PURPOSE To prevent excessive sliding and subsequent fixation failures in unstable intertrochanteric fractures with posteromedial comminution, extramedullary reduction through overlapping of the anteromedial cortices of both proximal and distal fragments as a buttress has been introduced. The purpose of this study was to compare the biomechanical properties between two reduction methods-intramedullary reduction and extramedullary reduction-in treating unstable intertrochanteric fractures with posteromedial comminution (AO/OTA classification 31-A2.2). MATERIALS AND METHODS Eight pairs of frozen human cadaveric femora were used. The femora of each pair were randomly assigned to one of two groups: the intramedullary reduction group or the extramedullary reduction group. A single axial load-destruction test was conducted after cephalomedullary nailing. Axial stiffness, maximum load to failure, and energy absorbed to failure were compared between the two groups. Moreover, the pattern of mechanical failure was identified. RESULTS The mean axial stiffness in the extramedullary reduction group was 27.3% higher than that in the intramedullary reduction group (422.7 N/mm vs. 332.0 N/mm, p=0.017). Additionally, compared with the intramedullary reduction group, the mean maximum load to failure and mean energy absorbed to failure in the extramedullary group were 44.9% and 89.6% higher, respectively (2,848.7 N vs. 1,966.5 N, p=0.012 and 27,969.9 N·mm vs. 14,751.0 N·mm, p=0.012, respectively). In the intramedullary reduction group, the mechanical failure patterns were all sliding and varus deformities. In the extramedullary reduction group, sliding and varus deformities after external rotation were noted in 3 specimens, sliding and varus deformities after internal rotation were noted in 3 specimens, and medial slippage was noted in 2 specimens. CONCLUSION In unstable intertrochanteric fractures with posteromedial comminution, the biomechanical properties of extramedullary reduction are superior to those of intramedullary reduction. Anteromedial cortex could be the proper buttress, despite a comminuted posteromedial cortex. It could help enhance the stability of the bone-nail construct.
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PURPOSE To obtain the accurate knowledge of the fundamental mechanical properties of the external fixator affected by variations in arrangements and structures.
We used newly developed external fixator, Anyfix, universal testing machine and plastic padding bone model which had similar structural properties to human tibia. The measured performance for seven different configurations of external fixators was its ability to control the motion of the bone fragment at the fracture site. Based on a unit of applied load, the corresponding displacement measured at the fracture site was used to described the stiffness of the fixation device for each load. Three stiffness moduli can be determined as axial stiffness, anterior posterior bending stiffness and lateral bending stiffness. RESULTS In basic configuration, all three stiffnesses for unilateral two plane external fixator showed marked increase than those for unilateral one plane model. Axial compression stiffness and bending stiffness were increased when ring component were located far from the fracture site. In modified configuration, all three stiffnesses were increased when the number of pin was increased and small sized ring was used. CONCLUSION The stiffness of the external fixator can be substantially increased by using unilateral two plane, locating the ring at far portion from the fracture site, using a small sized ring and increasing the number of pins.