Introduction

Ultra-high molecular weight polyethylene (UHMWPE) is the sole polymeric material currently used for weight- bearing surfaces in total joint replacement. However, the wear of UHMWPE in knee and hip prostheses after total joint replacement is one of the major restriction factors on the longevity of these implants. In order to minimize the wear of UHMWPE and to improve the longevity of artificial joints, it is necessary to clarify the factors influencing the wear of UHMWPE. A number of studies have investigated the factors influencing the wear of UHMWPE acetabular cup liner in hip prosthesis. Most of these studies, however, have focused on the main articulating surfaces between the femoral head and the polyethylene liner.

Introduction

Ultra-high molecular weight polyethylene (UHMWPE) is the sole polymeric material currently used for weight- bearing surfaces in total joint replacement. However, the wear of UHMWPE in the human body after total joint replacement causes serious clinical and biomechanical reactions. Therefore, the wear phenomenon of UHMWPE is now recognized as one of the major factors restricting the longevity of artificial joints. In order to minimize the wear of UHMWPE and to improve the longevity of artificial joints, it is necessary to clarify the factors influencing the wear mechanism of UHMWPE.

Introduction

Ultra-high molecular weight polyethylene (UHMWPE) is the sole polymeric material currently used for weight- bearing surfaces in total joint replacement. However, the wear of UHMWPE and the polyethylene wear debris generated in the human body after total joint replacement cause serious clinical and biomechanical reactions.

Therefore, the wear phenomenon of UHMWPE in total joint replacement is now recognized as one of the major factors restricting the longevity of these implants. In order to minimize the wear of UHMWPE and to improve the longevity of artificial joints, it is necessary to clarify the factors influencing the wear mechanism of UHMWPE.

Introduction

Wear phenomenon of ultra-high molecular weight polyethylene (UHMWPE) in hip and knee prostheses is one of the major restriction factors on the longevity of these implants. In retrieved hip prostheses with screw holes in the metal acetabular cup for fixation to the pelvis, the generation of cold flow into the screw holes is frequently observed on the backside of the UHMWPE acetabular cup liner. In most retrieved cases, the protruded areas of cold flow on the backside were located on the reverse side of the severely worn and deformed surface of the polyethylene liner. It would appear that the cold flow into screw holes contributes to increase of wear and damages of the polyethylene liner in hip prosthesis.

Ultra-high molecular weight polyethylene (UHMWPE) is the sole polymeric material currently used for weight-bearing surfaces in total joint arthroplasty. However, the wear phenomenon of UHMWPE components in knee and hip prostheses after total joint arthroplasty is one of the major restriction factors on the longevity of these implants. In order to minimize the wear of UHMWPE and to improve the longevity of artificial joints, it is necessary to clarify the factors influencing the wear mechanism of UHMWPE. In the microscopic surface observation of the virgin knee prosthesis with anatomical design, various grades of microscopic surface scratches and defects caused by machining and surface finishing processes during manufacture of the component were observed on the surface of the metallic femoral component [Fig. 1] (C. Cho et al, 2009), although the overall surface were finished at smoother level. It is thought that certain levels of the microscopic surface asperities caused by these surface damages in the metallic femoral component might contribute to increasing and/or accelerating wear of the UHMWPE tibial insert. Therefore, it is necessary to clarify quantitatively the influence of the microscopic surface asperities of the metallic components in virgin artificial joints on the wear of UHMWPE components.

The primary purpose of this study was to investigate the influence of the microscopic surface asperities of the virgin metallic femoral component on the wear of the UHMWPE tibial insert in the virgin knee prosthesis. In this study, the authors focused on the three-dimensional shape of the microscopic surface asperities as a factor influencing the wear mechanism of the UHMWPE tibial insert. The 3D microscopic surface profile measurement of the virgin metallic femoral component using a laser microscope and reproduction of the femoral component surface using 3D CAD software were performed [Fig. 2] in order to produce idealized 3D finite element models of the microscopic surface asperity of the femoral component based on actual measurement data. Elasto-plastic finite element contact analyses between idealized microscopic surface asperities and UHMWPE were also performed in order to investigate the influence of the three-dimensional shape of the microscopic surface asperities of the virgin metallic femoral component on the wear of the UHMWPE tibial insert. The analytical findings of this study suggest that the aspect ratio and shape ratio [Fig. 3] of the microscopic surface asperity of the virgin metallic femoral component have an important influence on increasing and/or accelerating wear of the UHMWPE tibial insert.

It is still unclear whether it is best, when revision surgery is required for replacement of an acetabular component, to treat femoral focal osteolysis with bone-grafting or instead to leave it untreated because the defect is too small and uncontained; the concern is to prevent bone graft from escaping into the hip joint. We hypothesized that progression of osteolysis can halted if the cause of particulate generation is removed and the femoral component is well osseointegrated. We prospectively followed 21 patients (24 hips) who underwent acetabular revision and curetting of proximal femoral osteolysis. These patients were followed for the minimum 3 years (mean, 4.3 years; range, 3–7.4 years).

At the time of the latest follow-up examination, all hips were judged to be stable and to have well-fixed acetabular cups and femoral stems. No hips had significant progression of the osteolytic defect through the follow-up period and none demonstrated any new osteolytic lesion.

Provided that a femoral component is bone ingrown with osseointegration sufficient to provide long-term stability, that the osteolytic defect is in the proximal aspect of the femur, and that the defect is uncontained, simple curettage is an effective alternative to additional bone-grafting.

The rate of failure of primary THA in patients with osteonecrosis of the femoral head is higher than that in patients who undergo THA because of other diagnoses. We examined the results of cementless THA performed with second-generation in a consecutive series of young patients with osteonecrosis of the femoral head.

Sixty-five consecutive primary THAs with insertion of a femoral stem with a circumferential proximal porous coating (HG Multilock prosthesis) and a cementless acetabular component (Harris-Galante II) were performed in 52 patients with osteonecrosis of the femoral head. These patients were followed prospectively and evaluated at a minimum of 10 years after surgery. Four patients (4 hips) died and three patients (3 hips) were lost to follow-up monitoring. The remaining 45 patients (58 hips) had a mean of 11.1 years (range, 10 to 13.4 years) of clinical and radiographic follow-up.

One stem (1.7%) was revised because of aseptic loosening. Eighteen cups (31%) were revised because of excessive polyethylene wear and osteolysis. One hip (1.7%) underwent revision of both acetabular and femoral component because of excessive polyethylene wear and osteolysis. The mean Harrsi Hip Score improved from 49 points before surgery to 92.8 points after surgery in patients who did not undergo reoperation. Osteolysis around the acetabular component was present in 22 hips (37.9%). Femoral osteolysis was seen in 9 hips (15.5%), and there was no osteolysis below the lesser trochanter in any hip.

Circumferentially porous-coated second-generation femoral prostheses provide excellent fixation in young patients with osteonecrosis of the femoral head. However, a high rate of polyethylene wear and osteolysis in these high-risk patients remains a challenging problem.

The wear phenomenon of ultra-high molecular weight polyethylene (UHMWPE) in knee and hip prostheses is one of the major restriction factors on the longevity of theses implants. Despite quite a number of studies on the wear of UHMWPE, the wear mechanism is not clear yet. In order to minimize the wear of UHMWPE and to improve the longevity of artificial joints, it is necessary to clarify the factors influencing the wear mechanism of UHMWPE. Especially for the artificial knee joint with anatomical design, the contact stresses in the UHMWPE tibial insert are generally higher than the yield stress of the material during normal gait. In addition, the predominant types of wear on reported simulator-tested and retrieved UHMWPE tibial inserts are delamination and pitting. These facts suggest that the fatigue fracture that causes micro-cracks both on and below the surface of the UHMWPE tibial insert and the generation of wear particles as fatigue type are closely related to the repeated plastic deformation. On the metallic femoral components of the retrieved knee prostheses with anatomical design, a number of microscopic scratches caused by various factors were observed. It is thought that microscopic surface asperities caused by this surface damage contribute to increasing and/or accelerating wear of the UHMWPE tibial insert. The primary objective of this study was to investigate the factors influencing the wear mechanism of UHMWPE tibial insert in knee prosthesis.

In this study, macroscopic and microscopic elasto-plastic contact analyses of the UHMWPE tibial insert based on macroscopic and microscopic geometrical measurements from retrieved knee prosthesis were performed using finite element method (FEM) in order to investigate the mechanical state, plastic deformation behavior in the UHMWPE tibial insert and microscopic wear of the polyethylene caused by microscopic surface asperity. For this purpose, the determinative method of the contact position between the femoral component and the UHMWPE tibial insert for the retrieved knee prosthesis was developed. The three-dimensional FEM model of the retrieved knee prosthesis with worn contact surfaces was produced. Three-dimensional microscopic surface profile measurements of damaged surface of a retrieved metallic femoral component by using a laser microscope and reproduction of the femoral component surface by using 3D CAD software were performed in order to produce the 3D FEM models of the microscopic asperity based on actual measurement data.

The analytical findings of this study suggest that maximum plastic strain below the surface is closely related to subsurface crack initiation and delamination of the retrieved UHMWPE tibial insert. The worn surface whose macroscopic geometrical congruity had been improved due to wear after joint replacement showed lower contact stress at the macroscopic level. The aspect ratio, shape ratio and indentation depth of the microscopic asperity have a significant effect on increasing and/or accelerating wear on the UHMWPE. Higher aspect ratios, shape ratios and indentation depths cause higher contact stresses and plastic strains in the UHMWPE. These are therefore significant factors influencing the wear mechanism of UHMWPE.