Stemless prosthesis designs in shoulder arthroplasty and their impact on stability

The use of stemless shoulder prostheses has gained increasing popularity over the past two decades due to their bone-preserving and less invasive approach. Anchorage of the humeral component in the humeral metaphysis requires both sufficient primary stability and an adequate contact area of the implant within the cancellous bone bed to ensure the mandatory osseointegration of the implant. Furthermore, osseointegration depends to a large extent on the patient's bone quality. Hence, stemless prosthesis designs represent a distinct challenge regarding biomechanical implant stability, which requires appropriate patient selection and anatomical reconstruction. While various implant systems have been developed, they can be distinguished in terms of their anchoring principles between impacted designs and screw-in designs. Moreover, certain designs incorporate a collar component that provides surface contact with the cortical frame at the metaphyseal interface, thereby offering a possible cortical force transmission. Initial biomechanical investigations have observed satisfactory primary stability with reasonable micromotion, dependent on bone density. However, comparative studies on primary stability are confined to finite element analyses, which have primarily highlighted differences in the contact area of anchoring components among various systems, as well as the correlation between stability and bone density. Comparative biomechanical studies are imperative for a more concrete evaluation of the stability provided by the anchoring principles of different stemless prosthesis designs.