An in vitro comparison of hollow ground and trocar points on threaded positive-profile external skeletal fixation pins in canine cadaveric bone.

OBJECTIVE

To compare the microstructural damage created in bone by pins with lathe-cut and rolled-on threads, and to determine the peak tip temperature and damage created by positive-profile external fixator pins with either hollow ground (HG) or trocar (T) tips during insertion.

STUDY DESIGN

An acute, in vitro biomechanical evaluation.

SAMPLE POPULATION

Twenty-seven canine tibiae.

METHODS

Lathe-cut thread design with T point (LT-T), rolled-on thread design with T point (RT-T), and rolled-on thread design with HG point (RT-HG) pins were evaluated. Twenty pins of each type were inserted under constant drilling pressure into 12 canine tibiae (12 diaphyseal and 8 metaphyseal sites per pin type). Peak pin tip temperature, drilling energy, end-insertional pin torque, and pullout force were measured for each pin. For the histologic study, five pins of each type were inserted into cortical and cancellous sites in 15 additional tibiae. Entry and exit damage, and thread quality were assessed from 100 micron histologic sections by using computer-interfaced videomicroscopy.

RESULTS

T-tipped pins reached higher tip temperature in both diaphyseal and metaphyseal bone compared with HG-tipped pins. RT-T pins had higher pullout strength (diaphyseal) and end-insertional torque compared with other combinations. No differences in drilling energy or insertional bone damage was found between the three pin types (P < .05).

CONCLUSIONS

T-tipped pins mechanically outperformed HG-tipped pins. Pin tip and thread design did not significantly influence the degree of insertional bone damage.

CLINICAL RELEVANCE

T-tipped pins may provide the best compromise between thermal damage and interface friction for maximizing performance of threaded external fixator pins.