The present invention provides an endoprosthesis for a joint between two bones which cannot be distracted, or which can be distracted only with difficulty. A suitable joint for the endoprosthesis is the sternoclavicular joint. The endoprosthesis has two components, one of which may be attached to the collar bone, and the other which may be attached to the breastbone. One component has a joint head which is insertable into the enclosure part of the second component. The joint head on one component is introduced from the side of the enclosure part on the second component by means of a lateral installation opening in the enclosure part. This arrangement allows little or no distraction of the bones to which the endoprosthesis components are attached when the components are connected.