Stiffness and Strength of a Modified External Skeletal Fixator for Orthopaedic Treatment of Animals

This study outlines the design of a cost effective external skeletal fixator which can be implanted on small animals. A modification for a commercially available Universal Mini External Fixator (UMEXTM) has been done on the biomechanical performance by using a cadaver canine tibia. The constituents of the design of the prototype system include a connecting bar (200 mm long and 6 mm in diameter), clamp I (dimension size in 20x10x10 mm), clamp II (dimension size in 10x10x10 mm), and transfixation pin (150 mm long and 4 mm in diameter). A negative profile partially threaded pin was designed because it is cheaper to manufacture. For this experimental bone testing, 80 canine tibia bones harvested from 40 canines were collected from the Centre for Protected Animals in Setapak, Kuala Lumpur. All the tibia bones were freshly harvested within 2 hours, frozen and then thawed just prior to testing. The Instron universal testing machine was used to axially compress the bone fragments. The specimen was attached to the machine with a steel-coring tool arrangement at either end and compressed at a constant displacement rate of 0.254 mm per second. Five specimens of each configuration were tested on an Instron Universal Testing Machine by placing a steel plate under compression load, and then recording the load/deformation curve and load at failure. Three variables were arranged in the test and that were categories in two and six of number of pins, 30 mm and 60 mm for proximity of fixator to bone and 750 and 900 of angle of position in direction of fixation pin to the bone. The degree of stiffness of this system was obtained from the load/displacement curve (N/mm). In preparation for the compression, six pins were inserted into the bone and then these pins were clamped to a connecting bar located 30 mm from the long bone. The average stiffness of this modified system was 29.525 N/mm. This is higher than the Universal Mini External Fixator (UMEXTM) which had a value of 12.774 N/mm. The results of this experiment works indicated that system arrangements greatly affect the degree of stiffness of the system. Therefore, the optimum variable for the compressive testing is using the six pins with 30 mm of proximity and 750 of angle of position in fixation can obtain in the fracture bone application. This optimal of modified external skeletal fixator can achieve the maximum load in 438.84 N compare with UMEXTM fixator just achieve the maximum load in 126.36 N. It may result in a decreased rate of pin loosening and thus prolong the function life of the external skeletal fixator system and lower the complication rate associated with its use.

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