ILIZAROV APPARATUS

DEVELOPMENT, CURRENT DESIGN

AND RECOMMENDATIONS

FOR IMPROVEMENT

M7 TEAMNicholas Chavez

Madison DryeRachel Fenberg

Reid LarsenPriscilla Mukudan

Submitted 17 December 2015for

Engineering 100.100.104

Dr. George T. WynarksyDr. Elizabeth S. Hildinger

Course Faculty

M7 Team 1

TABLE OF CONTENTS

Foreword…………………………………………………………………………………………….……..2

Summary………………………………………………………………..…………......…………...………2

Introduction…………………………………………………………….…………...………..……...……3

Anatomy of Physiology of Long Bones………………………………...………...…...………...3

Medical Problem Associated with the Ilizarov Apparatus…………....…………..........4

History and Development of the Ilizarov Apparatus………………....……...…………...5

Structure of the Ilizarov Apparatus……………………...…………....…………………………5

Function of the Ilizarov Apparatus………………………………...…………...………………..6

Materials Used in the Ilizarov Apparatus……………………...…………………………….. 7

Alternative Systems and Techniques………………………………….......……………………8

Limitations of the Ilizarov Apparatus……………………......…………………………………9

Research and Development of Limb Lengthening Devices…………………………….9

Design Recommendation………………………………………..…………………………………11

References…………...…………...…………...…………...…………...…………...…………...……...13

Appendix 1…………...…………...……………..…………...…………...…...…………...…………...17

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Foreword We were asked to investigate an orthopedic implant or medical device. We researched and gained expertise on the Ilizarov apparatus as a limb lengthening device. The purpose of this formal report is to give insight into the Ilizarov apparatus, its competing systems, and a design recommendation to address some of the limitations of the Ilizarov apparatus.

SummaryThe Ilizarov apparatus is a device that fixes limb length discrepancies (LLDs). LLDs are differences in the length of a limb. Long bones, like the tibia, are made of hard bone and are characterized by their tubular structure. After a fracture, these bones heal in a multi-step process. First, swelling occurs. Then, after the swelling goes down, a soft callus forms. Next, a hard callus forms on the soft callus, which connects the two bone segments into one bone. Lastly, the bone is remodeled and reshaped by bone cells (Kalfas).

LLDs can be caused by congenital defects, developmental problems, or fractures. Patients with LLDs may require treatment from the Ilizarov apparatus. External fixation as a solution for LLDs has been around for several centuries. External fixation is a treatment used to lengthen bones with a device that is outside of the body. External fixation has been developing quickly since it gained popularity in the 19th century.

The Ilizarov apparatus is an external fixation device that is made up of rings, Kirschner wires, adjustable telescopic rods, and wire fixation bolts. After the device is attached to the leg, a surgical cutting of the bone is performed. This procedure separates the bone segments. Distraction osteogenesis is the formation of new bone as the two bone segments are separated. During distraction osteogenesis, the device slowly separates the two bone segments at a rate of a quarter millimeter every six hours. The separation stimulates bone growth because the fracture site is under tension. This process is repeated continuously until the desired length is achieved. Once the bone has finished healing, the device is removed.

Stainless steel is typically used for all components of the Ilizarov apparatus. Carbon fiber can be used as a substitute material for the rings. Both materials have satisfactory material properties for the function of the Ilizarov apparatus.

An alternative system to the Ilizarov apparatus is the Taylor Spatial Frame. The Taylor Spatial Frame also uses the principle of distraction osteogenesis to lengthen the bone. However, the telescopic rods on the Taylor Spatial Frame are not perpendicular to the rings like in the Ilizarov apparatus. The Taylor Spatial Frame also uses half-pins instead of Kirschner wires to connect the bone to the rings.

There are limitations to the Ilizarov apparatus. From an engineering perspective, the Kirschner wires can plastically deform, leading to improper distraction. From a medical perspective, bacteria on the Kirschner wires can cause infections. The infection can loosen the Kirschner wires, resulting in device failure. Additionally, the patient also suffers from severe pain throughout the bone lengthening process. The Ilizarov apparatus is costly due to weekly X-rays needed to monitor the condition of the fracture site.

Current research and development of the Ilizarov apparatus has focused on adding a coating to the Kirschner wires, adding additional Kirschner wires, adding a monitoring component called Fracture Track, and developing an alternative system called the PRECICE intramedullary limb lengthening system. Adding a Kirschner wire coating would decrease the risk of infection. The addition of

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Kirschner wires would minimize plastic deformation. Fracture Track is a device that monitors the fracture site without the use of X-rays. The PRECICE system is entirely internal and distracts the bones with an external remote control rather than screws on the telescopic rods.

To address some of the limitations of the Ilizarov apparatus we recommend replacing the Kirschner wires with half-pins, adding Rancho cubes to the ring/half-pin junction, coating the half-pins with nanosilver, changing the half-pin material to titanium, and increasing the half-pin diameter. We hope that these recommendations decrease the rate of plastic deformation and minimize the risk of infection.

IntroductionA limb length discrepancy (LLD) is a difference in limb length. For our research we focused specifically on differences in tibial length. LLDs are significant because they affect 1 in 1000 individuals (Paley). Patients with LLDs suffer from pain that is caused by an abnormal gait. The change in gait causes an increased occurrence of stress fractures, risk of osteoarthritis in the hip of the long leg, and high incidence of knee and back pain. Currently, the Ilizarov apparatus, which lengthens the shorter leg, is the most common treatment for LLDs (Clarke).

Anatomy and Physiology of Long BonesLong bones are a type of bone characterized by their tubular structure and long length. The epiphyseal plate, or growth plate, seen in Figure 1, is made of cartilage and is the site at which bone growth begins (Petre).

The diaphysis is the long hollow portion in the middle of the bone. The diaphysis is mostly constructed of cortical, or dense, bone. This arrangement provides the bone with most of its strength without adding much weight to the bone.

The medullary cavity is the area of the diaphysis where there is no cortical bone. Blood vessels are housed inside the medullary cavity. Blood vessels carry vital nutrients, such as oxygen, calcium, and phosphate, to bone cells while removing waste products, such as carbon dioxide and dead bone tissue, away from the bone (Petre).

Bone cells are stored within the cortical bone as seen in Figure 2. These cells include osteoblasts and osteoclasts. Osteoblasts are cells that build bone, while osteoclasts are cells that destroy bone (Clarke). The non-cellular structure of bone is known as the extracellular matrix. The extracellular matrix is a collection of molecules between cells that provide strength to the bone (Petre). The Healing Process of Long BonesWhen a bone experiences trauma, the healing process occurs. This process happens in multiple stages, as shown in Figure 3.

Figure 2: Structure of a Long Bone (http://www.intechopen.com)

Figure 1: Anatomy of a Long Bone (http://droualb.faculty.mjc.edu)

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The first stage of the healing process, called latency, is a period during which no bone growth occurs and a haematoma is formed. A haematoma is swelling caused by a collection of blood outside of the blood vessels. This stage typically lasts five to ten days (Kalfas).

Next, during osteogenesis, osteoblasts secrete proteins, such as collagen, into the fracture site. This process creates a cartilaginous, or soft callus, which is a precursor to bone. A soft callus is made of cartilage and other compounds that form on the fracture site. This stage lasts about four to six weeks (Kalfas).

Once osteogenesis is complete, consolidation occurs. During this stage, calcium, phosphate, and other compounds are deposited onto the soft callus. This changes the soft callus into a hard callus, which is another precursor to bone. The hard callus connects the two bone fragments created from the fracture into one bone. This stage lasts about one to three months (Kalfas).

Finally, the remodeling stage occurs. During remodeling, the bone is reshaped by osteoblasts and osteoclasts. Where it is necessary, osteoblasts build new bone and osteoclasts destroy bone. During this stage, remodeling restores the bone to its original shape, strength, and structure. This stage typically lasts about five to eight months (Kalfas).

Medical Problem Associated with the Ilizarov ApparatusPatients typically require limb lengthening when they have a significant difference in leg length. Typically, differences in limb length of 3.5-4% must be treated with limb lengthening devices such as the Ilizarov apparatus. Patients who do not experience painful symptoms and deformities from discrepancies in limb length do not require treatment; however, patients with a deformity that is so severe that it affects their gait, require treatment (Limb Length Discrepancy-Ortho Info, 2007).

LLDs can occur because of congenital problems, developmental problems from a childhood disease, or fractures (Camak). Congenital LLDs are present from birth and worsen over time as the patient grows. Typically, the tibia or femur are the affected bones (Limb Length Discrepancy (LLD)). LLDs can also be caused by developmental problems. These developmental problems originate from illnesses in childhood. In the past, polio accounted for around one-third of LLDs. Polio can cause extreme muscle weakness and deterioration, which leads to LLDs. However, since the amount of people receiving polio vaccines has greatly increased, polio has become less of a cause of LLDs. (Common Disorders). Other developmental problems that cause LLDs include tumors from childhood cancer, Legg-Calve-Perthes disease, and cancer that requires radiation (Limb Length Discrepancy (LLD)). If a tumor requires bone removal in early childhood, that child can develop a LLD. Legg-Calve-Perthes disease is a condition where blood supply is interrupted to the femoral head of the hip joint. This disease causes fractures that lead to shortened limbs (Legg-Calve-Perthes disease).

Figure 3: Stages of bone healing (www.tube.medchrome.com)

Figure 4: Difference in bone length due limb length discrepancy (www.erikdalton.com)

Normal Latency Osetogenesis Consolidation Remodeling

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Fractures can also result in LLDs. If a fracture occurs in the growth plate of a bone, that bone will not develop as it normally should. Additionally, severe fractures in the leg may require parts of the bone to be completely removed. A significant amount of bone removal results in a LLD.

Patients with LLDs have an abnormal gait. Abnormal gaits can cause various problems throughout the body. Severe sciatica can occur as a result of LLDs (Keys To Recognizing And Treating Limb Length Discrepancy). Sciatica is a constant nerve pain that has been described as burning or tingling (What You Need to Know About Sciatica). Other problems that can result from LLDs include spinal problems, muscular weakness, and restriction in range of motion of various joints. The longer limb can be subjected to osteoarthritis of the hip joint and is at an increased risk of stress fracture. In the shorter limb, there can be severe knee pain and degenerative joint disease (Keys To Recognizing And Treating Limb Length Discrepancy). History and Development of the Ilizarov ApparatusHippocrates introduced external fixation in ancient Greece around 400 B.C. He described the use of traction, or pulling, to manipulate the length of the shortened bone (Nelson, 2015).

The first external fixator was designed to adjust the patella, or the kneecap, and was developed by the surgeon Joseph Malgaigne in the late 19th century. These minor advancements served as a framework for Alessandro Codivilla’s external skeletal traction, or limb pulling, system. As seen in Figure 5 of Appendix 1, Codivilla’s apparatus was the first of these devices to use a system of constant pulling to result in limb lengthening. Despite Codivilla’s efforts, the device failed (Brand, 2008).

External fixators prior to the invention of the Ilizarov apparatus were accompanied by complications like infection, poor circulation, and malunion. Malunion is an improper healing of the bone. In 1946, the surgeon Gavriil Ilizarov built on these successes and failures and created the first successful limb lengthening device.

The device did not gain much publicity until it was used to successfully treat an Olympic gold medalist. The success spread the use of the Ilizarov apparatus to the Western part of the world (Paley, 2015).

In 1990, American surgeon Charlie Taylor developed an alternative to the Ilizarov apparatus that he later named the Taylor Spatial Frame (TSF). The TSF, shown in Figure 6 of Appendix 1, is based on the Ilizarov apparatus, but has six fixed telescopic rods rather than the Ilizarov’s option for four or more. While the TSF is an innovative adaptation of the Ilizarov apparatus, the Ilizarov apparatus has more instances of success historically, so we chose to research the Ilizarov apparatus (“What is the TAYLOR SPATIAL FRAME,” 2015).

Structure of the Ilizarov ApparatusThe Ilizarov apparatus, as shown in Figure 7 of Appendix 1, is an external fixator that is comprised of rings, rods, wires, and wire fixation bolts (“Ilizarov principles of deformity correction”).

The rings are made of stainless steel, or more recently carbon fiber. As seen in Figure 8, the rings have circular holes in them so that the rods can be bolted to them (“Bone Fixator”).

The Ilizarov apparatus has two types of rods -- threaded rods and telescopic rods. Threaded rods are used to connect two rings that have a fixed distance between them. Telescopic rods, seen in Figure 9, are long rods with a screw in the middle. These rods have adjustable lengths. The length of

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the telescopic rod is adjusted by turning the screw in the middle. A minimum of four rods is required between a pair of rings (“Tri-Valley Orthopedic Specialists Inc”).

The wire shown in Figure 10 is called a Kirschner wire. Kirschner wires are made from stainless steel. One end of

the Kirschner wire is flat so it can be fixed to a drill, while the other end is pointed so it can be drilled through the bone. The Kirschner wires transfer, from the bone to the apparatus, the force applied by the body's weight.

There is no fixed size or number of rings and rods used for the Ilizarov apparatus. Instead, the size and number of rings and rods used depends on the needs of the patient. At least two wires are required for every ring (“Ilizarov principles of

deformity correction”). Wire fixation bolts, as illustrated in Figure 11, are made from stainless steel. The hole in the wire fixation

bolt exists so that the Kirschner wire can be slid through when connecting the Kirschner wire to the ring (“Truelok Ring Fixation System”).

Function of the Ilizarov ApparatusOnce the Ilizarov apparatus is in place, a corticotomy is performed. A corticotomy is the surgical cutting of the cortical bone without disturbing the medullary cavity. This allows for optimal bone healing as nutrients can go to the fracture site. The Ilizarov apparatus is based on Ilizarov’s principle of distraction osteogenesis as illustrated in Figure 12. Distraction is separation of the two bone segments created by the corticotomy. Osteogenesis is the formation of new bone at the cut site. Osteogenesis is initiated when the bone is subjected to tensile stress. When the screw is turned on the telescopic rods, the bone segments are pulled apart. This process subjects the bone segments to tensile stress. As the screws on the telescopic rods are turned a quarter millimeter every six hours, the bones are under constant tensile stress. This constant tensile stress allows for constant bone growth. The bone growth occurs as described in the anatomy section of this report. This process usually takes at least 12 months, but can vary depending on the desired length of distraction (“Ilizarov principles of deformity correction”).

Figure 12: Distraction Osteogenesis (Created by Reid Larsen)

Figure 8: Ring of the Ilizarov apparatus (indianorthopedic.com)

Figure 10: Kirschner Wire (www.micromed.com)

Figure 11: Wire fixation bolt (www.orthopediconlineshop.com)

Figure 9: Telescopic rod (hellotrade.com)

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Osteoblasts secrete proteins to the fracture site and begin constructing bone and its precursors. Ilizarov’s method pulls apart the bone segments at the same rate that bone growth occurs. This ensures that the bone continues to increase in length until the desired length is achieved. Once the desired length is achieved, distraction stops and the bone is remodeled. Once the bone has fully healed, the apparatus can be removed (Wheeless, III, C. R.).

Materials Used in the Ilizarov ApparatusStainless steel is typically used for the rings, the telescopic rods, the Kirschner wires, and the wire fixation bolts. Carbon fiber can be used as an alternative material for the rings.

Stainless Steel 304 (passive)Stainless steel 304 is a common stainless steel used in biomedical applications. It is typically used for all components of the Ilizarov apparatus. Figure 13 of Appendix 1 displays relevant properties and corresponding values of stainless steel 304.

Stainless steel 304 has a high yield strength of 215 MPa. This is important because the Kirschner wires should never plastically deform when they experience an impact. The Kirschner wires are susceptible to plastic deformation because they have a low geometric resistance to bending. If the Kirschner wires were to plastically deform, then they would move within the tibia. Kirschner wire movement can result in malunion, infection within the medullary cavity, or failure of the device. The fatigue strength of stainless steel 304 is also high at 180 MPa at 107 cycles. The high fatigue strength is important because the device undergoes dynamic loading when the patient walks while wearing the device. If stainless steel’s fatigue strength were lower, the device would have a higher probability of fracture. Stainless steel 304 has a high modulus of elasticity at 195 GPa. A high modulus of elasticity is important because the rings and telescopic rods are subjected to compressive stress from the weight of the body. Also, a high modulus of elasticity in the Kirschner wires would minimize the elastic deformation when they are tensioned. The device cannot undergo significant elastic deformation. Significant elastic deformation can result in malunion or device failure because the dynamics of the system would be affected (ASM Material Data Sheet).

The hardness of stainless steel 304 is 138 on the Knoop scale. Its hardness is sufficient to minimize potential scratching and indentation. Stainless steel 304 has a moderately high density of 8.19 g/cm3. While this is not optimal, the weight of the device is still light enough that the patient can walk with the device attached. Stainless steel 304 has very good corrosion resistance. Its corrosion resistance is important because the device is mostly external, and thus it is exposed to the elements. It is important that stainless steel does not corrode when exposed to the elements as corroded rings and telescopic rods could lead to device failure. Stainless steel 304 also has very good biocompatibility. Since the Kirschner wires are partially internal, it is necessary that they do not harm the tissues that they are exposed to and the tissues do not harm the Kirschner wires (ASM Material Data Sheet).

Carbon Fiber

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Carbon fiber is sometimes used as a substitute for stainless steel in the rings of the Ilizarov apparatus. Carbon fiber is a composite material made from chemically treated polymers (How is it made?). Figure 14 of Appendix 1 shows relevant properties of carbon fiber and their corresponding values.

Carbon fiber has a high fracture strength of 240 MPa. This high fracture strength makes carbon fiber a suitable material for the ring because it has a low probability of fracture if subjected to a sudden impact. Fracture would result in total failure of the device. Carbon fiber has a high fatigue strength of 138 MPa at 107 cycles. The high fatigue strength is important because the device is subjected to dynamic loading when the patient walks. If the rings were to fracture, the apparatus would have to be removed. While carbon fiber has a moderate modulus of elasticity of 73.1 GPa, it is suitable for use in the rings of the apparatus. Carbon fiber is suitable because the rings are not undergoing enough compressive stress to cause significant elastic deformation. Carbon fiber has a hardness of 150 Knoop. Its hardness is sufficient to minimize potential scratching and indentation. This is important because the ring is completely external. Carbon fiber has a very low density of 1.75 g/cm3. Carbon fiber is an excellent choice of material because the low density makes it easier for the patient to walk, as the rings are lightweight. Carbon fiber also has an excellent corrosion resistance, which is necessary because the ring is external and constantly exposed to the elements (Mechanical Properties of Carbon Fibre).

Alternative Systems and TechniquesThe Ilizarov apparatus is not the only treatment available for limb length discrepancies. A popular alternative to the Ilizarov apparatus is another external fixator called the Taylor Spatial Frame.

Structure of the Taylor Spatial FrameThe basic structure of the Taylor Spatial Frame can be seen in Figure 15. The Taylor Spatial Frame is made of rings, adjustable telescopic rods, Rancho cubes, and half-pins. The rings work like the rings in the Ilizarov apparatus. However, the telescopic rods are positioned between the two rings at an angle rather than vertically, as in the Ilizarov apparatus. The half-pins are responsible for anchoring the Taylor Spatial Frame to the bone like the Kirschner wires in the Ilizarov apparatus. The Rancho cubes are square-shaped bolts that hold the half-pins in place and add support to the ring/half-pin junction (“Taylor Spatial Frame,” 2015). Function of the Taylor Spatial FrameThe Taylor Spatial Frame, like the Ilizarov apparatus, is an external fixator used to lengthen a long bone when a patient suffers from limb length discrepancy. A patient who requires the Taylor Spatial Frame must first undergo a corticotomy. The surgeon then inputs information about the initial limb

Figure 15: Taylor Spatial Frame and its components (www.smith-nephew.com)

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length discrepancy, such as desired distraction length and the patient’s weight, into an advanced computer software. This software interprets the information and formulates a day-by-day treatment plan for the patient. This treatment plan sets the frequency and daily length of distraction (Bone Fixator).

The six telescopic rods can be independently lengthened or shortened according to a patient’s treatment plan. Turning the screws located on each telescopic rod lengthens the rod. One full rotation of the screw lengthens the telescopic rod about one millimeter. The distraction created by the lengthening of rods initiates distraction osteogenesis within the bone. (“What is the TAYLOR SPATIAL FRAME,” 2015).

Limitations of the Ilizarov ApparatusThe limitations of the Ilizarov apparatus were examined from three perspectives: the engineering, the medical, and the economic.

When the Kirschner wires plastically deform or loosen, a lack of internal stabilization occurs. The Ilizarov apparatus lacks internal stabilization because the bone segments, created by the corticotomy, are not held in a fixed position by the Kirschner wires. Plastic deformation occurs when the yield strength of the Kirschner wire is exceeded. This may be because of an improper number of rings and wires used on the Ilizarov apparatus. Wire tract infection can cause wire loosening. Wire tract infection is an infection that occurs around the Kirschner wire and is caused by bacteria on the surface of the wires. When a patient suffers from wire tract infection, the skin around the Kirschner wire loosens, allowing the Kirschner wires freedom of movement. The freedom of movement of the Kirschner wires disrupts the dynamics of the system (Hillard, Harrison, & Atkins, 1998).

From a medical perspective, the Ilizarov apparatus puts patients at high risk of wire tract infection and causes patients a lot of pain. The reported percentage of patients who were infected with wire tract infection is at least 11.3% (Fereira, Marais, 2012). Wire tract infection is not a serious complication as long as it is treated immediately. However, the infection can lead to wire loosening, which can cause device failure (Solomin, 2008). Additionally, patients who use the Ilizarov apparatus are subject to severe pain throughout the bone lengthening process (“The Use of the Ilizarov Method in Children for Limb Lengthening”).

The Ilizarov apparatus has high maintenance costs. Patients have to undergo weekly X-rays to monitor the condition of the fracture site. Since the Ilizarov apparatus normally has to remain on the patient for at least a year, this increases the cost of maintenance of the Ilizarov apparatus for patients, as X-rays are very expensive (Zuniga,2013).

Research and Development of the Limb Lengthening DevicesCurrent research and development of the Ilizarov apparatus has focused on adding a coating to the Kirschner wires, adding additional Kirschner wires, and adding a monitoring component called Fracture Track. An alternative device called the PRECICE intramedullary limb lengthening system is also under development.

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Wire Coating AdditionTo overcome the problem of wire tract infection, researchers are investigating material coatings that can be applied to the surface of the wire. Suggested materials include copper, silver, nanosilver, iodine, and nitric oxide. Although these materials have antibacterial properties, many can be toxic to humans. Thus, researchers are trying to limit the toxicity and maintain the antibacterial effect of the coating (Jennison).

Copper has shown antibacterial effects in vitro and in vivo (Jennison). However, copper is known to be toxic to humans in high doses, so further research is needed. Silver carries the same benefits and risks as copper. However, silver has been successfully used as an antibacterial agent in many clinical applications, so it is being researched as an antibacterial coating on the Kirschner wires (Jennison).

Nanosilver is an ultra-thin coating of silver ions, less than 100 nanometers thick, which can be applied to the surface of the Kirschner wire. Nanosilver’s high surface area to volume ratio would minimize silver’s toxic effects because much less silver is needed to cover the Kirschner wire. Preliminary testing on nanosilver has shown significantly more bacterial growth inhibition than the non-coated stainless steel used today in the Kirschner wires (Jennison).

Iodine is used as a common topical antiseptic and therefore has proven antibacterial properties. However, it can be toxic to humans and has shown negative effects on bone growth during remodeling. Therefore, iodine is not considered a viable coating material. Nitric oxide has been used as an antibiotic to cure many types of bacterial infections, so researchers are investigating its use as an antibacterial coating on the Kirschner wires. However, the physiological effects have only been studied on rats, so further testing is needed (Jennison). Addition of Kirschner WiresAs shown in Figure 16 of Appendix 1, The Kirschner wires in the Ilizarov apparatus have been proven to undergo plastic deformation throughout the lifetime of the device. Researchers found that if more than 50 N per wire is applied axially to the Ilizarov apparatus, then the wires undergo plastic deformation. Therefore, the researchers suggest adding supplemental wires so less than 50 N is applied per wire (Watson, 2003). Addition of Fracture TrackFracture Track is an external fixator supplement that is attached to the Ilizarov apparatus at the ring/wire junction. The device measures the load supported by the wires by measuring the wires’ natural deflection downward. From this information, Fracture Track calculates the force the Ilizarov apparatus is supporting. Because the total force, and the force supported by the Ilizarov apparatus is known, the force supported by the leg can be calculated using a complex algorithm. The calculated force supported by the leg correlates to the condition of the fracture site, allowing doctors to gain better insight into the appropriate distraction rate. This information would reduce the number of X-rays the patient undergoes, saving the patient’s money (Zuniga, 2013).

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PRECICE Intramedullary Limb Lengthening SystemTo minimize pain and complication rates, researchers led by Dr. Stuart Green, whom we contacted during our research, have developed an internal fixation device. The PRECICE intramedullary limb lengthening system is completely internal (Paley, 2015). The intramedullary rod of the PRECICE system is an extendable rod that is inserted into the medullary cavity of the tibia. Two to three screws aligned perpendicular to the rod are drilled through the bone to secure the intramedullary rod inside the bone, as shown in Figure 17 of Appendix 1. Distraction is initiated when the patient puts the external remote control within 10 centimeters of the intramedullary rod. The external remote control contains two magnets that spin about 30 times per minute. When held up to the leg, this spin creates an electric field and causes the magnet inside the intramedullary cavity to spin. This spinning turns the lead screw on the inside of the intramedullary rod, illustrated in Figure 18, causing distraction. The distraction mechanism is very similar to that of the Ilizarov apparatus except that in the PRECICE system the screw spins as a result of a magnetic remote control rather than a person turning a screw (Paley, 2015).

Plastic deformation has not been shown to be a problem with the PRECICE system. However, the device is still relatively new, so future application of the device will confirm or disprove this. Infection rates are lower in the PRECICE system, as the device is entirely internal. However, if infection occurs, it is much more difficult to treat because the entire device may have to be removed from the medullary cavity. Additionally, the device is significantly more expensive than the Ilizarov apparatus and a more invasive surgery is required to implement the PRECICE system. The PRECICE intramedullary limb lengthening system received FDA approval in 2013, but more improvements are still being made to the device (Paley, 2015).

Design RecommendationAs explained above, there is a high incidence of plastic deformation of the Kirschner wires and a high rate of infection around the Kirschner wires. To address these problems, we have come up with five improvements to the design of the Ilizarov apparatus. We recommend changing the Kirschner wires to half-pins, adding a nanosilver coating to the wires, adding Rancho cubes to the ring/half-pin junction, changing the half-pin material from stainless steel to titanium, and increasing the half-pin diameter.

Figure 18: The PRECICE System (http://ellipse-tech.com/precice-limb-lengthening-technology)

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Replacement of Kirschner Wires with Half-PinsWe recommend replacing the Kirschner wires of the Ilizarov apparatus with half-pins. The half-pins serve the same purpose as the Kirschner wires in the Ilizarov apparatus. The main difference between the Kirschner wires and the half-pins is the structure. The half-pin enters the bone but does not contact the medullary cavity as seen in Figure 19 (Wiesel, 2011). Most infections occur within the medullary cavity at the contact site with the Kirschner wire (Jennison, 2014). Replacement of the Kirschner wires with half-pins would minimize infections as there is no direct contact of the medullary cavity with the half-pin (Wiesel, 2011).

Addition of a Nanosilver CoatingWe recommend using a nanosilver coating to minimize the rate of infection. Nanosilver has antibacterial effects that reduce infection. Because of nanosilver’s high surface area to volume ratio, the nanosilver would have minimal toxic effects on the body and would not affect the half-pins’ strength (Jennison, 2014).

Addition of Rancho CubesWe recommend adding Rancho cubes, illustrated in Figure 20, to the structure of the Ilizarov apparatus. This is because the use of a half-pin increases the bending stress of the ring/half-pin junction. As a result, we recommend adding Rancho cubes like those in the Taylor Spatial Frame to increase the strength at the ring/half-pin junction (Wiesel, 2011).

Replacement of Stainless Steel with TitaniumWe recommend changing the half-pin material from stainless steel to titanium. Figure 21 shows relevant properties of stainless steel and titanium. As shown in Figure 21 of Appendix 1, titanium’s yield strength is over four times that of stainless steel. The increased yield strength would lower the possibility of plastic deformation. Titanium’s higher fatigue strength would decrease the probability of fracture due to dynamic loading. Stainless steel’s modulus of elasticity is greater than that of titanium. Titanium’s higher hardness would minimize potential scratching and indentation. As titanium’s density is about half that of stainless steel, the wires would be of lighter weight, which would be an additional benefit. Titanium and stainless steel have similar corrosion resistances and biocompatibilities (ASM Material Data Sheet).

Increase in the Pin DiameterOur team recommends increasing the half-pin diameter. Increasing the diameter of the half-pin compensates for titanium’s lower modulus of elasticity by maintaining the stiffness of the device.

Figure 20: Rancho Cubes (Adapted from http://www.fecad.com/Content/pix/jumbo/tsf_kinematic_1.PNG)

Half-pinMedullary Cavity

Figure 19: Cross Sectional View of Leg with Half-Pin (http://web.orthofix.com/Products/Pages/Galaxy-UNYCO.aspx?catid=21)

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The elastic modulus of a material multiplied by its geometric resistance to bending results in the stiffness of that material. Our calculations above demonstrate that the new half-pin diameter would have equivalent stiffness to that of the Kirschner wires. Our calculations found that increasing the diameter of the half-pin from 1.5mm to 1.7mm would maintain the half-pin’s stiffness.

In conclusion, we recommend replacing the Kirschner wires with half-pins, adding a nanosilver coating to the half-pins, adding Rancho cubes to the ring/half-pin junctions, changing the material of the half-pins from stainless steel to titanium, and finally increasing the diameter of the half-pins. We hope these recommendations address the high risk of infection and the Kirschner wires’ tendency to plastically deform.

References ASM Material Data Sheet. (n.d.). Retrieved December 16, 2015, from

http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MQ304A

Bone Fixator. Retrieved October 28, 2015 fromhttp://www.bonefixator.com/fracture_fixation/taylor_spatial_frame_2.html

Brand, R. (2008, September 18). Advances in Limb Lengthening and Reconstruction: Alessandro Codivilla, MD, 1861–1912. Retrieved November 12, 2015, from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2628242/

Clarke. Normal Bone Anatomy and Physiology. Retrieved November 9,2015, from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3152283/

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M7 Team 14

https://www.researchgate.net/publication/51322378_The_yielding_of_tensioned_fine_wires_in_the_Ilizarov_frame

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M7 Team 15

http://emedicine.medscape.com/article/1948532-overview#a2

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M7 Team 16

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M7 Team 17

Appendix 1:

Property Value

Yield Strength 215 MPa

Fatigue Strength (at 107 cycles) 180 MPa

Modulus of Elasticity 195 GPa

Hardness 138 Knoop

Density 8.19 g/cm3

Corrosion Resistance Very good

Biocompatibility Very good

Figure 13: Relevant properties of stainless steel 304 and their corresponding values

Figure 5: Codivilla’s external fixator (slideshare.net)

Figure 6: The Taylor Spatial Frame (What is the TAYLOR SPATIAL FRAME)

Figure 7: Structure of the Ilizarov apparatus (hellotrade.com)

M7 Team 18

Property Value

Fracture Strength 240 MPa

Fatigue Strength (at 107 cycles) 138 MPa

Modulus of Elasticity 73.1 GPa

Hardness (along the fiber texture) 150 Knoop

Density 1.75 g/cm3

Corrosion Resistance Excellent

Property Stainless Steel (304) Passive Value

Titanium (Ti-6Al-4V, Annealed) Value

Yield Strength 215 MPa 880 MPa

Figure 16: Plastic deformation of the Kirschner wires (http://medicaldevices.asmedigitalcollection.asme.org

Figure 17: Intramedullary Rod in Bone 9 (https://spokaneshrinershospital.wordpress.com)

Figure 14: Relevant properties of Carbon Fiber and their corresponding values

M7 Team 19

Fatigue Strength (at 107

cycles)180 MPa 510 MPa

Modulus of Elasticity 195 GPa 113.8 GPa

Hardness 138 Knoop 363 Knoop

Density 8.19 g/cm3 4.43 g/cm3

Corrosion Resistance Very Good Very Good

Biocompatibility Very Good Excellent

I have neither given nor received any unauthorized help on this assignment, nor have I concealed any violation of the Honor Code.

Figure 21: Relevant properties of stainless steel 304, titanium and their corresponding values