Reduction of pullout strength caused by reinsertion of 3.5-mm cortical screws

Pullout Strength of Pedicle Screws Inserted Using Three Different Techniques: A Biomechanical Study on Polyurethane Foam Block

Pullout strength is an important indicator of the performance and longevity of pedicle screws and can be heavily influenced by the screw design, the insertion technique and the quality of surrounding bone. The purpose of this study was to investigate the pullout strength of three different pedicle screws inserted using three different strategies and with two different loading conditions. Three pedicle screws with different thread designs (single-lead-thread (SLT) screw, dual-lead-thread (DLT) screw and mixed-single-lead-thread (MSLT) screw) were inserted into a pre-drilled rigid polyurethane foam block using three strategies: (A) screw inserted to a depth of 33.5 mm; (B) screw inserted to a depth of 33.5 mm and then reversed by 3.5 mm to simulate an adjustment of the tulip height of the pedicle screw and (C) screw inserted to a depth of 30 mm. After insertion, each screw type was set up with and without a cyclic load being applied to the screw head prior to the pullout test. To ensure that the normality assumption is met, we applied the Shapiro-Wilk test to all datasets before conducting the non-parametric statistical test (Kruskal-Wallis test combined with pairwise Mann-Whitney-U tests). All screw types inserted using strategy A had a significantly greater pullout strength than those inserted using strategies B and C, regardless of if the screw was pre-loaded with a cyclic load prior to testing. Without the use of the cyclic pre-load, the MSLT screw had a greater pullout strength than the SLT and DLT screws for all three insertion strategies. However, the fixation strength of all screws was reduced when pre-loaded before testing, with the MSLT screw inserted using strategy B producing a significantly lower pullout strength than all other groups (p < 0.05). In contrast, the MSLT screw using insertion strategies A and C had a greater pullout strength than the SLT and DLT screws both with and without pre-loading. In conclusion, the MSLT pedicle screw exhibited the greatest pullout strength of the screws tested under all insertion strategies and loading conditions, except for insertion strategy B with a cyclic pre-load. While all screw types showed a reduced pullout strength when using insertion strategy B (screw-out depth adjustment), the MSLT screw had the largest reduction in pullout strength when using a pre-load before testing. Based on these findings, during the initial screw insertion, it is recommended to not fully insert the screw thread into the bone and to leave a retention length for depth adjustment to avoid the need for screw-out adjustment, as with insertion strategy B.

Factors Associated With the Accuracy of Depth Gauge Measurements

Background: It is important to select appropriate screws in orthopedic surgeries, as excessively long or too short a screw may results failure of the surgeries. This study explored factors that affect the accuracy of measurements in terms of the experience of the surgeons, passage of drilled holes and different depth gauges.

Methods: Holes were drilled into fresh porcine femurs with skin in three passages, straight drilling through the metaphysis, straight drilling through the diaphysis, and angled drilling through the diaphysis. Surgeons with different surgical experiences measured the holes with the same depth gauge and using a vernier caliper as gold standard. The length of selected screws, and the time each surgeon spent were recorded. The measurement accuracy was compared based on the experiences of the surgeons and the passage of drilled holes. Further, parameters of depth gauges and 12-mm cortical bone screws from five different manufacturers were measured.

Results: A total of 13 surgeons participated in 585 measurements in this study, and each surgeon completed 45 measurements. For the surgeons in the senior, intermediate, and junior groups, the average time spent in measurements was 689, 833, and 785 s with an accuracy of 57.0, 42.2, and 31.5%, respectively. The accuracy and measurement efficiency were significantly different among the groups of surgeons (P &lt; 0.001). The accuracy of measurements was 45.1% for straight metaphyseal drilling, 43.6% for straight diaphyseal drilling, and 33.3% for angled diaphyseal drilling (P = 0.036). Parameters of depth gauges and screws varied among different manufacturers.

Conclusion: Both observer factor and objective factors could affect the accuracy of depth gauge measurement. Increased surgeon's experience was associated with improvements in the accuracy rate and measurement efficiency of drilled holes based on the depth gauge. The accuracy rate varied with hole passages, being the lowest for angled drilled holes.

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.

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.

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.

Helical blade compression failure occurred during PFNA implantation: A rare case and ingenious solution

RATIONALE

Femoral intertrochanteric fracture happens easily in the elderly, especially those with osteoporosis. As a standard intramedullary fixation implant, Proximal Femoral Nail Anti-rotation (PFNA) is applied to various types of stable or unstable intertrochanteric fractures of femur. Due to blade-related factors, such as cutting-through into the hip joint, cutting out or back out, there are endless postoperative failure cases of internal fixation, but reports about perioperative failure that the helical blade cannot be tightened are lacking. In this case, we firstly report an intraoperative blade compression failure and an effective technique to help surgeons out of the dilemma by using cannulated hexagonal screwdriver which was already included in the orthopedic instrument box.

PATIENT CONCERNS

An 81-year-old lady suffered left hip pain, swelling and limitation of activity, after slipping and falling when she was in the toilet.

DIAGNOSES

X-ray and computed tomography (CT)-scan showed comminuted left intratrochanteric fracture with a Jensen classification of IIa type.

INTERVENTIONS

The patient was treated by closed reduction and internal fixation with PFNA. We suffered an unprecedented problem that helical blade cannot be tightened by the blade impactor as usual. For fear of helical blade disintegration during removal and a significant decrease in pullout strength after reinsertions, we eliminated the dilemma by using a cannulated hexagonal screwdriver to tighten the "problem" helical blade.

OUTCOMES

The "problem" helical blade was finally locked by using the SW4.0 mm cannulated hexagonal screwdriver which was already included in the instrument box. The operation ended successfully after completing subsequent steps.

LESSONS

The cannulated hexagonal screwdriver is an effective instrument that can help surgeons out of the dilemma when the blade impactor fails to tighten the helical blade in PFNA implantation.

Comparison of the One-Time Accuracy of Simulated Freehand and Navigation Simulated Pedicle Screw Insertion

OBJECTIVE

To compare one-time accuracy rate between simulated freehand (SFH) and navigation simulated (NS) pedicle screw insertion, assuming no second chance to correct screws.

METHODS

A simulated, comparative, cross-sectional study was conducted on 69 patients undergoing lumbar spine surgery. An intraoperative registration system captured the planned point of entry and trajectory of pedicle screws for both SFH under direct visualization and NS under navigation-aided visualization. Pedicle screw insertion was simulated for each captured image (370 screws) using Surgimap. Rajasekaran's method helped evaluate the point of entry accuracy and trajectory.

RESULTS

Accuracy rate was better for the NS method (97.8%) than for the SFH method (63.8%). Of 370 screws in the SFH group, 134 penetrated the cortex, with 31 resulting in >4 mm penetration. Of 370 screws in the NS group, 8 penetrated the cortex, <4 mm penetration. Of 134 misplaced screws in the SFH group, 64 were due to error in the point of entry, 63 were due to error in the trajectory angle, and 7 were due to both errors. Of 8 errors in the NS group, 7 were due to the point of entry.

CONCLUSIONS

Intraoperative navigation had significantly better one-time accuracy of pedicle screw insertion than freehand insertion and should be used to avoid injury to the pedicle and surrounding tissue from screw reinsertion.

Accuracy in Screw Selection in a Cadaveric, Small-Bone Fracture Model

PURPOSE

Using a cadaveric model simulating clinical situations experienced during open reduction and internal fixation of proximal phalangeal fractures, the aim of this study was to evaluate the relationship between level of training and the rates of short, long, and ideal screw length selection based on depth gauge use without fluoroscopy assistance.

METHODS

A dorsal approach to the proximal phalanx was performed on the index, middle, and ring fingers of 4 cadaveric specimens, and 3 drill holes were placed in each phalanx. Volunteers at different levels of training then measured the drill holes with a depth gauge and selected appropriate screw sizes. The rates of short, long, and ideal screw selection were compared between groups based on level of training. Ideal screws were defined as a screw that reached the volar cortex but did not protrude more than 1 mm beyond it.

RESULTS

Eighteen participants including 3 hand fellowship-trained attending physicians participated for a total of 648 selected screws. The overall rate of ideal screw selection was lower than expected at 49.2%. There was not a statistically significant relationship between rate of ideal screw selection and higher levels of training. Attending surgeons were less likely to place short screws and screws protruding 2 mm or more beyond the volar cortex CONCLUSIONS: Overall, the rate of ideal screw selection was lower than expected. The most experienced surgeons were less likely to place short and excessively long screws.

CLINICAL RELEVANCE

Based on the low rate of ideal screws, the authors recommend against overreliance on depth gauging alone when placing screws during surgery. The low-rate ideal screw length selection highlights the potential for future research and development of more accurate technologies to be used in screw selection.

A biomechanical evaluation of different fixation strategies for posterolateral fragments in tibial plateau fractures and introduction of the 'magic screw'

BACKGROUND

Posterior plate fixation is biomechanically the strongest fixation method for posterolateral column fracture (PLCF) of the tibial plateau; however, there are inherent deficiencies and risks of a posterior approach. Thus, the 'magic screw' was proposed to enhance fixation stability of the lateral rafting plate used for PLCF. The purpose of this study was to re-examine and compare the stability of different fixation methods for PLCF.

METHODS

Synthetic tibiae models were used to simulate posterolateral split fractures. The fracture models were randomly assigned into three groups: Group A, fixed with posterolateral buttress plates; Group B, with lateral locking compression plates (LCP); and Group C fixed with lateral LCPs and one 'magic screw'. Gradually increased axial compressive loads were applied to each specimen.

RESULTS

There was a mean subsidence hierarchy of the posterolateral fragment at different load levels: Group A had the least subsidence, followed by Group C, and Group B had the most. There were no significant differences in the mean loads at different displacements between Group A and Group C. Group A had the highest axial stiffness. Additionally, there was a significant difference in axial stiffness between Group B and Group C.

CONCLUSION

Biomechanical stability of the combined fixation of the posteriorly positioned lateral rafting plate with the 'magic screw' was much closer to that of posterior plate fixation for split-type PLCF. The necessity of posterior fixation through a posterior approach may be reduced for selected patients.

Useful Intraoperative Technique for Percutaneous Stabilization of Bilateral Posterior Pelvic Ring Injuries

Treating patients with bilateral posterior pelvic ring injuries can be challenging. Placement of transiliac-transsacral style screws in available S1 or S2 osseous fixation pathways is becoming an increasingly common fixation method for these unstable injuries. We propose a percutaneous technique that sequences reduction and stabilization of 1 hemipelvis with at least 1 transiliac-transsacral screw and then uses the existing transiliac-transsacral screw and accompanying guide wires to assist in temporary stabilization and definitive fixation of the second hemipelvis.

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.].

Screw loosening and iliotibial band friction after posterolateral corner reconstruction

BACKGROUND

Many reconstruction techniques have already been developed for treating posterolateral corner (PLC) injuries, with still no consensus regarding what would be the best option. Some techniques use non-bone tunnel fixation, attaching the graft to the femur using a cortical screw with toothed washer. The main objective of the present study is to evaluate complications related to fixation performed by a screw and toothed washer technique.

METHODS

A prospective study with surgical reconstruction of the PLC structures of the knee between January 2008 and December 2009 was performed. PLC reconstruction included reconstruction of the lateral collateral ligament, popliteofibular ligament and popliteal muscle tendon. Fixation of the grafts in the femur was achieved by means of a 4.5mm screw with a toothed washer. The assessments were done using the following methods: objective IKDC, subjective IKDC, Lysholm and Tegner. Radiographic evaluations were performed immediately after the operation, at 3, 6 and 12months after surgery, and yearly thereafter. Complications were documented.

RESULTS

The mean subjective IKDC score after the operation was 69.2; Lysholm scale was 80.7. Two patients presented failure of reconstructions of the PLC. In the radiographic evaluations, signs of loosening of the screw with toothed washer in the femur were observed in eight cases (66.6%). Six patients (50%) complained of lateral pain after the operation.

CONCLUSION

The technique of femoral fixation was shown to be efficient in restoration of stability. However there was a high rate of complications secondary to implant, such as loosening of the screws and iliotibial tract friction.

LEVEL OF EVIDENCE

Case series, leve IV.

Mechanical effects of off-axis insertion of locking screws: should we do it?

OBJECTIVES

The purposes of this study were to evaluate the cantilevered bending strength and failure modes of locking screws inserted at various angles in a plate with fully circumferential threaded holes. As an additional measure, the amount of screw head prominence at these angles was also assessed.

METHODS

Standard 3.5-mm locking screws were inserted into round fully circumferential threaded holes through a standard straight 3.5-mm locking plate at various angles. The achieved angle of insertion and its prominence protruding from the far-bone side of the plate was measured using an optical luminescence technique. Each screw was then loaded at a constant rate until failure in a cantilevered bending scenario. The maximum cantilevered bending strength was measured, and the moment at failure was calculated.

RESULTS

There was a positive correlation between increasing insertion angle and increasing prominence; a higher screw insertion angle yielded greater prominence. Prominence values ranged from negligible to 2 mm. As screw insertion angle increased, the bending moment at failure decreased. Screws inserted to 3 degrees or below primarily failed through screw deformation at the minor diameter below the head, whereas screws inserted to greater than 3 degrees primarily failed through locking mechanism disengagement.

CONCLUSIONS

These findings indicate that cross threading may not be biomechanically advantageous and may change screw mode of failure. Based on these findings, screws inserted to 3 degrees or higher would reduce the bending moment at failure to approximately 50% of an orthogonally inserted screw.