning.
      Figure thumbnail gr3
      Figure 3Histogram of the anatomic femoral anteversion results.

      Acknowledgment

      The authors would like to acknowledge 360 Med Care for their support to this research.

      Conflicts of interest

      Dr. Brad Miles receives royalties from Corin; is a paid consultant for 360 Med Care; and has stock or stock options in 360 Med Care. Dr. J. Balakumar receives royalties from Medacta; is in speakers' bureau of or gave paid presentations for Medacta, Arthrex, and J&J; and is a paid consultant for Medacta and Stryker. Dr. J. Twiggs and M. Hardwick-Morris are paid employees of 360 Med Care. Dr. K. Kacker declares no potential conflicts of interest.
      For full disclosure statements refer to https://doi.org/10.1016/j.artd.2022.09.002.

      Appendix A. Supplementary data

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