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

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 (T_Plateau) 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 T_Plateau and aBMD, as well as correlations between T_Plateau and bone microarchitecture, have never been explored in human bone.The aim of this study was to determine whether T_Plateau, 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 T_Plateau was calculated from the insertion profile.T_Plateau 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 T_Plateau 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, T_Plateau, a strong predictor for insertion failure torque, is significantly dependent on bone microarchitecture (particularly SMI and BV/TV) and aBMD.