[Fusion implants of carbon fiber reinforced plastic]. / Fusionsimplantate aus kohlenstofffaserverstärktem Kunststoff.

Carbon fiber reinforced plastics (CFRP) are used in the medical field when high mechanical strength, innovative design, and radiolucency (see spinal fusion implants) are needed. During the manufacturing process of the material CFRP carbon fibers are embedded into a resin matrix. This resin material could be thermoset (e.g., epoxy resin EPN/DDS) or thermoplastic (e.g., PEAK). CFRP is biocompatible, radiolucent, and has higher mechanical capabilities compared to other implant materials. This publication demonstrates the manufacturing process of fusion implants made of a thermoset matrix system using a fiber winding process. The material has been used clinically since 1994 for fusion implants of the cervical and lumbar spine. The results of the fusion systems CORNERSTONE-SR C (cervical) and UNION (lumbar) showed no implant-related complications. New implant systems made of this CFRP material are under investigation and are presented.

Simulator trials to determine the wear of the combination aluminium oxide ceramic-carbon fibre reinforced plastic (CFRP) used as an insert in a hip socket.

Hip simulator trials were conducted to determine the initial wear between alumina femoral heads and carbon fibre reinforced plastic (CFRP, CAPROMAN) insert in a titanium socket. A force of 2500 N and a frequency of 0.857 H were applied. Using surface and sphericity measurement techniques, the amount of wear was determined. After 500,000 cycles, the centre of the head had moved by 10 microm into the insert, and the average radius increased by 2 microm. After 1 million cycles, the additional changes were less than 1 microm. Based on an examination of retrieved implants (wear rate: 6.1 microm/year) and based on the simulator results, the combination alumina-CFRP inserts could be approved for total hip replacement.

Tribological investigations of the wear couple alumina-CFRP for total hip replacement.

Wear debris is the main reason for aseptic loosening in total hip. Most troublesome is the wear of polyethylene cups. Ceramic femoral heads were introduced about 20 years ago. The combination ceramic-on-polyethylene reduces the wear rate and the loosening rate. But cups of polyethylene are still the weakest link in a hip prosthesis. Carbon fibre reinforced plastic (CFRP) was proposed as an alternative for polyethylene. Various test were performed to study the combination metal-on-CFRP, zirconia-on-CFRP, and alumina-on-CFRP. The simulator tests showed that the wear rate of alumina-on-CFRP is in the order of 1-3 microm per year. Based on investigation of retrieved implants the wear rate is 6.3 microm per year. Based on these results the combination alumina femoral heads (Biolox-forte) and CFRP cups (Caproman) could be approved for total hip replacement.

Wear characteristics of ceramic-on-ceramic for hip endoprostheses.

Femoral ball heads of aluminium oxide or zirconium oxide ceramics are used for total hip replacement. This application is based on their attractive tribological properties. Standard and state-of-the-art are femoral ball heads made of alumina (Al2O3) articulating against acetabular cups of polyethylene (PE-UHMW) and zirconia (Y-TZP) heads articulating against polyethylene cups. A very attractive wear couple is ceramic-on-ceramic. Some of the various ceramic combinations have been tested using the ring-on-disc method according to ISO 6474. The combination alumina/alumina was found to produce extremely low wear; catastrophic wear was found for the combinations alumina/Y-TZP and Y-TZP/Y-TZP.

[Tribological properties of carbon fiber-reinforced plastic. Experimental and clinical results]. / Tribologische Eigenschaften von kohlenstoffaserverstärktem Kunststoff (CFK). Experimentelle und klinische Ergebnisse.

Wear of the articulating components (especially PE-UHMW) of total hip endoprostheses is the most important technical factor limiting the functional lifetime. To minimize wear debris, ceramic heads, according to ISO 6474 (Al2O3), have been used, from 1969 paired with Al2O3 and since 1975 paired with PE-UHMW. Al2O3 balls articulating with cups made from CFRP have been in clinical use since 1988. Laboratory experiments and in-vivo testing showed minimized wear debris and mild biological response to wear products using CFRP (carbon fiber reinforced plastic) instead of PE-UHMW as the cup material. The articulating surfaces of retrieved ceramic heads (Al2O3-Biolox) and cementless CFRP cups (carbon fiber reinforced plastic, Caproman) were compared using sphericity measurement techniques, scanning electron microscopy (SEM) and roughness measurements (including advanced roughness parameters Rvk or Rpk according to ISO 4287). Altogether, the first results of the clinical study showed that the combination Al2O3-ball/CFRP-cup came up to the expected lower wear rates compared with the conventional combinations. The wear rates are comparable with the combination Al2O3/Al2O3 without the material-related problems of ceramic components in all ceramic combinations.

[Load bearing capacity of CFK hip prosthesis with ceramic femoral heads]. / Belastbarkeit von CFK-Hüftprothesen mit keramischen Kugelköpfen.

Wear of the articulating surfaces of mating components of total hip endoprostheses is the most important technical factor limiting the functional lifetime of the prosthesis. Biolox ceramic femoral heads have been used successfully throughout the world for more than 20 years. In vitro tests and clinical results show that the artificial modular hip joint system employing a Caproman stem and a ball made of Biolox or Biolox -forte is resistant to mechanical failure. The threshold values for burst load and cyclic load prescribed by the FDA are achieved or bettered. Clinical results support those of the in vitro tests. The femoral head made of Biolox or Biolox -forte used in combination with cups and stems made of Caproman thus complies with the required standards.

Wear characteristics of sliding pairs of zirconia (Y-TZP) for hip endoprostheses.

Femoral ball heads made of aluminium or zirconium oxide ceramics are used for total hip replacement. This application is based on their attractive tribological properties. Standard and state-of-the-art with good wear properties are femoral ball heads made of alumina (Al2O3) articulating against polyethylene (PE-UHMW) acetabular cups (since 1974) and zirconia (Y-TZP) heads articulating against polyethylene cups (since 1986). The best tribological combinations are alumina heads articulating against alumina cups. Zirconia has better mechanical properties (for example, bending strength or fracture toughness) than alumina. Therefore R&D is going on to design zirconia acetabular cups. The wear characteristics of zirconia (Y-TZP) articulating against itself were investigated using a ring-on-disc testing device according to ISO 6474. The high wear rate emphasized that medical-grade zirconia (Y-TZP according to ISO/ DIS 13356) should not be used for acetabular cups.

[Biomechanics of healing of cortical autotransplants after intramedullary nailing and device removal]. / Biomechanik der Heilung kortikaler Autotransplantate nach Marknagelung und Materialentfernung.

The aim of the study was to investigate the biomechanical properties of segmental cortical autografts subjected to extracorporeal treatment. For this purpose, 12 beagle dogs were submitted to tibial osteotomy, when 25 mm-long diaphyseal segments of bone were removed and treated outside the body by autoclaving or gamma radiation (Co, 5 kGy). Subsequently, osteosynthesis involving the use of medullary nailing was performed. After 36 weeks, the dogs were killed painlessly and, after removing the nails, the tibias were tested for the biomechanical properties. In comparison with the contralateral tibia, a mean stiffness of 47% and 32% was measured for tibia treated by irradiation and autoclaving, respectively, while bending showed an increase of up to 590%. These results show the need to allow plenty of time for healing in the case of weightbearing transplants. When bealing is undisturbed, no biomechanical differences are found between proximal and distal osteotomies, although in the X-ray image the osteotomy gap (fracture line) remains visible, and scintigraphic activity persists for a longer period. This means that the radiological aspect of the former does not necessarily reflect actual biomechanical strength.

[Preservation and sterilization of cortical bone--biomechanical studies of the rat]. / Konservierung und Sterilisation von Kortikalis--biomechanische Untersuchungen an der Ratte.

The biomechanical properties of cryopreserved, autoclaved, irradiated and chemically treated cortical bone harvested from rat tibiae were evaluated in three-point bending. Biomechanical parameters (maximum bending load, deflection, failure energy and stiffness) are not altered by cryopreservation (-70 degrees C, 28 days) or irradiation with doses of 1 kGy or 5 kGy. The maximum bending load of autoclaved (134 degrees C, 3 min) or irradiated (25 kGy) cortical bone is reduced to 82% and 74% compared that of controls tested immediately after death (100% = 74.4 +/- 6.6 N). Extension of the autoclaving time from 3 to 5 min or irradiation with 50 kGy diminishes the bending load (28% after autoclaving, 48% after irradiation), deflection and failure energy. Stiffness is altered by autoclaving only. If a sterilization process is combined with deep freezing, better biomechanical values are obtained by sterilizing the bone before deep freezing than vice versa. The bending loads for autoclaving and irradiation before deep freezing are 68.8 +/- 9.4 N and 50.8 +/- 7.9 N, as against 51.7 +/- 2.7 N and 43.1 +/- 3.9 N for autoclaving and irradiation of previously cryopreserved bone. Whereas treating bone with tetrahydrofurane does not alter its biomechanical behavior, 80% ethanol leads to a reduction of structural properties.

[Experimental studies of the biomechanical properties of hollow nails and solid nails for interlocking intramedullary nailing in the dog]. / Experimentelle Untersuchungen über die biomechanischen Eigenschaften von Hohlnägeln und Vollnägeln zur verriegelbaren Marknagelung am Hund.

Since locked intramedullary nailing of the tibia in dogs was followed by material failure of the conventional slotted hollow nails within days, special solid nails were developed and tested with respect to their mechanical properties. There was a clear advantage of the solid nails over the hollow nails in terms of bending resistance and yield force. In animal experiments--even in long-term experiments over nine months--no case of material failure occurred with the solid nails. The high torsional and bending resistance of the solid nails suggests the use of a mechanical device which, as our experience shows, permits reliable interlocking without the need for an image intensifier, that is to say, without radiation exposure.

[Biomechanical studies of change in the patellar tendon after transplant removal]. / Biomechanische Untersuchungen zur Veränderung der Patellarsehne nach Transplantatentnahme.

The reconstruction of the anterior cruciate ligament deficient knee with the patellar tendon is the "gold standard". Dislocation of the patella, rupture of the patellar tendon and fracture of the patella were reported to occur. In this biomechanical investigation on the changes of the patellar tendon following harvesting of a graft, 51 sheep knee underwent destructive testing at t0 (n = 11), 4 weeks p.op. (n = 5), 3 months p.op. (n = 14), 6 months p.op. (n = 15), and 12 months p.op. (n = 6). Harvesting of a graft produces a stiffness and strength of the patellar tendon of 50-70% of normal. There was no significant change of free patellar tendon length up to 12 months p.op. The cross-sectional area is definitely increased (p < 0.05). The tensile stress is always above normal, nevertheless the strength shows a massive decline until 6 months p.op. and does not regain normal strength by one year p.op. Stiffness shows comparable biomechanical pattern like tensile stress. There are time-dependent changes, the structural weakness is compensated by an increase of cross-sectional area. There is no restitution of the patellar tendon ad integrum, the remainder is a defect with scar tissue and alterated biomechanical properties. Revision surgery using the same host patellar tendon cannot be recommended.