Bioengineering Center, Institute of Materials and Structural Models, Central University of Venezuela
ABSTRACT
The objective of the present work was to design and build a prototype of a tool that facilitates the insertion and subsequent locking of an intramedullary nail to be used in surgeries whose main objective is to reduce and stabilize diaphyseal fractures in the first metacarpal bone. This system is an innovative design made up of four pieces that are properly coupled to the nail. On the one hand, we have the insertion handle, the cylindrical guides, and the connector, made of AISI 420 steel, and, on the other, the directional handle, made of an A356 cast aluminum alloy.
For the manufacture of the proposed final model, it was necessary to establish cutting sequences and parameters. It was also essential to carry out experimental tests to validate the functionality of the system.
1. INTRODUCTION
Damages that people refer to the upper limbs are common, specifically at the level of the hands, due to the innumerable personal and professional activities in which they participate under a wide range of loads. For a normal bone, the loads that produce fracture are typically extreme, with the exception of those with severe conditions of osteoporosis or certain pathologies that can produce fractures during the execution of normal and indispensable activities in daily life [3].
The mechanical properties of bone vary over a wide range, which can be affected by the presence of certain pathological processes that alter these properties. When a fracture occurs, then a series of complex biological and biomechanical events begins, directed towards the repair and restoration of its functions [3].
The progress achieved is aimed at understanding, controlling, and reinforcing the biological aspects of fracture healing. The mechanical environment will always be a crucial element in the healing of this injury, with strong interactions with biological factors. The clinical management of fractures must influence these two elements so that the original load-bearing capacity of the bone is restored as quickly as possible; the proportion in which this capacity is restored is closely related to the fracture stabilization method [3].
If a fracture is not treated with the proper technique, it can cause disability in the patient; an example of this is the fractures in the metacarpal bones at the level of the diaphysis. At present, the absence of suitable devices on the market for the treatment of this type of injury results in many patients losing limb mobility during and after fixation with existing mechanisms such as Kirschner wires that require immobilization with a plaster cast, which causes stiffness in the muscles which in turn prolongs recovery with excessively high treatments. This problem is of great importance because it is an indication that injured people may be totally or temporarily affected by their productive capacity, with the social consequences that this implies.
Biomechanics is an area of bioengineering created for the development and improvement of equipment and tools whose application reduces the difficulty and execution time of any treatment in patients with various injuries, and especially those related to bone pathologies.
The idea of creating a versatile instrument in Venezuela, whose application allows the management of a blocked intramedullary nail at the time of its placement for treatment of metacarpal bone fractures, is framed within the evolution of the DIGITUS PROJECT currently underway at the Bioengineering Center. Of the UCV. The main objective of this work is to design an intramedullary locking system for the treatment of some hand fractures; then, it is necessary to design a tool that is simple, inexpensive, and with sufficient strength for the insertion of the nail into the bone. On the other hand, the instrument must guide the surgeon to perform the locking with the fixation screws that will stabilize the implant, allowing the formation and consolidation of the bone callus.
In summary, it seeks to give continuity to the design of the first models made in the country of an intramedullary system; that is, to the device-instrumental set necessary in the operating room to reduce diaphyseal fractures of the first metacarpal bone, so that the surgeon can perform percutaneous surgery that is fast, effective, and as less invasive as possible that allows early mobility of the hand.
2. MATERIALS, MANUFACTURING PROCESSES, AND HEAT TREATMENT
Once the design of the prototype of the insertion instruments has been established, it is necessary to make known the processes involved for its manufacture. In the first place, the materials with which the parts that make up the tool were made were chosen through the management of criteria such as biocompatibility, mechanical, metallurgical, and physicochemical properties required and availability in the market. Then the processes to produce the parts of the final model were selected based on the materials and the technological and economic possibilities.
3. Selection of materials
The design of a piece that will fulfill a certain function necessarily includes the correct choice of the material with which it will be built. This selection is conditioned by the existing balance between the demands of the function, the properties offered by the available materials, and the manufacturing process [10].
As already mentioned, the parts of the tool need to be made with materials that respond adequately to the demands of use; then, it is necessary to start by establishing a reference standard for the choice of these materials. That pattern is the nail or fixation device itself, which was manufactured with 316L steel due to its known resistance to corrosion within the human body, good mechanical properties since it has been used successfully in the manufacture of other biomedical devices; for this reason, the parts that come into direct contact with It must be built with a material that has similar characteristics to ensure that when stress concentrations occur in the contact zones, the range of elastic behavior is not exceeded. In addition, the need to maintain a stable coupling with the assembly tool during the multiple uses to which it will be subjected is also taken into account.
AISI 420 steel
Normally, it is always difficult to proceed to choose the correct stainless steel correctly, since not only its resistance to corrosion must be taken into account, but also its mechanical properties, such as hardness and admissible load, resistance to wear and fatigue, and also the possibilities of improving these properties by heat treatment [10]. In this sense, AISI 420 (SAE 51420) steel was chosen for the manufacture of a large part of the accessories that make up the instruments, as it is hard enable martensitic chromium steel with heat treatment and offers good toughness and high resistance to corrosion. Corrosion and oxidation; this last property is essential to ensure the biocompatibility of these accessories with human tissues and fluids.
This steel is generally applied in parts that require high mechanical requirements together with resistance to corrosion: turbine blades, shafts, valve seats, general surgical instruments, cutlery, parts of the oil industry, etc.
The parts that deserved to be built in AISI 420 steel are the connector between the nail and the rest of the tool, the shaft with the threaded tip of the insertion handle, and the locking guide cylinders since they are the ones that enter into direct contact with the body at any given time.
Aluminum
A key piece within the concept of stability of the tool is the directional handle, which does not need to be manufactured in steel since, during its function, there are no notable stresses that compromise the stability of the said piece. Therefore, aluminum is presented as an alternative for the elaboration of this solid, since in general, its composition offers sufficient mechanical properties for this accessory to fulfill the function expected of it.
However, the behavior of pure aluminum has general virtues and defects. Among the former, it is worth highlighting the low density, the good electrical and thermal conductivities, the high
Resistance to corrosion, the low melting point and the relatively low temperatures of plastic deformation, the good surface appearance, the possibility of anodizing, etc. Regarding its characteristic deficiencies, it is worth mentioning poor mechanical properties, poor castability in the casting process, poor machinability, etc. [10]
To overcome these deficiencies of the pure metal, alloys have been developed with highly surprising results and that currently, depending on the technological process for which the alloys are intended, are classified into two families:
- Alloys for plastic deformation processes.
- Foundry alloys.
The basic characteristics that are needed in an alloy for the final construction of the directional handle are corrosion resistance, good machinability, and reasonable mechanical properties.
Since a prototype for In Vitro Testing of this accessory was built, it was not necessary to choose a particular alloy, and it was only a sufficient condition that it offered good machinability and that it could be easily purchased. In this sense, the aluminum alloy for casting A356 was used. However, if machining in the Numerical Control Center is viable for the production of the handle, the 6063 T5 alloys are also presented as an excellent alternative, since within its qualities are those mentioned above since it has already been used with success in the construction of parts used in the trauma area.
Polypropylene
This is the material from which the grips that will be used in the insertion instruments are made once their geometry proves to be effective during testing. These grips are distributed by REID Tool Supply Company [24].
3.2 Selection of manufacturing processes
Parts to be manufactured
Nail-Tool Connector, Directional Handle, Threaded End Insertion Handle, and Locking Guide Cylinders.
Machining
Most of the parts that make up the instruments for handling the intramedullary nail have geometries generated by solids of revolution that are made from solid cylinders of the aforementioned materials machined on the lathe. In addition, the design of said pieces also involves notches in whose manufacture it is necessary to have a milling machine to obtain the desired precision and shape; on the other hand, the milling process is also essential for the production of the directional handle given its geometry.
4. CONCLUSIONS AND RECOMMENDATIONS
Currently, the UCV Bioengineering Center develops intense activity in the area of biomechanics, with many work proposals aimed at the execution of projects in order to design and manufacture low-cost devices for the use of medicine. Venezuelan, capable of competing in quality with similar devices created in the international market.
The work reported here has made it possible to develop a methodological basis for the design and manufacture of surgical instruments aimed at the management and placement of intramedullary devices for the treatment of fractures in short bones of the human body, such as metatarsals in the foot and metacarpals at the level of the bones of the hand, where the suffering of this type of bone injury can lead to multiple problems of labor, economic, social and even psychological nature in the patient.
On the other hand, the achievements and setbacks found throughout this work showed that the subject in question presents a panorama that admits multiple engineering solutions to the problem posed, the geometries of the tool can be adapted depending on the requirements, criteria of the specialist, size of the series of production and means of manufacture available, among others.
4.1 Main conclusions
Regarding the tool
- A novel prototype of an instrument has been designed and built that has proven to be effective for the placement and guidance of the blockade of an intramedullary system aimed at the treatment of diaphyseal fractures occurring in the metacarpal bone of the thumb. The design validation of this device was carried out through “In Vitro” tests, the results of which confirmed that the geometry proposed for the parts that make up the tool fulfill the function for which they were designed.
- The prototype proposed together with the intramedullary nail allows a quick, simple, and minimally invasive surgical technique to be performed.
- The design and ergonomic characteristics of the prototype were evaluated and compared with other existing systems on the market for the placement of intramedullary nails in general, finding that simplicity, firm hold of the parts, and low weight are the qualities that distinguish the model proposed in this work.
The insertion instruments offer a safe and effective means to fulfill the main objective of the intramedullary system: to act as a rigid link, stabilizer of the fracture fragments for the formation and consolidation of the bone callus.