**7. Arthroplasty 3.0**

Third-generation implants have allowed for translation and compression forces that more closely resemble physiologic motion.

The Bryan cervical disc was under development as early as 1997 by Spinal Dynamics Corporation. This implant relied on the preservation of the natural vertebral concavities with convex titanium shells matching them. The convex portion of the implant has a rough porous coating for bony on-growth. The concave surface of the implant is surrounded by a flexible membrane and lubricant to reduce friction and prevent migration of wear debris. The inner polymer nucleus provides a full range of motion while also allowing for a full range of motion but without loading. The Bryan disc eliminated the need for chiseling of keels but required a complex endplate preparation rig and procedure to shape the vertebral endplates. Subsequent implants like M6 likewise require only a small amount of chiseling for stability.

The Orthotic M6 implant has additional design components that allow more physiologic motions and replicate the physiological phenomenon of progressive resistance to motion in all six degrees of freedom (**Figure 4**). This design enables the disc to move in all six degrees, with independent angular rotations (flexionextension, lateral bending, and axial rotation) along with independent translational motions (anterior–posterior and medial-lateral translations), as well as axial compression. This unique compressive ability has been thought to reduce adjacent segment disease specifically.

The M6 is a complex, multi-component implant that contains an artificial nucleus made of Viscoelastic polymer (PCU) designed to simulate the native nucleus structure. It lies adjacent to but is not fixated to two inner titanium endplates. This core nucleus is retained circumferentially between the titanium endplates by a fiber annulus matrix.

This Ultra High Molecular Weight Polyethylene (UHMWPE) fiber matrix is designed to simulate the native annular structure and is wound in a specific pattern, with multiple redundant layers. The matrix is wound around the core and through slots in the two Ti6Al4V titanium alloy inner endplates. Surrounding the flexible portions of the implant is a jacket of viscoelastic polymer (PCU) designed to minimize tissue in-growth and debris migration.

The inner plates are welded to outer plates the surface of which includes low profile fins and are coated with titanium plasma spray (TPS).

**Figure 4.** *The Orthofix M-6 implant allows for compressive axial loading.*
