Bone–implant interface strength and osseointegration: Biodegradable magnesium alloy versus standard titanium control

doi:10.1016/j.actbio.2010.08.020

Acta Biomaterialia

Volume 7, Issue 1, January 2011, Pages 432-440

https://doi.org/10.1016/j.actbio.2010.08.020 Get rights and content

Abstract

Previous research on the feasibility of using biodegradable magnesium alloys for bone implant applications mainly focused on biocompatibility and corrosion resistance. However, successful clinical employment of endosseous implants is largely dependent on biological fixation and anchorage in host bone to withstand functional loading. In the present study, we therefore aimed to investigate whether bone–implant interface strength and osseointegration of a novel biodegradable magnesium alloy (Mg–Y–Nd–HRE, based on WE43) is comparable to that of a titanium control (Ti–6Al–7Nb) currently in clinical use. Biomechanical push-out testing, microfocus computed tomography and scanning electron microscopy were performed in 72 Sprague–Dawley rats 4, 12 and 24 weeks after implantation to address this question. Additionally, blood smears were obtained from each rat at sacrifice to detect potential systemic inflammatory reactions. Push-out testing revealed highly significantly greater maximum push-out force, ultimate shear strength and energy absorption to failure in magnesium alloy rods than in titanium controls after each implantation period. Microfocus computed tomography showed significantly higher bone–implant contact and bone volume per tissue volume in magnesium alloy implants as well. Direct bone–implant contact was verified by histological examination. In addition, no systemic inflammatory reactions were observed in any of the animals. We conclude that the tested biodegradable implant is superior to the titanium control with respect to both bone–implant interface strength and osseointegration. These results suggest that the investigated biodegradable magnesium alloy not only achieves enhanced bone response but also excellent interfacial strength and thus fulfils two critical requirements for bone implant applications.

Introduction

Commonly used titanium and stainless steel implants as well as currently approved bioabsorbable polymers have specific drawbacks in bone surgery. The former are associated with stress shielding phenomena [1] and, particularly in paediatric and adolescent patients, require a second intervention for implant removal. The latter may cause adverse tissue reactions [2], [3], [4] and are not suitable for load-bearing applications due to their limited mechanical properties [5], [6]. Thus, new implant materials combining excellent strength retention properties, biodegradability and improved biocompatibility are desired in orthopaedic and trauma surgery.

Biodegradable magnesium alloys – a new class of degradable biomaterials – may be promising candidates and have recently attracted much attention. In contrast to commonly used titanium or steel implants, biodegradable magnesium alloys obviate the need for a second surgical intervention for implant removal and minimize stress shielding effects due to their elastic modulus, which is close to that of bone [1]. Furthermore, biodegradable magnesium alloys are more suitable for load-bearing applications due to their excellent mechanical properties [1]. The low corrosion resistance of magnesium – the major limitation for its use in musculoskeletal surgery – has been improved by appropriate alloy composition [7], [8], [9], [10], as well as by surface treatments and coatings [11], [12], [13], [14], [15], during the last few years. Moreover, several authors reported good biocompatibility [7], [8], [9], [16], [17], [18], [19], [20] of these alloys and there is some evidence that magnesium may have stimulatory effects on new bone formation [1], [8], [21].

The achievement of a mechanically stable bone–implant interface is particularly critical to the successful clinical use of orthopaedic implants. Hence, with regard to the feasibility of biodegradable magnesium alloys in musculoskeletal surgery, it is of utmost importance to assess their bone implant fixation strength in vivo and to relate these findings to those obtained for currently used titanium implants. Although the mechanical quality of the bone–implant interface can have a dramatic effect on the success or failure of the implanted degradable device, this issue has not yet been addressed.

Thus, it was the aim of the present study to evaluate bone–implant interface strength and osseointegration of a biodegradable magnesium alloy implant in a rat transcortical model. The study hypothesis was that the investigated magnesium alloy exhibits properties at least equal to those of a standard titanium rod.

Section snippets

Implants

In the present study, unthreaded uncoated smooth cylindrical-shaped pins were used. The investigated degradable implant was made of a new biodegradable magnesium alloy (chemical composition: Mg–Y–Nd–HRE; Young’s modulus 45 GPa, yield strength 200 MPa, ultimate tensile strength 280 MPa; surface roughness according to DIN EN ISO 4287: Ra = 0.76 ± 0.09 μm) based on the composition of WE43, developed and manufactured by Magnesium Elektron UK (Manchester, UK) and supplied by Synthes Corporation (Oberdorf,

Results

Seven of the 72 rats were lost intraoperatively due to anaesthetic complications or femoral fracture in consequence of drilling. All other animals tolerated the operation well and regained full weight-bearing capability. Healing occurred uneventfully and no signs of local inflammation, gross infection or tissue reaction could be observed clinically throughout the implantation period.

Discussion

In this study, we have shown that bone–implant interface strength and osseointegration of the investigated biodegradable magnesium alloy is significantly greater than that of a standard titanium alloy control in a rat transcortical model. We also demonstrated that in vivo degradation of this magnesium implant neither induces a systemic inflammatory reaction nor affects the cellular blood composition.

Conclusions

In the present study, we have shown that the tested biodegradable magnesium alloy rods yielded a significantly higher bone–implant interface strength than titanium controls, expressed by three well-established biomechanical parameters. Furthermore, μCT revealed an enhanced bone response to the investigated magnesium alloy, as indicated by significantly increased bone implant contact and higher BV/TV. In addition, no severe inflammatory reactions were observed in any test animal. Our results

Acknowledgements

The authors would like to thank Dipl.-Ing. Gerald Holzlechner for designing the test fixture and Dr. Michael Jamek for excellent technical assistance. Additionally we would like to thank Mag. Stefan Tangl for the histological work-up of the specimen. The study was partially financed by a third-party fund provided by the Synthes GmbH in Oberdorf, Switzerland and by the Lorentz Boehler Fund provided by the Lorentz Boehler Foundation, Vienna, Austria. S.B. is employed by the Synthes GmbH,

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¹: These authors contributed equally to this work.

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