Elizabeth P. Frankenburg

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 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. STUDY DESIGN 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. 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 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. RESULTS 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). 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 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.

Biomechanical Study of Sternal Closure Using Rigid Fixation Techniques in Human Cadavers

BACKGROUND We believe rigid plate fixation may be superior to wire fixation in sternal closure, as rigid fixation used in the craniofacial skeleton has shown greater stability, lower postoperative pain, and accelerated bone healing. We hypothesize that sterna fixed with titanium plates are more stable mechanically than sterna fixed with wires. METHODS The sterna from human cadavers were used in this two-phased study. Phase I compared wires to four-hole titanium straight plates. Phase II compared wires to four-hole titanium custom H plates. The sterna were tested biomechanically using all fixation methods. RESULTS Phase I showed no statistically significant difference in the stiffness or lateral displacement between the wired and straight plated sterna. Phase II showed a statistically significant greater stiffness (p < 0.05) and less lateral displacement (p < 0.05) in the custom plated sterna over the wired sterna. CONCLUSIONS Our results showed that custom titanium H plates were superior to wire fixation. Furthermore, our results established the importance of plate configuration in sternal fixation. Our study may have beneficial clinical implications, as decreased motion at the sternotomy site could mean less postoperative pain, a decreased incidence of infection, and accelerated bone healing.

Procollagen C Proteinase Enhancer 1 Genes Are Important Determinants of the Mechanical Properties and Geometry of Bone and the Ultrastructure of Connective Tissues

ABSTRACT Procollagen C proteinases (pCPs) cleave type I to III procollagen C propeptides as a necessary step in assembling the major fibrous components of vertebrate extracellular matrix. The protein PCOLCE1 (procollagen C proteinase enhancer 1) is not a proteinase but can enhance the activity of pCPs ∼10-fold in vitro and has reported roles in inhibiting other proteinases and in growth control. Here we have generated mice with null alleles of the PCOLCE1 gene, Pcolce, to ascertain in vivo roles. Although Pcolce− / − mice are viable and fertile, Pcolce − / − male, but not female, long bones are more massive and have altered geometries that increase resistance to loading, compared to wild type. Mechanical testing indicated inferior material properties of Pcolce − / − male long bone, apparently compensated for by the adaptive changes in bone geometry. Male and female Pcolce − / − vertebrae both appeared to compensate for inferior material properties with thickened and more numerous trabeculae and had a uniquely altered morphology in deposited mineral. Ultrastructurally, Pcolce − / − mice had profoundly abnormal collagen fibrils in both mineralized and nonmineralized tissues. In Pcolce − / − tendon, 100% of collagen fibrils had deranged morphologies, indicating marked functional effects in this tissue. Thus, PCOLCE1 is an important determinant of bone mechanical properties and geometry and of collagen fibril morphology in mammals, and the human PCOLCE1 gene is identified as a candidate for phenotypes with defects in such attributes in humans.

The thermal effects of intramedullary reaming.

Objective: To study the effect of tourniquet control on intramedullary reaming. Design: An experimental prospective nonsurvival animal study was performed using 5 mongrel dogs. A pneumatic tourniquet was randomized to either the right or left hind limb. Tibial intramedullary reaming was performed with progressively larger reamers. Cortical temperatures were measured using thermocouples inserted into the tibial diaphyseal cortex. Thermocouples were connected to an analog to digital converter that output continuous data that was collected on a computer. Upon completion of the procedure, the animals were killed. Results: The peak and low temperatures for each thermocouple with each reamer passage were recorded. Reamer sizes larger than the internal diameter of the intramedullary canal produced higher peak temperatures. The mean delta t (peak temperature minus low temperature) was calculated for each reamer passage. This measurement represents the overall amount of heat generated during each reamer passage. There were no significant differences between the 2 conditions (P = 0.8, paired t test). Temperatures decreased in between reamer exchange but did not return to baseline levels. Conclusions: Because similar temperatures were measured both with and without a tourniquet, the risk of thermal necrosis appears to be related more to the process of intramedullary reaming than to the tourniquet. Higher temperatures were measured with reamer sizes larger than the internal diameter of the intramedullary canal. Increasing the time interval between the passage of successive reamers may allow heat to dissipate and decrease the risk of thermal necrosis. The clinical practice of limited reaming (“ream-to-fit”) should minimize the occurrence of this complication.

Mouse model for thoracic congenital scoliosis.

This study sought to produce a dose–response curve for acute and chronic maternal carbon monoxide (CO) exposure versus vertebral anomalies in mouse offspring and to determine the critical day of exposure. In Part I, pregnant CD-1 mice were exposed to an acute dose of CO at 9 days of gestation. A positive dose–response relationship of acute maternal CO exposure and vertebral anomalies in the offspring was produced. In Part II, pregnant females were exposed to chronic CO for the first 11 days of gestation. Chronic exposure to CO did not produce significant vertebral anomalies. In Part III, pregnant females were exposed to an acute dose of 600 ppm of CO at gestation day 8, 9, or 10. Day 9 in this mouse breed is the critical day for maternal exposure to CO. The detected anomalies were predominately in the thoracic spine.

Biomechanics of Bone

The composition and structural integrity of the human skeleton have uniquely evolved, reflecting its need to balance four major functions: protection of vital organs, mechanical support and locomotion, mineral homeostasis, and hematopoiesis. During the past century numerous investigators have carefully attempted to document the mechanical and architectural properties of bone. These studies have provided insight into potential mechanisms of bone remodeling and also contributed to the assessment of fracture risk under normal, aging, or disease conditions.

Pubic symphysis repair strength in simulated bladder exstrophy using a sheep model

OBJECTIVES To investigate the mechanical strength of various pubic symphysis suture material in a simulated animal model of neonatal bladder exstrophy. METHODS Neonatal lamb pelves, which are the approximate size of neonatal human pelves, were used. Twenty-four neonatal lamb pelves were divided into four equal groups. A midline symphysotomy was made through the cartilaginous pubic symphysis in three groups and repaired using two figure-of-eight sutures (size 0) placed through the cartilaginous pubis using polypropylene, braided polyester, or polyglactin. The fourth group served as the control. The pelves were then tested to ultimate load in pure tension at a strain rate of 0.25 mm/s until failure. RESULTS There was a highly significant difference between the intact specimens and the repaired specimens (P = 0.0008). For the repaired specimens, there was a significant difference in the ultimate load normalized by pubic height between those repaired with polypropylene and polyglactin (P = 0.025), but not for those repaired with polypropylene and braided polyester (P = 0.11) or braided polyester and polyglactin (P = 0.11). CONCLUSIONS Braided resorbable sutures are recommended for pubic symphysis repair, because they have a lower tendency to cut out of the cartilage.