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 TE/TD = 4.39 +/- 0.90) were statistically stronger than defects treated with polymethylmethacrylate alone (TE/TD = 2.48 +/- 0.66; p less than 0.025) or by plating alone (TE/TD = 2.61 +/- 0.91; p less than 0.025), but torque-to-failure was only increased to approximately 56% of an intact bone. Plated intact bones (TE/TD = 5.33 +/- 0.41) were significantly weaker than intact bones (TE/TD = 8.50 +/- 2.52; p less than 0.001). Our results substantiate the need for using polymethylmethacrylate and internal fixation in combination when prophylactically fixing pathologic lesions of this proportion.