Improvement of Biodegradable Polymers Properties in Terms of Strength and Handling for the Use in Facial Bone Fractures

Abstract

Restoration of form and function of bone after trauma require adequate fixation by means of plating system. While titanium plates are considered the gold standard biodegradable plating system had gained popularity recently due to its reduction of infections, eliminate the need for second surgery to remove it and lack of interference with growth however its not without complication which include lack of strength difficult manipulation intraoperatively and causing sterile abscess, in this study the author will attempt The authors will attempt to measure the strength and stiffness of both biodegradable and titanium plating systems and assess the possibility of increasing the bulk of the polymers. Another objective is to review the possibility of using other alternative means for fixation of bone by exploring different types of polymers. This will be done by measuring the tensile strength, side bending and torsion of multiple polymers and titanium plating systems using test machine (Zwick/Roell TC-FR2, 5TS.D09, 2.5kN)

Introduction

The surgical treatment of craniomaxillofacial trauma involves the restoration of both form and function via a complex interplay between the facial bony skeleton and its soft tissue envelope. However, it was not until the introduction of open reduction and internal rigid fixation techniques for the facial skeleton that the basic orthopedic principles of accurate fracture reduction, bone fixation, and healing could be applied. The latter introduced the unprecedented ability to repair unstable and/or displaced bony fractures of the face, providing a stable foundation upon which to reestablish preinjury soft tissue contour1. Advances in the science of internal fixation, improvements in available plating materials and equipment, refinements in exposures to the facial skeleton, and an increase in the volume of facial trauma all fueled the rapid expansion of use of rigid internal fixation for facial fractures in the 1980s2. Metallic materials have and still being used for the fixation and stabilization of bony fractures since 1858 by hansmann3.

These materials remained ineffective until the introduction of alloys in the 20th century that was resistant to corrosion and improvement of the understanding of infections and aseptic technique eg include alloys of chromium, nickel, and molybdenum, or “stainless steel,” and later in 1936 Vitallium (an alloy of cobalt, chromium, and molybdenum)4. In the 1960s titanium plates started to be used and remains to be the best choice for use in facial fractures. Multiple complications is associated with these type of fixation materials including infection, the need for second surgery to remove it and its interference with growth in young patients which made scientists to the search of a better choice that can avoid this complications and improve outcomes of such conditions and that led them towards biodegradable materials which in theory will reduce infection, eliminate the need for removal and because its biodegradable it wouldn’t interfere with growth and it would serve its job by being there only as necessary for bone healing. The development of resorbable plates and screws for rigid internal fixation was a natural extension from biodegradable suture materials already available for many years to assist wound closure. The materials currently used to manufacture resorbable plating systems in common use are polymers of high-molecular-weight ?-hydroxy acids including polyglycolic acid (PGA) and polylactic acid (PLA).

Both materials are initially degraded by hydrolysis into lactic acid, which is then subsequently metabolized by the liver and excreted as carbon dioxide and water6. However, the rate at which they degrade differs. PGA degradation is rapid and although initially stiff, loses its mechanical strength by 6 weeks and is completely resorbed within a few months. Pure PGA hardware is no longer used because its brisk structural degradation provides insufficient structural support for bone healing and is also associated with local inflammatory reaction, osteolysis, and sterile abscesses. Alternatively, PLA is degraded at a rate far slower than that of PGA, often requiring several years and, thus, increasing the risk of foreign body–type reactions.

PLA’s stereoisomers, D- and L-lactide, however, have differing degradation characteristics allowing varying combinations of these isomers to confer shorter degradation rates. In a similar manner, mixtures of PGA and PLA, with increasing amounts of the former accelerating the rate of degradation, have been formulated to achieve the same effect9. With the presence of multiple types of companies supplying polyglycolic acid (PGA) and polylactic acid (PLA) different resorption rates is available(table 1) but generally it retains structure for up to 3 months and are resorbed by 1 to 2 years however despite that some studies showed materials persistence and formation of sterile abscess 9. Other complications include lack of strength available in metallic materials and the need to soften the polymers in water bath for at 55c for 15 sec to be able to adapt it to the fracture10. (figure 2 ) showing shapes of different types of plates and screws.

Objectives/significance The aim of this study proposal in general is to improve biomechanics by attempting to create a product that has the same strength of the titanium plating system and improve physical properties in terms of handling which would reduce surgical time and instruments needed with current available biodegradable systems available on today’s market. achieving a more desired product that overcome the disadvantages of both metallic and biodegradable materials which would have a positive impact on patients care and reduce medical cost which can only be done investigating different polymers and copolymers to gain better knowledge and understanding to solve these problems. The authors will attempt to measure the strength and stiffness of both biodegradable and titanium plating systems and assess the possibility of increasing the bulk of the polymers. Another objective is to review the possibility of using other alternative means for fixation of bone by exploring different types of polymers. Materials and methods to identify available biodegradable and titanium plating systems and its screws in market and record its general characteristics. The non-sterile titanium plates and screws will be sterilized.

We will use 4-hole extended plates for these tests. The plates and screws were fixed to 2 polymethylmethacrylate (PMMA) blocks that simulated bone segments. Two screws will be inserted in both PMMA blocks according to the prescriptions of the individual manufacturer (with prescribed burs and taps). The applied torque for inserting the screws will be measured to check whether it was comparable to the clinically applied torque.

The holes should be irrigated with water before insertion of the screws, to simulate the in-situ lubrication. The two PMMA blocks, linked by the plates device (1 plate and 4 screws) will be stored in a water tank containing water of 37.2 degrees Celsius for 24 hours to simulate the relaxation of biodegradable screws at body temperature. The plates and screws will be subjected to tensile, side bending, and torsion tests. The tensile test is performed as a standard loading test (figure 1).

Side bending tests will also be performed (figure 2). Torsion tests will be performed to subject the plating devices to high torque in order to simulate the most unfavourable situation (figure 3). The 2 PMMA blocks, linked by the plates, will mounted in a test machine (Zwick/Roell TC-FR2, 5TS. D09, 2.5kN Test machine. Force accuracy 0.2%, positioning accuracy 0.0001mm; Zwick/ Roell Nederland, Venlo, The Netherlands).

Regarding the tensile tests, the 2 PMMA blocks and thus the plate is subjected to a tensile force with a constant speed of 5 mm/min until fracture occurred (according to the standard ASTM D638M). For the side bending test the 2 PMMA blocks must be supported at their ends whereas the plates are loaded in the centre of the construction with a constant speed of 30 mm/min (with this speed the outer fibres are loaded as fast as the fibres of the plating system in the tensile test) until the plate was bended 30 degrees. For the torsion test the 2 PMMA blocks are twisted along the long axis of the plating system with a constant speed of 90 degrees/min (with this speed the outer fibres are loaded as fast as the fibres of the plating system in the tensile test) until the plate is turned 160 degrees. During testing the applied force is recorded by the load cell of the test machine. Both force and displacement were measured with a sample frequency of 500 hertz and graphically presented in force-displacement diagrams. During tensile tests, the strength of the plating system is monitored.

The stiffness is calculated for the tensile, side bending and torsion tests by linking the 25% and 75% points (to exclude inaccuracies of the start and end of the curves) of the maximum force on the force-displacement curves and determining the direction-coefficients of the curves11. Then we are going to assume a new sample that may be differs from the exist biodegradable polymers and have a different mechanical properties that overcome the disadvantages that we mentioned above.by making a mixture between different types of biodegradable polymers such as PLA (to add stiffness to the sample) and PGA (one of the best biodegradable polymers in mechanical properties)according to the results we will get from the tests done before and also using PHPV to add strength to the sample that will make the resultant have the desired mechanical properties.

References

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