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32 (); 78-84
doi:
10.1016/j.jor.2022.05.011

Comparative FE biomechanical and microbial adhesion analyses on an implanted humerus

, , , , , , , ,
a Dep. of Research and Internationalization, Via C. del mare 41, 98121, Messina, Italy
b Dep. of clinical e sperimental Medicine, University of Messina, 98125, Messina, Italy
c Orthopedic Institute “Franco Scalabrino” of Messina, Via Consolare Pompea, Messina, Italy
d Dep. of Biom., Dental Sciences and Morphological and Functional Images, University of Messina, 98125, Messina, Italy
e Radiology Unit, University Hospital A.O.U. “G. Martino”, 98125, Messina, Italy
f Physical Rehabilitation Medicine Unit, University Hospital A.O.U. “G. Martino”, 98125, Messina, Italy

∗Corresponding author: A. Tropea. atropea@unime.it

Received: , Accepted: ,
Professor P K Surendran Memorial Education Foundation
Disclaimer:
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Abstract

In this study a multi fragment humeral fracture, treated with locking plate system implant, was investigate and compared with a healthy humerus by the mining of a Finite Element (FE) analysis. Locking plate implant, in this case AxSOS 3® Titanium produced by Stryker is the preferred solution in presence of multiple fracture or osteoporosis.

Loading conditions were imposed by rotating of 52,5° respect the vertical axe, both the humeri (healthy and fractured), fixing distal end, and loading the top of bones with a vertical force of 543 N (Newton). This finite element analysis aimed to compare stability of implanted humerus, implant-bone interface, stress shielding, with those related to a healthy one. A microbial adhesion analysis was also performed on the implant's material.

Results obtained by FE analysis confirm a good agreement of the mechanical behavior of the models tested. The maximum values of the registered stressed, are of about 45 MPa (Mega Pascal) for the intact humerus and 113 MPa for the fractured one. Displacements, confirm higher values on the fractured humerus and no viable bacteria were found after microbial adhesion analyses.

Conclusion: comparison between healthy and fractured humerus showed an optimal stability of the implant, when contact surfaces optimization and screws insertion are correctly performed.

Keywords

Biomechanics
Finite element analysis
Locking plate
1

1 Introduction

Recent studies report humeral fractures as a quote of 10% of all traumatic bony events, and recently an increasing trend of this kind of trauma has been noticed, about 15% per year.1 The fracture of the humerus can be considered the third typology of fracture affecting elderly patients, and it is treated generally with a conservative surgical technique, which is based on three basic phases: reduction of fracture, holding of the reduced fracture, keeping it in a supported environment (such as a cast or splint) till its healing.2,3 Surgical intervention is preferable in presence of comminuted fractures or osteoporotic cases.4 Proximal Lateral Humerus Locking Plate System, AxSOS 3® Titanium produced by Stryker, is one of the most interesting solutions in treatment of this kind of fracture because of its locking screw technology. In vitro experimental investigations show a better mechanical behavior of fractured humerus implanted with locking plate instead of the non-locking plate one.5 Clinical data show complications due to the varus deviations of the fracture site, screws penetration on the gleno-humeral articulation, or painful collisions with bony parts of plate in acromial zone.6 Complications due screw perforation and mal reduction represent an important problem, especially in osteoporotic bone,7 while thickness of medial support results as a determinant factor in ensuring a correct plate fixation,8 for this reason, surgeons should first practice surgical interventions to adapt bony geometry, obtaining the optimal cortical contact.9 To avoid these complications many studies have been performed to optimize the design the locking-fixation system.10–13

Others FE studies on proximal humerus plates investigated screw configurations and bone cement augmentation,14–16 while other studies are focused on the optimization of the implant design parametrizing design characteristics. Other research carried on a FE analysis optimizing geometrical parameters to minimize maximum stresses.17 Experimental tests involving different surgical solutions should be applied on the same specimen, thus reducing variability connected with geometries, material properties etc.18 Computer simulations can predict the biomechanical behavior of bones and of bone-implant systems assessing a certain repeatability in the obtained results. FE models are generally based on a single case, use simplified material properties and simplified loading conditions. This finite element analysis (FEA) aims to compare the mechanical behavior of a healthy humerus and an implanted one investigating the stress shielding acting on the cortical bone, fragments, plate and screws. Furtherly a microbiological analysis has been performed on the material of the AxSOS 3® Titanium produced by Stryker, in order to verify its capacity to ensure an aseptic environment around the fracture situ.

2

2 Materials and methods

2.1

2.1 FE modellization

Starting from CT (Computed Tomography) data, obtained by scanning images at 0,5 mm of an healthy humerus belonging to a 45 years old male consentient patient, the Mimics Medical Imaging Software (The Materialise Group, Leuven, Belgium) was employed to generate a 3d model of the bone. Successively a 3Dimensional (3D) FE model of ten-node tetrahedral element of the humerus was developed. A second FE model was modelled by detaching and translating tetrahedral elements to simulate a three parts humeral fracture. On this second model the implant AxSOS 3, see Fig. 1(a), complete with its ten screws, was inserted, see Fig. 1(b). Screws were inserted collapsing coincident nodes at the cortical bone. Mechanical properties were selected by using an elastic modulus of 17.500 MPa and a Poisson ratio (v) of 0,3519,20 for the bony component, and E (Young Modulus) = 110 GPa (Giga Pascal) and v = 0.3 for the titanium alloy. A contact interface algorithm was selected by adopting a friction coefficient of 0,6 and a double face contact routine, to simulate interactions among the bony fragments.21 The model, as depicted in Fig. 1(b), was solved by using ANSYS software, fixing distal end of both the humeri (healthy and fractured). A rotation of 52,5° respect the vertical axe, was imposed to the bones to simulate the physiological load registered during 90° abduction of the humerus,22,23 and a vertical load of 543 N was applied at the top of both the humeri, around the articular surface, see Fig. 2.

(a) Three parts fractured humerus implanted with Locking Plate System, (b) FE model, (c) Particular of the Locking Plate System AxSOS 3® Titanium by Stryker.
Fig. 1 (a) Three parts fractured humerus implanted with Locking Plate System, (b) FE model, (c) Particular of the Locking Plate System AxSOS 3® Titanium by Stryker.
Loading and constrain setup.
Fig. 2 Loading and constrain setup.
2.2

2.2 Microbiological analysis

The plate without screws was tested to evaluate bacterial adhesion. In order to remove the surface contaminants, the plate was treated by ultrasonic rinsing for 10 min and rinsed in Millipore water for further 10 min, followed by drying in a thermostatic oven at 37 °C for 2 h. The strains used in this study, Staphylococcus aureus ATCC (American Type Culture Collection) 12600, Pseudomonas aeruginosa ATCC 27853 and Staphylococcus epidermidis ATCC 35984, were selected as the major causes of nosocomial infections.24 Strains were separately inoculated into 10 mL of sterile Tryptic Soy Broth and the bacterial suspensions were incubated for 24 h at 37 °C in a shaker at 250 rpm (revolutions per minute). Subsequently, bacterial pre-cultures were added to 200 mL growth medium and incubated for other 16 h at 37 °C. After incubation, bacteria were harvested by centrifugation at 6500 rpm for 5 min at 10 °C and washed twice with sterile PBS (phosphate-buffered saline) and suspended in TSB (Tryptic Boy Broth). Finally, the staphylococcal suspension was sonicated, while cooling in an ice bath in order to break bacterial aggregates.25 Bacterial suspensions of 108 CFU/mL (Colony Forming Unit/milliliter) were obtained by serial dilution in TSB and the absorbance correlating with cell viability was measured at a wavelength of 550 nm (nanometer).26 In order to evaluate the bacterial adhesion ability, the plate was immerged in a sterile polystyrene box containing 5 mL of TSB and incubated at 37 °C aerobically. After 24 h the growth medium, containing the non-adherent bacteria, was removed and replaced with 5 ml of fresh medium. For bacterial adhesion quantification, the plate was incubated furtherly, immersed in 5 mL of 0.1% w/v (weight/volume) dithiothreitol and mechanically stirred for 15 min at room temperature to detach adhered bacteria. Dilutions of the obtained fluids were plated onto Tryptic Soy Agar and incubated at 37 °C in aerobic conditions for 24 h for CFU count. After the contamination period, the decontamination protocol was performed using sodium bicarbonate for 1 min under aseptic conditions, according with da Silva et al.27 After 10-fold serial dilutions in saline, aliquots of 0.1 mL were posed onto Tryptic Soy Agar plates and incubated at 37 °C for 48 h. The colony-forming units grown were counted.

3

3 Results

Results obtained by FE analysis confirm a good agreement of the mechanical behavior of the models tested. As it is possible to notice by Fig. 3 the stress contour maps showed by the two humeri is quite similar. A bending stress, due to the not centered load, can be detected on the medial part of the bone, while also in the fractured area stress maintains quite low, confirming a good resistance and stiffness offered by the implant. The maximum values of the registered stressed, see Fig. 3, are of about 45 MPa for the intact humerus and 113 MPa for the fractured one. Displacements, reported on Fig. 4, confirm higher values on the fractured humerus. This was guessable, as the three bony fragments are linked each other only by the ten screws, which any case offer a good support to the entire structure, as displacements still remain within reasonable limits. The maximum displacement obtained are of about 5,6 mm for the intact humerus and 14 mm for the other one. In addition, the calculated equivalent elastic strain reflects perfectly the contour maps obtained for the equivalent von mises stress showing a value of 2,31E-003 in the first case and 5.88E-003. In Fig. 5(a) is reported a detailed stress contour map showed by the AxSOS 3 implant. As it is possible to notice stresses are quite low, peak stress of about 48 MPa, upper screws are much less solicited than the lower, which, any case, maintain a tolerable stress value 20–35 MPa. Plate support reacts very well to the imposed load revealing a stress concentration (28 MPa) on the left area. In Fig. 5(b), 5(c) and 5(d) is reported the detailed contour map of each bony fragment. On the first fragment (b) stress is localized on the lower part and amounts to 39 MPa, while the second fragment shows a value of 35 MPa. The maximum value of the registered stress is localized on the third part, related to the fixed area and the bending load, its computed value is 112 MPa. Finally, a graph of load versus displacement is reported in Fig. 6. The resulting three curves depict the behavior of each bony fragment. As it is possible to notice the maximum displacement, 14 mm, is localized on the fragment 2, followed by fragment 1 with 12 mm, and finally fragment 3 with 10 mm.

Contour maps of Eq. Von Mises stress for the healthy and fractured humeri.
Fig. 3 Contour maps of Eq. Von Mises stress for the healthy and fractured humeri.
Contour maps of displacements for the healthy and fractured humeri.
Fig. 4 Contour maps of displacements for the healthy and fractured humeri.
Contour maps of the equivalent Von Mises Stress registered on the AxSOS 3 (a), first bony fragment (b), second bony fragment (c), third bony fragment (d).
Fig. 5 Contour maps of the equivalent Von Mises Stress registered on the AxSOS 3 (a), first bony fragment (b), second bony fragment (c), third bony fragment (d).
Load vs. displacement curves referred to the bony fragments.
Fig. 6 Load vs. displacement curves referred to the bony fragments.
3.1

3.1 Results of microbiological analysis

Bacterial adhesion on the plate, see Table 1, was assessed in the first phase of incubation, reaching up 103 CFU/ml for all the bacteria tested. At the end of the incubation (t = 3) S. epidermidis ATCC 35984 and P. aeruginosa ATCC 27853 were 4.9 × 104 CFU/ml and 7.2 × 104 CFU/ml respectively, whereas S.aureus ATCC 12600 was 5.5 × 105 CFU/ml. The microorganisms adhesion ability on the surface recorded was comparable with the results obtained in a previous study.28 After application of the decontamination protocol, no viable bacteria were detected for all the strains tested, showing that the decontamination capacity of a clinical protocol treatment was very effective on the plate tested in this study.

Table 1 Bacterial adhesion on titanium disks and bacteria evaluation after decontamination.
Number of cell bacteria before decontamination Number of cell bacteria after decontamination
t1 t2 t3
S.aureus ATCC 12600 5.2 × 103 6.2 × 104 5.5 × 105 No viable bacteria
S. epidermidis ATCC 35984 4.8 × 103 6.7 × 103 4.9 × 104 No viable bacteria
P. aeruginosa ATCC 27853 5.1 × 103 7.1 × 104 7.2 × 104 No viable bacteria
Results expressed as CFU/ml
t1= 30 min
t2= 60 min
t3= 120 min
4

4 Discussion

The aim of this study provides is to furnish a computational comparative analysis of a healthy and a fractured humerus, at 90° arm abduction, to evaluate the stiffness offered by the implant AxSOS 3 and the maximum von Mises stress aging on the locking plate.

Experience in surgical technique foresees to ensure adequate surfaces contact among fragments and implant and to improve the stiffness of the system by inserting calcar screws. Implant's failures cases have been due probably to the stress concentration around these critical areas, as reported in literature,.29,30,and 31 A not adequate cohesion between fragments and locking plate can promote micro displacements that can increase their amplitude causing high stresses on the rotator cuff, a not complete or weak healing or in the worst case a failure of the implant.32–34

On the other hand, insertion of screws can be assumed as the optimal solution to ensure the adequate contact but leads inevitably to some risks related to a not healthy or enough dense bony part where screws era inserted, or the danger of injuring nerves.35,36 Results obtained in this paper confirm levels of stresses quite acceptable, especially in the distal fractured humerus, confirming a good resistance and stiffness offered by the system plate. Displacements are higher on the fractured humerus, but remain within reasonable limits.

The oval-shaped holes, located in the middle of the plate, should reduce stress in the implant. Examples of humeral plate's failures are reported in literature, related to the PHILOS plate (Synthes, Oberdorf, Switzerland).37,38 In the present study stresses are quite low with upper screws less solicited than the lower, moreover plate support reacts very well to the imposed load revealing a stress concentration. Limited axial motion improves fracture consolidation, and shear motion hampers bone formation in the osteotomy gap and reduces growing blood vessels across the gap.39,40

Reduction of the fracture fragments is a crucial aspect managing multi fragmented fracture patterns. The size of the surgical exposure required to achieve adequate reduction is strictly connected to the need of a required well positioning of the calcar screws.41

During bending, a high stress concentration can be detected at the left part of the plate, which could be due the stiffness differences between plate and bone. Further research on this field demonstrates, especially in cases of patients with osteoporotic bones, the possibility of extending fractures on the bone screws junction, in example by soliciting there may be a risk of further fracturing at the peripheral bone screw junction, in example by soliciting the fracture site according to the varus direction with a localized load.42 Another study evidenced as the thickness of medial cortex is a predominant factor, considering the complete system, regarding its stiffness for bending or traction/compression solicitations.43 In conclusion, in presence of complex distal fractures of the humerus, two are pregnant factors to be to take into consideration. First one, contact among the different fragments and locking plate must be verified and checked in each direction, trying to avoid or reduce micro displacements. The second aspect to be considered is the insertion and the number of screws. Much more inserted screws can guarantee the optimal support, but on the other hand, can damage the bony structure without offering any kind of fixation, if the local bony density cannot offer the sufficient robustness. These two factors, if correctly evaluated, can reduce the risk of the implant failure, and provide an optimal stability. The material used to realize the AxSOS 3 implant showed a good antibacterial microbial resistance capacity as no viable bacteria were detected during decontamination protocol carried out in this study.

Bacterial affinity towards implant surface represents an important factor to be evaluated since the cell adhesion can be considered the first stage of biofilm formation and its inhibition results in a biofilm-forming prevention.

It must be said that FE method has also many limitations. Setup testing is often simplified, also in the present study of the humerus, only a 90° of abduction configuration has been investigated, and the mineral bone density variation was not taken into account. In conclusion, the solution AxSOS 3 by Stryker, offers a good answer in terms of stability when applied in a fractured humerus, also biologically, no viable bacteria were detected for all the strains tested.

5

5 Conclusions

FE method represents an optimal alternative to reduce design and development costs by testing through non-invasive approach new implantable solutions or innovative surgical techniques, and great results have been reached in surgical devices adopted for complex proximal fractures of long bones. Therefore, FEA can provide more specific, accurate, and precise values for clinical practices in different field of application.44,45 The great advantage of FE method is related to the possibility of giving a complete landscape of information regarding the mechanical stress and strain aging on the model, making it possible to manage choices, solutions and improvements of surgical interventions and techniques. In this paper a numerical comparison between healthy and fractured humerus was performed, followed by a microbiological analysis. The obtained results confirm a good agreement of the mechanical behavior related to both the models tested and no viable bacteria were found after microbial adhesion analyses. The AxSOS 3 implant shows altogether low stresses, and the upper screws are much less solicited than the lower ones. During bending, a high stress concentration can be detected at the left part of the plate, which could be due the stiffness differences between plate and bone.

In conclusion, in presence of complex distal fractures of the humerus, two are pregnant factors to be to take into consideration: contact among the different fragments and locking plate and insertion and number of screws. An adequate contact between the different parts avoids micro-displacements, while the right number of screws guarantees an optimal support, without damaging the bony structure. These two factors, if correctly evaluated, can reduce the risk of the implant failure, and provide an optimal stability.

Declaration of competing interest

All Authors have nothing to disclose.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

IRB/ethical approval

The study was exempt from approval process of the Ethical Committee of the University of Messina.

CRediT authorship contribution statement

Tropea Alessia: microbial adhesion investigation, Writing; Tisano Adriana: Data curation, Writing; Bruschetta Antongiulio: Conceptualization, Data curation, Writing, Borzelli Daniele: Methodology, Data curation, Migliorato Alba: Conceptualization, Methodology, Nirta Giuseppe: Visualization, Investigation, Supervision, Writing – review & editing, Leonardi Giulia: Investigation, Supervision, Writing – review & editing, Trimarchi Fabio: Visualization, Supervision, review & editing, Alito Angelo: Visualization, Investigation, Data curation, writing.

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