Does cancellous screw insertion torque depend on bone mineral density and/or microarchitecture?

Abstract

During insertion of a cancellous bone screw, the torque level reaches a plateau, at the engagement of all the screw threads prior to the screw head contact. This plateau torque (TPlateau) was found to be a good predictor of the insertion failure torque (stripping) and also exhibited strong positive correlations with areal bone mineral density (aBMD) in ovine bone. However, correlations between TPlateau and aBMD, as well as correlations between TPlateau and bone microarchitecture, have never been explored in human bone. The aim of this study was to determine whether TPlateau, a predictor of insertion failure torque, depends on aBMD and/or bone microarchitecture in human femoral heads. Fifty-two excised human femoral heads were obtained. The aBMD and microarchitecture of each specimen were evaluated using dual X-ray Absorptiometry and micro-computed tomography. A cancellous screw was inserted into specimens using an automated micro-mechanical test device, and TPlateau was calculated from the insertion profile. TPlateau exhibited the strongest correlation with the structure model index (SMI, R=−0.82, p<0.001), followed by bone volume fraction (BV/TV, R=0.80, p<0.01) and aBMD (R=0.76, p<0.01). Stepwise forward regression analysis showed an increase for the prediction of TPlateau when aBMD was combined with microarchitectural parameters, i.e., aBMD combined with SMI (R2 increased from 0.58 to 0.72) and aBMD combined with BV/TV and BS/TV (R2 increased from 0.58 to 0.74). In conclusion, TPlateau, a strong predictor for insertion failure torque, is significantly dependent on bone microarchitecture (particularly SMI and BV/TV) and aBMD.