**4. Design rationale**

An arthroplasty device must replicate the native disc as much as possible. Three primary considerations include: maintaining intervertebral spacing, allowing for motion with the segment, and maintaining stability with the bones neighboring the segment. The initial stability with screw fixation was the primary focus of early implants while more recent implants relied on press-fit, teeth, and/or keels as well as ligamentum taxis for initial stability. Long-term stability involves ingrowth of bone into porous endplates while at the same time allowing for revision.

The placement of an artificial disc should be done with limited disruption of surrounding anatomy. Arthroplasty by nature relies on the integrity of the neighboring facet joints and ligaments for stability. Likewise, the functioning arthroplasty device should not overload the facets nor unload them.

Replicating motion in all planes but also constraining motion means the device has to mirror physiologic tissue in terms of biomechanics. In addition to allowing loading, flexion/extension, rotation, and lateral bending, the arthroplasty device should optimally allow for translation as well (**Figure 2**). Ideally, the device would have some natural shock absorption for axial forces. This proved to be a limiting factor in early devices but more modern devices have incorporated this.

**Figure 2.** *Flexion/extension views of the Centinel spine ProDisc-C at C5–6 show arthroplasty device flexing and extending with the spine.*

### *Cervical Arthroplasty DOI: http://dx.doi.org/10.5772/intechopen.102964*

The movement within the implant must be balanced by a stable bone-implant interface anchoring the implant. While a fusion allows for the remodeling of bone, arthroplasty is not afforded such long-term stability. The endplates must allow for a proper degree of bony on-growth while maintaining physiologic loads at this interface to reduce implant failure and endplate failure. The resilience of the implant over the patient's life span is also an important factor. In the event of implant failure, the design should allow for minimal impact from this failure and ideally offer a radiographic cue to its existence.

Implant material is another factor that must be considered in normal usage. Materials should be chosen that are biocompatible, durable, minimize wear debris, and have a minimal inflammatory response. Additionally, materials should be selected that minimize diagnostic imaging artifact at the index level, but certainly preserving visualization of the adjacent segments is essential.

## **5. History of arthroplasty**

While fusion has been the gold standard for over sixty years, arthroplasty designs have been developing over a similar time frame. Dr. Ulf Fernstrom studied a spherical intercorporeal endoprosthesis, or simply a stainless-steel ball, placed in the disc space in the late 1950s. He implanted 191 of his "Fernstrom Balls" in the cervical and lumbar spines of 101 patients [35]. The procedure was later abandoned over high failure rates with subsidence, migration, and hypermobility. Methylmethacrylate [36] was used as an alternative to the steel ball but did not gain much traction in the spine world.

Arthroplasty progress was somewhat dormant for approximately 30 years until the stainless-steel ball and socket implants from Bristol/Cummins were developed [37]. These advanced into a ball and trough design that allowed for translational movement to become the commercially available Prestige line from Medtronic. Charite was approved in 2004 as the first FDA-approved commercial spinal arthroplasty device (lumbar spine). Prestige ST was approved in 2007 as the first cervical arthroplasty device. This steel on steel implant was simple but its stainless-steel construction caused significant artifact on MR imaging. Some patients reported clicking sounds from the saddle joint (personal experience). The esthetics and dysphagia of an on-lay plate (Prestige-ST) as well as time-consuming implant procedure with four screw fixation.

Prestige LP was first marketed OUS in 2004 and approved by FDA in 2014. It was a less invasive approach in terms of fixation. As named, the LP design relied on lower-profile press-fit rales and antimigration teeth for fixation. It also had a titanium plasma spray for additional fixation. The implant was also made with a titanium ceramic composite material that provided better imaging characteristics. Arthroplasty implants designed up to this point allowed motion but no elasticity. The elasticity component is key for load-damping properties.

Early arthroplasty devices like the Bristol and Prestige-ST had a prominent four screw construct with a locking mechanism. Subsequent revisions like the Prestige-LP had a lower profile as so named along with no need for screw fixation.

Similar to the trends toward less invasive, more modern implants have also followed the trend toward more physiologic motion. Early arthroplasty devices mirrored general orthopedic implants with two articulating surfaces. In this spine, these first-generation implants relied on metal articulations attached to the endplate above and below the index disc (Bristol and Prestige). The early Bristol disc was a ball and socket which allows lateral bending, rotation, and flexion/extension but not translation. Prestige was created with a trough on the lower articulating surface in order to allow anterior/posterior translation.

General orthopedic implants evolved to incorporate a plastic spacer in hopes of reducing metallic wear debris while also providing better wear characteristics and a minor degree of shock absorption. A high molecular weight polyethylene core was juxtaposed between the metal surfaces. These second-generation devices reduced some of the metal-on-metal concerns but still lacked elasticity like a native disc. The ProDisc returned to a ball and socket approach with the bottom half of the polyethylene core anchored to the inferior endplate. The subsequently released Secure-C preserved the superior ball and socket design but had a saddle design on the inferior endplate articulating surface. This allowed for translation.
