Sex-based differences in neck selectivity in total hip arthroplasty using a modular femoral neck system

Background: Dislocation is a major complication of total hip arthroplasty (THA). The modular femoral neck system provides practical advantages by allowing adjustment of neck version and length in the presence of intraoperative instability. Anatomical studies have identified morphological differences in the hip joint between men and women. Despite sex-based differences in hip morphology, it remains unclear whether such differences affect neck selectivity in THA using a modular neck system and whether this approach achieves anatomical reconstruction, thereby reducing complications such as dislocation. This study aimed to investigate gender differences in neck selectivity in THA with the modular neck system and assess the clinical impact of the modular neck system. Methods: A total of 163 THAs using a modular neck system were included in this study. Data on the type of modular neck and intraoperative range of motion (ROM) were retrieved from patient records. Pre- and post-operative leg length differences (LLD) were examined as part of the radiographic assessment. Dislocation was investigated as a postoperative complication. Results: Neck selectivity did not significantly differ between men and women. The comparison of pre- and post-operative LLD revealed a tendency for varus necks to improve LLD more than version-controlled necks. Furthermore, no significant correlation was found between intraoperative ROM and neck selectivity, or postoperative dislocation and neck selectivity. Conclusions: This study on THA with a modular neck system provided valuable insights into sex-based differences in neck selectivity and highlighted the potential benefits of the modular neck system in addressing LLD and preventing postoperative dislocation.


Introduction
4][5] To reduce the incidence of dislocation, achieving optimal orientation of the implant placement should be prioritised. 6,7Intraoperative assessment of dislocation is a simple method for predicting instability after THA. 8 The modular femoral neck system has some practical advantages, allowing for the adjust-Anatomical studies have identified morphological differences in the hip joint between men and women.Women tend to have a shorter femoral neck and greater anteversion of the femoral neck. 13,14hile sex-based differences in hip morphology are recognised, it remains unclear whether these differences influence the choice of femoral neck for anatomical reconstruction with THA using a modular neck system.
Additionally, it is uncertain whether the modular neck system has clinical implications, namely, whether anatomical reconstruction is achieved and whether complications such as dislocation are reduced.
Therefore, we conducted this retrospective study to answer the following clinical questions.
(1) Is there sex-based difference in neck selectivity in THA with a modular neck system?
(2) Is there relationship between neck selectivity and hip function, complication and leg length differences (LLD)?
We hypothesised that there are sex differences in neck selectivity in THA with the modular neck system and that the modular neck system facilitates the acquisition of anatomical reconstruction and reduces postoperative dislocation.The results of this study will provide insights into whether the modular neck system can provide gender-specific care.This, in turn, may contribute to the pursuit of optimal treatment for individual patients, improving treatment efficacy and minimising complications.

Patient characteristics
Between July 2007 and March 2013, 185 THAs were performed in 168 patients at Teikyo University Chiba Medical Center.Among these 185 THAs, 163 procedures were conducted in 149 patients using a modular femoral neck system.The adoption of the modular neck system was based on a case-by-case decision by surgeons.However, it was not a deliberate or intentional choice.Data on the type of modular neck were retrieved from patient records.The following data were also retrieved from the patient records: patient demographics, such as age, height, weight, body mass index, side of operation, American Society of Anesthesiologists Physical Status (ASA-PS), diseases and surgical information, such as surgical duration, haemorrhage, surgical approach, femoral stem, bearing, acetabular cup, femoral head and intraoperative range of motion (ROM).Pre-and post-operative LLD were examined as radiographic assessment.Dislocation within the first year after surgery was investigated as a postoperative complication.Patients who underwent THA with a modular neck system between July 2007 and March 2013 were included, while patients with incomplete data were excluded from this study.

Surgical procedure
All surgeries were performed under general anaesthesia in a clean-air operating room by or under the supervision of the senior author (K.K.).A modified anterolateral approach by Dall 15 or a direct anterior approach 16 was used.The acetabular cup was fixed using a press-fit technique, and additional screw fixation was used for all cups except spiked cups.The femoral stems were selected to correspond to the size of the largest rasp.Intraoperative assessment of dislocation was performed for all patients to avoid dislocation, as well as intraoperative radiography to adjust the leg length.After these assessments, the optimal modular femoral neck was determined in accordance with the flowchart (Figure 1).The modular neck system used in the study includes 22 options: straight, varus/valgus, 8° anteverted/ retroverted, 15° anteverted/retroverted, anteverted varus, retroverted varus, anteverted valgus and retroverted valgus, each of which are subclassified into long and short types (Figure 2).

Statistical analysis
Fisher's exact test was employed to analyse categorical variables.For continuous variables, Student's t-test or the Mann-Whitney U test was used for two groups, and the Kruskal-Wallis test was used for multiple groups.
The Wilcoxon signed rank test was used to analyse changes in LLD between pre-and post-operation.All p-values were two-sided, and p-values less than 0.05 were considered statistically significant.All statistical analyses were performed with EZR version 1.52, 17 which is a graphical user interface for R version 4.02 (The R Foundation for Statistical Computing, Vienna, Austria).More precisely, it is a modified version of R commander designed to add statistical functions frequently used in biostatistics.

Backgrounds
Patient demographics are shown in Table 1.The study included 30 men (30 hips) and 119 women (133 hips).The mean height of men was significantly higher than that in women (164.8 ± 8.8 cm vs 150.1 ± 6.2 cm, p< 0.01).The mean weight was significantly greater in men than in women (62.7 ± 14.5 kg vs 54.4 ± 9.2 kg, p < 0.01).The ASA-PS significantly differed between men and women (p = 0.02), and post-hoc analyses revealed that ASA-PS 2 versus 3 was significantly different between men and women (p < 0.01).The disease necessitating surgery differed significantly between men and women (p < 0.01), and post-hoc revealed that osteoarthritis (OA) versus osteonecrosis of the femoral head and OA versus fracture were significantly different between men and women (p < 0.01 and p < 0.01, respectively).

Surgical information
Details of the surgical information are shown in Table 2. Surgical time and bleeding in one patient were not recorded.Six types of uncemented acetabular cups were used: Trilogy (Zimmer Biomet, Warsaw, IN, USA) was used for  122 hips, Conserve Plus (MicroPort Orthopedics, Memphis, TN, USA) for 17 hips, Trabecular (Zimmer Biomet, Warsaw, IN, USA) for 12 hips, Trident (Stryker Orthopaedics, Kalamazoo, MI, USA) for 11 hips and Continuum (Zimmer Biomet, Warsaw, IN, USA) was used for one hip.The use of acetabular cups differed significantly between men and women (p < 0.01), and post-hoc tests showed that Trilogy versus Conserve (spiked) was significantly different between men and women (p < 0.01).All cups were selected based on appropriate sizing.Metals on polyethylene bearing were used for 146 hips and metal on metal bearing for 17 hips, with a significant difference between men and women (p < 0.01).Two uncemented femoral stems were used: Profemur Z (MicroPort Orthopedics, Memphis, TN, USA) and Profemur TL (MicroPort Orthopedics, Memphis, TN, USA).The use of femoral stems did not significantly differ between men and women (p = 0.59).All stems were selected based on appropriate sizing.

Neck selectivity
The details of neck selectivity are shown in Figures 3  and 4.  Neck selectivity did not differ significantly between men and women (Table 3).In addition, neck selectivity did not differ significantly between men and women when analysed by disease (Table 4).Relationship between neck selectivity and radiographic analysis was shown in Table 5.The preoperative LLD was significantly greater in the long neck group than in the short neck group (p = 0.03).Comparison of pre-and post-operative LLD revealed a tendency for varus necks to improve LLD more than versioncontrolled necks.Additionally, no significant correlation was found between intraoperative ROM and neck selectivity or between the incidence of postoperative dislocation and neck selectivity (Table 6).

Discussion
Contrary to our hypothesis, this retrospective study on THAs with the modular neck system found no significant sex-based differences in neck selectivity.The results also revealed a tendency for varus necks to improve LLD, and version-controlled necks demonstrated potential effectiveness in preventing postoperative dislocation.Previous studies have included multiple surgeons or multiple methodologies, making it difficult to make direct comparisons.However, to our knowledge, this study was the first to report the results of THA performed by a single surgeon using a single method and concept.There was no correlation between intraoperative ROM and neck selectivity, nor between incidence of postoperative dislocation and neck selectivity.
However, there was no postoperative dislocation in the group used version-controlled neck.According to the flowchart in Figure 1, version-control necks are used when instability is observed during intraoperative movements.Despite the instability, the absence of dislocation rates suggested that the version-controlled neck increased dislocation resistance.These results demonstrated that the modular neck system, especially version-controlled neck may play a role in avoiding dislocation after THA.1][22] However, dislocation is multifactorial and cannot be completely prevented even when optimal implant positions are obtained.Furthermore, in cases of strong bone deformity, malalignment of the cup and femoral stem must be allowed to obtain a firm fixation, which may result in a deviation from the optimal position to prevent dislocation.For these reasons, modular neck system THA is effective in preventing dislocation because the femoral neck can be selected to prevent dislocation regardless of the position of the acetabular cup and femoral stem.
This study had several limitations.First, it was a retrospective study, which might have resulted in various biases.Second, the sample size was small, especially for men.Further studies are needed to clarify this issue.

Conclusions
This retrospective study provided valuable insights into sex-based differences in neck selectivity and highlighted the potential benefits of modular neck system in addressing LLD and preventing postoperative dislocation.

Figure 3 .
Figure 3. Distribution of modular femoral necks in men and women.

Table 4 .
Neck selectivity by analysis of each disease.

Table 5 .
Relationship between neck selectivity and radiographic analysis.