Even a thin layer of soft tissue may compromise the primary stability of cementless hip stems

Objective. Aim of the present study is to quantify the minimum thickness of mechanically non-bearing reg ions around a cementless prosthesis necessary to loose the implant enough to activate interface tissue differentiation. Design. A finite element model was used to predict the bone-implant micromo tion induced by stair-climbing joint loads for various thickness of non-bea ring tissue fully encapsulating the implant. Background. The results of a few published studies give indications on the amount of bone-implant relative micromotion that is required to initiate th e fibrous differentiation. On the contrary, very little is known on the eff ect of mechanically non-bearing regions at the interface on the stability o f the implant. Methods. A new modelling strategy was adopted, which allows the simulation Of Soft tissues layers down to 10 mum of thickness. This technique was used in combination with an accurate and extensively validated finite element m odel to investigate for an anatomical cementless stem design the effect of the thickness of the soft tissues layer on the induced micro-movements. Results. The stability of the implant was found extremely sensitive to the presence of soft tissue. Soft tissue layers of 300 mum were found sufficien t to compromise the osseointegration on most of the stem surface. Conclusions. This study supports the hypothesis that even thin layers of so ft tissue may create micro-movements large enough to activate adverse biolo gical effects.