From the initial CT scan to implantation, the entire virtual planning process, performed at the point of care, was highly reliable and posed no issues or unforeseen challenges. The accuracy of the CT images allowed a trustworthy segmentation using the available software. In the majority of cases, the WSPSIs' original design matched the osteotomy site well. The only case in which the gap occurred was due to overly high torque applied during the screw insertion. However, specimens 1L and 1R showed higher compressive strength in the biomechanical test, despite crack formation of the implants, indicating that the appearance of cracks due to screw pressure may not have a significant impact on the overall compressive strength during the bench test. To prevent future cracks caused by high bearing pressure, the placement of the WSPSIs' holes can be modified. Additionally, one could investigate the cutting of matching threads using a screw tap to increase the stability between implant and screw. Alternatively, screws of smaller size might be used. Due to polymer strength limitations another bioresorbable material, such as magnesium [22], might be used, however, further research must be done to determine the viability of the new solution.
The implants were successfully printed with medical-grade material. In this study, we focused mainly on the mechanical stability of the wedges. According to the results of the biomechanical study in Table 1, the construction can bear a force of at least 1’211 N. Many studies have been conducted to evaluate the forces occurring on the wrists while falling. For instance, the experiment by DeGoede et al. demonstrated that during a fall, the peak forces reach values of 1’021 ± 161 N when the shoulders are one meter above the ground [23]. Based on our results, the novel surgical method seems to be a promising approach for use in patients as, even in a fall scenario, the peak forces do not reach the connection strength's lowest values. However, we have not yet investigated the biological response to the implant material.
Polyesters based on lactic acid have been intensively researched for more than four decades for their biodegradable properties and their suitability for bone regeneration [24]. For the successful healing of the distal radius bone after corrective osteotomy, it is important to provide not only sufficient mechanical strength but also increase osteoconductivity and absorbability. Poly lactic acids are suitable for use for implants with medium mechanical strength, but lack bone bonding capabilities [24]. To improve bioactivity and osteointegration, certain inorganic additives such as β-TCP are frequently used [25]. Biodegradable poly lactic acids, such as PLDLLA copolymers, and osteoconductive additives such as β-TCP have been intensively researched independently and in combination [23–25]. These studies have shown PLDLLA/TCP composites to have good biocompatibility and mechanical properties close to bone.
To enhance the potential of healing and control the strength of the connection, additional design adjustments can be made, such as the incorporation of lattices or surface preparation [29]. To promote osteogenesis, the option of seeding the construct with cells or growth factors can be investigated. Some limitations, however, have to be considered. First, the investigation’s primary flaw is that the force has only been applied axially. The worst-case scenario of the connection has been tested, that is when the connection is the weakest due to not yet established bone ingrowth into an implant. However, axial forces are not the only loads acting on the wrists. Further tests with an increased number of experimental samples must be performed to evaluate the build’s capacity to withstand torsion, bending moments, and shear stresses, which, given the geometry of the construct, may be essential in establishing the viability of the solution. Fatigue testing of the samples and in vivo testing in animals should be considered in the future to investigate further investigate the mechanical stability and biological response in comparison to bone plate fixation, which is the current standard.
Since ceramic materials tend to have higher hardness than commercially used metals, the possibility of abrasion of the nozzle, during 3D-printing, via the composite was one of the crucial points of investigation. An EDX analysis revealed no metallic nozzle particles. It proves that the β-TCP can be safely printed without endangering patients to metal exposure of the machine feeding track. It also showed that the surface chemistry stays relatively constant throughout the degradation process which is important for cell adhesion. A further investigation shall be done to assess the adhesion and proliferation of the cells on the scaffold created by the APF-manufactured implant.
The mass-loss study revealed a gradual increase of mass of up to approx. 0.4 wt%, which is likely caused by precipitation of hydroxyapatite or other compounds on the implant surface during the immersion in the PBS [27]. The ratio of the organic-inorganic components was not significantly affected by the length of PBS exposure, but rather was related to the average pH level. The inorganic content level rises with increasing pH, suggesting that the PLDLLA part of the implant will degrade more quickly even with only a minor pH fall within the permitted range, or a pH of 7.4 ± 0.2. It is possible to draw the conclusion that the average inorganic content and pH values are related. The inorganic content is more pronounced in samples that have been deteriorated for one week or longer and increases as the average pH value decreases. As a result, a graph was made to display the relationship between the two variables.
To demonstrate the linear relationship between the pH value and the average inorganic concentration, a linear fitting has been done. The inorganic content decreases as the average pH increases. Reduced pH has a small impact on the β-TCP while having a significant impact on the rate of polymer breakdown. The resorption of the β-TCP is cell-mediated, where the β-TCP is reabsorbed as a result of the low pH fields of acid species generated by cells [28]. In the biodegradation study, there are no cells present in the PBS solution. No discernible β-TCP resorption is observed, suggesting that the small pH fluctuations are inadequate to damage the ceramic.
The calorimetry study revealed that the glass transition temperature has been significantly affected compared to the manufacturer’s specification. It reveals that the physical characteristics of the material are altered during the 3D-printing manufacturing process by heating the material over the melting point. The material pellets underwent mechanical and thermal processing in order for the specimens to be printed. They were kept at a temperature of 205°C and under the pressure of 200 MPa. Nevertheless, the glass transition temperature, at 50.26°C, remained significantly higher than body temperature. This suggests that the material is safe for implantation; however, caution is advised for subsequent post-processing and sterilization procedures involving temperatures beyond the glass transition point as they could change the material composition and stability.
In summary, the novel approach for the treatment of distal radius malunions have shown sufficient compressive and degradation stability, and could, based on the results reported in this study, be a viable option, in which the use of standard titanium plates, and, therefore, a second removal surgery, can be omitted.. The results are promising, laying the foundation for an innovative surgical approach to distal radius corrective osteotomy.