OTA 2005 Posters

Scientific Poster #50 Basic Science

A Biomechanical Comparison of Large Fragment Fixation Augmented with Biodegradable Calcium Phosphate and PMMA Bone Cements to Non-augmented Fixation in an Osteoporotic Bone Model

Cory A. Collinge, MD¹ (*); Bradley Merk, MD² (*);

Eugene P. Lautenschlager, PhD² (*);

¹Harris Methodist Hospital-Fort Worth and John Peter Smith Orthopaedic Surgery Residency Program, Fort Worth, Texas, USA;

²Department of Orthopedic Surgery, Northwestern University,

Chicago, Illinois, USA

Purpose: Fracture fixation in osteoporotic bone continues to be problematic despite recent advances in implant technologies. Polymethylmethacrylate (PMMA) has been used to augment screw fixation in osteoporotic bone; however, there are concerns about thermal tissue damage, difficulty of removal if necessary, and the difficulty of effectively introducing the material into a screw's pilot hole. Injectable biodegradable calcium phosphate (CaP) cements are available that might be useful for screw augmentation in osteoporotic bone and may avoid some of the undesirable properties of PMMA. The purpose of this study is to compare the biomechanical properties of standard large fragment screw and screw-plate fixation with similar constructs augmented by bone cements in an osteoporotic bone model.

Methods: Two experiments were performed. In experiment 1, an Instron machine (Canton, MA) was used to pull screws in uniaxial tension at 5 mm/min until failure (yield point). An open cell rigid polyurethane foam block was selected for this testing to avoid the heterogeneity of cadaver bone. Porous foam with a density of 7.5 lbs per cubic foot was selected to mimic properties of osteoporotic bone (Pacific Research Labs, Vashon, WA). 36-mm long, 4.5-mm diameter self-tapping cortical screws were placed using AO/ASIF technique to a depth of 30 mm. Four sets of 13 screws were tested: (1) screws without material for augmentation (control); (2) screws augmented with 1.5 to 2.0 cc of resorbable CaP (Norian SRS^®, Synthes, Paoli, PA); (3) screws augmented with 1.5 to 2.0 cc of resorbable CaP cement reinforced with polylactide-glycolide chopped fibers, or CaP-R (Norian SRS Reinforced^TM Synthes, Paoli, PA); and (4) screws augmented with 1.5 to 2.0 cc of PMMA (Simplex^® P, Stryker-Howmedica, Mahwah, NJ). After implantation, specimens were stored at 37° C in Ringer's lactate solution for 16 hours. Experiment 2 was designed to evaluate the biomechanical effects of screw augmentation with CaP cement on standard nonlocked plate-screw (NLP) and locked plate-screw (LP) constructs. Four sets of eight 6-hole 4.5-mm LCP plates (Synthes USA, Paoli, PA) were affixed to a similar bone model using three 30-mm 4.3-mm locked or 4.5-mm nonlocked screws placed into plate holes 1 through 3. NLP and LP constructs were tested by applying cantilever bending forces to test for failure by "plate lift-off" with no screw augmentation and with screw augmentation using CaP cement. The mean loads to failure for each screw and each screw-plate construct were analyzed using ANOVA (Statistica, StatSoft, Tulsa, OK).

Results: Experiment1: The mean pullout strengths (maximum load at failure) were 8.9 lbs for the nonaugmented screws (SD=7.4), 36.6 lbs for CaP SRS augmented screws (SD=8.0), 35.5 lbs for C-TR augmented screws (SD=10.7), and 20.2 lbs for PMMA augmented screws (SD=7.7) [P< 0.05 between all groups except screws augmented with CaP SRS versus CaP-R]. Pullout failure occurred by different modes; nonaugmented screws pulled out with no adherent bone model material, CaP augmented screws pulled out with a large "plug" of adherent CaP and bone model, and PMMA augmented screws pulled out with smaller amounts of PMMA (mostly proximal) with some bone model. Experiment2: The mean load at failure was 3.0 lbs for nonaugmented NL constructs (SD=1.5), 4.5 lbs. for nonaugmented LP constructs (SD=2.2), 10.9 lbs for CaP-R augmented NLP constructs (SD=2.8), and 14.9 lb. for CaP-R augmented LP constructs (SD=3.2) [P< 0.05 for all except nonaugmented NL constructs versus nonaugmented LP constructs).

Conclusion/Significance: We have demonstrated that screw augmentation with resorbable CaP bone cements offer a great ability to increase the mechanical strength of large fragment screw and screw-plate fixation in an osteoporotic bone model. Our data demonstrate that CaP cement increases large fragment fixation properties more than the use of locked plating or screw augmentation with PMMA alone. It is likely that the comparative ease of injection of the CaP cement over PMMA is a factor in this difference.