(H-286) Frequency Response of 3D-Printed Implants with Additive Surface Features

Introduction:: It has been documented that those occupations which predispose workers to elevated frequency loading display a propensity for low back pain at increased levels as compared to other occupations. With the advent of 3D printing, it is hypothesized that intervertebral implants could be constructed that display a frequency response in keeping with that of the human spine. Resonant frequency is the oscillation of a system at its natural or unforced resonance. Resonance occurs when a system stores and easily transfers, energy between different storage modes, such as Kinetic or Potential energy, as in a pendulum. Most systems have one resonant frequency with additional harmonic frequencies that diminish in contribution with deviation from the resonant frequency value. The goal of this study was to determine the dynamic frequency response of a 3D-printed implant employing various surface features. It was hypothesized that a design feature could be identified as a preferential element based on applied frequency response.


Materials and Methods:: Four implant designs of comparable dimensions were 3D-printed with identical matrix geometry. The implants differed by the inclusion of conventional surface features. The devices were all fabricated by the same manufacturer and identified as Base Matrix (BM, Xenix Medical, Orlando, FL), BM+Frame (BMF), BM+Teeth (BMT) and BM+Frame+Teeth (BMFT) (Figure 1A). For the devices containing a circumferential frame, the inner matrix was thinned to maintain comparable surface area to isolate the surface features of Frame and Teeth. Using 40 PCF bone blocks (Sawbones USA, Vashon Island, WA), each implant type was cycled for 600 cycles with load verses displacement data collected at cycle 5 and at 25 cycle intervals thereafter. Testing was conducted at 1, 5, 10, and 20 Hz (Figure 1B). The data acquisition rate was adjusted for each frequency to generate a minimum of 200 data points per cycle. A total of 5 tests were performed for each implant design at each frequency. The net displacement at the respective cycle counts were computed and averaged for each implant at each frequency and subjected to non-linear analysis to compute the initial displacement (Yo), the settling rate (Half-Life), and the total displacement (Span). Each parameter was analyzed using a 1-way ANOVA with a Holm-Sidak post-hoc test for comparison between implant types and frequency (Prism 9.4, Graphpad, San Diego, CA).


Results, Conclusions, and Discussions:: The BM design displayed comparable displacement (Span) to the BM+Frame design at 1 Hz and both were statistically reduced compared to the other designs. At the 20 Hz frequency, all designs displayed statistically elevated displacement compared to the BM design (P < 0.001). In the case of these implants, the BM design displayed the greatest displacement at the 5 Hz frequency (P < 0.001, Figure 1C). The settling rate (Half-Life) at 5 Hz for the BM design was statistically elevated as compared to all other designs (P < 0.001, Figure 1D) while the other designs were not statistically different from each other (P >0.65). This would indicate the BM design requires additional cycles to settle which is in keeping with increased spinal resonant displacement at the 5 Hz frequency. The resonant frequency of the lumbar spine has been reported to be in the 4-6 Hz range.^1,2 At resonance, the maximum energy and vibration are transmitted resulting in increased displacement. Based on these results, it appears that the BM design transmits the additional vibratory amplitudes that would allow the spine to resonate at the 5 Hz frequency. Alternatively, it could be interpreted that the BM design does not hinder the vibratory response of the spine. indicating that the BM design does not inhibit the spinal resonant frequency displacement transmission. The additional stiffening due to circumferential frames and serrated teeth appear to provide a perturbation to the natural 5 Hz resonance frequency of the human spine. The BM design examined in this study resulted in a similar resonance frequency allowing for increased vibratory displacement at 5 Hz in keeping with the cited reports of spinal resonance. Only the BM design displayed increased deflection at 5 Hz as compared to other frequencies. Other designs were suppressed or only displayed increased deflection at very high frequencies. Alternatively, it can be stated that the BM design does not restrict the spinal vibratory response. An appropriate vibratory response may be clinically important when treating patients exposed to high frequency occupations.


Acknowledgements (Optional): :

References (Optional): : 1. Pope MH, Magnusson M, Wilder DG. Kappa Delta Award. Low back pain and whole body vibration. Clin Orthop Relat Res. 1998 Sep;(354):241-8. doi: 10.1097/00003086-199809000-00029. PMID: 9755785.

2. Wilder DG, Woodworth BB, Frymoyer JW, Pope MH. Vibration and the human spine. Spine (Phila Pa 1976). 1982 May-Jun;7(3):243-54. doi: 10.1097/00007632-198205000-00008. PMID: 6214030.