Acetabular cup-native femoral head size discrepancy in primary total hip arthroplasty: A meta-analysis on intraoperative sizing verification.

Purpose: Intraoperative measurement of the native femoral head has been proposed as a simple cross-check for acetabular cup sizing in total hip arthroplasty (THA), but reported cup-head discrepancies vary. We synthesized the evidence to define an expected range and a pragmatic intraoperative alert threshold. Methods: A Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)-guided search of Cochrane Library, MEDLINE, Embase, Web of Science and Google Scholar (pages 1-20) from inception to 24 December 2025 identified studies reporting intraoperative femoral head diameter and implanted acetabular cup size. The primary outcome was the mean difference (Δ) between cup size and head diameter. Data were pooled using a random-effects model, 95% confidence interval (CI) and both 95% and 75% prediction interval (PI) were calculated. Results: Four studies (631 hips; mean age 66 years) were included. The pooled mean Δ was 2.93 mm (standard error [SE] 0.46). The 95% CI was 2.03-3.83 mm, and the 95% PI was -1.45 to 7.32 mm, indicating between-study variability. The 75% PI ranged from 1.30 to 4.57 mm. Across 556 hips with categorical data, the cup exceeded the head in 87.77% of cases; Δ was 0-2 mm in 36.33%, 2-4 mm in 33.27%, exactly 0 mm in 9.35%, >4 mm in 18.17% and negative in 2.88%. Conclusion: In primary THA, acetabular cups are around 2.93 mm larger than the excised femoral head, most often between 0 and 4 mm. This relationship supports femoral-head sizing as a rapid, no-cost intraoperative cross-check of reaming accuracy and final cup selection. A cup-head mismatch >4 mm should trigger verification of measurement, reaming progression and templating assumptions before altering component size. Level of Evidence: Level IV, systematic review and meta-analysis.