Materials in Orthopaedics

Millions of people worldwide enjoy improved quality of life owing to the implantation of orthopaedic devices that repair or replace parts of their skeleton. Demand for orthopaedic prostheses has increased substantially in the last decade owing to:
 
  • Longer life expectancy, resulting in the need for more implantations, revisions, and replacements;
  • The ageing population being more prone to degenerative bone diseases such as osteoarthritis and rheumatoid arthritis;
  • More young- to middle-aged candidates for implants due to sports injuries and vehicle accidents;
  • Accessible health care systems and improved surgical methods.
 
The development of orthopaedic implants and the growth in the implant market have been possible, in part, by the development of new and improved biomaterials. The global orthopaedics market was estimated to be AUD$15 billion in 2003 and rising 10% per annum. A major segment of the orthopaedics market is total joint prostheses such as hip and knee implants. Of critical importance to the longevity of these implants is their stable fixation in the skeleton. One major approach to fixation is the ingrowth of bone into roughened surfaces or porous coatings on the implant. The fundamental behaviour of bone on such surfaces is strongly influenced by the chemical composition and/or microstructure at the surface.
 
In terms of composition, we have shown that the incorporation of small amounts of biologically significant elements (such as Ca, Si, Mn, etc.) into bioinert ceramics can stimulate bone ingrowth into ceramic single tubes whereas other elements (such as Cr) can retard or even prevent bone ingrowth (see 2003 Annual Report). In terms of microstructure, we have shown that adhesion, proliferation, and migration of fibroblastic and osteoblastic cells on bioinert ceramic surfaces is strongly influenced by grain size and the amount, size (micro vs nano regime), and distribution of surface porosity (see 2004 Annual Report).
 
These two streams of work have now been combined and extended to the study of bone ingrowth into three-dimensional porous constructs having tightly controlled macro-pore geometries and pore surfaces of designed chemical composition and microstructure. Bioactive (hydroxyapatite) and bioinert (zirconia) ceramics containing layers of 250 µm diameter, straight , parallel channels with each layer being rotated 60° to adjacent layers have been implanted into a long-term sheep femoral model. The extent and amount of gross bone ingrowth into retrieved samples has been quantified by micro-CT. Overall, bone ingrowth appears to follow the same mechanism as that determined by the study of bone ingrowth into the single tubes: progressive ingrowth of a front of fibrovascular tissue into the pore leaving behind a layer of mature lamellar bone lining the pore surface (see Figure). The types, amounts, and distribution of tissues formed in the tubes are currently being determined by histological analysis of thin sections taken from the retrieved implants.
 
The development of ceramics having designed surface composition and nano/microstructure may improve fixation of orthopaedic implants in bone and, additionally, may find application in dental, facial, and cranial reconstruction and ligament and tendon repair. Ultimately, this research will contribute to a better quality of life for Australia’s ageing population.
 
 
Micro-CT image of bone ingrowth into 3-D porous bioinert construct
 
Owen Standard