Young Seok Lee and Heaseong Jeoung contributed equally to this study as co-first author.
Conventional osseointegration systems have been applied in patients requiring transhumeral or transfemoral amputation. However, the application of these systems to transradial amputation is limited by the small diameter of the radius and ulna. Our study compared the biomechanical stability of a novel osseointegration system with that of a conventional system used in transradial amputation through an analysis of finite element (FE) models.
We established three-dimensional FE models of transradial amputations, which were osseointegrated with both the novel and conventional systems. External loads were applied to the FE models with compressive force and tensile force along the long axis, horizontal shear force, and vertical shear force. The maximum equivalent stress (MES) and the distribution of stress through the radius and ulna were evaluated.
The MES of the radius and ulna was higher in the conventional system when compressive, tensile, and vertical shear forces were applied. However, when a horizontal shear force was applied, the opposite result was found. The distribution of stress was more effective in the novel system.
Three-dimensional FE modeling showed that the novel system enabled a lower stress level and a more even distribution of stress for osseointegration in transradial amputation.
Transradial amputation may result from a congenital condition, trauma, infection, or tumor [
The upper extremities are highly complex limbs with neurovascular bundles, lymphatics, muscles, and bones that come together to form a functional appendage used during daily activities [
Currently, only a few osseointegrated transcutaneous prosthetic systems (such as OPRA system [Integrum AB, Mölndal, Sweden] and ITAP system [Stanmore Medical Group, Stevenage, UK]) for use in humans exist but none are approved for general use in the United States [
In a review of the literature, no biomechanical study about osseointegrated implants for the transradial amputee was found. Because major upper-extremity amputees account for only 8% of the 1.5 million individuals living with limb loss [
From these points, we made a new design implant with several modifications. And in a similar way to Kaku et al. [
Ethics statement: This study was conducted after obtaining approval from the Institutional Review Board of Hanyang University Hospital (No. 2020-07-017). We obtained informed consent from the patient.
We established 3D FE models of transradial amputations, which were osseointegrated with novel and conventional systems. We used the Ansys Workbench ver. 17.0 software program (Ansys Inc., Canonsburg, PA, USA) to create FE models. Two FE models were created, one representing a transradial amputation with a conventional osseointegration implant and the other representing the same forearm bones with a novel osseointegration implant. The geometry of the forearm bones was acquired from CT scans (slice thickness, 1 mm) of a 34-year-old male patient’s forearm bones. For reading CT data, an open-source software (InVesalius) was used, and the computer-aided design exchange program was used to convert CT data into 3D remodeling format files. Because the implant had to be newly designed according to the curvature of the radius and ulna, Vascular Modeling Toolkit program was used to extract the centerline of the shape with curvature. The main cause of transradial amputation is trauma (up to 80%), which occurs in young men (aged 15–45 years) [
The FE model was fixed at the end of the amputation level of the radius and ulna and the interface between the implant and bone was modeled as a continuous bond. This arrangement implies ideal osseointegration was achieved without any relative motion at the interface. In other words, the implant was rigidly anchored in the bone, showing a fixed and similar type of bond at all prosthesis material interfaces. And we set the proximal end of the radius and ulna as a fixed condition. The biomechanical properties of the radius and ulna bones were composed of cortical bone and cancellous bone. The setting values are described in
External loads were applied to the FE models with compressive and tensile forces measuring 60 N along the long axis (Z-axis). Horizontal shear force (X-axis) and vertical shear force (Y-axis) to the FE models were similarly applied at a strength of 60 N. Various external loads have been applied in biomechanical studies on the forearm. We assumed that 60 N is the appropriate size for transradial amputee referring to Santoni et al.’s study [
Currently, there is no osseointegration system commercially available for transradial amputees. However, there have already been some tests to apply the existing osseointegration system modeled after the shape of a transhumeral or transfemoral implant applied in clinical cases. The conventional osseointegration system incorporates three main components: a threaded titanium implant (the fixture), a skin-penetrating cylindrical implant (the abutment), and a titanium screw (the abutment screw) [
When a loading force of 60 N was applied on the implant along the Y-axis (vertical shear force) and the Z-axis (compressive and tensile force), the maximum equivalent stress was higher in the conventional model rather than in the novel model except for in section 3 of the ulna. However, when a horizontal shear force of 60 N was applied (X-axis), the novel system received more stress than the conventional system in sections 2 and 3 of the radius and sections 1 and 2 of the ulna (
The stress at the proximal radius and ulnar shaft was decreased in the novel system when stress was loaded along the X-axis (horizontal shear), while that at the shaft of the radius was increased in the novel system condition (
Patients with amputated limbs have traditionally relied on a stump-socket interface for prosthetic attachment [
Osseointegrated implants are exposed to daily activities, consisting of flexion, extension, supination, and pronation of the elbow and radial deviation and ulnar deviation of the wrist. Our study showed that, when subjected to horizontal shear force, the radius and ulna received higher maximum equivalent stress in the novel system. In contrast, with compressive, tensile, and vertical shear forces, the radius and ulna received lower maximum equivalent stress in the novel system. Thus, more even stress distribution throughout the radius and ulna was observed in the novel system. The stress loaded along the X-axis (horizontal shear force) is assumed to be that loaded during radial and ulna deviation of the wrist, while the stress loaded along the Y-axis (vertical shear force) is assumed to be the force loaded during flexion and extension of the elbow and wrist. Compressive and tensile forces along the Z-axis are assumed to be the result of longitudinal movements characterized by changes in ulnar variance relative to the radial corner during forearm supination and pronation.
The novel system has a long stem, sharing the characteristic of an intramedullary nail. Osseointegration through the whole length of the radius and ulna was supposed to evenly distribute stress transferred from the distal end of the implants. Ding et al. [
Despite our good results, two problems of the novel system were recognized during FE analysis. First, the maximum equivalent stress of the radius and ulna in the horizontal shear force was higher in the novel system. Also, the stress distribution in section 3 of the ulna in the novel system was definitely less effective than that seen with the conventional system. Notably, the radius can experience physiologic bowing in the horizontal plane [
Stenlund et al. [
Our study has some limitations. First, there was a lack of simulation of the anisotropic material properties of human bone. Although the FE model was designed by distinguishing the material properties of cortical and cancellous bone, this does not fully reflect the material properties of human bone. Moreover, the amputated site may present a combination of damaged bone, callus, hematoma, and fibrotic tissues. Additional analysis is likely required in future research, incorporating detailed simulation of a more realistic bone model and some clinical cases. Second, it should be realized that this loading configuration does not represent a whole transradial amputation case. When applied in clinical practice, amputation levels and the loss of soft tissue including muscles vary between patients. Thus, these findings should be generalized with caution and unexpected disadvantages of the model can be found. Sufficient research is needed, including cadaver research before clinical application. Despite these limitations, however, we investigated the biomechanical properties of the novel and conventional implants for the transradial amputee and confirmed the chance of improvement of osseointegration in the forearm bone.
In conclusion, 3D FE models showed that the design of a novel system provides a lower stress level and more even stress distribution for osseointegration in transradial amputation. This novel osseointegration system is expected to reduce complications such as periprosthetic fracture and implant failure and to improve long-term implant survival.
The authors have nothing to disclose.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1A2C2101353) and the research fund of Hanyang University (HY-202000000000506).
The stress-distribution pattern and maximum equivalent stress were evaluated in each of the three sections (1, 2, 3) below: (A) X-axis and (B) Y and Z-axis.
The new upper-extremity implant design assessed in this study. (A) Radius implant and (B) ulnar implant.
Stress-distribution pattern when stress was loaded along the X-axis at the radius (A) and the ulna (B).
Stress-distribution pattern when stress was loaded along the Y-axis at the radius (A) and the ulna (B).
Stress-distribution pattern when stress was loaded along the Z-axis at the radius (A) and the ulna (B).
Bone material properties
Material property | Cortical bone | Cancellous bone |
---|---|---|
Young’s modulus (MPa) | ||
X direction | 6,900 | 680 |
Y direction | 8,500 | 1,210 |
Z direction | 18,400 | 2,050 |
Poisson’s ratio | ||
XY | 0.42 | 0.14 |
YZ | 0.31 | 0.10 |
XZ | 0.32 | 0.11 |
Shear modulus (MPa) | ||
XY | 2,400 | 330 |
YZ | 4,900 | 600 |
XZ | 3,600 | 400 |
Maximum equivalent stress at each section after application of an external load (60 N) along each axis
External load (60 N) application along the axis | Section 1 | Section 2 | Section 3 | |
---|---|---|---|---|
X-axis | ||||
Conventional system | Radius | 29.123 | 32.484 | 13.027 |
Ulna | 39.335 | 22.351 | 17.008 | |
Novel system | Radius | 24.194 | 42.453 | 14.63 |
Ulna | 104.09 | 28.816 | 16.03 | |
Y-axis | ||||
Conventional system | Radius | 52.21 | 31.281 | 27.361 |
Ulna | 15.329 | 23.733 | 21.674 | |
Novel system | Radius | 49.414 | 30.366 | 25.523 |
Ulna | 14.38 | 22.581 | 57.157 | |
Z-axis | ||||
Conventional system | Radius | 1.3949 | 1.9997 | 1.1668 |
Ulna | 1.1521 | 1.1741 | 3.0583 | |
Novel system | Radius | 0.95711 | 1.593 | 0.76815 |
Ulna | 0.75936 | 1.0219 | 3.9029 |