Understanding the Science Behind the Hype

Written by Bob Spiro, PhD and Jeffrey Kozak, MD | July 20, 2017 | Print  | Email

Five Considerations for Spine Surgeons When Evaluating Surface-Enhanced Interbody Fusion Devices for Optimal Clinical Outcomes from the Company that Wrote the Book

 

With over 2 million Plasmapore® surface-enhanced devices implanted to date and long-term clinical experience, Aesculap suggests five considerations for spine surgeons to use when evaluating these technologies:

 

1. A History of Surface Enhanced Technology

 

Surface-enhanced implants were first introduced to the orthopedic 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” devices. The philosophy behind this was to optimize the contact area between the implant and bone through a roughened, porous 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 biocompatibility to increase bone ingrowth, helping create a faster and more stable arthrodesis.

 

BS: One of the first titanium plasma spray (TPS) surface enhancements was developed 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 enhanced spinal interbody in 1995, followed by Plasmapore-modified PEEK (PlasmaporeXP) 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 treated with PlasmaporeXP 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 PlasmaporeXP enhanced spinal implants was an easy one.

 

2. Biocompatibility

 

Any material placed within the body has the potential to generate a cellular response as a mechanism to clear what is recognized as “foreign.” The basic cell biology underlying these inflammatory 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 release 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 potential 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 inflammation, 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, titanium debris was below the level reported to induce an inflammatory response in a preclinical animal model.3,4

 

JK: I have had no cases of osteolysis or delamination in the hundreds of Plasmapore®XP devices I’ve implanted in both anterior lumbar or TLIF applications.

 

General statements about TPS coatings on PEEK can be misleading 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 interface with the PlasmaporeXP surface. To date, there are no complaints of surface delamination or shear from over 30,000 clinical PlasmaporeXP 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 mechanical 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 implant surface with optimal porosity and pore size specifications for bone ingrowth and conduction. Clinical studies have found that fusion rates are significantly higher in patients treated with a porous Plasmapore implant when compared to patients treated with a smooth titanium implant.7

 

JK: While I do not have extensive experience with porous surfaces outside of PlasmaporeXP technology, my follow-up CT scans of Arcadius®XP L, a stand-alone PlasmaporeXP treated ALIF device, reveal a remarkable, near-immediate bony ingrowth into the surface. One of my ALIF patients returned to sedentary work six days after her procedure and was discharged from medical care with a solid fusion three months later without restrictions.

 

Designed with over 30 years of experience in TPS coatings on titanium, Aesculap’s Plasmapore XP surface on PEEK has the ideal porosity for bone ingrowth. This surface treatment has been tested side by side with and mimics Plasmapore, which has been studied in over 1,000 orthopedic and spine patients.2

 

4. The Move Away from Titanium

 

Although interbody fusion began with autograft, researchers began looking for a fatigue-resistant alternative, which led to the development of titanium alloy and stainless steel cages. Early issues with subsidence and visualization eventually led to the development of PEEK-OPTIMA® by Invibio® in 1999. This biomaterial quickly gained traction as a standard of care in interbody fusion due to its superior imaging properties and a modulus of elasticity that mimics bone.8

 

BS: Bone as an organ/tissue relies heavily on mechanical signals and stimulation to maintain and repair itself through a balance of anabolic (formation) and catabolic (resorption) processes. Implants placed in bone must be able to share and transmit this mechanical information in order maintain the right balance between these processes. The inherent material properties (modulus of elasticity) and design (shape, architecture) dictate the overall strength/stiffness of an implant. Titanium implants with inadequate design and strength/stiffness properties can lead to implant subsidence. PlasmaporeXP titanium-PEEK implants match the material properties of bone and tilt the balance of response toward formation as opposed to resorption.

 

JK: I have never used titanium implants. However, it is well known that imaging on the interspace is negligible.

 

To the greatest possible extent, the use of radiolucent PEEK-OPTIMA prevents artifact formation. This enables surgeons to assess the structures around the implant, even postoperatively. Recent studies show that chance of subsidence is up to six times less with PEEK-OPTIMA devices when compared to solid titanium interbodies.9 Despite these biomechanical advantages, PEEK-OPTIMA has been shown to exhibit a fibrous tissue response and does not have the ideal biocompatible properties of titanium.10 For this reason, many companies have found innovative ways to treat the surface of the PEEK-OPTIMA interbody with titanium without disrupting the modulus of elasticity.

 

5. Breaking Down Nanotechnology

 

The business end of the response to an implant takes place at a micro- (10-6 m) and nanoscale (10-9 m) level. Cellular components involved in the bone-forming process (MSCs, osteoblasts, osteoclasts) must be able to adhere, grow and differentiate in order to build new bone that integrates with the implant surface. These events are driven by the specific surface receptor-based interaction of cells with cell adhesive proteins that bind to the implant surface and signal bone-forming responses. The micro- and nanoscale topography and composition of the implant serve as the template for adhesive protein conformation and cell shape that direct the resulting cell response. The TPS Plasmapore surface enhancing process creates both microscale porosity and nanoscale topography optimal for osseointegration.2, 10

 

References
1Volkmann R, Bretschneider C, Eingartner C, Weller S. Revision arthroplasty – femoral aspect: the concept to solve high grade defects. International Orthopaedics. 2003;27(1 Suppl):S24-S28.

 

2, Data on file, Aesculap AG.

 

3. Cunningham BW, Hallab NJ, Hu N, Mcafee PC. Epidural application of spinal instrumentation particulate wear debris: a comprehensive evaluation of neurotoxicity using an in vivo animal model. Journal of Neurosurgery. Spine. 2013;19(3):336-350.

 

4. Kienle A, Graf N, Wilke H-J. Does impaction of titanium-coated interbody fusion cages into the disc space cause wear debris or delamination? The Spine Journal. 2016;16(2):235-242.

5. Vasconcellos L, Leite D, Oliveira F, Carvalho Y, Cairo C. Evaluation of bone ingrowth into porous titanium implant: histomorphometric analysis in rabbits. Brazilian Oral Research. 2010;24(4), 399-405.

 

6. Klawitter JJ, Bagwell JG, Weinstein AM, Sauer BW. An evaluation of bone growth into porous high density polyethylene. J Biomed Mater Res. 1976;10(2):311-23.

 

7.Takeuchi M, Yasuda M, Niwa A, et al. Plasmapore-Coated Titanium Cervical Cages Induce More Rapid and Complete Bone Fusion After Anterior Cervical Discectomy and Fusion as Compared to Noncoated Titanium Cages. World Neurosurgery. 2014;82(3-4):519-522.

 

8. Invibio® Biomaterial Solutions. PEEK-OPTIMA® Natural Typical Material Properties. www.invibio.com (10/2013).

 

9. Chen Y, Wang X, Lu X, Yang L, Yang H, Yuan W, Chen D. Comparison of titanium and polyetheretherketone (PEEK) cages in the surgical treatment of multi-level cervical spondylotic myelopathy: a prospective, randomized, control study with over 7-year follow-up. European Spine Journal. 7. 22(2013):1539–1546.

 

10. Cheng BC. Biomechanical pullout strength and histology of Plasmapore®XP Coated Implants: Ovine multi time point survival study. Aesculap Implant Systems. Whitepaper. 2013. (ART 129).

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