136. Relative vertebral motion after one- and two-level cervical disc arthroplasty

Abstract

<h3>BACKGROUND CONTEXT</h3> While cervical disc arthroplasty has been studied for some time, few studies examined the relative motion between respective vertebral bodies. <h3>PURPOSE</h3> The purpose of this study was to examine the effects on the relative motion of the vertebral bodies above, below and at the surgical index level. <h3>STUDY DESIGN/SETTING</h3> Human cervical spines from C2-C7 were subjected to continuous loading cycles under flexion, extension, and lateral bending. Three-dimensional motion data were acquired using electromagnetic sensors at C4-C5, C5-C6 and C6-C7. <h3>PATIENT SAMPLE</h3> Eight planned cadaver cervical spines were subjected to each loading mode under intact, index (C5-C6) and inferior (C6-C7) artificial disc implantations. Specimens displayed degenerated discs with reduced intervertebral disc height. <h3>OUTCOME MEASURES</h3> The motion at C4-C5, C5-C6 and C6-C7 was computed to compare implantation vs intact conditions. Computations involved removal of induced motion by the testing apparatus and subsequently computing the relative motion between vertebral levels. <h3>METHODS</h3> Human cadaver specimens were loaded in extension, flexion and lateral bending under displacement control of 3 mm for 20 cycles at 0.1Hz. The testing configuration permitted orientation and loading of the specimen without removal from the testing apparatus. The loading modes were obtained by rotating the specimen into the loading axis of the testing apparatus. Motion data were collected continuously at 60 frames/second. Artificial discs were implanted by experienced spine surgeons for one- and two-level implantations at C5-C6 and C6-C7. Data were compared using a one-way ANOVA with Dunnett's post-hoc tests. Significance was set at P <0.05. <h3>RESULTS</h3> Based on half of the specimens analyzed, in flexion, a trend (P >0.05) toward increased motion was observed as the number of implanted levels increased. An increase in motion by 44.8% was observed with implantation at the index level relative to the intact condition. Implantation of a device inferiorly resulted in a 43% increase in motion relative to the intact level. In extension, no significant differences were seen regardless of the level of implantation. Lateral bending displayed a trend toward decreased motion though this was not statistically significant. The examination of total relative motion between vertebral bodies may be an improved consideration when evaluating the effects of disc arthroplasty. <h3>CONCLUSIONS</h3> Based on the number of specimens currently analyzed, a trend toward increased flexural motion was observed. Examination of isolated vertebral level motion may lead to unforeseen biomechanical conditions arising at adjacent levels from the index level. Further studies to include increased sample population could offer more insight toward ensuring that overall cervical motion is not unduly influenced. <h3>FDA DEVICE/DRUG STATUS</h3> prodisc C (Approved for this indication)