3. The material rationale
A substantial part of the support for the clinical use of a pyrocarbon bearing surface in humeral arthroplasty rests on material properties demonstrated in laboratory studies. It is worth separating these properties from the possible clinical benefits inferred from them. Let's look at some of these studies.
Pyrolytic carbon is a graphite–carbon composite — the low-temperature isotropic (LTI) carbon that Bokros and colleagues developed at General Atomic in the 1960s and that has been the dominant bearing material for mechanical heart valves for roughly half a century.¹ Three of its properties are often used to justify its use as a shoulder bearing surface.
Elastic modulus close to bone. Pyrolytic carbon has a Young's modulus on the order of 20 GPa, within range of cortical bone (~15–20 GPa) and roughly an order of magnitude below cobalt-chromium (~210 GPa).² The argument is that a bearing surface whose stiffness approximates the bone it articulates against — rather than one ten times stiffer — transmits contact stress more physiologically and should therefore abrade subchondral bone less aggressively than metal.
Surface chemistry favorable to boundary lubrication. The load-bearing boundary lubricant of both native and prosthetic synovial joints is widely held to be surface-active phospholipid (SAPL), adsorbed as a film on the articular surface.³ Pyrolytic carbon's hydrophobic surface is hypothesized to adsorb these phospholipids and sustain the same boundary-lubrication film, lowering friction against bone. This is a frequently cited mechanism, but the pyrocarbon-specific link remains an inference from the general SAPL literature rather than a property directly demonstrated on the implant surface in vivo.
Reduced wear and favorable cell and tissue response, in vitro and in animals. In a shoulder wear simulator, the linearized bone-penetration rate, bone-volume-loss rate, and surface roughness for cobalt-chromium were roughly 30 times those for pyrocarbon; cobalt-chromium testing was halted at about 320,000 cycles because the bone interface had been consumed, whereas pyrocarbon ran to five million cycles.² In cultured chondrocytes, pyrocarbon supported cell growth without cytotoxicity and promoted type II collagen expression, generating a more cartilage-like matrix than either cobalt-chromium or plastic controls.⁴ An in vivo canine hip study found that cartilage articulating against LTI pyrocarbon showed significantly less gross wear, fibrillation, eburnation, glycosaminoglycan loss, and subchondral bone change than cartilage articulating against cobalt-chromium-molybdenum or titanium; survival analysis showed a 92% probability of cartilage survival against pyrocarbon at 18 months versus only 20% against either metal alloy.⁵ In a canine full-thickness-defect model, fibrocartilage regenerated at 86% of carbon-articulating defects versus 25% of metal, with surface cracking in 14% of carbon specimens versus 100% of metal.⁶
The clinical case built on these data is that pyrocarbon should generate less glenoid erosion than a metal hemiarthroplasty.
Two cautions should be considered. First, each finding presented above is preclinical — bench modulus, in vitro wear and cell culture, animal histology. None of it is, by itself, evidence of a potential patient-reported-outcome benefit exceeding the minimal clinically important difference relative to other bearing surfaces. Second, the material rationale describes the bearing surface only and does not necessarily predict the clinical performance of the implant itself.
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References
- Bokros JC. Carbon biomedical devices. Carbon. 1977;15(6):353–371.
- Klawitter JJ, Patton J, More R, Peter N, Podnos E, Ross M. In vitro comparison of wear characteristics of PyroCarbon and metal on bone: shoulder hemiarthroplasty. Shoulder Elbow. 2020;12(1 Suppl):11–22. doi:10.1177/1758573218796837. (Authors employed by / holding stock in the implant manufacturer.)
- Hills BA. Boundary lubrication in vivo. Proc Inst Mech Eng H. 2000;214(1):83–94. doi:10.1243/0954411001535264.
- Hannoun A, Ouenzerfi G, Brizuela L, Mebarek S, Bougault C, Hassler M, Berthier Y, Trunfio-Sfarghiu AM. Pyrocarbon versus cobalt-chromium in the context of spherical interposition implants: an in vitro study on cultured chondrocytes. Eur Cell Mater. 2019;37:1–15. doi:10.22203/eCM.v037a01.
- Cook SD, Thomas KA, Kester MA. Wear characteristics of the canine acetabulum against different femoral prostheses. J Bone Joint Surg Br. 1989;71(2):189–197.
- Kawalec JS, Hetherington VJ, Melillo TC, Corbin N. Evaluation of fibrocartilage regeneration and bone response at full-thickness cartilage defects in articulation with pyrolytic carbon or cobalt-chromium alloy hemiarthroplasties. J Biomed Mater Res. 1998;41(4):534–540.