Cervical disc arthroplasty (CDA) is a well known treatment for single level cervical disc degeneration. In multi level disc degeneration anterior cervical disc fusion (ACDF) is often combined with a CDA. In this hybrid construction, motion is preserved at the level of the CDA. This will give less stiffness in the neck and decreases stress at the adjacent level. However, there is a lack of literature about the long term efficacy, safety and complication rate of CDA in hybrid constructions. In this article we present two cases of spontaneous destruction of a M6-C prosthesis in a hybrid construction 5 years after surgery. One reason seems to be the polymer sheath who is used in the M6 prosthesis. Degradation of this sheath causes foreign body reactions with osteolysis in the vertebral bodies. However, the increased mechanical loading at the adjacent levels on the CDA due to the fused vertebrae could also be one of the reasons.