A device for assisting in performing an ankle arthroplasty on a non-resected bone surface of a tibia comprising: a posterior surface having a mating surface configured to engage the non-resected bone surface of the tibia in a matching manner on account of a patient-specific nature of the mating surface, the matching manner registering a single relative position of the mating surface relative to the non-resected bone surface of the tibia, the patient specific nature of the mating surface portion having been generated in the device prior to the device being brought into contact with the bone surface of the tibia or employed in the arthroplasty; a first guide member defining a transverse cutting instrument guide slot and having an anterior entrance and a posterior exit, wherein the transverse guide slot is configured to guide a cutting instrument along a transverse planar guide surface that is substantially orthogonal to a tibial mechanical axis of the tibia when the non-resected tibia surface is engaged to the mating surface in the matching manner; and a second guide member coupled to the first guide member , the second guide member defining an angled guide surface angled obliquely to the transverse guide slot, wherein the mating surface has a first curvature that is curved posteriorly from a central area of the mating surface toward a medial and a lateral edge of the mating surface so that the central area of the mating surface is positioned anterior to the medial and lateral edges of the mating surface, the device being characterized in that , when in contact with the non-resected bone surface, the first curvature of the mating surface at least partially wraps around the non-resected bone surface in a medial-to-lateral direction. a posterior surface having a mating surface configured to engage the non-resected bone surface of the tibia in a matching manner on account of a patient-specific nature of the mating surface, the matching manner registering a single relative position of the mating surface relative to the non-resected bone surface of the tibia, the patient specific nature of the mating surface portion having been generated in the device prior to the device being brought into contact with the bone surface of the tibia or employed in the arthroplasty; a first guide member defining a transverse cutting instrument guide slot and having an anterior entrance and a posterior exit, wherein the transverse guide slot is configured to guide a cutting instrument along a transverse planar guide surface that is substantially orthogonal to a tibial mechanical axis of the tibia when the non-resected tibia surface is engaged to the mating surface in the matching manner; and a second guide member coupled to the first guide member , the second guide member defining an angled guide surface angled obliquely to the transverse guide slot, wherein the mating surface has a first curvature that is curved posteriorly from a central area of the mating surface toward a medial and a lateral edge of the mating surface so that the central area of the mating surface is positioned anterior to the medial and lateral edges of the mating surface, the device being characterized in that , when in contact with the non-resected bone surface, the first curvature of the mating surface at least partially wraps around the non-resected bone surface in a medial-to-lateral direction. <EOS>