General Session: Biomechanics - Hall F
Presented by: A. Valdevit
A. Valdevit(1,2), A. Kedzierska(2), M.B. Gallagher(3), J.M. Schneider(3), P.F. Ullrich(3)
(1) SEA Limited, Biomedical Engineering, Columbus, OH, United States
(2) Stevens Institute of Technology, Biomedical Engineering, Hoboken, NJ, United States
(3) Titan Spine, LLC, Research, Mequon, WI, United States
Introduction: There is a paucity of work on implant/endplate interaction due to implant design to increase fusion rates. The aim of this study was to investigate if bulging of the vertebral endplate is inhibited by implants with a central strut thus limiting graft compression.
Methods: Fourteen porcine (L4, L5) vertebra were assigned to implant groups representing open or strutted devices (Figure1A). A transducer was placed on the endplate surface while 500N was applied to the implant at 1Hz for 500 cycles (Figure 1B). Load and endplate deformation was acquired at each time point, averaged, and subjected to regression with parameters of; K [or Rate (s-1)], Half-Life (s) and Span [or Compressive Change, (mm)] which were compared using unpaired t-tests.
Findings: An elevated Rate (K value) of endplate deformation (P< 0.01) was seen for open implants compared to strutted implants (Figure 2A). Expressing this as Half-Life (HL =ln2/K) represents the number of cycles or time required to achieve a 50% change towards settled endplate deformation (Figure 2B). A decreased Half-Life (P< 0.01) was seen for open implants compared to strutted implants. The Span parameter represents differences between initial endplate and final endplate deformation (Figure 2C) and displayed an elevated (P< 0.01) increase for open devices compared to strutted implants. Dynamic endplate bulging is represented by the K value (Rate) and Half-Life data. The results indicate open implants require less time to achieve endplate bulging. The endplate is therefore free to deform, permitting endplate expansion for graft compression. During Compressive Change (Span, Figure 2C), open implants provided 15µm of additional expansion. This distance is significant as osteoblasts differentiate at 28 um from capillaries in Static Osteogenesis (SO), while they must move during loading in Dynamic Osteogeneisis (DO).
Conclusions: Vertebral endplate bulging can be measured, and was increased for implants without a central strut. Open implants permit unimpeded endplate mechanics and may facilitate improved graft compression under cyclic loading. Clinical implications of unimpeded endplate bulging may result in graft compression and correlate to earlier fusion initiation by decreasing the distance osteoblasts must travel during DO. Deformation of the endplate in an attempt to improve graft compression is a consideration for future implant design.
Figure 1 and 2