Reference markers in computer aided orthopaedic surgery: rotational stability testings and clinical implications

Navigation procedures rely on the stability of the reference arrays (RA) fixed to the bony anatomy. The risk of inadvertent collision and unnoticed movements of the RA may occur, and limb movements might also provocate collisions. Consequently, relevant measurements failures during the navigated procedure might occur and reduce the overall precision of the system. The magnitude of torque to destabilize an RA from its bony-fixation is unknown. The purpose of this study was to determine the ability of standard RA’s to resist applied torque. A digital torque application device was developed to allow for precise torque application to the RA system at four cadavers. Clockwise, gradually increasing rotational force was applied to the RA in 1, 2, or 3 Nm, held for 1 s and released, repeated in 10 cycles. One pin fixation systems with 4.0 Schanz pins were used. A second RA was fixed 20 cm proximal to the tested RA. A navigation system was used to measure the relative positions of both RA’s during torque application. The rotational differences at torque application were calculated and statistically evaluated. Results revealed averaged 1.0° rotation [range (r), 1.0–1.1°] with first cycle of 1 Nm torque, the last cycle averaged 1.0° (r, 1.0–1.1°) with no significant differences in rotation between any of the cycles (p > 0.5). Torque of 2 Nm resulted in 1.8° rotation (r, 1.5–1.8°) with no significant increases between first and second cycle (p = 0.56), second and third trials (p = 0.35) while fourth cycle averaged 2.0°, with significant increase (p = 0.011). All subsequent cycles resulted in significant increases. Torque of 3 Nm produced 2.9° of rotation for initial cylce (r,2.5–3.3°), with significant increases with each cycle (p < 005). Torques of ≥2 Nm may cause loosening of the RA, thus may lose its original position relative to the bone. Surgeons using computer aided surgery systems should be aware of possible unrecognized movements of the RA, however, gentle collisions do not appear to cause significant motion or destabilization.