A prosthetic device comprising: a cup including a concave cup surface, the concave cup surface defined by at least one radius of curvature RC; and a head configured to fit within the cup, the head including a convex head surface configured to engage the concave cup surface, the convex head surface defined by at least one radius of curvature RH; in which the device has: (i) an articulation zone provided upon the convex head surface which subtends an angle (α) at the centre of the head, in which the head surface is configured to engage the cup surface in the articulation zone with RH is smaller than RC in the articulation zone, and (ii) a peripheral zone provided on the convex head surface outside of the articulation zone which includes an equatorial zone which subtends an angle (ε) at the centre of the head of at least 10°in which RC differs from RH in the peripheral zone by a second amount that is greater than the first amount but is no greater than 50 μm, and in which the angle (α) subtended by the articulation zone at the centre of the convex head surface is between 45° and 145° and the difference between RC and RH in the articulation zone is a first amount that is no greater than 40 μm, characterised in that the difference between RC and RH remains substantially constant in the equatorial zone, and the peripheral zone includes a transition zone between the articulation and equatorial zones in which the difference between RC and RH gradually increases in the transition zone moving from the articulation zone to the equatorial zone. a cup including a concave cup surface, the concave cup surface defined by at least one radius of curvature RC; and a head configured to fit within the cup, the head including a convex head surface configured to engage the concave cup surface, the convex head surface defined by at least one radius of curvature RH; in which the device has: (i) an articulation zone provided upon the convex head surface which subtends an angle (α) at the centre of the head, in which the head surface is configured to engage the cup surface in the articulation zone with RH is smaller than RC in the articulation zone, and (ii) a peripheral zone provided on the convex head surface outside of the articulation zone which includes an equatorial zone which subtends an angle (ε) at the centre of the head of at least 10°in which RC differs from RH in the peripheral zone by a second amount that is greater than the first amount but is no greater than 50 μm, (i) an articulation zone provided upon the convex head surface which subtends an angle (α) at the centre of the head, in which the head surface is configured to engage the cup surface in the articulation zone with RH is smaller than RC in the articulation zone, and (ii) a peripheral zone provided on the convex head surface outside of the articulation zone which includes an equatorial zone which subtends an angle (ε) at the centre of the head of at least 10°in which RC differs from RH in the peripheral zone by a second amount that is greater than the first amount but is no greater than 50 μm, and in which the angle (α) subtended by the articulation zone at the centre of the convex head surface is between 45° and 145° and the difference between RC and RH in the articulation zone is a first amount that is no greater than 40 μm, characterised in that the difference between RC and RH remains substantially constant in the equatorial zone, and the peripheral zone includes a transition zone between the articulation and equatorial zones in which the difference between RC and RH gradually increases in the transition zone moving from the articulation zone to the equatorial zone.