# Biomechanical Performance of the Cemented Hip Stem with Different Surface Finish
## 5 Discussion
### Effects of Interface Conditions of Stress  
Early studies based on numerical approaches also reported the maximum cement stress in the proximal or distal regions in response to stress within a range of 5.8-16 MPa [36; 39; 43; 44]. The variety of cement stress patterns was probably because different stem designs and loading conditions were used [36; 44]. [PERSON] et al. [44] reported that load distribution in a cemented artificial hip arthroplasty reconstruction was primarily affected by hip joint force and abductor muscles. Inclusions of theilio-tibial muscle force and the other muscle forces had relatively small effects on stress state of cemented hip composites. In their study, the peak cement stress predicted for bonded and debonded stems predicted under different loading configurations were approximately 3.5 and 9.0 MPa, respectively. In current study, the loading configuration was duplicated from previous study [36], in which theilio-tibial muscle forces were also included although they were measured to be smaller than other muscles. Under the same loading conditions, current cemented femoral models predicted that much of cements were stressed within 0-4 MPa, approximately 71%, while the model adopted in study of [PERSON] and [PERSON] predicted that there were 79% of cement to be stressed within 0-3 MPa. In addition, the bonded condition assumed at the cement-stem interface was another factor influencing the cemented femoral model that demonstrated different mechanical behaviors. [PERSON] and [PERSON] [45] reported that the peak cement stress greatly increased from 1.4 to 4.6 MPa when the bonding interface of the analysis model was changed from a fully bonded to debonded state. Similarly, [PERSON] et al. [35] found that the cement stresses generated by model with bonded and debonded interfaces were 2.4 and 4.9 MPa, respectively. In the studies described above, the cemented prosthetic models used in stress analyses were created by implementation of the frictional interface between the cement and the implant, while the friction coefficient was assumed to vary with surface roughness of the implant. As an empirical rule, a friction coefficient of 0.05-0.4 was assumed for polished stems and 0.5-1.0 for roughened stems [46; 47; 48]. This modeling approach adopted by [PERSON] et al. [49] led to the conclusion that roughened stems had a higher risk of cement failure than polished stems. However, these findings were in contrast with the results of a study conducted by [PERSON] et al. [42]. The inconsistent conclusions drawn from the smooth or rough stems were ascribed to the fact that the contact stiffness and frictional characteristics introduced by the surface-to-surface contact modeling approach were determined based on the stiffness of the material and the surface morphology of the bonded interfaces, which influenced the contact regions to a different extent under the applied loads [50]. In the surface contact mode, the joint load was only transferred through the node pairs, which were maintained at sticking or slipping contact under compressive stress. Therefore, the contact regions varying under the assumed contact conditions affected the load transferring path across the interface and then yielded different cement stress patterns, as previously reported [46; 47; 48; 49; 50; 35]. Unlike the surface contact mode with interface bonding conditions assumed for a polished or roughened stem in previous studies, in this study, we simulated the cemented interface by using an elastic layer with adequate stiffness. This was performed because joint load can also be delivered locally from the implant to the cement and bone under tensile loading as long as the node pairs on the bonded interfaces remain in the elastic bond state, unless the local bonding was destroyed, causing substantial separation between the pairs. The elastic interfacial characteristics (tensile and shear strengths) were experimentally verified to vary with the surface finish of the stem.