How can migration and subsidence be avoided?
While implant settling is a normal occurrence during the interbody fusion process, any further migration of cages into the adjacent endplates after the process of implant settling is undesirable. This further migration and subsidence can be avoided by secondary stability, which means osseointegration of the cage and solid fusion of the vertebral segment. This process might be influenced by an implant material design featuring lattice and pore structures.
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Bone ingrowth promotes secondary stability
The clinical outcome of lumbar spinal fusion surgery is correlated with bony fusion. [1] Achieving bone integration with an interbody implant is likely to aid fusion and improve implant longevity by limiting subsidence. [2] Any approach using interbody fusion cages therefore should promote bone ingrowth, act as a scaffold and work as a conductive material. [2] The ideal cage design provides a maximized contact surface area between cage and vertebral body, providing a better load distribution, therefore reducing risk of subsidence. [3] Another feature of a higher contact area is providing a bigger scaffold surface for a higher bone ingrowth probability and capability. [4] Taken together, improvement of contact area comes along with a higher potential and probability of secondary stability and complete bone integration of the cage. Thus, resulting in a solid fusion, improved fusion and lower subsidence rates. [5]
Unprocessed surfaces are generally inert and have limited ability to bond and interlock with surrounding bone. [2] Growth on the bone is the direct apposition of bone to the surface, whereas ingrowth involves the interlocking or bone growth “into” the surface of a material, requiring a three-dimensional structure.
Lattice structure may accelerate bone ingrowth
Load bearing implants have multiple requirements, such as low stiffness, high strength and high permeability. Lattice structures are able to reduce the stiffness of the implant / the elastic modulus, which minimizes the risk of subsidence after the process of implant settling while maintaining strength. [6] Lattice structure also increases surface area as scaffold and therfore may increase or inhence the bone ingrowth and consequently may reduce time till complete solid fusion.
Pore structure reduces the elastic modulus and facilitates secondary stability
The elastic modulus of titanium alloy can be reduced by manufacturing it in a pore structure. [7] Besides improving mechanical interlocking between bone and implant, the use of a pore structure goes along with another advantage. The stiffness mismatch between bone and implant, which may lead to stress-shielding, is diminished by utilizing a pore structure. By choosing the adequate porosity, mechanical properties like the properties of natural bone can be obtained. In addition, the presence of an interconnected pore structure in the contact surface between the implant and the bone facilitates the bone tissue growth. [4] Three-dimensionally interconnected pores allow an easy and fast penetration of bone-forming cells, and attachment and proliferation of vascularized new bone, thus providing a strong and durable implant-bone interaction and increased secondary stability. [8], [9]
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