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. This study aims to identify the achieved tightness for several variables, firstly to better understand factors related to achieving optimal intraoperative screw purchase and secondly to establish improved methodologies for future studies. 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 a further 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 surgical gloves, single surgical gloves, non-sterile nitrile gloves and, with and then without augmented feedback (using digitally displayed real-time achieved torque). 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. 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. 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. Further work is underway into the effect of other variables such as bone density and cortical thickness

Introduction. Loosening of the baseplate is one of the most common causes of failure in Reverse Shoulder Arthroplasty. To allow osteo-integration to occur and thus provide long-term stability, initial screws fixation plays a pivotal role. In particular, tightening torque and force of nonlocking screws are two parameters that are considered to have a clear impact on implant stability, yet the relation is not fully understood. For this reason, this study aims to define an experimental set-up, to measure force and torque in artificial bone samples of different quality, in order to estimate ranges of optimal surgical values and give guidelines to maximize screw fixation and therefore initial implant stability. Methods. A custom-made torque sensor (Figure 1a) was built and calibrated using a lever deadweight system. To measure the compression force generated by the screw head, three thin FlexiForce sensors (Tekscan, South Boston, US) were enclosed between two 3D printed plates with a central hole to allow screw insertion (Figure 1b). The tightening force, represented by the sum of the three sensors, was calibrated using a uniaxial testing machine (Zwick/Roell, Ulm, Germany). Multiple screw lengths (26mm, 32mm and 47mm) were selected in the protocol. Synthetic bone blocks (Sawbones; Malmö, Sweden) of 20 and 30 PCF were used to account for bone quality variation. To evaluate the effect of a cortical bone layer, for each density three blocks were considered with 0 mm (no layer), 1.5 mm and 3 mm of laminate foam of 50 PCF. The holes for the screws were pre-drilled in the same way as in the operation room. For each combination of screw dimensions and bone quality, ten measurements were performed by acquiring the signal of the insertion torque and tightening force until bone breaking. Results. The typical output signal shows a maximum in the torque and force measurements, corresponding to bone breaking. After failure, a drop in the torque is visible, while a residual force remains present. For the base case (20 PCF), both torque and force show increasing mean values with longer screws, passing from 0.39 Nm (26mm) to 1.12 Nm (47mm) and from 180 N (26mm) to 419 N (47mm) respectively. Similar patterns were observed when the cortical layer was present or the bone quality was increased. Discussion. The findings of this study demonstrate that tightening force and torque are strongly impacted by bone quality and screw length. As main outcome, the maximum torque values could be used in clinical practice as a safety threshold for the surgeon. Compression force could also be used as input parameter in stability predictions of numerical models. Since only bone substitute was used, future research should include the extension to cadaveric bones. For any figures or tables, please contact authors directly

Introduction. Self tapping bone screw has been widely used in the fixation of Arthroplasty implants and bone graft. But the unwanted screw or driver breakage can be a direct result of excessive driving torque due to the thread cutting resistance. Previous studies showed that bone drill bit cutting rake angle was a critical factor and was inversely related to the bone cutting efficiency. 1, 2, 3, 4. (Figure 1) However to date there was no data for how the rake angle could influence the performance of self tapping bone screw. The purpose of this study was to investigate the torque generated by the self tapping cortical screw in simulated bone insertion as a function of the screw tip cutting flute rake angle. Methods. Two 5 mm thick BM5166 polyurethane block were stacked together and drilled through with 2.5mm diameter holes. Five 30mm long 3.5 mm diameter Ti6AL4V alloy self tapping cortical screws with 0°rake angle cutting flutes (Figure 2) were inserted in the holes and driven by the spanner attached to the test machine (Z5.0TN/TC-A-10) with a displacement control of 3 revolutions/min and 30N constant axial loading. The screws were driven into the stacked polyurethane block for 8mm depth. The maximum driving torque was recorded. Procedure was repeated for five same screws but with 7° rake angle cutting flutes. (Figure 2) The driving torqueses were compared. Student t test was performed with confidence level of 95% was assumed. Results. The average insertion driving torque for the screw with 7° rake angle was 30% less torque required than that of the screw with 0° rake angle. (P=5.3E–06<0.05) (Table 1). Conclusion and discussion. Screw failure during the insertion would be a nightmare. A slight positive rake angle significantly decreases the torque resistance. This design feature can significantly increase the safety margin of the self tapping screw implants and screw driving instruments. The likely mechanism would be the positive rake angle generated a more efficient bone cutting instead of pushing. More study on driving torque as a function of wider rake angle change is warranted. For figures and tables, please contact authors directly