[Experimental studies on "less rigid" polyacetal plates for fracture fixation (author's transl)].

UNLABELLED

It is a well known fact that rigid and prolonged internal fixation of a fracture with a stiff plate yields osteoporosis. In order to solve this problem, so called "less rigid" plates made of various plastic materials have been tested by some workers experimentally, but they are not widely accepted yet for clinical use, for the plastic material used lacked sufficient strength to secure stable osteosynthesis. This paper presents a series of experimental studies using polyacetal (polyoxymethylene--POM) materials which have relatively larger strength, for the fixation of canine fractures.

MATERIALS AND METHODS

Experiment I--Alteration of material strength in vivo: Dumb-bell shaped test pieces were made of three different POM plastics; 1, acetal copolymer alone (M90), 2, acetal copolymer reinforced with 20% carbon fiber (CR20) and 3. 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 eight months. The results were compared with those before imbedding. Experiment II--Fracture healing study: After applying a POM plate or a stainless-steel plate (AO one-third tubular plate) on an adult canine femur with four stainless-stell 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 three 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.

RESULTS

Experiment I: A certain decrease in tensile strength was noted in the three tested materials. Experiment II: 1) With M90 plates, there was considerable tendency toward delayed union, which would have been due to insufficient strength of the plate. 2) 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 larger callus. 3) The ultimate strength of the united bone following CR20 of AO plate fixation ranged from 50 to 55% of that of the contralateral non-osteotomized femur. There was no significant difference between the two plate groups. 4) 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. 5) The strain at the fracture site was extremely large in the AOI 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 results of the experiments suggest possibilities of using reinforced polyacetal plates for better fracture fixation.