A BIOMECHANICAL COMPARISON OF PIN CONFIGURATIONS USED FOR PERCUTANEOUS PINNING OF DISTAL TIBIA FRACTURES IN CHILDREN Justin Brantley, MS1,2, Aditi Majumdar MD1, J. Taylor Jobe, MD1, Antony Kallur, MD1, Christina Salas, PhD1,2,3
ABSTRACT Background: Percutaneous pin fixation is often used in conjunction with closed-reduction and cast immobilization to treat pediatric distal tibia fractures. The goal of this procedure is to maintain reduction and provide improved stabilization, in effort to facilitate a more anatomic union. We conducted a biomechanical study of the torsional and bending stability of three commonly used pin configurations in distal tibia fracture fixation. Methods: A transverse fracture was simulated at the metaphyseal/diaphyseal junction in 15 synthetic tibias. Each fracture was reduced and fixed with two Kirschner wires, arranged in one of three pin configurations: parallel, retrograde, medial to lateral pins entering at the medial malleolus distal to the fracture (group A); parallel, antegrade, medial to lateral pins entering at the medial diaphysis proximal to the fracture (group B); or a cross-pin configuration with one retrograde, medial to lateral pin entering the medial malleolus distal to the fracture and the second an antegrade, medial to lateral pin entering at the medial diaphysis proximal to the fracture (group C). Stability of each construct was assessed by resistance to torsion and bending. Results: Resistance to external rotation stress was significantly higher in group A than group B (P = 0.044). Resistance to internal rotation stress was significantly higher in group C than group B (P = 0.003). There was no significant difference in torsional stiffness when comparing group A with group C. Under a medial-directed load, group B and C specimens were significantly stiffer
Department of Orthopaedics & Rehabilitation Center for Biomedical Engineering 3 Department of Mechanical Engineering, University of New Mexico, Albuquerque, NM Corresponding Author: Christina Salas, PhD Department of Orthopaedics & Rehabilitation, University of New Mexico MSC 10 5600, 1 University of New Mexico, Albuquerque, NM 87131 telephone: 505.272.5671; fax: 505.272-8098. E-mail: [email protected]. The authors declare no relevant conflict of interest 1 2
than those in group A (28 N/mm and 24 N/mm vs. 14 N/mm for A; P = 0.001 and P = 0.009, respectively) Conclusions: None of the three pin configurations produced superior results with respect to all variables studied. Group A configuration provided the highest resistance to external rotation forces, which is the most clinically relevant variable under short-cast immobilization. Parallel, retrograde, medial to lateral pins entering at the medial malleolus provide the greatest resistance to external rotation of the foot while minimizing the potential for iatrogenic injur y to soft tissue structures. INTRODUCTION Tibia fractures are the most common pediatric lower extremity fracture1,2 and the second most common fracture resulting in hospitalization.3 Most pediatric tibia fractures are isolated occurrences resulting from lowenergy, indirect torsional stress.4-6 About 50% of these fractures occur in the distal third of the tibia.5,7 Pediatric distal third tibia fractures have traditionally been described as complex injuries that can be challenging to manage because of articular involvement, proximity to the fibula, and open physes.8 For unstable fractures of long bones, intramedullar y fixation is a common surgical intervention as this permits earlier mobilization and facilitates optimal alignment.3,5 In pediatric distal tibia fractures, however, locked intramedullary rods are typically avoided because of the risk of physeal injury.9-11 External fixation was formerly the preferred procedure for treating distal tibial fractures, especially severely unstable fractures with soft tissue involvement or comminution.8,12 The use of external fixation has declined, however, because of recent reports of complications with the technique, including infection, skin necrosis, delayed union, limb overgrowth, and a need for re-manipulation.12 Elastic Stable Intramedullary Nailing (ESIN) is now commonly used for fixation of pediatric long-bone fractures. This approach is minimally invasive, with percutaneous placement of the nails.11,13-16 However, ESIN has been associated with complications resulting from insufficient nail thickness, malreduction during implant placement, and inadequate stabilization of fractures.2,17 Supplementation of cast immobilization with percutaneously Volume 36 133
J. Brantley, A. Majumdar, J. T. Jobe, A. Kallur, C. Salas
FIGURE 1. Kirschner-wire pin configurations. Left: parallel, retrograde pins entering medially through the medial malleolus and ending laterally, 15 mm proximal to the fracture site (group A). Middle: parallel, antegrade pins entering medially, centered 15 mm proximal to the fracture site, and ending at the fibular notch (group B). Right: cross-pin configuration with one retrograde pin entering through the medial malleolus and the second pin entering medially, 15 mm proximal to the fracture site and ending at the fibular notch (group C).
placed Kirschner wires across the fracture site has been reported to add adequate stabilization of unstable tibial fractures in children.5 Maintenance of alignment of the tibial shaft using this approach allows early mobilization and callus formation while minimizing infection risks.3,10,18 Percutaneous pinning is most appropriate for fractures that can be reduced by closed manipulation, such as simple, transverse fracture patterns.8 However, placement of percutaneous pins in the distal tibia can be difficult as a result of the need to avoid the tibiofibular and tibiotalar joints, muscular and tendinous structures, and other neurovascular structures. The ideal percutaneous pinning technique would be relatively simple and achieve maximal stabilization without injuring surrounding structures. Stability and union can be affected by torsional and bending stresses, especially those resulting from external rotation of the leg during immobilization. In the present biomechanical study we compared three pin configurations used to enhance stabilization of transverse fractures at the diaphyseal-metaphyseal junction in the distal tibia. We aimed to identify a technique that allows for enhanced structural stability with limited potential for injury of the surrounding soft-tissue. METHODS Fifteen fourth-generation synthetic composite tibiae (Pacific Research Laboratories, Vashon Island, WA) were used in this study. Each specimen was predrilled with an undersized hole (2.38 mm; 3/32-inch bit) to prevent deviations in pin trajectories. A transverse cut was then made 25 mm from the distal articular surface to simulate a simple transverse fracture of the distal tibia. Cutting and drilling were done with a custom jig to maintain consistent fracture locations and pin trajectories among 134 The Iowa Orthopedic Journal
FIGURE 2. Torsional testing construct for applying a fixed support at the distal end and a torsional moment at the proximal end.
the specimens in each group. Each fracture was reduced and fixed with two smooth 3.5 mm Kirschner wires placed in one of three pin configurations (Figure 1): (A), parallel, retrograde, medial to lateral pins entering at the medial malleolus distal to the fracture and ending laterally, 15 mm proximal to the fracture (group A; n=5), (B), parallel, antegrade, medial to lateral pins entering at the medial diaphysis centered 15 mm proximal to the fracture, and ending at the fibular notch (group B; n=5); or (C), a cross-pin configuration with one retrograde, medial to lateral pin entering the medial malleolus distal to the fracture and the second an antegrade, medial to lateral pin entering at the medial diaphysis 15 mm proximal to the fracture site and ending at the fibular notch (group C; n = 5). Biomechanical testing was performed using an MTS 858 Mini Bionix II servohydraulic machine (MTS Systems, Eden Prairie, MN). The specimens were loaded sequentially for six tests: external rotation, internal rotation, anterior-directed bending (fracture apex pos-
A Biomechanical Comparison of Pin Configurations
FIGURE 3. Bending construct for applying a fixed support at the proximal end with a roller support 10 cm proximal to the fracture site and a force through the distal fragment.
FIGURE 5. Bending stiffness results for the 3 pin configurations in 4 load directions. Error bars represent one standard deviation. Groups B and C had significantly more stiffness than Group A in the medial direction.
fracture line to concentrate the load directly through the distal fragment (Figure 3). The distal fragment was loaded to a maximum displacement of 4 mm at a rate of 0.5 mm/sec and unloaded. Each specimen was loaded sequentially three times, and then rotated 90 degrees, for each of the four bending directions. The mean value from the results of the three tests was used to compare the bending stiffness of the various pin configurations. FIGURE 4. Torsional stiffness results for the 3 pin configurations in internal and external rotation. Error bars represent one standard deviation. In external rotation, the difference between Groups A and B was significant. In internal rotation, the difference between Groups B and C was significant.
terior), lateral-directed bending (fracture apex medial), A posterior-directed bending (fracture apex anterior), and medial-directed bending (fracture apex lateral). For torsional loading, the tibia was rigidly attached to the actuator proximally and to an axial/torsional load cell distally (Figure 2). Each specimen was preloaded with 50 N in axial compression. Each specimen was then loaded from 0 to 5 N-mm in internal rotation at a rate of 0.5 N-mm/s, unloaded to zero, loaded to 5 N-mm in external rotation at a rate of 0.5 N-mm/s, and again unloaded to zero. This was repeated three times, and the stiffness was calculated from the linear region on the torque-rotation curve. The mean value of the three tests was used for comparisons between groups. In each of the bending tests, the tibia was oriented perpendicular to the actuator in a simply supported cantilever configuration. The proximal end was fixed, and a roller support was placed 10 cm proximal to the
Statistical Analysis One-way analysis of variance (ANOVA) was used to compare each of the 6 tests for each of the 3 pin configurations. The Tukey post-hoc test was used to determine which groups differed significantly from others. A P value of 0.05 was considered to represent a significant difference. RESULTS Torsional stiffness outcomes for external and internal rotation loads are shown in Figure 4. Group A had the highest mean value for torsional stiffness in external rotation (530 N-mm/degree).This was significantly higher than that for group B (330 N-mm/degree; P = 0.044). Group C had the highest mean value for torsional stiffness in internal rotation (400 N-mm/degree), and this was significantly higher than that for group B (220 N-mm/degree; P = 0.003). There was no statistical difference in torsional stiffness between groups A and C in external or internal rotation. Bending stiffness outcomes for anterior-, lateral-, medial-, and posterior-directed bending loads are shown in Figure 5. There was little difference between groups in values for resistance to bending under an anterior-, Volume 36 135
J. Brantley, A. Majumdar, J. T. Jobe, A. Kallur, C. Salas lateral-, or posterior-directed load. Under a medialdirected load, however, group B and C specimens were significantly stiffer than those in group A (28 N/mm and 24 N/mm vs. 14 N/mm for A; P = 0.001 and P = 0.009, respectively). DISCUSSION We conducted a biomechanical study to test the resistance to torsion and bending of three different pin configurations used to treat pediatric distal tibia fractures. We found that of three pin configurations studied, the group A configuration (parallel, retrograde, with pins entering medially through the medial malleolus and ending laterally, proximal to the fracture) provided the highest resistance to external rotation, but was A statistically different from group B C. Resistance to not bending was similar in group A to that of the other two configurations, except in the medial direction. Torsional stiffness in this orientation is clinically important because external rotation of the foot, possibly leading to rotational deformity, may occur in patients during immobilization.19 In addition, group A configuration may be easier and more safe to use than the other two configurations because penetration of muscles and other key structures may be avoided. Although cross pin configurations have been shown to provide more stability in clinical20,21 and biomechanical22,23 studies involving supracondylar fractures of the humerus, biomechanical24 and clinical25,26 investigations have suggested that cross-pinning does not produce improved results in distal tibia fractures. We found that the cross-pin configuration (group C) is only slightly more stiff in internal rotation than group A. This study has several limitations. Pediatric cadaver tibias are difficult to obtain thus requiring the need for a suitable synthetic model. The fourth-generation synthetic composites used have been found to accurately replicate the torsional, axial compressive, and bending characteristics of natural bone.27-30 Additionally, we tested only an isolated tibia fracture; thus, the contribution to bending stiffness and torsional stability of an intact fibula was not included in our assessment. In a previous study, Thambayah and Pereira assessed the role of the fibula in the torsional stability of the lower extremity and found that a fractured fibula resulted in a 5% loss in torsional stiffness and a completely removed fibula resulted in an 11% loss in stiffness.31 Thus, although the fibula provides some torsional stability, the tibia alone is still responsible for 90% of the rotational stiffness of the lower limb. This said, the presence of the fibula (both fractured and intact) should be considered in future work, since an intact fibula can present difficulty in successful reduction of tibial fractures and have been shown to induce varus deformity if remaining intact.6,32 In this study, a
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transverse fracture at the diaphyseal-metaphyseal junction was chosen as a model for evaluating the different configurations. Although a slightly less common fracture pattern among tibia fractures, in our experience this particular pattern can be more unstable and challenging to treat with a cast alone, often requiring supplemental percutaneous pinning to maintain reduction of displaced fractures. Thus, this fracture type was chosen as the mode of failure in which to test the different percutaneous pinning techniques. Furthermore, in order to standardize the methodology, holes were predrilled in the femurs prior to pin placement to prevent deviations in pin trajectory. This is not clinically feasible and may influence stiffness outcomes. Finally, because the bending and torsional tests were conducted on the same C specimen, weakening of the models may have occurred. However, because all stiffness testing was maintained within the elastic range of the specimens, it is unlikely that any permanent deformation occurred. In conclusion, this study demonstrated that two, parallel, retrograde, medial to lateral directed pins provide improved stability to a simple, transverse distal tibia fracture when exposed to external rotation forces. External rotation is the most common stress experienced during short-leg casting of the tibia due to the weight of the cast and tendency for external rotation of the foot. Crossed pinning is the most common configuration currently used but when doing so it is difficult to place the lateral pin and does interfere with placing the ankle in neutral position. Our study has shown adequate stability provided by the parallel retrograde pins initiating at the medial malleolus. It is less technically demanding, requires fewer intraoperative radiographs, and minimizes the risk of iatrogenic injury to surrounding structures. SOURCE OF FUNDING This study was funded through departmental research funds. No extramural financial support was received for this study. REFERENCES 1. Galano GJ, Vitale MA, Kessler MW, Hyman JE, Vitale MG. The most frequent traumatic orthopedic injuries from a national pediatric inpatient population. J Pediatr Orthop. 2005;25:39-44. 2. Pandya NK, Edmonds EW, Mubarak SJ. The incidence of compartment syndrome after flexible nailing of pediatric tibial shaft fractures. J Child Orthop. 2011;5:439-47. 3. Flynn JM, Skaggs DL, Sponseller PD, Ganley TJ, Kay RM, Leitch KK. The surgical management of pediatric fractures of the lower extremity. Instr Course Lect. 2003;52:647-59.
A Biomechanical Comparison of Pin Configurations 4. Cheng JC, Shen WY. Limb fracture pattern in different pediatric age groups: a study of 3,350 children. J Orthop Trauma. 1993;7:15-22. 5. Setter KJ, Palomino KE. Pediatric tibia fractures: current concepts. Curr Opin Pediatr. 2006;18:30-5. 6. Yang JP, Letts RM. Isolated fractures of the tibia with intact fibula in children: a review of 95 patients. J Pediatr Orthop. 1997;17:347-51. 7. Heinrich SD. Fractures of the shaft of the tibia and fibula. Fractures in children, Vol. 3. Fourth edition. 1996:1331-75. 8. Joveniaux P, Ohl X, Harisboure A, Berrichi A, Labatut L, Simon P, et al. Distal tibia fractures: management and complications of 101 cases. Int Orthop. 2010;34:583-8. 9. Soulier R, Fallat L. Irreducible Salter Harris type II distal tibial physeal fracture secondary to interposition of the posterior tibial tendon: a case report. J Foot Ankle Surg. 2010;49:399.e395-9. 10. Song KM, Sangeorzan B, Benirschke S, Browne R. Open fractures of the tibia in children. J Pediatr Orthop. 1996;16:635-9. 11. Kubiak EN, Egol KA, Scher D, Wasserman B, Feldman D, Koval KJ. Operative treatment of tibial fractures in children: are elastic stable intramedullary nails an improvement over external fixation? J Bone Joint Surg Am. 2005;87:1761-8. 12. Cullen MC, Roy DR, Crawford AH, Assenmacher J, Levy MS, Wen D. Open fracture of the tibia in children. J Bone Joint Surg Am. 1996;78:1039-47. 13. Ligier JN, Metaizeau JP, Prevot J, Lascombes P. Elastic stable intramedullary pinning of long bone shaft fractures in children. Z Kinderchir. 1985;40:20912. 14. Ligier JN, Metaizeau JP, Prevot J, Lascombes P. Elastic stable intramedullary nailing of femoral shaft fractures in children. J Bone Joint Surg Br. 1988;70:747. 15. Qidwai SA. Intramedullary Kirschner wiring for tibia fractures in children. J Pediatr Orthop. 2001;21:294-7. 16. Till H, Huttl B, Knorr P, Dietz HG. Elastic stable intramedullary nailing (ESIN) provides good longterm results in pediatric long-bone fractures. Eur J Pediatr Surg. 2000;10:319-22. 17. Lascombes P, Prevot J, Ligier JN, Metaizeau JP, Poncelet T. Elastic stable intramedullary nailing in forearm shaft fractures in children: 85 cases. J Pediatr Orthop. 1990;10:167-71. 18. Tosti R, Foroohar A, Pizzutillo PD, Herman MJ. Kirschner wire infections in pediatric orthopedic surgery. J Pediatr Orthop. 2015;35:69-73.
19. Phan VC, Wroten E, Yngve DA. Foot progression angle after distal tibial physeal fractures. J Pediatr Orthop. 2002;22:31-5. 20. Nacht JL, Ecker ML, Chung SM, Lotke PA, Das M. Supracondylar fractures of the humerus in children treated by closed reduction and percutaneous pinning. Clin Orthop Relat Res. 1983:203-9. 21. Weiland AJ, Meyer S, Tolo VT, Berg HL, Mueller J. Surgical treatment of displaced supracondylar fractures of the humerus in children. Analysis of fifty-two cases followed for five to fifteen years. J Bone Joint Surg Am. 1978;60:657-61. 22. Larson L, Firoozbakhsh K, Passarelli R, Bosch P. Biomechanical analysis of pinning techniques for pediatric supracondylar humerus fractures. J Pediatr Orthop. 2006;26:573-8. 23. Zionts LE, McKellop HA, Hathaway R. Torsional strength of pin configurations used to fix supracondylar fractures of the humerus in children.J Bone Joint Surg Am. 1994;76:253-6. 24. Lee SS, Mahar AT, Miesen D, Newton PO. Displaced pediatric supracondylar humerus fractures: biomechanical analysis of percutaneous pinning techniques. J Pediatr Orthop. 2002;22:440-3. 25. Solak S, Aydin E. Comparison of two percutaneous pinning methods for the treatment of the pediatric type III supracondylar humerus fractures. J Pediatr Orthop B. 2003;12:346-9. 26. Topping RE, Blanco JS, Davis TJ. Clinical evaluation of crossed-pin versus lateral-pin fixation in displaced supracondylar humerus fractures. J Pediatr Orthop. 1995;15:435-9. 27. Chong AC, Miller F, Buxton M, Friis EA. Fracture toughness and fatigue crack propagation rate of short fiber reinforced epoxy composites for analogue cortical bone. J Biomech Eng. 2007;129:487-93. 28. Cristofolini L, Viceconti M. Mechanical validation of whole bone composite tibia models. J Biomech. 2000;33:279-88. 29. Gardner MP, Chong AC, Pollock AG, Wooley PH. Mechanical evaluation of large-size fourth-generation composite femur and tibia models. Ann Biomed Eng. 2010;38:613-20. 30. Heiner AD. Structural properties of fourth-generation composite femurs and tibias. J Biomech. 2008;41:3282-4. 31. Thambyah A, Pereira BP. Mechanical contribution of the fibula to torsion stiffness in the lower extremity. Clin Anat. 2006;19:615-20. 32. Gordon JE, O’Donnell JC. Tibia fractures: what should be fixed? J Pediatr Orthop. 2012;32 Suppl 1:S52-61.
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ABSTRACT Background: Percutaneous pin fixation is often used in conjunction with closed-reduction and cast immobilization to treat pediatric distal tibia fractures. The goal of this procedure is to maintain reduction and provide improved stabilization, in effort to facilitate a more anatomic union. We conducted a biomechanical study of the torsional and bending stability of three commonly used pin configurations in distal tibia fracture fixation. Methods: A transverse fracture was simulated at the metaphyseal/diaphyseal junction in 15 synthetic tibias. Each fracture was reduced and fixed with two Kirschner wires, arranged in one of three pin configurations: parallel, retrograde, medial to lateral pins entering at the medial malleolus distal to the fracture (group A); parallel, antegrade, medial to lateral pins entering at the medial diaphysis proximal to the fracture (group B); or a cross-pin configuration with one retrograde, medial to lateral pin entering the medial malleolus distal to the fracture and the second an antegrade, medial to lateral pin entering at the medial diaphysis proximal to the fracture (group C). Stability of each construct was assessed by resistance to torsion and bending. Results: Resistance to external rotation stress was significantly higher in group A than group B (P = 0.044). Resistance to internal rotation stress was significantly higher in group C than group B (P = 0.003). There was no significant difference in torsional stiffness when comparing group A with group C. Under a medial-directed load, group B and C specimens were significantly stiffer
Department of Orthopaedics & Rehabilitation Center for Biomedical Engineering 3 Department of Mechanical Engineering, University of New Mexico, Albuquerque, NM Corresponding Author: Christina Salas, PhD Department of Orthopaedics & Rehabilitation, University of New Mexico MSC 10 5600, 1 University of New Mexico, Albuquerque, NM 87131 telephone: 505.272.5671; fax: 505.272-8098. E-mail: [email protected]. The authors declare no relevant conflict of interest 1 2
than those in group A (28 N/mm and 24 N/mm vs. 14 N/mm for A; P = 0.001 and P = 0.009, respectively) Conclusions: None of the three pin configurations produced superior results with respect to all variables studied. Group A configuration provided the highest resistance to external rotation forces, which is the most clinically relevant variable under short-cast immobilization. Parallel, retrograde, medial to lateral pins entering at the medial malleolus provide the greatest resistance to external rotation of the foot while minimizing the potential for iatrogenic injur y to soft tissue structures. INTRODUCTION Tibia fractures are the most common pediatric lower extremity fracture1,2 and the second most common fracture resulting in hospitalization.3 Most pediatric tibia fractures are isolated occurrences resulting from lowenergy, indirect torsional stress.4-6 About 50% of these fractures occur in the distal third of the tibia.5,7 Pediatric distal third tibia fractures have traditionally been described as complex injuries that can be challenging to manage because of articular involvement, proximity to the fibula, and open physes.8 For unstable fractures of long bones, intramedullar y fixation is a common surgical intervention as this permits earlier mobilization and facilitates optimal alignment.3,5 In pediatric distal tibia fractures, however, locked intramedullary rods are typically avoided because of the risk of physeal injury.9-11 External fixation was formerly the preferred procedure for treating distal tibial fractures, especially severely unstable fractures with soft tissue involvement or comminution.8,12 The use of external fixation has declined, however, because of recent reports of complications with the technique, including infection, skin necrosis, delayed union, limb overgrowth, and a need for re-manipulation.12 Elastic Stable Intramedullary Nailing (ESIN) is now commonly used for fixation of pediatric long-bone fractures. This approach is minimally invasive, with percutaneous placement of the nails.11,13-16 However, ESIN has been associated with complications resulting from insufficient nail thickness, malreduction during implant placement, and inadequate stabilization of fractures.2,17 Supplementation of cast immobilization with percutaneously Volume 36 133
J. Brantley, A. Majumdar, J. T. Jobe, A. Kallur, C. Salas
FIGURE 1. Kirschner-wire pin configurations. Left: parallel, retrograde pins entering medially through the medial malleolus and ending laterally, 15 mm proximal to the fracture site (group A). Middle: parallel, antegrade pins entering medially, centered 15 mm proximal to the fracture site, and ending at the fibular notch (group B). Right: cross-pin configuration with one retrograde pin entering through the medial malleolus and the second pin entering medially, 15 mm proximal to the fracture site and ending at the fibular notch (group C).
placed Kirschner wires across the fracture site has been reported to add adequate stabilization of unstable tibial fractures in children.5 Maintenance of alignment of the tibial shaft using this approach allows early mobilization and callus formation while minimizing infection risks.3,10,18 Percutaneous pinning is most appropriate for fractures that can be reduced by closed manipulation, such as simple, transverse fracture patterns.8 However, placement of percutaneous pins in the distal tibia can be difficult as a result of the need to avoid the tibiofibular and tibiotalar joints, muscular and tendinous structures, and other neurovascular structures. The ideal percutaneous pinning technique would be relatively simple and achieve maximal stabilization without injuring surrounding structures. Stability and union can be affected by torsional and bending stresses, especially those resulting from external rotation of the leg during immobilization. In the present biomechanical study we compared three pin configurations used to enhance stabilization of transverse fractures at the diaphyseal-metaphyseal junction in the distal tibia. We aimed to identify a technique that allows for enhanced structural stability with limited potential for injury of the surrounding soft-tissue. METHODS Fifteen fourth-generation synthetic composite tibiae (Pacific Research Laboratories, Vashon Island, WA) were used in this study. Each specimen was predrilled with an undersized hole (2.38 mm; 3/32-inch bit) to prevent deviations in pin trajectories. A transverse cut was then made 25 mm from the distal articular surface to simulate a simple transverse fracture of the distal tibia. Cutting and drilling were done with a custom jig to maintain consistent fracture locations and pin trajectories among 134 The Iowa Orthopedic Journal
FIGURE 2. Torsional testing construct for applying a fixed support at the distal end and a torsional moment at the proximal end.
the specimens in each group. Each fracture was reduced and fixed with two smooth 3.5 mm Kirschner wires placed in one of three pin configurations (Figure 1): (A), parallel, retrograde, medial to lateral pins entering at the medial malleolus distal to the fracture and ending laterally, 15 mm proximal to the fracture (group A; n=5), (B), parallel, antegrade, medial to lateral pins entering at the medial diaphysis centered 15 mm proximal to the fracture, and ending at the fibular notch (group B; n=5); or (C), a cross-pin configuration with one retrograde, medial to lateral pin entering the medial malleolus distal to the fracture and the second an antegrade, medial to lateral pin entering at the medial diaphysis 15 mm proximal to the fracture site and ending at the fibular notch (group C; n = 5). Biomechanical testing was performed using an MTS 858 Mini Bionix II servohydraulic machine (MTS Systems, Eden Prairie, MN). The specimens were loaded sequentially for six tests: external rotation, internal rotation, anterior-directed bending (fracture apex pos-
A Biomechanical Comparison of Pin Configurations
FIGURE 3. Bending construct for applying a fixed support at the proximal end with a roller support 10 cm proximal to the fracture site and a force through the distal fragment.
FIGURE 5. Bending stiffness results for the 3 pin configurations in 4 load directions. Error bars represent one standard deviation. Groups B and C had significantly more stiffness than Group A in the medial direction.
fracture line to concentrate the load directly through the distal fragment (Figure 3). The distal fragment was loaded to a maximum displacement of 4 mm at a rate of 0.5 mm/sec and unloaded. Each specimen was loaded sequentially three times, and then rotated 90 degrees, for each of the four bending directions. The mean value from the results of the three tests was used to compare the bending stiffness of the various pin configurations. FIGURE 4. Torsional stiffness results for the 3 pin configurations in internal and external rotation. Error bars represent one standard deviation. In external rotation, the difference between Groups A and B was significant. In internal rotation, the difference between Groups B and C was significant.
terior), lateral-directed bending (fracture apex medial), A posterior-directed bending (fracture apex anterior), and medial-directed bending (fracture apex lateral). For torsional loading, the tibia was rigidly attached to the actuator proximally and to an axial/torsional load cell distally (Figure 2). Each specimen was preloaded with 50 N in axial compression. Each specimen was then loaded from 0 to 5 N-mm in internal rotation at a rate of 0.5 N-mm/s, unloaded to zero, loaded to 5 N-mm in external rotation at a rate of 0.5 N-mm/s, and again unloaded to zero. This was repeated three times, and the stiffness was calculated from the linear region on the torque-rotation curve. The mean value of the three tests was used for comparisons between groups. In each of the bending tests, the tibia was oriented perpendicular to the actuator in a simply supported cantilever configuration. The proximal end was fixed, and a roller support was placed 10 cm proximal to the
Statistical Analysis One-way analysis of variance (ANOVA) was used to compare each of the 6 tests for each of the 3 pin configurations. The Tukey post-hoc test was used to determine which groups differed significantly from others. A P value of 0.05 was considered to represent a significant difference. RESULTS Torsional stiffness outcomes for external and internal rotation loads are shown in Figure 4. Group A had the highest mean value for torsional stiffness in external rotation (530 N-mm/degree).This was significantly higher than that for group B (330 N-mm/degree; P = 0.044). Group C had the highest mean value for torsional stiffness in internal rotation (400 N-mm/degree), and this was significantly higher than that for group B (220 N-mm/degree; P = 0.003). There was no statistical difference in torsional stiffness between groups A and C in external or internal rotation. Bending stiffness outcomes for anterior-, lateral-, medial-, and posterior-directed bending loads are shown in Figure 5. There was little difference between groups in values for resistance to bending under an anterior-, Volume 36 135
J. Brantley, A. Majumdar, J. T. Jobe, A. Kallur, C. Salas lateral-, or posterior-directed load. Under a medialdirected load, however, group B and C specimens were significantly stiffer than those in group A (28 N/mm and 24 N/mm vs. 14 N/mm for A; P = 0.001 and P = 0.009, respectively). DISCUSSION We conducted a biomechanical study to test the resistance to torsion and bending of three different pin configurations used to treat pediatric distal tibia fractures. We found that of three pin configurations studied, the group A configuration (parallel, retrograde, with pins entering medially through the medial malleolus and ending laterally, proximal to the fracture) provided the highest resistance to external rotation, but was A statistically different from group B C. Resistance to not bending was similar in group A to that of the other two configurations, except in the medial direction. Torsional stiffness in this orientation is clinically important because external rotation of the foot, possibly leading to rotational deformity, may occur in patients during immobilization.19 In addition, group A configuration may be easier and more safe to use than the other two configurations because penetration of muscles and other key structures may be avoided. Although cross pin configurations have been shown to provide more stability in clinical20,21 and biomechanical22,23 studies involving supracondylar fractures of the humerus, biomechanical24 and clinical25,26 investigations have suggested that cross-pinning does not produce improved results in distal tibia fractures. We found that the cross-pin configuration (group C) is only slightly more stiff in internal rotation than group A. This study has several limitations. Pediatric cadaver tibias are difficult to obtain thus requiring the need for a suitable synthetic model. The fourth-generation synthetic composites used have been found to accurately replicate the torsional, axial compressive, and bending characteristics of natural bone.27-30 Additionally, we tested only an isolated tibia fracture; thus, the contribution to bending stiffness and torsional stability of an intact fibula was not included in our assessment. In a previous study, Thambayah and Pereira assessed the role of the fibula in the torsional stability of the lower extremity and found that a fractured fibula resulted in a 5% loss in torsional stiffness and a completely removed fibula resulted in an 11% loss in stiffness.31 Thus, although the fibula provides some torsional stability, the tibia alone is still responsible for 90% of the rotational stiffness of the lower limb. This said, the presence of the fibula (both fractured and intact) should be considered in future work, since an intact fibula can present difficulty in successful reduction of tibial fractures and have been shown to induce varus deformity if remaining intact.6,32 In this study, a
136 The Iowa Orthopedic Journal
transverse fracture at the diaphyseal-metaphyseal junction was chosen as a model for evaluating the different configurations. Although a slightly less common fracture pattern among tibia fractures, in our experience this particular pattern can be more unstable and challenging to treat with a cast alone, often requiring supplemental percutaneous pinning to maintain reduction of displaced fractures. Thus, this fracture type was chosen as the mode of failure in which to test the different percutaneous pinning techniques. Furthermore, in order to standardize the methodology, holes were predrilled in the femurs prior to pin placement to prevent deviations in pin trajectory. This is not clinically feasible and may influence stiffness outcomes. Finally, because the bending and torsional tests were conducted on the same C specimen, weakening of the models may have occurred. However, because all stiffness testing was maintained within the elastic range of the specimens, it is unlikely that any permanent deformation occurred. In conclusion, this study demonstrated that two, parallel, retrograde, medial to lateral directed pins provide improved stability to a simple, transverse distal tibia fracture when exposed to external rotation forces. External rotation is the most common stress experienced during short-leg casting of the tibia due to the weight of the cast and tendency for external rotation of the foot. Crossed pinning is the most common configuration currently used but when doing so it is difficult to place the lateral pin and does interfere with placing the ankle in neutral position. Our study has shown adequate stability provided by the parallel retrograde pins initiating at the medial malleolus. It is less technically demanding, requires fewer intraoperative radiographs, and minimizes the risk of iatrogenic injury to surrounding structures. SOURCE OF FUNDING This study was funded through departmental research funds. No extramural financial support was received for this study. REFERENCES 1. Galano GJ, Vitale MA, Kessler MW, Hyman JE, Vitale MG. The most frequent traumatic orthopedic injuries from a national pediatric inpatient population. J Pediatr Orthop. 2005;25:39-44. 2. Pandya NK, Edmonds EW, Mubarak SJ. The incidence of compartment syndrome after flexible nailing of pediatric tibial shaft fractures. J Child Orthop. 2011;5:439-47. 3. Flynn JM, Skaggs DL, Sponseller PD, Ganley TJ, Kay RM, Leitch KK. The surgical management of pediatric fractures of the lower extremity. Instr Course Lect. 2003;52:647-59.
A Biomechanical Comparison of Pin Configurations 4. Cheng JC, Shen WY. Limb fracture pattern in different pediatric age groups: a study of 3,350 children. J Orthop Trauma. 1993;7:15-22. 5. Setter KJ, Palomino KE. Pediatric tibia fractures: current concepts. Curr Opin Pediatr. 2006;18:30-5. 6. Yang JP, Letts RM. Isolated fractures of the tibia with intact fibula in children: a review of 95 patients. J Pediatr Orthop. 1997;17:347-51. 7. Heinrich SD. Fractures of the shaft of the tibia and fibula. Fractures in children, Vol. 3. Fourth edition. 1996:1331-75. 8. Joveniaux P, Ohl X, Harisboure A, Berrichi A, Labatut L, Simon P, et al. Distal tibia fractures: management and complications of 101 cases. Int Orthop. 2010;34:583-8. 9. Soulier R, Fallat L. Irreducible Salter Harris type II distal tibial physeal fracture secondary to interposition of the posterior tibial tendon: a case report. J Foot Ankle Surg. 2010;49:399.e395-9. 10. Song KM, Sangeorzan B, Benirschke S, Browne R. Open fractures of the tibia in children. J Pediatr Orthop. 1996;16:635-9. 11. Kubiak EN, Egol KA, Scher D, Wasserman B, Feldman D, Koval KJ. Operative treatment of tibial fractures in children: are elastic stable intramedullary nails an improvement over external fixation? J Bone Joint Surg Am. 2005;87:1761-8. 12. Cullen MC, Roy DR, Crawford AH, Assenmacher J, Levy MS, Wen D. Open fracture of the tibia in children. J Bone Joint Surg Am. 1996;78:1039-47. 13. Ligier JN, Metaizeau JP, Prevot J, Lascombes P. Elastic stable intramedullary pinning of long bone shaft fractures in children. Z Kinderchir. 1985;40:20912. 14. Ligier JN, Metaizeau JP, Prevot J, Lascombes P. Elastic stable intramedullary nailing of femoral shaft fractures in children. J Bone Joint Surg Br. 1988;70:747. 15. Qidwai SA. Intramedullary Kirschner wiring for tibia fractures in children. J Pediatr Orthop. 2001;21:294-7. 16. Till H, Huttl B, Knorr P, Dietz HG. Elastic stable intramedullary nailing (ESIN) provides good longterm results in pediatric long-bone fractures. Eur J Pediatr Surg. 2000;10:319-22. 17. Lascombes P, Prevot J, Ligier JN, Metaizeau JP, Poncelet T. Elastic stable intramedullary nailing in forearm shaft fractures in children: 85 cases. J Pediatr Orthop. 1990;10:167-71. 18. Tosti R, Foroohar A, Pizzutillo PD, Herman MJ. Kirschner wire infections in pediatric orthopedic surgery. J Pediatr Orthop. 2015;35:69-73.
19. Phan VC, Wroten E, Yngve DA. Foot progression angle after distal tibial physeal fractures. J Pediatr Orthop. 2002;22:31-5. 20. Nacht JL, Ecker ML, Chung SM, Lotke PA, Das M. Supracondylar fractures of the humerus in children treated by closed reduction and percutaneous pinning. Clin Orthop Relat Res. 1983:203-9. 21. Weiland AJ, Meyer S, Tolo VT, Berg HL, Mueller J. Surgical treatment of displaced supracondylar fractures of the humerus in children. Analysis of fifty-two cases followed for five to fifteen years. J Bone Joint Surg Am. 1978;60:657-61. 22. Larson L, Firoozbakhsh K, Passarelli R, Bosch P. Biomechanical analysis of pinning techniques for pediatric supracondylar humerus fractures. J Pediatr Orthop. 2006;26:573-8. 23. Zionts LE, McKellop HA, Hathaway R. Torsional strength of pin configurations used to fix supracondylar fractures of the humerus in children.J Bone Joint Surg Am. 1994;76:253-6. 24. Lee SS, Mahar AT, Miesen D, Newton PO. Displaced pediatric supracondylar humerus fractures: biomechanical analysis of percutaneous pinning techniques. J Pediatr Orthop. 2002;22:440-3. 25. Solak S, Aydin E. Comparison of two percutaneous pinning methods for the treatment of the pediatric type III supracondylar humerus fractures. J Pediatr Orthop B. 2003;12:346-9. 26. Topping RE, Blanco JS, Davis TJ. Clinical evaluation of crossed-pin versus lateral-pin fixation in displaced supracondylar humerus fractures. J Pediatr Orthop. 1995;15:435-9. 27. Chong AC, Miller F, Buxton M, Friis EA. Fracture toughness and fatigue crack propagation rate of short fiber reinforced epoxy composites for analogue cortical bone. J Biomech Eng. 2007;129:487-93. 28. Cristofolini L, Viceconti M. Mechanical validation of whole bone composite tibia models. J Biomech. 2000;33:279-88. 29. Gardner MP, Chong AC, Pollock AG, Wooley PH. Mechanical evaluation of large-size fourth-generation composite femur and tibia models. Ann Biomed Eng. 2010;38:613-20. 30. Heiner AD. Structural properties of fourth-generation composite femurs and tibias. J Biomech. 2008;41:3282-4. 31. Thambyah A, Pereira BP. Mechanical contribution of the fibula to torsion stiffness in the lower extremity. Clin Anat. 2006;19:615-20. 32. Gordon JE, O’Donnell JC. Tibia fractures: what should be fixed? J Pediatr Orthop. 2012;32 Suppl 1:S52-61.
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