Introduction: Hydroxyapatite (HA) is a calcium-phosphate compound, which has been used in combination with β-tricalcium-phosphate (β-TCP) because of its biocompatibility and structural properties (e.g. by acting as a scaffold for bone ingrowth) resembling natural, healthy bone. However, HA/β-TCP mineral for implantation in bone is rigid and brittle and its mechanical properties are not comparable to natural healthy bone. Therefore, different attempts to reinforce and improve HA/β-TCP mineral scaffolds have been tried. Biocompatible polymers such as poly-lactic acid and glycolic acid have been added to the mineral scaffolds in order to increase the mechanical strength of the bone substitute without decreasing the porosity of the final composite scaffold. The appropriate ratio of the polymer to the mineral scaffold has not yet been elucidated.
Another possible polymer could be hyaluronic acid (HyA). HyA is a resilient component of the natural skeleton, and low molecular weight (LMw) HyA (30 kDa) has even been shown to increase osteogenic differentiation of mouse calvarial mesenchymal stem cells in vitro. HyA could make up a resilient composite substitute with osteoconductive as well as osteoinductive potential.
This study compares selected 3D-microarchitectural and mechanical properties of composite scaffolds with those of the pure HA/β-TCP.
Materials and Methods: Seven groups of different composite ceramic scaffolds and 1 group of pure HA/β-TCP (Tables 1 and 2) were produced and prepared in cubes (10x10x10 mm) by The Danish Technological Institute, and micro-CT scanned (VivaCT 40, Scanco Medical AG. Basserdorf, Switzerland) followed by 3D-reconstruction of images and 3D-microarchitechtural analysis. The 3D-microarchitectural properties of the composite scaffolds were compared to those of the pure scaffold, HA/β-TCP. The mean porosity, trabecular thickness (TbTh) and structural model indexes (SMI) of the composite scaffolds were calculated. SMI characterizes the 3D structure types of the scaffold as a certain amount of rods and plates. The value of SMI would lie between 0 and 3, when the structure consists of both rods and plates of equal thickness, depending of the volume ratio of rods and plates. An ideal plate structural model will reflect a higher mechanical strength and has a SMI value of 0. TbTh of young, healthy bone is approx. 0.19 ± 0.029 mm. Results were assessed by One-way ANOVA to compare any difference between groups, and p-values less than 0.5 % were considered significant.
Mechanical testing of the scaffolds was performed on an 858 Bionix MTS hydraulic material testing machine (MTS Systems Corporation, Minneapolis, Minnesota). A load cell of 1 kN was used, and a static strain-gauge extensometer (Model 632.11F-20; MTS Systems Corporation) was attached to the testing columns close to the specimen. Data were collected and converted to stress-strain data. Max load, max stress (strength) and Young's Modulus were calculated. The data were assessed by One-way ANOVA, and the post hoc multiple comparisons were adjusted using Bonferoni's test or Dunnett's test as appropriate. A p-value less than 0.5% was considered significant.
Results: 10 and 15% PLA (Groups 3, 4) significantly (p<0.001) increases the strength of HA/β-TCP without critically decreasing the porosity or compromising SMI. The remaining polymers (Groups 2,5,6,7 and 8) do not increase the strenght of HA/β-TCP significantly. Porosity in these groups are significantly different from HA/β-TCP, except for Groups 7 and 8.
Discussion and Conclusions: This study shows that both 10 and15% PLA significantly increases the mechanical strength of the composite scaffold whereas HyA had no effect on the mechanical parameters. PLA reduced scaffold porosity. In our next studies we will investigate the biological activity of the scaffolds, which we have found to hold the same properties as human bone.
Table 1: Selected 3D-microarchitectural properties of pure ceramic scaffold and composite scaffolds. The outer right column in Table 1 shows the results of One-way ANOVA when comparing porosity, TbTh and SMI of the composite scaffolds with that of pure HA/β-TCP (Group 1).
p-values from the comparisons of groups with G1
Pure ceramic (HA/β-TCP) (n = 6)
5% PLA (n = 5)
10% PLA (n = 6)
15% PLA (n = 6)
HyA-LMW (n = 5)
HyA-MMW (n = 6)
5% PLA+HyA-LMW (n = 6)
5% PLA+HyA-MMW (n = 6)
Table 2: Mechanical parameters from the mechanical test. The bottom row of Table 2 shows the results (One-way ANOVA) when comparing mechanical properties of all groups.