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Titanium mesh as a low-profile alternative for tension-band augmentation in patella fracture fixation: A biomechanical study Aaron J. Dickens a,*, Christina Salas a,b, LeRoy Rise a, Cristina Murray-Krezan c, Mahmoud Reda Taha a,b, Thomas A. DeCoster a, Rick J. Gehlert a a
Department of Orthopaedics and Rehabilitation, The University of New Mexico Health Sciences Center, MSC10 5600, 1 University of New Mexico, Albuquerque, NM 87131, United States Center for Biomedical Engineering, The University of New Mexico Health Sciences Center, MSC01 1141, 1 University of New Mexico, Albuquerque, NM 87131, United States c Division of Epidemiology, Biostatistics, and Preventive Medicine, Department of Internal Medicine, The University of New Mexico Health Sciences Center, MSC10 5550, 1 University of New Mexico, Albuquerque, NM 87131, United States b
A R T I C L E I N F O
A B S T R A C T
Article history: Accepted 19 February 2015
Objectives: We performed a simple biomechanical study to compare the fixation strength of titanium mesh with traditional tension-band augmentation, which is a standard treatment for transverse patella fractures. We hypothesised that titanium mesh augmentation is not inferior in fixation strength to the standard treatment. Methods: Twenty-four synthetic patellae were tested. Twelve were fixed with stainless steel wire and parallel cannulated screws. Twelve were fixed with parallel cannulated screws, augmented with anterior titanium mesh and four screws. A custom test fixture was developed to simulate a knee flexed to 908. A uniaxial force was applied to the simulated extensor mechanism at this angle. A non-inferiority study design was used to evaluate ultimate force required for failure of each construct as a measure of fixation strength. Stiffness of the bone/implant construct, fracture gap immediately prior to failure, and modes of failure are also reported. Results: The mean difference in force at failure was 23.0 N (95% CI: 123.6 to 77.6 N) between mesh and wire constructs, well within the pre-defined non-inferiority margin of 260 N. Mean stiffness of the mesh and wire constructs were 19.42 N/mm (95% CI: 18.57–20.27 N/mm) and 19.49 N/mm (95% CI: 18.64–20.35 N/mm), respectively. Mean gap distance for the mesh constructs immediately prior to failure was 2.11 mm (95% CI: 1.35–2.88 mm) and 3.87 mm (95% CI: 2.60–5.13 mm) for wire constructs. Conclusions: Titanium mesh augmentation is not inferior to tension-band wire augmentation when comparing ultimate force required for failure in this simplified biomechanical model. Results also indicate that stiffness of the two constructs is similar but that the mesh maintains a smaller fracture gap prior to failure. The results of this study indicate that the use of titanium mesh plating augmentation as a low-profile alternative to tension-band wiring for fixation of transverse patella fractures warrants further investigation. ß 2015 Elsevier Ltd. All rights reserved.
Keywords: Patella fracture Patella fixation Mesh plating Tension band Titanium mesh low-profile Biomechanics
Introduction Patella fractures account for 1% of all fractures in adults, with the highest incidence in men between 20 and 50 years old [1–3]. Most fractures occur from direct force applied to the distal femur with the knee in flexion, especially during falls and motor vehicle collisions [4,5]. Operative treatment is indicated in cases of open fracture, severe intra-articular comminution, disruption of the
* Corresponding author. Tel.: +1 505 272 4107; fax: +1 505 272 8098. E-mail address: [email protected] (A.J. Dickens).
extensor mechanism, and articular displacement of greater than 2 or 3 mm [6–8]. Many methods of patella fracture fixation have been proposed in the literature and include a variety of screw designs, Kirschner wires, compressive pins, stainless steel wire, braided suture, locking and non-locking plates, external fixators, and various combinations of these implants [6,9–21]. A wellaccepted construct for fixation of simple transverse patella fractures is an anterior tension-band with steel wires passed through partially threaded cannulated screws [6,10,22,23]. This technique provides high union rates for simple two-part fractures. However, removal of tension-band wire constructs may be required in up to 52% of cases because of the subcutaneous nature
http://dx.doi.org/10.1016/j.injury.2015.02.017 0020–1383/ß 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Dickens AJ, et al. Titanium mesh as a low-profile alternative for tension-band augmentation in patella fracture fixation: A biomechanical study. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.02.017
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Fig. 1. A Traditional stainless-steel wire tension-band construct with parallel partially-threaded cannulated screws. B 0.6 mm thick titanium mesh with four 2.3 mm 15 mm titanium cortical screws and parallel partially threaded cannulated screws.
of the patella, making it vulnerable to postoperative irritation from prominent twisted ends of the steel wires [24]. One study showed that current fixation techniques in patients younger than 60 years result in removal of implants in 40% of cases, and the authors recommended development of newer techniques to diminish skin irritation [25]. The unacceptably high rate of operations after patella implant surgery inspired us to investigate the use of lowprofile titanium mesh plating as an alternative to tension-band wiring. Titanium mesh is a biologically inert and effective implant commonly used in craniomaxillofacial surgery as a buttress for fracture fixation. It is easily contoured, has multiple holes for screw placement, and is very low-profile, which is necessary for subcutaneous implantation [26–31]. The goal of our study was to investigate mesh plating as a stable, low-profile, fixation augmentation option for treatment of simple transverse patella fractures. Materials and methods
[34]. A rasp was used to make opposing relief channels in each fragment to allow passage of nylon webbing (12.7 mm 1.9 mm; maximum load capacity 7784 N) to simulate the quadriceps and patellar tendons [20]. All specimens were then fixed with two 4.0 mm 36 mm partially threaded (1/3 thread length) cannulated screws (Orthopaedic Implant Company, Reno, NV), thereby achieving compression and reduction at the fracture site. The specimens were randomly divided into two groups, with 12 specimens in each group. In group I specimens, 18-gauge (1.02 mm) stainless steel wire (DS18 monofilament surgical steel, Ethicon, Somerville, NJ) was passed through the cannulated screws across the osteotomy site anteriorly in a figure-of-eight fashion. All of the wires were tensioned with a single twisted end (Fig. 1A). In group II specimens, 0.6-mm-thick titanium mesh (4 5 holes) was fixed to the anterior surface of the specimen with four 2.3 mm 15 mm titanium screws (Universal Neuro 2 Cranial Fixation System, Stryker Osteosynthesis, Germany). The mesh was centred on the osteotomy site, and 2.3-mm screws were placed at each corner peripherally, not through the central nylon webbing (Fig. 1B).
Study design Mechanical testing We performed a biomechanical study comparing titanium mesh with the traditional steel wire tension-band construct used for treatment of simple transverse patella fractures in a synthetic bone model. We hypothesised that the titanium mesh construct is not inferior to the traditional tension-band wire technique with respect to failure strength. Non-inferiority testing is commonly used when introducing a new treatment for a disorder when the new treatment may offer advantages over existing treatments, such as fewer complications, lower cost, versatility in application, or other clinical advantages [32]. Additionally, unlike the commonly used superiority testing, non-inferiority testing allows for consideration of clinical relevance at the onset of the study upon selection of the predefined non-inferiority margin (D) – the largest, clinically acceptable difference between a new treatment and a well-established treatment [33]. Fracture and fixation models Twenty-four synthetic medium left patellae were used in this study (Pacific Research Inc., Vashon, WA). A custom plaster mold was used to place parallel guide pins from proximal to distal in each specimen. They were then over-drilled with a cannulated drill bit. A second custom plaster mold was used to create a transverse fracture (OTA classification 34-C1.1) with a thin kerf saw blade
A custom-designed test fixture was developed to simulate a knee with attached extensor mechanism flexed at 908, similar to a model described by Schnabel et al. [35] (Fig. 2). The loading angle was chosen to simulate the load experienced in a stand-to-sit motion just prior to sitting. This has been shown to produce the highest normal and shear loads with the highest strains on the patella while weight bearing [35,36]. The nylon webbing ends were sewn together with industrial-grade thread to form fixedlength distal and proximal loops. These were then attached to the test fixture with a cable and pulley system. The patella specimens were centred on the trochlea of a total knee femoral prosthesis. A displacement controlled protocol was applied at a rate of 2 mm/s through the cable and pulley system attached to a servohydraulic actuator (Model 858, MTS systems, Eden Prairie, MN) [11,20]. Outcome measures from the MTS machine were actuator displacement and load cell force. Stiffness as a measure of stability was characterised by the most linear region of force-displacement plot. Ultimate force was characterised as the highest force sustained by the bone-implant construct immediately before failure. Failure was characterised as a permanent decrease in force observed in the force-displacement curve (Fig. 3). A digital SLR camera (EOS Rebel, Canon, Melville, NY) with full high definition video was positioned to capture the failure event of
Please cite this article in press as: Dickens AJ, et al. Titanium mesh as a low-profile alternative for tension-band augmentation in patella fracture fixation: A biomechanical study. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.02.017
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site immediately before catastrophic failure. Custom software allowed calibrated measurement of the fracture gap before failure, and gap displacement for each specimen was calculated from the mean values resulting from medial and lateral displacement measurements. The method of failure for each patella was based on observations from the failure video. Statistical analysis A non-inferiority study design in this proof-of-concept study was implemented to compare the mesh with the wire fixation [37]. On the basis of strength results described by Banks et al. [9], and with the aim of achieving 80% power with a one-sided a equal to 0.025 to detect a difference in ultimate force of 260 N (assuming SD = 380 N for each group) between mesh and wire groups, a noninferiority t-test determined that 19 synthetic patellae were required in each device group. The mesh was considered noninferior to wire fixation with respect to ultimate force if the lower bound of the 95% confidence interval for the difference between means of the two fixations did not exceed the pre-specified noninferiority margin (D = 260 N), e.g. the confidence interval should lie entirely to the right of D. The distribution of force measurements satisfied normality assumptions, and thus 95% confidence intervals based on the t-distribution were calculated for the difference in ultimate force. Stiffness of the constructs and fracture gap immediately prior to failure were also reported. The power analysis used PASS 11 software (PASS 11. NCSS, LLC. Kaysville, Utah, USA). Statistical analysis was performed by using SAS 9.3 (SAS Institute, Cary, NC, USA). Fig. 2. MTS setup with patella specimen centred on femoral prosthesis.
each specimen. Custom software based on the Hough transform feature-extraction technique was developed to allow for calibrated measurement analysis of still images acquired from the video. Calibration of all images was performed by using a metric ruler that was placed within the video field, in the plane of the anterior surface of the patella. Gap displacement was measured (in millimeters) at the medial and lateral edges of the osteotomy
Results Ultimate force at failure was 624.7 N (95% CI: 561.1 to 688.2 N) for the wire construct and 601.7 N (95% CI: 515.9 to 687.5 N) for the mesh construct. The mean difference in force at failure was 23.0 N (95% CI: 123.6 to 77.6 N), well within the pre-defined non-inferiority margin of 260 N. The mean stiffness of the wire and mesh constructs were 19.42 N/mm (95% CI: 18.57–20.27 N/ mm) and 19.49 N/mm (95% CI: 18.64–20.35 N/mm), respectively. Lateral gap displacement values for the wire and mesh constructs
Fig. 3. Force-displacement curves for twelve titanium mesh and twelve tension-band wire constructs.
Please cite this article in press as: Dickens AJ, et al. Titanium mesh as a low-profile alternative for tension-band augmentation in patella fracture fixation: A biomechanical study. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.02.017
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Fig. 4. A Fracture gap displacement the instant prior to catastrophic failure for the tension-band wire construct. B Fracture gap displacement the instant prior to catastrophic failure for the titanium mesh construct.
immediately prior to failure were 5.47 mm (95% CI: 3.36– 7.58 mm) and 3.06 mm (95% CI: 1.82–4.30 mm). Medial gap displacement for the wire and mesh constructs was 2.27 mm (95% CI: 1.68–2.86 mm) and 1.16 mm (0.78–1.55 mm). Mean gap displacement for the wire and mesh constructs was 3.87 mm (95% CI: 2.60–5.13 mm) and 2.11 mm (95% CI: 1.35–2.88 mm). Representative fracture gap displacement immediately prior to catastrophic failure for each construct is shown in Fig. 4. Modes of failure for the mesh constructs were as follows: 1. straightening of vertically crimped links with crack initiation at the superior aspect of the medial cannulated screw propagating to the superior medial mesh screw (n = 2); 2. symmetric fracture-gap increase with vertical-link straightening and mesh screw cutout from the superior portion of the medial and/or lateral cannulated screw (n = 7); and 3. lateral fracture gap increase with vertical-link straightening and mesh screw cutout from the superior aspect of the medial cannulated screw plus the inferior aspect of the lateral cannulated screw (n = 3). Modes of failure for the wire constructs were as follows: 1. large fracture gap increase and stretching of the wire with medial and lateral wire cutout from distal sawbone fragment (n = 7); and 2. large fracture-gap increase with stretching of the wire and medial cannulated screw cutout from the superior sawbone (n = 5). No catastrophic screw or implant failure occurred for either bonefixation type. The bone failed catastrophically, resulting in multifragmentation, in all cases. Discussion This study found that titanium mesh is not inferior to steel wire when using ultimate force as the clinically relevant outcome measure. A non-inferior ultimate force implies that the mesh construct can withstand physiologic loads in the range of the tension-band wire before catastrophic failure occurs. Additional analysis found no measured difference in stiffness between the two constructs. This finding suggests that the two constructs behave similarly when subjected to low static force. An added benefit is that the mesh showed a significantly smaller mean
fracture gap prior to failure (2.11 mm versus 3.87 mm). This is potentially important clinically because several studies have shown that more than 2.5–3 mm of displacement can increase the risk of post-operative arthritis, non-union and malunion [6–8]. A video review of specimen failure showed that the wire appeared to cut into the bone, whereas the mesh tended to stretch (straightening of vertically crimped links) before screw cutout and catastrophic failure. The mesh may have distributed the tensile load more evenly across the fracture gap, resulting in a smaller mean fracture gap. Based on results achieved with this synthetic mechanical model using a non-inferiority study design, we suggest that titanium mesh may be a viable alternative to the commonly used tension-band wiring technique. Previous studies have investigated low-profile plating options as an alternative to tension-band wiring in order to avoid complications associated with prominent wire ends. Banks et al. [9] used a 2.4/2.7 mm anterior locking X-plate (Synthes, West Chester, PA) for fixation of transverse patella fractures in a cadaveric model, with favourable results. Direct comparison of their force and stiffness results with those in our study is difficult because they used a cadaveric model under cyclical loading with the specimen tensioned in full knee extension. Banks et al. noted that in their clinical experience, the low-profile plate was not palpable and had a very low rate of implant removal. The titanium mesh used in the current study has a much lower profile (0.6 mm) and offers more screw-hole insertion options. Wild et al. [20] used peripheral plating as a low-profile option in a synthetic patella model. They found that fixed-angle peripheral plates were associated with more rigid fixation than tension-band wiring. The mesh tested in our study does not offer fixed-angle options but may be contoured to any surface. The limited fracture gap resulting from the mesh plate in our study shows a similar trend of more rigid fixation than wire constructs. A study by Dargel et al. [11] investigated patella fixation with interfragmentary lag screw, modified tension-band with K-wire, and three FFS large pins (OrthoFix, Bussolengo Verona, Italy) in a calf patella model. They found significant variation in ultimate loads, ranging from 323 N to 723 N. We found the ultimate force at failure was 624.66 N for the
Please cite this article in press as: Dickens AJ, et al. Titanium mesh as a low-profile alternative for tension-band augmentation in patella fracture fixation: A biomechanical study. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.02.017
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steel wire tension-band construct and 601.67 N for the titanium mesh construct in a synthetic patella model, within their ultimate force margins. The results of our study are similar to those of Carpenter et al. [6], who found that tension-band wiring with cannulated screws failed at a mean of 732 N (SD = 316) under monotonic loading in a cadaver model. Although a synthetic patella cannot reproduce the true behavior of a human patella, the synthetic bone in the present study provides a uniform material, similar in shape and bone density to the human patella, for directly comparing the two fixation methods. This eliminates variability in results commonly associated with bone quality and geometrical differences in smallsample cadaveric bone studies. Further testing in cadaveric models and subsequent in vivo analyses will be necessary to support the use of this mesh for treatment of patella fractures. Additionally, the loads applied to our synthetic models were non-cyclical. It is possible that screws used in the mesh plating treatment may loosen under cyclical loads, especially in cases of patients with poor bone quality. Further tests incorporating cyclic physiologic loads are needed. Although our pre-study power analysis indicated that 19 specimens would be required in each group in our study, only 12 patellae were available for testing. For this reason, a posthoc power analysis was performed on the ultimate force data to determine whether our sample size was sufficiently powered to claim non-inferiority within a margin of 260 N. Based on our results of a mean difference of 23.0 N, we found that with sample sizes of 12 fixations in each group, we had 80% power to determine non-inferiority to within 82.8 N and 95% power to determine non-inferiority to within 113.4 N. Our investigation was designed to test biomechanical properties of a tension-band construct with mesh augmentation for subcutaneous fixation in a simple patella fracture model. The mesh used is not approved by the Food and Drug Administration for patella fracture fixation. Use of this device for patella fracture fixation would be off-label as its use is restricted to minimal loadbearing applications. However, our findings that the mesh was non-inferior to the traditional tension-band wire construct in regard to ultimate strength suggest that further investigation of the device is warranted. A low-profile mesh designed specifically to resist tensile forces with a decreased propensity to cause knee pain or irritation and a reduction in the reoperation rate is needed. The results of our study may inspire innovative designs for robust low-profile plates used for subcutaneous fracture fixation under high-tensile loads. Conclusion Our biomechanical study using a simplified synthetic model suggested that low-profile mesh plating is non-inferior to tensionband wiring when evaluating ultimate force required for failure. The results also indicated that mesh plating is not different in stiffness, but limits fracture gap prior to failure. Further cadaveric and clinical investigations of mesh plating for treatment of patella fractures are warranted. Sources of funding Internal funding was provided by the Department of Orthopaedics and Rehabilitation, University of New Mexico Health Sciences Center. Statistical analysis was supported by the National Center for Research Resources and the National Center for Advancing Translational Sciences of the National Institutes of Health through grant number 8UL1TR000041, The University of New Mexico Clinical and Translational Science Center. No external sources of funding were obtained.
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Conflict of interest After completion, and based on the results of this study, several authors of this manuscript submitted a patent application for an alternative design of a mesh fracture fixation plate. Acknowledgements This information was presented at the 2014 Annual Orthopaedic Research Society Meeting in New Orleans, LA as a poster presentation. We acknowledge James Love for developing the software used for fracture gap measurement and thank Renee Robilliard for providing editorial assistance in the preparation of this manuscript. References [1] Ashby ME, Schields CL, Kariny JR. Diagnosis of osteochondral fractures in acute traumatic patellar dislocations using air arthrography. J Trauma 1975;15(11): 1032–3. [2] Nummi J. Operative treatment of patellar fractures. Acta Orthop Scan 1971;42(5): 437–8. [3] Bucholz RW, Court-Brown CM, Heckman JD, Tornetta III P. Fractures in adults. 7th ed. Lippincott; 2010. p. 1752–77. [4] Pailo AF, Malavolta EA, dos Santos ALG, Mendes MTR, de Rezende MU, Hernandez AJ, et al. Patellar fractures: a decade of treatment at IOT-HC-FMUSP part I: functional analysis. Acta Ortop Bras 2005;13(5):221–4. [5] Schmal H, Strohm PC, Niemeyer P, Reising K, Kuminack K, Sudkamp NP. Fractures of the patella in children and adolescents. Acta Orthop Belg 2010;76(5):644–50. [6] Carpenter JE, Kasman RA, Patel N, Lee ML, Goldstein SA. Biomechanical evaluation of current patella fracture fixation techniques. J Orthop Trauma 1997;1(5):351–6. [7] 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. [8] Luna-Pizarro D, Amato D, Arellano F, Lo´pez-Rojas P. Comparison of a technique using a new percutaneous osteosynthesis device with conventional open surgery for displaced patella fractures in a randomized controlled trial. J Orthop Trauma 2006;20(8):529–35. [9] Banks KE, Ambrose CG, Wheeless JS, Tissue CM, Sen M. An alternative patellar fracture fixation: a biomechanical study. J Orthop Trauma 2012;27(6):345–51. [10] Berg EE. Open reduction internal fixation of displaced transverse patella fractures with figure-eight wiring through parallel cannulated compression screws. J Orthop Trauma 1997;11(8):573–7. [11] Dargel J, Gick S, Mader K, Koebke J, Pennig D. Biomechanical comparison of tension band and interfragmentary screw fixation with a new implant in transverse patella fractures. Injury 2010;41(2):156–60. [12] Domby B, Henderson E, Nayak A, Erdog˘an M, Gutierrez S, Santoni BG, et al. Comparison of cannulated screw with tension band wiring versus compressive cannulated locking bolt and nut device in patella fracures – a cadaveric biomechanical study. J Orthop Trauma 2012;26(12):678–83. [13] Liang QY, Wu JW. Fracture of the patella treated by open reduction and external compressive skeletal fixation. J Bone Joint Surg Am 1987;69(1): 83–9. [14] Mao N, Ni H, Dinq W, Zhu X, Bai Y, Wang C, et al. Surgical treatment of transverse patella fractures by the cable pin system with a minimally invasive technique. J Trauma Acute Care Surg 2012;72(4):1056–61. [15] McGreal G, Reidy D, Joy A, Mahalingam K, Cashman WF. 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. [16] Melvin JS, Mehta S. Patella fractures in adults. J Am Acad Ortho Surg 2011;19(4):198–207. [17] Patel VR, Parks BG, Wang Y, Ebert FR, Jinnah RH. Fixation of patella fractures with braided polyester suture: a biomechanical study. Injury 2000;31(1):1–6. [18] Rabalais RD, Burger E, Lu Y, et al. Comparison of two tension-band fixation materials and techniques in transverse patella fractures: a biomechanical study. Orthopaedics 2008;31(2):128. [19] Thelen S, Schneppendahl J, Jopen E, et al. Biomechanical cadaver testing of a fixed-angle plate in comparison to tension wiring and screw fixation in transverse patella fractures. Injury 2012;43(8):1290–5. [20] Wild M, Eichler C, Thelen S, et al. Fixed-angle plate osteosynthesis of the patella – an alternative to tension wiring? Clin Biomech 2010;25(4):341–7. [21] Yammis I, Oguz E, Atesalp AS, Ozkan H, Ku¨rklu¨ M, Demiralp B, et al. Application of circular external fixator under arthroscopic control in comminuted patella fractures: technique and early results. J Trauma 2006;60(3):659–63. [22] Baran O, Manisali M, Cecen B. Anatomical and biomechanical evaluation of the tension band technique in patellar fractures. Int Orthop 2009;33(4):1113–7. [23] Fortis AP, Milis Z, Kostopoulos V, Tsantzalis S, Kormas P, Tzinieris N, et al. Experimental investigation of the tension band in fractures of the patella. Injury 2002;33(6):489–93.
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JINJ-6096; No. of Pages 6 Injury, Int. J. Care Injured xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
Titanium mesh as a low-profile alternative for tension-band augmentation in patella fracture fixation: A biomechanical study Aaron J. Dickens a,*, Christina Salas a,b, LeRoy Rise a, Cristina Murray-Krezan c, Mahmoud Reda Taha a,b, Thomas A. DeCoster a, Rick J. Gehlert a a
Department of Orthopaedics and Rehabilitation, The University of New Mexico Health Sciences Center, MSC10 5600, 1 University of New Mexico, Albuquerque, NM 87131, United States Center for Biomedical Engineering, The University of New Mexico Health Sciences Center, MSC01 1141, 1 University of New Mexico, Albuquerque, NM 87131, United States c Division of Epidemiology, Biostatistics, and Preventive Medicine, Department of Internal Medicine, The University of New Mexico Health Sciences Center, MSC10 5550, 1 University of New Mexico, Albuquerque, NM 87131, United States b
A R T I C L E I N F O
A B S T R A C T
Article history: Accepted 19 February 2015
Objectives: We performed a simple biomechanical study to compare the fixation strength of titanium mesh with traditional tension-band augmentation, which is a standard treatment for transverse patella fractures. We hypothesised that titanium mesh augmentation is not inferior in fixation strength to the standard treatment. Methods: Twenty-four synthetic patellae were tested. Twelve were fixed with stainless steel wire and parallel cannulated screws. Twelve were fixed with parallel cannulated screws, augmented with anterior titanium mesh and four screws. A custom test fixture was developed to simulate a knee flexed to 908. A uniaxial force was applied to the simulated extensor mechanism at this angle. A non-inferiority study design was used to evaluate ultimate force required for failure of each construct as a measure of fixation strength. Stiffness of the bone/implant construct, fracture gap immediately prior to failure, and modes of failure are also reported. Results: The mean difference in force at failure was 23.0 N (95% CI: 123.6 to 77.6 N) between mesh and wire constructs, well within the pre-defined non-inferiority margin of 260 N. Mean stiffness of the mesh and wire constructs were 19.42 N/mm (95% CI: 18.57–20.27 N/mm) and 19.49 N/mm (95% CI: 18.64–20.35 N/mm), respectively. Mean gap distance for the mesh constructs immediately prior to failure was 2.11 mm (95% CI: 1.35–2.88 mm) and 3.87 mm (95% CI: 2.60–5.13 mm) for wire constructs. Conclusions: Titanium mesh augmentation is not inferior to tension-band wire augmentation when comparing ultimate force required for failure in this simplified biomechanical model. Results also indicate that stiffness of the two constructs is similar but that the mesh maintains a smaller fracture gap prior to failure. The results of this study indicate that the use of titanium mesh plating augmentation as a low-profile alternative to tension-band wiring for fixation of transverse patella fractures warrants further investigation. ß 2015 Elsevier Ltd. All rights reserved.
Keywords: Patella fracture Patella fixation Mesh plating Tension band Titanium mesh low-profile Biomechanics
Introduction Patella fractures account for 1% of all fractures in adults, with the highest incidence in men between 20 and 50 years old [1–3]. Most fractures occur from direct force applied to the distal femur with the knee in flexion, especially during falls and motor vehicle collisions [4,5]. Operative treatment is indicated in cases of open fracture, severe intra-articular comminution, disruption of the
* Corresponding author. Tel.: +1 505 272 4107; fax: +1 505 272 8098. E-mail address: [email protected] (A.J. Dickens).
extensor mechanism, and articular displacement of greater than 2 or 3 mm [6–8]. Many methods of patella fracture fixation have been proposed in the literature and include a variety of screw designs, Kirschner wires, compressive pins, stainless steel wire, braided suture, locking and non-locking plates, external fixators, and various combinations of these implants [6,9–21]. A wellaccepted construct for fixation of simple transverse patella fractures is an anterior tension-band with steel wires passed through partially threaded cannulated screws [6,10,22,23]. This technique provides high union rates for simple two-part fractures. However, removal of tension-band wire constructs may be required in up to 52% of cases because of the subcutaneous nature
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Please cite this article in press as: Dickens AJ, et al. Titanium mesh as a low-profile alternative for tension-band augmentation in patella fracture fixation: A biomechanical study. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.02.017
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Fig. 1. A Traditional stainless-steel wire tension-band construct with parallel partially-threaded cannulated screws. B 0.6 mm thick titanium mesh with four 2.3 mm 15 mm titanium cortical screws and parallel partially threaded cannulated screws.
of the patella, making it vulnerable to postoperative irritation from prominent twisted ends of the steel wires [24]. One study showed that current fixation techniques in patients younger than 60 years result in removal of implants in 40% of cases, and the authors recommended development of newer techniques to diminish skin irritation [25]. The unacceptably high rate of operations after patella implant surgery inspired us to investigate the use of lowprofile titanium mesh plating as an alternative to tension-band wiring. Titanium mesh is a biologically inert and effective implant commonly used in craniomaxillofacial surgery as a buttress for fracture fixation. It is easily contoured, has multiple holes for screw placement, and is very low-profile, which is necessary for subcutaneous implantation [26–31]. The goal of our study was to investigate mesh plating as a stable, low-profile, fixation augmentation option for treatment of simple transverse patella fractures. Materials and methods
[34]. A rasp was used to make opposing relief channels in each fragment to allow passage of nylon webbing (12.7 mm 1.9 mm; maximum load capacity 7784 N) to simulate the quadriceps and patellar tendons [20]. All specimens were then fixed with two 4.0 mm 36 mm partially threaded (1/3 thread length) cannulated screws (Orthopaedic Implant Company, Reno, NV), thereby achieving compression and reduction at the fracture site. The specimens were randomly divided into two groups, with 12 specimens in each group. In group I specimens, 18-gauge (1.02 mm) stainless steel wire (DS18 monofilament surgical steel, Ethicon, Somerville, NJ) was passed through the cannulated screws across the osteotomy site anteriorly in a figure-of-eight fashion. All of the wires were tensioned with a single twisted end (Fig. 1A). In group II specimens, 0.6-mm-thick titanium mesh (4 5 holes) was fixed to the anterior surface of the specimen with four 2.3 mm 15 mm titanium screws (Universal Neuro 2 Cranial Fixation System, Stryker Osteosynthesis, Germany). The mesh was centred on the osteotomy site, and 2.3-mm screws were placed at each corner peripherally, not through the central nylon webbing (Fig. 1B).
Study design Mechanical testing We performed a biomechanical study comparing titanium mesh with the traditional steel wire tension-band construct used for treatment of simple transverse patella fractures in a synthetic bone model. We hypothesised that the titanium mesh construct is not inferior to the traditional tension-band wire technique with respect to failure strength. Non-inferiority testing is commonly used when introducing a new treatment for a disorder when the new treatment may offer advantages over existing treatments, such as fewer complications, lower cost, versatility in application, or other clinical advantages [32]. Additionally, unlike the commonly used superiority testing, non-inferiority testing allows for consideration of clinical relevance at the onset of the study upon selection of the predefined non-inferiority margin (D) – the largest, clinically acceptable difference between a new treatment and a well-established treatment [33]. Fracture and fixation models Twenty-four synthetic medium left patellae were used in this study (Pacific Research Inc., Vashon, WA). A custom plaster mold was used to place parallel guide pins from proximal to distal in each specimen. They were then over-drilled with a cannulated drill bit. A second custom plaster mold was used to create a transverse fracture (OTA classification 34-C1.1) with a thin kerf saw blade
A custom-designed test fixture was developed to simulate a knee with attached extensor mechanism flexed at 908, similar to a model described by Schnabel et al. [35] (Fig. 2). The loading angle was chosen to simulate the load experienced in a stand-to-sit motion just prior to sitting. This has been shown to produce the highest normal and shear loads with the highest strains on the patella while weight bearing [35,36]. The nylon webbing ends were sewn together with industrial-grade thread to form fixedlength distal and proximal loops. These were then attached to the test fixture with a cable and pulley system. The patella specimens were centred on the trochlea of a total knee femoral prosthesis. A displacement controlled protocol was applied at a rate of 2 mm/s through the cable and pulley system attached to a servohydraulic actuator (Model 858, MTS systems, Eden Prairie, MN) [11,20]. Outcome measures from the MTS machine were actuator displacement and load cell force. Stiffness as a measure of stability was characterised by the most linear region of force-displacement plot. Ultimate force was characterised as the highest force sustained by the bone-implant construct immediately before failure. Failure was characterised as a permanent decrease in force observed in the force-displacement curve (Fig. 3). A digital SLR camera (EOS Rebel, Canon, Melville, NY) with full high definition video was positioned to capture the failure event of
Please cite this article in press as: Dickens AJ, et al. Titanium mesh as a low-profile alternative for tension-band augmentation in patella fracture fixation: A biomechanical study. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.02.017
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site immediately before catastrophic failure. Custom software allowed calibrated measurement of the fracture gap before failure, and gap displacement for each specimen was calculated from the mean values resulting from medial and lateral displacement measurements. The method of failure for each patella was based on observations from the failure video. Statistical analysis A non-inferiority study design in this proof-of-concept study was implemented to compare the mesh with the wire fixation [37]. On the basis of strength results described by Banks et al. [9], and with the aim of achieving 80% power with a one-sided a equal to 0.025 to detect a difference in ultimate force of 260 N (assuming SD = 380 N for each group) between mesh and wire groups, a noninferiority t-test determined that 19 synthetic patellae were required in each device group. The mesh was considered noninferior to wire fixation with respect to ultimate force if the lower bound of the 95% confidence interval for the difference between means of the two fixations did not exceed the pre-specified noninferiority margin (D = 260 N), e.g. the confidence interval should lie entirely to the right of D. The distribution of force measurements satisfied normality assumptions, and thus 95% confidence intervals based on the t-distribution were calculated for the difference in ultimate force. Stiffness of the constructs and fracture gap immediately prior to failure were also reported. The power analysis used PASS 11 software (PASS 11. NCSS, LLC. Kaysville, Utah, USA). Statistical analysis was performed by using SAS 9.3 (SAS Institute, Cary, NC, USA). Fig. 2. MTS setup with patella specimen centred on femoral prosthesis.
each specimen. Custom software based on the Hough transform feature-extraction technique was developed to allow for calibrated measurement analysis of still images acquired from the video. Calibration of all images was performed by using a metric ruler that was placed within the video field, in the plane of the anterior surface of the patella. Gap displacement was measured (in millimeters) at the medial and lateral edges of the osteotomy
Results Ultimate force at failure was 624.7 N (95% CI: 561.1 to 688.2 N) for the wire construct and 601.7 N (95% CI: 515.9 to 687.5 N) for the mesh construct. The mean difference in force at failure was 23.0 N (95% CI: 123.6 to 77.6 N), well within the pre-defined non-inferiority margin of 260 N. The mean stiffness of the wire and mesh constructs were 19.42 N/mm (95% CI: 18.57–20.27 N/ mm) and 19.49 N/mm (95% CI: 18.64–20.35 N/mm), respectively. Lateral gap displacement values for the wire and mesh constructs
Fig. 3. Force-displacement curves for twelve titanium mesh and twelve tension-band wire constructs.
Please cite this article in press as: Dickens AJ, et al. Titanium mesh as a low-profile alternative for tension-band augmentation in patella fracture fixation: A biomechanical study. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.02.017
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Fig. 4. A Fracture gap displacement the instant prior to catastrophic failure for the tension-band wire construct. B Fracture gap displacement the instant prior to catastrophic failure for the titanium mesh construct.
immediately prior to failure were 5.47 mm (95% CI: 3.36– 7.58 mm) and 3.06 mm (95% CI: 1.82–4.30 mm). Medial gap displacement for the wire and mesh constructs was 2.27 mm (95% CI: 1.68–2.86 mm) and 1.16 mm (0.78–1.55 mm). Mean gap displacement for the wire and mesh constructs was 3.87 mm (95% CI: 2.60–5.13 mm) and 2.11 mm (95% CI: 1.35–2.88 mm). Representative fracture gap displacement immediately prior to catastrophic failure for each construct is shown in Fig. 4. Modes of failure for the mesh constructs were as follows: 1. straightening of vertically crimped links with crack initiation at the superior aspect of the medial cannulated screw propagating to the superior medial mesh screw (n = 2); 2. symmetric fracture-gap increase with vertical-link straightening and mesh screw cutout from the superior portion of the medial and/or lateral cannulated screw (n = 7); and 3. lateral fracture gap increase with vertical-link straightening and mesh screw cutout from the superior aspect of the medial cannulated screw plus the inferior aspect of the lateral cannulated screw (n = 3). Modes of failure for the wire constructs were as follows: 1. large fracture gap increase and stretching of the wire with medial and lateral wire cutout from distal sawbone fragment (n = 7); and 2. large fracture-gap increase with stretching of the wire and medial cannulated screw cutout from the superior sawbone (n = 5). No catastrophic screw or implant failure occurred for either bonefixation type. The bone failed catastrophically, resulting in multifragmentation, in all cases. Discussion This study found that titanium mesh is not inferior to steel wire when using ultimate force as the clinically relevant outcome measure. A non-inferior ultimate force implies that the mesh construct can withstand physiologic loads in the range of the tension-band wire before catastrophic failure occurs. Additional analysis found no measured difference in stiffness between the two constructs. This finding suggests that the two constructs behave similarly when subjected to low static force. An added benefit is that the mesh showed a significantly smaller mean
fracture gap prior to failure (2.11 mm versus 3.87 mm). This is potentially important clinically because several studies have shown that more than 2.5–3 mm of displacement can increase the risk of post-operative arthritis, non-union and malunion [6–8]. A video review of specimen failure showed that the wire appeared to cut into the bone, whereas the mesh tended to stretch (straightening of vertically crimped links) before screw cutout and catastrophic failure. The mesh may have distributed the tensile load more evenly across the fracture gap, resulting in a smaller mean fracture gap. Based on results achieved with this synthetic mechanical model using a non-inferiority study design, we suggest that titanium mesh may be a viable alternative to the commonly used tension-band wiring technique. Previous studies have investigated low-profile plating options as an alternative to tension-band wiring in order to avoid complications associated with prominent wire ends. Banks et al. [9] used a 2.4/2.7 mm anterior locking X-plate (Synthes, West Chester, PA) for fixation of transverse patella fractures in a cadaveric model, with favourable results. Direct comparison of their force and stiffness results with those in our study is difficult because they used a cadaveric model under cyclical loading with the specimen tensioned in full knee extension. Banks et al. noted that in their clinical experience, the low-profile plate was not palpable and had a very low rate of implant removal. The titanium mesh used in the current study has a much lower profile (0.6 mm) and offers more screw-hole insertion options. Wild et al. [20] used peripheral plating as a low-profile option in a synthetic patella model. They found that fixed-angle peripheral plates were associated with more rigid fixation than tension-band wiring. The mesh tested in our study does not offer fixed-angle options but may be contoured to any surface. The limited fracture gap resulting from the mesh plate in our study shows a similar trend of more rigid fixation than wire constructs. A study by Dargel et al. [11] investigated patella fixation with interfragmentary lag screw, modified tension-band with K-wire, and three FFS large pins (OrthoFix, Bussolengo Verona, Italy) in a calf patella model. They found significant variation in ultimate loads, ranging from 323 N to 723 N. We found the ultimate force at failure was 624.66 N for the
Please cite this article in press as: Dickens AJ, et al. Titanium mesh as a low-profile alternative for tension-band augmentation in patella fracture fixation: A biomechanical study. Injury (2015), http://dx.doi.org/10.1016/j.injury.2015.02.017
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steel wire tension-band construct and 601.67 N for the titanium mesh construct in a synthetic patella model, within their ultimate force margins. The results of our study are similar to those of Carpenter et al. [6], who found that tension-band wiring with cannulated screws failed at a mean of 732 N (SD = 316) under monotonic loading in a cadaver model. Although a synthetic patella cannot reproduce the true behavior of a human patella, the synthetic bone in the present study provides a uniform material, similar in shape and bone density to the human patella, for directly comparing the two fixation methods. This eliminates variability in results commonly associated with bone quality and geometrical differences in smallsample cadaveric bone studies. Further testing in cadaveric models and subsequent in vivo analyses will be necessary to support the use of this mesh for treatment of patella fractures. Additionally, the loads applied to our synthetic models were non-cyclical. It is possible that screws used in the mesh plating treatment may loosen under cyclical loads, especially in cases of patients with poor bone quality. Further tests incorporating cyclic physiologic loads are needed. Although our pre-study power analysis indicated that 19 specimens would be required in each group in our study, only 12 patellae were available for testing. For this reason, a posthoc power analysis was performed on the ultimate force data to determine whether our sample size was sufficiently powered to claim non-inferiority within a margin of 260 N. Based on our results of a mean difference of 23.0 N, we found that with sample sizes of 12 fixations in each group, we had 80% power to determine non-inferiority to within 82.8 N and 95% power to determine non-inferiority to within 113.4 N. Our investigation was designed to test biomechanical properties of a tension-band construct with mesh augmentation for subcutaneous fixation in a simple patella fracture model. The mesh used is not approved by the Food and Drug Administration for patella fracture fixation. Use of this device for patella fracture fixation would be off-label as its use is restricted to minimal loadbearing applications. However, our findings that the mesh was non-inferior to the traditional tension-band wire construct in regard to ultimate strength suggest that further investigation of the device is warranted. A low-profile mesh designed specifically to resist tensile forces with a decreased propensity to cause knee pain or irritation and a reduction in the reoperation rate is needed. The results of our study may inspire innovative designs for robust low-profile plates used for subcutaneous fracture fixation under high-tensile loads. Conclusion Our biomechanical study using a simplified synthetic model suggested that low-profile mesh plating is non-inferior to tensionband wiring when evaluating ultimate force required for failure. The results also indicated that mesh plating is not different in stiffness, but limits fracture gap prior to failure. Further cadaveric and clinical investigations of mesh plating for treatment of patella fractures are warranted. Sources of funding Internal funding was provided by the Department of Orthopaedics and Rehabilitation, University of New Mexico Health Sciences Center. Statistical analysis was supported by the National Center for Research Resources and the National Center for Advancing Translational Sciences of the National Institutes of Health through grant number 8UL1TR000041, The University of New Mexico Clinical and Translational Science Center. No external sources of funding were obtained.
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Conflict of interest After completion, and based on the results of this study, several authors of this manuscript submitted a patent application for an alternative design of a mesh fracture fixation plate. Acknowledgements This information was presented at the 2014 Annual Orthopaedic Research Society Meeting in New Orleans, LA as a poster presentation. We acknowledge James Love for developing the software used for fracture gap measurement and thank Renee Robilliard for providing editorial assistance in the preparation of this manuscript. References [1] Ashby ME, Schields CL, Kariny JR. Diagnosis of osteochondral fractures in acute traumatic patellar dislocations using air arthrography. J Trauma 1975;15(11): 1032–3. [2] Nummi J. Operative treatment of patellar fractures. Acta Orthop Scan 1971;42(5): 437–8. [3] Bucholz RW, Court-Brown CM, Heckman JD, Tornetta III P. Fractures in adults. 7th ed. Lippincott; 2010. p. 1752–77. [4] Pailo AF, Malavolta EA, dos Santos ALG, Mendes MTR, de Rezende MU, Hernandez AJ, et al. Patellar fractures: a decade of treatment at IOT-HC-FMUSP part I: functional analysis. Acta Ortop Bras 2005;13(5):221–4. [5] Schmal H, Strohm PC, Niemeyer P, Reising K, Kuminack K, Sudkamp NP. Fractures of the patella in children and adolescents. Acta Orthop Belg 2010;76(5):644–50. [6] Carpenter JE, Kasman RA, Patel N, Lee ML, Goldstein SA. Biomechanical evaluation of current patella fracture fixation techniques. J Orthop Trauma 1997;1(5):351–6. [7] 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. [8] Luna-Pizarro D, Amato D, Arellano F, Lo´pez-Rojas P. Comparison of a technique using a new percutaneous osteosynthesis device with conventional open surgery for displaced patella fractures in a randomized controlled trial. J Orthop Trauma 2006;20(8):529–35. [9] Banks KE, Ambrose CG, Wheeless JS, Tissue CM, Sen M. An alternative patellar fracture fixation: a biomechanical study. J Orthop Trauma 2012;27(6):345–51. [10] Berg EE. Open reduction internal fixation of displaced transverse patella fractures with figure-eight wiring through parallel cannulated compression screws. J Orthop Trauma 1997;11(8):573–7. [11] Dargel J, Gick S, Mader K, Koebke J, Pennig D. Biomechanical comparison of tension band and interfragmentary screw fixation with a new implant in transverse patella fractures. Injury 2010;41(2):156–60. [12] Domby B, Henderson E, Nayak A, Erdog˘an M, Gutierrez S, Santoni BG, et al. Comparison of cannulated screw with tension band wiring versus compressive cannulated locking bolt and nut device in patella fracures – a cadaveric biomechanical study. J Orthop Trauma 2012;26(12):678–83. [13] Liang QY, Wu JW. Fracture of the patella treated by open reduction and external compressive skeletal fixation. J Bone Joint Surg Am 1987;69(1): 83–9. [14] Mao N, Ni H, Dinq W, Zhu X, Bai Y, Wang C, et al. Surgical treatment of transverse patella fractures by the cable pin system with a minimally invasive technique. J Trauma Acute Care Surg 2012;72(4):1056–61. [15] McGreal G, Reidy D, Joy A, Mahalingam K, Cashman WF. 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. [16] Melvin JS, Mehta S. Patella fractures in adults. J Am Acad Ortho Surg 2011;19(4):198–207. [17] Patel VR, Parks BG, Wang Y, Ebert FR, Jinnah RH. Fixation of patella fractures with braided polyester suture: a biomechanical study. Injury 2000;31(1):1–6. [18] Rabalais RD, Burger E, Lu Y, et al. Comparison of two tension-band fixation materials and techniques in transverse patella fractures: a biomechanical study. Orthopaedics 2008;31(2):128. [19] Thelen S, Schneppendahl J, Jopen E, et al. Biomechanical cadaver testing of a fixed-angle plate in comparison to tension wiring and screw fixation in transverse patella fractures. Injury 2012;43(8):1290–5. [20] Wild M, Eichler C, Thelen S, et al. Fixed-angle plate osteosynthesis of the patella – an alternative to tension wiring? Clin Biomech 2010;25(4):341–7. [21] Yammis I, Oguz E, Atesalp AS, Ozkan H, Ku¨rklu¨ M, Demiralp B, et al. Application of circular external fixator under arthroscopic control in comminuted patella fractures: technique and early results. J Trauma 2006;60(3):659–63. [22] Baran O, Manisali M, Cecen B. Anatomical and biomechanical evaluation of the tension band technique in patellar fractures. Int Orthop 2009;33(4):1113–7. [23] Fortis AP, Milis Z, Kostopoulos V, Tsantzalis S, Kormas P, Tzinieris N, et al. Experimental investigation of the tension band in fractures of the patella. Injury 2002;33(6):489–93.
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