Strontium in the bone-implant interface.

Abstract

Total hip replacement surgery is being performed on an increasingly large part of the population and at increasingly younger age. Because we live and stay physically active longer, and since hip replacement surgery has become quite successful, the treatment is being offered to progressively more patients. Unfortunately, about 17% of hip replacement surgeries currently involve revisions. Consequently, the longevity of both the primary and revision implant is an issue and warrants further investigation. Implants undergoing early instability or even subsidence correlate with an increased risk of aseptic loosening, subsequently requiring revision. Thus, the goal is early fixation by osseointegration of the implant. For revision implants, this is an even greater challenge since an allograft is often needed during surgery to obtain immediate stability of the implant. Bone grafts are rapidly resorbed. Thus, instability of the prosthesis may develop before new bone formation is well established and can mechanically secure the prosthesis. Strontium is a dual action drug; being both bone anabolic and anti-catabolic. In the form of strontiumranelate, it is used in the treatment of osteoporosis. Strontium may potentially improve the early osseointegration and fixation of implants. This dissertation consists of three studies investigating the effect of strontium at the bone-implant interface. The questions were firstly, what is the optimal delivery method for strontium to the interface, and secondly, can strontium exercise its dual action at the interface? The studies were performed in a cementless, experimental gap model in canine. The effects of strontium were evaluated by histomorphometrical analysis of the osseointegration and mechanical push-out test of implant fixation. Different stereological methods were used for the histomorphometrical analysis of each study. The methods used were reviewed critically and found valid. Study I compared a 5% strontium-substituted hydroxyapatite (HA) coating with an HA coating after 4 weeks and 12 weeks observation time. We examined whether fixation of the implant was improved by the strontium substitution. It was found that fixation of the implant was not improved by the strontium substituted HA coating at any of the two time points. Study II compared a 5% strontium-doped HA bone graft extender with an HA bone graft extender. The bone graft extender was mixed with allograft and impacted around a titanium implant. The objective of this study was to determine whether strontium doping of the bone graft extender could protect the allograft from fast resorption and increase gap healing, leading to the improved fixation of the implant. We found that the strontium doping increased gap healing and protected the allograft, however, results of the mechanical test were inconclusive. The reason might have been that the increased gap healing had not yet reached the implant during the 4 weeks observation time, so ongrowth onto the implant was not improved. Study III investigated the effects of bioactive glass coating with a 0%, 10% or 50% strontium-substitution versus HA coating of grit-blasted titanium alloy implants. The goal was to determine whether fixation of the implant would be improved by the bioactive glass coating, and then further improved by the strontium-substitution of the coating in a dose-dependent manner. Unfortunately, the bioactive glass coating failed, presumably due to aluminum contamination originating from the grit-blasting powder. The HA coated implants were superior in all parameters of osseointegration and the mechanical fixation of the implants. These studies show the importance of performing further experimental investigation. Even when investigating a known agent for use in a new application. Strontium delivered as doping of an HA bone graft extender showed potential as a dual acting agent in the interface. However, delivery methods of strontium to the bone-implant interface clearly need further investigation.