Load‐transfer in the human vertebral body following lumbar total disc arthroplasty: Effects of implant size and stiffness in axial compression and forward flexion

Abstract

Adverse clinical outcomes for total disc arthroplasty (TDA), including subsidence, heterotopic ossification, and adjacent‐level vertebral fracture, suggest problems with the underlying biomechanics. To gain insight, we investigated the role of size and stiffness of TDA implants on load‐transfer within a vertebral body. Uniquely, we accounted for the realistic multi‐scale geometric features of the trabecular micro‐architecture and cortical shell. Using voxel‐based finite element analysis derived from a micro‐computed tomography scan of one human L1 vertebral body (74‐μm‐sized elements), a series of generic elliptically shaped implants were analyzed. We parametrically modeled three implant sizes (small, medium [a typical clinical size], and large) and three implant materials (metallic, E = 100 GPa; polymeric, E = 1 GPa; and tissue‐engineered, E = 0.01 GPa). Analyses were run for two load cases: 800 N in uniform compression and flexion‐induced anterior impingement. Results were compared to those of an intact model without an implant and loaded instead via a disc‐like material. We found that TDA implantation increased stress in the bone tissue by over 50% in large portions of the vertebra. These changes depended more on implant size than material, and there was an interaction between implant size and loading condition. For the small implant, flexion increased the 98th‐percentile of stress by 32 ± 24% relative to compression, but the overall stress distribution and trabecular‐cortical load‐sharing were relatively insensitive to loading mode. In contrast, for the medium and large implants, flexion increased the 98th‐percentile of stress by 42 ± 9% and 87 ± 29%, respectively, and substantially re‐distributed stress within the vertebra; in particular overloading the anterior trabecular centrum and cortex. We conclude that TDA implants can substantially alter stress deep within the lumbar vertebra, depending primarily on implant size. For implants of typical clinical size, bending‐induced impingement can substantially increase stress in local regions and may therefore be one factor driving subsidence in vivo.

ICR in human cadaveric specimens: An essential parameter to consider in a new lumbar disc prosthesis design

“…With respect to the biomechanical specifications, based on our study, the implant should meet the following requirements: Adapts the equations of the movement of the intact ICR of the joint to the post-surgical ICR [3 , 88] Behaves as a shock absorbing mechanism [58] Allows the load absorption within the implant so that adjacent levels do not overload [11] Biocompatible [37] Low implant wear [104] Different sizes to adapt to different patient geometries [59] Non-complex geometry in order to be readily manufactured [90] …”

“…At the ends of this bar 10 Kg weights were hung, in such a way that a flector moment of ±25.5Nm, as well as a 0 to 100 N compression was able to be applied. This simulated physiological loading conditions [11 , 97 , 118] . The superior portion of the system was free, in order to allow greater mobility.…”

The Role of Vertebral Porosity and Implant Loading Mode on Bone-Tissue Stress in the Human Vertebral Body Following Lumbar Total Disc Arthroplasty

“…Footplates were assigned material properties similar to cobalt-chromium (E ¼ 210 GPa, n ¼ 0.33 36 ). Elliptical plates were chosen to represent generic footplates, thereby assuming that subtle details of implant geometry have a secondary effect on tissue-level stress relative to footplate size 19 (see Sensitivity Study, Supplemental Digital Content 1, http:// links.lww.com/BRS/B750).…”

“…Previously, we used micro-computed tomography-(micro-CT-) based finite element analysis to study the bone biomechanics of a TDA-implanted vertebra. 19 A key advantage of this methodology is the high spatial resolution at which the bone is modeled: micro-CT-based models intrinsically incorporate the complete morphology of the calcified bone tissue, including the structural anisotropy and connectivity of the trabecular microstructure. Our prior results indicated that implant impingement induced by sagittal bending-more so than axial compression-generated high levels of bone-tissue stress, possibly increasing subsidence risk.…”

Can a Priori Unknown Values of Biomechanical Parameters Be Determined with Sufficient Accuracy in Mbs Using Sensitivity Analysis? Analyzing the Interaction Characteristics between Cervical Vertebra and Pedicle Screw

“…Although FE models are able to deal with stresses, deformations and complex material behavior, they require a high computational time [22]. The study of Bonnheim et all [23] included a CT-based FE analysis of a vertebral body implanted with prosthetic disc implants of various sizes and stiffness. The model was loaded with compression-and flexion-induced anterior impingement.…”