Aseptic loosening of small hydroxyapatite-coated quadrangular tapered femoral stems: camouflage in the Canadian Joint Replacement Registry

BACKGROUND: Small cementless hydroxyapatite-coated quadrangular tapered femoral stems are at increased risk of revision, but it is unclear whether this holds for appropriately-sized and undersized stems alike. This study aimed to assess the association between stem size and revisions for aseptic loosening. METHODS: Primary total hip arthroplasties with an Avenir (1547; Zimmer-Biomet), Corail (16,893; DePuy Synthes), or Polarstem (3403; Smith+Nephew) stem during 2012-2023 were identified from the Canadian Joint Replacement Registry. Cox proportional hazards models were used to estimate the association between stem/cup size and revision for aseptic loosening while adjusting for age, sex, and implant characteristics. Standardised stem sizing from A-K (smallest-largest) was utilised. RESULTS: Between different stems, the proportion of size A was 1.2-5.0% and of size B 3.6-12.2%. ⅓ (21/63) of aseptic loosening revisions occurred for size A/B. Increased risk of revision for aseptic loosening for size A stems was found with a hazard ratio (HR) of 46 (95% CI, 9-226) when paired with ⩾56-mm cups, 47 (14-157) when paired with 52-54-mm cups, and 17 (4-78) when paired with ⩽50-mm cups (reference size C+ with ⩾56-mm cups). Size B stems were also at increased risk of aseptic loosening, with an HR of 15 (95% CI, 4-52) and 9 (95% CI, 3-32) for ⩾56 and 52-54-mm cups respectively. CONCLUSIONS: The increased risk of revision of these small stems camouflages a highly increased risk of revision for aseptic loosening in undersized stems (small stem/big cup). This study was unable to ascertain whether this applies to other quadrangular tapered stems. Due to the small number of affected patients, it is important to repeat this study in other jurisdictions. Surgeons should be skeptical if a smaller size stem seems appropriate during broaching than preoperative templating or cup size suggests, as this may increase the risk of aseptic loosening.