In a first aspect the present invention provides a glenoid prosthesis assembly for use in shoulder joint arthroplasty to address conditions where deterioration of the rotator cuff mechanism of the shoulder joint is severe, the assembly comprising a first shell component to be affixed to a scapula by fixing means, and a smaller second component adapted to nest within the first component and having an articulation surface for articulating with a humeral head, the first component being over-sized relative to the glenoid surface of a scapula whereby the first component may be first affixed to at least two of the glenoid, acromion and coracoid processes of the scapula in use by the fixing means and the second component may be cemented in place within the first component allowing initial adjustability in the poise of the second component relative to the first component. Amongst other aspects the invention also provides a modular proximal humeral prosthesis system for use in shoulder joint arthroplasty which enables the proximal humeral prosthesis to be changed between anatomical and reverse of anatomical configurations to address differing degrees of shoulder joint deterioration.