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Objectives: Self-tapping cortical bone screws are commonly used for internal fixation of long bones. However, oftentimes the cutting flutes of these screws induce soft tissue irritation and patient discomfort. With the intent of reducing patient discomfort, this study investigated how changes in cutting flute design would affect the ease of insertion and holding power of these screws. We hypothesized that changes in cutting flute length and/or number of flutes would result in changes in the mean insertion torque and pull- out strength of the screw.
Methods: Six cortical bone screw designs were studied, each with a different flute number and/or flute length. The number of flutes varied from zero to four, and the flute length was either zero, 1.7 mm (one- third length), or 5.1 mm (full length). Screws were inserted bi-cortically from lateral to medial in the diaphyses of human cadaveric femurs. Each femur was scanned using dual-xray absorptiometry and regional bone mineral density (BMD) values were measured. Screws were inserted at seven locations along the diaphysis of each femur. The insertion torque was measured with a custom digital torque transducer and pull-out strength was measured using a servo-hydraulic load frame. Insertion torque and pull-out strength were normalized by dividing by the respective regional BMD values. The mean normalized insertion torque and normalized pull-out strength data were each statistically analyzed using a single factor analysis of variance and a Fischer's post-hoc test with a level of significance of 0.05.
Results: The mean normalized insertion torque of the screw design with four full-length flutes was significantly (p<0.05) lower than that of the one design with three full-length flutes, and the two designs with one-third length flutes. The mean normalized pull-out strength of the screw design with four full-length flutes was significantly (p<0.05) greater than that of all designs with fewer than three flutes.
Conclusions: The primary advantages of self-tapping screws include a decrease in 1) the number of instruments needed during surgery, 2) the number of surgical steps, and 3) the operation time. This study was designed to investigate the possibility of reducing the cutting flute length, while still maintaining the insertion and pull-out properties of similar screws with longer flutes. We found that the normalized insertion torque significantly increased and there was a trend toward a decrease in normalized pull-out strength as the number of flutes decreased. These results give insight into the effect of flute length and number on bi-cortical bone screws. The data suggest that screws with four, one-third length cutting flutes may be appropriate when attempting to minimize soft tissue irritation, but there is some compromise to screw performance.
Acknowledgments: This study was funded in part by Depuy-Ace Medical and the Rehabilitation R&D Center of the Veterans Affairs Palo Alto Health Care System.