Delrin acetal resins are thermoplastic polymers manufactured by the polymerization of formaldehyde. The use of Delrin in total hip prostheses was first suggested in the mid-1960s. The attraction of Delrin is that the creep resistance is ten times that of ultrahigh molecular weight polyethylene, the most usually used plastic material. In addition, Delrin is about five times harder than ultrahigh molecular weight polyethylene and thus will resist the abrasive action of bone chips and acrylic cement particles.
Since 1970, Delrin acetal polymer has been used in the Christiansen total hip prosthesis and in the Christiansen endoprosthesis. Up to the end of 1977, approximately 7000 surgical procedures had been carried out with the prosthesis. The results have been very good. Prostheses removed for cases of infection or loosening have not shown wear on the articulating Delrin components. Furthermore, histological studies of the surrounding tissue show a benign tissue response similar to that seen around polyethylene implants. The Christiansen prosthesis has been recommended as a Class II device by the U.S. Food and Drug Administration.
Studies on the performance of Delrin in the Christiansen total hip prosthesis have been carried out in a hip joint simulator at the University of Cincinnati. These investigations showed that the Delrin wear rate was similar to the wear rate of polyethylene in the Charnley or Charnley-Müller hip prosthesis configuration. In addition, studies on the simulator showed that although acrylic cement fragments caused deformation and scarring of polyethylene when entrapped between the joint surfaces, a similar phenomenon did not occur for Delrin.
The mechanical properties of Delrin have been investigated over many years, and a comparative table is shown, demonstrating the superiority of Delrin over ultrahigh molecular weight polyethylene, in relation to hardness, creep resistance, and strength.
Wear studies on Delrin in the laboratory have been carried out at several locations. The wear of Delrin has been shown to depend upon the test environment, the wear rate being lower with blood plasma as a lubricant than with distilled water. There is agreement on this between tests carried out at the University of Cincinnati and at the Royal National Orthopaedic Hospital in London. However, tests carried out at the Orthopedics Department of the University of California at Los Angeles (UCLA), show that the wear rate of Delrin does not depend upon the liquid test environment, although much higher wear rates will be obtained if the wear test is carried out under dry conditions. However, the tests at UCLA indicate that the wear rate of Delrin is acceptable for total hip prosthesis end use.
The results, taken in conjunction with the clinical findings, suggest that Delrin performs well in the Christiansen total hip prosthesis configuration. Work is at present under way to study the use of Delrin for a tibial plateau in an unconstrained knee prosthesis.