**3. Biomechanics**

Cervical disc arthroplasty attempts to replicate the normal kinematics of the subaxial cervical spine, defined as the segments from C3 to C7. The subaxial cervical spine contributes 60% of the flexion/extension motion of the cervical spine with each segment accounting for between 14 to 22 degrees of motion (Dvorak et al, 1993; Dvorak et al, 1991; Penning, 1978). The flexion/extension arc, together with translational movements due to slight relative facet motion, results in coupled motions. Coupled motions also occur with lateral bending and thus axial rotation. These coupled motions result in differences in the center of rotation of each motion segment. Since the center of rotation is not fixed, there are instantaneous centers of rotation. (ICR) Penning (Penning, 1978) established normalized ICR for each segment, which were further defined by Amevo et al (Amevo et al, 1991) The normalized ICR for each segment are shown in figure 5 (Bogduk & Mercer, 2000) and are grossly posterior and inferior to the center of the caudal vertebral endplate. Arthroplasty designs with a ball and socket articulation have a predetermined center of rotation (ie: ProDisc-C). These more constrained designs have to be implanted more precisely to match

The Porous-Coated Motion (PCM) Cervical Arthroplasty (Cervitech, Rockaway, New Jersey) consists of two cobalt-chrome-molybdenum (CoCrMo) endplates that have a titanium calcium phosphate porous coated backing for bone ingrowth. The device is inserted by a "press-fit" method, but the endplates have transverse serrated rows of teeth that resist migration. The bearing surface is an ultra high molecular weight polyethylene (UHMWPE) convex insert of large radius of curvature attached to the inferior endplate which articulates with the polished CoCrMo concave surface of the superior endplate.

Cervical disc arthroplasty attempts to replicate the normal kinematics of the subaxial cervical spine, defined as the segments from C3 to C7. The subaxial cervical spine contributes 60% of the flexion/extension motion of the cervical spine with each segment accounting for between 14 to 22 degrees of motion (Dvorak et al, 1993; Dvorak et al, 1991; Penning, 1978). The flexion/extension arc, together with translational movements due to slight relative facet motion, results in coupled motions. Coupled motions also occur with lateral bending and thus axial rotation. These coupled motions result in differences in the center of rotation of each motion segment. Since the center of rotation is not fixed, there are instantaneous centers of rotation. (ICR) Penning (Penning, 1978) established normalized ICR for each segment, which were further defined by Amevo et al (Amevo et al, 1991) The normalized ICR for each segment are shown in figure 5 (Bogduk & Mercer, 2000) and are grossly posterior and inferior to the center of the caudal vertebral endplate. Arthroplasty designs with a ball and socket articulation have a predetermined center of rotation (ie: ProDisc-C). These more constrained designs have to be implanted more precisely to match

**2.4 Porous-Coated Motion (PCM) Cervical Arthroplasty** 

(Figure 4)

(Courtesy of Paul McAfee, MD)

**3. Biomechanics** 

Fig. 4. Porous Coated Motion (PCM) Cervical Arthroplasty

the physiologic center of rotation to avoid increased strain on the facet joints. (Darden & Raposo, pending)

(Reprinted from: Bogduk N, Mercer S. Clin Biomechanics, 2000)

Fig. 5. Mean instantaneous axes of rotation for each motion segment of the cervical spine depicted with a dot. Two standard deviation range of distribution is located within the enclosed circles shown.

Cervical Disc Arthroplasty 505

Fig. 6-B.

Fig. 6-A, B. AP and lateral radiographs of ProDisc-C

In the cervical spine, the dominant plane of motion is sagittal. Constraint is therefore defined as limitation of anterior-posterior translational motion. (Huang et al, 2003) An

Fig. 6-A.

Fig. 6-A.

Fig. 6-B.

Fig. 6-A, B. AP and lateral radiographs of ProDisc-C

In the cervical spine, the dominant plane of motion is sagittal. Constraint is therefore defined as limitation of anterior-posterior translational motion. (Huang et al, 2003) An

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surgeries at the index level (3.4% ACDF, 1.9% Prestige ST). However, the rate of surgery at adjacent levels was statistically higher for ACDF (3.4% versus 1.1%, p=0.0492). Neurological success, defined as maintenance or improvement in the neurological exam, was better with Prestige ST (92.8%) than ACDF (84.3%). Clinically, the patients were evaluated by Shortform 36 (SF-36), Visual Analogue Scale (VAS) and the Neck Disability Index (NDI). While both groups improved significantly from the preoperative state, there was no statistically significant difference between the groups at final follow-up. Overall success was defined as an NDI improvement ≥ 15 points, maintenance of the neurological status and the absence of implant-related adverse events. The arthroplasty group showed overall success in 79.3% of the patients compared to 67.8% in the ACDF group. As a sidebar, the Prestige ST patients were able to return to work on average at 45 days postoperatively, compared to 61 days for

A small prospective study compared results for Prestige LP and ACDF at a minimum of two years follow-up. Single and multilevel procedures were evaluated by VAS, NDI, SF-36 and Japanese Orthopedic Association scores. Clinically, while both groups improved significantly, there was no statistical difference between them. Motion was preserved in the Prestige LP group at a mean of 13.9° on flexion/extension lateral radiographs of two years.

For the Bryan Cervical Disc Replacement, Goffin et al reported on the European experience, a multicenter, prospective, nonrandomized study, including both single-level and multi-level implants. Ninety-eight patients were evaluated at the 4 to 6 year followup point, 89 single-level patients and 9 two-level patients. The patients maintained improvement clinically at all evaluation periods. Approximately 90% of the patients had good or excellent results by Odum's criteria. The success rate for the arthroplasties, estimated by Kaplan-Meier analysis was 94% at 7 years postoperatively. One patient had removal of the arthroplasty for progressive spinal cord compression due to posterior

The Bryan FDA IDE study results at two years were published by Heller et al (Heller et al, 2009). Four hundred sixty-three patients enrolled, with 242 having a single-level Bryan Cervical Disc Replacement and 221 having single-level ACDF. The Bryan patients had statistically significantly improved NDI and VAS scores compared to the ACDF group at two years follow-up. Other clinical parameters improved equally between the two groups. Overall success at final follow-up was better in the Bryan patients (82.6%) versus the ACDF controls (72.7%), (p=0.010). As with the Prestige IDE patients, the Bryan patients returned to

Riew et al (Riew et al, 2008) evaluated a subset of patients enrolled in the Prestige ST or Bryan IDE studies that were determined to have a cervical myelopathy, defined as being hyperreflexic, having clonus or having a Nurick grade ≥ 1. In most of the patients, the cause of the myelopathy was a disc herniation. Because of enrollment criteria, multilevel cervical disease or patients with ossification of the posterior longitudinal ligament (OPLL) were excluded. A total of 107 patients in both studies were deemed myelopathic and underwent cervical disc arthroplasty. Compared to the ACDF patients, arthroplasty patients with myelopathy showed similar clinical improvement. There were no arthroplasty patients who deteriorated neurologically, suggesting that myelopathy confined to a single disc level without OPLL or retrovertebral osteophytes can be treated

the ACDF patients.

(Peng et al, 2011)

osteophytes. (Goffin et al, 2010)

work sooner than did the ACDF patients.

successfully with cervical disc arthroplasty.

unconstrained arthroplasty would allow unrestricted motion while a fully constrained arthroplasty would allow only flexion/extension without any anterior-posterior translation. However, compared to large joints, the differences in constraint in cervical disc arthroplasties are limited. (Darden & Raposo, pending) (Table 1)


Table 1. Summary of implant features and design characteristics.

A number of arthroplasties have been evaluated biomechanically versus simulated fusion in human cadaveric models. These routinely show increased adjacent segment motion and increased adjacent segment disc pressures in the fusion simulations compared to cervical disc arthroplasty. (DiAngelo et al, 2003; Dmitriev et al, 2005)
