164. Biomechanical analysis of the endplate fixation methods of cervical total disc replacement (TDR) prostheses to shear force expulsion

Abstract

<h3>BACKGROUND CONTEXT</h3> Current methods for achieving immediate endplate fixation with cervical TDR include the use of keels or spiked teeth. Nonphysiologic push-out or expulsion tests are typically used to measure the distraction force and mechanical work required to expulse the device. <h3>PURPOSE</h3> To determine the failure work and load required to expulse the upper and lower components of the 4 different cervical prostheses after undergoing multidirectional cyclic testing. Endplate configurations included single central keel, tripod keel, and spiked teeth. <h3>STUDY DESIGN/SETTING</h3> Each device was implanted in a cervical functional spinal unit (FSU) and cyclically loaded 2,000 times under physiological ranges of combined flexion/extension coupled with axial rotation at a rate of 2.5deg/s. Following cyclic testing, the specimens were disarticulated through the disc space, separated into upper and lower segments and mounted in a robotic testing platform for expulsion testing. <h3>PATIENT SAMPLE</h3> Sixteen (C4-C5) and 16 (C6-C7) FSUs were dissected from 8 male and 8 female cadaveric cervical spines with an average age of 55.9 +/- 9.7 years. Three FSUs were not usable due to poor tissue quality which reduced the sample size to 29 FSUs. <h3>OUTCOME MEASURES</h3> Shear expulsion force and distraction displacement were measured and combined to calculate the expulsion work, the resistive stiffness. The peak expulsion force was also obtained. Normality of the expulsion data was tested using the Shapiro-Wilk test (for p=0.05). If normality failed, a nonparametric Kruskal-Wallis test with Bonferroni correction was used to analyze the data between implant types. An additional test was done to compare the shear expulsion parameters between the upper and lower components of the same implant type. A similar normality test as above was used and a (2 tailed) Mann-Whitney U test was used here when normality failed. <h3>Methods</h3> Four ball and socket type cervical disc prostheses having different endplate fixation features were tested: 1. A clinically used and FDA approved single central keel (SCK), 2. A clinically used (outside the U.S.) low profile teeth (LPT) with domed endplate shape to allow a better fit in case of concave vertebral endplate, 3. A clinically used (outside the U.S.) tripod keel (TK), and 4. Small single keel (SSK). A minimum sample size of 6 implants per prosthesis type were used. Three additional TK disc prostheses and two additional LPT disc prostheses were implanted. A minimum of 6 implants per prosthesis type were used. Specimens were mounted in a robotic testing platform programmed to apply a 50N compressive load to the top surface of the disc, followed by an anterior shear force to the backside of the implant until the device was fully expulsed. <h3>Results</h3> Mean and standard deviations for peak expulsion forces were 69(41)N, 62(21)N, 110(71)N and 78(36)N, expulsion work were 321(225)Nmm, 399(161)Nmm, 463(305)Nmm, and 296(149)Nmm, and resistive stiffness were 81(61)N/mm, 44(42)Nmm, 163(207)Nmm, and 150(149)Nmm. No significant differences occurred between implant types for any of expulsion data. Further, no significant differences were found between the upper and lower components of any implant type except for the expulsion work term of the traditional central keel device (p=0.037). <h3>Conclusions</h3> The distraction failure strength of three different cervical disc prostheses was comparable to an existing FDA approved central-based keel designed disc prosthesis. <h3>FDA DEVICE/DRUG STATUS</h3> Prodisc C (Approved for this indication), Prodisc C Vivo (Investigational/Not approved), Prodisc C Nova (Investigational/Not approved), Prodisc C SK (Investigational/Not approved)