Long-term biostability of polyacetal (Delrin) implants.

Over the past two decades, polyacetal homopolymer (Delrin) has been used for heart valve occluders and in total hip replacements. Both types of implants are subject to mechanical loads that induce wear, and excessive wear could adversely affect the performance of the device. The present study examined components of Delrin heart valves and hip prostheses that had been recovered from patients after extended periods of use in vivo. The molecular properties of these were compared to non-implanted control components, manufactured during approximately the same time period, including comparison between mechanically loaded and non-loaded regions of the implants. The results of infrared spectroscopy, dilute solution viscosity and gel permeation chromatography provided no evidence of systematic degradation of the Delrin components due to long term exposure to biological fluids and cyclic loading, which would indicate the potential for a late decrease in resistance to wear.

Evaluation of wear in an all-polymer total knee replacement. Part 1: laboratory testing of polyethylene on polyacetal bearing surfaces.

The friction and wear characteristics of a prototype, all-polymer hip prosthesis were evaluated using a multi-station hip-joint simulator. The components consisted of 41 mm diameter polyacetal copolymer balls in combination with acetabular cups of UHMW polyethylene, and were compared to the conventional combination of cobalt-chromium alloy balls against UHMW polyethylene cups. The all-polymer combination exhibited about 20% lower friction and 39% lower wear of the polyethylene cups. The total volume of polymer wear from the all-polymer combination (i.e. including the wear of the polyacetal ball) was 23% less than the volume of polyethylene wear from the metal-polymer components. The appearance of the worn surfaces of the polyethylene cups in the scanning electron microscope were similar for the two combinations of materials, and the surfaces of the polyacetal balls showed only mild wear. Formaldehyde, a product of the degradation of polyacetal, was present in traces in some samples of lubricant from the test chambers with polyacetal balls. In this laboratory model, the friction and wear properties of a polyacetal copolymer ball in combination with an UHMW polyethylene demonstrated good potential for clinical application.

The wear behavior of ion-implanted Ti-6A1-4V against UHMW polyethylene.

Nitrogen-ion-implanted Ti-6A1-4V against UHMW polyethylene was tested on a joint simulator to evaluate the wear behavior under different conditions. It was concluded that the polymer wear rate was not affected by ion implantation. The metallic wear, on the other hand, was substantially reduced by this treatment. It was also found that ion implantation resulted in good protection of the metal surface against third-body wear by PMMA, while the introduction of C.P. titanium particles in the joint initiated tremendous wear. Although ion implantation improved the wear of titanium alloy against UHMW polyethylene, the durability of the implanted layer needs further study.

Difference in tissue response to nitrogen-ion-implanted titanium and c.p. titanium in the abdominal wall of the rat.

Ion implantation modifies the surface properties of different materials. We have compared the biological properties of titanium implanted with nitrogen with those of pure titanium. Implants were inserted in the abdominal wall of rats. The implants with surrounding tissue were excised after 1 and 6 weeks, and embedded in epoxy resin. The bulk metal was removed electrochemically and the tissue cut for light and electron microscopy. Using this technique the implant surface, formed by a thin oxide layer, remains and appears in sections as a dense line. After 1 week both types of implants were surrounded by a fluid space containing proteins and scattered macrophages but few polymorphonuclear granulocytes. The fluid space was wider around ion-implanted titanium (52 +/- 22 microns) than around pure titanium implants (15 +/- 3 microns). After 6 weeks the fluid space had largely disappeared around both type of implants. Around pure titanium implants macrophages and fibroblasts, quantified in 1-micron-thick sections in the light microscope, were present in about the same concentration in the inner tissue zone (within 25 microns from the implant surface). Around ion-implanted titanium macrophages predominated in the inner zone and multinuclear giant cells were present in almost all sections. Around both type of implants fibroblasts increased and macrophages decreased with increasing distance from the surface. In the electron microscope macrophages close to the surface of pure titanium were of small size and had an ultrastructure indicating a low activity. Macrophages close to ion-implanted titanium were large and had an active appearance as indicated by the presence of large amounts of endoplasmic reticulum and large Golgi areas in the cytoplasm. Our observations indicate that modification of the surface properties of titanium implants by ion implantation changes the biological properties.

Morphometrical studies of human femoral condyles.

Thirty-six cadaver knees were examined by various morphometrical methods in order to obtain mathematical and graphical data concerning the geometry of the lower end of the femora. It was found that the paramedian sagittal curves represented magnifications of a certain curvature pattern specific to the medial and the lateral femoral condyle. These curves could be expressed mathematically for each condyle. Furthermore, we found dimensional as well as geometrical intervariations between the different size groups in the knee sample. This could serve as an argument for preferring a different design for medial and lateral condyle components in knee prostheses.

Wear of ion-implanted pure titanium against UHMWPE.

Six specimens were manufactured from commercially pure titanium (grade IV) to resemble femoral head prostheses, and five of them ion implanted to increase the wear properties. Together with three cobalt-chrome controls they were run for one million cycles on a 10-station joint simulator. For the first time both the polymer and metallic wear were quantified in the same study. The purpose of the investigation was to estimate the potential for using commercially pure titanium as a bearing surface against ultrahigh-molecular-weight polyethylene for total joint application. It was concluded that the wear properties of titanium were improved by ion implantation, and that these positive effects could probably be enhanced with development of a better polishing technique, and by finding methods of extending the penetration depth of the nitrogen ions.

Osseointegrated knee prostheses. An experimental study in rabbits.

Osseointegration, defined as direct bone-to-implant contact, has been suggested as an approach for improving the results of arthroplasties of the major joints. An experimental investigation in femoral-patellar joint of the rabbit was performed to evaluate whether osseointegration could be achieved in a one-stage surgical procedure. After three months, it was found that the screws used for anchorage of the prosthesis showed an average direct bone-to-implant contact of 60% in three selected consecutive threads on each side. The range was 40-90%. These figures correspond well with previously reported ones from bone anchored titanium fixtures. It was therefore concluded that osseointegration of dynamically loaded screws used for anchorage of a femoral-patellar joint prosthesis may be achieved in spite of the fact that no controlled healing period for the screws was observed.

Implant fixation improved by close fit. Cylindrical implant-bone interface studied in rabbits.

Cylindric titanium implants of three different diameters were inserted and stabilized in a 3.7-mm burr hole in the rabbit tibia. The purpose of the study was to investigate the interfacial reaction to screw- and cylinder-shaped implants, and to determine if there is a critical gap at the insertion between bone and implant that prevents direct cortical bone apposition on the implant. The study indicated that this critical gap approached zero.

Acute tissue reactions to potassium alginate with and without colour/flavour additives.

The acute tissue reactions to potassium alginate, locally applied to a microvascular bed, were studied with the use of the vital microscopic hamster cheek pouch model combined with correlative histology. This experimental model permitted us to study microvascular permeability, blood flow, vessel diameters and leucocyte adhesion to vessel walls intravitally, and leucocyte migration and mastcell degranulation histologically. Deionized water alone and potassium alginate with flavour and colour mixed in saline was found to cause severe microvascular alterations, while potassium alginate, without flavour and colour, but mixed in saline and applied to the microvasculature resulted in a minor inflammatory reaction.

Osseointegration of titanium implants.

Forty-eight screws, six double cylindrical implants and six T-plates were inserted into the tibia or femur of 6 dogs. Two titanium screws were inserted into the proximal tibia of 5 patients to anchor a titanium mould. The implants were removed en bloc with adjacent bone tissue after 3 to 14 months. They were sectioned using a technique that allowed analysis of the intact tissue-to-metal specimens. Osseointegration, defined as a direct bone-to-implant contact without interposed soft tissue, was confirmed in all screw-shaped implants while the cylindres had only partial bone contact as did the T-plates. We conclude that osseointegrated implants may be applicable in joint reconstruction for arthrosis or rheumatoid arthritis.

Denervation of the wrist joint--an alternative in conditions of chronic pain.

Nine patients with chronic pain in the wrist secondary to lunatomalacia and non-union of the scaphoid bone were treated by denervation of the wrist joint. This operation was first presented by Wilhelm in 1966. We have been using a modified technique. The purpose of the operation is to achieve relief of pain by selective neurotomy without impairment of hand function and with preservation of wrist mobility. Eight patients were considerably improved; one was not helped at all. No appreciable progression of the pathological changes have occurred radiographically during the period of follow-up. We find our results encouraging and the method worth trying.