The 'critical trochanter angle' does not show superiority over the CCD angle in predicting varus stem alignment in cementless short-stem total hip arthroplasty.

PURPOSE: Varus positioning of the femoral stem can affect the hip offset (HO). The critical trochanter angle (CTA) was introduced in 2019 as a novel geometric angle, to predict varus stem alignment in cementless straight stem THA. The aim of this study was to evaluate the applicability of the CTA as a predictor for stem alignment in THA with a cementless neck resecting short stem. PATIENTS AND METHODS: In this retrospective study, 106 patients (index surgery 2014-2019) with unilateral THA and a morphologically healthy contralateral hip as a reference (Kellgren-Lawrence ≤ 1) were included. A cementless short stem with meta-diaphyseal fixation and press-fit cup was implanted in all cases. Stem alignment, CCD angle, CTA and offset reconstruction were measured on preoperative and 3 months postoperative AP radiographs of the pelvis. RESULTS: Preoperative lower CTA and lower CCD angle were positively correlated (r = 0.472; p < 0.001). Higher varus stem alignment is correlated with lower CTA (r = - 0.384; p < 0.001) and lower CCD angle (r = - 0.474; p < 0.001). A CTA of 23.1° or lower showed a sensitivity of 59.1% and a specificity of 87.1% (AUC: 0.733) and a CCD angle of 132.75° or lower a sensitivity of 68.2% and a specificity of 80.6% (AUC: 0.77) for a varus stem alignment > 3°. CONCLUSION: The CTA is also applicable in cementless THA with a neck resecting short stem to evaluate risk of intraoperative varus stem positioning. The CCD angle shows higher sensitivity with marginally lower specificity. Therefore, the CTA is not superior in predicting varus stem alignment in short-stem THA. LEVEL OF EVIDENCE: IV.

Evaluation of intra- and interobserver reliability in the assessment of the 'critical trochanter angle'.

BACKGROUND: The recently described 'critical trochanter angle' (CTA) is a novel parameter in the preoperative risk assessment of stem malalignment in total hip arthroplasty. As its reproducibility needs to be evaluated, the given study aims to investigate intra- and interobserver reliability. It is hypothesized that both analyses justify the clinical use of the CTA. METHODS: A total of 100 pelvic radiographs obtained prior to total hip arthroplasty were retrospectively reviewed by four observers with different levels of clinical experience. The CTA was measured twice by each observer at different occasions in the previously described technique. Intra- and interobserver reliability was evaluated using intraclass correlation coefficients (ICC) with confidence intervals (CI) and the Bland-Altman approach. RESULTS: The mean CTA in both measuring sequences was 20.58° and 20.78°. The observers' means ranged from 17.76° to 25.23°. Intraobserver reliability showed a mean difference of less than 0.5° for all four observers (95% limit of agreement: - 7.70-6.70). Intraobserver ICCs ranged from 0.92 to 0.99 (CI 0.88-0.99). For interobserver variation analysis, ICCs of 0.83 (CI 0.67-0.90) and 0.85 (CI 0.68-0.92) were calculated. CONCLUSION: Analyses concerning intra- and interobserver reliability in the assessment of the CTA showed 'very good' and 'good' results, respectively. In view of these findings, the use of the CTA as an additional preoperative parameter to assess the risk of intraoperative stem malalignment seems to be justified.

The 'critical trochanter angle': a predictor for stem alignment in total hip arthroplasty.

INTRODUCTION: Stem malalignment can affect offset reconstruction and may result in gluteal muscle insufficiency. In this retrospective study, a novel geometric angle named 'critical trochanter angle' (CTA) is described and investigated towards the risk of malposition of a collarless straight tapered hydroxyapatite-coated stem in primary total hip arthroplasty (THA). MATERIAL AND METHODS: A total of 100 cementless THA were implanted in patients with unilateral coxarthrosis via the direct anterior (n = 50) or direct lateral Hardinge approach (n = 50) in a two surgeon setting using the Corail® or Trendhip® stem (DePuy Synthes or Aesculap). Stem alignment was analysed in postoperative AP pelvic radiographs and correlated to the CTA: the angle crest was defined by the intersection of the femoral shaft and neck axis and the angle was measured between the shaft axis and a leg intersecting the vertex between the lateral and superoposterior facet of the trochanter. RESULTS: Forty-seven stems were implanted in varus (≥ + 1°), 42 in neutral (< + 1°/> - 1°) and 11 in valgus position (≤ - 1°). The mean critical trochanter angle was 25.0° (SD ± 7.5°), and there was a negative and statistically significant correlation to stem alignment (r = - 0.52; p ≤ 0.001) independent from the surgical approach. For stem malposition of 2° and above (n = 23), mean CTA was 17.2° for varus (n = 20) and 31.6° for valgus (n = 3). A CTA lesser or equal to 22.75° had a sensitivity of 90% and specificity of 80% for varus stem position of 2° or greater. Specificity raised to 100% with a cutoff CTA of 12.5° or lesser. CONCLUSION: Varus stem alignment in THA is associated with coxa vara deformity and a radiological low CTA. In preoperative planning, the critical trochanter angle can help to evaluate the risk for intraoperative stem malpositioning. If navigation or robotic assistance is not available when using this stem design, we recommend an intraoperative x-ray to verify correct implant positioning in patients with a CTA under 20° or above 30°.