Can A Universal Width Fit all? Exploring Optimal Dimensions For Cervical Disc Arthroplasty Implants

Introduction: Given the wide spectrum of implant widths, which are typically coupled to specific height and/or depth dimensions, and the desire to match native bony anatomy, as opposed to resecting/burring the bones to achieve implant-fit, there is need to clearly delineate subaxial cervical anatomy, especially at superior endplate(SEP). We aimed to determine normal inter-uncinate distance(IUD) across subaxial disc spaces and compare that to width-dimensions of currently marketed arthroplasty devices.

Methods: CT C-spines from a convenience sample of 50 young, healthy patients (29 males, 21 females), aged 23-28 years (mean: 25.1±1.3years), with mean BMI of 30.44kg/m2, for negative trauma evaluations were utilized. Normative reference values for linear parameters on SEP of subaxial cervical vertebrae (C4-C7) were obtained by measuring the IUD across two different reference points: uncinate base (transition point from flat central SEP to vertically projecting uncinate: A1, A2, A3) and between the uncinate process tips (B1, B2, B3) on coronal reconstructions at 3-disc space depths (anterior 1/3rd, 1/2 point, posterior 1/3rd).

Results: The mean IUD at the posterior-third(A3), was the limiting width at all levels, which only increased from 11.3±2mm at C4 to 13.4±2.9mm at C7. The 3rd quartile of mean A3 distance were: C4–12.61mm, C5–13.38mm, C6–15.18mm, C7–15.25mm. A1 and A2 were both greater than 14mm in majority of the subjects (C4–50%, C5–56%, C6–72%, C7–90%), whereas A1 and A2 were >17mm (most contacted depth by all marketed implants) only in 2% at C4, 4% at C5, 16% at C6 and 44% at C7.
While linear parameters differed significantly between the male and female sex, there was no statistically significant difference in mean A3 at any level (all p>0.05) or when grouped by BMI category.

Conclusion : IUD increases from cephalad to caudad and reduces from anterior to posterior at each level. For the vast majority of people (regardless of gender), the IUD is at least 14mm, especially in the most commonly CDA-treated disc spaces (C5-6, C6-7), suggesting that 14mm could serve as a one-width for all CDAs, especially if a primary goal of the surgical technique is to eliminate or significantly reduce the need for bony resection/burring to create a proper width-fit of the implant.