Instability arthropathy of the sternoclavicular joint is most commonly managed by rehabilitation, injection, and either arthroscopic or open debridement with or without interposition arthroplasty. When these options fail to achieve symptom relief, excision arthroplasty is an option. This is associated with a risk of persistent instability and incomplete pain relief. We describe two cases in which custom sternoclavicular joint replacement was performed for painful insufficiency of scapular suspension. Although the prostheses remained secure in both cases, the first case was complicated by persistent instability due to insufficient reconstruction of the intrinsic sternoclavicular capsular ligaments. This was recognised and corrected for in the second case, in which stability of the sternoclavicular joint replacement has been noted on more than a 5-year review. We conclude that sternoclavicular joint replacement is an option for patients in whom instability arthropathy remains an intrusive problem in daily life after other conventional treatments have not controlled symptoms. Stability of the joint remains a concern. We offer one method of achieving stability through an iterative design process including the ability to reconstruct the intrinsic capsular ligaments around the articulation.