The influence of material and design features on the mechanical properties of transpedicular spinal fixation implants

Abstract

This study describes the finding and performance of mechanical strength and corrosion testing procedures for comparative examination of multiple internal transpedicular spine fixators. Seven different implant models from five different manufacturers were compared regarding their bending strength and fatigue resistance. Because of the unacceptably high levels of time and material that they require, ISO and ASTM testing standards are not applicable to comparative testing. In addition, there is a lack of knowledge about clinically defined and proven strength-limit values. Therefore, actual standard testing procedures have been modified and extended to corrosion testing. Overall, the effort necessary to obtain reproducible comparative data has been reduced significantly. Although a reduced number of implants of each type were available for destructive testing, the results revealed fundamental differences in the tested implants between different materials and design features. During fatigue testing some of the implants showed poor corrosion properties. Because spinal fixation implants tend to be used as long-duration implants, corrosion testing as well as comparative strength testing with clinically successful implant models should be performed as preclinical evaluations.

Periprosthetic Electrochemical Corrosion of Titanium and Titanium-Based Alloys as a Cause of Spinal Fusion Failure

“…There are a wide variety of implant designs, including segmental and nonsegmental constructs, rigid and semirigid connectors, and different materials and surface treatments. 10,13 The surgeon should be aware of material and design considerations to balance relative benefits and limitations of different constructs. Approximately 35% of these constructs are made of stainless steel, and 65% are made of Ti-6Al-4V alloy.…”

“…The detected level of corrosion does not influence the mechanical endurance, but the effect on the organism might raise foreign-body reactions (metallosis), which can lead to bone resorption and implant loosening. 10 Three damage modes have been mostly observed on retrieved implants: wear (most common), corrosion, and fracture of fixation screws or longitudinal rods (least common). Wear and/or corrosion were already observed in implants retrieved before 1 full year of implantation, and it cannot be excluded that they might be one of the reasons for early removal (Ͻ2 years) of spinal devices.…”

“…[7][8][9] Other properties, such as wear and corrosion behavior, in addition to endurance, strength, and stiffness determine the clinical performance of transpedicular spinal devices. 10 Moreover, the environment within the human body is corrosive and aggressive for spinal stabilization devices, as it is for other implants. Corrosion effects and mechanical load act together and often influence each other.…”

The Biological Effects of Combining Metals in a Posterior Spinal Implant:In VivoModel Development Report of the First Two Cases

“…Further study with greater numbers is needed to answer the clinical concern of safety when mixing metals in spinal constructs, and we hope these pilot data will inform others who may wish to perform such studies. In vivo study such as this is ultimately required, in addition to ASTM and ISO testing standards for in vitro testing [26], and is clinically relevant to surgeons who initially implant and often revise patients particularly with nonfusion implant systems such as distraction based growing rods or growth guidance based procedures such as Shilla or Luque trolley.…”

Material dependent fretting corrosion in spinal fusion devices: Evaluation of onset and long‐term response

“…Current understanding of this has been largely based on analyzing retrieved posterior fusion devices for wear and corrosion . There are relatively few in vitro benchtop studies conducted to predict and guide implant design/performance involving the fretting corrosion response of posterior spinal systems. Specifically, there is a lack of information available on (1) the minimum angular displacement (in three anatomical planes) required to induce fretting onset which could be critical in understanding the effect of activities of daily living on the fretting corrosion onset; and (2) the long‐term fretting corrosion response of posterior fixation devices when subjected to a combination of axial load and angular displacements in anatomical planes.…”