Biomechanical Analysis of Fixation Strength in Unstable Intertrochanteric Femoral Fracture Models Based on the Caput–Collum–Diaphyseal Angle of Cephalomedullary Nails and Position of Lag Screws

<b>Background/Objectives:</b> The combined effect of femoral neck-shaft angle and lag screw position on unstable intertrochanteric fracture fixation has not been well established. This biomechanical study evaluated the effects of two caput-collum-diaphyseal (CCD) angles and two lag screw positions on construct stability. <b>Methods:</b> Twenty-four synthetic femurs with identical AO/OTA 31-A2.2 fracture gaps (2 mm) were fixed using cephalomedullary nails with CCD angles of either 125° or 130°, each with a central or inferior (calcar) lag screw (<i>n</i> = 6/group). Constructs were tested in a single-leg stance under preloading, cyclic loading (75-750 N, 10,000 cycles, and 2 Hz), and axial loading to failure. Lag screw migration was measured radiographically, and femoral head rotation was recorded using a three-dimensional coordinate-measuring device. Stiffness, failure load, and rotations were compared using the Kruskal-Wallis and Bonferroni post hoc tests. <b>Results:</b> The 125° inferior configuration showed the highest stiffness (188 ± 15 N/mm, <i>p</i> = 0.038) and failure load (1350 ± 97 N, <i>p</i> = 0.047), with the least screw migration (0.54 ± 0.11 mm, <i>p</i> = 0.003), significantly outperforming the 125° central and 130° central constructs. However, it exhibited greater varus collapse (2.25 ± 0.27°, <i>p</i> = 0.013) and axial rotation (~20-30% higher than others, <i>p</i> = 0.025). Screw position had a stronger effect on stability than the CCD angle, although the 130° inferior construct showed slightly less varus deformation. <b>Conclusions:</b> An inferior calcar-guided lag screw improves fixation strength and stiffness in unstable intertrochanteric fractures, particularly in those with a 125° nail. However, this configuration increases varus and rotational displacement, warranting adjunct measures to enhance rotational control in clinical applications.