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Abstract

Background

Minimally invasive cochlear implant surgery by using a microstereotactic frame demands solid connection to the bone. We aimed to determine the stability of commercially available orthodontic miniscrews to evaluate their feasibility for frame’s fixation. In addition, which substitute material most closely resembles the mechanical properties of the human temporal bone was evaluated.

Methods

Pull-out tests were carried out with five different types of orthodontic miniscrews in human temporal bone specimens. Furthermore, short fiber filled epoxy (SFFE), solid rigid polyurethane (SRPU50), bovine femur, and porcine iliac bone were evaluated as substitute materials. In total, 57 tests in human specimens and 180 tests in the substitute materials were performed.

Results

In human temporal bone, average pull-out forces ranged from 220 N to 285 N between screws. Joint stiffness in human temporal bone ranged between 14 N/mm and 358 N/mm. Statistically significant differences between the tested screws were measured in terms of stiffness and elastic energy. One screw type failed insertion due to tip breakage. No significant differences occurred between screws in maximum pull-out force. The average pull-out values of SFFE were 14.1 N higher compared to human specimen.

Conclusion

Orthodontic miniscrews provided rigid fixation when partially inserted in human temporal bone, as evidenced by pull-out forces and joint stiffness. Average values exceeded requirements despite variations between screws. Differences in stiffness and elastic energy indicate screw-specific interface mechanics. With proper insertion, orthodontic miniscrews appear suitable for microstereotactic frame anchoring during minimally invasive cochlear implant surgery. However, testing under more complex loading is needed to better predict clinical performance. For further pull-out tests, the most suitable substitute material is SFFE.

Graphical abstract

Keywords

minimally invasive cochlear implant surgery pull-out test orthodontic miniscrews bone substitute material

Introduction

For patients suffering from sensory deafness or profound hearing loss, a cochlear implant (CI) is the current treatment of choice.¹ Due to extensive research over the past years, indication criteria have been expanded to include patients with higher residual hearing,^2-4 unilateral deafness,⁵ and asymmetric hearing loss,⁶ as well as atypical causes of hearing loss (eg, vestibular schwannoma,⁷ Meniere’s disease,⁸ and tinnitus⁹). Especially, patients with residual hearing, mostly elderly patients with underlying diseases and associated higher risk of surgery-related complications, could benefit from a less invasive technique of CI surgery.

The standard procedure for implanting the CI, mastoidectomy posterior tympanotomy approach, is a traumatic and time-consuming surgery that requires extensive removal of the skull bone.^10,11 During the surgery, an array of stimulating electrodes is inserted into the cochlea, which ensures the transmission of the electrical pulses to the hearing nerve. In an effort to reduce both intraoperative trauma as well as the duration of surgery, and to increase patient’s safety, minimally invasive approaches of cochlear implantation have been investigated for more than 10 years. However, this approach is still experimental and has not been commonly performed yet.

The main idea of all procedures is to place the electrode array via a single bore-hole directly into the patient’s cochlea, avoiding extensive removal of the skull bone. Since important structures, for example, the facial nerve or chorda tympani, cannot be identified during surgery without resection of the temporal bone; unassisted manual drilling is no option. Several approaches to provide surgeons with the required accuracy in the submillimetric area with a maximum deviation of 0.3 mm¹² have been published: head-mounted systems,¹³ stationary robot systems,¹⁴ and microstereotactic frames (MSFs).^15-19

The workflow of the last-mentioned frame-based approach includes a bone-anchored fixation with three miniscrews onto the patient’s temporal bone (Figure 1), a segmentation of the patient’s individual anatomy and afterward the customization of an individual jig, that defines the drilling path to the basal turn of the cochlea.^20,21 In fact, the fixation of the frame to the skull plays a crucial role in the accuracy of the drilling process. If a rigid fixation is not ensured, the drilling can deviate from the calculated trajectory and hamper electrode insertion or, in the worst case, result in injury of nerval structures.

Figure 1.

Illustration of the reference frame and fixation with miniscrews.

For this specific application, Labadie et al demonstrated already in 2009 that bone anchors ensure adequate stability,²² these results were confirmed by Kobler et al.²³ However, experiments with miniscrews from orthodontics have not yet been published.

In this study, we investigated whether sufficient stability for the fixation of MSFs can also be ensured by orthodontic miniscrews which are commercially available and well-established medical products. These miniscrews are widely used in orthodontics as temporary fixation of midface trauma, mandibular fractures, or for the fixation of bone grafts due to their easy surgical insertion and their performance in establishing a solid and rigid connection to the bone.^24,25 They were mechanically characterized in several investigations demonstrating a high primary stability^26,27 which is important also for minimally invasive CI surgery. However, these miniscrews have never been tested in the mastoid bone and this study aims for filling this lack of knowledge.

Gracco et al showed a high stability at pull-out trails with miniscrews with a mean force of 183.4 N for all tested screws and with a comparable design to those screws investigated in this study.²⁸ In contrast, Yashwant et al showed lower forces in their pull-out trials with mini-implants ranged from 13.45 N to 61 N tested in sawbone samples²⁹ but with a significantly lower density than the samples tested in this study. These publications described considerable differences with regard to the stability of mini bone screws. Furthermore, in the aforementioned investigations, the bone screws were placed with their full length of 9 mm to 12 mm into the bone, which deviates from the fixation of the MSF. While it has been experimentally confirmed that a higher insertion depth also leads to greater stability,³⁰ screw-in depth at the lateral skull base is limited to 3.6 mm to 5 mm due to the limited thickness of the bone as shown in previous measurements with a custom-made software.³¹ The insertion depth of the screw for this special purpose is therefore only one-third of the whole screw and a review of literature could not show any experiments in which the miniscrews were only partially inserted into the bone or substitute material. Furthermore, the bone screws for the fixation frame will be placed in the temporal and the parietal bone, which mechanical characterizations differ from the jaw bone.^32-34

Due to the aforementioned limitations, a mechanical characterization of the fixation using mini bone screws for this special purpose needs to be performed. In this study, we present the results of the experiments, which aimed to show that a stable and reliable attachment to the lateral skull base is also feasible with mini bone screws.

In addition to the tests in the human temporal bone, pull-out tests were also performed in four different substitute materials. While experiments in human bone specimens provide the most reliable results for the clinical use, suitable substitutes for phantoms or training are also required. In this case, the limited availability of human bone samples is the major problem, as these specimens can only be obtained from body donors. Moreover, human specimens are more difficult to store and prepare. Polyurethan foams, for example, Sawbone^®, approximate the biomechanical properties of human bones with little limitations.³⁵ Studies to replace the human temporal bone have not been performed before. Based on studies related to the miniscrews, in which porcine iliac bone was frequently used as substitute material for the mandibula, this material was also tested. Moreover, bovine femur was included in our experiments, as it is characterized by comparable biomechanical characteristics to the human temporal bone.³⁶ The main advantage in these cases is the better availability compared to human bone. Storage and preparation require the same effort.

Materials and Methods

Pull-Out Test Setup

The mechanical pull-out testing was performed using a material testing machine (Zwick 1445; ZwickRoell GmbH & Co. KG), equipped with a 10 kN load cell. Recordings of deflection of the screw, applied force, and duration of the test were made with one measuring point per 0.01 mm deflection by the program Zwick PC-Software, V5.72 (ZwickRoell GmbH & Co. KG). The specimens were fixed on a base plate whereupon the screws were inserted 3.6 mm into the respective test material. The correct insertion depth was ensured by a spacer made individually for each screw tested. The spacer was also used to connect the construction to the test machine with a tension ring (Figure 2). Drilling a pilot hole was not required, as all screws are self-drilling. During the mounting of the specimens, attention was paid to a proper alignment of the longitudinal screw axis with the vertical pulling direction of the testing machine. First, a preload in tensile direction of 5 N was applied and held for five seconds. Then the pull-out test was performed at a velocity of 10 mm/min. The primary end point was the complete pull-out of the tested screw. The force-displacement curves were recorded by the software and the maximum force encountered was used for evaluation. The maximum displacement of a screw was defined at 5 mm, if this threshold was exceeded, the pull-out test was stopped. To compensate for the elongation of the steel cable, 19 tests were performed with a fixed screw up to a force of 300 N. Raw data of these trials were averaged and approximated by a fourth degree polynomial regression. In this manner, the elongation of the steel cable during the application of the force was mathematically subtracted. The calibrated displacement only refers to the screw-bone interface. The screws were inserted manually perpendicularly to the bone or sawbone surface up to the limit of the spacer. All screws were tightened to the predetermined depth until a solid connection to the bone was established. A total of 237 pull-out tests were performed in all described materials. In summary, all miniscrews were tested in human temporal bone; however, S1 did not yield interpretable results. S1 was tested in all substitute materials, while S2, S3, S4, and S5 were tested in SFFE and bovine femur as substitute materials. Table 1 summarizes the number of each pull-out test with regard to screw type and material.

Figure 2.

Illustration of the pull-out test setup: 1: baseplate; 2: specimen; 3: screw; 4: spacer; 5: tension ring.

Table 1.

Overview of Number of Performed Pull-Out Tests Per Screw in Each Investigated Material.

Material	S1	S2	S3	S4	S5
SFFE	17	14	15	15	15
SRPU50	15
Bovine femur	15	15	15	15	14
Porcine iliac bone	15
Human temporal bone	15	15	14	15	13

Abbreviations: S1: PSM quattro standard; S2: Promedia LeForte; S3: KLS maxdrive drill-free; S4: General Implants mini bone screw; S5: Promedia dual top; SFFE: short fiber filled epoxy; SRPU50: solid rigid polyurethane.

Specification of the Tested Materials

Human temporal bone was tested as reference material since the proposed fixation requires bone anchorage in this part of the skull. After receiving institutional review board approval (Ethics Committee of Hannover Medical School; 3512-2017), all human bone specimens were obtained from anonymized body donors. In addition, 2 artificial bone substitutes materials have been used: “solid rigid polyurethane” (SRPU50; Sawbones Europe) and SRPU with a 1.54 mm layer of short fiber filled epoxy (SFFE; Sawbones Europe). The SFFE was utilized for a realistic simulation of the cortical layer of the temporal bone.³⁷ In addition, two animal substitute materials were tested, porcine iliac bone and bovine femur. Only the plateau of the distal femur of the bovine femur was used as this part most closely resembles the structure of human bone.³⁸ All non-synthetic specimens were separated from soft tissue and stored frozen prior to the experiments. The samples were then thawed prior to the experiments which is known to have minor effect on the mechanical properties.³⁹ The mechanical properties of the materials, as available, are given in Table 2. For the testing of the sawbone specimens, a block measuring 15 mm × 15 mm × 30 mm was used for each pull-out test. Human bone samples and animal substitute materials were also trimmed to fit in the test setup. The distance between two drill holes in non-synthetic materials was at least 1.5 cm in each test. After each of the first five trials, a microscopic check was carried out to detect any damage to the bone. Afterwards, only macroscopic controls were carried out after each pull-out.

Table 2.

Overview of Mechanical Properties of the Investigated Substitute Materials and Human Temporal Bone. SFFE/SRPU50: Sawbones Europe.

Material	Density (g/cm³)	Tensile modulus longitudinal (GPa)	Tensile modulus transversal (GPa)	Tensile modulus (GPa)	Shear modulus (GPa)
SFFE⁴⁰	1.84	16	10
SRPU50⁴⁰	0.8			1.4689	0.178
Bovine femur³⁶	0.89-1.13	26.5	11		5.09
Porcine iliac bone⁴¹				13.6-28.2
Human temporal bone⁴²	1.87			13.4-23.4	4.7-7.1

Abbreviations: SFFE: short fiber filled epoxy; SRPU50: solid rigid polyurethane.

Specification of the Tested Screws

Five types of orthodontic miniscrews were tested (Figure 3 and Table 3).

Figure 3.

(a) Illustration of investigated bone screws. S1: PSM quattro standard (PSM Medical Solutions). S2: Promedia LeForte (Promedia Medizintechnik). S3: KLS maxdrive drill-free (Gebrüder Martin GmbH & Co KG). S4: General implants mini bone screw (General implants Germany GmbH). S5: Promedia dual top (Promedia Medizintechnik). (b) Detailed photograph of the flank at the tip of S3.

Table 3.

Overview of Miniscrews Investigated in This Study.

Label	Name	Manufacturer	Material	Diameter (mm)	Length (mm)	Intend use	Order no.
S1	Quattro standard	PSM	Titanium	2.0	9.0	Oral-maxillo-facial osteosythesis	33-12209
S2	LeForte	Promedia	Ti-6AI4V	2.0	10.0	Craniofacial osteosythesis	20 AT 010
S3	Maxdrive drill-free	KLS	Titanium	2.0	9.0	Oral-maxillo-facial osteosythesis	25-879-09-09
S4	Mini bone screw	General implants	Titanium	2.0	11.0	Craniofacial osteosythesis	20-712-11
S5	Dual-top	Promedia	Ti-6AI4V	2.0	10.0	Dental surgery	20 JA 010 N/S

PSM: PSM Medical Solutions; Promedia: Promedia Medizintechnik; KLS: Gebrüder Martin GmbH & Co KG; General implants: General Implants Germany GmbH.

All miniscrews were selected in advance as self-tapping and self-drilling, and all are distributed as such according to the manufacturer’s specifications. In contrast to previous tests,²³ all screws were commercially available with an original intended use in dental surgery or for craniofacial and maxillofacial osteosynthesis. The length of the screws ranged between 9 and 11 mm; however, all screws were only inserted into the material to a predetermined depth of 3.6 mm. The length of the miniscrews was chosen to ensure visibility of the screw heads above the skin level¹⁹ after insertion into the bone, which facilitates intraoperative anchoring of the frame. The screw diameter was 2 mm in all screws. The screwdrivers recommended by the manufacturers were used. All screws featured a cross-drive head, while all manufacturers recommended a classic screwdriver, only Promedia’s screwdriver could hold the entire screw head. Thread Design was the comparable for all screws. S2 to S5 were equipped with a flank at the tip (Figure 3b), whereas S1 was not equipped with such flank although it is also distributed as “self-drilling”.

Analysis

Graphical visualization and evaluation of the calibrated displacement was performed with Spyder, Version 4.1.5, with a customized protocol. The captured data was transferred to an Excel sheet (Microsoft Excel^® 2019). Means and standard deviations for peak load provide the descriptive statistics. Using Kruskal-Wallis test, each screw type’s measurements from independent samples were compared for significant differences. A significance level of p = .05 was set. All statistical analysis was performed with SSPS^® Version (28) (IBM Inc). Each force-displacement curve was analyzed individually to determine the joint stiffness, since the course of the pull-out was not linear over the entire test. A linear region was determined with a customized protocol in which the linear region was approximated using linear regression (least squares method) over at least 25 measurements. We iterated through different threshold values (from 0.05 to 10 N/mm) to find a suitable threshold. The threshold was used to determine if the linear regression line fitted for a segment of the data. Along with these measurements, the slope in the respective area was determined using the formula $k = \frac{Δ F}{Δ d}$ (Figure 4) where k is the joint stiffness. ΔF represents the change in the applied force in the linear region and is measured in N, Δd represents the change in displacement in the linear region and is measured in mm.

Figure 4.

Example of a force-displacement curve. Y-axis: force in N; X-axis: displacement in mm. Between the two crosses, a linear regression was applied.

With the given measurement data, the elastic energy was then determined using the formula: $elastic energy = \frac{1}{2} k {(Δ d)}^{2}$ .

k represents the stiffness determined before, measured in N/mm. Δd represents the change in displacement within the linear region and is measured in mm.

Results

The results of all pull-out tests divided by average force, standard deviation, range of the results, and calibrated displacement in human temporal bone as well as bovine femur and SFFE are given in Table 4.

Table 4.

Results of Pull-Out Tests Divided by Screw and Investigated Material.

Label	Average force	SD	Range	Calibrated displacement	n
(a) Results of pull-out tests in human temporal bone
S1
S2	230.9	124.7	513.8-36	0.49-2.00	14
S3	284.9	135.1	487.5-99.4	0.35-1.40	14
S4	220.6	101.7	395.4-77	1.88-3.34	15
S5	283.8	112	444.8-72.3	0.60-2.20	13
(b) Results of pull-out tests in bovine femur
S1	179.5	69.7	301.3-21.6	0.66-2.04	15
S2	150.6	63.1	238.4-40.7	0.73-3.55	14
S3	164	63.3	267.8-50.6	0.83-2.17	15
S4	189.7	79.2	290.2-56.8	1.60-4.30	15
S5	252.8	113	427.8-88.6	2.05-4.50	14
(c) Results of pull-out tests in SFFE
S1	232.5	44.6	337-171.7	1.01-1.37	17
S2	257.9	94.5	384.6-89.7	1.05-2.11	15
S3	217.9	75	350-91.2	0.9-1.29	15
S4	285.1	105.3	438.2-140	1.12-3.01	15
S5	315.1	54.3	404.2-194.7	2.02-3.53	15

All forces are given in N. Calibrated displacement is given in mm.

Abbreviation: SFFE: short fiber filled epoxy.

Bone Replacement Materials

S1 was evaluated in SRPU and bovine femur and also in pull-out tests with porcine iliac bone and pure sawbone. In 15 successful pull-out tests with porcine iliac bone, the average pull-out force was 127.7 N ± 80.7 N. In 15 successful pull-out tests with pure sawbone (SRPU50), the required average pull-out force was 187.1 N ± 59.2 N.

S2, S3, and S4 were tested in bovine femur and SFFE (Figure 5). In S2, one pull-out test had to be excluded from analysis due to an overtightening of the screw.

Figure 5.

Overview of results of pull-out tests displayed in boxplots regarding different materials. X-axis: tested material; Y-axis: pull-out forces in N; left: pull-out forces in human temporal bone; middle: pull-out forces in bovine femur; right: pull-out forces in SFFE; S2: red boxplots; S3: green boxplots; S4: blue boxplots; S5: yellow boxplots; SFFE: short fiber filled epoxy. Asterisk brackets show statistic significant difference.

Comparisons in bovine femur regarding pull-out force with the Kruskal-Wallis test were statistically insignificant. A statistical significance was found in the pull-out-test of laminated sawbone between S1 and S5 (P = .008). Comparisons between all other screws were statistically insignificant. In bovine femur, S3 demonstrated significantly higher joint stiffness compared to S2 (P = .08), S4 (P = .02), and S5 (P < .01). No statistically significant differences in joint stiffness were found between screws in the SFFE material (Figure 6).

Figure 6.

Overview of results of joint stiffness displayed in boxplots regarding different materials. X-axis: tested material; Y-axis: joint stiffness in N/mm; left: joint stiffness in human temporal bone; middle: joint stiffness in bovine femur; right: joint stiffness in SFFE; S2: red boxplots; S3: green boxplots; S4: blue boxplots; S5: yellow boxplots; SFFE: short fiber filled epoxy. Asterisk brackets show statistic significant difference.

In bovine femur, S3 demonstrated significantly lower elastic energy absorption than S4 (P = .05) and S5 (P < .05). In the SFFE material, significant differences were observed between S2 and S4 (P < .01), S3 and S4 (P < .01), and S3 and S5 (P = .03; Figure 7).

Figure 7.

Overview of results of elastic energy displayed in boxplots. X-axis: tested material; Y-axis: elastic energy in Nmm; left: elastic energy in human temporal bone; middle: elastic energy in bovine femur; right: elastic energy in SFFE; S2: red boxplots; S3: green boxplots; S4: blue boxplots; S5: yellow boxplots; SFFE: short fiber filled epoxy. Asterisk brackets show statistic significant difference.

Generally, in each test, the investigated material failed. No damage or failure of a screw, except for S1, was observed.

Human Temporal Bone

S1 could unexpectedly not be placed into the human temporal bone samples as, in most cases, a small piece of the screw-tip already broke off during insertion and could no longer be used to cut into the bone. A total of 15 attempts were performed to insert S1 correctly. However, this was only successful twice. In both following pull-out tests, a piece of the screw-tip remained in the bore hole (Figure 8).

Figure 8.

Comparison between S1 before drilling into the bone (left) and after the pull-out test (right).

In S3, in one case, the screw was overtightened and no pull-out was performed. One pull-out test was excluded from the analysis, as the pull-out force was less than 1 N so that it was evaluated as unnoticed overtightening of the miniscrew.

In S4, one test was excluded from the analysis, as the pull-out force was less than 1 N. In one case, a displacement of 5.0 mm was recorded, which was the predefined maximum of displacement. The test was stopped and the maximum force included into the evaluation.

In S5, in the pull-out tests in human temporal bone two screws and in bovine femur one screw was overtightened and no pull-out was performed.

All comparisons between the different screws types regarding pull-out strength (P > .05) in human temporal bone were statistically insignificant. The joint stiffness showed statistically significant differences between S3 and S4 (P = .01) as well as between S3 and S5 (P = .03) in human temporal bone. No significant differences were observed between the other screws (Figure 6). The elastic energy showed statistically significant differences between S3 and all other screws (S2, S4, S5) in human temporal bone, with P-values ranging from .01 to .04 (Figure 7).

Discussion

According to the recommendations of the Biometric Institute of our clinic, it was required to conduct 12 pull-out tests for each screw type in each material to demonstrate statistical significance. To ensure the attainment of this number even in the case of unsuccessful tests, the planned number of pull-out tests was increased to 15 for each screw type and substitute material.

Substitute Material

Regarding the investigation of a suitable substitute material, four different materials were tested. Laminated and non-laminated Sawbones were selected as artificial materials, as described. The natural materials evaluated included porcine iliac bone and bovine femur. No animal temporal bones were investigated because information regarding mechanical characterization was not available in literature at the time of the experiments. Porcine bone is often used as a replacement material for the mandible, representing one of the original applications of the reviewed miniscrews.^25,43,44 However, it exhibited significantly lower values in comparison to the human temporal bone, with an average pull-out force of 128 N ± 80.7 N, also in comparison to the bovine femur. The differences were statistically significant (P = .03), leading to the decision not to pursue this approach. These findings align with the results of Kobler et al.²³

A similar result was observed in the unlaminated Sawbone (SRPU50). With an average pull-out force of 187 N ± 59 N, this material showed a statistically significantly lower pull-out force than the laminated Sawbone (P = .01). Consequently, no further pull-out tests were conducted using this substitute material.

Bovine femur and laminated Sawbone were tested in this study with all screws. The bovine femur exhibited a larger standard deviation, attributed, as with the human temporal bone, to variations in bone-diploe interface. In all tested screws, bovine femur showed significant lower joint stiffness than the human temporal bone, while elastic energy was higher than in human temporal bone for all screws.

The mechanical behavior of SFFE with regard to joint stiffness and elastic energy, most closely resembling the human temporal bone among all tested replacement materials. The average pull-out values of the laminated Sawbone were only 14.1 N higher compared to the human specimen. This difference is considered acceptable, making it suitable for use as a substitute material for the human temporal bone in further pull-out experiments.

Pull-Out force in Human Temporal Bone

The initial study design involved testing all types of screws in each material, with S1 randomly selected to serve as a reference. Unfortunately, the experiments were started with pull-out tests in the substitute materials due to a lack of knowledge about the performance of S1 in human temporal bone specimens. Subsequently, as mentioned earlier, we discovered that none of the intended pull-out tests in human temporal bone specimens could be performed. As a result, S1 had to be excluded from further experiments.

This unexpected finding necessitated a modification of the original study protocol. Now all other screws were first evaluated in human temporal bone samples, as it is the most relevant and critical material. In contrast to S1, this was possible without remarkable problems for all other screw types.

Although S1 was also described as self-cutting and self-drilling, it differs from the rest of the miniscrews in a special feature: it does not have a flank at the tip (Figure 3b). In retrospect, we consider this design detail to be an important aspect of the screw that contributes to its ability to drill and to cut the temporal bone, and therefore, its stability.

The biomechanical behavior of the individual screws displayed some significant differences. S4 showed the smallest standard deviation (101.7 N), while the other screws showed larger variations, with standard deviations ranging from 112 N to 135 N. Notably, SFFE shows an exactly inverse trend.

During drilling, compressive forces primarily occur along the drilling axis. Pull-out forces may arise during inserting or removing the drill, but in the study by Kobler et al,^23,45 these forces amounted to a maximum of 26 N in untrained users. The mean forces were likely lower, but were not reported. The transverse interaction forces during drilling were consistently low, averaging between 1.0 N and 2.0 N, with a maximum reported value of 11.9 N.

Converting these transverse forces, which act roughly perpendicular to the skull surface, forces into tensile forces along the main axis of the miniscrews used for bone anchorage is highly speculative. It depends strongly on the geometry of the specific microstereotactic frame, including design details such as distances between bone screws, direction of force application, and its height above the skull surface. Therefore, a direct comparison of the values mentioned by Kobler et al²³ with the pull-out forces determined in this study is not feasible.

However, for a rough placement of the determined values in the context of expected drilling forces, we made the following assumptions: during drilling, compression forces primarily in axial direction are transferred to the bone screws via the ministereotactic frame. This is not critical for the stability of the bone anchoring. Transverse forces are on average negligibly small (≤2 N), and even the maximal transverse forces in a worst-case scenario described in of 12 N²³ does not seem to be crucial in case of a flat frame design, such as the one used in our group (which might differ in systems with longer legs and a more distal force transmission point, as in Kratchman et al⁴⁶). Therefore, assuming the strongest additive effects, it is unlikely that the forces in pull out direction would exceed 50 N.

Considering comprehensive safety margins are already included in these assumptions, further multiplication with additional safety factors does not seem justified. Therefore, we regard 50 N as an upper limit for reference forces. Furthermore, it is mentionable that the planned intraoperative setup (Figure 1) will distribute the applied forces among three miniscrews.^19,47 In another scenario with just one screw, the screw-in depth is increased to 6 mm¹⁷ which contributes to additional pull-out stability.

All the miniscrews tested^23,45 force successfully withstood the upper limit force, except for a single test by S2, which held only 36 N. Average pull-out forces in human temporal bone ranged between 220 N and 284 N. For comparison: in the mentioned study by Kobler et al, bone anchors showed a median pull-out strength of 392.9 N in the human specimens. However, the bone anchors were inserted 5 mm into the bone, which is 1.4 mm (28%) deeper than in this study.

There were also variations in the displacement of the miniscrews under load. While S4 exhibited a displacement of up to 3.34 mm, in some cases, the top layer of the bone was detached from the rest of the specimen during the experiment before the miniscrew was pulled out. The other types of screws showed calibrated displacements between 1.4 mm and 2.2 mm.

Based on the visual observation of the pull-out tests, we exclude that the screws were elongated by the reported values in the experiments. It is impossible to ascertain if and to what degree this displacement affects the stability of the bone-screw interface and thus the bone-screw interface. This relatively large deviation could indicate a slow failure with a detachment of the upper layer of the bone before the miniscrew pull-out rather than a pull-out from the bone material, potentially limiting accuracy. Furthermore, it could be a deformation resulting from the experimental setup that was not captured by the mathematical correction.

However, Rau et al have performed investigations regarding accuracy using three screws of type S3 to mount the reference frame of a surgical targeting system.¹⁵ The result was a high drilling accuracy with a mean position error of 0.41 mm. Therefore, we assume that a comparable large displacement does not occur in the clinical setting or has only minor effect on accuracy.

The joint stiffness in human temporal bone results align with the findings from the pull-out force testing, where S3 showed the highest average pull-out force compared to other screws. The significant differences observed between S3 and other screws in human bone suggested that S3 achieves a very rigid connection to the temporal bone. This could be attributed to subtle differences in thread pattern, shape, or material when compared to S2, S4, and S5. However, joint stiffness alone may not fully predict anchorage performance, as no significant differences were observed between screws in SFFE despite variations in pull-out force. Further mechanical testing could provide insights into the factors contributing to the higher joint stiffness observed for S3. Overall, these findings provide additional insights into the strength of the bone-implant interface achieved with different orthodontic miniscrews.

While S3 demonstrated the highest joint stiffness and maximum pull-out force, it showed significantly less elastic energy prior to failure compared to all other screws. As S3 achieves rigidity and resistance to pull-out, the low elastic energy value could point to only small deformation of the joint under load. In contrast, screws S2 and S4 exhibit more ductile interfaces, with lower maximum stiffness yet greater energy absorption before failure. Material factors likely also contribute, as differences in elastic energy were not observed between screws in SFFE. These findings highlight interrelated factors governing interface stability. Overall, S3 provides the strongest connection to bone, informing miniscrew selection for applications requiring enhanced anchorage strength and resistance to failure. Further testing under varied loading conditions is required to elucidate the mechanisms conferring high stiffness yet low ductility to the S3 bone-screw interface. However, the lack of significant differences between the miniscrews in SFFE in terms of joint stiffness emphasizes the influence of material properties on anchorage performance.

In terms of intraoperative handling, we found S5 to be the most convenient to use, as it was the only screwdriver that could hold the entire screw head, making the screwing more stable and easier to insert.

It might pose a clinical concern that one unnoticed overtightening of S3 and S4 was observed in one experiment with human material, although a manual check of the correct placement of the screw was performed after each screwing-in before the pull-out. If this issue arises during surgery, it could lead to a deviation of the trajectory due to the complete failure of the joint. In a similar setup for minimally invasive access to the cochlea, in which the frame was fixed with only one miniscrew onto the skull, the screw is tightened with a torque wrench to a predefined force to prevent overtightening.¹⁷

Human bone showed the highest variation of results compared to the tested substitute materials, possibly attributed to the variation of the cortical bone-diploe interface. Although age, gender, and ear side have only minor effects on temporal bone thickness and bone density,⁴⁸ their influence on pull-out force remains unclear. Given the bone structure, preoperative bone density measurements in imaging could be performed, followed by the use of preoperative indices (Screw Implantation Safety Index and Column Density Index) to identify optimal areas for screw placement.

An additional limitation of the presented tests is the exclusive use of experiments with bone specimens in an optimized configuration. In further investigations, the miniscrews should also be tested with the complete reference frame (Figure 1) including three bone screws, rather than examining an isolated, single screw, as the loading conditions are more complex in such a case. Moreover, in this study, only a load case of 90° along the screw axis was tested. While the longitudinal pull-out test provides valuable insights into bone-screw interface stability, this loading condition is simplified compared to clinical scenarios. In vivo, more complex forces arising from drilling and handling would act on the screws. Although sufficient for comparing basic interface rigidity, additional testing methods are warranted to replicate surgical conditions.

Transverse and oblique pull-out tests could determine strength against shear loads. Axial fatigue and cyclic loading tests would elucidate performance under repetitive forces and micromotions. Finally, experiments with the complete intraoperative setup using multiple screws could evaluate stability under combined multidirectional forces. While practical constraints often limit testing complexity, these techniques could better characterize construct strength for survival under intraoperative loads.

Our results provide preliminary data to guide miniscrew selection, but further verification is needed under diverse loading modalities. These experiments should also be conducted with the miniscrews only partially inserted into the bone, as their tapered design may make them more susceptible to lateral forces.

The investigated orthodontic miniscrews have proven excellent primary stability in orthodontic studies, resulting from the mechanical connection and the relationship between the miniscrew thread and the surrounding bone immediately after installation.²⁵ The higher loss rate described in several publications is a long-term complication that does not play a crucial role in cochlear implantation surgery, as the miniscrews are removed at the end of the procedure and do not have a sustainable load-bearing function.

Overall, none of the successfully tested miniscrews showed a statistically significant difference in withstanding forces in the pull-out test. S3 displayed significant higher stiffness, indicating a solid connection to the bone. However, the smallest elastic energy absorption also suggests a solid connection with only little deformation. Overall, all tested miniscrews seem suitable for fixation of a stereotactic frame at the lateral skull base due to sufficient stiffness and the ability to withstand the expected force.

S1 had to be excluded from the tests because no solid connection to the skull could be established. Due to the high displacement, S4 would not be our first choice, and we recommend conduction further tests before utilizing it for fixation.

Conclusion

This study is the first evaluation of the stability of orthodontic miniscrews at the lateral skull base and their feasibility for frame fixation in minimally invasive cochlear implantation surgery. Different substitute materials were compared for further experiments. Our findings demonstrate that laminated sawbone specimens, with an average pull-out force 14.1 N higher compared to the human specimens and comparable mechanical performance in terms of joint stiffness and elastic energy, are suitable as substitute material for the human temporal bone.

Our experiments revealed that all miniscrews withstood an adequate pull-out force in tests with human temporal bone specimens. The measured joint stiffness indicates that a solid bone connection can be established. Therefore, the investigated orthodontic miniscrews are suitable for fixation of a stereotactic frame at the lateral skull base.

Footnotes

Correction (June 2024):

This article has been updated with Figure 4 since its original publication.

Author Contributions

CM performed the experiments, drafted the manuscript, and designed the figures. CM, MK, BW, and TR were involved in planning, and TR, OM, and TL supervised the work. CM, BW, MK, and TR processed the experimental data, performed the analysis, CM and MK performed the statistics. TR, TL, and OM aided in interpreting the results and worked on the manuscript. All authors read and approved the final manuscript.

Availability of Data and Materials

The datasets during and/or analyzed during the current study available from the corresponding author on reasonable request.

Consent for Publication

Not applicable.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was funded in part by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy—EXC 2177/1—Project ID 390895286, project RA2751/4-1 and in part by the Federal Ministry of Education and Research of Germany (BMBF, grant number 13GW0367B).

Ethics Approval and Consent to Participate

Via institutional review board approval (Ethics Committee of Hannover Medical School; 3512-2017).

ORCID iDs

Christian Menke

Thomas S. Rau

References

Deep

Dowling

Jethanamest

Carlson

ML.

Cochlear implantation: an overview. J Neurol Surg B Skull Base. 2019;80(2):169-177. doi:10.1055/s-0038-1669411

Matin

Artukarslan

Illg

Lesinski-Schiedat

Lenarz

Suhling

MC.

Cochlear implantation in elderly patients with residual hearing. J Clin Med. 2021;10(19):4305. doi:10.3390/jcm10194305.4305

Varadarajan

Sydlowski

Anne

Adunka

OF.

Evolving criteria for adult and pediatric cochlear implantation. Ear Nose Throat J. 2021;100(1):31-37. doi:10.1177/0145561320947258

Sargsyan

Kanaan

Lenarz

Lesinski-Schiedat

Comparison of speech recognition in cochlear implant patients with and without residual hearing: a review of indications. Cochlear Implants Int. 2021;22(5):257-264. doi:10.1080/14670100.2021.1898111

Peters

JPM

van Heteren

JAA

Wendrich

, et al. Short-term outcomes of cochlear implantation for single-sided deafness compared to bone conduction devices and contralateral routing of sound hearing aids-Results of a Randomised controlled trial (CINGLE-trial). PLoS One. 2021;16(10):e0257447. doi:10.1371/journal.pone.0257447

Arndt

Laszig

Aschendorff

Hassepass

Beck

Wesarg

Cochlear implant treatment of patients with single-sided deafness or asymmetric hearing loss. HNO. 2017;65(Suppl 2):98-108. doi:10.1007/s00106-016-0297-5

Carlson

Breen

Driscoll

, et al. Cochlear implantation in patients with neurofibromatosis type 2: variables affecting auditory performance. Otol Neurotol. 2012;33(5):853-862. doi:10.1097/MAO.0b013e318254fba5

Hansen

Gantz

Dunn

Outcomes after cochlear implantation for patients with single-sided deafness, including those with recalcitrant Meniere’s disease. Otol Neurotol. 2013;34(9):1681-1687. doi:10.1097/MAO.0000000000000102

Buechner

Brendel

Lesinski-Schiedat

, et al. Cochlear implantation in unilateral deaf subjects associated with ipsilateral tinnitus. Otol Neurotol. 2010;31(9):1381-1385. doi:10.1097/MAO.0b013e3181e3d353

10.

Lenarz

Cochlear implant—state of the art. GMS Curr Top Otorhinolaryngol Head Neck Surg. 2018;16:Doc04. doi:10.3205/cto000143

11.

Carlson

ML.

Cochlear implantation in adults. N Engl J Med. 2020;382(16):1531-1542. doi:10.1056/NEJMra1904407

12.

Rau

Kreul

Lexow

, et al. Characterizing the size of the target region for atraumatic opening of the cochlea through the facial recess. Comput Med Imaging Graphics. 2019;77:101655. doi:10.1016/j.compmedimag.2019.101655

13.

Kobler

Nuelle

Lexow

, et al. Configuration optimization and experimental accuracy evaluation of a bone-attached, parallel robot for skull surgery. Int J Comput Assist Radiol Surg. 2016;11(3):421-436. doi:10.1007/s11548-015-1300-4

14.

Caversaccio

Wimmer

Anso

, et al. Robotic middle ear access for cochlear implantation: first in man. PLoS One. 2019;14(8):e0220543. doi:10.1371/journal.pone.0220543

15.

Rau

Witte

Uhlenbusch

Kahrs

Lenarz

Majdani

Concept description and accuracy evaluation of a moldable surgical targeting system. J Med Imaging (Bellingham). 2021;8(1):015003. doi:10.1117/1.JMI.8.1.015003

16.

Labadie

Balachandran

Noble

, et al. Minimally invasive image-guided cochlear implantation surgery: first report of clinical implementation. Laryngoscope. 2014;124(8):1915-1922. doi:10.1002/lary.24520

17.

Salcher

John

Stieghorst

, et al. Minimally invasive cochlear implantation: first-in-man of patient-specific positioning jigs. Front Neurol. 2022;13:829478. doi:10.3389/fneur.2022.829478

18.

Kobler

Kotlarski

Lexow

Rau

Majdani

Ortmaier

. Design Optimization of a Bone-Attached, Redundant and Reconfigurable Parallel Kinematic Device for Skull Surgery. 2014 IEEE International Conference on Robotics and Automation (ICRA), Hong Kong, China, 2014, pp. 2364–2371, doi: 10.1109/ICRA.2014.6907187.

19.

Rau

John

Kluge

, et al. Ex vivo evaluation of a minimally invasive approach for cochlear implant surgery. IEEE Trans Biomed Eng. 2023;70(1):390-398. doi:10.1109/TBME.2022.3192144

20.

Labadie

Balachandran

Mitchell

, et al. Clinical validation study of percutaneous cochlear access using patient-customized microstereotactic frames. Otol Neurotol. 2010;31(1):94-99. doi:10.1097/MAO.0b013e3181c2f81a

21.

Muller

Kahrs

Gaa

, et al. Workflow assessment as a preclinical development tool: surgical process models of three techniques for minimally invasive cochlear implantation. Int J Comput Assist Radiol Surg. 2019;14(8):1389-1401. doi:10.1007/s11548-019-02002-3

22.

Labadie

Mitchell

Balachandran

Fitzpatrick

JM.

Customized, rapid-production microstereotactic table for surgical targeting: description of concept and in vitro validation. Int J Comput Assist Radiol Surg. 2009;4(3):273-280. doi:10.1007/s11548-009-0292-3

23.

Kobler

Prielozny

Lexow

Rau

Majdani

Ortmaier

Mechanical characterization of bone anchors used with a bone-attached, parallel robot for skull surgery. Med Eng Phys. 2015;37(5):460-468. doi:10.1016/j.medengphy.2015.02.012

24.

Teekavanich

Uezono

Takakuda

Ogasawara

Techalertpaisarn

Moriyama

Evaluation of cortical bone microdamage and primary stability of orthodontic miniscrew using a human bone analogue. Materials (Basel). 2021;14(8):1825. doi:10.3390/ma14081825

25.

Nenen

Garnica

Rojas

Oyonarte

Comparison of the primary stability of orthodontic miniscrews after repeated insertion cycles. Angle Orthod. 2021;91(3):336-342. doi:10.2319/050120-375.1

26.

Pithon

Nojima

LI.

Primary stability of orthodontic mini-implants inserted into maxilla and mandible of swine. Oral Surg Oral Med Oral Pathol Oral Radiol. 2012;113(6):748-754. doi:10.1016/j.tripleo.2011.06.021

27.

Florvaag

Kneuertz

Lazar

, et al. Biomechanical properties of orthodontic miniscrews. An in-vitro study. J Orofac Orthop. 2010;71(1):53-67. doi:10.1007/s00056-010-9933-y

28.

Gracco

Giagnorio

Incerti Parenti

Alessandri Bonetti

Siciliani

Effects of thread shape on the pullout strength of miniscrews. Am J Orthod Dentofacial Orthop. 2012;142(2):186-190. doi:10.1016/j.ajodo.2012.03.023

29.

Yashwant

Dilip

Krishnaraj

Ravi

Does change in thread shape influence the pull out strength of mini implants? An in vitro study. J Clin Diagn Res. 2017;11(5):ZC17-ZC20. doi:10.7860/JCDR/2017/25774.9808

30.

Lin

JCY

Liou

EJW

Yeh

Evans

. A comparative evaluation of current orthodontic miniscrew systems. World J Orthod. 2007;8(2):136-144.

31.

Kluge

Rau

Lexow

, et al. Anatomische Bauraumanalyse für knochenverankerte Mini- Stereotaxiesysteme der lateralen Schädelbasis. In: Tagungsband der 14. Jahrestagung der Dtsch. Gesellschaft für Comput. und Robot. Chir. e.V. Proc. of 14th Annual Meeting of the German Society for Computer and Robot Assisted Surgery (CURAC), September 17-19 2015, Bremen, pp. 337–342.

32.

Schwartz-Dabney

Dechow

PC.

Variations in cortical material properties throughout the human dentate mandible. Am J Phys Anthropol. 2003;120(3):252-277. doi:10.1002/ajpa.10121

33.

Delille

Lesueur

Potier

Drazetic

Markiewicz

Experimental study of the bone behaviour of the human skull bone for the development of a physical head model. Int J Crashworthiness. 2007;12(2):1358-8265.

34.

Motherway

Verschueren

Van der Perre

Vander Sloten

Gilchrist

MD.

The mechanical properties of cranial bone: the effect of loading rate and cranial sampling position. J Biomech. 2009;42(13):2129-2135. doi:10.1016/j.jbiomech.2009.05.030

35.

Ayers

Clift

Gheduzzi

Morsellised sawbones is an acceptable experimental substitute for the in vitro elastic and viscoelastic mechanical characterisation of morsellised cancellous bone undergoing impaction grafting. Med Eng Phys. 2014;36(1):26-31. doi:10.1016/j.medengphy.2013.08.005

36.

Reilly

Burstein

AH.

The elastic and ultimate properties of compact bone tissue. J Biomech. 1975;8(6):393-405. doi:0021-9290(75)90075-5

37.

Schröder

von dem Berge

Lippert

. [Biomechanics of calvaria. VI. Ultimate bending stress and complex elastic modulus (author’s transl)]. Unfallheilkunde. 1977;80(9):391-395.

38.

Willis-Owen

Hearn

Keene

Costi

JJ.

Biomechanical testing of implant free wedge shaped bone block fixation for bone patellar tendon bone anterior cruciate ligament reconstruction in a bovine model. J Orthop Surg Res. 2010;5:66-66. doi:10.1186/1749-799X-5-66

39.

Shaw

Hunter

Gayton

Boivin

Prayson

MJ.

Repeated freeze-thaw cycles do not alter the biomechanical properties of fibular allograft bone. Clin Orthop Relat Res. 2012;470(3):937-943. doi:10.1007/s11999-011-2033-5

40.

Sawbone Europe. Biomechanical test materials properties. Accessed August 15, 2021. http://www.sawbone.com

41.

Kato

Koshino

Saito

Takeuchi

Estimation of Young’s modulus in swine cortical bone using quantitative computed tomography. Bull Hosp Jt Dis. 1998;57(4):183-186.

42.

Peterson

Dechow

PC.

Material properties of the human cranial vault and zygoma. Anat Rec A Discov Mol Cell Evol Biol. 2003;274(1):785-797. doi:10.1002/ar.a.10096

43.

Brosh

Rozitsky

Geron

Pilo

Tensile mechanical properties of swine cortical mandibular bone. PLoS One. 2014;9(12):e113229. doi:10.1371/journal.pone.0113229

44.

Migliorati

Drago

Dalessandri

, et al. On the stability efficiency of anchorage self-tapping screws: ex vivo experiments on miniscrew implants used in orthodontics. J Mech Behav Biomed Mater. 2018;81:46-51. doi:10.1016/j.jmbbm.2018.02.019

45.

Kobler

Wall

Lexow

, et al. An experimental evaluation of loads occurring during guided drilling for cochlear implantation. Int J Comput Assist Radiol Surg. 2015;10(10):1625-1637. doi:10.1007/s11548-015-1153-x

46.

Kratchman

Blachon

Withrow

Balachandran

Labadie

Webster

RJ.

Design of a bone-attached parallel robot for percutaneous cochlear implantation. IEEE Trans Biomed Eng. 2011;58(10):2904-2910. doi:10.1109/TBME.2011.2162512

47.

Rau

Zuniga

Salcher

Lenarz

A simple tool to automate the insertion process in cochlear implant surgery. Int J Comput Assist Radiol Surg. 2020;15(11):1931-1939. doi:10.1007/s11548-020-02243-7

48.

Talon

Visini

Wagner

Caversaccio

Wimmer

Quantitative analysis of temporal bone density and thickness for robotic ear surgery. Front Surg. 2021;8:740008. doi:10.3389/fsurg.2021.740008

Pull-Out Strength of Orthodontic Miniscrews in the Temporal Bone

Abstract

Background

Methods

Results

Conclusion

Keywords

Introduction

Materials and Methods

Pull-Out Test Setup

Specification of the Tested Materials

Specification of the Tested Screws

Analysis

Results

Bone Replacement Materials

Human Temporal Bone

Discussion

Substitute Material

Pull-Out force in Human Temporal Bone

Conclusion

Footnotes

Correction (June 2024):

Author Contributions

Availability of Data and Materials

Consent for Publication

Declaration of Conflicting Interests

Funding

Ethics Approval and Consent to Participate

ORCID iDs

References