An implant made in total or in parts of a biodegradable magnesium alloy consisting of Y: 2. Each of the axles is adapted to receive a bearing, and an anvil surface substantially concentric with the rotary axis. The molecular sieve is a natural zeolite of the clinoptilolite or chabasite species having an exchangeable cation. At least one guide aperture directs movement of a surgical instrument, wherein the shape andor position of the guide aperture is based, at least in part, on three or more anatomic reference points associated with the patellar.