Comparative Evaluation of In Vitro Mechanical Properties of Different Designs of Epoxy‐Pin External Skeletal Fixation Systems Surbhi Kuldeep Tyagi, MVSc, PhD, Hari Prasad Aithal, MVSc, PhD, Prakash Kinjavdekar, MVSc, PhD, Amarpal, MVSc, PhD, Abhijit Motiram Pawde, MVSc, PhD, Tuhin Srivastava, BTech, Kanti Prakash Tyagi, PhD, and Shongsir Warson Monsang, MVSc, PhD Division of Surgery, Indian Veterinary Research Institute, Bareilly, Uttar Pradesh, India
Corresponding Author Dr Hari Prasad Aithal, MVSc, PhD, Division of Surgery, Indian Veterinary Research Institute, Izatnagar, Uttar Pradesh 243 122, India. E‐mail: [email protected], [email protected]. Submitted January 2012 Accepted December 2013 DOI:10.1111/j.1532-950X.2013.12128.x
Objectives: To compare in vitro biomechanical properties of different designs of epoxy‐pin external skeletal fixator (ESF) constructs. Study Design: Mechanical testing study. Sample Population: Four epoxy‐pin ESF design constructs (uniplanar [EU], multiplanar‐I [EM‐I], multiplanar‐II [EM‐II], and circular [EC]) were mechanically tested in compression, bending, and torsion. Methods: Four different designs of free‐form epoxy‐pin external fixator constructs were developed using 1.5 mm K‐wires and epoxy resin mounted in an ultra‐high density polyethylene rod (20 mm diameter). Three‐point fixation was done in each fragment, and the distance between fixation wires, and between the rod and the side bars was kept constant in all the designs. A 5 mm gap was maintained at the center of the fixation rod to simulate an unstable fracture condition. The fixator constructs (n ¼ 12 of each design) were subjected to mechanical testing in axial compression, bending, or torsion. Load–deformation curves were generated and mechanical properties were compared between construct types. Results: EU was the weakest design. Under compression, constructs EM‐I, EM‐II, and EC were similar. Under bending, EM‐I and EM‐II had similar strength, whereas EC was strongest. Under torsion, EC was strongest, followed by EM‐II, EM‐I, and EU; EM‐II provided double the rotational stability of EM‐I. Conclusions: Overall, EC followed by EM‐II epoxy‐pin fixator designs had better mechanical strength.
External skeletal fixators (ESF) are typically made from metal, most commonly stainless steel. Other materials used for fabrication of ESF are carbon fiber, aluminum, and polymers. Aluminum and carbon are lighter in weight and relatively radiolucent compared to stainless steel; however, carbon is costly and aluminum must be thicker than stainless steel to achieve comparable rigidity.1,2 Replacement of the external metallic component with nonmetallic polymer material like polymethylmethacrylate (PMMA) or epoxy putty offers advantages like less weight, less expensive, pin direction need not be influenced by the direction and location of the connecting bar/ring and pin diameter is not limited by clamp size.3,4 In clinical practice, fractures typically result from a combination of forces/loads, like compression, bending, and torsion, and hence fixation devices should be strong enough to resist these forces.5 Acrylic ESF when compared mechanically with stainless steel ESF were stronger in axial compression, craniocaudal bending, and torsion and as strong in mediolateral
bending.6 When compared with methacrylates, epoxy putties had similar strength and greater apparent modulus.7 Epoxy is a polymer available in resin form, and when mixed with a hardener it solidifies to form a very strong material. Nonmedical grade epoxy is readily available as a sealant. Because of its mechanical properties it can be used as a component of ESF system just like acrylics. Epoxy adhesive and cast materials have been used to construct free‐form external fixators in birds.7–9 Epoxy putty is easy to handle, inexpensive, and has suitable setting times and mechanical properties.7 Recently epoxy‐pin fixators have been used for treatment of open long bone fractures and joint luxations in dogs, small ruminants, calves, and foals with good success.10,11 In our earlier clinical studies,11,12 multiplanar epoxy‐pin ESF systems developed using 1.2–1.5 mm fixation pins and 15– 20 mm diameter side bars provided stable fixation of fractures below the stifle or elbow joint in dogs weighing up to 40 kg. In calves and foals weighing up to 100 kg, 2.0–3.0 mm wires, and 20–25 mm side bars were adequate for stable fixation of
Veterinary Surgery 43 (2014) 355–360 © Copyright 2014 by The American College of Veterinary Surgeons
355
Mechanical Evaluation of Epoxy‐Pin External Skeletal Fixator
fractures at middle or distal third of radius/tibia, metacarpus, and metatarsus. Our purpose was to compare the in vitro biomechanical characteristics of different designs of free‐form ESF systems developed using epoxy putty.
Preparation of ESF Constructs ESF constructs were prepared using ultra high density polyethylene (UHDPE) 20 mm diameter rods (Metalon1‐ Ashoka steels, Chabri Bazaar, Delhi, India). Two segments of 70 mm UHDPE rods each were used, keeping a gap of 5 mm between fragments to simulate an unstable fracture condition. Fixation pins (1.5 mm diameter K‐wires) made of 316‐L stainless steel (Nebula Surgicals Pvt. Ltd., Gujarat, India) were passed through the UHDPE rods at fixed distance. Initial pins were inserted 10 mm from the gap and then space at 20 mm intervals (Fig 1). The distance between pins in orthogonal
356
planes was kept to minimum (1 mm). Pins were passed in the same line, parallel to each other, in uniplanar design. In multiplanar and circular designs, pins were crossed at nearly 90° angle taking care that the pins did not interfere with each other. Uniplanar Design (EU). Pins were in 1 plane and were then bent (at 90°) at a distance of 40 mm from the rod towards the gap, and joined to each other using adhesive tape to make a temporary scaffold (Fig 1A).
MATERIALS AND METHODS
Figure 1
Tyagi et al.
Multiplanar‐I (EM‐I). The crossed pins in the same planes were bent towards the fracture site and joined by adhesive tape (Fig 1B). Multiplanar‐II (EM‐II). The crossed pins in the same planes were bent towards the fracture site and joined using adhesive tape. The 2 side bars on each side were then joined proximally and distally using additional wire pieces, so that 2 rectangular frames were formed on both sides (Fig 1C).
Cranial and dorsal views of epoxy ESF designs. A: Uniplanar (EU); (B) multiplanar‐1 (EM‐I); (C) multiplanar‐II (EM‐II); and (D) circular (EC).
Veterinary Surgery 43 (2014) 355–360 © Copyright 2014 by The American College of Veterinary Surgeons
Tyagi et al.
Circular (EC). After making the side bars as in Multiplanar‐I design, all 4 side bars were joined proximally and distally using additional wire pieces, thereby, 2 rings were constructed 1 each at the proximal and distal end of the side bars (Fig 1D). The epoxy‐resin (M‐Seal1 Phataphat, Pidilite Industries Ltd., Daman, India) was mixed with the hardener for 1– 2 minutes, till a uniform color dough was formed. During standardization, all 3 types of M‐seal preparations available commercially namely regular (red), fast curing (blue), and quick set (white) were tested and fast curing epoxy was selected for construction of fixators. The side bars and rings of the fixator were constructed by molding the epoxy on the pins by incorporating the bent pins within the mold. The epoxy dough was then squeezed around the pins for good contact with the pin surface. All the fixator components were assembled together to form a temporary scaffold and then epoxy applied at once for construction of bars and rings. The diameter of the epoxy column was kept uniform at 20 mm throughout. The epoxy was allowed to harden. The internal temperature (°C) of the epoxy dough was measured during hardening. Readings were taken at 15 seconds intervals during the first minute, then at 30 seconds intervals up to 5 minutes, and subsequently at 1 minute intervals up to 1 hour. Mean temperature at different intervals and the maximum temperature attained were recorded. Side bars were constructed at uniform distance of 20 mm from the central UHDPE rod. Total length and diameter of the fixators were 145 mm (including 5 mm gap) and 100 mm, respectively, including the connecting bars. Total weight of the different designs of the ESF and technical easiness and difficulties encountered during the construct preparation were noted.
Mechanical Evaluation of Epoxy‐Pin External Skeletal Fixator
applied at 20°/min. Torque and degrees required for construct failure, where plastic deformation started, were recorded. Shear stress, shear strain, and stiffness were calculated.13 Statistical Analysis Data obtained during compression (stress, strain, stiffness, and modulus of elasticity), bending (bending moment, stiffness, and modulus), and torsion (shear stress, shear modulus, and stiffness) testing were analyzed using ANOVA, and mean differences were tested for statistical significance by Duncan’s multiple range test (DMRT)14 using software (Statistical Package for Social Sciences version 15.0). Significance was recorded at P
Corresponding Author Dr Hari Prasad Aithal, MVSc, PhD, Division of Surgery, Indian Veterinary Research Institute, Izatnagar, Uttar Pradesh 243 122, India. E‐mail: [email protected], [email protected]. Submitted January 2012 Accepted December 2013 DOI:10.1111/j.1532-950X.2013.12128.x
Objectives: To compare in vitro biomechanical properties of different designs of epoxy‐pin external skeletal fixator (ESF) constructs. Study Design: Mechanical testing study. Sample Population: Four epoxy‐pin ESF design constructs (uniplanar [EU], multiplanar‐I [EM‐I], multiplanar‐II [EM‐II], and circular [EC]) were mechanically tested in compression, bending, and torsion. Methods: Four different designs of free‐form epoxy‐pin external fixator constructs were developed using 1.5 mm K‐wires and epoxy resin mounted in an ultra‐high density polyethylene rod (20 mm diameter). Three‐point fixation was done in each fragment, and the distance between fixation wires, and between the rod and the side bars was kept constant in all the designs. A 5 mm gap was maintained at the center of the fixation rod to simulate an unstable fracture condition. The fixator constructs (n ¼ 12 of each design) were subjected to mechanical testing in axial compression, bending, or torsion. Load–deformation curves were generated and mechanical properties were compared between construct types. Results: EU was the weakest design. Under compression, constructs EM‐I, EM‐II, and EC were similar. Under bending, EM‐I and EM‐II had similar strength, whereas EC was strongest. Under torsion, EC was strongest, followed by EM‐II, EM‐I, and EU; EM‐II provided double the rotational stability of EM‐I. Conclusions: Overall, EC followed by EM‐II epoxy‐pin fixator designs had better mechanical strength.
External skeletal fixators (ESF) are typically made from metal, most commonly stainless steel. Other materials used for fabrication of ESF are carbon fiber, aluminum, and polymers. Aluminum and carbon are lighter in weight and relatively radiolucent compared to stainless steel; however, carbon is costly and aluminum must be thicker than stainless steel to achieve comparable rigidity.1,2 Replacement of the external metallic component with nonmetallic polymer material like polymethylmethacrylate (PMMA) or epoxy putty offers advantages like less weight, less expensive, pin direction need not be influenced by the direction and location of the connecting bar/ring and pin diameter is not limited by clamp size.3,4 In clinical practice, fractures typically result from a combination of forces/loads, like compression, bending, and torsion, and hence fixation devices should be strong enough to resist these forces.5 Acrylic ESF when compared mechanically with stainless steel ESF were stronger in axial compression, craniocaudal bending, and torsion and as strong in mediolateral
bending.6 When compared with methacrylates, epoxy putties had similar strength and greater apparent modulus.7 Epoxy is a polymer available in resin form, and when mixed with a hardener it solidifies to form a very strong material. Nonmedical grade epoxy is readily available as a sealant. Because of its mechanical properties it can be used as a component of ESF system just like acrylics. Epoxy adhesive and cast materials have been used to construct free‐form external fixators in birds.7–9 Epoxy putty is easy to handle, inexpensive, and has suitable setting times and mechanical properties.7 Recently epoxy‐pin fixators have been used for treatment of open long bone fractures and joint luxations in dogs, small ruminants, calves, and foals with good success.10,11 In our earlier clinical studies,11,12 multiplanar epoxy‐pin ESF systems developed using 1.2–1.5 mm fixation pins and 15– 20 mm diameter side bars provided stable fixation of fractures below the stifle or elbow joint in dogs weighing up to 40 kg. In calves and foals weighing up to 100 kg, 2.0–3.0 mm wires, and 20–25 mm side bars were adequate for stable fixation of
Veterinary Surgery 43 (2014) 355–360 © Copyright 2014 by The American College of Veterinary Surgeons
355
Mechanical Evaluation of Epoxy‐Pin External Skeletal Fixator
fractures at middle or distal third of radius/tibia, metacarpus, and metatarsus. Our purpose was to compare the in vitro biomechanical characteristics of different designs of free‐form ESF systems developed using epoxy putty.
Preparation of ESF Constructs ESF constructs were prepared using ultra high density polyethylene (UHDPE) 20 mm diameter rods (Metalon1‐ Ashoka steels, Chabri Bazaar, Delhi, India). Two segments of 70 mm UHDPE rods each were used, keeping a gap of 5 mm between fragments to simulate an unstable fracture condition. Fixation pins (1.5 mm diameter K‐wires) made of 316‐L stainless steel (Nebula Surgicals Pvt. Ltd., Gujarat, India) were passed through the UHDPE rods at fixed distance. Initial pins were inserted 10 mm from the gap and then space at 20 mm intervals (Fig 1). The distance between pins in orthogonal
356
planes was kept to minimum (1 mm). Pins were passed in the same line, parallel to each other, in uniplanar design. In multiplanar and circular designs, pins were crossed at nearly 90° angle taking care that the pins did not interfere with each other. Uniplanar Design (EU). Pins were in 1 plane and were then bent (at 90°) at a distance of 40 mm from the rod towards the gap, and joined to each other using adhesive tape to make a temporary scaffold (Fig 1A).
MATERIALS AND METHODS
Figure 1
Tyagi et al.
Multiplanar‐I (EM‐I). The crossed pins in the same planes were bent towards the fracture site and joined by adhesive tape (Fig 1B). Multiplanar‐II (EM‐II). The crossed pins in the same planes were bent towards the fracture site and joined using adhesive tape. The 2 side bars on each side were then joined proximally and distally using additional wire pieces, so that 2 rectangular frames were formed on both sides (Fig 1C).
Cranial and dorsal views of epoxy ESF designs. A: Uniplanar (EU); (B) multiplanar‐1 (EM‐I); (C) multiplanar‐II (EM‐II); and (D) circular (EC).
Veterinary Surgery 43 (2014) 355–360 © Copyright 2014 by The American College of Veterinary Surgeons
Tyagi et al.
Circular (EC). After making the side bars as in Multiplanar‐I design, all 4 side bars were joined proximally and distally using additional wire pieces, thereby, 2 rings were constructed 1 each at the proximal and distal end of the side bars (Fig 1D). The epoxy‐resin (M‐Seal1 Phataphat, Pidilite Industries Ltd., Daman, India) was mixed with the hardener for 1– 2 minutes, till a uniform color dough was formed. During standardization, all 3 types of M‐seal preparations available commercially namely regular (red), fast curing (blue), and quick set (white) were tested and fast curing epoxy was selected for construction of fixators. The side bars and rings of the fixator were constructed by molding the epoxy on the pins by incorporating the bent pins within the mold. The epoxy dough was then squeezed around the pins for good contact with the pin surface. All the fixator components were assembled together to form a temporary scaffold and then epoxy applied at once for construction of bars and rings. The diameter of the epoxy column was kept uniform at 20 mm throughout. The epoxy was allowed to harden. The internal temperature (°C) of the epoxy dough was measured during hardening. Readings were taken at 15 seconds intervals during the first minute, then at 30 seconds intervals up to 5 minutes, and subsequently at 1 minute intervals up to 1 hour. Mean temperature at different intervals and the maximum temperature attained were recorded. Side bars were constructed at uniform distance of 20 mm from the central UHDPE rod. Total length and diameter of the fixators were 145 mm (including 5 mm gap) and 100 mm, respectively, including the connecting bars. Total weight of the different designs of the ESF and technical easiness and difficulties encountered during the construct preparation were noted.
Mechanical Evaluation of Epoxy‐Pin External Skeletal Fixator
applied at 20°/min. Torque and degrees required for construct failure, where plastic deformation started, were recorded. Shear stress, shear strain, and stiffness were calculated.13 Statistical Analysis Data obtained during compression (stress, strain, stiffness, and modulus of elasticity), bending (bending moment, stiffness, and modulus), and torsion (shear stress, shear modulus, and stiffness) testing were analyzed using ANOVA, and mean differences were tested for statistical significance by Duncan’s multiple range test (DMRT)14 using software (Statistical Package for Social Sciences version 15.0). Significance was recorded at P
