Preoperative factors associated with alignment change from planned metaphyseal- to diaphyseal-dominant fixation in calcar-guided short-stem total hip arthroplasty.

PURPOSE: Calcar-guided short stems allow a certain degree of alignment variability and may result in different fixation patterns within the proximal femur. Consequently, postoperative fixation pattern does not always correspond to preoperative planning and may influence offset or leg-length reconstruction. This study aimed to identify preoperative factors associated with deviation from planned metaphyseal-dominant fixation in total hip arthroplasty (THA). METHODS: Sixty-two consecutive primary THAs using a calcar-guided short stem were analyzed. All hips were preoperatively planned using CT-based three-dimensional templating to achieve metaphyseal-dominant fixation. Preoperative variables included patient demographics, Dorr classification, neck-shaft angle, and a planned stem depth parameter defined as the height difference between the femoral saddle and the planned stem shoulder (planned stem height). Postoperative fixation pattern was classified as metaphyseal-dominant or diaphyseal-dominant, and factors associated with postoperative diaphyseal-dominant fixation were analyzed. RESULTS: Metaphyseal-dominant fixation was observed in 43 hips, whereas 19 hips demonstrated diaphyseal-dominant fixation. Multivariable analysis showed that a lower planned stem height and a larger neck-shaft angle were independently associated with postoperative diaphyseal-dominant fixation. The cutoff values were - 4.4 mm for planned stem height and 134.5° for neck-shaft angle. CONCLUSION: A low planned stem position, often observed in hips with a relatively short femoral neck, and a larger neck-shaft angle increase the likelihood of deviation toward diaphyseal-dominant fixation. When these features are present, surgeons should consider planning with diaphyseal-dominant fixation or selecting an alternative stem design.