A comparison of screw insertion torque and pullout strength

Biomechanical comparison of a new undercut thread design vs the V-shape thread design for pedicle screws

BACKGROUND CONTEXT

Thread shape is regarded as an important factor influencing the fixation strength and osseointegration of bone screws. However, commercial pedicle screws with a V-shaped thread are prone to generating stress concentration at the bone-screw interface, thereby increasing the risk of screw loosening. Thus, modification of the pedicle-screw thread is imperative.

PURPOSE

This study aimed to investigate the fixation stability of pedicle screws with the new undercut thread design in comparison to pedicle screws with a V-shaped thread.

STUDY DESIGN

In vitro cadaveric biomechanical test and finite element analysis (FEA).

METHODS

Pedicle screws with the undercut thread (characterized by a flat crest feature and a tip-facing undercut feature) were custom-manufactured, whereas those with the V-shaped thread were procured from a commercial supplier. Fixation stability was assessed by the cyclic nonpullout compressive biomechanical testing on cadaveric female osteoporotic vertebrae. The vertical displacement and rotation angle of the 2 types of pedicle screws were calculated every 100 cycles to evaluate their resistance to migration and rotation. FEA was conducted to investigate the stress distribution and bone damage at the bone-screw interface for both types of pedicle screws.

RESULTS

Biomechanical testing revealed that the pedicle screws with the undercut thread exhibited significantly lower vertical displacement and rotation angles than the pedicle screws with the V-shape thread (p<0.05). FEA results demonstrated a more uniform stress distribution in the bone surrounding the thread in the undercut design than in the V-shape design. Additionally, bone damage resulting from the pedicle screw was lower in the undercut design than in the V-shape design.

CONCLUSIONS

Pedicle screws with an undercut thread are less prone to migration and rotation and thus more stable in the bone than those with a V-shape thread.

CLINICAL SIGNIFICANCE

The undercut thread design may reduce the incidence of pedicle-screw loosening.

A Novel Bone-Screw-Fastener Demonstrates Greater Maximum Compression Force Before Failure Compared With a Traditional Buttress Screw

OBJECTIVES

This study compared the maximal compression force before thread stripping of the novel bone-screw-fastener (BSF) with the traditional buttress screw (TBS) in synthetic osteoporotic and cadaveric bone models.

METHODS

The maximum compression force of the plate-bone interface before loss of screw purchase during screw tightening was measured between self-tapping 3.5-mm BSF and 3.5-mm TBS using calibrated load cells. Three synthetic biomechanical models were used: a synthetic osteoporotic diaphysis (model 1), a 3-layer biomechanical polyurethane foam with 50-10-50 pounds-per-cubic-foot layering (model 2), and a 3-layer polyurethane foam with 50-15-50 pounds-per-cubic-foot layering (model 3). For the cadaveric metaphyseal model, 3 sets of cadaveric tibial plafonds and 3 sets of cadaveric tibial plateaus were used. A plate with sensors between the bone and plate interface was used to measure compression force during screw tightening in the synthetic bone models, while an annular load cell that measured screw compression as it slid through a guide was used to measure compression in the cadaver models.

RESULTS

Across all synthetic osteoporotic bone models, the BSF demonstrated greater maximal compression force before stripping compared with the TBS [model 1, 155.51 N (SD = 7.77 N) versus 138.78 N (SD = 12.74 N), P = 0.036; model 2, 218.14 N (SD = 14.15 N) versus 110.23 N (SD = 8.00 N), P < 0.001; model 3, 382.72 N (SD = 20.15) versus 341.09 N (SD = 15.57 N), P = 0.003]. The BSF had greater maximal compression force for the overall cadaver trials, the tibial plafond trials, and the tibial plateau trials [overall, 111.27 N vs. 97.54 N (SD 32.32 N), P = 0.002; plafond, 149.6 N versus 132.92 N (SD 31.32 N), P = 0.006; plateau, 81.33 N versus 69.89 N (SD 33.38 N), P = 0.03].

CONCLUSIONS

The novel bone-screw-fastener generated 11%-65% greater maximal compression force than the TBS in synthetic osteoporotic and cadaveric metaphyseal bone models. A greater compression force may increase construct stability, facilitate early weight-bearing, and reduce construct failure.

Optimization of Pedicle Screw Parameters for Enhancing Implant Stability Based on Finite Element Analysis

OBJECTIVE

To improve implant stability parameters, including pedicle screw (PS) outer diameter, thread depth, and pitch, by finite element analysis.

METHODS

Insertion and pullout of the PS were simulated by finite element analysis, and the precision of simulation was evaluated by comparison with mechanical tests. Influences of the parameters on the maximum insertion torque and maximum pullout force were analyzed by computational simulations, including single-factor analysis and orthogonal experiments.

RESULTS

The simulation results agreed with the mechanical test results. The order of parameters influencing insertion torque and pullout force was outer diameter > pitch > thread depth. When the pilot hole diameter is 0.1 mm larger than the inner diameter of the PS, the calculated Pearson correlation coefficient between the maximum insertion torque and maximum pullout force was r = 0.99. The optimized PS had a maximum insertion torque of 485.16 N·mm and a maximum pullout force of 1726.33 N, 23.9% and 9.1% higher, respectively, than the values of standard screws.

CONCLUSIONS

The presently used models are feasible for evaluating the implant stability of PSs. The maximum insertion torque and maximum pullout force of PSs are highly correlated and can be improved by increasing the outer diameter and decreasing pitch. Although with the parameters of the PS, pedicle size and bone mineral density are 2 additional factors to consider for better implant stability.

Low-speed instrumented drill press for bone screw insertion

Screw insertion torque is a widely used/effective method for quantifying fixation strength in orthopedic implant research for different screw geometries, implantation sites, and loads. This work reports the construction of an open-source instrumented benchtop screw insertion device for a total cost of $7545 ($492 + $7053 for equipped sensors), as well as validation of the device and an example use-application. The insertion device is capable of recording the axial load, rotational speed, and applied torque throughout the screw insertion process at 10 samples per second, as demonstrated in the validation test. For this combination of bone analog (20 PCF Sawbones©), screw, and loading, the resolution of the torque sensor was 25% of the maximum measured torque; a different model torque sensor would be required to meet ASTM F543-17, which specifies a resolution of 10% of the maximum torque. This system is optimized for fastener insertion at speeds of 120 rpm or less and axial loading up to 50 N.

A radiologic determination of the different screw cutting patterns in cut and uncut orthopedic cortical screws using a novel imaging technique

OBJECTIVE

We hypothesize that cut screws will deform in a manner that increases the core and outer diameters of the screw hole compared to uncut controls, and effects will be more pronounced in titanium screws.

MATERIALS AND METHODS

We used biomechanical polyurethane foam blocks to simulate cortical bone. We organized four groups of stainless steel and titanium cut and uncut screws. Blocks were fitted with a jig to ensure perpendicular screw insertion. We imaged the blocks using digital mammography and measured them using PACS software. Power analysis determined a power of 0.95 and an alpha error of 0.05.

RESULTS

Highly statistically significant differences in core diameter were found after cutting stainless steel and titanium screws. Cutting stainless steel screws increased core diameter by 0.30 mm (95% CI, 0.16 to 0.45; p < .001). Titanium screws' core diameter increased by 0.45 mm (95% CI, 0.30 to 0.61; p < .001). No significant differences were found in the outer diameters of stainless steel and titanium screws after cutting.

CONCLUSION

Titanium and stainless steel screw tracts demonstrated screw core diameter and screw thread pattern deformation after cutting. Titanium screws demonstrated more significant effects.

Relationship between thread depth and fixation strength in cancellous bone screw

BACKGROUND

Clarifying the effect of each parameter of screw design on its fixation strength is critical in the development of any type of screw. The purpose of this study was to clarify the relationship between the thread depth and fixation strength of metal screws for cancellous bone.

METHODS

Nine types of custom-made screws with the only changed variable being the thread depth were manufactured. Other elements were fixed at a major diameter of 4.5 mm, a thread region length of 15 mm, a pitch of 1.6 mm, and a thread width of 0.20 mm. The pull-out strength and insertion torque of each screw were measured for each of two foam-block densities (10 or 20 pcf). The correlation between the thread depth of the screw and the mechanical findings were investigated with single regression analysis.

RESULTS

Regardless of the foam-block density, the pull-out strength significantly increased as the thread depth increased from 0.1 mm to 0.4 mm; after that, the increase was more gradual (p < 0.01, respectively). The relationship between the thread depth and insertion torque was similar. In addition, the insertion torque tended to be more strongly affected by screw depth than the pull-out strength (2.6 times at 20 pcf and 1.4 times at 10 pcf).

CONCLUSIONS

The pull-out strength of 4.5-mm-diameter metal screws in a cancellous bone model was found to be biphasic, although linearly correlated with the change in screw depth in both phases. The boundary of the correlation was 0.4 mm regardless of the density of the bone model, with the effect of screw depth on pull-out strength beyond that being small in comparison.

Evaluation of the efficacy of modal analysis in predicting the pullout strength of fixation bone screws

Background

Pilot hole preparation has been shown to have an impact on the short and long-term stability of the screw fixation constructs.

Purpose

Investigation and comparison of two nondestructive modal analysis methods with conventional insertion torque (IT) and pullout tests in optimum pilot hole diameter detection.

Methods

Twenty conical core titanium screws were embedded in high-density polyethylene blocks with different pilot hole diameters. The maximum IT was recorded for each screw during implantation. Then, two modal analysis methods including accelerometer (classical modal analysis [CMA]) and acoustic modal analysis (AMA) were carried out to measure the natural frequency (NF) of the bone-screw structure. Finally, stiffness (S), pullout force (Fult), displacement at Fult (dult) and energy dissipation (ED) were obtained from the destructive pullout test.

Results

The IT increased, as the pilot hole diameter decreased. The maximum value of IT was observed in the smallest pilot hole diameter. The same trend was found for the Fult and the first NF derived from both modal methods except for 5.5 mm pilot hole diameter. The natural NFs derived from CMA and AMA showed high correlations in different groups (R2 = 0.94) and did not deviate from y = x hypothesis in linear regression analysis. The Fult, dult, and ED were measured 4800 ± 172 N, 3.10 ± 0.08 mm and 14.23 ± 1.10 N.mm, respectively.

Discussion

No significant change was observed in "S" between the groups. The highest Fult and first NF were obtained for the 5.5 mm pilot hole diameter. Both CMA and AMA were found to be reliable methods and can promote the undesirable contradiction between Fult and IT.

Biomechanical comparison of the undercut thread design versus conventional buttress thread for the lag screw of the dynamic hip screw system

Objective: To compare the fixation stability of the lag screw with a undercut thread design for the dynamic hip screw (DHS) system versus the lag screw with the conventional buttress thread. Methods: The lag screws with the undercut thread (a flat crest feature, a tip-facing undercut feature) and buttress thread were both manufactured. Fixation stability was investigated using cyclic compressive biomechanical testing on custom osteoporotic femoral head sawbone. The forces required for the same vertical displacement in the two types of lag screw were collected to evaluate the resistance to migration. Varus angle was measured on X-ray images to assess the ability in preventing varus collapse. Finite element analysis (FEA) was performed to analyze the stress and strain distribution at the bone-screw interface of the two types of lag screws. Results: The biomechanical test demonstrated that the force required to achieve the same vertical displacement of the lag screw with the undercut thread was significantly larger than the lag screw with conventional buttress thread (p < 0.05). The average varus angles generated by the undercut and buttress threads were 3.38 ± 0.51° and 5.76 ± 0.38°, respectively (p < 0.05). The FEA revealed that the region of high-stress concentration in the bone surrounding the undercut thread was smaller than that surrounding the buttress thread. Conclusion: The proposed DHS system lag screw with the undercut thread had higher migration resistance and superior fixation stability than the lag screw with the conventional buttress thread.

Novel Dual-Threaded Pedicle Screws Provide Fixation Stability That Is Comparable to That of Traditional Screws with Relative Bone Preservation: An In Vitro Biomechanical Study

Replacement with larger diameter screws is always used in pedicle screw loosening but carries a risk of pedicle wall violation. A pedicle screw with more preserved bone stock is the preferred primary fixation choice. The purpose of this study was to evaluate whether a newly designed proximal-conical dual-thread screw with less bone occupancy provides fixation strength comparable to that of a traditional screw. Six types of pedicle screws based on three different shapes (cylindrical, conical, and proximal-conical) and two thread profiles (single-thread and dual-thread) were grouped. Conical and proximal-conical screws differed mainly in the slope of the outer diameter from the hub to the tip. Conical screws had an outer diameter (6.5 mm) that differed from the hub and tapered by 30% to an outer diameter (4.5 mm) at the tip and proximal-conical screws had the same outer diameter from the hub and tapered by 30% (4.5 mm) at 20 mm from the hub and then maintained the outer diameter (45 mm) to the tip. A total of 36 L4 Sawbones® vertebrae were used in the study and six trials for each screw group. The results of the imaging, screw volume in bone, insertion torque, and pullout force were analyzed. For screws with the same shape, insertion torque and pullout force were significantly higher for those in the dual-thread groups than for those in the single-thread groups (p < 0.05). For screws with the same thread profile, there was no significant difference in either biomechanical test between the different screw shapes (p > 0.05). Our results demonstrated that these proximal-conical dual-thread screws, with the property of relative bone stock preservation, display a comparable biomechanical performance to traditional dual-thread screws and a better performance than single-thread screws. This screw design could serve as the primary pedicle screw choice to reduce revision difficulty.

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.

Avoiding Glenoid Baseplate Fixation Failure by Altering Surgical Technique for Varying Bone Densities

Glenoid baseplate failure is one of the causes of revision and poor outcomes in reverse shoulder arthroplasty (RSA). The objective of this study was to determine whether alterations in surgical technique can improve time-zero fixation of the baseplate in varying bone densities. A secondary objective was to identify whether preoperative radiographic glenoid sclerosis width was associated with the implementation of these techniques.

Lag Screw Trajectory in Supination-External Rotation Fractures: Does the Direction of the Fibula Lag Screw Have an Effect?

Background

The fracture obliquity of supination-external rotation injury of the fibula is often amenable to lag screw insertion. The purpose of the study was to determine whether biomechanical differences exist between lag screws inserted from an anterior to posterior direction and from a posterior to anterior direction and the thickness of the anterior and posterior fibular cortices were correlated with biomechanical testing.

Methods

Ten cadaver fibulae were harvested and submitted to material testing following 3.5-mm cortical screw insertion from either an anterior to posterior direction or a posterior to anterior direction. Screw torsional insertion strength and axial pullout strength were measured. Computed tomography images of 40 consecutive patients undergoing preoperative planning for fractures excluding the fibula were examined to define fibular cortical thickness and correlate anatomic findings with the biomechanical testing.

Results

The axial pullout strength of lag screws inserted from posterior to anterior was significantly greater than that of lag screws inserted from anterior to posterior (p < 0.05). Screw insertion torque measurements demonstrated a similar trend although the data did not reach statistical significance (p = 0.056). The anterior cortex of the distal fibula exhibited a radiographically greater thickness than that of the posterior cortex at the same level (p < 0.001).

Conclusions

For oblique fractures of the distal fibula, posterior to anterior lag screw insertion exhibited improved biomechanical properties when compared with a similar screw inserted from anterior to posterior. These results correlated with the thicker cortical bone present along the anterior fibula.

Impact on periosteal vasculature after dual plating of the distal femur: a cadaveric study

Although dual plating of distal femur fractures has been described for injuries at risk of varus displacement, the vascular insult to the medial distal femur utilizing this technique is unknown. The aim of this study was to evaluate the perfusion of the medial distal femoral periosteal arteries after supplemental medial plating of the distal femur.

Methods

Fifteen human fresh-frozen cadaveric femora were thawed and randomized to lateral locked plating alone or with supplemental medial plate fixation. Conventional submuscular medial plating was performed using a 12-hole small fragment plate and multiple cortical screws. The superficial femoral artery was injected with latex dye. Specimens were dissected. The patency of the medial distal femoral periosteal vessels was evaluated.

Results

Four vessels were consistently observed traversing the distal medial femur: the transverse and descending (d-MMPA) branches of the medial metaphyseal periosteal artery, and the transverse and longitudinal branches of the descending geniculate artery. The anterior longitudinal arch (ALA) was present in 13 of 15 specimens and was fed by the d-MMPA. The median number of periosteal arteries occluded by the medial plate was 2 (6 out of 8 specimens). The d-MMPA was occluded in 6 of 8 medially plated femurs, resulting in a complete lack of perfusion of the ALA.

Conclusions

Submuscular medial plating of the distal femur compressed the d-MMPA in the majority of specimens. This vessel gives rise to the ALA, which lacked perfusion in these specimens. This vascular insult could affect the healing of metaphyseal distal femur fractures treated with dual plating.

Biomechanical Comparison of Fixation Stability among Various Pedicle Screw Geometries: Effects of Screw Outer/Inner Projection Shape and Thread Profile

The proper screw geometry and pilot-hole size remain controversial in current biomechanical studies. Variable results arise from differences in specimen anatomy and density, uncontrolled screw properties and mixed screw brands, in addition to the use of different tapping methods. The purpose of this study was to evaluate the effect of bone density and pilot-hole size on the biomechanical performance of various pedicle screw geometries. Six screw designs, involving three different outer/inner projections of screws (cylindrical/conical, conical/conical and cylindrical/cylindrical), together with two different thread profiles (square and V), were examined. The insertional torque and pullout strength of each screw were measured following insertion of the screw into test blocks, with densities of 20 and 30 pcf, predrilled with 2.7-mm/3.2-mm/3.7-mm pilot holes. The correlation between the bone volume embedded in the screw threads and the pullout strength was statistically analyzed. Our study demonstrates that V-shaped screw threads showed a higher pullout strength than S-shaped threads in materials of different densities and among different pilot-hole sizes. The configuration, consisting of an outer cylindrical shape, an inner conical shape and V-shaped screw threads, showed the highest insertional torque and pullout strength at a normal and higher-than-normal bone density. Even with increasing pilot-hole size, this configuration maintained superiority.

3D pull-out finite element simulation of the pedicle screw-trabecular bone interface at strain rates

Biomedical experimental studies such as pull-out (PO), screw loosening experience variability mechanical properties of fresh bone, legal procedures of cadaver bone samples and time-consuming problems. Finite Element Method (FEM) could overcome experimental problems in biomechanics. However, material modelling of bone is quite difficult, which has viscoelastic and viscoplastic properties. The study presents a bone material model which is constructed at the strain rates with the Johnson-Cook (JC) material model, one of the robust constitutive material models. The JC material constants of trabecular bone are determined by the curve fitting method at strain rates for the 3D PO finite element simulation, which defines the screw-bone interface relationship. The PO simulation is performed using the Abaqus/CAE software program. Bone fracture mechanisms are simulated with dynamic/explicit solutions during the PO phenomenon. The paper exposes whether the strain rate has effects on the PO performance. Moreover, simulation reveals the relationship between pedicle screw diameter and PO performance. The results obtained that the maximum pull-out force (POF) improves as both the screw diameter and the strain rate increase. For 5.5 mm diameter pedicle screw POFs were 487, 517 and 1708 N at strain rate 0.00015, 0.015 and 0.015 s^-1, respectively. The FOFs obtained from the simulation of the other screw were 730, 802 and 2008 N at strain rates 0.00015, 0.0015 and 0.015, respectively. PO phenomenon was also simulated realistically in the finite element analysis (FEA).

Can barb thread design improve the pullout strength of bone screws?

AIMS

To draw a comparison of the pullout strengths of buttress thread, barb thread, and reverse buttress thread bone screws.

METHODS

Buttress thread, barb thread, and reverse buttress thread bone screws were inserted into synthetic cancellous bone blocks. Five screw-block constructs per group were tested to failure in an axial pullout test. The pullout strengths were calculated and compared. A finite element analysis (FEA) was performed to explore the underlying failure mechanisms. FEA models of the three different screw-bone constructs were developed. A pullout force of 250 N was applied to the screw head with a fixed bone model. The compressive and tensile strain contours of the midsagittal plane of the three bone models were plotted and compared.

RESULTS

The barb thread demonstrated the lowest pullout strength (mean 176.16 N (SD 3.10)) among the three thread types. It formed a considerably larger region with high tensile strains and a slightly smaller region with high compressive strains within the surrounding bone structure. The reverse buttress thread demonstrated the highest pullout strength (mean 254.69 N (SD 4.15)) among the three types of thread. It formed a considerably larger region with high compressive strains and a slightly smaller region with high tensile strains within the surrounding bone structure.

CONCLUSION

Bone screws with a reverse buttress thread design will significantly increase the pullout strength. Cite this article: Bone Joint Res 2021;10(2):105-112.

Drilling Energy Correlates With Screw Insertion Torque, Screw Compression, and Pullout Strength: A Cadaver Study

INTRODUCTION

To determine whether drilling energy correlates with bone mineral density (BMD), maximum insertion torque (MIT), maximum screw compression, and pullout strength (POS).

METHODS

Ten cadaver tibias were used for testing. Unicortical pilot holes were drilled and the drilling energy measured. Drill site bone quality was determined with microcomputed tomography. Drill holes were randomly assigned to POS or MIT testing using 3.5-mm cortical screws engaging only the near cortex. Pearson correlation coefficients were calculated to determine the relationship between drilling energy, BMD, POS, MIT, and maximum screw compression.

RESULTS

Drilling energy was correlated with BMD (P < 0.001). Compared with BMD, drilling energy had a better correlation with MIT, maximum screw compression, and POS. Maximum screw compression also correlated with MIT (P = 0.012).

CONCLUSIONS

Drilling energy better correlates with MIT, maximum screw compression, and POS compared with BMD in cadaver cortical bone. Dynamically measuring drilling energy may help inform the orthopaedic surgeon as to the quality of the bone before insertion of implants.

Biomechanical evaluation of the screw preload values used in the plate placement for bone fractures

The purpose of this study is to examine the effects of screw preload values on the bone-plate system. The preload value was taken differently in the literature range from 50 N to 3000 N. These preload value were examined in this study. The finite element method was used to calculate the strain and stress on the models. The long bone, plate and screws were modeled as 3D using CAD software. The finite element models were created using Ansys Workbench software. The convergence and validation study were made for the correct results. The 400 N axial load was applied to the proximal end of bone. The distal end of the bone fixed for boundary condition. The preload values were applied to the screws differently. The results of the finite element analysis were compared and evaluated. The results showed that when the preload values increased, the von Mises stresses and strains on the bone and plate system increased. The critical preload value of the screw is the 500 N. The upper values of this critical value can be damaged bone and plate system. The critical region of the bone is the holes where the screw inserted. In conclusion, the preload values of the screw should not exceed the 500 N for the successful fixation.

Pullout Strength After Multiple Reinsertions in Radial Bone Fixation

Background: Due to bone cutting loss from self-tapping screws (STS), progressive destruction of bone can occur with each reinsertion during surgery. When considering the use of jigs that utilize multiple insertions such as those seen in ulnar and radial shortening osteotomy systems, or scenarios where a screw needs to be removed and reinserted due to some technical issue, this can be concerning, as multiple studies examining the effects of multiple reinsertions and the relationship between insertional torque and pullout strength have had mixed results. Methods: Insertional torque and pullout strength were experimentally measured following multiple reinsertions of STS for up to 5 total insertions for various densities and locations along radial sawbone shafts. Results: Torque and pullout strength were significantly greater in middle segments of the radial shaft. Our trials corroborate previous literature regarding a significant reduction in fixation between 1 and 2 insertions; beyond this, there was no significant difference between pullout strength across all segment locations as well as bone densities for 3 to 5 insertions. There was a moderate to high correlation of insertional torque to pullout strength noted across all bone densities and segments (Pearson r = 0.663, P < .001). Conclusion: While reinsertion of STS between 1 and 2 insertions has been shown to significantly differ in pullout strength, beyond this, there does not appear to be a significant difference in up to 5 insertions at any specific region of radial bone across a range of sawbone densities. Further insertions may be considered with caution.

Relationship Between Insertion Torque and Compression Strength in the Reverse Total Shoulder Arthroplasty Baseplate

Reverse shoulder arthroplasty is a well-established procedure, however, there is limited data in the literature regarding adequate insertion torque and the resulting compression for glenoid baseplate fixation. In this biomechanical study, we evaluated the relationship between insertion torque and baseplate compression by simultaneously measuring the insertion torque and axial compressive forces generated by two reverse shoulder arthroplasty baseplates with central screw design. Three different bone surrogates were chosen to mimic clinical scenarios where differences in compression achieved during baseplate insertion may exist due to varying bone quality. Epoxy resin sheets were combined with the bone surrogates to simulate the glenoid vault. A digital torque gauge was used to measure insertion torque applied to the baseplate, while compression data were collected continuously from a load cell. A strong positive correlation was found between baseplate compression and insertion torque. Among the lower density bone surrogates, neither baseplate design reached maximum insertion torque (6.8 Nm) due to material strip-out. This phenomenon did not occur in denser bone surrogates. Both baseplate designs experienced a significant increase in mean baseplate compression as insertion torque increased and were found to behave similar in the denser bone surrogates. The results presented here suggest that larger compressive forces can be achieved with an increase in insertion torque in denser bone surrogates, but caution must be used when trying to achieve fixation in poor-quality bone. Clinically, this could be useful preoperatively to minimize baseplate failure, and in further studies regarding baseplate design for improved initial fixation and stability. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:871-879, 2020.

MULTI-OBJECTIVE OPTIMIZATION FOR SURGICAL DRILLING OF HUMAN FEMURS: A METHODOLOGY FOR BETTER PULL-OUT STRENGTH OF FIXATION USING TAGUCHI METHOD BASED ON MEMBERSHIP FUNCTION

Drilling through bone is one of the common cutting processes involved in many of the orthopedic surgeries. In bone drilling, spindle speed, feed rate, diameter of the drill bit, drill bit geometry and method of cooling are the important parameters to influence the in-situ temperature, drill thrust force and quality characteristics of the drilled hole. Because of the selection of inappropriate drilling parameters, uncontrolled large drilling forces, continuous increase in temperature and mechanical damage to the local host bone were observed. As these adverse effects lead to poor bone–implant contact and often a revision surgery, performing a surgical drilling with optimal parameters is essential to succeed in the surgical procedure. It was observed that in addition to the variations in apparent bone density, the orientation of osteons influences the drilling thrust force and temperature in bone drilling. Ten adult cadaveric human femurs from the age group of 32–65 years were considered and drilling experiments were conducted on proximal-diaphysis, mid-diaphysis and distal-diaphysis regions in the longitudinal, radial and circumferential directions. Bone drilling with different spindle speeds (500, 1000 and 1500[Formula: see text]rpm), feed rates (40, 60 and 80[Formula: see text]mm/min), and apparent density in the range of 0.98[Formula: see text]g/cm3 to 1.98[Formula: see text]g/cm3 was investigated in this work using a 3.20[Formula: see text]mm diameter surgical drill-bit. The generation of in-situ temperature as well as thrust force at each target location was measured using [Formula: see text]-type thermocouple and Kistler[Formula: see text] dynamometer, respectively. Taguchi method based on membership function was used to optimize the drilling process. Then the efficacy of the method in reducing the in-situ temperature and thrust force, and quality of the drilled hole in respect of anatomical region and drilling direction was investigated using pull-out strength of the bone screws. Results revealed that the optimal parameters obtained from the Taguchi method based on membership function could simultaneously minimize the temperature as well as thrust force in bone drilling. The proposed method can be adopted to minimize the temperature and thrust force, and choose the best location nearest to the defect site for strong implant fixation by using CT datasets of the patient as the only input.

Screw insertion torque as parameter to judge the fixation. Assessment of torque and pull-out strength in different bone densities and screw-pitches

BACKGROUND

Pull-out strength is a critical parameter to judge screw fixation in orthopaedic implants. However, the insertion torque is the main feeling in the hand of a surgeon relating to the strength of synthesis. The correlation between pull-out strength and torque is not completely understood. This creates uncertainty about the key-question: Should the torque be considered a valid parameter to judge the quality of fixation?

METHODS

Using the ASTM F543 as reference, three screws differing only in pitch (1.5, 2.1, 2.8 mm pitches) were tested in three foam-block densities (10, 15, and 20 pcf). The correlation was investigated by assessing the role of density and screw geometry.

FINDINGS

Torque was related to pull-out strength in all configurations (R = 0.979, P = 0.000). No difference in pull-out strength was found when screws were tightened to a range of 71.6%, SD = 7.6, of torque to fail (P > 0.05). Torque and pull-out strength were stratified according to density that influenced the two parameters up to 524% (P < 0.000). Pitch determined pull-out strength up to 33% (P < 0.000) while the 2.1 mm screw pitch showed the highest pull-out strength and torque in all configurations.

INTERPRETATION

Insertion torque was demonstrated to be a valid parameter to judge the quality of bone under fixation and therefore, the strength of the synthesis. Surgeons should not tighten the screws to values approaching torque to fail to obtain the highest pull-out strength. Density was the main factor influencing pull-out strength and torque. Pitch is another parameter deciding screw holding capacity.

Kirschner wire prepared pilot holes improve screw pullout strength in synthetic osteoporotic-type bone

OBJECTIVES

To compare the pullout strength and maximal insertional torque of pilot holes prepared with the traditional twist drill bit versus a smooth Kirschner wire.

METHODS

Pilot holes were prepared using a drill press with either a 2.5 mm twist drill bit or a 2.5 mm smooth Kirschner wire into 2 distinct polyurethane foam densities representing severe and mild osteoporotic bone. 3.5 mm cortical and 4.0 mm cancellous screws were then inserted freehand into the prepared holes. All permutations of pilot hole type, screw size and foam density were tested for maximum pullout strength and maximum insertional torque.

RESULTS

Kirschner wire prepared pilot holes resulted in significantly higher pullout load than drill bit holes in low density blocks (P < 0.001), but not in high density blocks (P = 0.232). There was no statistical difference (P > 0.05) for maximum insertional torque in the pilot hole preparation type.

CONCLUSION

In severely osteoporotic bone, Kirschner wire pilot hole preparation may improve screw pullout strength.

Fully threaded sacroiliac lag screws have higher load to failure when compared to partially threaded screws: A biomechanical study

The purpose of this study is to compare biomechanical properties of fully and partially threaded iliosacral screws. We hypothesise that fully threaded screws will have a higher yield force, and less deformation than partially threaded screws following axial loading. Twenty sawbone blocks were uniformly divided to simulate vertical sacral fractures. Ten blocks were affixed with fully threaded iliosacral screws in an over-drilled, lag-by-technique fashion whilst the remaining ten were fixed with partially threaded lag-by-design screws. All screws measured 7.3-mm x 145 mm, and were inserted to a 70% of calculated maximal insertional torque, ensuring uniform screw placement throughout across models. Continuous axial loads were applied to 3 constructs of each type to failure to determine baseline characteristics. Five hundred loading cycles of 500 N at 1 Hz were applied to 4 constructs of each type, and then axially loaded to failure. Force displacement curves, elastic, and plastic deformation of each construct was recorded. Fully threaded constructs had a 428% higher yield force, 61% higher stiffness, 125% higher ultimate force, and 66% lower yield deformation (p < 0.05). The average plastic deformation for partially threaded constructs was 336% higher than fully threaded constructs (p = 0.071), the final elastic deflection was 10% higher (p = 0.248), and the average total movement was 21% higher (p = 0.107). We conclude from this biomechanical study that fully threaded, lag-by-technique iliosacral screws can withstand significantly higher axial loads to failure than partially threaded screws. In addition, fully threaded screws trended towards exhibiting a significantly lower plastic deformation following cyclical loading.

3D-printed PLA/HA composite structures as synthetic trabecular bone: A feasibility study using fused deposition modeling

Additive manufacturing has significant advantages, in the biomedical field, allowing for customized medical products where complex architectures can be achieved directly. While additive manufacturing can be used to fabricate synthetic bone models, this approach is limited by the printing resolution, at the level of the trabecular bone architecture. Therefore, the aim of this study was to evaluate the possibilities of using fused deposition modeling (FDM) to this end. To better mimic real bone, both in terms of mechanical properties and biodegradability, a composite of degradable polymer, poly(lactic acid) (PLA), and hydroxyapatite (HA) was used as the filament. Three PLA/HA composite formulations with 5-10-15 wt% HA were evaluated, and scaled up human trabecular bone models were printed using these materials. Morphometric and mechanical properties of the printed models were evaluated by micro-computed tomography, compression and screw pull out tests. It was shown that the trabecular architecture could be reproduced with FDM and PLA by applying a scaling factor of 2-4. The incorporation of HA particles reduced the printing accuracy, with respect to morphology, but showed potential for enhancement of the mechanical properties. The scaled-up models displayed comparable, or slightly enhanced, strength compared to the commonly used polymeric foam synthetic bone models (i.e. Sawbones). Reproducing the trabecular morphology by 3D printed PLA/HA composites appears to be a promising strategy for synthetic bone models, when high printed resolution can be achieved.

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

BACKGROUND

Millions of non-locking screws are manually tightened during surgery each year, but their insertion frequently results in overtightening and damage to the surrounding bone. We postulated that by calculating the torque limit of a screw hole, using bone and screw properties, the risk of overtightening during screw insertion could be reduced. Additionally, predicted maximum torque could be used to identify optimum screw torque, as a percentage of the maximum, based on applied compression and residual pullout strength.

METHODS

Longitudinal cross-sections were taken from juvenile bovine tibial diaphyses, a validated surrogate of human bone, and 3.5 mm cortical non-locking screws were inserted. Fifty-four samples were used to define the association between stripping torque and cortical thickness. The relationship derived enabled prediction of insertion torques representing 40 to 100% of the theoretical stripping torque (T_str) for a further 170 samples. Screw-bone compression generated during insertion was measured, followed immediately by axial pullout testing.

FINDINGS

Screw-bone compression increased linearly with applied torque up to 80% of T_str (R² = 0.752, p < 0.001), but beyond this, no significant further compression was generated. After screw insertion, with all screw threads engaged, more tightening did not create any significant (R² = 0.000, p = 0.498) increase in pullout strength.

INTERPRETATION

Increasing screw tightness beyond 80% of the maximum did not increase screw-bone compression. Variations in torques below T_str, did not affect pullout forces of inserted screws. Further validation of these findings in human bone and creation of clinical guidelines based on this research approach should improve surgical outcomes and reduce operative costs.

Cannulated Screw Prominence in Tension Band Wiring of Patella Fractures Increases Fracture Gapping: A Cadaver Study

BACKGROUND

Transverse patella fractures are often treated with cannulated screws and a figure-of-eight anterior tension band. A common teaching regarding this construct is to recess the screws so that their distal ends do not protrude beyond the patella because doing so may improve biomechanical performance. However, there is a lack of biomechanical or clinical data to support this recommendation.

QUESTION

In the treatment of transverse patella fractures, is there a difference between prominent and recessed cannulated screw constructs, supplemented by tension banding, in terms of gap formation from cyclic loading and ultimate load to failure?

METHODS

Ten pairs of fresh-frozen cadaver legs (mean donor age, 72 years; range, 64-89 years) were randomized in a pairwise fashion to prominent or standard-length screws. In the prominent screw group, screw length was 15% longer than the measured trajectory, resulting in 4 to 6 mm of additional length. Each patella was transversely osteotomized at its midportion and fixed with screws and an anterior tension band. Gap formation was measured over 40 loaded flexion-extension cycles (90° to 5°). Ultimate load to failure was assessed with a final monotonic test after cyclic loading. Areal bone mineral density (BMD) of each patella was measured with dual energy x-ray absorptiometry (DEXA). There was no difference in BMD between the recessed (1.06 ± 0.262 g/cm) and prominent (1.03 ± 0.197 g/cm) screw groups (p = 0.846). Difference in gap formation was assessed with a Wilcoxon Rank Sum Test. Ultimate load to failure and BMD were assessed with a paired t-test.

RESULTS

Patella fractures fixed with prominent cannulated screws demonstrated larger gap formation during cyclic loading. Median gap size at the end of cyclic loading was 0.13 mm (range, 0.00-2.92 mm) for the recessed screw group and 0.77 mm (range, 0.00-7.50 mm) for the prominent screw group (p = 0.039; 95% confidence interval [CI] difference of geometric means, 0.05-2.12 mm). There was no difference in ultimate failure load between the recessed screw (891 ± 258 N) and prominent screw (928 ± 268 N) groups (p = 0.751; 95% CI difference of means, -226 to 301 N). Ultimate failure load was correlated with areal BMD (r = 0.468; p = 0.046).

CONCLUSIONS

In this cadaver study, when using cannulated screws and a figure-of-eight tension band to fix transverse patella fractures, prominent screws reduced the construct's ability to resist gap formation during cyclic loading testing.

CLINICAL RELEVANCE

This biomechanical cadaver study found that the use of prominent cannulated screws for the fixation of transverse patella fractures increases the likelihood of interfragmentary gap formation, which may potentially increase the risk of fracture nonunion and implant failure. These findings suggest that proximally and distally recessed screws may increase construct stability, which may increase the potential for bony healing. The findings support further laboratory and clinical investigations comparing recessed screws supplemented by anterior tension banding with other repair methods that are in common use, such as transosseous suture repair.

A New Fastener With Improved Bone-To-Implant Interface Shows Superior Torque Stripping Resistance Compared With the Standard Buttress Screw

OBJECTIVE

The conventional AO buttress screw used for fracture fixation relies on a historic buttress thread design, which is prone to stripping at the bone-implant interface. We hypothesized that a new Bone-Screw-Fastener with an innovative interlocking thread design demonstrates increased resistance to torque stripping forces compared with the buttress screw, without compromising pullout strength.

METHODS

A biomechanical model was established in 6 matched pairs of adult human cadaveric tibiae to test torque resistance between the 3.5 mm Bone-Screw-Fastener and the 3.5 mm cortical AO buttress screw until failure. Uniaxial pullout testing of both screw types was performed as an internal control experiment.

RESULTS

The 3.5 mm Bone-Screw-Fastener had a significantly increased resistance to torque failure compared with the standard 3.5 mm AO buttress screw (P = 0.0145). In contrast to the buttress screws, none of the Bone-Screw-Fasteners stripped from the bone but rather failed at the screwdriver-implant interface in terms of a metal-on-metal failure. The internal control experiments revealed no significant difference in axial pullout strength between the 2 implants (P = 0.47).

CONCLUSIONS

These data demonstrate the superiority of the new Bone-Screw-Fastener over the conventional AO buttress screw regarding protection from torque stripping forces. In addition, the new thread design that interlocks to the bone does not sacrifice axial pullout resistance conveyed by the buttress screw. Future controlled trials will have to validate the in vivo relevance of these findings in a clinical setting.

BIOMECHANICAL STUDY OF PEDICLE SCREW FIXATION STRENGTH: ASSOCIATION OF SCREW MALPOSITION AND SCREW INSERTION TORQUE

Pedicle screws have been widely used for the treatment of spinal diseases, but improper screw placement is not uncommon and may lead to neurovascular injuries and reduced screw fixation strength. This study aimed to investigate the feasibility of using real-time screw insertion torque monitoring to prevent screw penetration. Commercially available synthetic L4 vertebrae were divided in to seven test groups based on different screw placements. Screw insertion torque and maximal pullout strength were compared among groups. The results indicated that the insertion torque gradually increased when the screw tip was within vertebral cancellous bone without penetration. However, an instantaneous decrease of torque value was observed once the screw tip penetrated the cortex wall. When compared to the control group, higher pullout strength was found for the groups with medial cortex penetration. However, vertebrae with medial cortex penetration may lead to the concern of neurovascular damage. Meanwhile, lower pullout strength was found for the groups with lateral cortex penetration and end-plate penetration, which may lead to the concern of screw loosening. We concluded that pedicle screw penetration can be judged using real-time screw insertion torque monitoring during surgery, which may aid surgeons in avoiding neurovascular injury and reduction of screw fixation strength.

Using a corkscrew-tipped telescopic nail in the treatment of osteogenesis imperfecta: a biomechanical study and preliminary results of 17 consecutive cases

High complication rates were reported with the telescopic nail technique systems. To overcome such technical difficulties, we designed a corkscrew-tipped telescopic nail (CTTN). We biomechanically compared its pullout strength with that of two other tip designs. We used CTTN in 17 patients with osteogenesis imperfecta and reported their preliminary results. Average patient age was 82.6 months, and mean follow-up was 32.0±6 months. Telescoping and osteotomy site healing were assessed using radiological studies. Successful telescoping with event-free osteotomy site healing was achieved in 94.1% of patients; limited telescoping and delayed union were detected in one case each. Our results show that CTTN provides sufficient pullout strength and reduced complication rates compared with other designs.

Does thread shape affect the fixation strength of the bioabsorbable interference screws for anterior cruciate ligament reconstructions? A biomechanical study

Background

The purpose of this study was to compare the biomechanical behaviour of two bioabsorbable interference screws with different geometries.

Methods

Two different pitch (2.5 and 5 mm) bioabsorbable interference screws, both 9 × 30 mm, were tested. Tests were performed with forty bovine digital extensor tendons and skeletally mature porcine tibiae. Two protocols of cyclic tests at 1 Hz were performed: 1000 cycles from 50 to 250 N, and 5000 cycles from 100 to 300 N (n = 10 for each type of test and screw). After the cyclic loading, a final ramp displacement until failure at 0.5 mm/s was applied.

Results

The stiffness after the cyclic phase of the tests was not statistically different between the two screws (1000th cycle: 2.5 mm pitch 280.3 ± 56.4 N/mm, 5 mm pitch 275.2 ± 65.0 N/mm, P = .965; 5000th cycle: 2.5 mm pitch 281.3 ± 66.4 N/mm, 5 mm pitch 286.1 ± 79.4 N/mm, P = .814). The yield load was not significantly different between the screws (1000 cycle tests: 2.5 mm pitch 482.2 ± 120.2 N, 5 mm pitch 495.9 ± 131.3 N, P = .508; 5000 cycle tests: 2.5 mm pitch 476.4 ± 65.3 N, 5 mm pitch 494.3 ± 39.2 N, P = .391). No correlation was found between the insertion torque and yield load (1000 cycle tests, R² = 0.013; 5000 cycle tests, R² = 0.006).

Conclusions

The pitch of bioabsorbable interference screws does not seem to affect fixation strength. Also, the authors recommend not to use insertion torque alone to estimate the fixation strength.

Biomechanical and histologic assessment of a novel screw retention technology in an ovine lumbar fusion model

BACKGROUND CONTEXT

Screw loosening is a prevalent failure mode in orthopedic hardware, particularly in osteoporotic bone or revision procedures where the screw-bone engagement is limited.

PURPOSE

The objective of this study was to evaluate the efficacy of a novel screw retention technology (SRT) in an ovine lumbar fusion model.

STUDY DESIGN/SETTING

This was a biomechanical, radiographic, and histologic study utilizing an ovine lumbar spine model.

METHODS

In total, 54 (n=54) sheep lumbar spines (L₂-L₃) underwent posterior lumbar fusion (PLF) via pedicle screw fixation, connecting rod, and bone graft. Following three experimental variants were investigated: positive control (ideal clinical scenario), negative control (simulation of compromised screw holes), and SRT treatments. Biomechanical and histologic analyses of the functional spinal unit (FSU) were determined as a function of healing time (0, 3, and 12 months postoperative).

RESULTS

Screw pull-out, screw break-out, and FSU stability of the SRT treatments were generally equivalent to the positive control group and considerably better than the negative control group. Histomorphology of the SRT treatment screw region of interest (ROI) observed an increase in bone percentage and decrease in void space during healing, consistent with ingrowth at the implant interface. The PLF ROI observed similar bone percentage throughout healing between the SRT treatment and positive control. Less bone formation was observed for the negative control.

CONCLUSIONS

The results of this study demonstrate that the SRT improved screw retention and afforded effective FSU stabilization to achieve solid fusion in an otherwise compromised fixation scenario in a large animal model.

Dual Motor Drill Continuously Measures Drilling Energy to Calculate Bone Density and Screw Pull-out Force in Real Time

INTRODUCTION

Low bone density complicates the surgical management of fractures. Screw stripping in osteoporotic bone leads to decreased fixation strength and weakening of the fixation construct. If low density could be detected during drilling, augmentation may be performed to prevent screw stripping. Furthermore, continuous monitoring of the drill bit depth and bone density can allow detection of the far cortex where density suddenly increases, providing immediate and accurate screw length measurement and reducing the risk of overpenetration or plunge in osteoporotic bone. Therefore, a dual motor drill was created to calculate bone density and pull-out force in real time. The purpose of this study was to determine whether real-time monitoring of drill bit torque and depth could be used to estimate bone density and pull-out force. We hypothesized that the calculated drilling energy could be used to determine density and would correlate with pull-out force.

METHODS

Drilling and screw insertion were performed using a validated composite unicortical bone model. Screws of 5-, 10-, and 20-mm length were placed into blocks of known densities (10, 20, 30, and 40 pounds per cubic foot). During creation of holes by the dual motor drill, drilling energy was recorded and used to calculate density. Calculated bone density was then compared with the known density of the block. The drill bit was exchanged for a screwdriver, and screw insertion energy was recorded in a similar fashion during screw placement. Screws were then subjected to maximal axial pull-out force testing with a material testing device. Recorded drilling energy and screw insertion energy were then correlated with the measured pull-out force.

RESULTS

Calculated bone density correlated very strongly with the known control density, confirming the accuracy of density calculations in real time. Drilling energy and screw insertion energy correlated very strongly with the measured pull-out force by destructive testing confirming ultimate pull-out force could be quantified during drilling or placement of a screw.

DISCUSSION

Our results confirmed that a dual motor drill can accurately and immediately allow determination of bone density and screw pull-out force before placing a screw. This knowledge could allow a surgeon to perform augmentation or alter surgical technique to prevent screw stripping and loss of fixation as well as detect the far cortex and prevent overpenetration in osteoporotic bone.

Local bone quality measurements correlates with maximum screw torque at the femoral diaphysis

BACKGROUND

Successful fracture fixation depends critically on the stability of the screw-bone interface. Maximum achievable screw torque reflects the competence of this interface, but it cannot be quantified prior to screw stripping. Typically, the surgeon relies on the patients' bone mineral density and radiographs, along with experience and tactile feedback to assess whether sufficient compression can be generated by the screw and bone. However, the local bone quality would also critically influence the strength of the bone-screw interface. We investigated whether Reference Point Indentation can provide quantitative local bone quality measures that can inform subsequent screw-bone competence.

METHODS

We examined the associations between the maximum screw torque that can be achieved using 3.5 mm, 4.5 mm, and 6.5 mm diameter stainless steel screws at the distal femoral metaphysis and mid-diaphysis from 20 cadavers, with the femoral neck bone mineral density and the local measures of bone quality using Reference Point Indentation.

FINDINGS

Indentation Distance Increase, a measure of bone's resistance to microfracture, correlated with the maximum screw stripping torque for the 3.5 mm (p < 0.01; R = 0.56) and 4.5 mm diameter stainless steel screws (p < 0.01; R = 0.57) at the femoral diaphysis. At the femoral metaphysis, femoral neck bone mineral density significantly correlated with the maximum screw stripping torque achieved by the 3.5 mm (p < 0.01; R = 0.61), 4.5 mm (p < 0.01; R = 0.51), and 6.5 mm diameter stainless steel screws (p < 0.01; R = 0.56).

INTERPRETATION

Reference Point Indentation can provide localized measurements of bone quality that may better inform surgeons of the competence of the bone-implant interface and improve effectiveness of fixation strategies particularly in patients with compromised bone quality.

Achievable accuracy of hip screw holding power estimation by insertion torque measurement

BACKGROUND

To ensure stability of proximal femoral fractures, the hip screw must firmly engage into the femoral head. Some studies suggested that screw holding power into trabecular bone could be evaluated, intraoperatively, through measurement of screw insertion torque. However, those studies used synthetic bone, instead of trabecular bone, as host material or they did not evaluate accuracy of predictions. We determined prediction accuracy, also assessing the impact of screw design and host material.

METHODS

We measured, under highly-repeatable experimental conditions, disregarding clinical procedure complexities, insertion torque and pullout strength of four screw designs, both in 120 synthetic and 80 trabecular bone specimens of variable density. For both host materials, we calculated the root-mean-square error and the mean-absolute-percentage error of predictions based on the best fitting model of torque-pullout data, in both single-screw and merged dataset.

FINDINGS

Predictions based on screw-specific regression models were the most accurate. Host material impacts on prediction accuracy: the replacement of synthetic with trabecular bone decreased both root-mean-square errors, from 0.54 ÷ 0.76 kN to 0.21 ÷ 0.40 kN, and mean-absolute-percentage errors, from 14 ÷ 21% to 10 ÷ 12%. However, holding power predicted on low insertion torque remained inaccurate, with errors up to 40% for torques below 1 Nm.

INTERPRETATION

In poor-quality trabecular bone, tissue inhomogeneities likely affect pullout strength and insertion torque to different extents, limiting the predictive power of the latter. This bias decreases when the screw engages good-quality bone. Under this condition, predictions become more accurate although this result must be confirmed by close in-vitro simulation of the clinical procedure.

NUMERICAL ANALYSIS OF TEMPERATURE, SCREWING MOMENT AND THRUST FORCE USING FINITE ELEMENT METHOD IN BONE SCREWING PROCESS

In this study, the bone screwing process carried out with M3.5 cortex screw for stabilization after reduction of femur shaft fracture was investigated both experimentally and numerically. The numerical analyses were performed based on the finite element method using Deform-3D software. The friction, material model, the loading and boundary conditions were exactly identified for finite element analyses. An analytic model and software were developed, which calculate the process parameters such as screwing power and thrust power, heat transfer coefficients in order to determine the temperature distributions occurring in the screw and bone material (sawbones) during screwing process. As a result, the screwing moment and thrust force values decrease with increase of spindle speed. On the contrary, temperature values of screw and sawbones increase with increase of spindle speed. A good consistency between the results obtained from both experimental and numerical simulations was found during the bone screwing process.

Screw Stripping After Repeated Cortical Screw Insertion-Can We Trust the Cancellous "Bailout" Screw?

BACKGROUND

During osteosynthesis standard nonlocking cortical screws often require reinsertion, raising concern over possible decrease in their effectiveness. This study aims to quantify that potential loss of fixation with reinsertions as well as examine the ability of a cancellous "bailout screw" to regain insertion torque in a previously stripped screw hole.

METHODS

Four different types of bone surrogates were chosen to represent normal cortical bone, osteoporotic cortical bone, high-density (normal) cancellous bone, and low-density (osteoporotic) cancellous bone; nonlocked 3.5-mm cortical screws were inserted into the predrilled holes 1, 2, 3, 4, or 5 times before being torqued maximally to the point of stripping. A 4.0-mm cancellous "bailout" screw was then placed into the same hole and torqued until stripping. Torque was measured continuously using a torque-measuring screwdriver and maximal insertion torque (MIT) of 3.5 and 4.0 screws before stripping was recorded.

RESULTS

MIT decreased with reinsertion of nonlocked cortical screws. By the third reinsertion in all but the normal bone surrogates, the screws lost approximately one third to one half of their original MIT (50%-71% of original torque). The bailout screw succeeded in restoring the original MIT in the osteoporotic cancellous bone surrogate and the normal cortical bone surrogate. In the normal cancellous and osteoporotic cortical bone surrogates, the bailout screw was only able to restore an average of 50% (range 31%-63%) of the original MIT.

CONCLUSIONS

Screw reinsertion may significantly reduce the MIT of 3.5-mm nonlocked cortical screws. Use of the bailout cancellous screw for a stripped cortical screw should be expected to restore MIT only in normal cortical bone and osteoporotic cancellous bone. In other scenarios, the bailout screw should not be expected to uniformly restore full insertion torque.

Cancellous Screws Are Biomechanically Superior to Cortical Screws in Metaphyseal Bone

Cancellous screws are designed to optimize fixation in metaphyseal bone environments; however, certain clinical situations may require the substitution of cortical screws for use in cancellous bone, such as anatomic constraints, fragment size, or available instrumentation. This study compares the biomechanical properties of commercially available cortical and cancellous screw designs in a synthetic model representing various bone densities. Commercially available, fully threaded, 4.0-mm outer-diameter cortical and cancellous screws were tested in terms of pullout strength and maximum insertion torque in standard-density and osteoporotic cancellous bone models. Pullout strength and maximum insertion torque were both found to be greater for cancellous screws than cortical screws in all synthetic densities tested. The magnitude of difference in pullout strength between cortical and cancellous screws increased with decreasing synthetic bone density. Screw displacement prior to failure and total energy absorbed during pullout strength testing were also significantly greater for cancellous screws in osteoporotic models. Stiffness was greater for cancellous screws in standard and osteoporotic models. Cancellous screws have biomechanical advantages over cortical screws when used in metaphyseal bone, implying the ability to both achieve greater compression and resist displacement at the screw-plate interface. Surgeons should preferentially use cancellous over cortical screws in metaphyseal environments where cortical bone is insufficient for fixation. [Orthopedics.2016; 39(5):e828-e832.].

Effect of Real-Time Feedback on Screw Placement Into Synthetic Cancellous Bone

OBJECTIVES

The objective of this study is to evaluate whether real-time torque feedback may reduce the occurrence of stripping when inserting nonlocking screws through fracture plates into synthetic cancellous bone.

METHODS

Five attending orthopaedic surgeons and 5 senior level orthopaedic residents inserted 8 screws in each phase. In phase I, screws were inserted without feedback simulating conventional techniques. In phase II, screws were driven with visual torque feedback. In phase III, screws were again inserted with conventional techniques. Comparison of these 3 phases with respect to screw insertion torque, surgeon rank, and perception of stripping was used to establish the effects of feedback.

RESULTS

Seventy-three of 239 screws resulted in stripping. During the first phase, no feedback was provided and the overall strip rate was 41.8%; this decreased to 15% with visual feedback (P < 0.001) and returned to 35% when repeated without feedback. With feedback, a lower average torque was applied over a narrower torque distribution. Residents stripped 40.8% of screws compared with 20.2% for attending surgeons. Surgeons were poor at perceiving whether they stripped.

CONCLUSIONS

Prevention and identification of stripping is influenced by surgeon perception of tactile sensation. This is significantly improved with utilization of real-time visual feedback of a torque versus roll curve. This concept of real-time feedback seems beneficial toward performance in synthetic cancellous bone and may lead to improved fixation in cancellous bone in a surgical setting.

Two-Finger Tightness: What Is It? Measuring Torque and Reproducibility in a Simulated Model

OBJECTIVES

Residents in training are often directed to insert screws using "two-finger tightness" to impart adequate torque but minimize the chance of a screw stripping in bone. This study seeks to quantify and describe two-finger tightness and to assess the variability of its application by residents in training.

METHODS

Cortical bone was simulated using a polyurethane foam block (30-pcf density) that was prepared with predrilled holes for tightening 3.5 × 14-mm long cortical screws and mounted to a custom-built apparatus on a load cell to capture torque data. Thirty-three residents in training, ranging from the first through fifth years of residency, along with 8 staff members, were directed to tighten 6 screws to two-finger tightness in the test block, and peak torque values were recorded. The participants were blinded to their torque values.

RESULTS

Stripping torque (2.73 ± 0.56 N·m) was determined from 36 trials and served as a threshold for failed screw placement. The average torques varied substantially with regard to absolute torque values, thus poorly defining two-finger tightness. Junior residents less consistently reproduced torque compared with other groups (0.29 and 0.32, respectively).

CONCLUSIONS

These data quantify absolute values of two-finger tightness but demonstrate considerable variability in absolute torque values, percentage of stripping torque, and ability to consistently reproduce given torque levels. Increased years in training are weakly correlated with reproducibility, but experience does not seem to affect absolute torque levels. These results question the usefulness of two-finger tightness as a teaching tool and highlight the need for improvement in resident motor skill training and development within a teaching curriculum. Torque measuring devices may be a useful simulation tools for this purpose.

Time-elapsed screw insertion with microCT imaging

Time-elapsed analysis of bone is an innovative technique that uses sequential image data to analyze bone mechanics under a given loading regime. This paper presents the development of a novel device capable of performing step-wise screw insertion into excised bone specimens, within the microCT environment, whilst simultaneously recording insertion torque, compression under the screw head and rotation angle. The system is computer controlled and screw insertion is performed in incremental steps of insertion torque. A series of screw insertion tests to failure were performed (n=21) to establish a relationship between the torque at head contact and stripping torque (R(2)=0.89). The test-device was then used to perform step-wise screw insertion, stopping at intervals of 20%, 40%, 60% and 80% between screw head contact and screw stripping. Image data-sets were acquired at each of these time-points as well as at head contact and post-failure. Examination of the image data revealed the trabecular deformation as a result of increased insertion torque was restricted to within 1mm of the outer diameter of the screw thread. Minimal deformation occurred prior to the step between the 80% time-point and post-failure. The device presented has allowed, for the first time, visualization of the micro-mechanical response in the peri-implant bone with increased tightening torque. Further testing on more samples is expected to increase our understanding of the effects of increased tightening torque at the micro-structural level, and the failure mechanisms of trabeculae.

In vitro biomechanical study of pedicle screw pull-out strength based on different screw path preparation techniques

BACKGROUND

Poor screw-to-bone fixation is a clinical problem that can lead to screw loosening. Under-tapping (UT) the pedicle screw has been evaluated biomechanically in the past. The objective of the study was to determine if pedicle preparation with a sequential tapping technique will alter the screw-to-bone fixation strength using a stress relaxation testing loading protocol.

MATERIALS AND METHODS

Three thoracolumbar calf spines were instrumented with pedicle screws that were either probed, UT, standard-tapped (ST), or sequential tapped to prepare the pedicle screw track and a stress relaxation protocol was used to determine pull-out strength. The maximum torque required for pedicle screw insertion and pull-out strength was reported. A one-way ANOVA and Tukeys post-hoc test were used to determine statistical significance.

RESULTS

The pedicle screw insertion torques for the probed, UT, ST and sequentially tapped (SQT) techniques were 5.09 (±1.08) Nm, 5.39 (±1.61) Nm, 2.93 (±0.43) Nm, and 3.54 (±0.67) Nm, respectively. There is a significant difference between probed compared to ST (P ≤ 0.05), as well as UT compared to both ST and SQT (P ≤ 0.05). The pull-out strength for pedicle screws for the probed, UT, ST and SQT techniques was 2443 (±782) N, 2353(±918) N, 2474 (±521) N, and 2146 (±582) N, respectively, with no significant difference (P ≥ 0.05) between techniques.

CONCLUSIONS

The ST technique resulted in the highest pull-out strength while the SQT technique resulted in the lowest. However, there was no significant difference in the pull-out strength for the various preparation techniques and there was no correlation between insertion torque and pull-out strength. This suggests that other factors such as bone density may have a greater influence on pull-out strength.

"Turn-of-the-Nut" Method Is Not Appropriate for Use in Cancellous Bone

OBJECTIVE

The level to which bone screws are tightened is determined subjectively by the operating surgeon. It is likely that the tactile feedback that surgeons rely on is based on localized tissue yielding, which may predispose the screw-bone interface to failure. A limited number of studies have investigated the ratio between clinical tightening torque and stripping torque. The purpose of this study was to measure, for the first time, the ratio between yield torque (T yield) and stripping torque (T max) during screw insertion into the cancellous bone and to compare these torques with clinical levels of tightening reported in the literature. Additionally, a rotational limit was investigated as a potential end point for screw insertion in cancellous bone.

METHODS

A 6.5-mm outer diameter commercial cancellous bone screw was inserted into human femoral head specimens (n = 89). Screws were inserted to failure, while recording insertion torque, compression under the screw head, and rotation angle.

RESULTS

The median, interquartile ranges, and coefficient of variation were calculated for each of the following parameters: T yield, T max, T yield/T max, slope, T plateau, and rotation angle. The median ratio of T yield/T max and rotation angle was 85.45% and 96.5 degrees, respectively. The coefficient of variation was greatest for the rotation angle compared with the ratio of T yield/T max (0.37 vs. 0.12).

CONCLUSIONS

The detection of yield may be a more precise method than the rotation angle in cancellous bone; however, bone-screw constructs that exhibit a T yield close to T max may be more susceptible to stripping during insertion. Future work can identify factors that influence the ratio of T yield/T max may help to reduce the incidence of screw stripping.