EXTERNAL SKELETAL FIXATION
0195-5616/92 $0.00
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CLASSIFICATION AND NOMENCLATURE OF EXTERNAL FIXATORS Simon C. Roe, BVSc, MVSt, MS,
MACVS~
Although external skeletal fixation has been used in fracture management for many years, consistent nomenclature has not been developed. 4 Initially there was little need for classification of fixators because there were few devices and only one or two configurations. As in other areas of surgery, the devices were named for the developer. The Stader reduction splint19 and the Kirschner-Ehmer splint12 are examples cited in the early veterinary literature. The Roger Anderson spline and the Hoffmann apparatus 16 were first popularized in human fracture management. As understanding of fracture healing improved, the science of external fixation also advanced, and the number of devices and possible configurations grew rapidly. ELEMENTS OF AN EXTERNAL FIXATOR Pin
The external frame of the fixator is connected to the bone by pins or wires. Pins are made of rigid stainless steel and may be smooth or threaded. the thread may be cut into the pin or built onto the pin, the latter being stronger but more expensive. Pins are classified as half if they penetrate only one skin surface and the bone and full if they transfix the limb and penetrate two skin surfaces. Wires, made rigid by being placed under tension, may also be used to connect frames to the . bone fragments. 17 From the Department of Companion Animal and Special Species Medicine, North Carolina State University College of Veterinary Medicine, Raleigh, North Carolina VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE VOLUME 22 • NUMBER 1 • JANUARY 1992
11
12
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Connecting Rod or Bar
The connecting rod or bar may join a group of pins together or span the fracture site to immobilize the bone fragments. Stainless steel rods are primarily used in veterinary surgery, although free-form fixators using plaster or acrylics to bridge across pins can also be applied.9 In humans, adjustable rods are used with some systems so that continuous distraction can be applied fo lengthen a limb.I Clamp
The Kirschner system (Kirschner Co ., Aberdeen, MD) for veterinary surgery has double and single clamps. Double clamps join two connecting rods. Single clamps attach a pin to a connecting rod. Similar principles are used in the Synthes system (Synthes, Paoli, PA). The Hoffman device (Howmedica, Rutherford, NJ), used in humans, contains a clamp that holds a number of pins. The clamp is then attached to the connecting rod. EARLY NOMENCLATURE
In veterinary literature, fixators were termed Kirschner-Ehmer splints. The standard Kirschner-Ehmer splint employed double clamps.7 A more rigid splint that employed only single clamps became known as the modified Kirschner-Ehmer splint.6 Other popular terms for configurations were the half Kirschner-Ehmer splint (two half pins and a connecting rod) that is used in conjunction with an intramedullary pin and the full or through-and-through Kirschner-Ehmer splint, in which the pins transfix the limb and connecting bars span the fracture along two skin surfaces. 20 NEED FOR CLASSIFICATION
With the resurgence in popularity of external fixation, there has been a great increase in the number of scientific communications about these devices. Unfortunately there is inconsistency within the veterinary profession as evidenced by a survey of small animal surgeons in which one configuration was known by a number of different popular names. 18 Classification has become necessary so that readers can interpret configurations and be able to compare devices employing similar principles. The major obstacle to developing a comprehensive classification system is the growing number of devices and the free-form nature of their application. Improved understanding of the mechanics of fixators reveals that differences in any part of the device may influence overall stability. 5, 15 For a classification system to inform a
CLASSIFICATION AND NOMENCLATURE OF EXTERNAL FIXATORS
13
reader accurately of the characteristics of a particular frame, information such as number of pins, pin diameter, pin length, connecting rod length, and diameter should be included. These factors influence the overall strength of the device but are rarely described because of the cumbersome nature in presenting this information. In 1978, a classification system was proposed by Hierholzer et aP4 that categorized fixators according to pin type. Type I fixators employed half pins, type II used full pins, and type III consisted of a combination of half-pin and full-pin techniques. The disadvantage of this system is the absence of descriptive terms to convey the pattern of the devices. Also, readers must have prior knowledge of the categories to understand the configuration. A more descriptive system, which accommodates a wider range of devices, has been proposed by Behrens.3 , 4 In this system, fixators are either simple pin, clamp, or ring forms. They are broadly categorized into unilateral or bilateral frames and can be one-plane or two-plane devices.4 A classification system similar to that proposed by Hierholzer et aP4 has been used in recent veterinary texts. 9, 10, 11 Embellishments were added to categorize the popular configurations used in veterinary orthopedics. Another modification of the Hierholzer system has been proposed by Toombs. 21 Following, the various suggested nomenclatures are presented. Devices The Kirschner and Synthes systems employed in veterinary surgery are examples of a simple pin fixator . Clamps attach the pins directly to the connecting rods. Clamp fixators are those in which all pins are held in one clamp, which is then connected to the rod. The Hoffmann device is an example in this category. The third category is the ring fixator . Circular or semicircular frame elements wrap around the limb and hold the pins or wires. Connecting rods join these elements to stabilize the fracture. 17 Frame Configuration Six frame configurations are presented diagrammatically in Figure 1. The various names that have been applied to these fixators are discussed next. Those frames that have only half pins are called type I or unilateral frames. These devices may be in one or two planes. One-plane unilateral frames are commonly used in veterinary surgery, In the old nomenclature, parts A and B of Figure I, were known as the standard and modified Kirschner-Ehmer. In the current systems, the adjectives double-clamp and single-bar precede unilateral or type I. These qualifiers provide additional information concerning frame structure. The frame
,... ~
A
B
c
Unilateral Single Bar Type Ia
Unilateral Double Bar Double Bar Type Ia
Double
Clamp ~\. . ' - ""
Connecting _ Bar
Fixation _ _ _ _ _ _ _..J Pin
Unilateral Double Clamp Double Clamp Type Ia
D
E
F
Cross·Section
Unilateral Two Plane Quadrilateral Type Ib
.... U1
Bilateral Type II
Figure 1. Six diagrams of frames used in veterinary orthopedics. Each is named according to the two classification systems described in the text.
Bilateral Two Plane Type III
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depicted in part C in Figure 1 is termed a double-bar unilateral or type la fixator and has gained popularity because of its greater rigidity when compared with the single bar configuration (Fig. 1B).8 The frame configuration depicted by part D in Figure 1 has two offset unilateral frames that brace each other. This configuration has been referred to as a quadrilateral9 • II or biplanar,1O type Ib frame or a unilateral two-plane frame. 4 Toombs 21 divides the type I category into type 1a for those uniplanar frames (Fig. lA, B, C) and type Ib for biplanar frames (Fig. 1D). Fixators with transfixing pins are categorized as bilateral or type II frames (Fig. IE) . One-plane bilateral frames have greater stability than one-plane unilateral frames. Connecting rods present on both sides of the limb provide greater rigidity.8 In the old nomenclature, these were generally termed full Kirschner-Ehmer splints. When full and half pins are employed in a fixator, they are termed type III or two-plane bilateral frames (Fig. IF). By definition, ring fixators are two-plane bilateral frames because pins or wires enter from many angles.
Conclusions
Because of the complexity associated with describing fixators so that stability can be understood, classification systems categorize only the general frame type and structure. As long as the principles of application that maximize device strength have been followed, the properties of a fixator can be reasonably conveyed by its general classification. The categories presented in this article are most helpful in describing frames applied to the lower extremities. External fixators in other areas, such as the pelvis or maxilla, are less structured, and no classification method has been developed to describe accurately the resulting arrangement of pins, clamps, and rods. Although the nomenclature derived from Hierholzer et aP4 has been used extensively in veterinary literature, it is a difficult system to understand because the categories may be general and nonspecific regarding frame characteristics. To define each category, the terms unilateral, bilateral, uniplanar, and biplanar are needed. I believe that the descriptive nomenclature proposed by Behrens,4 which uses these adjectives as category names, may be more valuable. Behrens' system does not require prior knowledge of the category definition to permit interpretation of the fixator. This is particularly helpful for students and nonspecialists who are trying to understand external skeletal fixation. Consistency in the veterinary and human orthopedic literature would also benefit communication, particularly in studies of basic principles that have broad application.
CLASSIFICATION AND NOMENCLATURE OF EXTERNAL FIXATORS
17
SUMMARY Classification of external fixators has graduated from simple device names to a more descriptive system based on frame configuration. Fixators are of the simple pin, clamp, or ring type. Two classification systems are currently being used to categorize fixators. In one, they are defined as type I, type II, or type III fixators . .In the second classification system, the categories are more descriptive. Fixators are broadly defined either as unilateral or bilateral. Within each of these categories, there are one-plane or two-plane frames. In veterinary orthopedics, further adjectives, such as double-clamp, single-bar, two-pin single-bar, and double-bar, are employed to describe some of the more popular configurations. Many configurations are not easily definable because complex arrangements of pins and clamps are sometimes necessary to adapt the frame to the fracture bone successfully. References 1. Aldegheri R, Renzi-Brivio L, Agostini S: The callotasis method of limb lengthening.
Clin Orthop ReI Res 241 :137, 1989 2. Anderson R: An ambulatory method of treating fractures of the shaft of the femur. Surg Gynecol Obstet 52:865, 1936 3. Behrens F: A classification of external fixators. In Uhthoff HK (ed): Current Concepts of External Fixation of Fractures. Berlin, Springer-Verlag, 1982, p 15 4. Behrens F: A primer of fixator devices and configurations. Clin Orthop ReI Res 241 :5, 1989 5. Behrens F, Johnson W: Unilateral external fixation: Methods to increase and reduce frame stiffness. Clin Orthop ReI Res 241:48, 1989 6. Brinker WO, Flo GL: Principles and application of external skeletal fixation. Vet Clin North Am Small Anim Pract 5:197, 1975 7. Brinker WO, Piermattei DL, Flo GL: Fractures: Classification, diagnosis and treatment. In Handbook of Small Animal Orthopedics and Fracture Treatment. Philadelphia, WB Saunders, 1983, p 2 8. Egger EL: Static strength evaluation of six external skeletal fixation configurations. Vet Surg 12:130, 1983 9. Egger EL: The use of external skeletal fixation Kirschner-Ehmer apparatus: Half-pin splintage and full-pin splintage. In Whittick WG (ed): Canine Orthopedics. Philadelphia, Lea & Febiger, 1990, p 248 10. Egger EL: External skeletal fixation. In Bojrab J (ed): Current Techniques in Small Animal Surgery. Philadelphia, Lea & Febiger, 1990, p 816 11. Egger EL, Greenwood KM: External skeletal fixation. In Slatter DH (ed): Textbook of Small Animal Surgery. Philadelphia, WB Saunders, 1985, p 1972 12. Ehmer EA: Bone pinning in fractures of small animals. J Am Vet Med Assoc 110:14, 1947 13. Grill F: Correction of complicated extremity deformities by external fixation . Clin Orthop ReI Res 241:166, 1989 14. Hierholzer G, Kleining R, Horster G, Zemenides P: External fixation: Classification and indications. Arch Orthop Trauma Surg 92:175, 1978 15. Johnson WD, Fischer DA: Skeletal stabilization with a multiplane external fixation device. Biomechanical evaluation and finite element model analysis. Clin Orthop ReI Res 180:34, 1983 16. Karlstrom G, Olerud S: Percutaneous pin fixation of open tibial fractures. Double frame anchorage using the Vidal-Adrey method. J Bone Joint Surg [Am) 57:915, 1975
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17. Paley 0, Catagni MA, Argnani F, et al: Ilizarov treatment of tibial nonunions with bone loss. Clin Orthop Rei Res 241 :146, 1989 18. Roe Sc, Johnson AL: Nomenclature of external fixators in veterinary orthopedics. Letter to the editor. Vet Surg 14:255, 1985 19. Stader 0: Treating fractures of long bones with reduction splint. North Am Vet 20:55, 1939 20. Straws RC: The Kirschner-Ehmer splint in small animal orthopedics. Mod Vet Pract 65:503, 184 21. Toombs JP: Nomenclature of external skeletal fixatio~. In Proceedings of the ACVS Forum, New Orleans, 1990
Address reprint requests to Simon C. Roe, BVSc, MVSt, MS, MACVSc Department of Companion Animal and Special Species Medicine North Carolina State University 4700 Hillsborough Street Raleigh, NC 27606
0195-5616/92 $0.00
+ .20
CLASSIFICATION AND NOMENCLATURE OF EXTERNAL FIXATORS Simon C. Roe, BVSc, MVSt, MS,
MACVS~
Although external skeletal fixation has been used in fracture management for many years, consistent nomenclature has not been developed. 4 Initially there was little need for classification of fixators because there were few devices and only one or two configurations. As in other areas of surgery, the devices were named for the developer. The Stader reduction splint19 and the Kirschner-Ehmer splint12 are examples cited in the early veterinary literature. The Roger Anderson spline and the Hoffmann apparatus 16 were first popularized in human fracture management. As understanding of fracture healing improved, the science of external fixation also advanced, and the number of devices and possible configurations grew rapidly. ELEMENTS OF AN EXTERNAL FIXATOR Pin
The external frame of the fixator is connected to the bone by pins or wires. Pins are made of rigid stainless steel and may be smooth or threaded. the thread may be cut into the pin or built onto the pin, the latter being stronger but more expensive. Pins are classified as half if they penetrate only one skin surface and the bone and full if they transfix the limb and penetrate two skin surfaces. Wires, made rigid by being placed under tension, may also be used to connect frames to the . bone fragments. 17 From the Department of Companion Animal and Special Species Medicine, North Carolina State University College of Veterinary Medicine, Raleigh, North Carolina VETERINARY CLINICS OF NORTH AMERICA: SMALL ANIMAL PRACTICE VOLUME 22 • NUMBER 1 • JANUARY 1992
11
12
ROE
Connecting Rod or Bar
The connecting rod or bar may join a group of pins together or span the fracture site to immobilize the bone fragments. Stainless steel rods are primarily used in veterinary surgery, although free-form fixators using plaster or acrylics to bridge across pins can also be applied.9 In humans, adjustable rods are used with some systems so that continuous distraction can be applied fo lengthen a limb.I Clamp
The Kirschner system (Kirschner Co ., Aberdeen, MD) for veterinary surgery has double and single clamps. Double clamps join two connecting rods. Single clamps attach a pin to a connecting rod. Similar principles are used in the Synthes system (Synthes, Paoli, PA). The Hoffman device (Howmedica, Rutherford, NJ), used in humans, contains a clamp that holds a number of pins. The clamp is then attached to the connecting rod. EARLY NOMENCLATURE
In veterinary literature, fixators were termed Kirschner-Ehmer splints. The standard Kirschner-Ehmer splint employed double clamps.7 A more rigid splint that employed only single clamps became known as the modified Kirschner-Ehmer splint.6 Other popular terms for configurations were the half Kirschner-Ehmer splint (two half pins and a connecting rod) that is used in conjunction with an intramedullary pin and the full or through-and-through Kirschner-Ehmer splint, in which the pins transfix the limb and connecting bars span the fracture along two skin surfaces. 20 NEED FOR CLASSIFICATION
With the resurgence in popularity of external fixation, there has been a great increase in the number of scientific communications about these devices. Unfortunately there is inconsistency within the veterinary profession as evidenced by a survey of small animal surgeons in which one configuration was known by a number of different popular names. 18 Classification has become necessary so that readers can interpret configurations and be able to compare devices employing similar principles. The major obstacle to developing a comprehensive classification system is the growing number of devices and the free-form nature of their application. Improved understanding of the mechanics of fixators reveals that differences in any part of the device may influence overall stability. 5, 15 For a classification system to inform a
CLASSIFICATION AND NOMENCLATURE OF EXTERNAL FIXATORS
13
reader accurately of the characteristics of a particular frame, information such as number of pins, pin diameter, pin length, connecting rod length, and diameter should be included. These factors influence the overall strength of the device but are rarely described because of the cumbersome nature in presenting this information. In 1978, a classification system was proposed by Hierholzer et aP4 that categorized fixators according to pin type. Type I fixators employed half pins, type II used full pins, and type III consisted of a combination of half-pin and full-pin techniques. The disadvantage of this system is the absence of descriptive terms to convey the pattern of the devices. Also, readers must have prior knowledge of the categories to understand the configuration. A more descriptive system, which accommodates a wider range of devices, has been proposed by Behrens.3 , 4 In this system, fixators are either simple pin, clamp, or ring forms. They are broadly categorized into unilateral or bilateral frames and can be one-plane or two-plane devices.4 A classification system similar to that proposed by Hierholzer et aP4 has been used in recent veterinary texts. 9, 10, 11 Embellishments were added to categorize the popular configurations used in veterinary orthopedics. Another modification of the Hierholzer system has been proposed by Toombs. 21 Following, the various suggested nomenclatures are presented. Devices The Kirschner and Synthes systems employed in veterinary surgery are examples of a simple pin fixator . Clamps attach the pins directly to the connecting rods. Clamp fixators are those in which all pins are held in one clamp, which is then connected to the rod. The Hoffmann device is an example in this category. The third category is the ring fixator . Circular or semicircular frame elements wrap around the limb and hold the pins or wires. Connecting rods join these elements to stabilize the fracture. 17 Frame Configuration Six frame configurations are presented diagrammatically in Figure 1. The various names that have been applied to these fixators are discussed next. Those frames that have only half pins are called type I or unilateral frames. These devices may be in one or two planes. One-plane unilateral frames are commonly used in veterinary surgery, In the old nomenclature, parts A and B of Figure I, were known as the standard and modified Kirschner-Ehmer. In the current systems, the adjectives double-clamp and single-bar precede unilateral or type I. These qualifiers provide additional information concerning frame structure. The frame
,... ~
A
B
c
Unilateral Single Bar Type Ia
Unilateral Double Bar Double Bar Type Ia
Double
Clamp ~\. . ' - ""
Connecting _ Bar
Fixation _ _ _ _ _ _ _..J Pin
Unilateral Double Clamp Double Clamp Type Ia
D
E
F
Cross·Section
Unilateral Two Plane Quadrilateral Type Ib
.... U1
Bilateral Type II
Figure 1. Six diagrams of frames used in veterinary orthopedics. Each is named according to the two classification systems described in the text.
Bilateral Two Plane Type III
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depicted in part C in Figure 1 is termed a double-bar unilateral or type la fixator and has gained popularity because of its greater rigidity when compared with the single bar configuration (Fig. 1B).8 The frame configuration depicted by part D in Figure 1 has two offset unilateral frames that brace each other. This configuration has been referred to as a quadrilateral9 • II or biplanar,1O type Ib frame or a unilateral two-plane frame. 4 Toombs 21 divides the type I category into type 1a for those uniplanar frames (Fig. lA, B, C) and type Ib for biplanar frames (Fig. 1D). Fixators with transfixing pins are categorized as bilateral or type II frames (Fig. IE) . One-plane bilateral frames have greater stability than one-plane unilateral frames. Connecting rods present on both sides of the limb provide greater rigidity.8 In the old nomenclature, these were generally termed full Kirschner-Ehmer splints. When full and half pins are employed in a fixator, they are termed type III or two-plane bilateral frames (Fig. IF). By definition, ring fixators are two-plane bilateral frames because pins or wires enter from many angles.
Conclusions
Because of the complexity associated with describing fixators so that stability can be understood, classification systems categorize only the general frame type and structure. As long as the principles of application that maximize device strength have been followed, the properties of a fixator can be reasonably conveyed by its general classification. The categories presented in this article are most helpful in describing frames applied to the lower extremities. External fixators in other areas, such as the pelvis or maxilla, are less structured, and no classification method has been developed to describe accurately the resulting arrangement of pins, clamps, and rods. Although the nomenclature derived from Hierholzer et aP4 has been used extensively in veterinary literature, it is a difficult system to understand because the categories may be general and nonspecific regarding frame characteristics. To define each category, the terms unilateral, bilateral, uniplanar, and biplanar are needed. I believe that the descriptive nomenclature proposed by Behrens,4 which uses these adjectives as category names, may be more valuable. Behrens' system does not require prior knowledge of the category definition to permit interpretation of the fixator. This is particularly helpful for students and nonspecialists who are trying to understand external skeletal fixation. Consistency in the veterinary and human orthopedic literature would also benefit communication, particularly in studies of basic principles that have broad application.
CLASSIFICATION AND NOMENCLATURE OF EXTERNAL FIXATORS
17
SUMMARY Classification of external fixators has graduated from simple device names to a more descriptive system based on frame configuration. Fixators are of the simple pin, clamp, or ring type. Two classification systems are currently being used to categorize fixators. In one, they are defined as type I, type II, or type III fixators . .In the second classification system, the categories are more descriptive. Fixators are broadly defined either as unilateral or bilateral. Within each of these categories, there are one-plane or two-plane frames. In veterinary orthopedics, further adjectives, such as double-clamp, single-bar, two-pin single-bar, and double-bar, are employed to describe some of the more popular configurations. Many configurations are not easily definable because complex arrangements of pins and clamps are sometimes necessary to adapt the frame to the fracture bone successfully. References 1. Aldegheri R, Renzi-Brivio L, Agostini S: The callotasis method of limb lengthening.
Clin Orthop ReI Res 241 :137, 1989 2. Anderson R: An ambulatory method of treating fractures of the shaft of the femur. Surg Gynecol Obstet 52:865, 1936 3. Behrens F: A classification of external fixators. In Uhthoff HK (ed): Current Concepts of External Fixation of Fractures. Berlin, Springer-Verlag, 1982, p 15 4. Behrens F: A primer of fixator devices and configurations. Clin Orthop ReI Res 241 :5, 1989 5. Behrens F, Johnson W: Unilateral external fixation: Methods to increase and reduce frame stiffness. Clin Orthop ReI Res 241:48, 1989 6. Brinker WO, Flo GL: Principles and application of external skeletal fixation. Vet Clin North Am Small Anim Pract 5:197, 1975 7. Brinker WO, Piermattei DL, Flo GL: Fractures: Classification, diagnosis and treatment. In Handbook of Small Animal Orthopedics and Fracture Treatment. Philadelphia, WB Saunders, 1983, p 2 8. Egger EL: Static strength evaluation of six external skeletal fixation configurations. Vet Surg 12:130, 1983 9. Egger EL: The use of external skeletal fixation Kirschner-Ehmer apparatus: Half-pin splintage and full-pin splintage. In Whittick WG (ed): Canine Orthopedics. Philadelphia, Lea & Febiger, 1990, p 248 10. Egger EL: External skeletal fixation. In Bojrab J (ed): Current Techniques in Small Animal Surgery. Philadelphia, Lea & Febiger, 1990, p 816 11. Egger EL, Greenwood KM: External skeletal fixation. In Slatter DH (ed): Textbook of Small Animal Surgery. Philadelphia, WB Saunders, 1985, p 1972 12. Ehmer EA: Bone pinning in fractures of small animals. J Am Vet Med Assoc 110:14, 1947 13. Grill F: Correction of complicated extremity deformities by external fixation . Clin Orthop ReI Res 241:166, 1989 14. Hierholzer G, Kleining R, Horster G, Zemenides P: External fixation: Classification and indications. Arch Orthop Trauma Surg 92:175, 1978 15. Johnson WD, Fischer DA: Skeletal stabilization with a multiplane external fixation device. Biomechanical evaluation and finite element model analysis. Clin Orthop ReI Res 180:34, 1983 16. Karlstrom G, Olerud S: Percutaneous pin fixation of open tibial fractures. Double frame anchorage using the Vidal-Adrey method. J Bone Joint Surg [Am) 57:915, 1975
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ROE
17. Paley 0, Catagni MA, Argnani F, et al: Ilizarov treatment of tibial nonunions with bone loss. Clin Orthop Rei Res 241 :146, 1989 18. Roe Sc, Johnson AL: Nomenclature of external fixators in veterinary orthopedics. Letter to the editor. Vet Surg 14:255, 1985 19. Stader 0: Treating fractures of long bones with reduction splint. North Am Vet 20:55, 1939 20. Straws RC: The Kirschner-Ehmer splint in small animal orthopedics. Mod Vet Pract 65:503, 184 21. Toombs JP: Nomenclature of external skeletal fixatio~. In Proceedings of the ACVS Forum, New Orleans, 1990
Address reprint requests to Simon C. Roe, BVSc, MVSt, MS, MACVSc Department of Companion Animal and Special Species Medicine North Carolina State University 4700 Hillsborough Street Raleigh, NC 27606
