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

Area of Science:

• Orthopedic Surgery
• Biomedical Engineering
• Radiology

Background:

• Stem malalignment in total hip arthroplasty (THA) can lead to complications like offset reconstruction issues and gluteal muscle insufficiency.
• A novel geometric measurement, the Critical Trochanter Angle (CTA), is introduced to assess the risk of malpositioning for specific uncemented hip stems.

Purpose of the Study:

• To investigate the Critical Trochanter Angle (CTA) as a predictor for stem alignment in primary total hip arthroplasty (THA).
• To evaluate the correlation between CTA and the positioning of collarless straight tapered hydroxyapatite-coated stems.

Main Methods:

• Retrospective analysis of 100 cementless THA cases using direct anterior or direct lateral Hardinge approaches.
• Measurement of stem alignment on postoperative AP pelvic radiographs and correlation with the preoperatively assessed CTA.
• Definition of CTA based on the intersection of the femoral shaft and neck axis, measured between the shaft axis and a line connecting specific trochanteric facets.

Main Results:

• A statistically significant negative correlation (r=-0.52, p≤0.001) was found between CTA and stem alignment, independent of surgical approach.
• A CTA ≤ 22.75° demonstrated 90% sensitivity and 80% specificity for detecting varus stem positioning (≥2°).
• A CTA ≤ 12.5° achieved 100% specificity for varus stem positioning.

Conclusions:

• Varus stem alignment in THA is associated with coxa vara deformity and a lower CTA.
• The CTA is a valuable tool for preoperative planning to evaluate the risk of intraoperative stem malpositioning.
• Intraoperative X-rays are recommended for stems with CTA < 20° or > 30° when navigation or robotic assistance is unavailable.