1 Introduction

Templating before total hip arthroplasty (THA) is essential for optimal pre-operative planning. Acetabular and femoral component size and position can be estimated for intraoperative execution and assessment. The vertical and horizontal offset required to restore the hip centre of rotation (COR) were measured to determine which reconstructive options needed to be available to make intra-operative adjustments. Restoration of the COR has been demonstrated as a key factor in achieving good long-term functional outcomes.^1–8

The neck shaft angle and the native horizontal offset could be varied, and information regarding the predominant variants is not known.

We hypothesized that there was a variation in the native horizontal offset and the neck shaft angle in patients who underwent unilateral THA in our unit over the past 3 years. We also hypothesized that the pre-op templating, when corroborated with intraoperative assessment, achieves optimal offset restoration.

2 Materials and Methods

After institutional review board approval (IRB Min. No. 2409009 dated 04.09.2024), all patients undergoing a unilateral THA for osteoarthritis between 2022 and 2024 were included in this study. Consent was taken from all patients. A total of 150 patients (103 males) had a mean age of 43.5 years (range 14–73). All patients with unilateral hip disease above the age of 18 years were included. Patients with bilateral hip disease and Crowe 4 dysplasia were excluded. The neck shaft angle and the horizontal offset were measured on standard anteroposterior (AP) X-rays of the pelvis and recorded during THA templating.

Templating was performed on the AP Pelvis for all patients with a 3 cm metal ball strapped to the hip.⁹ This reproducible technique described has been the standard procedure for preoperative templating in our unit. Templating and measurements were calculated using the PACS (Picture archiving and communication systems, GE, USA). Standard templates provided were used to assess the actual acetabular size and the acetabular COR. Femoral templating estimated the size and horizontal offset that would be appropriate to restore the native offset that was measured on the contralateral hip. The normal hip was templated first to assess the acetabular and femoral size, and particular attention was paid to the horizontal offset that would be required for the affected hip undergoing a THA.

All THAs were performed through a standard modified Hardinge approach. Adequate surgical exposure allowed for the correct placement of the acetabular component in the correct orientation. The transverse acetabular ligament was used to ensure proper restoration of the hip COR.

The femur was prepared in standard fashion with a broach-only system [Depuy Corail (Johnson and Johnson, USA)], and sizing was deemed appropriate once rotational stability of the broach was achieved. Several offset options were available, including short neck, standard offset, and high offset. The offset was trialled at the time of trial reduction, and the intra-operative assessment was compared to the pre-operative templated plan. All patients were allowed immediate weight-bearing with crutch ambulation for 6 weeks.

The Chi-square test was used for the association between categorical variables. The Pearson correlation coefficient was used for the continuous variables. Tests were two-sided at α = 0.05 level of significance. The measurements before and after THA were correlated, and statistical analysis was performed using the Statistical Package for the Social Sciences (SPSS) software, Version 21.0 (Armonk, NY: IBM Corp).

3 Results

The pre-operative neck shaft angle and the horizontal offset were assessed, and the neck shaft angle was classified as normal (120–135°) in 107 hips (71.3 %), coxa vara <120° in 13 hips (8. 7 %), and coxa valga >135° in 30 hips (20 %). The native horizontal offset varied from 2.58 to 7 cm with a mean of 4.50 cm (SD 0.67). The mean pre-operative neck shaft angle was 129.4° (range 103–142°) (SD 6.55). The neck shaft angle variation was from 103 to 142°. The variation in offset and neck shaft angle was useful in planning for offset restoration at THA.

Post-operative offset assessment was categorised as restored, decreased, or increased offset. The post-operative horizontal offset was 3.2–6.11 (Mean 4.54, SD 0.58). Offset was restored ( ± 5 mm) in 107 hips (71.4 %), reduced (< less than 5 mm) in 17hips (11.3 %), and increased (> more than 5 mm) in 26 hips (17.3 %) (Fig. 1). The difference between the pre- and post-operative femoral offset was 0.41(SD 0.56) (Fig. 2). Two authors performed post-operative measurements in a blinded fashion, and the intra-class coefficient was 0.98 (p-value <0.01).

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Fig. 1 Distribution of offset restoration after THA. Offset was restored ( ± 5 mm) in 107 hips (71.4 %), reduced (< less than 5 mm) in 17hips (11.3 %), and increased (> more than 5 mm) in 26 hips (17.3 %).

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Fig. 2 Comparison of preop femoral offset, postop femoral offset, and distribution of femoral offset compared to preop.

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The standard offset femoral component was used in 124 hips (82.7 %) (Fig. 3). Offset was restored in 87 hips (70.2 %), increased in 22 hips (17.7 %), and decreased in 15 hips (12.1 %) with the standard offset femur. The short-neck femoral component used in 20 hips achieved offset restoration in 18 hips (90 %) (Fig. 4). The high-offset femoral component used in 6 hips restored offset in 2 hips (33.3 %) (Fig. 5).

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Fig. 3 A 26-year-old male with left hip arthritis secondary to avascular necrosis. a: Templating on the diseased left hip shows a standard offset requirement b:Pelvis radiograph showing both hips c: Templating on the normal right hip shows a standard offset d. Total hip arthroplasty of left hip with a standard offset stem achieved horizontal offset restoration and an excellent postoperative outcome. [Preop HHS:61.2, Postop HHS:93.5].

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Fig. 4 A 53-year-old male with right chronic hip arthritis secondary to inflammatory etiology, a. Templating on the diseased right hip shows a short offset requirement b. Pelvis radiograph showing both hips,c. Templating on the normal left hip shows a short offset, d. Total hip arthroplasty of right hip with a short offset stem achieved a horizontal offset restoration and an excellent postoperative outcome. [Preop HHS:48.2, Postop HHS:92.5] Neck shaft angle 122. 5°, horizontal offset 5.28 cm, unaffected left hip and postop 5.14 cm right hip.

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Fig. 5 A 48 year old female with left hip arthritis secondary to post septic sequelae. A: Templating on the diseased left hip shows a high offset requirement B:Pelvis radiograph showing both hips C: Templating on the normal right hip shows a high offset stem. D:Total hip arthroplasty of left hip with a high offset stem achieved a horizontal offset restoration and an excellent postoperative outcome. [Preop HHS:48.6, Postop HHS:93.5].

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A total of 89.7 % (96 out of 107) patients with restoration of horizontal offset had excellent Harris hip scores, with the score improving from a mean of 47.4–91.6. One hundred and fourteen patients had excellent HHS (90–100), and 25 had good scores (80–90) at a mean follow-up of 15. 38 months. HHS was fair (70–80) in 9 patients with 5 patients having an increased postoperative offset. 2 patients with poor HHS (60–70) had a reduced postoperative horizontal offset.

4 Discussion

Templating before THA gives optimal information regarding horizontal and vertical offset, which needs to be restored for the restoration of hip biomechanics.^1,2,7,10 Our purpose was to measure the variation in horizontal offset and neck shaft angle at primary THA measured by pre-operative templating and correlated to the post-operative offset restoration and functional outcome. This series shows a considerable variation in the native offset and the neck shaft angle, which needs to be considered in primary THA planning.

The femoral component used for all hips was the 135-degree neck-shaft angle for all offset options. The Depuy Corail stem (Johnson and Johnson, USA) has three neck options – short neck, standard neck, and high offset for optimal offset restoration.

In our series, the offset restoration correlated with the Harris Hip score, which was excellent in 89.7 % (96 out of 107 hips with restored offset). Offset restoration with the 135-degree femoral component was possible, with the variation in the neck-shaft angle observed in our series (Figs. 6 and 7). Information regarding the variation in the neck shaft angle and offset did not necessarily translate to the need for neck shaft angle and offset options at THA. The offset restoration had a significant association with the Harris hip score. Our series demonstrated excellent and good HHS associated with the restoration of the normal offset, and those with decreased offset had a higher proportion of poor and fair outcomes. Five patients (55.6 %) with increased postoperative offset had fair outcome (HHS 70–80). Two patients with reduced offset had poor Harris hip scores. Failure to restore horizontal offset is associated with decreased function.^4,7,11,12

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Fig. 6 Comparison and distribution of the preoperative and postoperative femoral offset.

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Fig. 7 The mean pre-operative neck shaft angle on the normal side was 129° (SD 6.55), while the corresponding mean femoral offset was 4.50 cm (SD 0.67). The scatter plot demonstrated a clear inverse relationship between the two parameters, with increasing neck shaft angle associated with a reduction in femoral offset. Pearson's correlation analysis confirmed this strong negative correlation (r = −0.608, df = 148, p < 0.001), indicating that greater valgus alignment of the proximal femur is consistently linked with a smaller horizontal femoral offset.

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The increase in offset in 22 hips (17.7 %) with a standard offset femoral component, and the restoration of offset in 90 % (18 out of 20) with the short neck implant, implies the prevalence of reduced native horizontal femoral offset in this series of patients. This offset variation needs to be considered when planning for THA.

Limitations of the study include the fact that although the standard X-rays were used, CT scans could provide an alternative assessment of the version.¹³ Correlation of the preoperative femoral offset of the unaffected hip to the postoperative offset was the goal of this study. Hip centre restoration was achieved, and the global offset was not considered in this series, as the aim was to study the horizontal offset variation. The sample size included in this series is not large, and the measurements were done on standard pelvis AP x-rays.⁴ The mean follow-up was 15.4 months; longer follow-up could provide an ideal outcome assessment.

5 Conclusion

The restoration of offset is essential for hip centre restoration and a good functional outcome. This series demonstrates the variation in offset and neck shaft angle that needs to be considered at THA. Offset restoration in our series was achieved with a single neck shaft angle femur component (135°) with multiple offset options.

Declaration of patient consent form

Appropriate consent was taken after IRB approval.

Ethical approval and patient consent

IRB Min. No. 2409009 dated 04.09.2024.

Credit author statement

Alen John Alex contributed to methodology, software, validation, formal analysis, investigation, resources, and data curation.

Augustine Jaison Paul contributed to methodology, software, validation, investigation, resources, and data curation.

Jozy Timothy contributed to software, validation, investigation, resources, data curation,

Gokulprasath contributed to software, validation, formal analysis, investigation, resources, and data curation.

Pradeep M Poonnoose contributed to methodology, software, validation, formal analysis, investigation, and resources.

Anil Thomas Oommen contributed to Conceptualization, methodology, software, validation, formal analysis, investigation, resources, data curation, writing – original draft, review, editing, visualization, supervision, and project administration.

Funding

No funding or financial support was received.