Femoral antetorsion after calcar‐guided short‐stem total hip arthroplasty: A cadaver study

Abstract Calcar‐guided short stems in total hip arthroplasty (THA) permit surgeons to successfully reconstruct postoperative femoroacetabular offset, accurately restore leg length, and adequately re‐establish a wide range of caput‐collum‐diaphyseal angles. However, their effect on femoral antetorsion is less known. Indeed, controlling antetorsion of the femoral stem can be challenging because of the differences in individual femoral geometry and curvature. Therefore, we investigated if calcar‐guided short‐stem THA alters femoral antetorsion and compared it with conventional‐stem THA. Using 12 Thiel‐fixed, full‐body cadaver specimens from donors without known hip disorders, we compared an uncemented calcar‐guided femoral short‐stem prosthesis with an uncemented conventional straight‐stem prosthesis. In a paired study setup, each specimen received a calcar‐guided short stem on one side and a conventional stem on the other. On the acetabular side, all specimens received a press‐fit, monobloc acetabular cup. Femoral antetorsion angles were measured using the Waidelich method, and pre‐ and post‐operative angles of both sides were recorded. The mean preoperative femoral antetorsion angles were similar in both groups (24.8°  ± 7.5° vs. 23.8° ± 6.1°, p = 0.313). Mean postoperative femoral antetorsion angles were 23.0° ± 5.5° in short‐stem and 13.5° ± 7.1° in conventional‐stem hips. Short‐stem hips had a small but nonsignificant difference in femoral antetorsion angles pre‐ and post‐operatively (1.8° ± 3.2°, p = 0.109), while the difference for conventional‐stem hips was much larger and highly significant (10.3° ± 5.8°, p < 0.001). Calcar‐guided short‐stem THA effectively restores femoral antetorsion. However, how this affects long‐term clinical outcomes and complications warrants further exploration.


| INTRODUCTION
The goal of total hip arthroplasty (THA) is to restore normal hip anatomy and biomechanics. To achieve this objective, crucial factors affecting the patient's anatomy, including leg length, caput-collumdiaphyseal (CCD) angle, femoroacetabular offset, and femoral antetorsion, must be considered. [1][2][3][4] To date, several hip prosthesis designs have been developed to accurately reconstruct the hip joint. Among them are calcar-guided short stems, which were designed to optimally adapt to the proximal femur's anatomy and restore hip biomechanics. Their curved design renders individual stem positioning possible in a wide range of varus and valgus alignments, 5 a conclusion confirmed by a clinical study in which calcar-guided short-stem prostheses allowed for accurate hip joint reconstruction in the studied patient population. 6 In addition, calcar-guided short stems permit surgeons to successfully reconstruct postoperative femoroacetabular offset, accurately restore leg length, and adequately re-establish a wide range of CCD angles in most patients, contributing to the restoration of anatomical hip geometry and favorable midterm clinical outcomes. 1,[5][6][7][8][9][10][11] In addition to the abovementioned parameters referring to the anteroposterior view, antetorsion of the femoral component also plays an important role in proper hip stability. 2,4,12,13 Both excessive antetorsion and retrotorsion can lead to impingement and instability. 13 Correct component torsion is therefore necessary to achieve an impingement-free range of motion and prevent wellknown complications of THA such as instability, dislocation, and component wear. 2,13,14 Conventional stems have led to substantial reduction in femoral antetorsion angles after THA compared with preoperative values. 4 However, to our knowledge, the long-term clinical significance of this has not been studied. Although calcar-guided short stems allow good anatomical reduction in the anteroposterior view, their effect on femoral antetorsion is less known.
For this reason, we carried out an in vitro study to examine (1) whether calcar-guided short-stem THA alters postoperative femoral antetorsion and (2) how calcar-guided short-stem THA compares with conventional-stem THA. We hypothesized that the femoral antetorsion angle would not change significantly from preoperative values after calcar-guided short-stem THA.

| Selection and preparation of cadaver specimens
The study included 12 Thiel-fixed full-body cadaver specimens from donors without known hip disorders, such as dysplasia, fracture sequelae, and proximal femoral bone defects, used according to institutional guidelines. The cadavers were prepared according to the standard protocol described by Thiel, 15 using modified Thiel solutions. 16

| Surgical approach and implants
An anterolateral approach was used in the supine position without a traction table in all cadaver specimens. In all procedures, the hip was externally rotated. With 90°of knee flexion, the ipsilateral lower leg thus ended up in the horizontal plane, parallel to the operation table.
Both conventional and short stems were implanted according to the accompanying surgical techniques. In conventional stems, the implants were inserted with an antetorsion perpendicular to the lower leg, and the femoral neck did not guide the implant during insertion. In short stems, the implants were inserted along the partially preserved femoral neck (so-called "calcar-guided" implant insertion), which determined the final position of the implant.
Three senior orthopedic surgeons (J.H., C.A., and R.O.) from the same clinic operated on four cadaver specimens each (left and right side). The three surgeons had operated on over 100 cases using both implants included in this study, and therefore, no learning curve was associated with calcar-guided short-stem implantation. 17 On the femoral side, an uncemented calcar-guided femoral shortstem prosthesis (optimys stem; Mathys Ltd Bettlach) was implanted and compared with an uncemented conventional straight-stem prosthesis similar to the Zweimüller design (CBH stem; Mathys Ltd Bettlach). Instead of actual implants, three-dimensionally printed stems made of polylactide and 28-mm trial heads made of polyphenylsulfone 18 were used to minimize image artifacts. However, the implant bed was prepared using standard instruments. On the acetabular side, a press-fit, monobloc acetabular cup (RM Pressfit; Mathys Ltd Bettlach) was implanted in all cases.
All cadaver specimens were randomly assigned to the surgeons (four specimens each). In a paired study setup, calcar-guided shortstem implants were implanted on one side and conventional-stem implants on the other based on random side allocation, making sure that both groups had the same number of left and right implants.

| Image acquisition and measurement of femoral antetorsion angles
All pre-and post-operative measurements were recorded with a computed tomography (CT) scanner (Somatom Emotion 6, Siemens) using the corresponding three-dimensional imaging software (Syngo Via, Siemens). All scans were performed helically with a vertical gantry using a layer thickness of 1.25 mm. We centered the specimens with feet parallel in a supine position and scanned them from the iliac crest to the knee joint. All images were reconstructed with an overlapping reconstruction technique to obtain images with a slice thickness of 0.8 mm.
We used a hard kernel (UH90) and a bone window setting to obtain high-quality images for radiographic evaluation.

| Statistical analysis
We used descriptive statistics (means, SDs, and 95% confidence intervals) to describe specimen characteristics and outcome variables at the measurement point. Paired t tests were used to determine pre-and post-operative differences in each specimen, and the Shapiro-Wilk test was applied to test for normality. The sample size was determined based on a paired scenario assuming a within-patient correlation of 0.85, yielding power of 80%. Statistical analysis was performed with SAS version 9.4 (SAS Institute Inc). A value of p < 0.05 (two-sided) was considered statistically significant.

| DISCUSSION
In this paired in vitro study, we examined whether calcar-guided short- Improper component torsion of the hip is known to limit the range of motion and increase the risk of complications, such as impingement, instability, dislocation, and component wear. 13,14,26 Short femoral stems may reduce the risk of these complications due to their ability to accurately reconstruct the hip joint and restore hip biomechanics. In fact, the Australian National Joint Registry reported a lower cumulative incidence of dislocation with short stems than with conventional stems over a 15-year period. 27 We suspect that the lower dislocation rates with short-stem THA could be attributed to the good restoration of femoral antetorsion we saw in our study.
However, a possible correlation between femoral antetorsion and dislocation rates, along with other complications and clinical outcomes, requires clinical investigation.
We acknowledge some limitations of our study. First, we used a limited number of samples. That said, 12 cadavers were found to be sufficient to reach statistical significance, according to our sample size estimation. Second, we used Thiel-fixed instead of fresh-frozen cadavers. However, Thiel-fixed cadavers have a texture similar to fresh-frozen cadavers and offer several advantages, including slower tissue deterioration and the absence of disease transmission. 28 Thielfixed cadavers are therefore considered suitable for use in orthopedic applications. 28 Third, because femoral antetorsion can largely depend on the surgical technique, results may vary across clinics. Finally, we used three-dimensionally printed short stems and trial heads to minimize imaging artifacts, and therefore, our results may be applicable only to the stems evaluated. However, we did use the same procedures, approach, and instrumentation as in a standard in vivo THA, making the procedure comparable to a real-life clinical scenario.
Although polyphenylsulfone stems are smoother than metallic stems due to their nonporous surface, a good press-fit can be achieved when using the right size. In this study, the appropriate size of the implants was chosen preoperatively.
Nevertheless, our study has several strengths. In particular, all surgeons involved in this study practice in the same clinic and therefore use the same surgical technique, limiting any technical differences.
Additionally, we followed the Waidelich method for femoral antetorsion measurement using CT images in transverse planes. Due to the fixed anatomical landmarks, available both pre-and post-operatively, this method is known to have high accuracy and reproducibility, as well as low intra-and inter-observer variability. 20 In conclusion, our study demonstrated that calcar-guided shortstem THA effectively restored femoral antetorsion. However, whether the lower dislocation rates of short-stem implants result from the good restoration of femoral antetorsion warrants further exploration.

ACKNOWLEDGMENTS
We thank Dr. Dominik Pfluger at numerics data GmbH for the statistical analysis, Mathys Ltd. for partially funding this study, and Medical Minds GmbH for medical writing and editorial support. The work was partially supported by Mathys Ltd Bettlach. Funds sponsored statistical analysis through an independent consultant as well as medical writing and editorial support from a medical writing agency. No other external sources were involved. Mathys Ltd. Bettlach had no role in the analysis or interpretation of the data, or the decision to submit results.

AUTHOR CONTRIBUTIONS
All authors contributed equally to data collection, analysis, and writeup. All authors have read and approved the final submitted manuscript.