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 regions 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 micromotion induced by stair-climbing joint loads for various thickness of non-bearing 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 the fibrous differentiation. On the contrary, very little is known on the effect of mechanically non-bearing regions at the interface on the stability of the implant. Methods. A new modelling strategy was adopted, which allows the simulation of soft tissues layers down to 10 μm of thickness. This technique was used in combination with an accurate and extensively validated finite element model 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 μm were found sufficient to compromise the osseointegration on most of the stem surface. Conclusions. This study supports the hypothesis that even thin layers of soft tissue may create micro-movements large enough to activate adverse biological effects.