Biomedical subjects
Publications and source records attributed to G Willmann.
Hydroxyapatite ceramic for surgical implants is a material that meets the ASTM F 1185-88 standard. A standard defines specifications for the manufacturer, in particular in the area of quality control. A standard like this provides the necessary basis for clinical use, but only the approval of the public health department provides a fully adequate basis for broader application.
OBJECTIVE: In THA, ceramic-on-ceramic wear couples are increasingly used. A restricted range of motion (ROM) due to unfavourable implant design or positioning may cause impingement or dislocation, which can result in failure of ceramic inserts. METHODS: By means of a 3-D CAD program different hip joint movements were simulated and the effects of ceramic hip implant design and position on the range of motion were analysed. RESULTS: To offer sufficient ROM and to minimise risk of impingement and dislocation, inclination angle of the acetabular cup should be 45 degrees, cup anteversion 15 degrees and stem antetorsion 0 degrees to 10 degrees. In regard to implant design, acetabular cups with slightly- recessed ceramic inserts should be used. Prosthetic systems with an elevated liner or with a mushroom-shaped femoral head are associated with limited ROM and increased risk of mechanical failure. The ratio of head to neck diameter should never be less than 2 : 1. Larger heads not only increase ROM, but also the stability of the prosthesis against dislocation. Thereby, the wear rate of ceramic-on-ceramic couples is not increased, in contrast to polyethylene. CONCLUSION: Considering certain criteria for ceramic hip implants regarding implant positioning, design and handling, ceramic-on-ceramic couples can be used with low risk of revision surgery and they can also reduce the prosthesis loosening associated with wear in young and active patients.
INTRODUCTION: Cementless hip arthroplasty is not completely satisfactory--even with macroporous structured surface. Medium term results are sometimes disappointing because of insufficient secondary stability. Thus our aim was to improve fixation by additional coating with hydroxyapatite. METHODS: 200 consecutive patients who had hydroxyapatite-coated cementless hip replacement with a macroporous hip prosthesis (09/92-03/96) were studied prospectively. All patients were included in a prospective follow up schedule according to the criteria of Johnston et al. As measure of clinical outcome we calculated Harris Hip Score as well as Enghs Score to assess radiologic fixation and stability. RESULTS: 91% could be followed up regularly. No revision because of aseptic loosening had to be done. Analysis of clinical results showed almost painless patients from early after the operation--especially no thigh pain. Average HHS after 2 years was 97. Radiological evaluation showed early and complete osteointegration of all components. According to Engh-Score they are stable and well fixed. CONCLUSION: HA coated macroporous implants provide some striking advantages, which encouraged us to continue with this system. By early and secure bony ingrowth a fibrous interface is avoided and thereby also long-lasting thigh pain.
Plasma spraying of hydroxyapatite coatings onto hip joint and knee endoprosthesis implants and dental implants is state of the art. Nevertheless, there is still a lack of data on the long-term behavior of such coatings. Using the rotating-bending test DIN 50,113 it is shown that parts plasma-sprayed with 200 microns OSPROVIT exhibit delamination only after fracture of the metallic component. At a load of 400 MPa more than 1.10(9) loading cycles were achieved.
Stems and cups for total hip replacement (THR) must be coated with hydroxyapatite (HA), which enhances the osseo-integration of the implant. The aim of the present report was to investigate the basic influence of coating on the biomechanical situation, for example, the mechanical stress in the femur and at the femur-implant interface. Using the finite element method, three types of stems for THR were investigated: non-coated, completely HA-coated, and proximally HA-coated. All three stems were of the same design, and had identical material properties. In addition, all other conditions were identical. The results show that, in terms of the nature of the coating, the HA-coated stem is biomechanical superior to the non-coated stem, and the proximally coated stem is superior to the completely coated stem.
The present investigation focuses on total hip replacement using ceramic acetabular components. The relationship between the position of the cup and the range of motion (ROM) was investigated. A limited range of motion may cause impingement, which is defined as contact between the femoral neck and the rim of the acetabular cup. Impingement may result in wear, chipping, fracture or dislocation of the femoral head. Joint movements were simulated in a three-dimensional CAD program. The results obtained underscore the importance of correct positioning and design of the cup for achieving a ROM as close to the physiological situation as possible. With ceramic cups, the inclination angle should not be more than 45 degrees, and the antetorsion angle between 10 and 15 degrees. If the cup is too vertical, the risk of dislocation and fracture of the ceramic increases. If, on the other hand, the angle of inclination is too small, flexion and abduction will be greatly limited. The study shows that acetabular components with non-recessed ceramic inserts should not be used. Slight recession of the insert helps to avoid impingement. The ROM is reduced and the risk of impingement appreciably increased when mushroom-shaped femoral heads (XL heads) or ceramic inserts protected by a polyethylene ring are used.