An implantable sensor assembly (10, 87, 94, 108) comprising: a fixation member (30, 88, 96, 110) expandable from a low profile configuration to an expanded configuration adapted to engage the walls of a vessel to maintain its position in the vessel; a sensor housing containing sensing elements adapted to sense a physiological parameter of a patient, the housing being attached to the fixation member (30, 88, 96, 110) by means comprising: the fixation member (30, 88, 96, 110) having a longitudinally extending attachment strut (44, 90, 100, 116) having a non-circular cross section; the sensor housing having at least one pair of plastically deformable tabs (46, 48) extending from the housing and spaced to define a longitudinally extending channel receptive to the attachment strut (44, 90, 100, 116), the channel being configured and dimensioned to prevent rotation of the attachment strut relative to the sensor housing when the strut is contained within the channel and to align the housing with the attachment strut; at least one of the tabs of at least one of the pairs of tabs having a portion protruding beyond the attachment strut when the attachment strut is contained in the channel, whereby the attachment strut (44, 90, 100, 116) can be placed transversely into the channel and the protruding tab portion may be plastically deformed to overlie the strut to secure the housing to the fixation member (30, 88, 96, 110). a fixation member (30, 88, 96, 110) expandable from a low profile configuration to an expanded configuration adapted to engage the walls of a vessel to maintain its position in the vessel; a sensor housing containing sensing elements adapted to sense a physiological parameter of a patient, the housing being attached to the fixation member (30, 88, 96, 110) by means comprising: the fixation member (30, 88, 96, 110) having a longitudinally extending attachment strut (44, 90, 100, 116) having a non-circular cross section; the sensor housing having at least one pair of plastically deformable tabs (46, 48) extending from the housing and spaced to define a longitudinally extending channel receptive to the attachment strut (44, 90, 100, 116), the channel being configured and dimensioned to prevent rotation of the attachment strut relative to the sensor housing when the strut is contained within the channel and to align the housing with the attachment strut; at least one of the tabs of at least one of the pairs of tabs having a portion protruding beyond the attachment strut when the attachment strut is contained in the channel, whereby the attachment strut (44, 90, 100, 116) can be placed transversely into the channel and the protruding tab portion may be plastically deformed to overlie the strut to secure the housing to the fixation member (30, 88, 96, 110). the fixation member (30, 88, 96, 110) having a longitudinally extending attachment strut (44, 90, 100, 116) having a non-circular cross section; the sensor housing having at least one pair of plastically deformable tabs (46, 48) extending from the housing and spaced to define a longitudinally extending channel receptive to the attachment strut (44, 90, 100, 116), the channel being configured and dimensioned to prevent rotation of the attachment strut relative to the sensor housing when the strut is contained within the channel and to align the housing with the attachment strut; at least one of the tabs of at least one of the pairs of tabs having a portion protruding beyond the attachment strut when the attachment strut is contained in the channel, whereby the attachment strut (44, 90, 100, 116) can be placed transversely into the channel and the protruding tab portion may be plastically deformed to overlie the strut to secure the housing to the fixation member (30, 88, 96, 110).