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A double button adjustable loop device is biomechanically equivalent to tension band wire in the fixation of transverse patellar fractures—A cadaveric study
G Model JINJ 6984 No. of Pages 7
Injury, Int. J. Care Injured xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
Full length article
A double button adjustable loop device is biomechanically equivalent to tension band wire in the fixation of transverse patellar fractures—A cadaveric study Fucai Hana , Christopher Jon Pearceb,c, David Q.K. Ngd , Amit K. Ramruttunc , Desmond Y.R. Chonge , Diarmuid Murphya , Chin Tat Lima,* , Bernard C.S. Leeb a Department of Orthopaedic Surgery, University Orthopaedics, Hand & Reconstructive Microsurgery Cluster, National University Hospital, National University Health System, Singapore b Department of Orthopaedic Surgery, Ng Teng Fong General Hospital, Jurong Health Services, Singapore c Department of Orthopaedic Surgery, Yong Loo Lin School of Medicine, National University of Singapore, Singapore d Department of Biomechanical Engineering, National University of Singapore, Singapore e Engineering Cluster, Singapore Institute of Technology, Singapore
A R T I C L E I N F O
Keywords: Patella fracture Tightrope Adjustable loop device Tension band wire Complications of tension band wire
A B S T R A C T
Introduction: Tension-band wire fixation of patellar fractures is associated with significant hardwarerelated complications and infection. Braided polyester suture fixation is an alternative option. However, these suture fixations have higher failure rates due to the difficulty in achieving rigid suture knot fixation. The Arthrex syndesmotic TightRope, which is a double-button adjustable loop fixation device utilizing a 4-point locking system using FibreWire, may not only offer stiff rigid fixation using a knotless system, but may also obviate the need for implant removal due to hardware related problems. The aim of our study is to compare the fixation rigidity of patella fractures using Tightrope versus conventional tension-band wiring (TBW) in a cadaveric model. Materials and methods: TBW fixation was compared to TightRope fixation of transverse patella fractures in 5 matched pairs of cadaveric knees. The knees were cyclically brought through 0–90 of motion for a total of 500 cycles. Fracture gapping was measured before the start of the cycling, and at 50, 100, 200 and 500 cycles using an extensometer. The mean maximum fracture gapping was derived. Failure of the construct was defined as a displacement of more than 3 mm, patella fracture or implant breakage. Results: All but one knee from each group survived 500 cycles. The two failures were due to a fracture gap of more than 3 mm during cycling. There was no significant difference in the mean number of cycles tolerated. There was no implant breakage. There was no statistical significant difference in mean maximum fracture gap between the TBW and TightRope group at all cyclical milestones after 500 cycles (0.3026 0.4091 mm vs 0.3558 0.7173 mm, p = 0.388). Conclusions: We found no difference between the TBW and Tightrope fixation in terms of fracture gapping and failure. With possible lower risk of complications such as implant migration and soft tissue irritation, we believe tightrope fixation is a feasible alternative in fracture management of transverse patella fractures. ã 2016 Elsevier Ltd. All rights reserved.
Introduction Transverse patellar fractures account for approximately 1% of all skeletal injuries. The conventional method of utilizing a tension-
* Corresponding author at: Department of Orthopaedic Surgery, University Orthopaedics, Hand & Reconstructive Microsurgery Cluster, National University Hospital, National University Health System, NUHS Tower Block, Level 11, 1E Kent Ridge Road, 119228 Singapore. E-mail address: [email protected] (C.T. Lim).
band principle mode of fixation was first suggested by Pauwels in 1935 [1], and this is still advocated by the AO foundation (Arbeitsgemeinschaft für Osteosynthesefragen). Multiple authors have shown good biomechanical and clinical results with the use of this technique [2–4]. Although this principle is still commonly employed, tension-band fixation of transverse patellar fractures with two stainless-steel K-wires and a figure-of-eight stainless steel wire has been associated with significant hardware-related complications. These include hardware migration, breakage, prominence and infection [4–9]. Symptomatic hardware has been
http://dx.doi.org/10.1016/j.injury.2016.11.013 0020-1383/ã 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: F. Han, et al., A double button adjustable loop device is biomechanically equivalent to tension band wire in the fixation of transverse patellar fractures—A cadaveric study, Injury (2016), http://dx.doi.org/10.1016/j.injury.2016.11.013
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reported in up to 60% of cases and often results in additional surgery [7]. In addition, multiple authors have reported early fixation failure when tension-band wiring alone was employed in the fixation of transverse fractures, with a large proportion due to inadequate fixation with excessive soft-tissue interposition [10]. Improved fracture fixation stability and decreased fixation failure has been reported using other devices such as cannulated screws, cables, and locking plates [11–13]. Other authors have demonstrated the successful use of alternative materials such as braided polyester sutures, screws and bioabsorbable implants to treat these fractures [6,14–17]. Indeed there is evidence to suggest that the use of a tensionband principle may not apply, or be the most appropriate form of fixation in these fractures [18]. Tension-band wiring of transverse fractures may actually work by static compression by the wires as opposed to dynamic intermittent compression as conventionally taught [19]. The use of braided polyester sutures as an alternative to stainless steel wires has been described with equivalent, if not better results in terms of the stability of the fixation as well as reduced complication rates [6,16,17]. These sutures, however, may have higher failure rates, with Gosal et al. reporting a 6% suture failure rate [6]. There was also concern regarding the difficulty in achieving rigid fixation when securing the knots with these sutures. To address knot reliability, various suture techniques had been studied to determine the rigidity of the fixation and security of the knots [17]. In a recent biomechanical study, No. 5 FibreWire (Arthrex, Naples, FL, USA) has been shown to have superior maximum tensile force when compared to 18 -gauge stainless steel wires [20]. Depending on the knot technique, double stranded Fibrewire also had equivalent, if not better failure strength when compared to stainless steel wire in the figure-of-eight configuration. FibreWire was also able to maintain its initial stiffness until failure at tensile forces beyond the maximum tensile force of the stainless steel wire [20]. In a separate study, double stranded FibreWire utilized in figure-of-eight tension band had improved interfragmentary compression and decreased fracture displacement compared to 1.0 mm and 1.25 mm stainless steel wire [21]. In light of the current understanding that rigid fixation of transverse fractures may result in less fixation failures, and that sutures significantly reduce the incidence of hardware complications and reoperation rates, we set out to assess the use of the Arthrex TightRope in a parallel configuration in the fixation of transverse patellar fractures. Materials and methods Specimen preparation 7 pairs of fresh frozen cadaveric knees with no history of bone disease were obtained. All specimens were thawed at room temperature on the day of experiment. The knee joints and surrounding soft tissue were kept moist throughout the duration of the testing. All soft tissues around the knee joints were dissected leaving the joint capsule, ligaments and extensor mechanism intact. Care was taken to ensure the medial and lateral retinaculum were preserved. In order to facilitate accurate reduction, two 2.0 mm K-wires were drilled from the superior pole to the inferior pole of each patella before the patellar fracture was created. The two wires were subsequently removed. An oscillating saw was then used to create a transverse fracture through the widest diameter of the patella. 3 cm of medial and lateral retinaculum was also severed along the edges of the fracture to simulate a traumatic patellar fracture with rupture of the retinaculum.
Fig. 1. Tension band wire fixation showing steel wires (black arrows) and extensometer (white arrows).
Tension band wire fixations were then performed on one of each pairs of cadaveric knees. Two 2.0 mm stainless steel K-wires were inserted into the previously drilled holes in the proximal and distal fragments of the patella. Thereafter, an 18-gauge stainless steel wire was laid around the superior and inferior ends of the K wires with a figure-of-eight configuration on the anterior surface of the patella. The wires were then tightened by manually twisting the wires until a stable fixation was achieved (Fig. 1). The medial and lateral retinaculum were repaired using a running stitch with No. 1 vicryl suture. Tightrope fixations were then performed on the contralateral cadaveric knees. A 3.5 mm drill bit was used to enlarge the 2.0 mm drill holes that were created before the fracture. The needle and pull-through sutures attached to the tightrope were passed through the drill hole from the superior to the inferior pole of the patella. The oblong button attached to the end of the tight rope was then pulled though the patella. This button was then flipped to sit on the inferior pole of the patella. The soft tissues around the superior and inferior poles of the patellae were dissected to ensure that the buttons sat directly onto bone. The trailing button was then tightened down onto the superior pole of the patella until a stable fixation was achieved. This procedure was then repeated using a second tightrope along the other previously drilled hole (Fig. 2). The medial and lateral retinaculum were repaired using a running stitch with No. 1 vicryl suture. Anterior and lateral radiographs of the knee joints were then performed to ensure proper reduction of the fractures and placement of the implants. (Figs. 3 and 4) In preparation for the mounting of the cadaveric limb for mechanical testing, a transverse osteotomy was created over the
Please cite this article in press as: F. Han, et al., A double button adjustable loop device is biomechanically equivalent to tension band wire in the fixation of transverse patellar fractures—A cadaveric study, Injury (2016), http://dx.doi.org/10.1016/j.injury.2016.11.013
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Fig. 3. (Continued)
Fig. 2. Tightrope fixation showing Tightrope buttons (black arrows) and extensometer (white arrows).
Fig. 3. (a and b) Anterior-posterior and lateral radiographs of tightrope fixation.
proximal femur perpendicular to the shaft of the femur. The femur was then firmly secured onto the loading device using steel clamps along the length of the femur. Two No. 5 Ethibond were then secured onto the quadriceps tendon in whipstitch fashion. The quadriceps tendon was attached to the actuator to effect knee extension (Fig. 5).
Fig. 4. (a and b) Anterior-posterior and lateral radiographs of tension band wire fixation.
Please cite this article in press as: F. Han, et al., A double button adjustable loop device is biomechanically equivalent to tension band wire in the fixation of transverse patellar fractures—A cadaveric study, Injury (2016), http://dx.doi.org/10.1016/j.injury.2016.11.013
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Fig. 4. (Continued)
Biomechanical experiment To measure the amount of displacement at the fracture site, a DVRT extensometer (M-DVRT-6, LORD microstrain sensing systems, USA) was mounted onto the specimens after fracture fixation. One 18-gauge pin was mounted in each fracture fragment, 5 mm from the facture site. The extensometer had a gauge length of 6 mm. The extensometer was zeroed at the start of the initial test cycle. The tensioning experiment was performed by an electrohydraulic testing machine (MTS Bionix II, MTS systems, USA). The actuator of the machine applied tension to the quadriceps tendon, extending the knee from 90 degree flexion to full extension. Through progressive cyclic loading of the cadaver prior to creation of the patellar fracture, the necessary load required to achieve full extension was determined to be less than 300N. Therefore, a maximum load of 300N was used during subsequent testing. The knee was cyclically brought from full extension to 90 flexion with a force of up to 300N [15,16,22]. The knees were cycled at a velocity of 2.54 mm/s [22]. Each specimen was subjected to 500 cycles of tensioning [13]. Fracture gapping was measured before the start of the cycling, at 50, 100, 200 and 500 cycles. At each designated measurement cycle, fracture gapping at full extension, 60 flexion and 90 flexion were recorded. Only the maximum gapping at each measurement cycle was used. The mean of these maximum gapping recordings was then calculated (mean maximum displacement). Failure of the construct was defined as a displacement of more than 3 mm during testing [23,24], patellar fracture or implant breakage. Statistical analysis Statistical analysis was performed using IBM SPSS version 20. A Dangostino-Pearson test was used to assess for normality of data. A paired t-test was used for parametric data analysis and Wilcoxon test for non-parametric data.
Fig. 5. Set up of biomechanical tensile loading of the patellar fracture fixation.
Results The two different methods of fixation were tested on paired cadavers to eliminate other contributing factors such as bone quality and to allow direct comparison. The first two pairs of cadavers were discarded. They were used for experimental surgical procedure, to test the study protocol and to ensure the final surgical procedure was reproducible. Four out of the remaining five limbs from each group survived 500 cycles while one each failed at 200 cycles (Table 1). The limb that failed in each group belonged to two different cadavers. The two failures were due to a fracture gap of more than 3 mm during cycling. In the tightrope specimen, the proximal tightrope button cut through the proximal patella bone. In the TBW specimen, there was gapping noted without failure of the implants. In the remaining cadavers, there was no significant difference in the mean number of cycles tolerated. There was no implant breakage. Fracture gaps were measured at full extension, 60 and 90 flexion. The mean maximum fracture gap after 50th, 100th, 200th and 500th cycle are shown in Table 1 and Fig. 6. The fracture gapping after 50 cycles was slightly larger in the TBW group, although this difference was small (0.3253 vs 0.2033) and not statistically significant (p = 0.356). There was no significant
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Table 1 Difference of mean maximum fracture gaps between Tightrope fixation and Tension band wire fixation. Osteosynthesis Mean maximum fracture gap before 1st cycle (mm)
Mean maximum fracture gap after 50th cycle (mm)
Mean maximum fracture gap after 100th cycle (mm)
Mean maximum fracture gap after 200th cycle (mm)
Mean maximum fracture gap after 500th cycle (mm)
Tension band wire fixation Tight rope fixation p value
0
0.3253 0.4243
0.3890 0.4848
0.5057 0.6218
0.3026 0.4091
0
0.2033 0.3349
0.4031 0.6691
0.6842 1.0846
0.3558 0.7173
–
0.356
0.630
0.306
0.388
difference in mean maximum gap between the two groups at all the different cyclical milestones. The maximum fracture gap in the TBW and Tightrope groups was noted at the 200th cycle (0.5057 mm vs 0.6842 mm, p = 0.306). At the end of 500 cycles, the fracture gaps were not significantly different between the TBW group and the TightRope group (0.3026 0.4091 mm vs 0.3558 0.7173 mm, p = 0.388). Discussion The Arthrex TightRopeTM was devised for the treatment of syndesmotic injuries in the ankle. The TightRope comprises a double stranded No. 5 Fibrewire which when employed allows compression between two buttons that are seated on cortical bone. FibreWire is a blend of ultra-high molecular weight polyethylene multifilament core with a braided polyester jacket that confers it improved strength and resistance to elongation. The major advantage of this system is that it reduces the need for implant removal due to hardware related problems, as is commonly seen with tension-band wiring. The extracortical fixation in the TightRope may reduce the reliance on good cancellous bone
quality for reliable fixation, as is required in cancellous screw fixation. However, we note that the mode of failure in the TightRope group in our study was via cut-through of the button through cortical bone. This may be a potential mode of failure in this technique. Separately, the TightRope’s 4-point locking system also potentially addresses the issue of poor knot and loop security as seen when using FibreWire alone, which would affect the rigidity of the fixation construct. FibreWire has also been able to maintain its initial stiffness until failure [20]. In our study, we chose to study the fracture gap at 50 cycles to determine the initial stability of the fixation. We believe there may be an initial gap widening due to soft tissue interposition of the implants and loosening of the construct due to elongation of the fixation materials. There was a non-significant higher degree of widening of the fracture gap in the tension band group (0.3252 mm) compared to tightrope group (0.2033 mm) (p = 0.356). This may be due to material elongation. It has previously been shown that above 250N, 18-gauge stainless steel wire has been unable to maintain its initial stiffness until failure, unlike FibreWire [20] This same study showed no difference in the strength of 18G stainless steel wire compared to double stranded No. 5 Fibrewire with a sliding knot in a tension-band construct. In the tightrope group, we
Fig. 6. Graph showing mean maximum fracture gaps at different cycles intervals.
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dissected the soft tissue around the superior and inferior pole and ensure the buttons sat directly on the bone. Dissecting tissues around the figure-of-8 stainless steel wires would not be feasible as this would involve a circumferential excessive stripping of soft tissues. In the event of usage in patients, we would also suggest cycling the knee after tightening the tightrope without cutting the tightening sutures. After initial cycling with possible fracture gap widening, the tightrope can be further tightened to take up the slack. In anterior tension band wiring, by eccentric laying of the wires, there is a theoretical tension-band effect in compressing the fracture with flexion of the knee joint [1]. This has been shown not to be the case in a cadaveric study in transverse olecranon fractures, and that the success of fixation may be due to static compression by the wires rather than dynamic and intermittent compression via the tension-band effect [19]. The tightrope works by direct compression of the fracture. Direct compression with screws has also been shown to provide better compression than TBW [11]. We believe that TightRope fixation via extraosseous double-button compression may be more reliable than cancellous screw fixation which relies on the quality of the cancellous bone for screw purchase for compression and rigidity. However that is beyond the scope of this cadaveric study and may be studied in a separate study. We also studied the maximum fracture gap after 200 cycles because the greatest change in displacement has been shown to be during the initial 200 cycles. In a trial on synthetic patellae by Thelen et al. a steady state was reached after the initial 200 cycles with no notable alteration thereafter in a 10000 cycle loading protocol [13]. There was no significant difference in the mean maximum gapping at 200 cycles between the tension band wiring group and the tightrope group in our study. We further compared the maximum fracture gap at 500 cycles, which was also not significantly different. It should also be noted that the mean maximum gapping in the cadavers that did not fail were all submillimeter in magnitude, and this may be of clinical significance in the stability of the fixation. We are unable to conclusively explain the two fixation failures in our study. We can only postulate that this may have been due to fixation issues or poor bone quality. Due to the limited number of cadavers and lack of objective bone mineral density, we do not have enough information to provide recommendations regarding which patients may be at high risk of fixation failure with the TightRope. One limitation of our study was the small sample size. The sample size may not be large enough to detect the difference between the different groups. Another limitation was the lack of bone mineral density measurement. However, the use of paired cadavers should have eliminated this possible confounding factor and allow direct comparison even without knowledge of the quality of the bone. Although we cannot conclusively show TightRope fixation to be superior to tension-band wiring, we found that there was no significant difference between the two groups in terms of stability in our current study. With possible lower risk of complications related to prominent hardware and soft tissue irritation, we believe TightRope fixation employing extraosseous compression may be a feasible alternative in transverse patellar fracture fixation as compared to TBW. Alteration to the TightRope design, such as having a larger button such as the Arthrex DogBone button (designed for use in acromioclavicular joint fixation), could be used to minimize risk of cut-through of the implants in soft cortical bone. This technique of fixation may be useful in facilitating arthroscopic-assisted, minimally invasive fixation of transverse patella fractures.
Conflict of interest Christopher J Pearce: Paid speaker for Smith & Nephew and Depuy Synthes and on the editorial board for KSSTA(Knee Surgery, Sports Traumatology, Arthroscopy). Bernard C Lee: Paid speaker for Arthrex. All the other authors have no conflicts of interest associated with the work performed and represented in the submitted manuscript. All authors, their immediate family, and any research foundations with which they are affiliated did not receive any financial payments or other benefits from any commercial entity related to the subject of this article Acknowledgements This investigation was performed with the assistance of the National University Health System (Singapore) Junior Pitch for Funds 2013 and provision of Tightropes by Malex Medical Asia (Singapore) for the conduct of this study. Mr Hazlan Bin Sanusi (laboratory technologist), Department of Orthopaedic Surgery, National University of Singapore for assistance with the conduct of this study.
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Injury, Int. J. Care Injured xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
Full length article
A double button adjustable loop device is biomechanically equivalent to tension band wire in the fixation of transverse patellar fractures—A cadaveric study Fucai Hana , Christopher Jon Pearceb,c, David Q.K. Ngd , Amit K. Ramruttunc , Desmond Y.R. Chonge , Diarmuid Murphya , Chin Tat Lima,* , Bernard C.S. Leeb a Department of Orthopaedic Surgery, University Orthopaedics, Hand & Reconstructive Microsurgery Cluster, National University Hospital, National University Health System, Singapore b Department of Orthopaedic Surgery, Ng Teng Fong General Hospital, Jurong Health Services, Singapore c Department of Orthopaedic Surgery, Yong Loo Lin School of Medicine, National University of Singapore, Singapore d Department of Biomechanical Engineering, National University of Singapore, Singapore e Engineering Cluster, Singapore Institute of Technology, Singapore
A R T I C L E I N F O
Keywords: Patella fracture Tightrope Adjustable loop device Tension band wire Complications of tension band wire
A B S T R A C T
Introduction: Tension-band wire fixation of patellar fractures is associated with significant hardwarerelated complications and infection. Braided polyester suture fixation is an alternative option. However, these suture fixations have higher failure rates due to the difficulty in achieving rigid suture knot fixation. The Arthrex syndesmotic TightRope, which is a double-button adjustable loop fixation device utilizing a 4-point locking system using FibreWire, may not only offer stiff rigid fixation using a knotless system, but may also obviate the need for implant removal due to hardware related problems. The aim of our study is to compare the fixation rigidity of patella fractures using Tightrope versus conventional tension-band wiring (TBW) in a cadaveric model. Materials and methods: TBW fixation was compared to TightRope fixation of transverse patella fractures in 5 matched pairs of cadaveric knees. The knees were cyclically brought through 0–90 of motion for a total of 500 cycles. Fracture gapping was measured before the start of the cycling, and at 50, 100, 200 and 500 cycles using an extensometer. The mean maximum fracture gapping was derived. Failure of the construct was defined as a displacement of more than 3 mm, patella fracture or implant breakage. Results: All but one knee from each group survived 500 cycles. The two failures were due to a fracture gap of more than 3 mm during cycling. There was no significant difference in the mean number of cycles tolerated. There was no implant breakage. There was no statistical significant difference in mean maximum fracture gap between the TBW and TightRope group at all cyclical milestones after 500 cycles (0.3026 0.4091 mm vs 0.3558 0.7173 mm, p = 0.388). Conclusions: We found no difference between the TBW and Tightrope fixation in terms of fracture gapping and failure. With possible lower risk of complications such as implant migration and soft tissue irritation, we believe tightrope fixation is a feasible alternative in fracture management of transverse patella fractures. ã 2016 Elsevier Ltd. All rights reserved.
Introduction Transverse patellar fractures account for approximately 1% of all skeletal injuries. The conventional method of utilizing a tension-
* Corresponding author at: Department of Orthopaedic Surgery, University Orthopaedics, Hand & Reconstructive Microsurgery Cluster, National University Hospital, National University Health System, NUHS Tower Block, Level 11, 1E Kent Ridge Road, 119228 Singapore. E-mail address: [email protected] (C.T. Lim).
band principle mode of fixation was first suggested by Pauwels in 1935 [1], and this is still advocated by the AO foundation (Arbeitsgemeinschaft für Osteosynthesefragen). Multiple authors have shown good biomechanical and clinical results with the use of this technique [2–4]. Although this principle is still commonly employed, tension-band fixation of transverse patellar fractures with two stainless-steel K-wires and a figure-of-eight stainless steel wire has been associated with significant hardware-related complications. These include hardware migration, breakage, prominence and infection [4–9]. Symptomatic hardware has been
http://dx.doi.org/10.1016/j.injury.2016.11.013 0020-1383/ã 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: F. Han, et al., A double button adjustable loop device is biomechanically equivalent to tension band wire in the fixation of transverse patellar fractures—A cadaveric study, Injury (2016), http://dx.doi.org/10.1016/j.injury.2016.11.013
G Model JINJ 6984 No. of Pages 7
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reported in up to 60% of cases and often results in additional surgery [7]. In addition, multiple authors have reported early fixation failure when tension-band wiring alone was employed in the fixation of transverse fractures, with a large proportion due to inadequate fixation with excessive soft-tissue interposition [10]. Improved fracture fixation stability and decreased fixation failure has been reported using other devices such as cannulated screws, cables, and locking plates [11–13]. Other authors have demonstrated the successful use of alternative materials such as braided polyester sutures, screws and bioabsorbable implants to treat these fractures [6,14–17]. Indeed there is evidence to suggest that the use of a tensionband principle may not apply, or be the most appropriate form of fixation in these fractures [18]. Tension-band wiring of transverse fractures may actually work by static compression by the wires as opposed to dynamic intermittent compression as conventionally taught [19]. The use of braided polyester sutures as an alternative to stainless steel wires has been described with equivalent, if not better results in terms of the stability of the fixation as well as reduced complication rates [6,16,17]. These sutures, however, may have higher failure rates, with Gosal et al. reporting a 6% suture failure rate [6]. There was also concern regarding the difficulty in achieving rigid fixation when securing the knots with these sutures. To address knot reliability, various suture techniques had been studied to determine the rigidity of the fixation and security of the knots [17]. In a recent biomechanical study, No. 5 FibreWire (Arthrex, Naples, FL, USA) has been shown to have superior maximum tensile force when compared to 18 -gauge stainless steel wires [20]. Depending on the knot technique, double stranded Fibrewire also had equivalent, if not better failure strength when compared to stainless steel wire in the figure-of-eight configuration. FibreWire was also able to maintain its initial stiffness until failure at tensile forces beyond the maximum tensile force of the stainless steel wire [20]. In a separate study, double stranded FibreWire utilized in figure-of-eight tension band had improved interfragmentary compression and decreased fracture displacement compared to 1.0 mm and 1.25 mm stainless steel wire [21]. In light of the current understanding that rigid fixation of transverse fractures may result in less fixation failures, and that sutures significantly reduce the incidence of hardware complications and reoperation rates, we set out to assess the use of the Arthrex TightRope in a parallel configuration in the fixation of transverse patellar fractures. Materials and methods Specimen preparation 7 pairs of fresh frozen cadaveric knees with no history of bone disease were obtained. All specimens were thawed at room temperature on the day of experiment. The knee joints and surrounding soft tissue were kept moist throughout the duration of the testing. All soft tissues around the knee joints were dissected leaving the joint capsule, ligaments and extensor mechanism intact. Care was taken to ensure the medial and lateral retinaculum were preserved. In order to facilitate accurate reduction, two 2.0 mm K-wires were drilled from the superior pole to the inferior pole of each patella before the patellar fracture was created. The two wires were subsequently removed. An oscillating saw was then used to create a transverse fracture through the widest diameter of the patella. 3 cm of medial and lateral retinaculum was also severed along the edges of the fracture to simulate a traumatic patellar fracture with rupture of the retinaculum.
Fig. 1. Tension band wire fixation showing steel wires (black arrows) and extensometer (white arrows).
Tension band wire fixations were then performed on one of each pairs of cadaveric knees. Two 2.0 mm stainless steel K-wires were inserted into the previously drilled holes in the proximal and distal fragments of the patella. Thereafter, an 18-gauge stainless steel wire was laid around the superior and inferior ends of the K wires with a figure-of-eight configuration on the anterior surface of the patella. The wires were then tightened by manually twisting the wires until a stable fixation was achieved (Fig. 1). The medial and lateral retinaculum were repaired using a running stitch with No. 1 vicryl suture. Tightrope fixations were then performed on the contralateral cadaveric knees. A 3.5 mm drill bit was used to enlarge the 2.0 mm drill holes that were created before the fracture. The needle and pull-through sutures attached to the tightrope were passed through the drill hole from the superior to the inferior pole of the patella. The oblong button attached to the end of the tight rope was then pulled though the patella. This button was then flipped to sit on the inferior pole of the patella. The soft tissues around the superior and inferior poles of the patellae were dissected to ensure that the buttons sat directly onto bone. The trailing button was then tightened down onto the superior pole of the patella until a stable fixation was achieved. This procedure was then repeated using a second tightrope along the other previously drilled hole (Fig. 2). The medial and lateral retinaculum were repaired using a running stitch with No. 1 vicryl suture. Anterior and lateral radiographs of the knee joints were then performed to ensure proper reduction of the fractures and placement of the implants. (Figs. 3 and 4) In preparation for the mounting of the cadaveric limb for mechanical testing, a transverse osteotomy was created over the
Please cite this article in press as: F. Han, et al., A double button adjustable loop device is biomechanically equivalent to tension band wire in the fixation of transverse patellar fractures—A cadaveric study, Injury (2016), http://dx.doi.org/10.1016/j.injury.2016.11.013
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Fig. 3. (Continued)
Fig. 2. Tightrope fixation showing Tightrope buttons (black arrows) and extensometer (white arrows).
Fig. 3. (a and b) Anterior-posterior and lateral radiographs of tightrope fixation.
proximal femur perpendicular to the shaft of the femur. The femur was then firmly secured onto the loading device using steel clamps along the length of the femur. Two No. 5 Ethibond were then secured onto the quadriceps tendon in whipstitch fashion. The quadriceps tendon was attached to the actuator to effect knee extension (Fig. 5).
Fig. 4. (a and b) Anterior-posterior and lateral radiographs of tension band wire fixation.
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Fig. 4. (Continued)
Biomechanical experiment To measure the amount of displacement at the fracture site, a DVRT extensometer (M-DVRT-6, LORD microstrain sensing systems, USA) was mounted onto the specimens after fracture fixation. One 18-gauge pin was mounted in each fracture fragment, 5 mm from the facture site. The extensometer had a gauge length of 6 mm. The extensometer was zeroed at the start of the initial test cycle. The tensioning experiment was performed by an electrohydraulic testing machine (MTS Bionix II, MTS systems, USA). The actuator of the machine applied tension to the quadriceps tendon, extending the knee from 90 degree flexion to full extension. Through progressive cyclic loading of the cadaver prior to creation of the patellar fracture, the necessary load required to achieve full extension was determined to be less than 300N. Therefore, a maximum load of 300N was used during subsequent testing. The knee was cyclically brought from full extension to 90 flexion with a force of up to 300N [15,16,22]. The knees were cycled at a velocity of 2.54 mm/s [22]. Each specimen was subjected to 500 cycles of tensioning [13]. Fracture gapping was measured before the start of the cycling, at 50, 100, 200 and 500 cycles. At each designated measurement cycle, fracture gapping at full extension, 60 flexion and 90 flexion were recorded. Only the maximum gapping at each measurement cycle was used. The mean of these maximum gapping recordings was then calculated (mean maximum displacement). Failure of the construct was defined as a displacement of more than 3 mm during testing [23,24], patellar fracture or implant breakage. Statistical analysis Statistical analysis was performed using IBM SPSS version 20. A Dangostino-Pearson test was used to assess for normality of data. A paired t-test was used for parametric data analysis and Wilcoxon test for non-parametric data.
Fig. 5. Set up of biomechanical tensile loading of the patellar fracture fixation.
Results The two different methods of fixation were tested on paired cadavers to eliminate other contributing factors such as bone quality and to allow direct comparison. The first two pairs of cadavers were discarded. They were used for experimental surgical procedure, to test the study protocol and to ensure the final surgical procedure was reproducible. Four out of the remaining five limbs from each group survived 500 cycles while one each failed at 200 cycles (Table 1). The limb that failed in each group belonged to two different cadavers. The two failures were due to a fracture gap of more than 3 mm during cycling. In the tightrope specimen, the proximal tightrope button cut through the proximal patella bone. In the TBW specimen, there was gapping noted without failure of the implants. In the remaining cadavers, there was no significant difference in the mean number of cycles tolerated. There was no implant breakage. Fracture gaps were measured at full extension, 60 and 90 flexion. The mean maximum fracture gap after 50th, 100th, 200th and 500th cycle are shown in Table 1 and Fig. 6. The fracture gapping after 50 cycles was slightly larger in the TBW group, although this difference was small (0.3253 vs 0.2033) and not statistically significant (p = 0.356). There was no significant
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Table 1 Difference of mean maximum fracture gaps between Tightrope fixation and Tension band wire fixation. Osteosynthesis Mean maximum fracture gap before 1st cycle (mm)
Mean maximum fracture gap after 50th cycle (mm)
Mean maximum fracture gap after 100th cycle (mm)
Mean maximum fracture gap after 200th cycle (mm)
Mean maximum fracture gap after 500th cycle (mm)
Tension band wire fixation Tight rope fixation p value
0
0.3253 0.4243
0.3890 0.4848
0.5057 0.6218
0.3026 0.4091
0
0.2033 0.3349
0.4031 0.6691
0.6842 1.0846
0.3558 0.7173
–
0.356
0.630
0.306
0.388
difference in mean maximum gap between the two groups at all the different cyclical milestones. The maximum fracture gap in the TBW and Tightrope groups was noted at the 200th cycle (0.5057 mm vs 0.6842 mm, p = 0.306). At the end of 500 cycles, the fracture gaps were not significantly different between the TBW group and the TightRope group (0.3026 0.4091 mm vs 0.3558 0.7173 mm, p = 0.388). Discussion The Arthrex TightRopeTM was devised for the treatment of syndesmotic injuries in the ankle. The TightRope comprises a double stranded No. 5 Fibrewire which when employed allows compression between two buttons that are seated on cortical bone. FibreWire is a blend of ultra-high molecular weight polyethylene multifilament core with a braided polyester jacket that confers it improved strength and resistance to elongation. The major advantage of this system is that it reduces the need for implant removal due to hardware related problems, as is commonly seen with tension-band wiring. The extracortical fixation in the TightRope may reduce the reliance on good cancellous bone
quality for reliable fixation, as is required in cancellous screw fixation. However, we note that the mode of failure in the TightRope group in our study was via cut-through of the button through cortical bone. This may be a potential mode of failure in this technique. Separately, the TightRope’s 4-point locking system also potentially addresses the issue of poor knot and loop security as seen when using FibreWire alone, which would affect the rigidity of the fixation construct. FibreWire has also been able to maintain its initial stiffness until failure [20]. In our study, we chose to study the fracture gap at 50 cycles to determine the initial stability of the fixation. We believe there may be an initial gap widening due to soft tissue interposition of the implants and loosening of the construct due to elongation of the fixation materials. There was a non-significant higher degree of widening of the fracture gap in the tension band group (0.3252 mm) compared to tightrope group (0.2033 mm) (p = 0.356). This may be due to material elongation. It has previously been shown that above 250N, 18-gauge stainless steel wire has been unable to maintain its initial stiffness until failure, unlike FibreWire [20] This same study showed no difference in the strength of 18G stainless steel wire compared to double stranded No. 5 Fibrewire with a sliding knot in a tension-band construct. In the tightrope group, we
Fig. 6. Graph showing mean maximum fracture gaps at different cycles intervals.
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dissected the soft tissue around the superior and inferior pole and ensure the buttons sat directly on the bone. Dissecting tissues around the figure-of-8 stainless steel wires would not be feasible as this would involve a circumferential excessive stripping of soft tissues. In the event of usage in patients, we would also suggest cycling the knee after tightening the tightrope without cutting the tightening sutures. After initial cycling with possible fracture gap widening, the tightrope can be further tightened to take up the slack. In anterior tension band wiring, by eccentric laying of the wires, there is a theoretical tension-band effect in compressing the fracture with flexion of the knee joint [1]. This has been shown not to be the case in a cadaveric study in transverse olecranon fractures, and that the success of fixation may be due to static compression by the wires rather than dynamic and intermittent compression via the tension-band effect [19]. The tightrope works by direct compression of the fracture. Direct compression with screws has also been shown to provide better compression than TBW [11]. We believe that TightRope fixation via extraosseous double-button compression may be more reliable than cancellous screw fixation which relies on the quality of the cancellous bone for screw purchase for compression and rigidity. However that is beyond the scope of this cadaveric study and may be studied in a separate study. We also studied the maximum fracture gap after 200 cycles because the greatest change in displacement has been shown to be during the initial 200 cycles. In a trial on synthetic patellae by Thelen et al. a steady state was reached after the initial 200 cycles with no notable alteration thereafter in a 10000 cycle loading protocol [13]. There was no significant difference in the mean maximum gapping at 200 cycles between the tension band wiring group and the tightrope group in our study. We further compared the maximum fracture gap at 500 cycles, which was also not significantly different. It should also be noted that the mean maximum gapping in the cadavers that did not fail were all submillimeter in magnitude, and this may be of clinical significance in the stability of the fixation. We are unable to conclusively explain the two fixation failures in our study. We can only postulate that this may have been due to fixation issues or poor bone quality. Due to the limited number of cadavers and lack of objective bone mineral density, we do not have enough information to provide recommendations regarding which patients may be at high risk of fixation failure with the TightRope. One limitation of our study was the small sample size. The sample size may not be large enough to detect the difference between the different groups. Another limitation was the lack of bone mineral density measurement. However, the use of paired cadavers should have eliminated this possible confounding factor and allow direct comparison even without knowledge of the quality of the bone. Although we cannot conclusively show TightRope fixation to be superior to tension-band wiring, we found that there was no significant difference between the two groups in terms of stability in our current study. With possible lower risk of complications related to prominent hardware and soft tissue irritation, we believe TightRope fixation employing extraosseous compression may be a feasible alternative in transverse patellar fracture fixation as compared to TBW. Alteration to the TightRope design, such as having a larger button such as the Arthrex DogBone button (designed for use in acromioclavicular joint fixation), could be used to minimize risk of cut-through of the implants in soft cortical bone. This technique of fixation may be useful in facilitating arthroscopic-assisted, minimally invasive fixation of transverse patella fractures.
Conflict of interest Christopher J Pearce: Paid speaker for Smith & Nephew and Depuy Synthes and on the editorial board for KSSTA(Knee Surgery, Sports Traumatology, Arthroscopy). Bernard C Lee: Paid speaker for Arthrex. All the other authors have no conflicts of interest associated with the work performed and represented in the submitted manuscript. All authors, their immediate family, and any research foundations with which they are affiliated did not receive any financial payments or other benefits from any commercial entity related to the subject of this article Acknowledgements This investigation was performed with the assistance of the National University Health System (Singapore) Junior Pitch for Funds 2013 and provision of Tightropes by Malex Medical Asia (Singapore) for the conduct of this study. Mr Hazlan Bin Sanusi (laboratory technologist), Department of Orthopaedic Surgery, National University of Singapore for assistance with the conduct of this study.
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Please cite this article in press as: F. Han, et al., A double button adjustable loop device is biomechanically equivalent to tension band wire in the fixation of transverse patellar fractures—A cadaveric study, Injury (2016), http://dx.doi.org/10.1016/j.injury.2016.11.013