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Biomedical subjects

G Willmann

Publications and source records attributed to G Willmann.

At least 19 recordsLinked to original sources
PubMed1998

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.

Aluminum Oxide↗
PubMed1998

[Ceramic cups for hip endoprostheses. 5: Consideration of designs].

Using the combination ceramic on ceramic for hip prostheses, the wear rate can be reduced to less than 1 micron per million cycles. More than 20 years of experience are now available for various concepts involving alumina ceramics in THR. In the early days, monolithic ceramic cups were employed. However, since alumina ceramic has an unsatisfactory potential for bone integration, soft tissue forms at the interface, with the result that the sockets migrate and penetrate. To improve bony integration, monoblock cups surfaced with ceramic beads were introduced. For the past 10 years, there has been a trend towards the use of modular acetabular components comprising a metal shell and a liner made of Biolox forte ceramic. Two concepts for the fixation of the ceramic liners are employed: locking of the ceramic in the metal shell (CeraLock) and the sandwich concept with a polyethylene layer interposed between liner and shell. The basic design and important aspects such as diameter, range of motion, and the possibility for revision, clinical experience, and trends are discussed.

Ceramics↗
PubMed1998

[Polyethylene in total endoprosthetics--a dead end for permanent implants?].

Faced with a growing number of younger patients in need of total joint arthroplasty, orthopaedic surgeons are demanding long-term implants for total joint replacement. An assessment of the long-term mechanical behaviour is vivo of all biomaterials in common use in prostheses is in progress. In the present paper, we discuss the long-term reliability of polyethylene (UHMWPE) in joint replacement in terms of tribology and the different biomechanical situation in various human joints. Long-term failure mechanisms for polyethylene in total hip arthoplasty (THA) differ from those seen in total knee arthroplasty (TKA) although the same materials are used. It is thus questionable whether improvements in wear rates of polyethylene in THA would be helpful in TKA.

Biomechanical Phenomena↗
PubMed1998

[Ceramic cups for hip endoprostheses. 4: Never mix and match].

Femoral ball heads for total hip replacement are made of various materials, e.g. cobalt chrome alloy, coated metal, or various ceramics. Standard sockets are made of polyethylene, alumina ceramics, or CFRP. Due to tribological reasons some of the combinations of head and socket that seem possible are prohibited. The approved and prohibited combinations are reviewed and reasons for not approving are discussed.

Acetabulum↗
PubMed1998

Ceramics for total hip replacement--what a surgeon should know.

Numerous clinical results reported in the literature prove that ceramic femoral heads reduce wear, osteolysis, and revision rates in total hip replacements (THRs). Femoral heads and cups made out of alumina ceramics were introduced about 25 years ago, and femoral heads made out of zirconia were introduced about 10 years ago. Today, the wear couples of alumina-on-polyethylene or zirconia-on-polyethylene have become the standard in THR. The wear couple offering the lowest wear rate is alumina-on-alumina. This article discusses the state of the art and lists advantages as well as problems with the various materials used for prostheses in THR.

Aluminum Oxide↗
PubMed1997

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.

Aluminum Oxide↗
PubMed1997

[Standardization of zirconia ceramics for hip endoprostheses].

Bioceramics are used in a wide range of different clinical applications, the most important being femoral heads in total hip replacement. Because of their very attractive mechanical properties, zirconia ceramics (ZrO2) have recently been used as implant material. At the present time, two medical-grade types of zirconia are available: Mg-PSZ and Y-TZP. For the Y-TZP, an international standard (ISO) was issued. The standard is discussed, and the properties that are of importance in the field of orthopaedics are identified.

Biomechanical Phenomena↗
PubMed1997

[Load bearing capacity of CFK hip prosthesis with ceramic femoral heads].

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.

Biomechanical Phenomena↗
PubMed1997

[Ceramic cups for hip endoprostheses. 3: On the problem of osseointegration of monolithic cups].

The combination of ceramic with ceramic offers the option of minimising wear and the need for revision in total hip replacement. At first, uncemented monolithic ceramic sockets were implanted. Some of these did not prove successful. The prerequisites for fixation of an implant by bony integration are discussed i.e. the structural and surface compatibility of an implant. The reasons for the high revision rate associated with monolithic ceramic sockets are discussed. Using monolithic ceramic sockets only contact osteogenesis can in principle be achieved, which does not suffice for good long-term fixation. In many cases, soft tissue can be found at the ceramic socket/bone interface, and the ceramic sockets may as a result migrate and penetrate, creating conditions that elevate the rate of wear to unacceptable levels.

Biomechanical Phenomena↗
PubMed1996

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.

Aluminum↗
PubMed1996

[Ceramic acetabulum cup inserts for hip endoprostheses].

The articulating components of a total hip prosthesis are the spherical femoral head and the acetabular cup. Particularly high rates of wear are seen with cups made of polyethylene, and the abrasion particles cause osteolysis, which often makes surgical revision necessary. Despite considerable progress in the development of total hip replacement systems over the last 20 years, there is still a need to eliminate or further minimize the problem of osteolysis, which is also referred to as polyethylene disease. Clinical experience over the last 20 years has shown that by using Biolox (medical grade alumina) femoral heads and acetabular cups it is possible to achieve very low wear rates, which histological studies have shown to be readily tolerated. The secondary conditions needed to develop modern modular ceramic acetabular cup inserts are discussed. For the fixation of cup inserts, the well known taper fixation concept has proved to be of value. A concept taking into account the needs of the surgeon and the manufacturing constraints applying to ceramic materials is proposed.

Acetabulum↗
PubMed1996

[Design considerations for improving the acetabular cup of the ESKA hip endoprosthesis by using the ceramic/ceramic wear couple].

Since 1987, cementless THR acetabular components with coarsely reticulate surfaces (S&G, ESKA) have been implanted in the Department of Orthopaedic Surgery of the Alfried Krupp Hospital in Essen. By 31st December 1995, 590 of these components had been implanted. Follow-up of the first 51 of these completely cement free THRs revealed loosening in an only single case (2%). Despite these good medium-term results, however, it must be expected that over a ten-year period, the life of the THR will decrease, the probable reason being increasing debris caused by polyethylene (PE) wear leading to aseptic loosening. An alternative to the PE/ceramic system of opposing materials would be ceramic/ceramic. Experience to date has shown that the biological inertness of ceramic does not allow ingrowth and bony integration to take place. Against this background an acetabular component employing a metal backing with the 3-dimensional reticulate surface mentioned above and a Biolox ceramic cup insert articulating with a Biolox ceramic femoral head was designed. The result is a cement free modular THR with optimal bony integration properties and articulating surfaces. To investigate the superiority of this new concept, a comparative study involving two groups of 50 patients each, one group receiving the PE/ceramic system, the other the new ceramic/ceramic system, has been initiated.

Acetabulum↗
PubMed1996

[Ceramic acetabulum components for hip endoprostheses. 2: Evaluating design and safety].

In the field of hip arthroplasty, there is a clearly recognizable towards the use of modular acetabular cups involving metal-backed sockets into which cup inserts made of either polyethylene or alumina ceramic (Biolox forte) can be fitted. The design aspects for this concept have already been discussed in [17]. In the present paper, we discuss the question of component reliability and the criteria for testing acetabular cups with ceramic inserts. To date, neither standards nor government regulations for ceramic cups are available. In the absence of such regulations, those laid down bei the American Food and Drug Administration (FDA), together with our own longterm experience with femoral ceramic heads are used as a guide for the testing of ceramic cup inserts.

Acetabulum↗
PubMed1995

[Increasing the safety of ceramic femoral heads for hip prostheses].

Since 1974 Biolox ceramic femoral ball heads have been used successfully for artificial modular hip joints. The revision rate due to ball head fracture is lower than 0.02%. This is an extremely low value. In this article it is shown how the safety of a ceramic ball head can be improved using the procedures of HIPing, engraving by laser technique, and 100% proof testing. By applying these means the materials properties density, grain size, grain size distribution, and the strength of the ball head, i.e. the fracture load can be improved significantly.

Ceramics↗
PubMed1995

[Materials for hip endoprostheses--alternatives to standard materials].

A review of current clinically applied biomaterials for the femoral heads and acetabular cups of total hip prostheses (UHMWPE, CoCrMo alloys, alumina and zirkonium) in terms of their resistance to wear is presented. Further developments in metallic and ceramic materials over the last few years have now left UHMWPE as the weakest link in the prosthesis material chain. Alternative materials aimed at improving the tribological properties of the femoral head/acetabular head system are presented. New surface hardening and coating techniques for titanium alloys, polymer coatings, optimized UHMWPE, as well as various carbon-fibre-reinforced synthetic materials are described. The advantages, disadvantages and prospects of these materials are compared with those of standard materials and documented in experimental and clinical studies.

Biocompatible Materials↗
PubMed1995

[A new port catheter system of aluminum oxide ceramics].

Implantable port catheter systems are becoming increasingly important, as they often permit out-patient treatment for many indications that would otherwise require hospitalization. Moreover, they also increase the safety/reliability of infusion therapy in critical inpatients. For a variety of reasons, the materials used so far, i.e. steel, titanium and various plastics have not been completely satisfactory. The main disadvantage of metallic systems is the formation of artefacts in tomographic images, while the shortcomings of plastics are mechanical, e.g. chip formation and early membrane failure. Against this background, a port catheter system made of alumina ceramic, which is largely free of the disadvantages of the other materials, was developed. The expected advantages in terms of complication rate and radiological artefacts, were fully confirmed by the evaluation of 160 monitored patients.

Aluminum Oxide↗