Hip screw augmentation with an in situ-setting calcium phosphate cement: an in vitro biomechanical analysis

This study was performed to determine whether a new, in situ-setting calcium phosphate cement would have sufficient mechanical integrity to reinforce compression screw fixation of unstable intertrochanteric fractures. We compared the cut-out resistance of screws augmented with calcium phosphate cement to the cut-out resistance of screws augmented with polymethylmethacrylate (PMMA). We used PMMA as the standard for comparison because it is currently used clinically. Our hypothesis was that initial fixation strength with PMMA and calcium phosphate cement augmentation would not be significantly different from one another.

Cut-out testing of compression hip screws in paired human cadaveric proximal femurs was performed before and 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 internal 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.

Ten paired human femurs (mean age, 75 +/- 9.2 years) were implanted with Richards AMBI compression hip screws. Basicervical osteotomies were then performed, yielding isolated proximal fragments for mechanical testing. Preaugmentation cut-out tests were performed under displacement control, with cut-out continuing to five millimeters at two millimeters per second. The screws were then removed, and the screw tracks were filled with 2.0 cubic centimeters of PMMA (one side) or calcium phosphate cement (contralateral side). After augmentation, the screws were reinserted and the cements were allowed to harden for twenty-four hours. Postaugmentation testing followed the protocols for preaugmentation testing, and the initial fixation strength of screws augmented with calcium phosphate cement was compared with the initial fixation strength of screws augmented with PMMA using a two-way repeated measures analysis of variance.

The cut-out behavior of screws augmented with calcium phosphate cement was not significantly different from the cut-out behavior of screws augmented with PMMA. With calcium phosphate cement, yield strength increased by 15.8 percent (from 1,354 +/- 632 newtons to 1,568 +/- 320 newtons); with PMMA, the yield strength increased by 26.8 percent (from 1,477 +/- 526 newtons to 1,834 +/- 225 newtons). However, only the increase with PMMA augmentation was significant at p < 0.05). The energy to yield increased significantly (41 percent, p < 0.05) with both types of augmentation (from 2,399 +/- 1,186 newton-millimeters to 3,378 +/- 857 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 newtons per millimeter) and fell slightly with calcium phosphate cement augmentation (10 percent, from 457 +/- 201 newtons per millimeter to 411 +/- 663 newtons per millimeter).

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 materials may have promise as substitutes for PMMA in the salvage of compression hip screw fixation in elderly osteopenic patients with complex intertrochanteric fractures and that further study of their use in this application is warranted.