1 Introduction

Coronal malalignment of the femoral stem in THA has been associated with the risk of subsidence.¹ The biomechanical implications of altered strain distribution, contact stress at the cortex with the stem tip and abnormal femoral loading have all been described in previous studies.² Clinical and functional implications are less significant in current reported literature.³

Many attempts have been made to understand the predictor variables that significantly contribute to this outcome.^4,5 We describe the predictors of varus stem malalignment including a novel radiographic measurement incorporating the greater trochanteric (GT) height into a simple ratio (Femoral Access Ratio (FAR)) that is unaffected by the degree of femoral rotation during radiographic measurement. We assess patients at a minimum of 10 years with respect to their revision rates and functional outcome scores.

2 Materials and methods

2.2

2.2 Data collection and analysis

Institutional Review Board approval was received prior to commencement of this study. All data was collected from our institutional arthroplasty registry which was established in 2005. Parameters collected included all demographic variables, details of the operative procedure, specifications of implants inserted and radiographic measurements in both the preoperative and immediate postoperative setting.

Radiographic measurements were calculated by two authors. Standardisation of pelvic radiographs for analysis was ensured by centring the image on the pubic symphysis. Radiographers were instructed to position patients with 15° of hip internal rotation to demonstrate the true femoral neck length and offset on plain film. Neutral flexion of 0° was also required for accurate radiographic images. This technique allowed for standardisation between patients. All radiographs were taken in the supine position.

Primary outcome measures included the coronal alignment of the femoral stem relative to the proximal diaphysis of the femur defined as the Stem-Proximal Diaphysis Angle coronal (SPDAc) (Fig. 1). Clinical outcome measures included all-cause revision rates at a minimum of 10 years and functional outcome measures at a minimum of 10 years using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score. WOMAC was originally developed in 1982 to specifically evaluate the physical function, stiffness and pain of hip and knee joints in patients with osteoarthritis.⁶

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Fig. 1 Stem-Proximal Diaphysis Angle coronal (SPDAc).

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Predictor variables of note included the novel Femoral Access Ratio (FAR) score. This ratio considers the greater trochanteric height and how this impacts on the surgeons ability to insert a femoral stem in perfect collinearity with the anatomical axis of the femoral diaphysis. As illustrated in Fig. 2, the FAR score consists of 2 measurements on plain radiograph of the proximal femur. Line B is a horizontal line beginning at the proximal aspect of the lesser trochanter (LT) and spanning the width of the proximal femur from outer cortex to outer cortex. Line A is a perpendicular line to line B that extends from the most superior tip of the greater trochanter (GT) down to line B. The FAR score is a ratio that is then calculated by dividing line A by line B. This avoids the need for absolute measurements and allows easy comparison between observers regardless of differing magnifications.

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Fig. 2 Femoral access ratio (FAR) (A/B).

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The interobserver variability of the FAR score was calculated using Cohen's kappa co-efficient (κ). This coefficient was described by Cohen in 1960 and has been utilised since then to measure agreement levels between observers for a large range of scoring systems.⁷ The below equation demonstrates how Kappa was evaluated, where Pο is the observed agreement among raters and Pε is the probability of agreement by chance. When κ = 1, there is complete agreement between the observers:κ=Pο−Pε1−Pε

In this case, the kappa coefficient was 0.91 demonstrating an almost perfect agreement between observers. Other predictor variables measured included gender, age, BMI, operating surgeon, surgical approach, GT height on radiograph and the prosthesis used.

3 Results

3.2

3.2 Varus Predictors

The approach used was not predictive of coronal malalignment (p = 0.27) although the posterior approach did have a trend toward less varus stem malalignment than the modified Hardinge and the direct lateral approach. On multivariate analysis, it was found that 3 predictors were associated with stem varus malalignment: 1- increasing BMI (p = 0.048), 2- FAR score of less than 1 (p = 0.009) and 3- decreasing GT height (p = 0.013).

3.2.1

3.2.1 BMI

We found that the percentage of stems with varus malalignment increased with an increasing BMI. With a BMI of less than 25, 45.5% of stems were in varus. For a BMI between 25 and 29.9, the rate of varus stem malalignment was 57.14%. With a BMI over 30, 76.9% of stems were deemed to be in varus (Fig. 3). This relationship remained significant even after controlling for confounding predictor variables on multivariate analysis. When considering only varus stems, there was a correlation on regression analysis between increasing BMI and increasing SPDAc as depicted in Fig. 4.

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Fig. 3 Percentage of stems in varus according to BMI grouping.

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Fig. 4 BMI correlation with varus stem angulation.

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3.2.2

3.2.2 FAR score

The FAR score ranged from 0.21 to 1.27 (mean 0.96, σ = 0.16). Taking a FAR score of 1 was found to be a useful threshold for determining the likelihood of varus stem malpositioning in this cohort. Patients with a FAR score of 1 or greater had a 36.6% chance of varus stem malalignment whereas a FAR score of less than 1, significantly increased the likelihood of varus malalignment to 68.4% (p = 0.009).

3.2.3

3.2.3 GT height

Perhaps the most significant component of the FAR score is the GT height. The absolute measurement of the GT height (Line A of the Femoral Access Ratio) ranged from 2.81 to 7.06 cm. A mean GT height of 5.36 cm was noted (σ = 0.66) for the entire cohort. The femoral diaphyseal width was not found to be significantly associated with coronal malposition of the stem (p = 0.32).

4 Discussion

The impact of coronal malalignment of the femoral stem in THA is uncertain. Biomechanical studies have suggested that uncemented femoral stems in varus lead to higher load transmission and an altered strain distribution in the femur when loaded.² This same study concludes that stem stability was not compromised due to this malalignment however. More recent work by Reina et al. assessed stem survivorship and functional patient outcomes at 8 years for malaligned stems (>3°from neutral axis) and neutral stems.³ This study analysed 978 neutral stems and 50 malaligned stems. Survivorship exceeded 97% in both groups at 8 years and clinical outcomes measured using the Harris Hip Score were comparable at the same time point. Some studies suggest that for uncemented stems, a varus position may be expected and not necessarily a technical failure.⁸

Historically however, cemented femoral components seem to be more intolerant of coronal malalignment.⁹ This association is largely felt to be attributable to a deficient cement mantle at certain locations around a malaligned stem. Barrack et al. have demonstrated the importance of achieving an optimal homogenous cement mantle around the femoral stem.¹⁰ Cementation technique is essential in achieving optimal outcomes also.¹¹ This in turn introduces an operator-dependent predictor variable that is not encountered with the insertion of uncemented femoral stems.

We found that there were 3 variables predictive of varus malalignment after multivariate analysis: BMI, GT height and FAR score. Numerous morphological bony features have been reported to impact on the femoral stem alignment to date. Wang et al. describe the effect of the femoral lateroversion. They found that GT overhang beyond the femoral centreline significantly increases the risk of coronal malalignment.⁴ A concern with using a reference point that varies in the horizontal plane is that femoral rotation will alter the measurement. This may of course be controlled with standardising the femoral version during radiographic imaging, but it is still subject to inaccuracy. By using the trochanteric height as a reference point, one can avoid the issue of femoral rotation as this remains constant regardless of the femoral version during radiographs. This may of course be altered by flexing the femur, however, we maintain that it is easier for a radiographer to ensure that a hip is in 0° flexion than it is to ensure that a hip is in a certain degree of rotation (e.g. 15°anteversion). In addition, the use of ratio measurements in place of absolute measurements is desirable as it negates the need to incorporate radiographic magnification into measurements taken. We see the effective use of this mathematical characteristic in other areas of hip arthroplasty research.¹² For this reason, we developed the FAR score as it is easy to use, has an almost perfect interobserver reliability and is independent of the hips rotational position during radiographic imaging. Murphy et al. have documented the association between coxa vara and an increased trochanteric height.⁸ Coxa vara was then predictive of varus stem malalignment in that study. In this series, by comparison, we note that an increasing trochanteric height is associated with a lower risk of varus stem position. We postulate that a higher GT will allow the operating surgeon to easily manoeuvre the stem shoulder under the GT thereby reducing the risk of a varus stem position.

The negative impact of BMI on THA outcomes has been extensively documented in relation to infection, instability, increased operative times and a host of other complications.^13–15 Regarding the impact of BMI on the incidence of stem malpositioning, little evidence is available. In 2013, Elson et al. reported the incidence of femoral malposition in 211 morbidly obese patients and compared the findings with a control group of 211 normal patients.¹⁶ We corroborate these findings with an increased incidence of varus malalignment with an increasing patient BMI. With a BMI over 30, there was a 77% chance of varus malalignment versus 45% with a BMI of less than 25. Regression analysis also confirmed a linear increase in varus angulation for an increasing BMI value. This relationship would support the findings proposed by Elson et al. and gives cause for renewed vigilance in surgeons selecting appropriate candidates for hip arthroplasty.

The outcomes of malaligned stems have been described in a number of settings. Kutzner et al. found that valgus stems were more problematic than varus stems regarding subsidence but clinical outcomes were unaffected. Reina et al. concluded based on their large uncemented cohort that slight malalignment does not negatively impact survivorship or clinical outcomes.¹⁷ We report that varus stems do not have an increased revision rate or reduced functional outcome at a minimum of 10 years. To our knowledge, this is the longest reported follow-up period regarding outcomes of malaligned femoral stems. It appears that a varus femoral stem may not impact significantly on long-term outcomes as once thought. There is a concern however regarding poor cement mantles with varus stems and with an altered load distribution that may lead to thigh pain and other potential complications.² For these reasons we recommend that surgeons still aim for a collinear stem and diaphysis. A number of techniques have been described to increase this outcome: Trochantoplasty as described by Yoo et al. is a technique that may improve positioning,¹⁸ attention to the neck resection level and angle may also improve position⁵ as may consideration of the trochanteric height and FAR score as described in this study.

5 Conclusion

Varus stem malalignment can be predicted by reduced GT height and FAR score. Specifically a FAR score of less than 1 leads to a significantly increased incidence of varus stem malalignment. The severity of varus malalignment also increases in a linear fashion with an increasing BMI. Long term clinical and functional outcomes at a minimum of 10 years in this cohort are seemingly unaffected by coronal malalignment.

The Femoral Access Ratio score is a useful simple preoperative radiographic measurement that can predict the likelihood of femoral stem varus malalignment in cemented femoral stems. We recommend a particular awareness when cementing femoral stems into a femur with a FAR score of less than 1. Surgeons should also be aware of an increasing BMI and how it contributes to increasing varus stem malalignment in these patients.

Submission declaration

We, the authors, declare that this work is not under consideration for publication elsewhere, that its publication is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out, and that, if accepted, it will not be published elsewhere in the same form, in English or in any other language, including electronically without the written consent of the copyright-holder.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Ethics declaration

Institutional ethical approval was sought and granted for completion of this study. We, the authors, declare that the work described has been carried out in accordance with The Code of Ethics of the World Medical Association (Declaration of Helsinki) for experiments involving humans. The manuscript is in line with the Recommendations for the Conduct, Reporting, Editing and Publication of Scholarly Work in Medical Journals and aims for the inclusion of representative human populations (sex, age and ethnicity) as per those recommendations.

CRediT authorship contribution statement

G. Sheridan: Conceptualization, Formal analysis, Writing - original draft. H. Hughes: Data curation, Writing - review & editing. A. Welch-Phillips: Visualization, Investigation. P. Kenny: Writing - review & editing. G. O'Toole: Supervision, Validation. J. O'Byrne: Supervision, Validation, Writing - review & editing.