The effect of adding tobramycin to Simplex P cement on femoral stem micromotion as measured by radiostereometric analysis: a 2-year randomized controlled trial

Background

Previous in vitro research on addition of antibiotics to bone cement has found no statistically significant deterioration in mechanical properties. However, no clinical studies have compared the performance of tobramycin-laden bone cement with that of standard bone cement (Simplex P).

Patients and Methods

23 patients (25 hips) were randomized to receive an Exeter (Stryker Orthopaedics) femoral stem cemented with either Simplex P (standard) or Simplex T (tobramycin-laden) cement. There were 2 years of follow-up, with scheduled radiostereometric (RSA) examinations.

Results

All stems migrated distally and showed some degree of retroversion. No clinically significant differences in stem subsidence or retroversion were found between the Simplex T and Simplex P cement groups after 2 years. Overall subsidence was less than in previous studies, probably due to a postponed initial post-surgical examination. Rates of subsidence in both cement groups were consistent with those from previous studies of Exeter stems.

Interpretation

Subsidence of the femoral stem after 2 years was similar in the Simplex T (tobramycin-laden) and Simplex P (standard) groups.When antibiotic-laden bone cement (ABLC) was introduced in 1970, there were concerns that the addition of antibiotic powder to bone cement could compromise the mechanical properties of the cement, and therefore increase the risk of aseptic loosening of arthoplasty components (Murray 1984, Lundberg and Hedlund 2007). Since then, numerous in vitro studies have shown that the addition of less than 2 g of antibiotic to 40 g cement powder has a negligible effect on the mechanical strength and fixation properties of bone cements (Davies and Harris 1991, Klekamp et al. 1999, Bourne 2004). However, there is some debate about the appropriateness of laboratory testing of cement properties (Nottrott et al. 2008) since regulatory standards require that bone cement be tested after 24 h of ageing under dry conditions at 23°C (an environment very unlike that of the human body), and not over an extended time (Nottrott et al. 2008). Furthermore, success in the laboratory does not guarantee long-term clinical success, as seen with the disastrous outcomes with Boneloc cement (Gebuhr et al. 2000). This underscores the need for rigorous clinical testing and in vivo measurements of new products prior to adopting them for routine use (Thanner et al. 1995, Hallan et al. 2006).Large cohort studies have shown that the prophylactic use of ABLC is associated with a lower risk of infection-based revision (Havelin et al. 1995, Engesæter et al. 2003). Unfortunately, such studies may have underestimated the true aseptic loosening-based failure rates, as some patients with loose implants may never undergo revision (Soderman et al. 2001). Radiostereometric analysis (RSA) has also been used to investigate the performance of new antibiotic-laden cements (Adalberth et al. 2002, Hallan et al. 2006). Hallan et al. (2006) compared the extent and patterns of migration between Charnley total hips randomly cemented with either Refobacin-loaded Palamed G or gentamicin-loaded Palacos R, and found similar migration patterns. Such studies are useful for comparing different ABLCs, but studies that directly examine the effect of adding antibiotic to a specific cement on the risk aseptic loosening are needed. Thus, we determined whether the addition of tobramycin to Simplex P cement increases the risk of long-term aseptic loosening as predicted by implant micromotion detected by RSA. We selected tobramycin-laden Simplex cement (Simplex T), as it was a relatively new ABLC introduced into North America at the start of this study in 2003.