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Biomechanical evaluation of the tension band wiring principle. A comparison between two different techniques for transverse patella fracture fixation
Accepted Manuscript Title: Biomechanical evaluation of the tension band wiring principle. A comparison between two different techniques for transverse patella fracture fixation Authors: Ivan Zderic, Karl Stoffel, Christoph Sommer, Dankward H¨ontzsch, Boyko Gueorguiev PII: DOI: Reference:
S0020-1383(17)30362-5 http://dx.doi.org/doi:10.1016/j.injury.2017.05.037 JINJ 7263
To appear in:
Injury, Int. J. Care Injured
Accepted date:
27-5-2017
Please cite this article as: Zderic Ivan, Stoffel Karl, Sommer Christoph, H¨ontzsch Dankward, Gueorguiev Boyko.Biomechanical evaluation of the tension band wiring principle.A comparison between two different techniques for transverse patella fracture fixation.Injury http://dx.doi.org/10.1016/j.injury.2017.05.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Biomechanical evaluation of the tension band wiring principle. A comparison between two different techniques for transverse patella fracture fixation.
Ivan Zderica, MSc, Karl Stoffelb, c, PhD, Christoph Sommerd, MD, Dankward Höntzsche, MD, Boyko Gueorguieva, PhD
a
AO Research Institute Davos, Davos, Switzerland
b
Cantonal Hospital Baselland, Bruderholz, Switzerland
c
University of Basel, Basel, Switzerland
d
Cantonal Hospital Graubuenden, Chur, Switzerland
e
BG Klinik Tübingen, Tübingen, Germany
Corresponding author: Ivan Zderic AO Research Institute Davos, Clavadelerstrasse 8, 7270 Davos, Switzerland E-Mail: [email protected]
Abstract Purpose: The aim of this study was to investigate the validity of the dynamic compression principle of tension band wiring in two techniques for patella fracture treatment.
Methods: Twelve human cadaveric knees with simulated transverse patella fractures were assigned to two groups for treatment with tension band wiring using Kirschner (K-) wires or cannulated screws. Biomechanical testing was performed over three knee movement cycles between 90° flexion and 0° extension. Pressure distribution in the fracture gap and fracture site displacement was evaluated at the 3rd cycle in 15° steps, namely 90º-75º-60º-45º-30º-15º-0º extension and 0°-15º-30º-45º-60º-75º-90º flexion phase.
Results: Mean anterior and posterior interfragmentary pressure in group with K-wires ranged within 0.16 – 0.40 MPa/0.12 – 0.35 MPa, and 0.37 – 0.59 MPa/0.10 – 0.30 MPa with cannulated screws. These changes remained non-significant in both groups and loading phases (P≥0.171). Mean anterior and posterior fracture site displacement in group with K-wires ranged within -0.01 – 0.53 mm/0.11 – 0.74 mm, and 0.11 – 0.55 mm/-0.10 –
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0.50 mm with cannulated screws. Anterior displacement remained without significant changes in both groups and loading phases (P≥0.112). However, posterior displacement underwent a significant increase in this regard for K-wires (P≤0.047), but not for cannulated screws (P≥0.202), in the course of knee extension. Significantly smaller displacement at the posterior fracture site was detected in group using cannulated screws compared to Kwires at 60° and 75° extension phase (P≤0.017), as well as at 45°, 60° and 75° flexion phase (P≤0.018). The critical value of 2 mm displacement at the posterior fracture site was not reached for any specimen and fixation technique. Knee extension was accompanied by synchronous increase in quadriceps pulling force.
Conclusions: Tension band wiring fulfills from a biomechanical point of view the requirements for sufficient fixation stability in transverse patella fracture fixation. It should, however, rather be considered as a static fixation principle than a dynamic one. Tension band wiring with cannulated screws was found advantageous over Kirschner wires with regard to interfragmentary movements at the posterior fracture site.
Key words: Transverse patella fracture, Tension band wiring, Kirschner wire, Cannulated screw, Biomechanics, Interfragmentary pressure
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Introduction The incidence of patella fractures, usually caused by direct trauma of the anterior knee surface, is approximately 0.5-1.5% of all fractures [1]. Among them, most common are the transverse patella fractures [2]. Operative treatment is indicated for displaced fractures with a disrupted extensor mechanism [1]. High contact forces acting between the patella and the femur require restoration and preservation of a smooth articular surface in order to prevent posttraumatic osteoarthritis [3]. This can only be achieved if the two fragments are anatomically reduced and subsequently firmly fixed.
Tension band wiring is one of the most common treatment methods for transverse patella fracture fixation [3]. Its main principle in all modifications is to counteract muscle traction, maintain the fracture reduced and possibly transform the tensile forces between the quadriceps muscle and the anterior tibia tuberosity into compression at the articular patella cortex during knee flexion [4]. The latter should stabilize the fracture and enhance bone healing in an environment of large distraction forces by closing the fracture gap and maintaining the interfragmentary contact [5]. Similar techniques are applied for olecranon and malleolus fracture fixation [6].
Although tension band wiring is currently one of the most frequently used fixation techniques for patella fracture fixation, poor clinical outcomes have been reported in up to 55% of the cases [1]. Moreover, lots of previous studies are not in favor of this principle by stating that other implants, such as compression screws (single or in combination with tension band wiring), perform better in terms of fixation stability, complication rates or functional outcomes [2, 3, 7-25]. Currently, there is no existing evidence yet, based on pressure assessment in the patella fracture gap and/or interfragmentary movements, that this principle is valid in the surgical practice. Specifically, there are no existing studies on pressure distribution in the patellar fracture gap, displacement and rotational movement at the fracture site, related to knee flexion and extension during the early mobilization phase, proving that tension band wiring by using Kirschner (K-) wires or cannulated screws is a effective surgical treatment.
Therefore, the aim of the present study was to investigate whether the principle of tension band wiring applies for transverse patella fracture fixation during the extension and flexion phase of knee movement. Referring to previous findings on tension band wiring of the olecranon [25], we examined the following hypotheses for the tension band wiring of the knee in the acute postoperative phase of knee movement:
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1) Tension band wiring using K-wires and cannulated screws does not induce a significant change upon knee motion in terms of interfragmentary pressure and movement; 2) Interfragmentary displacements do not exceed the 2 mm-threshold upon knee motion with these two fixation techniques, satisfying the requirements for primary stability; 3) There are no significant differences between the two techniques in terms of interfragmentary pressure and movement.
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Materials and Methods Specimens and preparation Twelve fresh-frozen (-20°C) human cadaveric knees (5 left and 7 right) with distal femur and proximal tibia parts, including soft tissue, were used in this study. The sample size was determined based on a priori power analysis for seven-step repeated measures test using GLIMMPSE web-based power and sample size program (http://glimmpse.samplesizeshop.org). Presuming 0.1 MPa initial (90° knee flexion angle) posterior pressure continuously increasing by 0.1 MPa until 0° full knee extension for tension band wiring using K-wires, and 0.3 MPa posterior pressure continuously increasing by 0.3 MPa using cannulated screws, an initial intermeasurement correlation of 0.6 with 0.1 decay rate, a sample size of n = 5 would be required to detect a significant response of the posterior pressure upon knee movement under a power of 0.839 and level of significance 0.05. To account for unexpected irregularities during testing, a sample size of n = 6 was chosen for each group. All donors gave their informed consent within the donation of anatomical gift statement during their lifetime. The specimens were randomly assigned to two study groups with six specimens each for treatment with tension band wiring techniques using either K-wires or cannulated screws. Radiographic assessment assured intact knee joints without any pathology in all specimens. A physiological range of motion between 130° flexion and full knee extension was proven by physical examination. The specimens were thawed at room temperature prior to preparation and biomechanical testing.
Each knee was prepared following a procedure as previously described by Schnabel et al. The femur was transected 12 cm proximally and the tibia 15 cm distally to the knee joint by using a handsaw. The fibula was removed and soft tissue was stripped off preserving the joint capsule, ligaments and the extensor mechanism intact. The proximal 6 cm of the femur and the distal 6 cm of the tibia were embedded in polymethylmethacrylate (PMMA, Suter Kunststoffe AG, Fraubrunnen, Switzerland). A steel rod was secured into the tibia canal during embedding. With the knee in full extended position, a transverse patella fracture was simulated via an osteotomy between the superior and inferior poles of the patella using an oscillating saw. The extensor retinaculum was divided in line with the transverse cut.
An electronic pressure sensor with thickness of 0.1 mm, spatial resolution of 0.695 mm2 and a total matrix area of 1023 mm2 (Tekscan 5033, 46 x 32 sensels, 38.3 mm x 26.7 mm, Tekscan Inc., South Boston, USA) was inserted into the osteotomy gap from anterior to posterior direction. Two 0.5 mm thick rubber foils were attached on both sides of the sensor to prevent uneven stress distribution over the sensing area. Prior to insertion, each
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sensor was calibrated on a calibration device (trublu Eichgerät, novel GmbH, Munich, Germany) applying a twopoint power law calibration at 0.3 MPa and 0.6 MPa pressure with Mid-2 sensitivity (I-Scan, Tekscan Inc., South Boston, USA). The calibration succeeded prior to foil attachment and resulted in an approximate saturation pressure of 112 MPa. One sensor was used per specimen.
Surgical technique Prior to instrumentation the created osteotomy gap was anatomically reduced and stabilized using two pointed forceps. For the tension band wiring technique using K-wires two 1.8 mm K-wires were inserted through the osteotomy from the proximal to the distal pole. A 1.0 mm cerclage was then placed in a figure-of-eight pattern through the ligamentous structures and around the K-wires close to the patella surface. Two twisted knots, dividing the cerclage wire in two sections of equal length, were tightened with surgical pliers. The distal ends of both K-wires were bent over towards the patella to prevent slipping of the cerclage. A specimen instrumented with tension band wiring using K-wires is shown in Figure 1a.
The tension band wiring technique with cannulated screws was performed by inserting two 1.2 mm K-wires through the osteotomy from the proximal to the distal pole. A 2.7 mm cannulated drill bit was then inserted over the K-wires to drill out the pilot holes. A short-threaded self-drilling 4.0 mm titanium alloy (TAN) cannulated screw was inserted in antegrade direction over each K-wire using a cannulated screw driver. Optimal screw length was measured individually for each specimen, taking into account that the screw tip should remain completely intraosseous. A 1.0 mm cerclage wire was inserted into the cannula of the screws and fixed in a similar way as with tension band wiring using K-wires. A specimen instrumented with tension band wiring using cannulated screws is shown in Figure 1b.
In both groups the K-wires were inserted after positioning of the electronic sensor in the fracture gap, ensuring hereby its stable trans-fixation via perforation at the entry points. Proper instrumentation was checked via radiographic examination of all specimens as shown in Figure 1c-d. All instrumentation procedures were performed by a single surgeon according to the surgical techniques as prescribed by the manufacturer. All implants were provided by the same manufacturer (DePuy Synthes, Zuchwil, Switzerland). Finally, three retroreflective marker sets were attached to the proximal patella fragment, distal patella fragment and proximal tibia for optical motion tracking during biomechanical testing.
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Biomechanical Testing Biomechanical testing was performed on a servohydraulic test system (Bionix 858.20; MTS Systems, Eden Prairie, MN, USA) in a test setup similar to Schnabel et al., as shown in Figure 2. Each femur was fixed proximally to the base of the testing frame in horizontal position. Pulling force was introduced to the quadriceps tendon via an inextensible steel cable, attached via a pulley to a 25 kN load cell being interconnected to the machine actuator. The quadriceps tendon was sutured between two custom steel plates, connected to the steel cable with a Ti-Cron 5 suture (Covidien plc, Dublin, Ireland). Weight of the lower leg was simulated attaching a disc of 3.1 kg load to the steel rod, fixed in the tibia canal during specimen's embedding, at a distance of 25 cm distally to the knee joint, establishing same moment arm around the knee for each specimen equivalent to that of an average person weighing 70 kg [15].
The loading protocol was adapted from previously published work [25, 26]. One test with three ramped, continuous load cycles, ranging from 90º knee flexion to full knee extension was performed in displacement control of the machine actuator at 1/6 Hz for each specimen and stabilization technique to simulate active knee extension and passive knee flexion of a sitting patient. After each cycle the actuator was paused for 5 seconds, keeping the knee at 90° flexion. The range for actuator displacement required to reach these extreme positions was assessed by extending and flexing the knee in a manually controlled actuator displacement one-cycle test prior to main test start.
Data acquisition and analysis Machine data in terms of force (equivalent to pulling tendon force, N) and actuator displacement (proportional to knee angle, °), as well as pressure sensor output signals (MPa) were continuously recorded during each biomechanical test at 64 Hz and 30 Hz, respectively.
For pressure distribution analysis, the area of each pressure sensor was divided in one posterior and one anterior region, defined as the areas of the osteotomy plane closer or more distant to the knee joint surface, respectively (Figure 3). Posterior and anterior pressure was calculated by dividing the force acting upon the respective region to its area by using software package I-Scan (Tekscan Inc., South Boston, MA, USA). While performing the first two loading cycles for preconditioning purposes, the third cycle was considered for evaluation and investigated during both extension and flexion phases of knee movement in 15° steps as follows: 90º - 75º - 60º - 45º - 30º -
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15º - 0º extension phase and 0° - 15º - 30º - 45º - 60º - 75º - 90º flexion phase. Begin and end of a loading cycle were characterized by an acute pressure change in the continuous time-pressure graph. Considering the temporal distance between these two points as the duration of one full loading cycle, and the linear relationship between time and knee angle based on the ramped (not sinusoidal) nature of each cycle, the time scale was converted into a continuous knee angle scale using trivial linear conversion formula. Consequently, the distances between the 13 knee angle positions were characterized by 12 sections of equal length. Finally, maximum anterior and posterior pressure was determined for each specimen and loading phase together with the respective knee angle at which maximum pressure was reached.
A three-dimensional optical Motion Tracking system with five Qualisys ProReflex MCU cameras (Qualisys AB, Gothenburg, Sweden) was used to capture movements of the marker sets, attached to the patella fragments and the proximal tibia at 100 Hz throughout the test. The system was found to operate at a resolution of 10 µm, an accuracy of 1.39%-2.37%, and a precision of 1.9-3.9 µm [27].
Fracture site displacements (mm) at the most anterior (ventral) and posterior (dorsal) aspects of the patella along the tibia axis, as well as interfragmentary rotation around the mediolateral axis were calculated from the motion tracking data in the same fashion (knee angles, loading phases and cycles) as for pressure distribution analysis. Software packages Qualisys Track Manager (v.1.10.282, Qualisys AB) and Matlab (v.2015a, The MathWorks, Natick, MA, USA) were used for this purpose. A value of 2 mm posterior displacement was defined as failure criterion in accordance with previous studies [3, 8, 25].
Statistical analysis was carried out using SPSS software package (v.21, IBM SPSS, Armonk, NY, USA). Data was screened for normality of distribution with Shapiro-Wilk test. General Linear Model Repeated Measures test was applied to assess influence of knee motion on posterior and anterior pressure, as well as on the posterior and anterior fracture site displacements and interfragmentary rotation. It was employed for each loading mode separately with each loading mode comprising seven repeated measurements (90º - 75º - 60º - 45º - 30º - 15º - 0º extension phase and 0° - 15º - 30º - 45º - 60º - 75º - 90º flexion phase). Independent samples t-test was conducted to detect significant differences between the fixation techniques. Differences between anterior and posterior pressure, as well as between anterior and posterior fracture site displacements were explored with paired samples t-tests. Level of significance was set to 0.05 for all statistical tests.
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Results All biomechanical tests were absolved without catastrophic failure of the specimens and with a normal distribution of the data for all parameters of interest. The critical value of 2 mm displacement at the posterior fracture site was not reached within the performed three test cycles for any specimen and fixation technique.
Interfragmentary pressure Mean anterior and posterior pressure in group using K-wires was within the range 0.16 – 0.40 MPa/0.12 – 0.35 MPa, and 0.37 – 0.59 MPa/0.10 – 0.30 MPa in group using cannulated screws, respectively. The course of pressure in both sites is shown in Figure 4 for each fixation technique and loading phase of the knee separately. Maximum pressures and corresponding knee angles, at which these maxima were reached, are shown in Table 1. Both anterior and posterior pressure, measured at the predefined knee angles, did not change significantly in the course of each loading phase (P ≥ 0.171) for each fixation technique. Furthermore, no significant differences in anterior and posterior pressure were detected between the two fixation techniques for each knee angle and loading phase (P ≥ 0.102), with one exception showing a trend for higher anterior pressure with cannulated screws than K-wires at 45° extension phase (P = 0.053). Finally, the comparison between anterior and posterior pressure of each specimen revealed no significant differences within each group, knee angle and loading phase (P ≥ 0.102).
Fracture site displacements Mean anterior and posterior fracture site displacement in group using K-wires was within the range -0.01 – 0.53 mm/0.11 – 0.74 mm, and 0.11 – 0.55 mm/-0.10 – 0.50 mm in group using cannulated screws, respectively. The course of fracture site displacement in both sites is shown in Figure 5 for each fixation technique and loading phase of the knee separately. Anterior fracture site displacement in each group did not change significantly in the course of each loading phase and cycle (P ≥ 0.112). However, posterior fracture site displacement underwent a significant change in this regard in group using K-wires (P ≤ 0.047) but not cannulated screws (P ≥ 0.202). Displacement at the anterior fracture site revealed no significant differences between the two fixation techniques for each knee angle and loading phase (P ≥ 0.09). On the contrary, significantly smaller displacement at the posterior fracture site was detected in group using cannulated screws compared to K-wires at 60° and 75° extension phase (P ≤ 0.017), as well as at 45°, 60° and 75° flexion phase (P ≤ 0.018). Finally, the comparison between the displacements at the anterior and posterior fracture cites of each specimen resulted in significantly
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higher values of the former in group using K-wires at 15° extension phase, at full knee extension and at 15° flexion phase (P ≤ 0.037).
Interfragmentary rotation Mean interfragmentary rotation in group using K-wires was within the range -2.14 – 0.56°, and 0.16 – 1.03° using cannulated screws. The course of this rotation in both groups is shown in Figure 6a-b) for each loading phase of the knee separately. It changed significantly in the course of each loading phase in group using K-wires (P ≤ 0.030), but not cannulated screws, the latter showing only a trend for change in flexion phase (P ≥ 0.062). Finally, significantly higher rotation in terms of gap opening at the posterior fracture site was registered in group using K-wires compared to cannulated screws at 0° and 15° extension phase and at 15° flexion phase (P ≤ 0.047).
Quadriceps pulling force Mean quadriceps pulling force in group using K-wires was within the range 3 – 389 N, and 6 – 384 N using cannulated screws. The course of this force in both groups is shown in Figure 6c-d) for each loading phase of the knee separately.
Discussion The present study investigated biomechanically the validity of the tension band wiring principle for transverse patella fracture fixation by means of pressure measurements in the fracture gap and interfragmentary movement analysis. Fixation strength of two state-of-the-art tension band wiring techniques with the use of K-wires or cannulated screws was quantified.
The first (null-) hypothesis that tension band wiring would not induce significant changes upon knee motion was partially rejected, namely for posterior displacement and interfragmentary rotation in the group using K-wires. On the other hand, neither anterior nor posterior pressure change significantly responded to the course of cyclic knee motion.
The highest interfragmentary posterior pressure is theoretically expected when the highest distraction forces act upon the anterior site of the patella. Taking into account diverse parameters influencing fragment stability, such
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as lever arm between patella and tibio-femoral joint, patellofemoral joint reaction forces as well as quadriceps muscle forces, the highest distraction forces can be expected for movements in the range between approximately 30° knee flexion and full knee extension [28]. Considering both fixaton techniques, loading phases and evaluated cycles, the maximum posterior pressure was realized at knee angles between 8° and 49°. This is approximately the knee angle with highest lever arm between the patella and the tibio-femoral joint [29], as well as with highest expected patellofemoral joint reaction forces [30]. From this point of view, our findings are in agreement with the principle of tension band wiring. However, the changes in posterior pressure between the predefined knee angles, observed within a loading phase for both fixation techniques failed to statistically substantiate this theory. Based on motion tracking data the measured posterior fracture site displacement continuously increased from 90° flexion to full knee extension in both groups, thereby significantly changing in group using K-wires. Similar progression was observed for quadriceps pulling forces reaching a peak at full knee extension in each group. This synchronous increase in fragment distraction and pulling force seems to be contradictory to the principle of tension band wiring, but is in agreement with the majority of previously published work [7, 10, 13, 25].
Although interfragmentary pressure did not change considerably upon knee movement, it was present for both the anterior and posterior fracture regions in the groups, indicating maintained stability of the fixed fragments. Moreover, both anterior and posterior regions revealed comparable pressure, which is an indication for homogeneous pressure distribution over the whole fracture site. Preserved sufficient stability was also reflected by the motion tracking data, showing that critical values defining loss of reduction, reported to be between 2 mm [3, 8, 25] and 3 mm [31] displacement at the posterior fracture site, have not been reached in any of the groups, accepting hereby the second hypothesis of this study. Although, or precisely because this stability remained to the biggest part indifferent in the course of knee movement and between anterior and posterior sites, or even decreased with acting pulling forces, as found and reported above, we anticipate considering tension band wiring rather as a static than a dynamic fixation technique. However, it was shown to fulfill the requirements for adequate fracture stabilization, which is in contrast to reported findings of some previous studies claiming the opposite [8, 9, 11]. However, a direct comparison to other published work is infeasible due to different study design and evaluated parameters.
Tension band wiring with use of cannulated screws showed significantly less posterior fracture site displacement than with K-wires for some knee angles. The latter can be considered as the method of preferable choice if additional stabilization is required, which is not surprising, considering the additional compressive strength that
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short-threaded screws provide. Their use has been favored in previous publications [15, 18]. Surprisingly, there were no significant differences between the two techniques with regard to anterior fracture site displacement. Furthermore, no clear advantage could be ascribed to any of the two fixation methods with respect to interfragmentary pressure, which remained at the same level. This fact could be referred to distraction forces acting on the patella, being potentially high enough to destroy the interface between the trabecular structures and the threaded part of the screw, and herewith its anchorage. Furthermore, the screw heads could have migrated, at least on a microscopic scale, into the bony structures. In both cases an abrupt loss of compression would have occurred.
There was a distinct difference in interfragmentary rotations around the mediolateral axis between the two fixation techniques. Whereas during knee extension the fragments in group using K-wires rotated predominantly toward posterior site opening, the predominant rotation in group using cannulated screws was toward anterior site opening. Posterior site opening in group using K-wires could have been facilitated by the cerclage, inducing anterior bending of the K-wires. Its highest values were detected at full knee extension, the state with highest pulling forces. In group using cannulated screws the rigid screws seemed to have withstood the bending moments during tightening of the cerclages, thus resulting in cerclage-bone interface as the weakest link and leading to predominant anterior wedge opening, which was maximal at 45° knee flexion phase.
The limitations of the present work were similar to those inherent to all cadaveric biomechanical studies with first and most important a limited number of tested specimens and scattering of the measured data, rendering statistical power possibly lower than initially predicted. Second, non-paired cadaveric knees were used, disabling a pairwise and thus more meaningful comparison. Third, each pressure sensor had to be perforated with K-wires or two cannulated screws, both differing in diameter and potentially unevenly affecting the measurements. The humidity-sensitive sensors were harmed by exposing them to a cadaveric environment. In addition, rubber foils that were attached to the sensors may have influenced the results. Some negative values indicated bone compaction, but this may in fact be the result of foil deformation. Forth, the setup used in this study could have been too aggressive, simulating exercising forces during active knee extension. Clinically, patients would usually not undergo such extreme movements immediately after surgery. However, the aim was to investigate the validity of the tension band principle, and therefore the use of higher forces was justified. Fifth, the performed three loading cycles deem at a first glance not enough to investigate fixation stability of these two techniques. However, we focused on the investigation of the dynamic principle, which is basically applicable to all
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subsequent loading cycles. In addition, in a similar biomechanical study performed by Patel et al. [26], the authors reported no significant decrease in quality of fixation after loading three knees in a long-term cyclic test. Relying on these findings the consideration of the third loading cycle for evaluation of fixation stability is justified. Finally, knots of the cerclage wire were twisted according to surgeon's feeling for each specimen individually, making the instrumentation less standardized and comparable.
The validity of the tension band was directly assessed in the fracture gap by using calibrated pressure sensors. In addition, a motion tracking system, capable to detect interfragmentary movements at high resolution, was used. These two approaches represented a methodological strength of this study.
Conclusions Tension band wiring fulfills from a biomechanical point of view the requirements for sufficient fixation stability in transverse patella fracture fixation. It should, however, rather be considered as a static fixation principle than a dynamic one. Tension band wiring with cannulated screws was found advantageous over Kirschner wires with regard to interfragmentary movements at the posterior fracture site.
Conflict of Interest The authors are not compensated and there are no other institutional subsidies, corporate affiliations, or funding sources supporting this work unless clearly documented and disclosed. Acknowledgments DePuy Synthes is acknowledged for providing all implants. We thank very much Dr. Tomas Nicolino for the instrumentation of all specimens.
Funding This investigation was performed with the assistance of the AO Foundation via the AOTK System.
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[16] Parent S, Wedemeyer M, Mahar AT, Anderson M, Faro F, Steinman S, et al. Displaced olecranon fractures in children: a biomechanical analysis of fixation methods. J Pediatr Orthop. 2008;28:147-51. [17] Chang SM, Ji XL. Open reduction and internal fixation of displaced patella inferior pole fractures with anterior tension band wiring through cannulated screws. Journal of orthopaedic trauma. 2011;25:366-70. [18] Tian Y, Zhou F, Ji H, Zhang Z, Guo Y. Cannulated screw and cable are superior to modified tension band in the treatment of transverse patella fractures. Clin Orthop Relat Res. 2011;469:3429-35. [19] Rahm S, Lionetto S, Hotz T, Kaech K. Screw fixation as an alternative to tension band wiring in displaced transverse patella fractures: a retrospective, single center study. BRITISH JOURNAL OF SURGERY: WILEYBLACKWELL COMMERCE PLACE, 350 MAIN ST, MALDEN 02148, MA USA; 2010. p. 21-. [20] Gehr J, Friedl W. [Problems in osteosynthesis of patella fractures with the AO tension belt and consequences for new implants. The XS nail]. Chirurg. 2001;72:1309-17; discussion 17-8. [21] Friedl W. Zuggurtungsnagel. Trauma und Berufskrankheit. 2004;6:S225-S34. [22] Luna-Pizarro D, Amato D, Arellano F, Hernandez A, Lopez-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. Journal of orthopaedic trauma. 2006;20:529-35. [23] Thelen S, Schneppendahl J, Jopen E, Eichler C, Koebke J, Schonau 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;43:1290-5. [24] Sadri H, Stern R, Singh M, Linke B, Hoffmeyer P, Schwieger K. Transverse fractures of the olecranon: a biomechanical comparison of three fixation techniques. Arch Orthop Trauma Surg. 2011;131:131-8. [25] Brink PR, Windolf M, de Boer P, Brianza S, Braunstein V, Schwieger K. Tension band wiring of the olecranon: is it really a dynamic principle of osteosynthesis? Injury. 2013;44:518-22. [26] Patel VR, Parks BG, Wang Y, Ebert FR, Jinnah RH. Fixation of patella fractures with braided polyester suture: a biomechanical study. Injury. 2000;31:1-6. [27] Liu H, Holt C, Evans S. Accuracy and repeatability of an optical motion analysis system for measuring small deformations of biological tissues. J Biomech. 2007;40:210-4. [28] Carpenter JE, Kasman R, Matthews LS. Fractures of the patella. Instr Course Lect. 1994;43:97-108. [29] Krevolin JL, Pandy MG, Pearce JC. Moment arm of the patellar tendon in the human knee. Journal of biomechanics. 2004;37:785-8. [30] Reilly DT, Martens M. Experimental analysis of the quadriceps muscle force and patello-femoral joint reaction force for various activities. Acta orthopaedica Scandinavica. 1972;43:126-37.
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[31] Benjamin J, Bried J, Dohm M, McMurtry M. Biomechanical evaluation of various forms of fixation of transverse patellar fractures. Journal of orthopaedic trauma. 1987;1:219-22.
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Figure captions Fig. 1 Photographs (a, b) and radiographic images (c, d) of two exemplified specimens instrumented with tension band wiring using K-wires (a, c) and cannulated screws (b, d)
Fig. 2 Test setup with an instrumented specimen mounted for mechanical testing
Fig. 3 a) Evaluation principle showing the division of the sensing area in an anterior (green bordered) and a posterior (red bordered) region; b) Retrograde view on the fracture gap with the sensor schematically divided in two halves
Fig. 4 Course of anterior (green) and posterior (red) pressure in the fracture gap for tension band wiring using Kwires (a-b) and cannulated screws (c-d), shown separately for extension (a, c) and flexion (b, d) phase in terms of mean and standard deviation (dashed lines) values
Fig. 5 Course of anterior (green) and posterior (red) fracture site displacement for tension band wiring using Kwires (a-b) and cannulated screws (c-d), shown separately for extension (a, c) and flexion (b, d) phase in terms of mean and standard deviation (dashed lines) values
Fig. 6 Course of interfragmentary rotation (a, b) and quadriceps pulling force (c, d) for tension band wiring using K-wires (green) and cannulated screws (red), shown separately for extension (a, c) and flexion (b, d) phase in terms of mean and standard deviation (dashed lines) values
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18
19
20
21
22
23
Table 1: Maximum anterior and posterior pressure and angle at maximum pressure shown in terms of mean and standard deviation values for each fixation technique and loading phase.
Maximum pressure (MPa) Angle at maximum pressure (°)
K-wires Cann. Screws K-wires Cann. Screws
Extension phase Anterior Posterior 0.40 0.38 (0.41) (0.27) 0.79 0.32 (0.43) (0.38) 15 (30) 45 (19)
Flexion phase Anterior Posterior 0.40 0.40 (0.41) (0.20) 0.71 0.33 (0.34) (0.37) 8 (15) 35 (24)
49 (41)
45 (44)
23 (29)
24
19 (14)
S0020-1383(17)30362-5 http://dx.doi.org/doi:10.1016/j.injury.2017.05.037 JINJ 7263
To appear in:
Injury, Int. J. Care Injured
Accepted date:
27-5-2017
Please cite this article as: Zderic Ivan, Stoffel Karl, Sommer Christoph, H¨ontzsch Dankward, Gueorguiev Boyko.Biomechanical evaluation of the tension band wiring principle.A comparison between two different techniques for transverse patella fracture fixation.Injury http://dx.doi.org/10.1016/j.injury.2017.05.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Biomechanical evaluation of the tension band wiring principle. A comparison between two different techniques for transverse patella fracture fixation.
Ivan Zderica, MSc, Karl Stoffelb, c, PhD, Christoph Sommerd, MD, Dankward Höntzsche, MD, Boyko Gueorguieva, PhD
a
AO Research Institute Davos, Davos, Switzerland
b
Cantonal Hospital Baselland, Bruderholz, Switzerland
c
University of Basel, Basel, Switzerland
d
Cantonal Hospital Graubuenden, Chur, Switzerland
e
BG Klinik Tübingen, Tübingen, Germany
Corresponding author: Ivan Zderic AO Research Institute Davos, Clavadelerstrasse 8, 7270 Davos, Switzerland E-Mail: [email protected]
Abstract Purpose: The aim of this study was to investigate the validity of the dynamic compression principle of tension band wiring in two techniques for patella fracture treatment.
Methods: Twelve human cadaveric knees with simulated transverse patella fractures were assigned to two groups for treatment with tension band wiring using Kirschner (K-) wires or cannulated screws. Biomechanical testing was performed over three knee movement cycles between 90° flexion and 0° extension. Pressure distribution in the fracture gap and fracture site displacement was evaluated at the 3rd cycle in 15° steps, namely 90º-75º-60º-45º-30º-15º-0º extension and 0°-15º-30º-45º-60º-75º-90º flexion phase.
Results: Mean anterior and posterior interfragmentary pressure in group with K-wires ranged within 0.16 – 0.40 MPa/0.12 – 0.35 MPa, and 0.37 – 0.59 MPa/0.10 – 0.30 MPa with cannulated screws. These changes remained non-significant in both groups and loading phases (P≥0.171). Mean anterior and posterior fracture site displacement in group with K-wires ranged within -0.01 – 0.53 mm/0.11 – 0.74 mm, and 0.11 – 0.55 mm/-0.10 –
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0.50 mm with cannulated screws. Anterior displacement remained without significant changes in both groups and loading phases (P≥0.112). However, posterior displacement underwent a significant increase in this regard for K-wires (P≤0.047), but not for cannulated screws (P≥0.202), in the course of knee extension. Significantly smaller displacement at the posterior fracture site was detected in group using cannulated screws compared to Kwires at 60° and 75° extension phase (P≤0.017), as well as at 45°, 60° and 75° flexion phase (P≤0.018). The critical value of 2 mm displacement at the posterior fracture site was not reached for any specimen and fixation technique. Knee extension was accompanied by synchronous increase in quadriceps pulling force.
Conclusions: Tension band wiring fulfills from a biomechanical point of view the requirements for sufficient fixation stability in transverse patella fracture fixation. It should, however, rather be considered as a static fixation principle than a dynamic one. Tension band wiring with cannulated screws was found advantageous over Kirschner wires with regard to interfragmentary movements at the posterior fracture site.
Key words: Transverse patella fracture, Tension band wiring, Kirschner wire, Cannulated screw, Biomechanics, Interfragmentary pressure
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Introduction The incidence of patella fractures, usually caused by direct trauma of the anterior knee surface, is approximately 0.5-1.5% of all fractures [1]. Among them, most common are the transverse patella fractures [2]. Operative treatment is indicated for displaced fractures with a disrupted extensor mechanism [1]. High contact forces acting between the patella and the femur require restoration and preservation of a smooth articular surface in order to prevent posttraumatic osteoarthritis [3]. This can only be achieved if the two fragments are anatomically reduced and subsequently firmly fixed.
Tension band wiring is one of the most common treatment methods for transverse patella fracture fixation [3]. Its main principle in all modifications is to counteract muscle traction, maintain the fracture reduced and possibly transform the tensile forces between the quadriceps muscle and the anterior tibia tuberosity into compression at the articular patella cortex during knee flexion [4]. The latter should stabilize the fracture and enhance bone healing in an environment of large distraction forces by closing the fracture gap and maintaining the interfragmentary contact [5]. Similar techniques are applied for olecranon and malleolus fracture fixation [6].
Although tension band wiring is currently one of the most frequently used fixation techniques for patella fracture fixation, poor clinical outcomes have been reported in up to 55% of the cases [1]. Moreover, lots of previous studies are not in favor of this principle by stating that other implants, such as compression screws (single or in combination with tension band wiring), perform better in terms of fixation stability, complication rates or functional outcomes [2, 3, 7-25]. Currently, there is no existing evidence yet, based on pressure assessment in the patella fracture gap and/or interfragmentary movements, that this principle is valid in the surgical practice. Specifically, there are no existing studies on pressure distribution in the patellar fracture gap, displacement and rotational movement at the fracture site, related to knee flexion and extension during the early mobilization phase, proving that tension band wiring by using Kirschner (K-) wires or cannulated screws is a effective surgical treatment.
Therefore, the aim of the present study was to investigate whether the principle of tension band wiring applies for transverse patella fracture fixation during the extension and flexion phase of knee movement. Referring to previous findings on tension band wiring of the olecranon [25], we examined the following hypotheses for the tension band wiring of the knee in the acute postoperative phase of knee movement:
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1) Tension band wiring using K-wires and cannulated screws does not induce a significant change upon knee motion in terms of interfragmentary pressure and movement; 2) Interfragmentary displacements do not exceed the 2 mm-threshold upon knee motion with these two fixation techniques, satisfying the requirements for primary stability; 3) There are no significant differences between the two techniques in terms of interfragmentary pressure and movement.
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Materials and Methods Specimens and preparation Twelve fresh-frozen (-20°C) human cadaveric knees (5 left and 7 right) with distal femur and proximal tibia parts, including soft tissue, were used in this study. The sample size was determined based on a priori power analysis for seven-step repeated measures test using GLIMMPSE web-based power and sample size program (http://glimmpse.samplesizeshop.org). Presuming 0.1 MPa initial (90° knee flexion angle) posterior pressure continuously increasing by 0.1 MPa until 0° full knee extension for tension band wiring using K-wires, and 0.3 MPa posterior pressure continuously increasing by 0.3 MPa using cannulated screws, an initial intermeasurement correlation of 0.6 with 0.1 decay rate, a sample size of n = 5 would be required to detect a significant response of the posterior pressure upon knee movement under a power of 0.839 and level of significance 0.05. To account for unexpected irregularities during testing, a sample size of n = 6 was chosen for each group. All donors gave their informed consent within the donation of anatomical gift statement during their lifetime. The specimens were randomly assigned to two study groups with six specimens each for treatment with tension band wiring techniques using either K-wires or cannulated screws. Radiographic assessment assured intact knee joints without any pathology in all specimens. A physiological range of motion between 130° flexion and full knee extension was proven by physical examination. The specimens were thawed at room temperature prior to preparation and biomechanical testing.
Each knee was prepared following a procedure as previously described by Schnabel et al. The femur was transected 12 cm proximally and the tibia 15 cm distally to the knee joint by using a handsaw. The fibula was removed and soft tissue was stripped off preserving the joint capsule, ligaments and the extensor mechanism intact. The proximal 6 cm of the femur and the distal 6 cm of the tibia were embedded in polymethylmethacrylate (PMMA, Suter Kunststoffe AG, Fraubrunnen, Switzerland). A steel rod was secured into the tibia canal during embedding. With the knee in full extended position, a transverse patella fracture was simulated via an osteotomy between the superior and inferior poles of the patella using an oscillating saw. The extensor retinaculum was divided in line with the transverse cut.
An electronic pressure sensor with thickness of 0.1 mm, spatial resolution of 0.695 mm2 and a total matrix area of 1023 mm2 (Tekscan 5033, 46 x 32 sensels, 38.3 mm x 26.7 mm, Tekscan Inc., South Boston, USA) was inserted into the osteotomy gap from anterior to posterior direction. Two 0.5 mm thick rubber foils were attached on both sides of the sensor to prevent uneven stress distribution over the sensing area. Prior to insertion, each
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sensor was calibrated on a calibration device (trublu Eichgerät, novel GmbH, Munich, Germany) applying a twopoint power law calibration at 0.3 MPa and 0.6 MPa pressure with Mid-2 sensitivity (I-Scan, Tekscan Inc., South Boston, USA). The calibration succeeded prior to foil attachment and resulted in an approximate saturation pressure of 112 MPa. One sensor was used per specimen.
Surgical technique Prior to instrumentation the created osteotomy gap was anatomically reduced and stabilized using two pointed forceps. For the tension band wiring technique using K-wires two 1.8 mm K-wires were inserted through the osteotomy from the proximal to the distal pole. A 1.0 mm cerclage was then placed in a figure-of-eight pattern through the ligamentous structures and around the K-wires close to the patella surface. Two twisted knots, dividing the cerclage wire in two sections of equal length, were tightened with surgical pliers. The distal ends of both K-wires were bent over towards the patella to prevent slipping of the cerclage. A specimen instrumented with tension band wiring using K-wires is shown in Figure 1a.
The tension band wiring technique with cannulated screws was performed by inserting two 1.2 mm K-wires through the osteotomy from the proximal to the distal pole. A 2.7 mm cannulated drill bit was then inserted over the K-wires to drill out the pilot holes. A short-threaded self-drilling 4.0 mm titanium alloy (TAN) cannulated screw was inserted in antegrade direction over each K-wire using a cannulated screw driver. Optimal screw length was measured individually for each specimen, taking into account that the screw tip should remain completely intraosseous. A 1.0 mm cerclage wire was inserted into the cannula of the screws and fixed in a similar way as with tension band wiring using K-wires. A specimen instrumented with tension band wiring using cannulated screws is shown in Figure 1b.
In both groups the K-wires were inserted after positioning of the electronic sensor in the fracture gap, ensuring hereby its stable trans-fixation via perforation at the entry points. Proper instrumentation was checked via radiographic examination of all specimens as shown in Figure 1c-d. All instrumentation procedures were performed by a single surgeon according to the surgical techniques as prescribed by the manufacturer. All implants were provided by the same manufacturer (DePuy Synthes, Zuchwil, Switzerland). Finally, three retroreflective marker sets were attached to the proximal patella fragment, distal patella fragment and proximal tibia for optical motion tracking during biomechanical testing.
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Biomechanical Testing Biomechanical testing was performed on a servohydraulic test system (Bionix 858.20; MTS Systems, Eden Prairie, MN, USA) in a test setup similar to Schnabel et al., as shown in Figure 2. Each femur was fixed proximally to the base of the testing frame in horizontal position. Pulling force was introduced to the quadriceps tendon via an inextensible steel cable, attached via a pulley to a 25 kN load cell being interconnected to the machine actuator. The quadriceps tendon was sutured between two custom steel plates, connected to the steel cable with a Ti-Cron 5 suture (Covidien plc, Dublin, Ireland). Weight of the lower leg was simulated attaching a disc of 3.1 kg load to the steel rod, fixed in the tibia canal during specimen's embedding, at a distance of 25 cm distally to the knee joint, establishing same moment arm around the knee for each specimen equivalent to that of an average person weighing 70 kg [15].
The loading protocol was adapted from previously published work [25, 26]. One test with three ramped, continuous load cycles, ranging from 90º knee flexion to full knee extension was performed in displacement control of the machine actuator at 1/6 Hz for each specimen and stabilization technique to simulate active knee extension and passive knee flexion of a sitting patient. After each cycle the actuator was paused for 5 seconds, keeping the knee at 90° flexion. The range for actuator displacement required to reach these extreme positions was assessed by extending and flexing the knee in a manually controlled actuator displacement one-cycle test prior to main test start.
Data acquisition and analysis Machine data in terms of force (equivalent to pulling tendon force, N) and actuator displacement (proportional to knee angle, °), as well as pressure sensor output signals (MPa) were continuously recorded during each biomechanical test at 64 Hz and 30 Hz, respectively.
For pressure distribution analysis, the area of each pressure sensor was divided in one posterior and one anterior region, defined as the areas of the osteotomy plane closer or more distant to the knee joint surface, respectively (Figure 3). Posterior and anterior pressure was calculated by dividing the force acting upon the respective region to its area by using software package I-Scan (Tekscan Inc., South Boston, MA, USA). While performing the first two loading cycles for preconditioning purposes, the third cycle was considered for evaluation and investigated during both extension and flexion phases of knee movement in 15° steps as follows: 90º - 75º - 60º - 45º - 30º -
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15º - 0º extension phase and 0° - 15º - 30º - 45º - 60º - 75º - 90º flexion phase. Begin and end of a loading cycle were characterized by an acute pressure change in the continuous time-pressure graph. Considering the temporal distance between these two points as the duration of one full loading cycle, and the linear relationship between time and knee angle based on the ramped (not sinusoidal) nature of each cycle, the time scale was converted into a continuous knee angle scale using trivial linear conversion formula. Consequently, the distances between the 13 knee angle positions were characterized by 12 sections of equal length. Finally, maximum anterior and posterior pressure was determined for each specimen and loading phase together with the respective knee angle at which maximum pressure was reached.
A three-dimensional optical Motion Tracking system with five Qualisys ProReflex MCU cameras (Qualisys AB, Gothenburg, Sweden) was used to capture movements of the marker sets, attached to the patella fragments and the proximal tibia at 100 Hz throughout the test. The system was found to operate at a resolution of 10 µm, an accuracy of 1.39%-2.37%, and a precision of 1.9-3.9 µm [27].
Fracture site displacements (mm) at the most anterior (ventral) and posterior (dorsal) aspects of the patella along the tibia axis, as well as interfragmentary rotation around the mediolateral axis were calculated from the motion tracking data in the same fashion (knee angles, loading phases and cycles) as for pressure distribution analysis. Software packages Qualisys Track Manager (v.1.10.282, Qualisys AB) and Matlab (v.2015a, The MathWorks, Natick, MA, USA) were used for this purpose. A value of 2 mm posterior displacement was defined as failure criterion in accordance with previous studies [3, 8, 25].
Statistical analysis was carried out using SPSS software package (v.21, IBM SPSS, Armonk, NY, USA). Data was screened for normality of distribution with Shapiro-Wilk test. General Linear Model Repeated Measures test was applied to assess influence of knee motion on posterior and anterior pressure, as well as on the posterior and anterior fracture site displacements and interfragmentary rotation. It was employed for each loading mode separately with each loading mode comprising seven repeated measurements (90º - 75º - 60º - 45º - 30º - 15º - 0º extension phase and 0° - 15º - 30º - 45º - 60º - 75º - 90º flexion phase). Independent samples t-test was conducted to detect significant differences between the fixation techniques. Differences between anterior and posterior pressure, as well as between anterior and posterior fracture site displacements were explored with paired samples t-tests. Level of significance was set to 0.05 for all statistical tests.
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Results All biomechanical tests were absolved without catastrophic failure of the specimens and with a normal distribution of the data for all parameters of interest. The critical value of 2 mm displacement at the posterior fracture site was not reached within the performed three test cycles for any specimen and fixation technique.
Interfragmentary pressure Mean anterior and posterior pressure in group using K-wires was within the range 0.16 – 0.40 MPa/0.12 – 0.35 MPa, and 0.37 – 0.59 MPa/0.10 – 0.30 MPa in group using cannulated screws, respectively. The course of pressure in both sites is shown in Figure 4 for each fixation technique and loading phase of the knee separately. Maximum pressures and corresponding knee angles, at which these maxima were reached, are shown in Table 1. Both anterior and posterior pressure, measured at the predefined knee angles, did not change significantly in the course of each loading phase (P ≥ 0.171) for each fixation technique. Furthermore, no significant differences in anterior and posterior pressure were detected between the two fixation techniques for each knee angle and loading phase (P ≥ 0.102), with one exception showing a trend for higher anterior pressure with cannulated screws than K-wires at 45° extension phase (P = 0.053). Finally, the comparison between anterior and posterior pressure of each specimen revealed no significant differences within each group, knee angle and loading phase (P ≥ 0.102).
Fracture site displacements Mean anterior and posterior fracture site displacement in group using K-wires was within the range -0.01 – 0.53 mm/0.11 – 0.74 mm, and 0.11 – 0.55 mm/-0.10 – 0.50 mm in group using cannulated screws, respectively. The course of fracture site displacement in both sites is shown in Figure 5 for each fixation technique and loading phase of the knee separately. Anterior fracture site displacement in each group did not change significantly in the course of each loading phase and cycle (P ≥ 0.112). However, posterior fracture site displacement underwent a significant change in this regard in group using K-wires (P ≤ 0.047) but not cannulated screws (P ≥ 0.202). Displacement at the anterior fracture site revealed no significant differences between the two fixation techniques for each knee angle and loading phase (P ≥ 0.09). On the contrary, significantly smaller displacement at the posterior fracture site was detected in group using cannulated screws compared to K-wires at 60° and 75° extension phase (P ≤ 0.017), as well as at 45°, 60° and 75° flexion phase (P ≤ 0.018). Finally, the comparison between the displacements at the anterior and posterior fracture cites of each specimen resulted in significantly
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higher values of the former in group using K-wires at 15° extension phase, at full knee extension and at 15° flexion phase (P ≤ 0.037).
Interfragmentary rotation Mean interfragmentary rotation in group using K-wires was within the range -2.14 – 0.56°, and 0.16 – 1.03° using cannulated screws. The course of this rotation in both groups is shown in Figure 6a-b) for each loading phase of the knee separately. It changed significantly in the course of each loading phase in group using K-wires (P ≤ 0.030), but not cannulated screws, the latter showing only a trend for change in flexion phase (P ≥ 0.062). Finally, significantly higher rotation in terms of gap opening at the posterior fracture site was registered in group using K-wires compared to cannulated screws at 0° and 15° extension phase and at 15° flexion phase (P ≤ 0.047).
Quadriceps pulling force Mean quadriceps pulling force in group using K-wires was within the range 3 – 389 N, and 6 – 384 N using cannulated screws. The course of this force in both groups is shown in Figure 6c-d) for each loading phase of the knee separately.
Discussion The present study investigated biomechanically the validity of the tension band wiring principle for transverse patella fracture fixation by means of pressure measurements in the fracture gap and interfragmentary movement analysis. Fixation strength of two state-of-the-art tension band wiring techniques with the use of K-wires or cannulated screws was quantified.
The first (null-) hypothesis that tension band wiring would not induce significant changes upon knee motion was partially rejected, namely for posterior displacement and interfragmentary rotation in the group using K-wires. On the other hand, neither anterior nor posterior pressure change significantly responded to the course of cyclic knee motion.
The highest interfragmentary posterior pressure is theoretically expected when the highest distraction forces act upon the anterior site of the patella. Taking into account diverse parameters influencing fragment stability, such
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as lever arm between patella and tibio-femoral joint, patellofemoral joint reaction forces as well as quadriceps muscle forces, the highest distraction forces can be expected for movements in the range between approximately 30° knee flexion and full knee extension [28]. Considering both fixaton techniques, loading phases and evaluated cycles, the maximum posterior pressure was realized at knee angles between 8° and 49°. This is approximately the knee angle with highest lever arm between the patella and the tibio-femoral joint [29], as well as with highest expected patellofemoral joint reaction forces [30]. From this point of view, our findings are in agreement with the principle of tension band wiring. However, the changes in posterior pressure between the predefined knee angles, observed within a loading phase for both fixation techniques failed to statistically substantiate this theory. Based on motion tracking data the measured posterior fracture site displacement continuously increased from 90° flexion to full knee extension in both groups, thereby significantly changing in group using K-wires. Similar progression was observed for quadriceps pulling forces reaching a peak at full knee extension in each group. This synchronous increase in fragment distraction and pulling force seems to be contradictory to the principle of tension band wiring, but is in agreement with the majority of previously published work [7, 10, 13, 25].
Although interfragmentary pressure did not change considerably upon knee movement, it was present for both the anterior and posterior fracture regions in the groups, indicating maintained stability of the fixed fragments. Moreover, both anterior and posterior regions revealed comparable pressure, which is an indication for homogeneous pressure distribution over the whole fracture site. Preserved sufficient stability was also reflected by the motion tracking data, showing that critical values defining loss of reduction, reported to be between 2 mm [3, 8, 25] and 3 mm [31] displacement at the posterior fracture site, have not been reached in any of the groups, accepting hereby the second hypothesis of this study. Although, or precisely because this stability remained to the biggest part indifferent in the course of knee movement and between anterior and posterior sites, or even decreased with acting pulling forces, as found and reported above, we anticipate considering tension band wiring rather as a static than a dynamic fixation technique. However, it was shown to fulfill the requirements for adequate fracture stabilization, which is in contrast to reported findings of some previous studies claiming the opposite [8, 9, 11]. However, a direct comparison to other published work is infeasible due to different study design and evaluated parameters.
Tension band wiring with use of cannulated screws showed significantly less posterior fracture site displacement than with K-wires for some knee angles. The latter can be considered as the method of preferable choice if additional stabilization is required, which is not surprising, considering the additional compressive strength that
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short-threaded screws provide. Their use has been favored in previous publications [15, 18]. Surprisingly, there were no significant differences between the two techniques with regard to anterior fracture site displacement. Furthermore, no clear advantage could be ascribed to any of the two fixation methods with respect to interfragmentary pressure, which remained at the same level. This fact could be referred to distraction forces acting on the patella, being potentially high enough to destroy the interface between the trabecular structures and the threaded part of the screw, and herewith its anchorage. Furthermore, the screw heads could have migrated, at least on a microscopic scale, into the bony structures. In both cases an abrupt loss of compression would have occurred.
There was a distinct difference in interfragmentary rotations around the mediolateral axis between the two fixation techniques. Whereas during knee extension the fragments in group using K-wires rotated predominantly toward posterior site opening, the predominant rotation in group using cannulated screws was toward anterior site opening. Posterior site opening in group using K-wires could have been facilitated by the cerclage, inducing anterior bending of the K-wires. Its highest values were detected at full knee extension, the state with highest pulling forces. In group using cannulated screws the rigid screws seemed to have withstood the bending moments during tightening of the cerclages, thus resulting in cerclage-bone interface as the weakest link and leading to predominant anterior wedge opening, which was maximal at 45° knee flexion phase.
The limitations of the present work were similar to those inherent to all cadaveric biomechanical studies with first and most important a limited number of tested specimens and scattering of the measured data, rendering statistical power possibly lower than initially predicted. Second, non-paired cadaveric knees were used, disabling a pairwise and thus more meaningful comparison. Third, each pressure sensor had to be perforated with K-wires or two cannulated screws, both differing in diameter and potentially unevenly affecting the measurements. The humidity-sensitive sensors were harmed by exposing them to a cadaveric environment. In addition, rubber foils that were attached to the sensors may have influenced the results. Some negative values indicated bone compaction, but this may in fact be the result of foil deformation. Forth, the setup used in this study could have been too aggressive, simulating exercising forces during active knee extension. Clinically, patients would usually not undergo such extreme movements immediately after surgery. However, the aim was to investigate the validity of the tension band principle, and therefore the use of higher forces was justified. Fifth, the performed three loading cycles deem at a first glance not enough to investigate fixation stability of these two techniques. However, we focused on the investigation of the dynamic principle, which is basically applicable to all
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subsequent loading cycles. In addition, in a similar biomechanical study performed by Patel et al. [26], the authors reported no significant decrease in quality of fixation after loading three knees in a long-term cyclic test. Relying on these findings the consideration of the third loading cycle for evaluation of fixation stability is justified. Finally, knots of the cerclage wire were twisted according to surgeon's feeling for each specimen individually, making the instrumentation less standardized and comparable.
The validity of the tension band was directly assessed in the fracture gap by using calibrated pressure sensors. In addition, a motion tracking system, capable to detect interfragmentary movements at high resolution, was used. These two approaches represented a methodological strength of this study.
Conclusions Tension band wiring fulfills from a biomechanical point of view the requirements for sufficient fixation stability in transverse patella fracture fixation. It should, however, rather be considered as a static fixation principle than a dynamic one. Tension band wiring with cannulated screws was found advantageous over Kirschner wires with regard to interfragmentary movements at the posterior fracture site.
Conflict of Interest The authors are not compensated and there are no other institutional subsidies, corporate affiliations, or funding sources supporting this work unless clearly documented and disclosed. Acknowledgments DePuy Synthes is acknowledged for providing all implants. We thank very much Dr. Tomas Nicolino for the instrumentation of all specimens.
Funding This investigation was performed with the assistance of the AO Foundation via the AOTK System.
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References [1] Wendl K, Zinser R, Hochstein P, Grützner PA. Frakturen der Kniescheibe. Trauma und Berufskrankheit. 2002;4:30-7. [2] Wild M, Eichler C, Thelen S, Jungbluth P, Windolf J, Hakimi M. Fixed-angle plate osteosynthesis of the patella - an alternative to tension wiring? Clin Biomech (Bristol, Avon). 2010;25:341-7. [3] Schnabel B, Scharf M, Schwieger K, Windolf M, Pol B, Braunstein V, et al. Biomechanical comparison of a new staple technique with tension band wiring for transverse patella fractures. Clin Biomech (Bristol, Avon). 2009;24:855-9. [4] Rowland SA, Burkhart SS. Tension band wiring of olecranon fractures. A modification of the AO technique. Clin Orthop Relat Res. 1992:238-42. [5] Nerlich M, Weigel B. Patella. AO Principles of Fracture Management Thieme, Stuttgart. 2000:483-97. [6] Josten C, Muhr G. Tension band principle. AO principles of fracture management Thieme, New York. 2000:187-92. [7] Hutchinson DT, Horwitz DS, Ha G, Thomas CW, Bachus KN. Cyclic Loading of Olecranon Fracture Fixation Constructs. The Journal of Bone & Joint Surgery. 2003;85:831-7. [8] Smith ST, Cramer KE, Karges DE, Watson JT, Moed BR. Early complications in the operative treatment of patella fractures. Journal of orthopaedic trauma. 1997;11:183-7. [9] Wilson J, Bajwa A, Kamath V, Rangan A. Biomechanical comparison of interfragmentary compression in transverse fractures of the olecranon. J Bone Joint Surg Br. 2011;93:245-50. [10] Brill W, Hopf T. [Biomechanical study of various osteosynthesis procedures in transverse patella fractures]. Unfallchirurg. 1987;90:162-72. [11] Baran O, Manisali M, Cecen B. Anatomical and biomechanical evaluation of the tension band technique in patellar fractures. International orthopaedics. 2009;33:1113-7. [12] Wang L, Cui Y. [Misunderstanding about the tension band principle in the treatment of patella fractures]. Zhongguo gu shang= China journal of orthopaedics and traumatology. 2010;23:125-7. [13] Labitzke R. [Proper and improper tension band fixation exemplified by patellar fracture]. Chirurg. 1997;68:638-42. [14] Issendorff WDv, Ahlers J, Ritter G. Die Drahtzuggurtungsosteosynthese—Untersuchungen zu Spannung und Fixierung des Osteosynthesedrahtes. Unfallchirurgie. 1990;16:277-85. [15] Carpenter JE, Kasman RA, Patel N, Lee ML, Goldstein SA. Biomechanical evaluation of current patella fracture fixation techniques. Journal of orthopaedic trauma. 1997;11:351-6.
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[16] Parent S, Wedemeyer M, Mahar AT, Anderson M, Faro F, Steinman S, et al. Displaced olecranon fractures in children: a biomechanical analysis of fixation methods. J Pediatr Orthop. 2008;28:147-51. [17] Chang SM, Ji XL. Open reduction and internal fixation of displaced patella inferior pole fractures with anterior tension band wiring through cannulated screws. Journal of orthopaedic trauma. 2011;25:366-70. [18] Tian Y, Zhou F, Ji H, Zhang Z, Guo Y. Cannulated screw and cable are superior to modified tension band in the treatment of transverse patella fractures. Clin Orthop Relat Res. 2011;469:3429-35. [19] Rahm S, Lionetto S, Hotz T, Kaech K. Screw fixation as an alternative to tension band wiring in displaced transverse patella fractures: a retrospective, single center study. BRITISH JOURNAL OF SURGERY: WILEYBLACKWELL COMMERCE PLACE, 350 MAIN ST, MALDEN 02148, MA USA; 2010. p. 21-. [20] Gehr J, Friedl W. [Problems in osteosynthesis of patella fractures with the AO tension belt and consequences for new implants. The XS nail]. Chirurg. 2001;72:1309-17; discussion 17-8. [21] Friedl W. Zuggurtungsnagel. Trauma und Berufskrankheit. 2004;6:S225-S34. [22] Luna-Pizarro D, Amato D, Arellano F, Hernandez A, Lopez-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. Journal of orthopaedic trauma. 2006;20:529-35. [23] Thelen S, Schneppendahl J, Jopen E, Eichler C, Koebke J, Schonau 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;43:1290-5. [24] Sadri H, Stern R, Singh M, Linke B, Hoffmeyer P, Schwieger K. Transverse fractures of the olecranon: a biomechanical comparison of three fixation techniques. Arch Orthop Trauma Surg. 2011;131:131-8. [25] Brink PR, Windolf M, de Boer P, Brianza S, Braunstein V, Schwieger K. Tension band wiring of the olecranon: is it really a dynamic principle of osteosynthesis? Injury. 2013;44:518-22. [26] Patel VR, Parks BG, Wang Y, Ebert FR, Jinnah RH. Fixation of patella fractures with braided polyester suture: a biomechanical study. Injury. 2000;31:1-6. [27] Liu H, Holt C, Evans S. Accuracy and repeatability of an optical motion analysis system for measuring small deformations of biological tissues. J Biomech. 2007;40:210-4. [28] Carpenter JE, Kasman R, Matthews LS. Fractures of the patella. Instr Course Lect. 1994;43:97-108. [29] Krevolin JL, Pandy MG, Pearce JC. Moment arm of the patellar tendon in the human knee. Journal of biomechanics. 2004;37:785-8. [30] Reilly DT, Martens M. Experimental analysis of the quadriceps muscle force and patello-femoral joint reaction force for various activities. Acta orthopaedica Scandinavica. 1972;43:126-37.
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Figure captions Fig. 1 Photographs (a, b) and radiographic images (c, d) of two exemplified specimens instrumented with tension band wiring using K-wires (a, c) and cannulated screws (b, d)
Fig. 2 Test setup with an instrumented specimen mounted for mechanical testing
Fig. 3 a) Evaluation principle showing the division of the sensing area in an anterior (green bordered) and a posterior (red bordered) region; b) Retrograde view on the fracture gap with the sensor schematically divided in two halves
Fig. 4 Course of anterior (green) and posterior (red) pressure in the fracture gap for tension band wiring using Kwires (a-b) and cannulated screws (c-d), shown separately for extension (a, c) and flexion (b, d) phase in terms of mean and standard deviation (dashed lines) values
Fig. 5 Course of anterior (green) and posterior (red) fracture site displacement for tension band wiring using Kwires (a-b) and cannulated screws (c-d), shown separately for extension (a, c) and flexion (b, d) phase in terms of mean and standard deviation (dashed lines) values
Fig. 6 Course of interfragmentary rotation (a, b) and quadriceps pulling force (c, d) for tension band wiring using K-wires (green) and cannulated screws (red), shown separately for extension (a, c) and flexion (b, d) phase in terms of mean and standard deviation (dashed lines) values
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Table 1: Maximum anterior and posterior pressure and angle at maximum pressure shown in terms of mean and standard deviation values for each fixation technique and loading phase.
Maximum pressure (MPa) Angle at maximum pressure (°)
K-wires Cann. Screws K-wires Cann. Screws
Extension phase Anterior Posterior 0.40 0.38 (0.41) (0.27) 0.79 0.32 (0.43) (0.38) 15 (30) 45 (19)
Flexion phase Anterior Posterior 0.40 0.40 (0.41) (0.20) 0.71 0.33 (0.34) (0.37) 8 (15) 35 (24)
49 (41)
45 (44)
23 (29)
24
19 (14)