A hemi-arthroplasty bone j oint implant comprising: a distal first part configured to be implanted into bone, a proximal second part, and a hemi-arthroplasty articulating coupling (121, 123) between the first and second parts, wherein: the second part and the hemi-arthroplasty coupling (100, 123, 121) are removable in situ during revision surgery, and the first part comprises an engagement feature, the implant further comprises a converter comprising: a replacement coupling for replacing the hemi arthroplasty coupling by engagement with the engagement feature, and a replacement second part (220, 210) which is implantable, wherein the replacement coupling is arranged to engage on one side with the engagement feature of the first part and to engage with the replacement second part (210, 220), characterised in that the proximal second part is configured to slide on bone in a translational manner, wherein the implant is configured for multi-axial motion with translational movement of the second part over a bone and rotation of the first part about the articulating coupling. a distal first part configured to be implanted into bone, a proximal second part, and a hemi-arthroplasty articulating coupling (121, 123) between the first and second parts, wherein: the second part and the hemi-arthroplasty coupling (100, 123, 121) are removable in situ during revision surgery, and the first part comprises an engagement feature, the implant further comprises a converter comprising: a replacement coupling for replacing the hemi arthroplasty coupling by engagement with the engagement feature, and a replacement second part (220, 210) which is implantable, wherein the replacement coupling is arranged to engage on one side with the engagement feature of the first part and to engage with the replacement second part (210, 220), characterised in that the proximal second part is configured to slide on bone in a translational manner, wherein the implant is configured for multi-axial motion with translational movement of the second part over a bone and rotation of the first part about the articulating coupling. the second part and the hemi-arthroplasty coupling (100, 123, 121) are removable in situ during revision surgery, and the first part comprises an engagement feature, the implant further comprises a converter comprising: a replacement coupling for replacing the hemi arthroplasty coupling by engagement with the engagement feature, and a replacement second part (220, 210) which is implantable, wherein the replacement coupling is arranged to engage on one side with the engagement feature of the first part and to engage with the replacement second part (210, 220), characterised in that the proximal second part is configured to slide on bone in a translational manner, wherein the implant is configured for multi-axial motion with translational movement of the second part over a bone and rotation of the first part about the articulating coupling. a replacement coupling for replacing the hemi arthroplasty coupling by engagement with the engagement feature, and a replacement second part (220, 210) which is implantable, wherein the replacement coupling is arranged to engage on one side with the engagement feature of the first part and to engage with the replacement second part (210, 220), characterised in that the proximal second part is configured to slide on bone in a translational manner, wherein the implant is configured for multi-axial motion with translational movement of the second part over a bone and rotation of the first part about the articulating coupling.