A spacer device for assisting a surgeon during knee surgery on a patient comprising: a housing that defines an axis (a) and includes an upper wall having an upper surface and a lower surface and a pair of side members (112a-b) that comprise respective, internal, opposed side walls (112a-b) that extend axially and distally from a respective end portion of the lower surface of the upper wall, each of the side walls (112a-b) comprising an inwardly projecting tab (113a-b) at a distal end thereof, the lower surface, the side walls (112a-b) and the inwardly projecting tabs (113a-b) defining a pair of channels (114a-b) therebetween; and a support portion having a first end portion and a second end portion, the second end portion capable of being at least partially disposed between the side walls (112a-b) of the housing and the first end portion including a first pair of lateral projections (124a-b); and in which the upper surface of the upper wall and a first end of the support portion are suitable for positioning adjacent a resected distal femoral surface, a posterior femoral surface and/or a resected proximal tibial surface; wherein the second end portion of the support portion comprises a second pair of lateral projections (125a-b) capable of being at least partly disposed and axially slidable within the pair of channels (114a-b) of the side walls (112a-b) that facilitates axial slidable movement of the support portion relative to the housing to define a first space and a second space between the first pair of lateral projections (124a-b) and the respective inwardly projecting tabs (113a-b) of the side members (112a-b) for receiving one or more spacer elements (201,202) and/or a force sensor therein. a housing that defines an axis (a) and includes an upper wall having an upper surface and a lower surface and a pair of side members (112a-b) that comprise respective, internal, opposed side walls (112a-b) that extend axially and distally from a respective end portion of the lower surface of the upper wall, each of the side walls (112a-b) comprising an inwardly projecting tab (113a-b) at a distal end thereof, the lower surface, the side walls (112a-b) and the inwardly projecting tabs (113a-b) defining a pair of channels (114a-b) therebetween; and a support portion having a first end portion and a second end portion, the second end portion capable of being at least partially disposed between the side walls (112a-b) of the housing and the first end portion including a first pair of lateral projections (124a-b); and in which the upper surface of the upper wall and a first end of the support portion are suitable for positioning adjacent a resected distal femoral surface, a posterior femoral surface and/or a resected proximal tibial surface; wherein the second end portion of the support portion comprises a second pair of lateral projections (125a-b) capable of being at least partly disposed and axially slidable within the pair of channels (114a-b) of the side walls (112a-b) that facilitates axial slidable movement of the support portion relative to the housing to define a first space and a second space between the first pair of lateral projections (124a-b) and the respective inwardly projecting tabs (113a-b) of the side members (112a-b) for receiving one or more spacer elements (201,202) and/or a force sensor therein.