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Abstract

The purpose of this study was to investigate the effect of varying the outer diameter of screws of a vertebral fixation system by submitting them to mechanical tests and photoelasticity. The pullout mechanical test was performed in 20 swine lumbar vertebrae, divided into two groups based on the screw outer diameter: 5.0 and 6.0 mm. The maximal pullout strengths and stiffness were evaluated. For the photoelasticity, eight models were used and divided into the same groups. The maximal pullout strength was 974.12 ± 144.44 N in the 5.0 mm screws and 1537.42 ± 326.95 N in the 6.0 mm screws (P ┼ 0.001). The stiffness was 418.60 ± 62.58 10³N/m in the 5.0 mm screws and 502.12 ± 133.45 10³N/m in the 6.0 mm screws (P = 0.09). The mean ± SD shear stress of the 5.0 mm screws was 12.90 ± 1.87 KPa and 11.99 ± 2.01 KPa for the 6.0 mm screws. Thus, the 5.0 mm screw had lower pullout and higher shear stress, suggesting that this screw is more susceptible to loosening.

1. Introduction

Vertebral fixation system bone implants are used in the treatment of deformities, degenerative diseases, tumors, infections, and fractures of the spine [1–3]. The anchorage of the screws in the vertebrae is important for the performance of biomechanical movements.

Screws can become displaced and give problems of instability to the fixed segment during the time that the screws are implanted in the bone tissue [4–6]. A failure in the anchorage of a fixation system may be related to the mechanical resistance of implants or the quality of the vertebral bone tissue [7]. The screws may have an appropriate mechanical resistance to pullout in order to avoid loosening during correction procedures. Screw loosening mainly happens in the vertebrae located in the extremities of scoliotic curves.

The pullout strength is an important parameter that predicts the fixation and stabilization of fixed segments [8]. Under a pullout force, stress is formed around the implants, and its distribution can be evaluated by special techniques [9, 10].

Regardless of the many studies focused on technology and engineering to improve the pullout resistance of implants, fixation failure still occurs and causes the loosening of vertebral alignment and segment stability, generating severe complications [4]. The incidence of bone screw loosening ranges from 0.6 to 11% and can be even higher when the implants are inserted in osteoporotic bones. The pullout strength is influenced by many factors, including the outer diameter of screws [11].

Mechanical analyses are used to evaluate the mechanical resistance of implants by means of measuring the load that the implant was submitted to before its failure. However, studies concerning the stress distribution around the screws are necessary. Photoelasticity is one of the techniques that allows this kind of analysis, and for this reason, it has currently been a target of many studies. It is an experimental technique that permits a qualitative and quantitative analysis of the inner stress in materials by the observation of optical effects. This technique is used to evaluate the stresses or strains of transparent elastic bodies by means of light physical effects [12]. This technique is applied to study models with complicated shapes with a complex load distribution, or both [13, 14].

Thus, the purpose of this study was to investigate the behavior and the effect of varying the outer diameter of screws of a vertebral fixation system by submitting them to mechanical tests of the pullout strength and photoelasticity.

2. Materials and Methods

2.1. Pullout Mechanical Test

In order to realize the mechanical pullout strength, we used 20 lumbar vertebrae (L1 to L6) of Landrace pork that weighed approximately 81.20 kg and were 150 days old. The vertebrae were harvested, cleaned of soft tissues, and kept in a freezer at −20°C. Each vertebra was thawed overnight to room temperature before testing. Initially, bones were kept at 4°C and then in room temperature. The pedicle and processes were dissected in order to allow an appropriate placement of the bone in a universal testing machine (EMIC DL 10000, Brazil). The vertebrae were randomly assigned into two groups according to the outer diameter of the screw: (1) 5.0 USS I pedicle screw and (2) 6.0 USS I pedicle screw.

Stainless steel screws of the USS vertebral fixation system (Synthes) were used. The screws’ outer diameters were 5.0 and 6.0 mm. The 5.0 mm screw had an inner core of 3.8 mm, and the 6.0 mm screw had an inner diameter of 4.8 mm. Both screws were 50.0 mm in length, had a 2.0 mm thread pitch, and had a 0.6 mm thread height (Figure 1).

Figure 1:

The (a) 5 mm and (b) 6 mm screws (Synthes) used in the study.

The screws were inserted at a depth of 30.0 mm, in the lateral face of the vertebral body, next to the pedicle. The screw was inserted after making a pilot hole with a drill. The vertebrae that resulted in incorrect screw placement were excluded.

The pilot hole was made by a drill of the same diameter as the inner diameter of the screws. Therefore, for the 5.0 mm screw, we used the 3.8 mm drill, and for the 6.0 mm screw, we used the 4.8 mm drill. The pilot holes were performed with different awls based on manufacturer recommendation (Synthes).

Each specimen with the screw inserted in the lateral face of the vertebral body was fixed by a vice at the base of the universal testing machine, and we used a pin to connect the screw head to the steel cable which provided the pullout and allowed the measurement of the analyzed parameter (Figure 2).

Figure 2:

Schematic drawing of the screw pullout from a vertebral body.

The pullout mechanical test was performed by means of the EMIC universal testing machine using a load cell of 2000 N, a load application speed of 10 mm/min performed parallel to the long axis of the screw with an accommodation time of 30 seconds, and a preload of 50 N. The experimental model with the screw inserted in the lateral faces of the vertebral body was fixed in the base of the universal testing machine. The screw head was connected to a steel cable (Figure 2).

During the mechanical testing, a graph of the load versus the deformation was plotted by a computer by software Tesc 1.13. By means of these graphs, the maximal pullout strength and stiffness were recorded. Maximum pullout strength was defined as peak force before any negative deflection on the load-deformation curve. Figure 3 represents the pullout strength graph as recorded by the computer load versus displacement.

Figure 3:

Pullout strength graph.

The data were analyzed using a Student's t-test, and the level of significance was set at 5%.

2.2. Photoelastic Analysis

Photoelastic models were made with a regular geometry through an acrylic mold in order to allow the analysis of the internal stresses around the screws.

Screws were inserted in the models at 30.0 mm. The flexible photoelastic epoxy resin (Polipox) has a Young's Modulus of 4.51 MPa and a Poisson's ratio of 0.4. The resin was positioned in the acrylic mold, with a proportion of 2.2 mL of resin to 1.0 mL of catalyzer (an amine base), to obtain the photoelastic models. The models had a 12.0 mm thickness, were 58.0 mm wide, and were 50.0 mm in length (Figure 4).

Figure 4:

The photoelastic models with the 5 and 6 mm pedicle screws. (a) Lateral view, (b) frontal view of the model with a 5 mm pedicle screw. Observe that only 30 mm of the screw (with thread) were inserted into the photoelastic model.

These models were selected due to its simple shape that simulates a plane surface of bone located next to a screw outer diameter. Two experimental groups were set up according to the outer diameter of the screw. Each experimental group consisted of four photoelastic models, for a total of eight models to be studied. One group consisted of models using screws measuring 5.0 mm in outer diameter, and the other consisted of models using screws measuring 6.0 mm.

The photoelastic resin was calibrated using a circular disk under a compressive load, obtaining an optical constant (f_r) of 0.21 N/mm fringe. This constant was used to calculate the shear stress (τ).

The photoelastic analyses were performed with a transmission polariscope by applying a pullout strength on the screw's head that was inserted in the photoelastic models. A 100 N Kratos load cell was used. The stresses produced by the screws were evaluated qualitatively and quantitatively.

For the qualitative analysis, the stress distribution around the screws was observed, emphasizing the starting site, type of growth, and the site of the highest concentration.

The quantitative analysis was performed by the application of a pullout force of 7.5 N in the region with the highest stress concentration. The shear stresses were measured at standardized points in the region next to the tip of the screw. The locations of the points were always the same, even in different screws. However, there was a difference of 0.5 mm due to the difference of the outer diameters of the screws. Figure 5 represents the distribution of the seven analyzed points.

Figure 5:

Schematic view of the seven points located next to the tip of the screw.

The shear stress (τ) was determined using the optical stress law by means of the Tardy compensation method [15]

τ = \frac{σ_{1} - σ_{2}}{2} = \frac{N \times f_{r}}{2 \cdot h} .

(1)

In (1), σ₁ and σ₂ are the principal stresses, N is the fringe order, f_r is the optical constant value of the photoelastic material (0.21 N/mm fringe), and h is the thickness of the photoelastic model (mm).

The data were analyzed using an analysis of covariance—ANOVA one-way analysis of variance to test the differences between the groups. For a comparative analysis of the experimental groups, a post hoc Bonferroni test was performed. The level of significance was set at 5% (P ≤ 0.05).

3. Results

3.1. Pullout Mechanical Test

The mean values and standard deviations of the maximal pullout strengths were 974.12 ± 144.44 N for the 5.0 mm screws and 1537.42 ± 326.95 N for the 6.0 mm screws, which were significantly different (P ┼ 0.001) (Figure 6). The maximal pullout strength was 37% higher in the 6.0 than in the 5.0 mm screws.

Figure 6:

Mean values and standard deviation of the maximal pullout strength of 5 and 6 mm screws.

The mean values and standard deviations of the stiffness were 418.60 ± 62.58 10³N/m in the 5.0 mm screws and 502.12 ± 133.45 10³N/m in the 6.0 mm screws. There was no significant difference (P = 0.09) (Figure 7). The stiffness was 16.5% higher in the 6.0 than in the 5.0 mm screws.

Figure 7:

Mean values and standard deviation of the stiffness of 5 and 6 mm screws.

3.2. Analysis by the Photoelastic Method

For the qualitative analysis, the fringe order along the crest of the screws was observed in all of the photoelastic models. In all models, the starting site of the fringe order and the site of the highest concentration were located on the tip of the screws, growing in a helicoidal manner according to the format of the screws, as illustrated in Figure 8.

Figure 8:

The stress gradient in (a) 5 mm and (b) 6 mm screws emphasizing the regions next to the tip of the screws.

The quantitative analysis was performed in order to measure the shear stress in the seven pree stablished points of the photoelastic models. Figure 7 shows the behavior of the mean values of the strain tension that the 5.0 and 6.0 mm screws underwent a load of 7.5 N. The mean ± SD shear stress of the 5.0 mm screw was 12.90 ± 1.87 KPa and 11.99 ± 2.01 KPa for the 6.0 mm screw (Figure 9). A significant difference (P = 0.04) was observed between the two screws; the 5.0 mm screw showed higher shear stress values. This behavior would be similar to bone, except that the quantitative values of the shear stress would be different due to the bone/screw and resin/screw interactions. Thus, with higher values of load, there is an increase in the stress level in the same proportion.

Figure 9:

Mean of the shear stress values for the 5 and 6 mm screws.

A significant difference between the set of preestablished points was observed (P ┼ 0.001). The values in points 6 and 7 were lower than points 3, 4, and 5. Points 1 and 2 had intermediate values.

4. Discussion

Screws are able to resist strain load, flexion, and pullout [9, 10]. These mechanical properties are related to the dimensions and the geometry of the components and also to the quality of the bone tissue.

The resistance of an implant to pullout is proportional to the thread surface in contact with the bone tissue [10, 16–18], the thread number per length [19–22], and the outer diameter [11, 23, 24]. Kwok et al. reported that the pullout strength is influenced by the outer diameter of the screws [11]. These authors performed a comparative study among screws with different outer diameters. They observed that a 6.0 mm screw has 47% more pullout strength than a 5.0 mm screw. In this study, we found an increase of 37% in a 6.0 mm screw compared to a 5.0 mm screw. We believe that the 37% increase in pullout load resulting from different size screw can, however, suppress the potential effects of variation in pedicle morphology and could be different. Another concern refers to the screw insertion. In our study, the screws were partially inserted which is not similar to inserting a screw fully in the bone, and it can give different values and may explain the statistical differences found in our study when compared to previous study [11]. However, both the 5 and 6 mm screws were partially inserted in the bone, and the difference found between them was then due to the outer diameter of screw.

The static mechanical tests were performed in this study with the purpose of evaluating the mechanical resistance of implants to pullout. In these tests, we applied an axial load that differs from the physiological movements in the spine that may cause loosening of the screw. However, static mechanical tests are experimental procedures used to simulate these analyses in vitro [23, 25]. Mechanical tests are widely accepted as a method to quantify the mechanical resistance of implants [11]. There are different protocols for pullout mechanical test in the literature; some authors used pullout speed of 1 mm/min [25], 2 mm/min [23], and 5 mm/min [4, 8], and others used a higher speed of 60 mm/min [5, 11, 26]. Previous studies were performed in our laboratory involving pullout tests with different speeds and, based on our results (unpublished data), we have used 10 mm/min as our standard speed of pullout tests. A recent paper was published in the scientific literature using 10 mm/min as the pullout speed [27].

The photoelasticity technique utilized the pullout strength for the stability of the different measurements of the screws fixed into the photoelastic model. The homogeneity and symmetry of the models allowed a faithful comparison concerning the outer diameter of the screw and the stress distribution. The inner stress was quantified by means of fringe orders and the calculation of shear stress. The results of the pullout strength obtained during the mechanical tests with the shear stress obtained by photoelasticity were correlated.

In photoelasticity, the models constructed with screws fused into the photoelastic resin simulate a screw submitted to a chronic postsurgical period, as occurs in clinical practice in surgeries using a vertebral fixation system.

In the qualitative analysis, the photoelasticity started to increase at the tip of the screws, and its distribution accompanied the geometry of each screw. In the quantitative analysis, we evaluated the shear stress and concluded that the tip of the screw had the highest stress, between points 3 and 5. This region is the most critical, with a greater propensity to screw loosening. The highest fringe orders were observed next to the tip, in the first threads ridges, especially in the peak of the crests, which is the most susceptible area to shear. These results may be explained by the differences in the screw design (outer and inner diameter, thread pitch, and height of thread).

In the current study, we observed that the 5.0 mm screw had a higher shear stress than the 6.0 mm screw by photoelasticity. By mechanical testing, we found that the 6.0 mm screw had a higher pullout strength than the 5.0 mm screw. These results agree with those reported by Kwok et al., Barber et al., and Gayet et al., [11, 26, 28].

5. Conclusion

Our data showed, by means of mechanical tests, that the maximal pullout strength was significantly (37%) higher in a screw with a greater outer diameter (6.0 mm versus 5.0 mm). This finding was confirmed by photoelasticity. We observed that the shear stress in the 5.0 mm screws was higher than in the 6.0 mm screws. In addition, we concluded that the tip of the screw has the highest concentration of stress, and the areas more susceptible to shear are next to the tip, in the first threads ridges.

Footnotes

Financial Disclosure

We certify that no party having a direct interest in the results of the research supporting this article has or will confer a benefit on us or on any organization with which we are associated and, if applicable, we certify that all financial and material support for this research (e.g., NIH or NHS grants) and work is clearly identified in the title page of the paper.

Acknowledgment

This project was funded by the Foundation for Research Support of the State of São Paulo (FAPESP).

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Biomechanical Study of the Pullout Resistance in Screws of a Vertebral Fixation System

Abstract

1. Introduction

2. Materials and Methods

2.1. Pullout Mechanical Test

2.2. Photoelastic Analysis

3. Results

3.1. Pullout Mechanical Test

3.2. Analysis by the Photoelastic Method

4. Discussion

5. Conclusion

Footnotes

Financial Disclosure

Acknowledgment

References