Laura J. M. Hirvinen, DVM; Alan S. Litsky, MD, ScD; Valerie F. Samii, DVM; Steven E. Weisbrode, VMD, PhD; Alicia L. Bertone, DVM, PhD American Journal of Veterinary Research August 2009 70 8 964-972 Objective—To compare biomechanical strength, interface quality, and effects of bone healing in bone-implant interfaces that were untreated or treated with calcium phosphate cement (Ca-cement), magnesium phosphate cement (Mg-cement), or polymethylmethacrylate (PMMA) in horses. Animals—6 adult horses. Procedures—4 screw holes were created (day 0) in each third metacarpal and third metatarsal bone of 6 horses. In each bone, a unicortical screw was placed in each hole following application of Ca-cement, Mg-cement, PMMA, or no treatment (24 screw holes/treatment). Screws were inserted to 2.82 N m torque. Horses were euthanized and bones were harvested at day 5 (16 screw holes/treatment) or day 182 (8 screw holes/treatment). Radiography, biomechanical testing, histomorphometry, and micro–computed tomography were performed to characterize the bone-implant interfaces. Results—Use of Mg-cement increased the peak torque to failure at bone-implant interfaces, compared with the effects of no treatment and Ca-cement, and increased interface toughness, compared with the effects of no treatment, Ca-cement, and PMMA. Histologically, there was 44% less Ca-cement and 69% less Mg-cement at the interfaces at day 182, compared with amounts present at day 5. Within screw threads, Ca-cement increased mineral density, compared with PMMA or no treatment. In the bone adjacent to the screw, Mg-cement increased mineral density, compared with PMMA or no treatment. One untreated and 1 Ca-cement–treated screw backed out after day 5. Conclusions and Clinical Relevance—In horses, Mg-cement promoted bone-implant bonding and adjacent bone osteogenesis, which may reduce the risk of screw loosening.