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Biomechanical evaluation and finite element analysis of axial-loading simulated experiment of wrist fracture

This study combined mechanical experiments and finite element analysis (FEA), and verified each other, to assess the biomechanical analysis and effect of wrist fracture, providing theoretical basis for the simulation experiments of wrist fracture and optimal design of wrist protector. Six cadaveric wrists were included to create experimental specimens. After grouping, the wrist models were axially loaded under physiological load of 600 N, the stress magnitude and distribution of experimental group and control group were obtained. Moreover, a three-dimensional (3D) wrist finite element model (FEM) of a healthy volunteer was developed to verify the rationality and effectiveness of wrist models.


Background
The incidence of wrist fracture is widespread currently, accounting for 6.7% − 11% of the total body fracture, which inevitably results in high medical costs [1,2]. The wrist joint is mainly composed of a bony structure and other small joints, which is a composite joint between the forearm and palm [3,4]. Moreover, not only the bony structure and other small joints have a clear division of labor in structure, but also correspond to each other in function [5,6]. As the most complicated joint of human body [7], the mechanical mechanism of wrist joint is more complicated. Once the wrist joint is injured, it is likely to cause secondary damage to other wrist structures [8], affecting the function of relevant parts of the upper limbs. Thus, a thorough and detailed study of wrist joint has significance clinical significance.
Currently, researches on preventing wrist fractures are still limited on drugs in the treatment of osteoporosis, osteoporosis fracture predicted by the Quantitative computed tomography (QCT). [9][10][11][12]. However, from the perspective of biomechanics and FEA, it is of great practical significance to explore the prevention and treatment of wrist fractures.
Based on this, this study combined mechanical experiment of cadaveric wrist models with finite element (FE) wrist simulation analysis, and verified each other, which contributes to assessing the biomechanical analysis and effect evaluation of wrist protector. Moreover, it also provided the theoretical basis for the simulation experiment of wrist fracture and the optimal design of wrist protector.

Models and materials
Six wrist samples were collected from cadavers in individuals with an age at mortality of 20-50 years at the Department of Human Anatomy of Basic Medicine, Nanchang University 4 (Nanchang, Jiangxi, China). Prior to storage at -20℃, in order to exclude the skeletal defects, dislocations, lesions and tumors, the X-ray examination and bone mineral density (BMD) of wrists were performed. An unsealed cuboid container (stainless steel materials) with the specification of 80 mm×80 mm×100 mm was designed to display the wrists.
PALACOS bone cements with low viscosity, 40 g×20 boxes (Heraus, Germany) were used to implant the wrists vertically into the cuboid container according to the filling technology of bone cement [13], and the bone cements were embedded and fixed until the wrists were completely firm without any slight movement. Moreover, the wrists were equipped with a stress sensing system device (Taizhou, Zhejiang, China), a microelectronic universal testing machine (Jinan, Shandong, China) and four static strain test units (Jinan, Shandong, China), which were connected by wires. Four static strain test units were respectively marked as A, B, C and D units, of which unit A represented the stress of radioulnar distal palmar unit; unit B represented the stress of radioulnar distal dorsal unit; unit C represented the stress of radioulnar proximal palmar unit; unit D represented the stress of radioulnar proximal dorsal unit. Ethical approval was obtained from Institutional Review Board of Jiangxi Provincial People's Hospital Affiliated to Nanchang University.
The computed tomography (CT) data was obtained from a 30-year-old healthy Chinese volunteer, whose age and forearm size strictly met the requirements of above cadaver specimens. In order to ensure the normal anatomy of wrist, the X-ray of his wrist was also performed to exclude the fracture, lesions and other conditions. The experimental devices in this current study included a dual-source CT (Siemens, Berlin, Germany), a computer

Biomechanical testing methods
The biomechanical experiment in this study belonged to the impact test of static loading within the yield strength. The specimens wearing wrist protectors were divided into experimental group, and the naked wrist specimens were divided into control group. In experimental group, the wrist protectors were made of 20 mm thick soft sponge materials and 2 mm thick polypropylene hard materials. Then, the mechanical axial compression mode was selected, the strain sensing system on wrist models was installed and the static resistance strain gauges were connected. With the palm facing upward, the wrist models were fixed according to the above method, 90 degrees perpendicular to the ground, and the pressure hammer was aimed at the center of the navicular and lunar bone. The initial loading speed was set at 2 mm/min, and the loading range was 0-600 N (Fig. 1). The strain values of all target units were recorded for every 20 N load, and each specimen was tested three times under the same conditions.

Construction of 3D FEMs
The volunteer was scanned transversely by CT, ranging from proximal forearm to fingertip.
During scanning, the volunteer referred to the direction of axial-loading and the posture maintained when the wrist models were tested. It was determined that the established wrist model was set to 75 degrees of dorsiflexed and 10 degrees of pronated, the scanned data was saved in the Digital Imaging and Communications in Medicine (DICOM) format.
Then, the data was input into the Mimics19.0 software to initially establish the 3D geometric model of wrist. The contours of forearm cortex, loose tissue and surrounding soft tissue were extracted by the tools of Thresholding and Manual drawing, and the STereoLithography (STL) format data were obtained. Then, input the data into the Geomagic Studio 12.0 software, and conducted with deeper level of hole filling and smoothing for each part of model to prevent the occurrence of poor grids. Finally, the data 6 of generated solid 3D model was imported into Abaqus 6.51 software for statistical analysis.

Statistical analysis
The data obtained above were statistically analyzed by SPSS 22.0 software (SPSS, Chicago, USA), and was assessed by the paired t-test. P < 0.05 was regarded as the difference with statistical significance.

Results
The experimental results of control group  (Fig. 2).

The comparison of results between control group and experimental group
The comparison of stress peak and decline between two groups was derived as (Table 1).
In all 8 groups, besides the radioulnar proximal and dorsal units, there was no significant stress difference between the control group and experimental group at the late stage of experiment. However, the stresses of remaining 6 units in experimental group were decreased by 44% compared with control group on average (Fig. 3).

The constitution of 3D FEM
Based on the extension, flexion, retraction and rotation of normal human wrist, the 3D  Table 2).

The stress distribution in the models of control group and experimental group.
At late stage of experiments, the stress distribution in the models of control group and experimental group was shown in (Fig. 4). The results of FEA well confirmed the conclusions of the biomechanical experiments above. That is, besides the radioulnar proximal and dorsal units, there was no significant stress color difference between the control group and experimental group at the end of experiments. However, the stress color of the remaining 6 units in experimental group were lighter than those in control group, indicating the stresses of the experiment group were less than those of control group. This result further verified the rationality and effectiveness of the biomechanical experiments above.

Discussion
As the aging process of population continues to evolve, the increase in proportion of the elderly has led to a surge in patients with wrist fractures to a certain extent [14][15][16].
Especially in middle-aged and elderly women, due to the menopause and estrogen loss, the physical function and ability to deal with emergencies has declined inevitably, which brings about the high incidence of wrist fractures [13,[17][18][19]. In addition, wrist fractures are also common in young and middle-aged people during daily exercise and work [20,21]. In order to improve the prevention ability of wrist fracture in different ages and update the wrist protectors with poor effectiveness in the current state, this study combined the mechanical experiment of cadaveric wrists with FE wrist simulation analysis to evaluate the effect of the wrist external forces from multiple angles.
During axial-loading experiments, it is found that despite the complex anatomical structure of wrist joint, the external forces were mainly transmitted from navicular and lunar bone down to the radioulnar joints, and then continued to the proximal end of the forearm. In this process, the radioulnar joint was regarded as a composite joint, and the compressive deformation occurred on the radioulnar palmar units, presenting as the compressive stress. The tensile deformation occurred on the radioulnar dorsal units, presenting as tensile stress. In addition, under the normal circumstances, during the deformation of the radioulnar joint under external forces, the palmar and dorsal units with the same axis distance had the same bending moments [22]. However, in this experiment, the axial center was biased to the dorsal units under the impact, and all units had larger palmar bending moments and smaller dorsal bending moments, which explains that the absolute values of the palmar units were greater than those of the stress of dorsal units.
Secondly, according to the comparison of two groups, it is found that wearing wrist protector can effectively reduce the stress on radioulnar distal palmar, radioulnar proximal palmar and radioulnar distal dorsal units, while has no obvious influence on radioulnar proximal dorsal units. In experimental group, the stresses of radioulnar distal palmar and dorsal units were apparently reduced by 44% compared with the control group on average, which was related to the absorption and shunting of the impact load on the wrist protector. Hence, when designing and improving the wrist protector, it is reasonable to place the stress center on the radioulnar distal palmar and dorsal units. Similar to the findings of this experiment, Sun et al. [23] have designed a hip protector and screened 3 volunteers to perform a certain intensity of simulated human side fall test. The results indicated that average peak impact force could reach (1738.88 ± 215.66) N in the group without hip pad, while the average peak impact force in the group with hip pad increased apparently to (1907.44 ± 441.42) N. This result reflected that wearing a hip protector can increase the peak impact force of hip, which could prevent the occurrence of hip fracture to a certain extent.
In addition, certain limitations in this study should be recognized and pointed out. Firstly, the cadaveric specimens lack the natural soft tissue tension and stress protection mechanism of normal human body, which is unable to accurately reflect the true stress and strain of normal human wrist [24][25][26]. Subsequently, the experimental sample size was only six, which was relatively small. Thirdly, the force mechanism of wrist fracture caused by external force impact is complex. However, this study was limited to the vertical axial-loading of wrist joint, which simplified the actual force of human body.
Ultimately, the FEA method also has certain limitations. The FE simulation can only be approximated to the real situation, and the authenticity and validity of results need to be Tables Table 1 The comparison of stress peak and decline between two groups