The main objectives of optimising a mechanical system are to reduce costs and increase performance. Thanks to additive manufacturing, it is possible to exploit shape optimisation to place the material where it is needed. This optimisation implies a gain in material and mass but also changes in mechanical behaviour, for example in the connections. Thus, the boundary conditions on the functional surfaces are different before and after the topological optimisation. The objective of this paper is to show that following a topological optimisation, the functional surfaces (i.e. the non-design spaces) have to be redesigned and/or adapted. For this purpose, it is proposed to study a mechanism using two approaches. The first is the simple optimisation of a mechanism along different optimisation paths. The second approach consists in optimising while considering the inter-linkage efforts. The proposed methodology will be applied on a case study which will demonstrate the impact on the performance with an improvement of the functional surfaces. The result of the study shows that by adapting these nondesign spaces, the mechanical strength of the parts is improved with the same gain in mass.