Preliminary Evaluation of Germanium Based Glass Polyalkenoate Cements

Introduction

Glass polyalkenoate cements (GPCs) possess intrinsic characteristics that make them attractive for orthopaedic applications. Conventional aluminosilciate based GPCs, frequently used in dental applications, are contraindicated for orthopaedic use due to the substantial release of Al3+ ions. Consequently, alternative GPC chemistries based on Ca-Sr-Zn-Si glasses have been proposed to enable the expanded clinical utility of these materials, particularly with respect to vertebroplasty (VP) and kyphoplasty (KP); palliative procedures to stabilize vertebral body compression fractures and return mechanical integrity to a vertebral body. The ideal bone cement for VP and KP (i.e. the minimum characteristics for alternative GPCs), is described, at minimum, as being: (i) injectable for 5-10 minutes, (ii) exhibits at least 30MPa of compression strength, (iii) radiopaque, and (iv) produces minimal heat during setting [1, 2]. However, Zn-based GPCs demonstrate an inability to balance such handling properties with the requisite levels of mechanical strength. To elucidate this point, Zn-based GPC compositions that exceeds 30 MPa in compression strength, set too quickly (c. 1-2 minutes) to be clinically practical. The objective of this work is to improve the clinical viability of Zn based GPCs for the applications of VP and KP. Specifically, this requires the prolongation of their setting reactions. To achieve this, we hypothesize that changing the GPC chemistry by adding germanium (Ge) and zirconium (Zr) to the Zn-Si glass system will slow the GPC setting reaction due to their respective abilities to increase polymerization of the glass network and its resistance to acid degradation.

Materials and Methods

Table 1 displays the molar composition of 11 experimental glasses and the ZnGPC control. Glass compositions were synthesized via a melt-quench technique, with the resultant frit ground and sieved to sub 45 µm powder, and subsequently annealed. Cements were prepared by blending powdered glasses with polyacrylic acid (Mw = 12,700 g/mol, 50 wt% concentration with deionized water) at a ratio of 1:0.75. The GPCs were evaluated on the basis of working time (Wt), setting time (St), 24 hour compression strength (σc), setting exotherm (TSET; in ambient temperature of 25 ËšC), and radiopacity. Experimental results were compared using a one-way analysis of variance with a Tukey post-hoc test to determine any significant differences (p=0.05).

Results

It was observed that including Ge in the glass composition significantly increases GPC Wt to 6 mintues and St to 16 minutes (i.e. DG202 vs. ZnGPC control in Table 2). When Zr is also included in the glass composition, the setting reaction is extended even further (DG208: Wt = 10 min, St = 36 min). The influence of Zr on the reactivity of Si based glass networks is strongly compositionally dependent, evident by the considerable difference in handling properties. For example, both Wt and St for DG206 and 207 where significantly shorter than ZnGPC, but contrary to these results, DG204 did not set. The strongest compositions after 24 hours was the ZnGPC control followed by DG209, but the difference between them did not reach the point of significance. Since DG204 failed to set, mechanical data could not be determined. The weakest materials were also the slowest setting ones, with DG203 being the weakest. The quicker setting cements produce the biggest exotherms, where the ZnGPC was the highest at 40 ËšC, 15 ËšC above the ambient temperature. The Ge containing GPCs exhibited modest exotherms, setting between 2-7 ËšC above ambient temperatures. The radiopacity for the GPCs ranged between 1.8 and 3.1 mm of Al, with the Ge containing GPCs being more radiopaque than the Si based compositions.

Discussion and Conclusion

The most significant outcome of this work is the development of Al-free GPCs that meet the minimum characteristics required for ideal VP and KP bone cements. That is to say, the inclusion of Ge and Zr into ZnGPCs produces cements with compression strengths in excess of the required 30 MPa whilst exhibiting working times up to 10 minutes. Although working time is not an accurate representation of cement's injectability, the working times in excess of 5 minutes signify the potential of these materials to attain the desired levels of injectability. In addition to the balance between handling and mechanical properties, these materials are sufficiently radiopaque to yield visual contrast against bone under x-rays, and low setting temperatures significantly reduce the potential of thermal necrosis of bone, compared to current clinical bone cements. This work shows that including both Ge and Zr in Ca-Sr-Zn-Si glasses significantly improves the viability of Zn based GPCs for use in clinical settings.

Table 1 - Glass compositions (mol. fraction). The ZnGPC control is in the first row, followed by the 11 experimental compositions.

Table 2 - Observed properties for the ZnGPC control and the 11 experimental GPC compositions. Radiopacity quantified as equivalent thickness of aluminum.

References

1. Heini, P.F. and U. Berlemann, Bone substitutes in vertebroplasty. European Spine Journal, 2001. 10 Suppl 2: p. S205-13. 2. Jansen, J., et al., Injectable calcium phosphate cement for bone repair and implant fixation. Orthopedic Clinics of North America, 2005. 36(1): p. 89-95.

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