Effects of radiofrequency energy on human chondromalacic cartilage: an assessment of insulation material properties

The objective of this study was to establish guidelines for the selection of an insulation material used to surround the electrode of radiofrequency energy (RFE) probes used for thermal chondroplasty. These guidelines were established by identifying which insulation materials resulted in the least amount of chondrocyte death while smoothing the surface of chondromalacic cartilage. RFE causes electrolyte oscillation and molecular friction in the tissue to heat it and subsequently smooth the surface. Material properties investigated included the coefficient of thermal expansion (CTE), thermal conductivity (TC), and volume resistivity (VR). Fresh human chondromalacic cartilage samples of Outerbridge grades II and III were obtained from patients undergoing total knee arthroplasty. Stiffness measurements were taken pretreatment and posttreatment. RFE was applied to a 1-cm² area for 15 s in a paintbrush treatment pattern. The insulation materials evaluated included Macor® (↓CTE,↓TC,?VR; in relation to CTE=10×10^-6/° C @ 20°C, TC=3 W/mK, VR=1×10¹⁴ ohm*cm), zirconia toughened alumina (ZTA) and 99.5% alumina (↓CTE,?TC,?VR), aluminum nitride (↓CTE,?TC,↓VR), Teflon® (PTFE) (?CTE,↓TC,?VR), partially stabilized zirconia (YTZP) (↓CTE,↓TC,↓VR), and Ultem® (?CTE,↓TC,↓VR). There were no significant differences between pretreatment and posttreatment stiffness of the cartilage for any material investigated. Subjectively scored scanning electron microscopy (SEM) images revealed that the surfaces of all samples treated with RFE were relatively smooth with melted fronds. Prototype probes made with Macor®, 99.5% alumina, and ZTA had TC≤30 W/mol*K and resulted in a mean of 35% less cell death (176±56 μm, 130±48 μm, and 114±33μm, respectively) than aluminum nitride, PTFE, and YTZP (246±68 μm, 231±108 μm, and 195±89 μm, respectively). Macor®, 99.5% alumina, and ZTA prototype probes all had VR≥1×10¹⁴ ohm*cm and resulted in a mean 37% less cell death th- an aluminum nitride or YTZP. There was no apparent relationship between CTE and the depth of chondrocyte death.