Biomechanical evaluation of hybrid, bicortical and univrotical, screw configurations for internal bone late fixation of long bone fracture : an in-vitro study of porcine femur bone models / by Gabriel Jonas Gonzalez Quintero.
ABSTRACT Midshaft fractures of the femur are commonly seen in clinical cases. A fracture is a break through the bone that compromises bone stability and its surroundings. After a fracture, the bone must go through bone healing to recover its full stability and function. Internal bone plate fixation of midshaft femur fractures is one of the leading ways that surgeons treat transverse fractures of the femur bone. The purpose of the implant plate is to provide structure and stability while the bone regenerates. Current clinical applications prefer the use of 6 bicortical non-locking screws in a 7 hole dynamic compression plate, considered as the traditional method, for the internal fixation of midshaft femur transverse fractures. This configuration provides a secure structure to allow for long-term rehabilitation, but it is also very invasive to the bone. The traditional method does not necessarily provide the best mechanical performance possible. This study proposes the use of hybrid configurations of screws for an alternative method of the bone plate fixation. Hypothetically, the combinations combine 4 unicortical and 2 bicortical screws into the plate/screw to bone interface to provide a balance between level of support and invasiveness offered by the implant. The goal of this study is to conduct an in-vitro, a physical, and a statistical analysis to better understand the implications of the hybrid configurations and compare their performance to the traditional method. The overall purpose of this study is to identify the best configuration of bone plate fixation for rehabilitation of a fractured femur bone. The study was conducted using porcine femur models. Porcine femurs are known to be anatomically and mechanically similar to the human femur . Four groups were considered for this study. The control group was based on the traditional method of bone plate fixation, consisting of 6 bicortical screws. Each of the three testing groups had four unicortical and two bicortical screws, each with a different placement for the bicortical screws. The bicortical screws for Groups 2, 3, and 4 were located in the innermost, middle, and outermost holes of the plate, respectively. Seven bone samples were made for each group (n=7) following the same procedure of bone plate fixation for each group. A 10 mm transverse fracture was created at the midshaft of the femur to simulate the fractured bone. Each sample was then fixated at both ends of the femur through an epoxy. All bones were tested on the Materials Testing System located in the orthopedics lab of Mercer University, School of Engineering. Three analyses were conducted to test the performance of each configuration: an experimental in-vitro analysis of mechanical properties, a theoretical analysis of force interactions, and a statistical analysis for of significant difference of the data. The in-vitro investigation was done through a material analysis of the construct. Axial compression and axial failure tests were implemented to simulate the mechanical behavior of the construct under elastic and plastic deformation. In the axial failure test, pre- and post-cyclic assessments were made and the axial stiffness was calculated for each group. The average axial pre-stiffness was 909 ± 117 N/mm for Group 1 (the control group), 958 ± 104 N/mm for Group 2, 1083 ± 287 N/mm for Group 3, and 1096 ± 445 N/mm for Group 4. Overall, the configurations were ranked based on pre-stiffness performance in the following order: Group 4 > Group 3 > Group 2 > Group 1. The average axial post-stiffness was 1181 ± 156 N/mm for Group 1, 1046 ± 162 N/mm for Group 2, 1160 ± 207 N/mm for Group 3, and 1240 ± 521 N/mm for Group 4. Overall, the configurations were ranked based on post-stiffness performance in the following order: Group 4 > Group 1 > Group 3 > Group 2. The average axial stiffness was 407 ± 145 N/mm for Group 1 (the control group), 445 ± 91 N/mm for Group 2, 460 ± 143 N/mm for Group 3, and 680 ± 225 N/mm for Group 4. The average axial yield strength was 3910.13 ± 1776.638 N for Group 1 (the control group), 4268 ± 1837 N for Group 2, 5107 ± 2608 N for Group 3, and 7002 ± 2187 N for Group 4. The average ultimate failure force was 4949 ± 2678 N for Group 1 (the control group), 5743 ± 3026 N for Group 2, 6065 ± 3052 N for Group 3, and 8499 ± 1492 N for Group 4. Overall, the configurations were ranked based on the axial failure performance in this order: Group 4 > Group 3 > Group 2 > Group 1. The theoretical analysis studied the forces and moments acting on the implant to bone interface. This analysis was done through the use of free body diagrams. An analysis was performed for both the static (in equilibrium) and dynamic (not in equilibrium) behaviors of each configuration. From the static analysis it was determined that bicortical screws create larger forces on the bone than unicortical screws. The presence of more bicortical screws can result in higher wearing for the bone, as the bone cortex must create counteracting forces under axial loading. Thus, more bicortical screws results in higher bone wear at the implant interface. From the dynamic analysis it was concluded that if bicortical screws are placed farther from the fracture gap, they are able to provide a higher stiffness for the system due to a greater moment arm from the bicortical screw to the fracture gap. Based on these results, Group 4 represents a better theoretical model than Groups 1, 2, and 3. The statistical analysis was done through Minitab 17. The groups were tested for distribution normality and statistical significance in each of the variables. Most groups presented a normal distribution of the data. A total of 4 cases came out to be non-normally distributed, which only meant that these cases were not able undergo the statistical significance test. ANOVA analysis was done for those cases that presented a normally distributed data. Most variables presented no statistically significant difference between the groups. However, there were two cases, the axial stiffness under equal variances and the ultimate failure force under unequal variances, that had a p-value lower than 0.05. For these two variables there was enough evidence to show that the values were statistically significantly different and these were not attributed to chance. The material and physical analyses agreed with each other on the conclusions made. Overall, it was found that Group 4 offers a less invasive model than Group 1 and also greater stability and resistance to deformation than Groups 1, 2, and 3. In addition, the statistical analysis gave evidence that certain variables do represent the actual averages of the overall population. In conclusion, this study recommends the use of Group 4’s configuration as a more suitable implant for rehabilitation purposes of midshaft transverse fracture of femur bone. Keywords: DCP, Transverse Fracture, Femur Bone, Internal Fixation, Bone Plate, Unicortical and Bicortical Screw, Stiffness, Elastic and Plastic Deformation, and Configuration.
Gonzalez Quintero, Gabriel Jonas