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Short and long-term stabilities of cementless implants are strongly determined by the interfacial load transfer between implants and bone tissue. Stress-shielding effects arise from shear stresses due to the difference of material properties between bone and the implant. It remains difficult to measure the stress field in periprosthetic bone tissue. This study proposes to investigate the dependence of the stress field in periprosthetic bone tissue on i) the implant surface roughness, ii) material properties of bone and of the implant, iii) the bone-implant contact ratio. To do so, a microscale 2-D finite element model of an osseointegrated boneimplant interface was developed where the surface roughness was modeled by a sinusoidal surface. The results show that the isostatic pressure is not affected by the presence of the bone-implant interface while shear stresses arise due to the combined effects of a geometrical singularity (for low surface roughness) and of shear stresses at the bone-implant interface (for high surface roughness). Stress-shielding effects are likely to be more important when the bone-implant contact ratio value is low, which corresponds to a case of relatively low implant stability. Shear stress reach a maximum value at a distance from the interface comprised between 0 and 0.1 time roughness wavelength $\lambda$ and tend to 0 at a distance from the implant surface higher than $\lambda$, independently from bone-implant contact ratio and waviness ratio. A comparison with an analytical model allows validating the numerical results. Future work should use the present approach to model osseointegration phenomena., Comment: Journal of Orthopaedic Research, Wiley, In press

The primary stability of the femoral stem (FS) implant determines the surgical success of cementless 15 hip arthroplasty. During the insertion, a compromise must be found for the number and energy of 16 impacts that should be sufficiently large to obtain an adapted primary stability of the FS and not too high 17 to decrease fracture risk. The aim of this study is to determine whether a hammer instrumented with a 18 force sensor can be used to monitor the insertion of FS. 19 Cementless FS of different sizes were impacted in four artificial femurs with an instrumented hammer, 20 leading to 72 configurations. The impact number when the surgeon empirically felt that the FS was fully 21 inserted was noted Nsurg. The insertion depth E was assessed using video motion tracking and the impact 22 number Nvid corresponding to the end of the insertion was estimated. For each impact, two indicators 23 noted I and D were determined based on the analysis of the variation of the force as a function of time. 24 The pull-out force F was significantly correlated with the indicator I (R${}^2$ =0.67). The variation of D was 25 analyzed using a threshold to determine an impact number Nd, which is shown to be closely related to 26 Nsurg and Nvid, with an average difference of around 0.2. This approach allows to determine i) the moment 27 when the surgeon should stop the impaction procedure in order to obtain an optimal insertion of the FS 28 and ii) the FS implant primary stability. This study paves the way towards the development of a decision 29 support system to assist the surgeon in hip arthroplasty., Comment: Journal of mechanical behavior of biomedical materials, Elsevier, 2018