Background <p>Stem sizing may influence periprosthetic femoral fracture (PPF) resistance after cemented total hip arthroplasty (THA), but whether this effect is consistent across cemented stem systems with distinct design philosophies remains incompletely understood. This study investigated how stem sizing influences torsional fracture resistance in an osteoporotic femoral model, using two cemented stem systems—the Exeter polished taper-slip system and the Charnley–Marcel–Kerboull (CMK) polished line-to-line system.</p> Methods <p>Four cemented stem constructs were tested using osteoporotic composite femoral analogues (model 3503; Sawbones): Exeter V40 44-3, Exeter V40 44-4, CMK 203, and CMK 303 (<i>n</i> = 6 per group). Stem selection was based on CT-derived canal morphology and surgeon judgment of fit, with paired upsizing within the Exeter taper-slip system and paired downsizing within the CMK line-to-line system. Compression–torsion testing was performed under a 2 kN axial load with a 2 N·m internal rotation preload, followed by internal rotation to 40° over 1&#xa0;s. The primary outcome was fracture torque at failure; secondary outcomes were fracture pattern and Vancouver classification.</p> Results <p>Fracture torque differed significantly among constructs (Kruskal–Wallis test, <i>p</i> = 0.012). In exploratory, unadjusted pairwise comparisons, the CMK 303—the largest construct—had the highest fracture torque (<i>p</i> = 0.013 versus each of the other three constructs); the principal contrast (CMK 303 versus Exeter 44-3) remained significant after Holm correction, whereas the other three constructs did not differ significantly. Median fracture torque was 74.85 N·m (IQR 72.63–81.54), 79.70 N·m (IQR 77.09–82.99), 80.08 N·m (IQR 78.43–82.81), and 94.30 N·m (IQR 86.51–98.90) for the Exeter 44-3, Exeter 44-4, CMK 203, and CMK 303, respectively. These values were 3- to fourfold higher than reported in vivo torsional moments at the hip during routine activity, consistent with an acute traumatic loading condition. Fractures were predominantly Vancouver B2 type (21/24 specimens: all Exeter 44-3 and CMK 203 specimens, four of six Exeter 44-4, and five of six CMK 303).</p> Conclusions <p>Stem sizing influenced torsional PPF resistance in this osteoporotic femoral model. The CMK 303, the largest construct tested, demonstrated greater fracture torque than the other three constructs, whereas constructs of similar size showed comparable resistance regardless of stem system. These findings support the concept that the maximum feasible stem size accommodated within a given femur may be a key determinant of acute torsional fracture resistance in cemented osteoporotic constructs, with implications that may extend across cemented stem systems; because the constructs differed in geometry, fixation philosophy, and the manufacturer-recommended cement together, these results are interpreted as construct-level effects rather than as the effect of fixation philosophy alone.</p>