The effect of different insertion variables on achieved screw tightness and stripping rates for non-locking screws

Background
Nearly a quarter of screws cause damage during insertion by stripping the bone, reducing pullout strength by over 80%. Studies assessing surgically achieved tightness have predominately shown that variations between individual surgeons can lead to underpowered investigations. Further to the variables that have been previously explored, several basic aspects related to tightening screws have not been evaluated with regards to how they affect screw insertion.
Objectives
This study aims to identify the achieved tightness and stripping rates for several variables, firstly to better understand factors related to achieving optimal intraoperative screw purchase and secondly to establish improved methodologies for future studies.
Study Design & Methods
Two torque screwdrivers were used consecutively by two orthopaedic surgeons to insert 60 cortical, non-locking, stainless-steel screws of 3.5 mm diameter through a 3.5 mm plate, into custom-made 4 mm thick 20 PCF sheets of Sawbone, mounted on a custom-made jig. Screws were inserted to optimal tightness subjectively chosen by each surgeon. The jig was attached to a bench for vertical screw insertion, before 60 screws were inserted using the first torque screwdriver with the jig mounted vertically, enabling horizontal screw insertion. Following the decision to use the first screwdriver to insert the remaining screws in the vertical position for the other variables, the following test parameters were assessed with 60 screws inserted per surgeon: without gloves, double or single surgical gloves, non-sterile nitrile gloves and, with and then without augmented feedback (using digitally displayed real-time achieved torque), three Sawbone denisties (10, 20 and 40 PCF) and seven cortical thicknesses (1, 2, 3, 4, 5, 6 and 7 mm). For all tests, except when augmented feedback was used, the surgeon was blinded to the insertion torque. Once the stopping torque was reached, screws were tightened until the stripping torque was found; this being used to calculate tightness (stopping/stripping torque ratio). Screws were recorded to have stripped the material if the stopping torque was greater than the stripping torque. Following tests of normality, Mann-Whitney-U comparisons were performed between and combining both surgeons for each variable, with Bonferroni corrections for multiple comparisons.
Results
There was no significant (p=0.29) difference in the achieved tightness between different torque screw drivers nor different jig positions (p=0.53). The use of any gloves led to significant (p<0.001) increases in achieved tightness compared to not using gloves for one surgeon but made no difference for the other (p=0.38-0.74). Using augmented feedback was found to virtually eliminate stripping. For one surgeon average tightness increased significantly (p<0.001) when torque values were displayed from 55 to 75%, whilst for the other, this was associated with significantly decreases (p<0.001), 72 to 57%; both surgeons returned to their pre-augmentation tightness when it was removed. For both, lower density or thinner specimens (< 3 mm) lead to greater stripping rates (p<0.05).
Conclusions
Individual techniques make a considerable difference to the impact from some variables involved when inserting screws. However, the orientation of screws insertion and the type of screwdriver did not affect achieved screw tightness. Using visual feedback reduces rates of stripping and investigating ways to incorporate this into clinical use are recommended. Special attention to bone stripping should be paid when inserting into low density material and/or in the presence of thin cortices.