## 3 Discussion
### Degradation  
Degradation behaviour of biodegradable materials is a critical factor affecting healing of bone fracture. Similar to sterilization, in the course of degradation, the PLA molecules undergo chemical reactions such as bond cleavage, chain crosslinking but, furthermore, oxidation when exposed to UV light, air or oxygen [38]. As a result of this degradation, on the macro level, the PLA rich composite might show deformation or brittle fraction that affect its physicochemical properties. A number of studies investigated the degradation of PLA based composites comprising HA, however, they show conflicting outcome of the impact of bioceramics on degradation mechanism.  
[PERSON] et al. [23] showed that inclusion of HA into PLA matrix could accelerate degradation at the interface of polymer matrix/particles. Similarly, studies by [PERSON] et al. [24] and [PERSON] et al. [25] reported that the inclusion of beta-TCP or soluble calcium phosphate glass led to an increase the degradation rate of PLA; this increase in degradation of PLA with the presence of Ca-P fillers might be due to the hydrophilicity of the fillers and their structural arrangement in the particle/matrix interface [26]. In our study, the distribution of pores in composites substantially increased after incubation for 12 weeks, which was a result of substantial degradation of \(M_{\rm w}\) and weight loss of composites.  
On the other hand, [PERSON] et al. [39] reported that the PLA composites without inclusion of fillers enabled absorption more water and showed greater loss in its weight compared to the composites with included HA or tricalcium phosphate (TCP) fillers, when immersing the materials in simulated body fluid (SBF) for 12 weeks. Similarly, a study by [PERSON] [27] showed a slower degradation of composite composed of PLA and beta-TCP compared to unfilled PLA; this might be due to the fact that mineral incorporation into PLA composite improved the thermal stability of polymer as suggested by [PERSON] et al. [40].  
Besides reduction of \(M_{\rm w}\), our study showed that inclusion of HA led to reduction of composites weight loss in the course of incubation; this indicates that the incorporation of HA in a form of whiskers or hexagonal rods into polymer matrix might prevent its degradation.  
In the course of degradation, the thermal properties of PLA composites were assessed. The \(T\)g reduction from pure polymer to a form of degraded composite was the results of polymeric chain cleavage [41]. Given the fact that above \(T\)g the amorphous polymer (phase) reaches viscoelastic state in which the fragments of polymer chains are able to change its conformation, the longer polymer chain the \(T\)g is greater. Thus, the shorter chains formed in the course of degradation require lower temperature to gain ability for the chain movements that is translated into reduction of \(T\)g. In contrast to some reports [28; 42], our polymers showed no cold crystallization behaviour in the course of degradation. In the second heating, DSC showed only one \(T\)g peak in all tested materials, indicating that the amorphous structure was formed.  
In terms of the compressive strength of our tested composites, it is noticed that the property sometimes raised after degradation compared to their starting counterparts, although it was not substantially change. A study by [PERSON] et al. [26] investigated the degradation of composites made of PLA and nHA and suggested that short molecules of PLA degradation might play a role of plasticizer and improve the toughness of the composites to some extent; this might explain the variation or increase in mechanical properties of composite after degradation.