Strength reduction and the effects of treatment of long bones with diaphyseal defects involving 50% of the cortex

Currently, three criteria are accepted as indications for prophylactic internal fixation of metastatic disease in long bone, including lesions (a) destroying 50% or more of the cortex; (b) 2.5 cm or greater in diameter; or (c) with pain unrelieved by radiation therapy. Using an oblong defect configuration in which one half of the cross‐sectional area was destroyed, canine femora were torsion‐tested at high speed to determine (a) the actual strength reduction incurred by a lesion destroying 50% of the cortical circumference, and (b) the effects and benefits of internal fixation using polymethylmethacrylate and/or a six‐hole compression plate on such a defect. The femurs with a 50% circumferential cortical defect demonstrated only 12.7 ± 3.8% of intact strength. Defects treated with a combination of plating (all screws bicortical) and polymethylmethacrylate (torque to failure 4.39 ± 0.90 times greater than the defect alone or T_E/T_D = 4.39 ± 0.90) were statistically stronger than defects treated with polymethylmethacrylate alone (T_E/T_D = 2.48 ± 0.66; p < 0.025) or by plating alone (T_E/T_D = 2.61 ± 0.91; p < 0.025), but torque‐to‐failure was only increased to approximately 56% of an intact bone. Plated intact bones (T_E/T_D = 5.33 ± 0.41) were significantly weaker than intact bones (T_E/T_D = 8.50 ± 2.52; p < 0.001). Our results substantiate the need for using polymethylmethacrylate and internal fixation in combination when prophylactically fixing pathologic lesions of this proportion.