HIP SCREW AUGMENTATION WITH AN IN-SITU SETTING CALCIUM-PHOSPHATE CEMENT - AN IN-VITRO BIOMECHANICAL ANALYSIS

Objectives/Hypothesis: This study was performed to determine whether a new, in situ-setting calcium phosphate cement would have sufficient m echanical integrity to reinforce compression screw fixation of unstabl e intertrochanteric fractures. We compared the cut-out resistance of s crews augmented with calcium phosphate cement to the cut-out resistanc e of screws augmented with polymethylmethacrylate (PMMA). We used PMMA as the standard for comparison because it is currently used clinicall y. Our hypothesis was that initial fixation strength with PMMA and cal cium phosphate cement augmentation would not be significantly differen t from one another. Study Design: Cut-out testing of compression hip s crews in paired human cadaveric proximal femurs was performed before a nd after augmentation with PMMA or calcium phosphate cement. Bilateral testing was performed to allow pairwise comparisons of the materials used for augmentation, and repeated testing was done to provide an int ernal control for the effects of bone quality. The initial fixation of screws augmented with calcium phosphate cement was compared with that of screws augmented with PMMA. Methods: Ten paired human femurs (mean age, 75 +/- 9.2 years) were implanted with Richards AMBI compression hip screws. Basicervical osteotomies were then performed, yielding iso lated proximal fragments for mechanical testing. Preaugmentation cut-o ut tests were performed under displacement control, with cut-out conti nuing to five millimeters at two millimeters per second. The screws we re then removed, and the screw tracks were filled with 2.0 cubic centi meters of PMMA (one side) or calcium phosphate cement (contralateral s ide). After augmentation, the screws were reinserted and the cements w ere allowed to harden for twenty-four hours. Postaugmentation testing followed the protocols for preaugmentation testing, and the initial fi xation strength of screws augmented with calcium phosphate cement was compared with the initial fixation strength of screws augmented with P MMA using a two-way repeated measures analysis of variance. Results: T he cut-out behavior of screws augmented with calcium phosphate cement was not significantly different from the cut-out behavior of screws au gmented with PMMA. With calcium phosphate cement, yield strength incre ased by 15.8 percent (from 1,354 +/- 632 newtons to 1,568 +/- 320 newt ons); with PMMA, the yield strength increased by 26.8 percent (from 1, 477 +/- 526 newtons to 1,834 +/- 225 newtons). However, only the incre ase with PMMA augmentation was significant atp < 0.05. The energy to y ield increased significantly (41 percent, p < 0.05) with both types of augmentation (from 2,399 +/- 1,186 newton-millimeters to 3,378 +/- 85 7 newton-millimeters for calcium phosphate cement, and from 2,635 +/- 1,113 newton-millimeters to 3,741 +/- 426 newton-millimeters for PMMA) , whereas the stiffness increased only slightly with PMMA augmentation (6.2 percent, from 481 +/- 180 newtons per millimeter to 511 +/- 92 n ewtons per millimeter) and fell slightly with calcium phosphate cement augmentation (10 percent, from 457 +/- 201 newtons per millimeter to 411 +/- 663 newtons per millimeter). Conclusions: The in situ-setting calcium phosphate cement investigated in this study compared favorably with PMMA in a single-cycle cut-out test of augmented compression hip screws in senile trabecular bone. Our results suggest that these mate rials may have promise as substitutes for PMMA in the salvage of compr ession hip screw fixation in elderly osteopenic patients with complex intertrochanteric fractures and that further study of their use in thi s application is warranted.