Mechanical testing of an absorbable hybrid fusion cage for the cervical spine

Abstract

Conventional fusion devices ("cages") are often used to join two vertebrae of the human spine and generally remain in the body for a lifetime and can theoretically lead to any complications. Therefore, an absorbable hybrid fusion cage consisting of a magnesium skeleton infiltrated with an absorbable polymer [poly-ε-caprolactone (PCL)] has been developed. The primary objective of the cage is to ensure an adequate stiffness of the disc space directly after the operation and to encourage the ingrowth of the new bone tissue to secure long life stability. Once a sufficiently rigid bone connection is formed, the implant should be absorbed. The purpose of this first study on the new absorbable fusion cage was to investigate the mechanical properties in vitro. Tensile tests were performed with tensile specimens type 1BA according to DIN EN ISO 527 made of PCL foamed using controlled expansion of saturated polymers (CESP). Furthermore, cyclic compression tests and compression tests with steady movement were performed with different designs of the new cage. Compression tests were also performed with vertebral endplates of ovine cadaveric spines. Foaming of PCL resulted in a modulus of elasticity of 135 MPa, which is approximately one third of unfoamed PCL. The results indicate that the initial compression strength of the implants should be adequate for the implantation in the cervical spine.

Quantitative analysis of near-implant magnesium accumulation for a Si-containing coated AZ31 cage from a goat cervical spine fusion model

“…Though the authors attributed the fusion failure to the hindering of new bone ingrowth by the PCL coating, we considered that the failure might be due to the excessive intervertebral Mg accumulation: 1) The blood supply environment of the endplate-treated disc space after anterior cervical discectomy and fusion (ACDF) is different from that of cortical bone or cancellous bone, which is believed to influence the corrosion rates of Mg-based implants and the absorption of released Mg ions [ 11 ]. 2 ) Differences in the stress stimulation between Mg-based cages and screws/plates also determine the unique characteristics of the resulting cage corrosion rate and intervertebral Mg accumulation [ 12 , 13 ]. As a result, we decided to conduct a quantitative study of near-cage Mg accumulation using a goat cervical spine fusion model.…”

Preliminary results in anterior cervical discectomy and fusion with an experimental bioabsorbable cage – clinical and radiological findings in an ovine animal model

“…This material combination was chosen after in vitro -investigation of the mechanical properties of both substances with demonstration of an adequate initial compression strength for implantation in the cervical spine (Kauth et al 2012). The favourable characteristics of magnesium alloys are the ability to degradation, a similar elasticity modulus to bone, a stimulation effect to bone growth and good biocompatibility (Shi et al 2010, Staiger et al 2006, Xu et al 2007, Zeng et al 2008).…”

“…The PCL is known for its bioabsorbability with slow degradation, and it is radiolucent and not toxic (Albertsson and Karlsson 1995, Hiljanen-Vainio et al 1996, Kronental 1975). The idea to combine these two kinds of material as a hybrid cage was to ensure a sufficient primary stability by the magnesium skeleton and to prevent too fast degradation by infiltration of the magnesium alloy with the PCL (Kauth et al 2012). The goal of our investigation is to show the clinical findings, complications and the radiographic results of the harvested spinal segments (3, 6, 12, and 24 weeks after surgery) and to compare the osseointegration of the magnesium-PCL implant with the autologous bone graft.…”

Bioabsorbable Interbody Magnesium-Polymer Cage

“…[19][20][21][22][23][24] Issues concern a low primary stability with development of cracks and some foreign body reactions with infl ammatory signs, leading to widespread skepticism of the value of bioabsorbable implants and limited use in clinical practice. 25 The properties of magnesium alloys that are attractive for use in medical disciplines are their degradation ability, a similar elasticity modulus to bone, a stimulation effect on bone growth, and good biocompatibility. It consisted of a magnesium structure and was infi ltrated with a polymer (poly-ε-caprolactone, PCL).…”

“…The material combination had been extensively tested in vitro and showed suffi cient initial compression strength for implantation in the cervical spine. 25 The 2 aims of this study were: fi rst, to show the degradation kinetics of this magnesium-polymer implant, and second, to evaluate the biomechanical and histological fi ndings in comparison with an autologous bone graft. [26][27][28][29] The polymer PCL is known for its bioabsorbability with slow degradation, and it is radiolucent and not toxic.…”