Review article: Polyethylene wear and osteolysis in total hip arthroplasty

Journal of Orthopaedic Surgery, Jun 2001 by Zhu, Y H, Chiu, K Y, Tang, W M

ABSTRACT

Polyethylene wear has been accepted as a major cause of osteolysis in total hip arthroplasty. Submicron particles, which are secondary to abrasive wear, migrate into the effective joint space and stimulate a foreign-body response resulting in bone loss which is mainly mediated by macrophages. Diagnosis depends on serial radiographic evaluation and frequent followup. Polyethylene wear and osteolysis can be prevented by reducing the wear such as using a small femoral head, adaptive polyethylene thickness, suitable surgical techniques, non-polyethylene articulation, etc. The presence of cement or circumferential coatings may also retard the distal migration of particles. Medicines such as NSAIDs and bisphosphonate appear to inhibit the progress of osteolysis. As far as treatment, revision surgery is able to reconstruct the joint by replacing partial or total prosthesis and repair the defect by bone grafting according to intraoperative assessment.

Key words: polyethylene, wear, osteolysis, total hip arthroplasty

INTRODUCTION

Periprosthetic osteolysis after total hip arthroplasty now constitutes one of the most common complications and the leading reason for revision after primary replacement 15,9 and also makes joint reconstruction much more difficult. Although much research work has been done both in animals and patients, the exact mechanism of osteolysis still remains unclear. However, wear particles, especially ultra-high molecular weight polyethylene (UHMWPE) has been gradually accepted to represent a major contributor to the bone loss phenomenon. Research work reveals that many different manifestations of wear are due to the complex interaction of both biological and mechanical factors.

PATHOGENSIS

Initial studies focused on cement failure and mechanical instability as the reasons for postoperative osteolysis. But it is now strongly suggested that biological consequences of wear in both cemented and cementless total hip arthroplasty lead to a series of changes resulting in bone loss,39 due to generation of particles and their access to periprosthetic interface.3

Such a `particle disease' suggests that many kinds of materials are responsible for this process. However, analyses of failed prostheses and the recognition of problems associated with rapidly progressive osteolysis28,2 indicate that polyethylene particles are the major culprit initiating an inflammatory process that results in osteolysis and contributes significantly to implant failure.21

STAGE 1: GENERATION OF PARTICLES

In prosthetic joints, the relevant wear mechanisms include adhesion, abrasion and fatigue. The production of these submicron particles in the hip is mainly secondary to abrasive wear.10 From a mechanical perspective, implant geometry and material properties are the major factors influencing the generation of wear debris and osteolysis that follows. Joint forces and kinematics combined with the contact surface geometries and material properties determine the cyclic stresses that lead to particle generation. Though polyethylene particles are produced in a variety of sizes, studies using different methods have all demonstrated that particles resulting from wear are very small - 90% are less than 10 m with an average diameter of 0.3-0.5 (mu)m which is submicron. 8 The reason is that those larger than 10mm are not easily ingested by the macrophage. Apart from the small size, they are present in large amounts at about 1.4 x 10^sup 10^ in billions to trillions.24

STAGE 2: MIGRATION OF PARTICLES

To understand more accurately the migration of the submicron polyethylene particles, the concept of effective joint space30 including all periprosthetic regions is important. It is not only the space within the hip capsule, but also the entire area surrounding the joint into which particles can escape and still be in contact with bone. It leads to the hypothesis that particulate debris, which often forms primarily at the prosthetic articulation, is able to penetrate the periprosthetic interface and migrate extensively3l That is why lysis can occur at the tip of the femoral stem or at the dome of the acetabulum.

Based on the erroneous premise that the lytic process was regarded as cement disease, the major innovation of the 1980s was to use cementless components for total hip replacement. But for all various cementless femoral components, the femoral osteolysis comes on earlier, progressively and extensively.40 Polyethylene particles may enter into the interface between the prosthesis and the bone through the so-called effective joint surface. With the current development of modern techniques, femoral lysis among patients with cemented femoral components using second or third generation cementing techniques remains low4,32 though it still can be found in patients treated with first generation techniques (Fig. 1). Thus, femoral lysis with cementless components is believed to be more common since its prevalence is significantly higher when compared with cement ones, which suggests that the use of cement with new techniques may protect these femurs against lysis in a way that cementless may not. The explanation seems to be that cement seals off the femoral cavity and delays the ingress of particulate polyethylene.