# Biomechanical Performance of the Cemented Hip Stem with Different Surface Finish
## 5 Discussion
### Limitations and Implications  
For further application in evaluating the performance of cemented prosthetic implant with various surface textures, there are some limitations to the proposed method. As presented in the analysis model, the cement-stem interfaces around stem were assumed to have homogenous characteristics. This assumption did not consider the variation of locally substantial bonding between the bone cement and the metallic stem which can be processed to uneven or different surface morphology by blasting method with multiply grit sizes, causing the strength to scatter in a wider range [13; 14]. In addition, the interface characteristics, which were the vital factors in modeling the cemented prosthetic stem model, were experimentally measured as the fracture force over the bonding area of interface specimens, also termed apparent strengths. As indicated in published literature [7; 11; 12; 30], during the preparation of the interface specimens, the use of cements of different viscosity and metal specimens with different surface treatment might produce porosity to some extent at interfaces, thus contributing to the high variation of the interface characteristics. Consequently, as suggested in [14], experiments with smaller scale interface specimens may be more appropriate to describe the micro-mechanical characteristic of the interface as well as the local failure behavior [59].  
Another shortcoming of the study is that the fatigue failure behavior was not concerned as we focused only on static strength tests, and the failure of cemented interfaces and fracture of cement mantles were assessed under physiological gait loading of static mode. Therefore, the analysis results can only reflect the influence of the surface roughness on short-term fixation function of a cemented implant. Clinically, the failure of cemented hip prostheses is well known as a long-term failure scenario, which was characterized as the fatigue fracture propagation of the cement mantles and the progressive debonding of the cemented interface with the increasing gait loadings [4; 5]. In vitro studies on commercial cemented synthetic femurs have observed the formation of micro-cracking initiated at the stem-cement interface and then propagating towards the cortical bone of the femur under cyclic loadings [4; 60]. To examine such fatigue failure behavior, the fatigue properties of the cemented interface associated with the fatigue failure model should be taken into consideration in finite element modeling of the cemented model. Essentially, the fatigue debonding behavior of the stem-cement interface could be modeled by means of the Paris law model integrated with the experimentally measured crack propagation rate under cyclic loadings [61; 62]. It could also be predicted through the use of linear elastic fracture mechanics integrated with the measurement of the fracture property [51; 62].  
To gain deeper insight into fixation failure of the cemented interface, the mutual influence of cement-stem and cement-bone interfaces should be investigated. Our previous study [32; 62] demonstrated the effectiveness of the interface spring element for modeling the debonding behavior of the cement-bone interface, in which the interfacial fracture toughness was quantified by implementing the experimental measured interfacial static strengths into the finite element model of bimaterial specimen. Our future studies will extend the experimental and computational model proposed in this study to investigate the long-term mechanical failure of cemented hip prostheses based on the interaction of two bimaterial interfaces.