The long-term in vivo behavior of polymethyl methacrylate bone cement in total hip arthroplasty

Background and purpose

The long-term success of cemented total hip arthroplasty (THA) has been well established. Improved outcomes, both radiographically and clinically, have resulted mainly from advances in stem design and improvements in operating techniques. However, there is concern about the durability of bone cement in vivo. We evaluated the physical and chemical properties of CMW1 bone cements retrieved from patients undergoing revision THA.

Methods

CMW1 cements were retrieved from 14 patients who underwent acetabular revision because of aseptic loosening. The time in vivo before revision was 7–30 years. The bending properties of the retrieved bone cement were assessed using the three-point bending method. The molecular weight and chemical structure were analyzed by gel permeation chromatography and Fourier-transform infrared spectroscopy. The porosity of the bone cements was evaluated by 3-D microcomputer tomography.

Results

The bending strength decreased with increasing time in vivo and depended on the density of the bone cement, which we assume to be determined by the porosity. There was no correlation between molecular weight and time in vivo. The infrared spectra were similar in the retrieved cements and in the control CMW1 cements.

Interpretation

Our results indicate that polymer chain scission and significant hydrolysis do not occur in CMW1 cement after implantation in vivo, even in the long term. CMW1 cement was stable through long-term implantation and functional loading.The concept behind Charnley low-friction arthroplasty was established in the 1960s, and the fundamental principles have remained unchanged since then. Several clinical studies have recently reported the long-term success of total hip arthroplasty (THA). Wroblewski et al. (2009) reported good results using Charnley low-friction arthroplasty with a follow-up of 30–40 years. Overall, 90% of hips were free from pain, and activity was normal in 59% of the patients. Carrington et al. (2009) reported the results of the Exeter Universal cemented femoral component after 15–17 years. With an endpoint of revision for aseptic loosening, the survivorship at 17 years was 100% for the femoral component and 90% for the acetabular component. With all reasons for reoperation as the endpoint, the survivorship was 81%. A variety of cemented stems designed according to various concepts have been used, and several improvements have been incorporated into the operating techniques (Madey et al. 1997, Noble et al. 1998, Scheerlinck and Casteleyn 2006). Although self-curing polymethyl methacrylate (PMMA) bone cements have been used for fixation of the implants for the past 50 years, the composition of the cements has remained essentially unaltered. The ultra-long clinical and radiographic success of cemented THA may depend on the mechanical and chemical longevity of the bone cements in vivo.Several authors have reported on the in vivo behavior of PMMA bone cement in the implanted joint. Some studies have shown aging of PMMA in vivo. Hughes et al. (2003) showed a decrease in molecular weight and hydrolysis of PMMA associated with long-term implantation. Looney and Park (1986) reported a reduction in flexural strength but not in compressive strength. Fernandez-Fairen and Vazquez (1983) analyzed the compressive properties of the retrieved CMW1 cements and found a decrease in the compressive modulus and strength after long implantation periods. By contrast, Ries et al. (2006) concluded that the most important factor for the mechanical properties of bone cement in vivo is not the implant duration but the porosity. It remains unknown whether the mechanical and chemical properties of bone cement change in vivo, and how these changes affect the long-term outcome of cemented THA. We investigated various properties, including molecular weight, chemical structure, bending properties, density, and porosity in retrieved bone cements.