Patients with amputated limbs have traditionally relied on a stump-socket interface for prosthetic attachment.7 This type of prosthetic design is associated with several problems, such as skin changes, increased energy expenditure during ambulation, and disuse osteopenia.12–14 Several types of osseointegration systems have been developed to overcome these problems and are currently clinically available; however, these osseointegration systems also encountered several problems of their own, which are infection, periprosthetic bone fracture, bone loss due to stress shielding, and implant failure.15,16 Furthermore, we are concerned that the conventional design available for trans-humeral and trans-femoral amputees is not suitable for the trans-radial amputee. Specifically, the short length of the conventional system inserted into a long bone with a small diameter increases the risk of periprosthetic fracture due to targeted stress concentration.
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 al17 reported that increasing the diameter and length of the implant decreased the stress and strain on the alveolar crest per biomechanical analysis of high-quality FE models of complete range mandibles. Baggi et al18 also reported the influence of implant diameter and length on the stress distribution of osseointegrated implants related to crestal bone geometry. Stress values and concentration areas are decreased for cortical bone when the implant diameter is increased, whereas more effective stress distributions for cancellous bone were observed with increasing implant length.18 In the case of trans-radial amputation, the medullary diameter of the radius and ulna is not a modifiable factor, but the length of the implant is. The results of our study were consistent with those of previous studies in that the implant length affected the stress distribution and maximum equivalent stress.
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 Sect. 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 plane19 and adaptation to this physiologic bowing by deep insertion of the long stem seems to be the cause of greater maximum equivalent stress. In the implant design, the diameter of shaft of the novel radial implant was 3.0 mm(supplement). To decrease the stress value in relation to horizontal force, a larger radial-implant diameter can be considered. Also, increased stress around the distal longitudinal grooves of both the radius and ulna implants was observed. There is a tendency to increase the stress value in the angled location rather than the cylindrical or smooth location in FE analysis. According to Xiaobin et al20, when the surfaces are angled, the stress concentration is large when forces are applied from multiple directions. Conversely, when the surfaces were rounded, it was found that the stress concentration was relatively low for the same external forces. For clinical application of the implants, a cylindrical design rather than an angular groove modality might reduce bone absorption in the insert area.
Stenlund et al21 found that the elastic strains and principal compressive stresses in the bone in direct contact with the abutment at the most distal location may indicate unfavorable bone remodeling, resulting in a decrease in bone density with time. Meanwhile, Nebergall15 et al reported that the transmission of loads to the contact point between the bone and implant can cause cortical thinning or bone resorption. This results a high risk of loosening of that area, together with various complications in the distal portion, including implant loosening and periprosthetic fractures. Ultimately, the need for reoperation to replace the entire implant will increase.22
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 trans-radial 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. Despite these limitations, however, we investigated the biomechanical properties of the novel and conventional implants for the trans-radial amputee and confirmed the chance of improvement of osseointegration in the forearm bone.
In conclusion, three-dimensional FE models showed that the design of a novel system provides a lower stress level and more even stress distribution for osseointegration in trans-radial amputation. This novel osseointegration system is expected to reduce complications such as periprosthetic fracture and implant failure and to improve long-term implant survival.