Biomechanical properties of small bone screws

Cutting Flute and Thread Design on Self-Tapping Pedicle Screws Influence the Insertion Torque and Pullout Strength

Self-tapping screws are commonly used in trauma and maxillofacial surgery and are increasingly used for pedicle screw insertions. In order to evaluate how the quantity and length of cutting flutes on self-tapping pedicle screws affect the insertion torque and pullout strength, eight different self-tapping pedicle screw designs were evaluated. All screws had a threaded length of 35 mm and featured variations in the number of leads, as well as the length and quantity of cutting flutes. Five samples of each design were inserted into pre-drilled, untapped holes (ø2.7 mm, length 35 mm) in sawbone blocks of density 20 PCF. The insertion torque and pullout strength were measured according to ASTM F543. The results showed that screws with a longer cutting flute of 9.5 mm had a lower mean maximum insertion torque than screws with shorter 2.9 mm cutting flutes. Pedicle screws with a double-lead thread design had a greater insertion torque than their single-lead counterparts, and the use of three cutting flutes produced a lower torque than two cutting flutes. The results demonstrated a greater pullout strength in screws with a single-lead thread rather than a double-lead, three cutting flutes instead of two, and a longer length for the cutting flute. In conclusion, to provide immediate stability and reduce the surgical insertion time, a single-lead, self-tapping pedicle screw incorporating three long cutting flutes is recommended because of the significantly greater pullout strength. This design could also reduce the risk of implant loosening in comparison to double-lead, self-tapping pedicle screw designs.

Mechanical characterization of bone anchors used with a bone-attached, parallel robot for skull surgery

Bone-attached robots and microstereotactic frames, intended for deep brain stimulation and minimally invasive cochlear implantation, typically attach to a patient's skull via bone anchors. A rigid and reliable link between such devices and the skull is mandatory in order to fulfill the high accuracy demands of minimally invasive procedures while maintaining patient safety. In this paper, a method is presented to experimentally characterize the mechanical properties of the anchor-bone linkage. A custom-built universal testing machine is used to measure the pullout strength as well as the spring constants of bone anchors seated in four different bone substitutes as well as in human cranial bone. Furthermore, the angles at which forces act on the bone anchors are varied to simulate realistic conditions. Based on the experimental results, a substitute material that has mechanical properties similar to those of cranial bone is identified. The results further reveal that the pullout strength of the investigated anchor design is sufficient with respect to the proposed application. However, both the measured load capacity as well as the spring constants vary depending on the load angles. Based on these findings, an alternative bone anchor design is presented and experimentally validated. Furthermore, the results serve as a basis for stiffness simulation and optimization of bone-attached microstereotactic frames.

Bivariate optimization of orthodontic mini-implant thread height and pitch

PURPOSE

Mini-implants have been used as anchorage for years, but failure is common in clinical practice. Mini-implant design is a critical factor affecting its stability. The aim of this study was to evaluate the effect of continuous and simultaneous variations of thread height and pitch on the biomechanical properties of an orthodontic mini-implant.

METHOD

A 3D finite element model, composed of a posterior maxilla section and an orthodontic mini-implant, was created. Mini-implant thread height ranged from 0.10 to 0.40 mm, and thread pitch ranged from 0.50 to 2.00 mm. Effects of the implant thread height and pitch on the maximum Von Mises stresses in maxilla and mini-implant, as well as maximum displacements in the mini-implant, were evaluated by a finite element method. Bivariate analysis was used to determine the optimal range of thread height and pitch.

RESULTS

Variation of thread height and pitch decreased the maximum Von Mises stresses in cortical bone, cancellous bone and mini-implant by 54.9, 78.4 and 23.6 %, respectively. The maximum displacement in the mini-implant decreased by 21.8 %.

CONCLUSION

Maxillary stress and mini-implant stability were influenced by mini-implant thread height and pitch. Increased thread height with a thread pitch of 1.20 mm was better for orthodontic mini-implant in the maxillary posterior region. Thread height played a more significant role than the thread pitch in reducing maxillary stress and enhancing orthodontic mini-implant stability.

Horizontal pull-out strength of orthodontic infrazygomatic mini-implant: an in vitro study

PURPOSE

New modified mini-implants have recently come into use for reinforcing skeletal anchorage in orthodontic application. The aim of this study was to investigate the influence of the design of a mini-implant on its mechanical strength.

MATERIALS AND METHODS

We measured the insertion torques and horizontal pull-out strengths of 3 brands of infrazygomatic mini-implants (AbsoAnchor, Bioray, and Lomas; 2 mm for all). Five implants of each brand were manually driven 6 mm into the artificial bone. Significant differences in various parameters among the brands were investigated with the Kruskal-Wallis test.

RESULTS

There was no significant relationship between insertion torque and horizontal pull-out strength. The Bioray mini-implants had significantly greater horizontal pull-out strength than the AbsoAnchor mini-implants.

CONCLUSIONS

The design of the mini-implant can influence its insertion torque and horizontal pull-out strength. In our findings, the horizontal pull-out strength of all mini-implants placed in the infrazygomatic crest was significantly greater than the orthodontic force applied.

Mechanical properties of self-drilling orthodontic micro-implants with different diameters

OBJECTIVE

The hypothesis to be tested is that peak-insertion torque of self-drilling micro-implants of an appropriate diameter correlates with peak-removal torque mechanically.

MATERIALS AND METHODS

A total of 360 self-drilling micro-implants composed of five different types were used. They (24 of each group) were inserted in three types of artificial bone with the use of a driving torque tester at a speed of 15 rpm. Insertion torque was measured during the placement, while the removal torque was measured within 3 days after insertion.

RESULTS

Most of the micro-implants in type A sheared before they were completely inserted in 40-pounds per cubic foot bone. The implants in other types were successfully inserted without implant breakage and bone fracture in all bone densities. There was a statistically significant correlation between insertion torque and removal torque (r > or = 0.43543, P = .0001). There were significant differences in insertion and removal torque among the diameters of implants and bone densities with an increasing tendency. The torque loss rates reduced as the diameter of the implant and bone density increased.

CONCLUSIONS

Micro-implants with a diameter of less than 1.3 mm are unsuitable for insertion into a bone with a density greater than 40 pounds per cubic foot mechanically when one is using a self-drilling technique.

Biomechanical and histological comparison of self-drilling and self-tapping orthodontic microimplants in dogs

INTRODUCTION

The purpose of this study was to compare the influences of different implant modalities on orthodontic microimplants and surrounding tissues biomechanically and histologically.

METHODS

Fifty-six titanium alloy microimplants placed on the buccal side of the maxillae and the mandibles in 2 dogs were divided into 2 groups of 28; one group of microimplants was self-drilling, and the other was self-tapping. Approximately 200 g of continuous and constant forces were applied immediately between 2 microimplants by stretching closed nickel-titanium coil springs for 9 weeks. Peak insertion torque and removal torque were recorded immediately after the implants were placed and when the dogs were killed, respectively. Undecalcified sections of the microimplants and the surrounding tissues were studied with light microscope and fluorescent microscope.

RESULTS

Success rates were higher in the self-drilling group (93%) than in self-tapping group (86%). Higher peak insertion torque and peak removal torque values were seen in the self-drilling group in both the maxilla and the mandible. A tendency to fracture was found in self-drilling group. The percentage of bone-to-implant contact values was greater in the self-drilling group.

CONCLUSIONS

Self-drilling microimplants can provide better anchorage and can be recommended for use in the maxilla and in thin cortical bone areas of the mandible.

Effects of surgical errors on small fragment screw fixation

OBJECTIVES

The purpose of this study is to determine the effects of technical errors that occur during the application of small fragment screw fixation and to assess which screw holes can be salvaged.

INTERVENTION

Testing of screw pullout from a bone substitute model on a universal testing instrument (Instron Corp., Canton, MA).

OUTCOME MEASUREMENTS

Testing was performed on 9 sets of 12 small fragment screws applied to a bone substitute model using the instruments available in a small fragment set (Synthes, Paoli, Pa). In the first 2 sets, 3.5-mm cortical screws and 4.0-mm cancellous screws were placed using the proper instrumentation according to recommended AO/ASIF techniques. The other 7 sets were inserted using "incorrect" methods: a single step was altered intentionally to assess its influence on fixation strength. The third set of screws included 3.5-mm cortical screws placed after drilling the pilot hole with a 3.5-mm drill. For the fourth set, the 2.5-mm drill was used, but the hole was tapped using the 4.0-mm cancellous tap before insertion of a 3.5-mm cortical screw. In set five, 4.0-mm cancellous screws were placed after tapping the hole with a 3.5-mm cortical tap. Set 6 included cancellous screws placed without tapping. The seventh set included 3.5-mm cortical screws that were placed according to recommended methods, and then removed and replaced into the screw hole. Set number 8 included 3.5-mm cortical screws, which were inserted correctly and then stripped by overtightening. The ninth set included 3.5-mm cortical screws that were stripped as those in set 8; the stripped screws were removed, the holes were packed with bone material, and the screws were replaced. All screws were inserted to a thread depth of 32 mm.

RESULTS

Drilling a 3.5-mm pilot hole for a 3.5-mm cortical screw and "stripping" the screw by overtightening resulted in 76% and 82% less pullout strength, respectively, than when the proper technique was used (P<0.01). Use of the wrong tap before placement of a 3.5-mm cortical or 4.0-mm cancellous screw decreased pullout strength by 12% and 11%, respectively (P<0.01). Exchanging screws of similar geometry had no significant effect on screw pullout strength (P>0.1). Inserting a 4.0-mm cancellous screw without tapping actually increased pullout strength by 4% (P<0.01).

CONCLUSIONS

Alterations from recommended techniques for the placement of orthopedic screws had varying effects on screw fixation, as assessed by the pullout strength. Clinically, these findings indicate that, in some cases, a screw hole that was not initially placed according to the optimal technique may be salvaged. Finally, the authors recommend that careful vigilance be maintained at all times in surgery and that fixation be applied according to sound principles in an effort to avoid some of these problems.

Immediate Occlusal Loading of Freestanding Implants Using Cortical Satellite Implants: Preliminary Report of a Prospective Study

Freestanding implants with mandibular overdentures are used frequently after 3 months' healing time. Immediate full loading may be applied to this approach if sufficient primary stability is provided. The present study evaluates the success rate of two single-standing interforaminal implants stabilized with cortical satellite implants and loaded immediately with overdentures. Twenty patients (five male and 15 female; age, 45-87 years) received two single-standing titanium screw implants (Semados, Bego, Bremen, Germany). All implants were stabilized during healing time with cortical satellite implants (2.0-mm bone screws; Medartis, Basel, Switzerland) via individual connectors attached to the implant abutment subgingivally. Mandibular overdentures were incorporated immediately after surgery using ball attachments as retentive elements. The patients were under no restrictions concerning diet and loading. The satellite implants were removed after 3 months. All implants were osseointegrated after 10 months' mean observation time. The mean Periotest value was -4.9, and the mean marginal bone loss was 0.7 mm. No dropouts were observed, and 19 of 20 patients would recommend the treatment to a close friend. Preliminary follow-up data indicate that cortical stabilization of two anterior mandibular implants with satellite implants leads to osseointegration of the implants under immediate load conditions with an overdenture. This concept contributes to reduction of prosthetic treatment costs and permits immediate enhancement of masticatory function.

Stability consideration for internal maxillary distractors

PURPOSE

Stability in distractor design ensures distraction osteogenesis healing with good bone regenerate formation. The aim of this study was to compare the holding strengths of different fixation systems for maxillary distractor design on bone pieces of different thicknesses.

MATERIAL AND METHODS

Cross-sectional images of 10 dry skulls were obtained by computer tomography and the bone thickness of the maxillae were measured according to five individual anatomical regions (paranasal, infra-orbital, posterior sinus wall, zygomatic and alveolar regions). According to the measurements, the screws of 1.5 and 2mm in diameter and the three-screw mini-plates in triangular and straight configurations were evaluated for holding strength by pull-out tests on fresh animal bone pieces of defined thickness.

RESULTS

The paranasal and zygomatic regions of the human skulls had the thickest cortical bone (4mm) followed by the alveolar region (2mm). In the bones of 2 and 4mm thickness, the 2mm screws were confirmed stronger than the 1.5mm ones in pull-out tests. However, the pull-out behaviour of screws of different diameters in 1mm thick bones and the mini-plates in two different configurations showed no significant differences.

CONCLUSION

This study confirms that the paranasal and zygomatic bones are the thickest for fixation of internal maxillary distractors. Fixation screws of 2mm diameter in either triangular or straight miniplates can produce good stabilization for distractors.

Enhancement of primary stability of dental implants using cortical satellite implants

PURPOSE

This study aims to assess the effect of satellite implants on the primary stability of dental implants placed in fresh extraction sites in vitro.

METHOD

34 titanium screw implants (3.75 mm x 10 mm; Bego, Bremen, Germany) were inserted in premolar- and molar-fresh extraction sites in domestic pig mandibles. Periotest (PT) values were assessed before and after insertion of one vestibular and one lingual 1.7-mm bone screw (Mondeal, Tuttlingen, Germany) as a satellite implant was connected to the implants with a 0.6-mm microplate welded to the implant abutment.

RESULTS

The average PT values were 2.9 without satellite implants, -1.0 with one satellite implant, and -2.5 with two satellite implants during horizontal testing, and 3.0, 1.4, and 0.4, respectively, for vertical testing.

CONCLUSION

Satellite implants increase the horizontal stability of implants in fresh extraction sites. Differences for horizontal PT assessment were significant on a 0.01 level of confidence. Implants in extraction sites may be loaded immediately, if vertical stabilization is provided by cortical bone and if horizontal PT values show sufficient stability after satellite implant insertion.

Effect of cutting flute design on cortical bone screw insertion torque and pullout strength

OBJECTIVE

To determine the effect of the number and length of cutting flutes on the insertion torque and pullout strength for self-tapping 4.5-millimeter cortical bone screws.

DESIGN

Screws were self-tapped in the diaphysis of human cadaver femurs. Each of the six screw types studied had different designs with varying cutting flute lengths and numbers. Bone mineral density, insertion torque, and pullout strength were measured.

SETTING

The study was conducted at an experimental biomechanics laboratory associated with a university medical center.

OUTCOME MEASUREMENTS

Insertion torque and pullout strength were normalized by the local bone mineral density.

RESULTS

The mean normalized insertion torque of the design with four full-length cutting flutes was less than the design with three full-length flutes and the two designs with one-third length flutes (p < 0.05). The mean normalized pullout strength of the screw with four full-length flutes was significantly greater than that of all screws with fewer than three flutes (p < 0.05).

CONCLUSIONS

Priorities for a cutting flute design should ideally include ease of screw insertion, minimal soft tissue irritation, and maximal screw holding power. Screws with more than two flutes were easier to insert and did not cause cortical damage during insertion. The screw with four full-length flutes showed a trend toward being the easiest to insert and having the greatest holding strength.

Complications of internal fixation of maxillofacial fractures with microplates

PURPOSE

The aim of this retrospective study was to evaluate the complications of open reduction and internal fixation of maxillofacial fractures with microplates.

PATIENTS AND METHODS

In 44 patients with maxillofacial trauma, fractures of the maxillofacial skeleton were treated by open reduction and internal fixation using a 1.0-mm and 1.5-mm microsystem. Simultaneously occurring fractures of the mandible or frontozygomatic suture were treated with a 2.0-mm miniplate system. Perioperative and postoperative complications were traced using patient charts, operation reports, and radiographs. The average follow-up was 46.8 months (range, 31 to 54 months).

RESULTS

A total of 124 1.0-mm microplates and 546 1.0-mm microscrews, and 17 1.5-mm microplates and 75 1.5-mm microscrews, was used. The perioperative complication rate was 1.2% for the 1.0-mm screws (use of four emergency screws, breakage of one screw in the dense frontozygomatic suture area, and an insertion of a screw in a premolar root). The postoperative complication rate was 0.8% for the 1.0-mm screws (screw dislocation without clinical implication). No complications were observed with the 1.5-mm system. Plate-related infection did not occur. All fractures healed well. Three patients asked for plate removal because of a vague, persisting pain in the treated area. After removal, only one patient was free of pain. A loose 1.5-mm screw was found in this patient.

CONCLUSION

The overall complication rate for microsystems was 2.0%. Both microsystems proved to be a reliable modality to fix fractures of the maxillofacial skeleton. Complications can be considered incidental and of neglectable clinical significance.

Biomechanical evaluation of titanium, biodegradable plate and screw, and cyanoacrylate glue fixation systems in craniofacial surgery

Choice of appropriate fixation after reduction of displaced bone fragments or advancement of osteotomized segments requires knowledge of the maximal force to which these segments can be subjected. The present study was performed to obtain a biomechanical comparison of a variety of resorbable fixation systems as an alternative to metal plates and screws. Sheep cadaver parietal bone segments were osteotomized and fixed with one of six methods of fixation: (A) titanium plates and screws consisting of (1) miniplates and 2.0-mm-diameter screws; (2) midface plates and 1.5-mm screws; (3) microplates and 1.0-mm screws; (B) resorbable systems consisting of combinations of butyl-2-cyanoacrylate glue and biodegradable polylactic acid/polyglycolic acid copolymer plates and 2.0-mm screws as follows: (1) direct glue fixation of segments; (2) resorbable plates fixed to bone segments with cyanoacrylate glue; (3) resorbable plates fixed with resorbable screws. Compression testing was performed upon bone segments advanced and fixed across a central gap, and distraction testing was performed on bone segments fixed in direct contact. Force to failure in both distraction and compression was significantly greater in bone segments fixed with titanium miniplates than with any other method of fixation. Segments fixed with plates and screws, either nonresorbable or resorbable, achieved stronger fixation in distraction than in compression for all plate sizes tested. Resorbable plate and screw fixation was as strong as standard titanium midface and microplating systems in distraction, and stronger than the latter techniques in compression. With compressive forces of relapse, fixation with glue and resorbable plates was as strong as standard titanium midface and microplating systems. However, with distractive forces of relapse, glue fixation of either the bone segments or resorbable plates was weaker than both titanium and resorbable alternatives in which plates and screws were used. These findings may have direct impact on the choice of fixation devices used to support osteotomized or fractured bone segments, which are subjected to persistent muscular and soft-tissue pull.

Influence of different pilot hole sizes on torque measurements and pullout analysis of osteosynthesis screws

When screws are inserted in thick cortical bone, a small pilot hole size, corresponding to the core diameter of the screw, can result in high torsional stress, leading to screw fracture. The aim of this study was to enlarge the drill size up to a critical pilot hole size (CPHS) which, if exceeded, means a rapid decrease in the screw holding power. 1.5 and 2 mm titanium screws were inserted in discs of polyvinylchloride (PVC), wood and porcine mandibular bone with thicknesses differing between 2 to 4 mm, using an increasing pilot hole size between 66% and 95% of the screw external diameter. Torque measurements and pullout tests were performed and the CPHS was calculated. In torque measurements, the CPHS of microscrews ranged between 83% and 85% of the screw external diameter (SED). The CPHS of miniscrews lay between 80% and 90% of SED. In pullout analysis, the CPHS of microscrews ranged between 83% and 89% of SED; the CPHS of miniscrews lay between 79% and 91% of SED. The mean of the CPHS was calculated to be approximately 85% of the SED. Up to this critical point, the pilot hole size may be increased without affecting the holding power of the screws.