Augmentation of Anterior Vertebral Body Screw Fixation by an Injectable, Biodegradable Calcium Phosphate Bone Substitute

Study Design. A biomechanical study to evaluate the effects of a biodegradable calcium phosphate (Ca-P) bone substitute on the fixation strength and bending rigidity of vertebral body screws. Objectives. To determine if an injectable, biodegradable Ca-P bone substitute provides significant augmentation of anterior vertebral screw fixation in the osteoporotic spine. Summary of Background Data. Polymethylmethacrylate (PMMA) augmented screws have been used clinically; however, there is concern about thermal damage to the neural elements during polymerization of the PMMA as well as its negative effects on bone remodeling. Injectable, biodegradable Ca-P bone substitutes have shown enhanced fixation of pedicle screws. Methods. Sixteen fresh cadaveric thoracolumbar vertebrae were randomly divided into two groups: control (no augmentation) (n = 8) and Ca-P bone substitute augmentation (n = 8) groups. Bone–screw fixation rigidity in bending was determined initially and after 105 cycles, followed by pullout testing of the screw to failure to determine pullout strength and stiffness. Results. The bone–screw bending rigidity for the Ca-P bone substitute group was significantly greater than the control group, initially (58%) and after cyclic loading (125%). The pullout strength for Ca-P bone substitute group (1848 ± 166 N) was significantly greater than the control group (665 ± 92 N) (P < 0.01). Stiffness in pullout for the Ca-P bone substitute groups (399 ± 69 N/mm) was significantly higher than the control group (210 ± 51 N/mm) (P < 0.01). Conclusion. This study demonstrated that augmentation of anterior vertebral body screw fixation with a biodegradable Ca-P bone substitute is a potential alternative to the use of PMMA cement.