Reinforcement of osteosynthesis screws with brushite cement

The fixation of osteosynthesis screws remains a severe problem for fracture repair among osteoporotic patients. Polymethyl-methacrylate (PMMA) is rout inely used to improve screw fixation, but this material has well-known draw backs such as monomer toxicity, exothermic polymerization, and nonresorbabi lity. Calcium phosphate cements have been developed for several years, Amon g these new bone substitution materials, brushite cements have the advantag e of being injectable and resorbable. The aim of this study is to assess th e reinforcement of osteosynthesis screws with brushite cement, Polyurethane foams, whose density is close to that of cancellous bone, were used as bon e model. A hole was tapped in a foam sample, then brushite cement was injec ted, Trabecular osteosynthesis screws were inserted. After 24 h of aging in water, the stripping force was measured by a pullout test. Screws (4.0 and 6.5 mm diameter) and two foam densities (0.14 and 0.28 g/cm(3)) were compa red. Cements with varying solid/liquid ratios and xanthan contents were use d in order to obtain the best screw reinforcement, During the pull-out test , the stripping force first increases to a maximum, then drops to a steady- state value until complete screw; extraction. Both maximum force and platea u value increase drastically in the presence of cement. The highest strippi ng force is observed for 6.5-mm screws reinforced with cement in low-densit y foams, In this case, the stripping force is multiplied by 3.3 in the pres ence of cement, In a second experiment, cements with solid/liquid ratio ran ging from 2.0 to 3.5 g/mL were used with 6.5-mm diameter screws. In some co mpositions, xanthan was added to improve injectability. The best results we re obtained with 2.5 g/mL cement containing xanthan and with 3.0 g/mL cemen ts without xanthan, A 0.9-kN maximal stripping force was observed with nonr einforced screws, while 1.9 kN was reached with reinforced screws, These fi rst results are very promising regarding screw reinforcement with brushite cement. However, the polyurethane foam model presents noninterconnected por osity and physiological liquid was not modelized. (Bone 25: 95S-98S; 1999) (C) 1999 by Elsevier Science Inc, All rights reserved.