**6. Summary**

While autograft use is common for bone grafting procedures, it is associated with limitations, including donor site morbidity, limited quantities, and unsatisfactory biological activity. For ACDF, autograft use has steadily declined in favor of alternatives, primarily structural bone allografts and PEEK cages [18, 30]. Favorably, each has mechanical properties similar to autograft, with comparable elastic modulus and sufficient strength for intended applications. However, *in vivo* models demonstrate lack of osseointegration for PEEK, as well as fibrous tissue growth. Poor integration can lead to graft subsidence and pseudarthrosis, and ultimately pain, immobility and sensory loss. In contrast, structural allografts act as an osteoconductive scaffold demonstrating osseointegration in a rat model and have a long history of successful clinical use. These differences are reflected in clinical outcomes, as detailed in this chapter and summarized in **Table 3**. As shown, fusion rates when using PEEK cages were generally lower than when structural allografts were used. Moreover, the use of PEEK cages, and cages in general, in ACDF surgery presented a significantly higher rate of pseudarthrosis vs. structural allografts, leading to a greater rate for the need for subsequent revision surgery.

Due to lack of osseointegration of PEEK reported both pre-clinically and clinically, researchers have modified its surface or sought other materials in attempt to improve clinical outcomes. There are promising advances in porous and titanium coatings and clinical efficacy is being assessed.

In conclusion, although conventional PEEK cages have similar elastic modulus as structural allografts and autografts, they display poorer osseointegration characteristics compared to human bone implants. Comparative clinical analyses indicate that structural allografts yield higher fusion rates and lower incidence of pseudarthrosis than conventional PEEK cages in ACDF procedures.
