Non-locking screw insertion: No benefit seen if tightness exceeds 80% of the maximum torque

Development and initial validation of a novel thread design for nonlocking cancellous screws

High failure rates have been associated with nonlocking cancellous screws with a typical buttress thread in patients with osteoporotic bone. This study aimed to develop a novel thread design and compare its fixation stability with that of a typical buttress thread. Nonlocking cancellous screws with a novel thread design (proximal flank angle of 120 degrees, a flat crest feature, a tip-facing undercut feature) and nonlocking cancellous screws with a typical buttress thread were manufactured using stainless steel. Fixation stabilities were evaluated individually by the axial pullout and lateral migration tests, and they were evaluated in pairs together with a dynamic compression plate in an osteoporotic bone substitute (10 PCF polyurethane foam per ASTM F1839) under cyclic craniocaudal and torsional loadings. Pullout strength and lateral migration resistance for the individual screw test and the force, torque, and number of cycles required to achieve specific displacement and torsion for the multi-screw test were comparatively analyzed between both screw types. A finite element analysis model was constructed to analyze the stress distributions in the bone tissue adjacent to the threads. The biomechanical test revealed the novel undercut thread had superior axial pullout strength, lateral migration resistance, and superior fixation stability when applied to a dynamic compression plate under cyclic craniocaudal loading and torsional loading than those in the typical buttress thread. The finite element analysis simulation revealed that the novel thread can distribute stress more evenly without high-stress concentration at the adjacent bone tissue when compared to that of a typical buttress thread.

Augmented screwdrivers can increase the performance of orthopaedic surgeons compared with use of normal screwdrivers

Orthopaedic screws insertion can be trivialised as a simple procedure, however it is frequently performed poorly. Limited work exists defining how well surgeons insert screws or whether augmented screwdrivers can aid surgeons to reduce stripping rates and optimise tightness. We aimed to establish the performance of surgeons inserting screws and whether this be improved with screwdriver augmentation. 302 orthopaedic surgeons tightened 10 non-locking screws to what they determined to be optimum tightness into artificial bone sheets. The confidence in the screw purchase was given (1-10). A further 10 screws were tightened, using an augmented screwdriver that indicated when a predetermined optimum tightness was reached. The tightness for unstripped insertions under normal conditions and with the augmented screwdriver were 81% (95% CI 79-82%)(n = 1275) and 70% (95% CI 69-72%)(n = 2577) (p < 0.001). The stripping rates were 58% (95% CI 54-61%) and 15% (95% CI 12-17%) respectively (p < 0.001). The confidences when using the normal and augmented screwdrivers respectively were 7.2 and 7.1 in unstripped insertions and 6.2 and 6.5 in stripped insertions. Performance improved with an augmented screwdriver, both in reduced stripping rates and greater accuracy in detecting stripping. Augmenting screwdrivers to indicate optimum tightness offer potentially enormous clinical benefits by improving screw fixation.

Compression generated by cortical screws in an artificial bone model of an equine medial femoral condylar cyst

Objective

Determine compression generated by lag and neutral screws over 12 h using two bone analogs.

Study design

Experimental study.

Sample population

Bone analogs were made of composite synthetic bone (CSB) or three‐dimensional printed polylactic acid (PLA). Analogs had a 2 mm exterior shell with a 10 mm thick internal layer of open‐cell material.

Methods

Bone analogs were opposed, making a 4‐sided box with open ends. A central channel contained the sensor and the screws passed through it to engage both paired analogs. Four screw/analog conditions were tested: neutral and lag screw with bicortical engagement, neutral and lag screw with unicortical engagement. All screws were tightened to 2 Nm torque and compression values recorded at 0, 0.5, 1, 2, 6, and 12 h (six trials per condition). Medians were compared across groups for statistical significance.

Results

There was no difference in median compression between lag and neutral bicortical screws. For PLA, greater median compression was generated by neutral (median 437 N) and lag (median 379 N) bicortical screws compared to neutral unicortical screws (median 208 N, p < .001); lag bicortical screws generated greater median compression than lag unicortical screws (median 265 N, p = .012). For CSB, lag bicortical screws (median 293 N) generated greater median compression than neutral unicortical screws (median 228 N, p = .008).

Conclusion

Lag and neutral screws generated similar compression. Bicortical screws had higher median compression than unicortical screws in bone analogs.

Clinical significance

Neutral screws generate compression in cancellous bone analogs that can be increased with bicortical bone engagement.

Orthopedic screws insertion simulation with immediate feedback enhances surgical skill

BACKGROUND

Screw insertion to bones is a fundamental skill in orthopedic, spine and cranio-maxillofacial surgery. Applying the correct tightening torque is critical when compressing and fixating bone fragments. Overtightening yields in plastic deformation of the bone and destruction of the screw-bone interface, damaging the construct's stability. The surgeon is required to achieve sufficient hold and compression without stripping the bone. Several studies have investigated these skills, demonstrating much potential to enhance the future surgeons' capabilities. This study presents a novel training module, combining direct tightening followed by deliberate striping with immediate feedback suggested to enhance the surgeon's tactile perception and improve skill.

METHODS

A prospective single-blinded cohort study was run. Twenty surgeons from various disciplines, excluding orthopedic and maxillo-facial surgeons, were trained using an orthopedic screws insertion model, comprised of synthetic bones. Training sessions considered inserting 40 screws into normal and osteoporotic bone models, experiencing deliberate stripping of the screws and feedback for their performance in three different sessions.

FINDINGS

Success rate increased between sessions - by 24% to 48% in normal bone, and by 37% to 52% in osteoporotic bone. Stripping rate decreased between sessions - by 37.5% to 18.5% in normal bone, and by 29% to 14% in osteoporotic bone. Average ratio between tightening torque and maximum possible torque before bone stripping improved gradually and consistently from 67.3% to 81.6% in normal bone (p < 0.001), and slightly from 76.4% to 77.5% in osteoporotic bone (p = 0.026).

INTERPRETATION

Immediate feedback with deliberate stripping and external feedback using a digital torque measuring screwdriver may improve cortical screw insertion technique in the surgeons' community.

Screw tightness and stripping rates vary between biomechanical researchers and practicing orthopaedic surgeons

BACKGROUND

Screws are the most frequently inserted orthopaedic implants. Biomechanical, laboratory-based studies are used to provide a controlled environment to investigate revolutionary and evolutionary improvements in orthopaedic techniques. Predominantly, biomechanical trained, non-surgically practicing researchers perform these studies, whilst it will be orthopaedic surgeons who will put these procedures into practice on patients. Limited data exist on the comparative performance of surgically and non-surgically trained biomechanical researchers when inserting screws. Furthermore, any variation in performance by surgeons and/or biomechanical researchers may create an underappreciated confounder to biomechanical research findings. This study aimed to identify the differences between surgically and non-surgically trained biomechanical researchers' achieved screw tightness and stripping rates with different fixation methods.

METHODS

Ten orthopaedic surgeons and 10 researchers inserted 60 cortical screws each into artificial bone, for three different screw diameters (2.7, 3.5 and 4.5 mm), with 50% of screws inserted through plates and 50% through washers. Screw tightness, screw hole stripping rates and confidence in screw purchase were recorded. Three members of each group also inserted 30 screws using an augmented screwdriver, which indicated when optimum tightness was achieved.

RESULTS

Unstripped screw tightness for orthopaedic surgeons and researchers was 82% (n = 928, 95% CI 81-83) and 76% (n = 1470, 95% CI 75-76) respectively (p < 0.001); surgeons stripped 48% (872/1800) of inserted screws and researchers 18% (330/1800). Using washers was associated with increased tightness [80% (95% CI 80-81), n = 1196] compared to screws inserted through plates [76% (95% CI 75-77), n = 1204] (p < 0.001). Researchers were more accurate in their overall assessment of good screw insertion (86% vs. 62%). No learning effect occurred when comparing screw tightness for the first 10 insertions against the last 10 insertions for any condition (p = 0.058-0.821). Augmented screwdrivers, indicating optimum tightness, reduced stripping rates from 34 to 21% (p < 0.001). Experience was not associated with improved performance in screw tightness or stripping rates for either group (p = 0.385-0.965).

CONCLUSIONS

Surgeons and researchers showed different screw tightness under the same in vitro conditions, with greater rates of screw hole stripping by surgeons. This may have important implications for the reproducibility and transferability of research findings from different settings depending on who undertakes the experiments.

Lateral migration resistance of screw is essential in evaluating bone screw stability of plate fixation

Conventional evaluation of the stability of bone screws focuses on pullout strength, while neglecting lateral migration resistance. We measured pullout strength and lateral migration resistance of bone screws and determined how these characteristics relate to screw stability of locking plate (LP) and dynamic compression plate (DCP) fixation. Pullout strength and lateral migration resistance of individual bone screws with buttress, square, and triangular thread designs were evaluated in polyurethane foam blocks. The screw types with superior performance in each of these characteristics were selected. LP and DCP fixations were constructed using the selected screws and tested under cyclic craniocaudal and torsional loadings. Subsequently, the association between individual screws’ biomechanical characteristics and fixation stability when applied to plates was established. Screws with triangular threads had superior pullout strength, while screws with square threads demonstrated the highest lateral migration resistance; they were selected for LP and DCP fixations. LPs with square-threaded screws required a larger force and more cycles to trigger the same amount of displacement under both craniocaudal and torsional loadings. Screws with triangular and square threads showed no difference in DCP fixation stability under craniocaudal loading. However, under torsional loading, DCP fixation with triangular-threaded screws demonstrated superior fixation stability. Lateral migration resistance is the primary contributor to locking screw fixation stability when applied to an LP in resisting both craniocaudal and torsional loading. For compression screws applied to a DCP, lateral migration resistance and pullout strength work together to resist craniocaudal loading, while pullout strength is the primary contributor to the ability to resist torsional loading.

Variations in non-locking screw insertion conditions generate unpredictable changes to achieved fixation tightness and stripping rates

BACKGROUND

Screws are the most commonly inserted orthopaedic implants. However, several variables related to screw insertion and tightening have not been evaluated. This study aimed firstly to assess the effect of insertion variables on screw tightness, secondly to improve methodologies used by researchers when testing screw insertion techniques and thirdly to assess for any learning or fatigue effects when inserting screws.

METHODS

Two surgeons tightened a total of 2280 non-locking, 3.5 mm cortical screws, with 120 screws inserted to what they felt to be optimum tightness whilst varying each of the following factors: different screwdrivers for measuring torque, screwdriver orientation, gloves usage, dominant/non-dominant hand usage, awareness to the applied torque (blinded, unblinded and re-blinded), four bone densities and seven cortical thicknesses. Screws were tightened to failure to determine stripping torque, which was used to calculate screw tightness - ratio between stopping and stripping torque.

FINDINGS

Screw tightness increased with glove usage, being blinded to the applied torque and with denser artificial bone and with thinner cortices. Considering all the insertions performed, the two surgeons stopped tightening screws at difference values of tightness ((77% versus 66% (p < 0.001)). A learning effect was observed with some parameters including sterile gloves usage and non-dominant hand application.

INTERPRETATION

Different insertion conditions frequently changed screw tightness for both surgeons. Given the influence of screw tightness on fixation stability, the variables investigated within this study should be carefully reported and controlled when performing biomechanical testing alongside practicing screw insertion under different conditions during surgical training.

Stripping torques in human bone can be reliably predicted prior to screw insertion with optimum tightness being found between 70% and 80% of the maximum

AIMS

To devise a method to quantify and optimize tightness when inserting cortical screws, based on bone characterization and screw geometry.

METHODS

Cortical human cadaveric diaphyseal tibiae screw holes (n = 20) underwent destructive testing to firstly establish the relationship between cortical thickness and experimental stripping torque (Tstr), and secondly to calibrate an equation to predict Tstr. Using the equation's predictions, 3.5 mm screws were inserted (n = 66) to targeted torques representing 40% to 100% of Tstr, with recording of compression generated during tightening. Once the target torque had been achieved, immediate pullout testing was performed.

RESULTS

Cortical thickness predicted Tstr (R2 = 0.862; p < 0.001) as did an equation based on tensile yield stress, bone-screw friction coefficient, and screw geometries (R2 = 0.894; p < 0.001). Compression increased with screw tightness up to 80% of the maximum (R2 = 0.495; p < 0.001). Beyond 80%, further tightening generated no increase in compression. Pullout force did not change with variations in submaximal tightness beyond 40% of Tstr (R2 = 0.014; p = 0.175).

CONCLUSION

Screw tightening between 70% and 80% of the predicted maximum generated optimum compression and pullout forces. Further tightening did not considerably increase compression, made no difference to pullout, and increased the risk of the screw holes being stripped. While further work is needed for development of intraoperative methods for accurate and reliable prediction of the maximum tightness for a screw, this work justifies insertion torque being considerably below the maximum.Cite this article: Bone Joint Res 2020;9(8):493-500.

Locking plate constructs benefit from interfragmentary lag screw fixation with decreased shear movements and more predictable fracture gap motion in simple fracture patterns

BACKGROUND

A mechanical characterisation of lag screw fixation plus locking plate - although clinically widely used as either "mixed fixation concept" or absolutely stable fixation - is so far missing. This study aimed to evaluate the influence of an interfragmentary lag screw on the resulting motion at the fracture site of locking plate constructs using a simple fracture at the distal femur.

METHODS

Human cadaver femora were in vitro loaded in torsion and axial bending-compression with and without lag screw fixation next to a locking plate fixation. In addition, two plate working lengths were tested. Interfragmentary movement was measured optically.

FINDINGS

Axial interfragmentary movement is reduced with lag screw (102 mm plate working length, 1000 N, mean): 0.28 mm versus 0.82 mm. With lag screw, the fracture gap stays closed with mean normal interfragmentary movement ≤0.03 mm. Fracture gap tends to open without lag screw: normal interfragmentary movement up to -0.29 mm. Reduction of shear interfragmentary movement was observed throughout all tested loads and groups. Mean true shear remains generally low with lag screw (≤0.42 mm) compared to without lag screw (≤1.46 mm). We also found that interfragmentary movement variance decreases with lag screw, especially for longer plate working length.

INTERPRETATION

An interfragmentary lag screw next to locking bridge plating reduces fragment motion in vitro for a simple fracture pattern and provides a sufficient tool to decrease detrimental shear movements. Prospective clinical trials with interfragmentary lag screw fixation should prove these findings in wide clinical use to treat simple fracture patterns.