Experimental studies on "less rigid" polyacetal plates for fracture fixation
Kusunose, K.
Nihon Seikeigeka Gakkai Zasshi 56(5): 399-414
1982
ISSN/ISBN: 0021-5325 PMID: 7108319 Document Number: 187762
Rigid and prolonged internal fixation of a fracture with a stiff plate yields osteoporosis. To solve this problem, so called less rigid plates made of various plastic materials were tested by some workers experimentally, but they are not widely accepted yet for clinical use, for the plastic material used lacked sufficient strength for stable osteosynthesis. A series of experiments using polyacetal (polyoxymethylene-POM) materials which have relatively more strength, for the fixation of canine fractures are presented. Alteration of material strength in vivo: Dumb-bell shaped test pieces were made of 3 different POM plastics; acetal copolymer alone (M90), acetal copolymer reinforced with 20% carbon fiber (CR20) and acetal copolymer with 20% fluorine (YF20). Tensile strength was measured after imbedding the test piece between the vastus lateralis and the biceps femoris of dogs for 8 mo. The results were compared with those before imbedding. Fracture healing study: After applying a POM plate or a stainless-steel plate (AO [Osteosynthesis Association] 1/3 tubular plate) on an adult canine femur with 4 stainless-steel screws, the femur was osteotomized at the center with a Gigli saw to produce a transverse fracture. Postoperative roentgenograms were taken periodically. After sufficient bone union on roentgenogram, the animal was sacrificed to excise the femur for 3 point bending test applying up to 30 kg load posteriorly to the femur to measure ultimate strength, deflexion at the center and strains at the anterior portion of the bone. A certain decrease in tensile strength was noted in the 3 tested materials. With M90 plates, there is considerable tendency toward delayed union, which would be due to insufficient strength of the plate. No significant difference in the period required for bone union was noted between the CR20 and the AO plate groups, while the former tended to produce a larger callus. The ultimate strength of the united bone following CR20 or AO plate fixation ranged from 50-55% of that of the contralateral non-osteotomized femur. There was no significant difference between the 2 plate groups. The deflexion at the center was significantly less in the CR20 group under low load, irrespective to the difference in the amount of the ultimate strength. The strain at the fracture site was extremely large in the AO group compared to those at the other sites, which would indicate uneven strength distribution, while the strains were relatively even in the CR20 group as in the non-osteotomized femur. The possibilities of using reinforced polyacetal plates for better fracture fixation are discussed.