**3. Implants**

### **3.1 Arthroplasty implants**

Since 1955 there have been several accounts of a variety of implants which were the harbinger of the modern day cervical arthroplasty. Initial reports of disc replacements ranged from methylmethacrylate injections to unconstrained spheres composed of various substances ranging from silicone, rubber, and stainless steel [18–20]. Early arthroplasty designs never achieved much in the form of widespread practical application as they were forsaken after sparse clinical use. However, early success with lumbar disc replacements ushered in a new era of spinal arthroplasty in the cervical spine. In 1991 the Bristol/Cummins disc is credited as the first of the modern articulating Cervical Artificial Disc Replacement (C-ADR) devices which was implanted in 20 patients and was reported to be functional in several for as long as 12 years postoperatively [21]. Since then an array of designs have flooded the marketplace with materials ranging from metal-on-metal, metal-on-plastic, non-articulating metal bonded to plastic, plastic embedded in cloth, polymer fibers wound around a polycarbonate urethane core, and PEEK on PEEK [17, 22–24]. Despite a wide array of designs and formulations, manufacturers have been unable to emulate and reproduce the mechanical and load bearing properties of the innate human disc. Therefore the various axial and shear loads are still being transferred to the index and neighboring adjacent levels. In order to offset these loads different bearing designs have been conceived, each of which vary based on the amount of impedance, restraint and stability they confer to the spinal unit and dorsal facet joints. Implants without any mechanical impedance built in are considered unconstrained and allow for significant mobility while sacrificing some implant stability. Constrained devices impede movement of the spinal unit within the range of normal physiologic motion and infer greater implant stability by removing shear forces on the facet joints, but in turn place significant stress on the surfaces at the vertebral endplate-implant junction. Semiconstrained implants allow motion just outside the physiologic norm in effort to theoretically decrease the mechanical stresses felt at both at the facet joints and the interface between the implant and the bony surfaces. While often successful, these varied designs have also brought with them a concomitant range of complications with documented occurrences of extrusions, heterotopic ossification, osteolysis, and hardware failure [16, 23, 25–28].

### **3.2 Fusion implants**

Following implantation of the disc replacement a successful cervical hybrid arthroplasty is conditional upon a solid foundation in the form of the adjacent fusion. Fusion implants can be grouped into stand alone versus standard plate and interbody cage designs. Often Allograft, Carbon fiber, Polyetheretherketone (PEEK), and titanium (Ti) have been designated for interbody cage designs. Each material varies in regards to their unique biocompatibility, surface topography, osseointegration, and imaging characteristics. Some implant manufacturers are also borrowing traits from other designs in efforts to improve upon attributes they lack. For example some designers are taking the

once inert PEEK cage and bio-actively coating them with either Hydroxyapatite (HA) or Ti, creating a composite design in efforts to improve osseous integration.
