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FiberWire® is superior in strength to stainless steel wire for tension band fixation of transverse patellar fractures
Injury, Int. J. Care Injured 40 (2009) 1200–1203
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
FiberWire1 is superior in strength to stainless steel wire for tension band fixation of transverse patellar fractures P.B. Wright a,b,*, V. Kosmopoulos a,b, R.E. Cote´ c, T.J. Tayag c, A.D. Nana a,b a
Bone and Joint Research Center, Department of Orthopaedic Surgery, University of North Texas Health Science Center, 3500 Camp Bowie Boulevard, Fort Worth, TX 76107, USA Department of Orthopaedic Surgery, John Peter Smith Hospital, 1500 South Main Street, Fort Worth, TX 76104, USA c Department of Engineering, College of Science and Engineering, Texas Christian University, 2800 South University Drive, Fort Worth, TX 76129, USA b
A R T I C L E I N F O
A B S T R A C T
Article history: Accepted 28 April 2009
Background: The metal implants used to achieve fixation of displaced transverse patellar fractures are associated with implant failure, postoperative pain and a significant re-operation rate. Recent studies have examined braided suture as a possible alternative to stainless steel wire to increase patient satisfaction and decrease re-operation rates, but suture has not demonstrated clearly superior fixation strength. FiberWire1 is a reinforced braided polyblend suture that has demonstrated superior characteristics to the previous sutures studied and has not to our knowledge been examined as a material for tension band fixation of transverse patellar fractures. Methods: Materials testing was performed on repeated samples of No. 5 FiberWire suture and 18-gauge stainless steel wire. The strength and stiffness of each material was measured. The two materials were then used for tension band fixation on a novel transverse patellar fracture model and tested to failure by three-point bending. The constructs included a single stainless steel wire, a single-strand FiberWire tied with a sliding knot, double-strand FiberWire tied with sliding knots and double-strand FiberWire tied with a Wagoner’s Hitch. The fixation strength and stiffness of the constructs were measured. Findings: Unlike stainless steel, FiberWire maintained its initial stiffness until failure. Furthermore, during three-point-bend testing, double-strand FiberWire was found to have a significantly higher failure load than stainless steel wire when the suture was tied and locked under the tension produced by a modified Wagoner’s Hitch. Interpretation: FiberWire is a potentially superior alternative to stainless steel wire in tension band fixation of transverse patellar fractures. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: Patellar fracture Tension band Suture fixation FiberWire Injury
Introduction Internal fixation using metal implants in configurations based on the tension band principle remains the mainstay of treatment for operative transverse patellar fractures.14 However, metal implants are associated with complications in 18–50% of patients.2,6,13,16 Hardware may be prominent causing irritation, inhibition of motion and possible sinus formation and, over time, wires may fragment and migrate. Recent investigations2,4,5,11,12 have examined heavy suture as a possible alternative to conventional metal implants. Heavy suture is easier to place accurately in the soft tissues, conforms more readily to bony structures, is less likely to fragment over time and appears to be associated with greater patient
* Corresponding author at: Department of Orthopaedic Surgery, John Peter Smith Hospital, 1500 South Main Street, Fort Worth, TX 76104, USA. Tel.: +1 817 9271370; fax: +1 817 9273955. E-mail addresses: [email protected], [email protected] (P.B. Wright). 0020–1383/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.injury.2009.04.011
satisfaction and decreased re-operation rates. Should it be determined, therefore, that suture is at least as effective as stainless steel wire in the maintenance of a tension band under deforming forces, the clinical use of the suture is justified to decrease complication rates and increase patient satisfaction. The No. 5 Ethibond (Ethicon, Somerville, NJ, USA) and No. 5 TiCron (Davis and Geck, Gosport, Hampshire, UK) braided polyester sutures have been studied in patellar fracture models for this purpose. To our knowledge, FiberWire1 (Arthrex, Naples, FL, USA) suture has not been studied as a possible substrate for tension band fixation of patellar fractures. FiberWire is a Food and Drug Administration (FDA)-approved, biocompatible, braided polyblend suture consisting of two polyester strings and a polyethylene string. It has demonstrated superior strength compared to braided polyester sutures in experimental studies.7,15 The present study aims (1) to evaluate differences in stiffness and failure strength between No. 5 FiberWire, with and without a surgical knot, and 18gauge stainless steel wire, with and without a compression twist; and (2) to evaluate the effectiveness of FiberWire as a tension band construct using a novel three-point-bend model.
P.B. Wright et al. / Injury, Int. J. Care Injured 40 (2009) 1200–1203
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Fig. 1. Schematics of the stainless steel three-point-bend model.
Materials and methods Repeated samples of No. 5 FiberWire suture and 18-gauge stainless steel wire were tensile tested to failure under displacement control at a rate of 5 mm/min using a materials testing system (Instron, Norwood, MA, USA). To simulate the conditions after uniting opposite ends of the material to secure a tension band construct, tensile tests were then performed with a knot in the centre of a set of new FiberWire samples and a twist in the centre of a set of new 18-gauge stainless steel wires. Specifically, the opposite ends of the FiberWire were tied with a sliding knot followed by three square knots, and the opposite ends of the stainless steel wire were secured with seven visually confirmed symmetric twists. Stiffness and failure strength were calculated for each sample in both of these protocols. Stiffness was calculated as the slope of a best-fit line for the linear portion of the force– deformation curve. Failure strength was determined to be the highest load tolerated by the material. The aforementioned preliminary series of tests, to address the first aim, were performed simply to characterise the suture materials and fastening methods in isolation. After these preliminary tests, the two materials were used for tension band fixation of a novel three-point-bend model (Fig. 1). The three-point-bend model was machined from stainless steel with an anterior convex surface to mimic a symmetrical patella and flanges superior and inferior to permit placement in a threepoint-bend jig (Fig. 2). To reproduce the Lo¨tke figure-of-eight anterior tension band technique, two parallel longitudinal tunnels were drilled in the model to allow the passage of the FiberWire or stainless steel wire.9 After the suture or wire was secured, the model was loaded in three-point bending at a rate of 5 mm/min using the aforementioned material testing machine (see Fig. 2). The three-point-bending configuration stretches the tension band fixation leading to an anterior fracture gap. An anterior fracture gap of 3 mm was defined as construct failure14 and was calculated from a start point of zero using the following equation: displacement ½mm fracture gap ½mm ¼ 32 sin tan1 25:4
The equation was derived based on the geometry of the threepoint-bend model. The gap was also confirmed by the use of a micrometre at the end of each trial. Failure strength for the construct was thus identified at this critical fracture gap. The four constructs tested were: (A) single stainless steel wire with two (bilateral) compression twists; (B) single-strand FiberWire tied with a Mu¨ller sliding knot10; (C) double-strand FiberWire tied with individual sliding knots; and (D) double-strand FiberWire tied with a modified Wagoner’s Hitch5 (Fig. 3). Based on the first six repeated samples, power analyses were performed to calculate the needed sample size. To compare and identify significant differences between the four different tension band constructs, a one-way balanced analysis of variance (ANOVA) was employed. To specifically identify which pairs of means (i.e. which of the four conditions) were significantly different, if any,
Fig. 2. The patella model in the three-point-bending protocol. In three-point bending, two outer points of the model are supported (shown with an ‘*’), and as the actuator from the testing machine displaces downwards at a central point (noted with an ‘’), the suture begins to stretch and a fracture gap starts to develop at the most anterior point (noted with a ‘#’).
P.B. Wright et al. / Injury, Int. J. Care Injured 40 (2009) 1200–1203
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Fig. 3. The four anterior tension band constructs tested included (A) a single stainless steel wire with two compression twists, (B) a single-strand FiberWire tied with a sliding knot, (C) a double-strand FiberWire tied with individual sliding knots, and (D) a double-strand FiberWire tied with a modified Wagoner’s Hitch.
Table 1 The average stiffness and maximum tensile force (Fmax) of the materials tested.
Stainless steel wire FiberWire Stainless steel with twist FiberWire with knot
Stiffness (N/mm)
Fmax (N)
157.6 8.4 56.6 5.7
494.8 541.5 180.9 255.8
Tukey’s honestly significant difference (hsd) multiple comparison test was used (a = 5%). Results The power analyses revealed that a maximum of nine repeated samples were necessary to maintain a power of 0.80 at a 5% significance level. The tension tests resulted in relatively small differences in failure strength (9%) and large differences in stiffness between the stainless steel and FiberWire (180%) (Table 1). The introduction of the sliding knot in the tensile tests of the FiberWire resulted in a 32% and 53% decrease in stiffness and strength, respectively. The twist in the stainless steel wire resulted in a 64% and 63% reduction in stiffness and strength, respectively. The average load needed for construct failure in the threepoint-bending protocol for each construct is shown in Fig. 4. A one-
Fig. 4. Boxplot showing the clear advantage in failure strength of the double-strand FiberWire tied with a modified Wagoner’s Hitch (D) as compared to the other threepoint-bending constructs (A–C). The central mark is the median, the edges of the box are the 25th and 75th percentiles, the whiskers extend to the most extreme data points considered not to be outliers, and the outliers are plotted individually using a +. Notation: stainless steel (A); FiberWire single strand (B); FiberWire double strand (C); and FiberWire double strand with Wagoner’s Hitch (D).
way ANOVA confirmed that differences in mean failure strength between the tested conditions were significant (p < 0.0001). Results from the hsd multiple comparison test showed significant differences in failure strength between the stainless steel wire (636.0 N) and the single-strand FiberWire with a sliding knot (343.4 N, p < 0.01), and the stainless steel wire with the modified Wagoner’s Hitch FiberWire (1337.4 N, p < 0.01) constructs. No significant differences were found when the strength of the stainless steel wire was compared to the double-strand FiberWire with sliding knot construct (636.0 N vs. 580.7 N). Significant differences were also identified between the single- and doublestrand FiberWire with sliding knots (p < 0.01), and the singlestrand FiberWire and Wagoner’s Hitch FiberWire constructs (p < 0.01). Finally, significant differences were found between the double-strand FiberWire with sliding knots and Wagoner’s Hitch FiberWire constructs (p < 0.01). Discussion Patellar fractures are a common and significant cause of disability in modern orthopaedics. They constitute approximately 1% of all traumatic fractures14 and as many as 5.5% of all fractures in patients over 65 years of age.8 One-third to two-thirds will require surgical intervention1,3 and despite excellent union rates, a significant portion of postoperative morbidity can be attributed to the use of metal implants,2,6,13,16 including patient pain, disability and the need for another surgery to remove the hardware. In 2001, Gosal et al.4 specifically examined the re-operation rate in 37 patients randomised between the AO-modified tension band using metal wires and a Pyrford suture technique performed with a No. 5 Ethibond. They found a 38% hardware removal rate secondary to pain and hardware failure when metal implants were used, and reported no incidence of symptoms related to the use of heavy suture. The suture fixation group, however, demonstrated a 6% failure rate identifying that the material and method used could lead to fixation failure before conventional metallic implants. This is the likely reason why suture has not been routinely recommended for the osteosynthesis of patellar fractures. Previous experiments designed to test suture as a tension band for patellar fractures have been studied in cadaveric models with little standardisation as to size, shape and bone or soft-tissue quality. The reliability and reproducibility of the drilling of the parallel longitudinal tunnels, for example, and overall placement of implants, have also depended upon the researcher. We submit that, with this number of uncontrolled variables, the design of the tested implants and techniques may not have been the only variables affecting the outcome of the trials.
P.B. Wright et al. / Injury, Int. J. Care Injured 40 (2009) 1200–1203
Our study was designed to test FiberWire, a suture with superior strength15 and likely better properties to promote osteosynthesis than Ethibond, on a model that would eliminate the variables of previous studies and allow multiple trials to confirm statistical significance. Unfortunately, our study’s greatest strength is also a weakness with respect to extrapolating the results to the clinical setting. Without the use of cadaveric specimens, we are unable to determine how patella bone and surrounding soft tissue would respond to FiberWire, although this can be inferred from previous studies examining suture in cadaveric specimens.5,11,12 In addition, in vivo the patella is loaded not only in bending, but also in tension. We inferred the tension capabilities of FiberWire through materials testing and compared them to stainless steel (first aim of this study), but we did not specifically examine this phenomenon with our patella model. Instead, we chose an extreme loading protocol to better understand the tolerance and behaviour of the materials studied at their limits so that the results could be extrapolated to a clinical worstcase scenario. This study specifically evaluated the use of FiberWire in tension band fixation of a transverse patellar fracture using a novel threepoint-bend model. Mechanical tests were performed to compare FiberWire to a current standard of treatment employing stainless steel wire. Initial material testing demonstrated that at higher tensile forces (>250 N) the stainless wire was unable to maintain its superiority in stiffness when compared to the FiberWire. In contrast, the FiberWire, even beyond the maximum tensile force of the stainless wire, was able to maintain its initial stiffness until failure. In terms of the three-point-bending protocol, our results indicate that the double-strand FiberWire tied with sliding knots and the 18-gauge stainless steel wire with double twists required a similar load for construct failure. The load required for construct failure, however, was increased substantially when the FiberWire was tied with a modified Wagoner’s Hitch. This was likely due to the material behaviour of the FiberWire, and the mechanical advantage of the modified Wagoner’s Hitch, which allowed the surgeon to tie and lock the suture under greater tension than that produced by the sliding knot. Conclusions This study clearly demonstrates that a FiberWire tied with a modified Wagoner’s Hitch is superior in strength to stainless steel in the maintenance of a tension band under force. The clinical implications of this study are profound because we have identified a suture configuration that will be less likely to fail than that previously studied in vivo.4 In addition, our method is performed with a material that is less likely to cause postoperative pain and require removal than the metal implants routinely used in modern
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orthopaedic surgery. We therefore conclude that the in vivo study of FiberWire for the fixation of patellar fractures is justified and will likely result in greater patient satisfaction and decreased reoperation rates. Conflict of interest statement The authors do not have any financial or personal affiliations with other people or organisations that could inappropriately influence (bias) their work. Acknowledgements We thank Drs. Peter Raven, Robert Bunata and David Lichtman for their support of this research. The FiberWire used in our study was generously donated by Arthrex, Inc., Naples, FL, USA. References 1. Bostrom A. Fracture of the patella: a study of 422 patellar fractures. Acta Orthop Scand Suppl 1972;143:22–71. 2. Chatakondu SC, Abhaykumar S, Elliott DS. The use of non-absorbable suture in the fixation of patellar fractures: a preliminary report. Injury 1998;29:23–7. 3. Edwards B, Johnell O, Redlund-Johnell I. Patellar fractures: a 30-year follow-up. Acta Orthop Scand 1989;60:712–4. 4. Gosal HS, Singh P, Field RE. Clinical experience of patellar fracture fixation using metal wire or non-absorbable polyester—a study of 37 cases. Injury 2001;32(2): 129–35. 5. Hughes SC, Stott PM, Hearnden AJ, Ripley LG. A new and effective tension-band braided polyester suture technique for transverse patellar fracture fixation. Injury 2007;38(2):212–22. 6. Hung LK, Chan KM, Chow YN, Leung PC. Fractured patella: operative treatment using the tension band principle. Injury 1985;16:343–7. 7. Komatsu F, Mori R, Uchio Y. Optimum surgical suture material and methods to obtain high tensile strength at knots: problems of conventional knots and the reinforcement effect of adhesive agent. J Orthop Sci 2006;11(1):70–4. 8. Koo D, Adams DA, Copeland TM, et al. Incidence and costs to Medicare of fractures among Medicare beneficiaries aged greater than or equal to 65 years— United States, July 1991–June 1992. CDC Morb Mortal Wkly Rep 1996;45(41): 877–83. 9. Lo¨tke PA, Ecker ML. Transverse fractures of the patella. Clin Orthop Relat Res 1981;158:180–4. 10. Mast J, Jakob R, Ganz R. Planning and reduction technique in fracture surgery, 1st ed., New York: Springer-Verlag; 1989. p. 250. 11. McGreal G, Reidy D, Joy A, et al. The biomechanical evaluation of polyester as a tension band for the internal fixation of patellar fractures. J Med Eng Technol 1999;23(2):53–6. 12. Patel VR, Parks BG, Wang Y, et al. Fixation of patellar fractures with braided polyester suture: a biomechanical study. Injury 2000;31(1):1–6. 13. Smith ST, Cramer KE, Karges DE, et al. Early complications in the operative treatment of patellar fractures. J Orthop Trauma 1997;11(3):183–7. 14. Whittle AP, Wood II GW. Fractures of the lower extremity. In: Canale ST, editor. Campbell’s operative orthopaedics. 10th ed., Philadelphia: Mosby; 2003. p. 2796–804. 15. Wright PB, Budoff JE, Yeh ML, et al. Strength of damaged suture: an in vitro study. Arthroscopy 2006;22(12):1270–5. e3. 16. Wu CC, Tai CL, Chen WJ. Patellar tension band wiring: a revised technique. Arch Orthop Trauma Surg 2001;121(1/2):12–6.
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
FiberWire1 is superior in strength to stainless steel wire for tension band fixation of transverse patellar fractures P.B. Wright a,b,*, V. Kosmopoulos a,b, R.E. Cote´ c, T.J. Tayag c, A.D. Nana a,b a
Bone and Joint Research Center, Department of Orthopaedic Surgery, University of North Texas Health Science Center, 3500 Camp Bowie Boulevard, Fort Worth, TX 76107, USA Department of Orthopaedic Surgery, John Peter Smith Hospital, 1500 South Main Street, Fort Worth, TX 76104, USA c Department of Engineering, College of Science and Engineering, Texas Christian University, 2800 South University Drive, Fort Worth, TX 76129, USA b
A R T I C L E I N F O
A B S T R A C T
Article history: Accepted 28 April 2009
Background: The metal implants used to achieve fixation of displaced transverse patellar fractures are associated with implant failure, postoperative pain and a significant re-operation rate. Recent studies have examined braided suture as a possible alternative to stainless steel wire to increase patient satisfaction and decrease re-operation rates, but suture has not demonstrated clearly superior fixation strength. FiberWire1 is a reinforced braided polyblend suture that has demonstrated superior characteristics to the previous sutures studied and has not to our knowledge been examined as a material for tension band fixation of transverse patellar fractures. Methods: Materials testing was performed on repeated samples of No. 5 FiberWire suture and 18-gauge stainless steel wire. The strength and stiffness of each material was measured. The two materials were then used for tension band fixation on a novel transverse patellar fracture model and tested to failure by three-point bending. The constructs included a single stainless steel wire, a single-strand FiberWire tied with a sliding knot, double-strand FiberWire tied with sliding knots and double-strand FiberWire tied with a Wagoner’s Hitch. The fixation strength and stiffness of the constructs were measured. Findings: Unlike stainless steel, FiberWire maintained its initial stiffness until failure. Furthermore, during three-point-bend testing, double-strand FiberWire was found to have a significantly higher failure load than stainless steel wire when the suture was tied and locked under the tension produced by a modified Wagoner’s Hitch. Interpretation: FiberWire is a potentially superior alternative to stainless steel wire in tension band fixation of transverse patellar fractures. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: Patellar fracture Tension band Suture fixation FiberWire Injury
Introduction Internal fixation using metal implants in configurations based on the tension band principle remains the mainstay of treatment for operative transverse patellar fractures.14 However, metal implants are associated with complications in 18–50% of patients.2,6,13,16 Hardware may be prominent causing irritation, inhibition of motion and possible sinus formation and, over time, wires may fragment and migrate. Recent investigations2,4,5,11,12 have examined heavy suture as a possible alternative to conventional metal implants. Heavy suture is easier to place accurately in the soft tissues, conforms more readily to bony structures, is less likely to fragment over time and appears to be associated with greater patient
* Corresponding author at: Department of Orthopaedic Surgery, John Peter Smith Hospital, 1500 South Main Street, Fort Worth, TX 76104, USA. Tel.: +1 817 9271370; fax: +1 817 9273955. E-mail addresses: [email protected], [email protected] (P.B. Wright). 0020–1383/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.injury.2009.04.011
satisfaction and decreased re-operation rates. Should it be determined, therefore, that suture is at least as effective as stainless steel wire in the maintenance of a tension band under deforming forces, the clinical use of the suture is justified to decrease complication rates and increase patient satisfaction. The No. 5 Ethibond (Ethicon, Somerville, NJ, USA) and No. 5 TiCron (Davis and Geck, Gosport, Hampshire, UK) braided polyester sutures have been studied in patellar fracture models for this purpose. To our knowledge, FiberWire1 (Arthrex, Naples, FL, USA) suture has not been studied as a possible substrate for tension band fixation of patellar fractures. FiberWire is a Food and Drug Administration (FDA)-approved, biocompatible, braided polyblend suture consisting of two polyester strings and a polyethylene string. It has demonstrated superior strength compared to braided polyester sutures in experimental studies.7,15 The present study aims (1) to evaluate differences in stiffness and failure strength between No. 5 FiberWire, with and without a surgical knot, and 18gauge stainless steel wire, with and without a compression twist; and (2) to evaluate the effectiveness of FiberWire as a tension band construct using a novel three-point-bend model.
P.B. Wright et al. / Injury, Int. J. Care Injured 40 (2009) 1200–1203
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Fig. 1. Schematics of the stainless steel three-point-bend model.
Materials and methods Repeated samples of No. 5 FiberWire suture and 18-gauge stainless steel wire were tensile tested to failure under displacement control at a rate of 5 mm/min using a materials testing system (Instron, Norwood, MA, USA). To simulate the conditions after uniting opposite ends of the material to secure a tension band construct, tensile tests were then performed with a knot in the centre of a set of new FiberWire samples and a twist in the centre of a set of new 18-gauge stainless steel wires. Specifically, the opposite ends of the FiberWire were tied with a sliding knot followed by three square knots, and the opposite ends of the stainless steel wire were secured with seven visually confirmed symmetric twists. Stiffness and failure strength were calculated for each sample in both of these protocols. Stiffness was calculated as the slope of a best-fit line for the linear portion of the force– deformation curve. Failure strength was determined to be the highest load tolerated by the material. The aforementioned preliminary series of tests, to address the first aim, were performed simply to characterise the suture materials and fastening methods in isolation. After these preliminary tests, the two materials were used for tension band fixation of a novel three-point-bend model (Fig. 1). The three-point-bend model was machined from stainless steel with an anterior convex surface to mimic a symmetrical patella and flanges superior and inferior to permit placement in a threepoint-bend jig (Fig. 2). To reproduce the Lo¨tke figure-of-eight anterior tension band technique, two parallel longitudinal tunnels were drilled in the model to allow the passage of the FiberWire or stainless steel wire.9 After the suture or wire was secured, the model was loaded in three-point bending at a rate of 5 mm/min using the aforementioned material testing machine (see Fig. 2). The three-point-bending configuration stretches the tension band fixation leading to an anterior fracture gap. An anterior fracture gap of 3 mm was defined as construct failure14 and was calculated from a start point of zero using the following equation: displacement ½mm fracture gap ½mm ¼ 32 sin tan1 25:4
The equation was derived based on the geometry of the threepoint-bend model. The gap was also confirmed by the use of a micrometre at the end of each trial. Failure strength for the construct was thus identified at this critical fracture gap. The four constructs tested were: (A) single stainless steel wire with two (bilateral) compression twists; (B) single-strand FiberWire tied with a Mu¨ller sliding knot10; (C) double-strand FiberWire tied with individual sliding knots; and (D) double-strand FiberWire tied with a modified Wagoner’s Hitch5 (Fig. 3). Based on the first six repeated samples, power analyses were performed to calculate the needed sample size. To compare and identify significant differences between the four different tension band constructs, a one-way balanced analysis of variance (ANOVA) was employed. To specifically identify which pairs of means (i.e. which of the four conditions) were significantly different, if any,
Fig. 2. The patella model in the three-point-bending protocol. In three-point bending, two outer points of the model are supported (shown with an ‘*’), and as the actuator from the testing machine displaces downwards at a central point (noted with an ‘’), the suture begins to stretch and a fracture gap starts to develop at the most anterior point (noted with a ‘#’).
P.B. Wright et al. / Injury, Int. J. Care Injured 40 (2009) 1200–1203
1202
Fig. 3. The four anterior tension band constructs tested included (A) a single stainless steel wire with two compression twists, (B) a single-strand FiberWire tied with a sliding knot, (C) a double-strand FiberWire tied with individual sliding knots, and (D) a double-strand FiberWire tied with a modified Wagoner’s Hitch.
Table 1 The average stiffness and maximum tensile force (Fmax) of the materials tested.
Stainless steel wire FiberWire Stainless steel with twist FiberWire with knot
Stiffness (N/mm)
Fmax (N)
157.6 8.4 56.6 5.7
494.8 541.5 180.9 255.8
Tukey’s honestly significant difference (hsd) multiple comparison test was used (a = 5%). Results The power analyses revealed that a maximum of nine repeated samples were necessary to maintain a power of 0.80 at a 5% significance level. The tension tests resulted in relatively small differences in failure strength (9%) and large differences in stiffness between the stainless steel and FiberWire (180%) (Table 1). The introduction of the sliding knot in the tensile tests of the FiberWire resulted in a 32% and 53% decrease in stiffness and strength, respectively. The twist in the stainless steel wire resulted in a 64% and 63% reduction in stiffness and strength, respectively. The average load needed for construct failure in the threepoint-bending protocol for each construct is shown in Fig. 4. A one-
Fig. 4. Boxplot showing the clear advantage in failure strength of the double-strand FiberWire tied with a modified Wagoner’s Hitch (D) as compared to the other threepoint-bending constructs (A–C). The central mark is the median, the edges of the box are the 25th and 75th percentiles, the whiskers extend to the most extreme data points considered not to be outliers, and the outliers are plotted individually using a +. Notation: stainless steel (A); FiberWire single strand (B); FiberWire double strand (C); and FiberWire double strand with Wagoner’s Hitch (D).
way ANOVA confirmed that differences in mean failure strength between the tested conditions were significant (p < 0.0001). Results from the hsd multiple comparison test showed significant differences in failure strength between the stainless steel wire (636.0 N) and the single-strand FiberWire with a sliding knot (343.4 N, p < 0.01), and the stainless steel wire with the modified Wagoner’s Hitch FiberWire (1337.4 N, p < 0.01) constructs. No significant differences were found when the strength of the stainless steel wire was compared to the double-strand FiberWire with sliding knot construct (636.0 N vs. 580.7 N). Significant differences were also identified between the single- and doublestrand FiberWire with sliding knots (p < 0.01), and the singlestrand FiberWire and Wagoner’s Hitch FiberWire constructs (p < 0.01). Finally, significant differences were found between the double-strand FiberWire with sliding knots and Wagoner’s Hitch FiberWire constructs (p < 0.01). Discussion Patellar fractures are a common and significant cause of disability in modern orthopaedics. They constitute approximately 1% of all traumatic fractures14 and as many as 5.5% of all fractures in patients over 65 years of age.8 One-third to two-thirds will require surgical intervention1,3 and despite excellent union rates, a significant portion of postoperative morbidity can be attributed to the use of metal implants,2,6,13,16 including patient pain, disability and the need for another surgery to remove the hardware. In 2001, Gosal et al.4 specifically examined the re-operation rate in 37 patients randomised between the AO-modified tension band using metal wires and a Pyrford suture technique performed with a No. 5 Ethibond. They found a 38% hardware removal rate secondary to pain and hardware failure when metal implants were used, and reported no incidence of symptoms related to the use of heavy suture. The suture fixation group, however, demonstrated a 6% failure rate identifying that the material and method used could lead to fixation failure before conventional metallic implants. This is the likely reason why suture has not been routinely recommended for the osteosynthesis of patellar fractures. Previous experiments designed to test suture as a tension band for patellar fractures have been studied in cadaveric models with little standardisation as to size, shape and bone or soft-tissue quality. The reliability and reproducibility of the drilling of the parallel longitudinal tunnels, for example, and overall placement of implants, have also depended upon the researcher. We submit that, with this number of uncontrolled variables, the design of the tested implants and techniques may not have been the only variables affecting the outcome of the trials.
P.B. Wright et al. / Injury, Int. J. Care Injured 40 (2009) 1200–1203
Our study was designed to test FiberWire, a suture with superior strength15 and likely better properties to promote osteosynthesis than Ethibond, on a model that would eliminate the variables of previous studies and allow multiple trials to confirm statistical significance. Unfortunately, our study’s greatest strength is also a weakness with respect to extrapolating the results to the clinical setting. Without the use of cadaveric specimens, we are unable to determine how patella bone and surrounding soft tissue would respond to FiberWire, although this can be inferred from previous studies examining suture in cadaveric specimens.5,11,12 In addition, in vivo the patella is loaded not only in bending, but also in tension. We inferred the tension capabilities of FiberWire through materials testing and compared them to stainless steel (first aim of this study), but we did not specifically examine this phenomenon with our patella model. Instead, we chose an extreme loading protocol to better understand the tolerance and behaviour of the materials studied at their limits so that the results could be extrapolated to a clinical worstcase scenario. This study specifically evaluated the use of FiberWire in tension band fixation of a transverse patellar fracture using a novel threepoint-bend model. Mechanical tests were performed to compare FiberWire to a current standard of treatment employing stainless steel wire. Initial material testing demonstrated that at higher tensile forces (>250 N) the stainless wire was unable to maintain its superiority in stiffness when compared to the FiberWire. In contrast, the FiberWire, even beyond the maximum tensile force of the stainless wire, was able to maintain its initial stiffness until failure. In terms of the three-point-bending protocol, our results indicate that the double-strand FiberWire tied with sliding knots and the 18-gauge stainless steel wire with double twists required a similar load for construct failure. The load required for construct failure, however, was increased substantially when the FiberWire was tied with a modified Wagoner’s Hitch. This was likely due to the material behaviour of the FiberWire, and the mechanical advantage of the modified Wagoner’s Hitch, which allowed the surgeon to tie and lock the suture under greater tension than that produced by the sliding knot. Conclusions This study clearly demonstrates that a FiberWire tied with a modified Wagoner’s Hitch is superior in strength to stainless steel in the maintenance of a tension band under force. The clinical implications of this study are profound because we have identified a suture configuration that will be less likely to fail than that previously studied in vivo.4 In addition, our method is performed with a material that is less likely to cause postoperative pain and require removal than the metal implants routinely used in modern
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orthopaedic surgery. We therefore conclude that the in vivo study of FiberWire for the fixation of patellar fractures is justified and will likely result in greater patient satisfaction and decreased reoperation rates. Conflict of interest statement The authors do not have any financial or personal affiliations with other people or organisations that could inappropriately influence (bias) their work. Acknowledgements We thank Drs. Peter Raven, Robert Bunata and David Lichtman for their support of this research. The FiberWire used in our study was generously donated by Arthrex, Inc., Naples, FL, USA. References 1. Bostrom A. Fracture of the patella: a study of 422 patellar fractures. Acta Orthop Scand Suppl 1972;143:22–71. 2. Chatakondu SC, Abhaykumar S, Elliott DS. The use of non-absorbable suture in the fixation of patellar fractures: a preliminary report. Injury 1998;29:23–7. 3. Edwards B, Johnell O, Redlund-Johnell I. Patellar fractures: a 30-year follow-up. Acta Orthop Scand 1989;60:712–4. 4. Gosal HS, Singh P, Field RE. Clinical experience of patellar fracture fixation using metal wire or non-absorbable polyester—a study of 37 cases. Injury 2001;32(2): 129–35. 5. Hughes SC, Stott PM, Hearnden AJ, Ripley LG. A new and effective tension-band braided polyester suture technique for transverse patellar fracture fixation. Injury 2007;38(2):212–22. 6. Hung LK, Chan KM, Chow YN, Leung PC. Fractured patella: operative treatment using the tension band principle. Injury 1985;16:343–7. 7. Komatsu F, Mori R, Uchio Y. Optimum surgical suture material and methods to obtain high tensile strength at knots: problems of conventional knots and the reinforcement effect of adhesive agent. J Orthop Sci 2006;11(1):70–4. 8. Koo D, Adams DA, Copeland TM, et al. Incidence and costs to Medicare of fractures among Medicare beneficiaries aged greater than or equal to 65 years— United States, July 1991–June 1992. CDC Morb Mortal Wkly Rep 1996;45(41): 877–83. 9. Lo¨tke PA, Ecker ML. Transverse fractures of the patella. Clin Orthop Relat Res 1981;158:180–4. 10. Mast J, Jakob R, Ganz R. Planning and reduction technique in fracture surgery, 1st ed., New York: Springer-Verlag; 1989. p. 250. 11. McGreal G, Reidy D, Joy A, et al. The biomechanical evaluation of polyester as a tension band for the internal fixation of patellar fractures. J Med Eng Technol 1999;23(2):53–6. 12. Patel VR, Parks BG, Wang Y, et al. Fixation of patellar fractures with braided polyester suture: a biomechanical study. Injury 2000;31(1):1–6. 13. Smith ST, Cramer KE, Karges DE, et al. Early complications in the operative treatment of patellar fractures. J Orthop Trauma 1997;11(3):183–7. 14. Whittle AP, Wood II GW. Fractures of the lower extremity. In: Canale ST, editor. Campbell’s operative orthopaedics. 10th ed., Philadelphia: Mosby; 2003. p. 2796–804. 15. Wright PB, Budoff JE, Yeh ML, et al. Strength of damaged suture: an in vitro study. Arthroscopy 2006;22(12):1270–5. e3. 16. Wu CC, Tai CL, Chen WJ. Patellar tension band wiring: a revised technique. Arch Orthop Trauma Surg 2001;121(1/2):12–6.