Measurement of Fastening Torque of Rod Securing Spinal Implant Screws

1. Introduction

Spinal instrumentation systems commonly rely on rod–screw constructs to achieve mechanical stability and maintain spinal alignment. The integrity of these constructs critically depends on the secure fixation of individual components, particularly the tightening of set screws that lock the rods within pedicle screw heads. Insufficient tightening may result in construct loosening and loss of stability, whereas excessive torque may lead to implant damage or compromised mechanical performance.

To ensure the reliability and safety of spinal implants, several standardized testing protocols have been established. Widely used standards [1,2] define methodologies for evaluating the mechanical behavior of spinal implant constructs, including static and fatigue performance as well as the properties of interconnection mechanisms. While these standards provide a robust framework for assessing structural integrity under controlled laboratory conditions, they do not specify clinically relevant tightening torque values for rod-securing screws in many implant systems. Moreover, they do not account for the variability introduced by manual torque application during surgical procedures.

In clinical practice, although some implant manufacturers provide torque-limiting screwdrivers and recommend specific torque values, in many cases the applied torque remains dependent on the surgeon’s subjective judgment. This introduces a potential source of variability that may influence construct stability and long-term outcomes. Despite extensive biomechanical investigations focusing on implant design, fixation strength, and load distribution [3,4,5,6,7,8,9,10,11,12,13,14,15], relatively little attention has been given to the actual torque applied intraoperatively and its variability among surgeons.

Furthermore, the influence of surgical technique—such as hand positioning, number of hands used, and surgeon orientation relative to the implant—on torque generation remains poorly understood. These factors may significantly affect the magnitude of applied torque and contribute to inconsistencies in implant fixation.

The aim of the present study was therefore to quantify the magnitude and variability of tightening torque applied by spine surgeons using a standardized experimental model and to evaluate the effect of handling technique on torque generation.

2. Materials and Methods

2.1. Study Design and Model

A realistic thoracolumbar plastic spine model (Biocalderoni, Budapest, Hungary) was mounted on a portable platform at a height corresponding to standard surgical positioning. Bilateral pedicle tulip-headed screws were inserted in all lumbar vertebrae. Screw heads were modified by incorporating high-tensile female screws (Figure 1).

The measurement apparatus consisted of three components:

• Standard surgical screwdriver (geometrically identical to instruments used in operating theaters)
• Calibrated torque-limiting screwdriver (Torqueleader Quickset, Guildford, UK) for calibration
• Calibrated dial-type torque wrench (WERA 7112BDS, Wuppertal, Germany) for loosening torque measurement

2.2. Calibration Phase (First Measurement Series)

To establish the tightening-loosening torque ratio for each screw, we performed sequential tightening at known torque values using the calibrated torque-limiting screwdriver, followed by loosening torque measurement with the torque wrench. This process was repeated three times. The first series was considered a "break-in" phase and excluded from ratio calculations; ratios were derived from the second and third measurements.

Preliminary analysis revealed that the tightening-loosening ratio varied slightly with torque magnitude. Therefore, two distinct ratios were assigned to each screw: one for tightening torques below 3.5 Nm and another for values exceeding this threshold.

2.3. Surgeon Assessment Phase (Second Measurement Series)

Twenty-two spine surgeons from six Hungarian institutions participated voluntarily. Participants were instructed to tighten screws on the model exactly as they would during surgery. The following parameters were documented for each maneuver:

• Grip technique (thumb-upward, thumb-downward, or two-handed)
• Hand dominance (right or left)
• Standing position relative to the model (right or left side)
• Institutional routine use of torque-measuring instruments

Measurement Protocol:

• The spine model was secured horizontally at surgical height with pre-loosened screws
• Surgeons tightened screws in any order using the provided standard screwdriver (Figure 2)
• Loosening torque was measured using the calibrated torque wrench, recording maximum torque
• All data were documented in standardized measurement protocols

2.4. Control Phase (Third Measurement Series)

To assess potential screw wear from repeated use, we repeated the calibration measurements (as described in the first series) following the surgeon assessment phase. This allowed detection of any significant changes in screw-screwdriver interface characteristics.

2.5. Statistical Analysis

Comparisons between tightening-loosening ratios from the first and third measurement series, as well as torque values associated with different grip techniques, were analyzed using two-tailed Student's t-tests with a significance level of 5%. Data are presented as means with standard deviations.

3. Results

Individual surgeon results and institutional averages are presented in Figure 3 and Figure 4. Substantial inter-surgeon variability was observed, with individual mean torques ranging from 1.8 Nm to 8.9 Nm (overall mean: 4.52 Nm, SD: 1.84 Nm). Institutional averages showed somewhat less variability than individual values.

The control calibration series revealed no significant differences in tightening-loosening ratios between pre- and post-assessment measurements (p=0.412), confirming that screw wear did not significantly influence results.

3.1. Grip Technique Effects

Significant differences in applied torque were observed based on grip technique (Figure 5). Two-handed grip produced significantly higher torque (mean: 6.2 Nm, SD: 1.3 Nm) compared to thumb-downward (mean: 3.9 Nm, SD: 1.1 Nm) and thumb-upward (mean: 4.1 Nm, SD: 1.2 Nm) techniques (p=0.003 for both comparisons). No significant difference was detected between the two one-handed techniques (p=0.382).

3.2. Positioning Effects

Surgeons consistently stood perpendicular to the model on either the right or left side. Analysis of screw position relative to surgeon standing side revealed that screws on the contralateral side were tightened significantly more (4.79 Nm, SD: 1.6 Nm) compared to ipsilateral screws (4.25 Nm, SD: 1.5 Nm, p=0.011).

3.3. Hand Dominance

Only one left-handed surgeon participated, precluding meaningful statistical analysis of handedness effects.

3.4. Torque-Measuring Instrument Use

No significant correlation was found between institutional routine use of torque-measuring instruments and mean applied torque (p=0.287).

4. Discussion

This study demonstrates substantial variability in screw fastening torque among spine surgeons, with individual mean values ranging nearly fivefold (1.8-8.9 Nm). This finding is particularly concerning given that many spinal implant systems lack specific torque guidelines [5]. Such variability could contribute to inconsistent construct stability and potentially compromise clinical outcomes [16,17].

The significant differences associated with grip technique and surgeon positioning highlight the influence of ergonomic factors on applied torque. Two-handed tightening, which is not standard surgical practice, produced significantly higher torque and should be discouraged to prevent excessive loading. The observation that contralateral screws are tightened more forcefully suggests that awkward positioning may compromise tactile feedback, prompting surgeons to apply additional force.

Interestingly, institutional use of torque-measuring instruments did not correlate with torque magnitude. This may reflect inconsistent instrument use, inadequate calibration, or surgeons overriding torque limits based on tactile sensation. It is also possible that surgeons familiar with torque instruments develop better awareness of force application, paradoxically leading to more variable torque values when these instruments are unavailable.

The clinical implications of our findings are significant. Optimal screw tightening torque is critical for construct stability, as inadequate tightening may lead to implant loosening and failure, while excessive torque may risk screw stripping or bone damage [18,19]. The wide variability observed in this study suggests that some patients may receive suboptimal constructs due to inconsistent surgical technique.

4.1. Limitations

Several limitations should be acknowledged. First, the plastic spine model may not fully replicate the tactile feedback of human bone, potentially influencing surgeon behavior. Second, the study was limited to a single screwdriver geometry; results might differ with other instrument designs. Third, the relatively small sample size and single-country setting may limit generalizability. Finally, we did not assess the impact of surgeon experience or fatigue on torque application.

4.2. Recommendations

Based on our findings, we recommend that implant manufacturers provide:

• Clear specifications for optimal tightening torque for their systems
• Calibrated torque-limiting screwdrivers with each implant set
• Surgeon education on the importance of standardized torque application

Hospitals and surgical departments should consider implementing routine torque measurement and documentation as part of quality assurance in spinal surgery.

5. Conclusions

Significant variability exists in the torque applied by spine surgeons during screw fastening, influenced by grip technique, surgeon positioning, and individual practice patterns. These findings support the routine use of torque-limiting instruments in spinal instrumentation to ensure consistent, optimal construct stability and potentially improve clinical outcomes. Future research should investigate the relationship between applied torque and long-term clinical results, as well as develop simplified torque-limiting devices suitable for routine surgical use.