The Changes in the Removal Torque of Screws after Local Heating

Suh, Sung Wook; Yoo, Jeong Joon; Kim, Hee Joong

Abstract

PURPOSE
The extraction of well-fixed metallic implants can be extremely demanding and time consuming, resulting in damage surrounding bone. Therefore, a less traumatic method of removing a well-fixed implant from bone is necessary. Our hypothesis was that a well-fixed implant can be extracted more easily if it is heated to a high temperature. The present study was conducted to evaluate the effect of local heating on the removal torque of well-fixed titanium screws.
MATERIALS AND METHODS
A titanium screw (2.7 mm diameter, 12 mm length) was bilaterally inserted into the distal femur of 14 New Zealand White rabbits at identical locations. Five rabbits were sacrificed at 8 weeks, and 9 rabbits were sacrificed at 20 weeks after screw insertion. Both femurs were harvested, and screw removal torques were measured using a torque gauge. Before removal, the screws in right femurs were heated with an electric soldering iron and then cooled with dry ice, whereas screws in the left femur were removed without heating. After removing the screws, Hematoxylin & Eosin staining was performed to allow the histological assessment of bone tissues around the implants, and picrosirius red F3BA staining was performed to assess the extent of bone damage. Statistical differences between the heat treated and untreated groups were determined using the Wilcoxon Signed Ranks test and differences with respect to the duration of implantation were evaluated by the Mann-Whitney test. A p value of & .05 was considered to be significant.
RESULTS
The removal torques for the heated screws were significantly lower than those for the non-heated screws for both 8 and 20 weeks groups. For non-heated screws, removal torques for the screws implanted 20 weeks was significantly higher than those of the screws implanted for 8 weeks (p & .05). H & E staining showed that the screw-bone interface was denatured in heat treated group. Polarized microscopy after picrosirius red F3BA staining showed that the collagen in the thermally damaged region had denatured, resulting in a loss of its natural birefringence. The maximal depth of the thermally damaged region from the screw/bone interface was less than 400 micrometer.
CONCLUSION
Controlled local heating may be a less traumatic method of removing a well-fixed implant from bone.