Becker's Spine Review

15 Executive Briefing Sponsored by: Understanding the Science Behind the Hype Five Considerations for Spine Surgeons When Evaluating Surface-Enhanced Interbody Fusion Devices for Optimal Clinical Outcomes from the Company that Wrote the Book By Bob Spiro, PhD and Jeffrey Kozak, MD With over 2 million Plasmapore® surface-enhanced devices im- planted to date and long-term clinical experience, Aesculap sug- gests five considerations for spine surgeons to use when evaluat- ing these technologies: 1. A History of Surface Enhanced Technology Surface-enhanced implants were first introduced to the ortho- pedic space as a potential solution for aseptic loosening of hip replacements.In the 1970s, the development of porous titanium surface enhancements marked a shift toward "cement-free" de- vices. The philosophy behind this was to optimize the contact area between the implant and bone through a roughened, po- rous surface that promotes biological fixation. This improved bone-implant interface not only helped increase the likelihood of survival but also helped improve load transfers, thereby avoiding stress shielding complications. This technology was eventually applied to spinal implants with the goal of improving biocom- patibility to increase bone ingrowth, helping create a faster and more stable arthrodesis. BS: One of the first titanium plasma spray (TPS) surface enhancements was devel- oped by Aesculap (Tuttlingen, Germany) in 1986 to encourage biological fixation on hip prostheses, helping to ensure long- term implant survivorship. 1 Following nine years of clinical experience in total joints, Aesculap launched the first Plasmapore en- hanced spinal interbody in 1995, followed by Plasmapore-modified PEEK (Plasmapor- e XP ) interbody implants in 2012. These implants are still widely used today, highlighting a strong safety and efficacy record with no evidence of any detrimental biologic response to the porous surface in over 30 years of clinical experience. 2 JK: I was attracted to spinal implants treat- ed with Plasmapore XP surface enhancement as a mechanism to overcome the well- known fibrous envelope surrounding PEEK implants without coatings. Given Aesculap's extensive experience with Plasmapore in the hip implant arena, the decision to use Plasmapore XP enhanced spinal implants was an easy one. 2. Biocompatibility Any material placed within the body has the potential to gener- ate a cellular response as a mechanism to clear what is recog- nized as "foreign." The basic cell biology underlying these inflam- matory responses is well studied at the basic science level. Small particles (less than 10 microns in diameter) can be ingested by immune cells (macrophages, dendritic cells), resulting in the re- lease of factors that drive a localized inflammatory response. This type of inflammatory activity in the vicinity of a bone implant has the potential to induce osteolysis that can lead to postoperative loosening of the implant. To date, animal studies support that more than 600,000,000 particles are likely needed to induce this type of reaction. 3 For this reason, current implant designs focus on enhancing the bone-implant interface and limiting the poten- tial for release of debris. BS: The description of the foreign body response to debris from titanium spinal implants and the potential for osteolysis is often grossly oversimplified. Beyond preclinical models of in- flammation, it has not been possible to determine the critical concentration of titanium particulate debris that results in these postoperative clinical complications. Biomechanical testing of a surface-enhanced interbody under excessive impaction force generated only a few thousand particles in the 1 to 10 micron range. Even under such extreme parameters, which are well above in vivo forces reflected by the current ASTM standard, tita- nium debris was below the level reported to induce an inflamma- tory response in a preclinical animal model. 3,4 JK: I have had no cases of osteolysis or delamination in the hun- dreds of Plasmapore® XP devices I've implanted in both anterior lumbar or TLIF applications. General statements about TPS coatings on PEEK can be mislead- ing as not all coatings are the same. Aesculap developed a two- stage proprietary process that results in a coating-to-substrate shear and bond strength of over 30MPa. 6 This value well exceeds the ASTM requirements and the maximum physiological shear forces that occur in the lumbar spine. Tests have shown that the PEEK material will shear from itself before it shears from the in- terface with the Plasmapore XP surface. To date, there are no com- plaints of surface delamination or shear from over 30,000 clinical Plasmapore XP implantations since 2012. 2 3. The Importance of Implant Composition and Architecture Historically, there is a large amount of evidence showing that the formation and conduction of new bone (osteoconduction) requires a template with a composition, architecture and me- chanical properties that mimic natural bone. In particular, there is a requirement for an interconnected macro- and micro-scale pore structure large enough to support the ingrowth of blood vessels and capillaries that support bone forming cells. 5 Implant designs that incorporate a 20 to 40 percent overall porosity and 40 to 135 µm pore diameter result in optimal bone ingrowth and conduction. 6 BS: The Plasmapore surface enhancing process creates an im- plant surface with optimal porosity and pore size specifications Dr. Bob Spiro Dr. Jeffrey Kozak

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