We hypothesise that the stiffness of the acetabular component influences the stresses transmitted to bone. Thus stress shielding or stress overload of the underlying host bone may be influenced by the choice of fixation method. In addition, we believe that the so called “brake drum effect” plays a significant role in the development of rim stresses and subsequent failure of fixation.

We have constructed a jig which allows the direct comparison, under controlled conditions, of contact stresses measured behind the acetabular component of polyethylene controls, uncemented metal backe cups and cemented all polyethylene cups, under physiological load. The design of the jig also allows measurement of stresses transmitted to the acetabular rim of the same three prostheses in order to confirm the presence and magnitude of the brake drum effect. The contact stresses are measured by miniature pressure transducers which are inserted through specially drilled holes in the shell of the jig so that the transducer is flush with the prosthesis under test. A total of 6 transducers are arranged in concentric circles radiating away from the prosthetic dome, so that contact stresses may be directly measured in various parts of the acetabular bed under conditions that reproduce as closely as possible the situation in a total hip prosthesis in vivo. Thus our method can be compared to other laboratory and theorectical techniques for investigation into stress transmission around acetabular components. The transducers were callibrated prior to each test to ensure parity of test results. The transducers were prestressed to ensure contact prior to each test. 6 polyethylene uncemented liners were tested alone in the jig to act as a control. In the same fashion, the same 6 polyethylene components were tested firstly in an uncemented, metal back acetabular component, and then as a cemented, all polyethylene component.

The results indicate that significantly less stress is transmitted to bone when metal back components are used as compared to cemented components and controls. The data confirms that the brake drum effect occurs in both cemented and uncemented prostheses, leading to at least the absence of compressive forces at the prosthetic rim and in some circumstances tensile forces.

Introduction: We hypothesise that the fixation method of the acetabular component influences stress transmission to the host bone in vivo. We believe that the frequency of appearance of radiolucent lines at the prosthetic rim is directly related to the brake drum effect whereby compressive forces at the dome of a semi-rigid body leads to tensile forces at the rim.

Method: A series of miniature pressure transducers were manufactured and positioned at the prosthetic/ bone interface of an acetabular component of a total hip arthroplasty (THA) in a jig designed to replicated the loading conditions of a THA in vivo. The transducers were arranged in a series of five concentric rings spaced from the centre of the acetabular dome to the prosthetic rim. A total of six transducers was used. Three separate experiments were performed: 1. a polyethylene component alone to act as a control. 2. a polyethylene component surrounded by a cement mantle and 3. a polyethylene component surrounded by a metal-back. A separate jig was constructed to provide a cement mantle of the same thickness as the metal back. The stress transmitted to the host bone was measured in each case.

Results: The results indicated that successively less stress was transmitted when changing from controls to cemented then to metal-backed cups. Both cemented and uncemented cups demonstrated at the very least absence of compressive forces at the prosthetic rim and in some cases tensile forces, indicating that the brake drum effect is likely.

Tissue reaction to wear particles from metal implants may play a major role in the aseptic loosening of implants. We used electron microprobe elemental analysis to determine the chemical composition of wear particles embedded in the soft tissues around hip and knee implants from 11 patients at revision surgery for aseptic loosening. The implants were made of cobalt-chromium-molybdenum alloy or titanium-aluminium-vanadium alloy. Histological examination showed a widespread giant-cell reaction to the particles. Elemental analysis showed that the chemical composition of the particles was different from that of the implanted alloys: cobalt and titanium were reduced, often down to zero, whereas chromium and aluminium persisted. Our findings indicate that corrosion is continually changing the shape, size and chemical composition of the implanted alloy. This may alter the biochemical environment of the tissue surrounding an implant to favour bone resorption.

We report the finding of sodium- and phosphorus-based crystallisation in abnormal human articular cartilage. We prepared five chondromalacic, five osteoarthritic and four macroscopically normal specimens of patellar cartilage by a cryofracturing technique and examined them in a scanning electron microscope. An energy-dispersive X-ray microanalysis system was used to identify the crystals, which were found in only three of the five chondromalacic specimens. Star-shaped crystals were seen either individually or in clusters in the matrix of the cartilage. They consisted of sodium and phosphorus, and we have found no previous reports of such findings. The calcified zone, the bone, and the articular surface were free from crystals.

We report the experimental use of three different biological implants to restore articular surface defects: glutaraldehyde-fixed bovine meniscal xenograft, glutaraldehyde-fixed bovine costal cartilage xenograft, and viable osteochondral allografts. The grafts were implanted in the knees of 19 goats who were allowed free-field activity and were studied for up to one year. The natural articular surfaces of meniscal fibrocartilage provided excellent articular surfaces at all times. Equally good articular surfaces were restored by host tissue growth covering costal cartilage grafts at six months, but by 12 months this surface had degenerated. The majority of the allografts survived and integrated with the host at six months, but many showed signs of failure at 12 months. Only three out of seven ungrafted defects healed completely at six months and the healed surfaces were degenerating at 12 months.