Few histological studies on bone substitutes in human cervical spine are available and the biological processes of bone substitutes are not well documented. The authors studied four failure cases of cervical interbody fusion: two cases with hydroxyapatite (HA), one case with beta-tricalcium phosphate ceramic (beta-TCP) and one case with xenograft (bovine bone). Clinical data showed that all the patients experienced neck pain with or without numbness of upper extremity due to fusion failure. Successful fusions were achieved after the salvage surgeries in which autograft were used. Radiographs showed that radiolucent lines were present in all cases. Two HA substitutes fractured without complications. One of them sank into the vertebral body. Some small beta-TCP fragments were found under the microscope. Histological study demonstrated only a few newly formed bones at the interface of the substitutes. The fragments of HA were encapsulated by fibrous tissue. The degradation process and bone regeneration were more active in beta-TCP than in HA. The intertrabecular spaces of bovine bone were filled with fibrous tissue. The results suggest that a porous calcium phosphate ceramic with special design might assure bone ingrowth and meet the mechanical requirements in cervical interbody fusion. The complications of these materials in the cervical spine should be highlighted.