Porous materials for bone engineering

Maintaining bone geometric and structural integrity is a necessity for normal mobility. After fracture, bone disease or other conditions resulting in skeletal loss or compromise, porous materials offer the possibility for near faultless replacement of the normal bone material. Ceramics, and to a lesser extent, metals, are the predominant porous materials currently used in bone engineering. This latter term is used as a blanket term for orthodontics, orthopedics and related fields in which the replacement of bone is either required or selectively chosen. Because bone is a porous material, there is a physiological rationale for the use of porous materials in its replacement. Moreover, porous bone implant material is advantageous for the early incorporation of the implant into or apposed to the bony tissue surrounding it. There is, however, a difference in the size and extent of the bone and implant porosities for optimal bone incorporation of the material. This review intends to clarify both the nature of and reason for this difference. Accordingly, a review of the principal types of porous materials (organics, ceramics, metals, metallorganics and organoapatites and composites) used in bone engineering will be provided. This will springboard a consideration of the important engineering considerations of material property matching, machining and forming, corrosion and biocompatibility, fatigue and lifecycle, coating, and interfacial properties. The importance of matching the porous material to the particular bone engineering application will then be discussed. In providing this review, the authors hope to bring an appreciation of the complexity of the field to the fore, while also demonstrating how much has already been accomplished due to the efforts of many research groups. The ultimate porous bone implant, perhaps, is yet to be designed; however, there is reason to believe that such a material is not long in coming. We hope to demonstrate some possible pathways to this material.