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Pullout strength of a novel hybrid fixation technique (Tape Locking Screw™) in soft-tissue ACL reconstruction: A biomechanical study in human and porcine bone
Accepted Manuscript Title: Pullout Strength of a Novel Hybrid Fixation Technique (Tape Locking Screw TM ) in Soft-Tissue ACL Reconstruction:A Biomechanical Study in Human and Porcine Bone Author: Mark Ayzenberg Dillon Arango Grigory E. Gershkovich Praveen S Samuel Minn Saing PII: DOI: Reference:
S1877-0568(17)30049-X http://dx.doi.org/doi:10.1016/j.otsr.2017.01.006 OTSR 1689
To appear in: Received date: Revised date: Accepted date:
11-8-2016 11-1-2017 19-1-2017
Please cite this article as: Ayzenberg M, Arango D, Gershkovich GE, Samuel PS, Saing M, Pullout Strength of a Novel Hybrid Fixation Technique (Tape Locking Screw TM ) in Soft-Tissue ACL Reconstruction:A Biomechanical Study in Human and Porcine Bone, Orthopaedics and Traumatology: Surgery and Research (2017), http://dx.doi.org/10.1016/j.otsr.2017.01.006 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.
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Original article
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Pullout Strength of a Novel Hybrid Fixation Technique (Tape Locking Screw ™) in Soft-Tissue ACL Reconstruction:A Biomechanical Study in Human and Porcine Bone
4 Mark Ayzenberg,Dillon Arango, Grigory E Gershkovich, Praveen S Samuel,Minn Saing
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Einstein Medical Center 5501 Old York Road Willowcrest Building 4th Floor Philadelphia, PA 19141
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Corresponding author t:
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[email protected]
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ABSTRACT
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Introduction
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A novel hybrid anterior cruciate ligament(ACL) reconstruction technique known as Tape Locking Screw™ (TLS)is
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gaining popularity. Utilizing a suspension-type construct in conjunction with an interference screw,thistechnique has
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demonstrated successful initial clinical results with the use of quadruple hamstring graft. However, there is currently
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limited data available on the biomechanical strength of this fixation. This study investigates the pullout strength of
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theconstructin human distal femora as well as in a porcine model.The construct is tested in isolation, without the use
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of any graft. We hypothesized that the pullout strength of this construct would be similar to or better than current
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fixation systems available.
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Materials and Methods
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The Tape Locking Screwhybrid fixation system was implanted into twenty-two fresh frozen human distal femora
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(50 – 89 years old) randomized to 10x20mm titanium or polyether ether ketone (PEEK) screws by a single sports
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fellowship trained orthopedic surgeon.Given that the graft is secured to polyethylene terephthalate tape within the
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construct, the construct was implanted without any graft in order to isolate the device for biomechanical testing.
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After implantation, a tensile force was applied directly to the loop of tape at a loading rate of 5 mm/min using an
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electromechanical testing system. The failure load was calculated from the resultant load-displacement curve.
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Specimens were then visually examined for mode of failure. Similar biomechanical tests were performed on sixteen
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porcine femora.
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Results
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In the human model, the mean pullout strength was 523 ± 269 N with the PEEK screw and578 ± 245 N with the
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titanium screw. In the porcine femur model, mean strength was 616 ± 177 Nwith PEEK, 584 ± 245 N with titanium.
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There was no statistically significant difference in failure loads between these four groups. Tape slippage at the
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screw bone interface was the primary mode of failure in all the groups tested.
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Discussion
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Our results demonstratethat the hybrid technique provides excellent pullout strength in comparison to other soft-
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tissue ACL fixation methods, with tape slippage being the mode of failure in all specimens tested. This data, in
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addition to the advantages of the TLS system, support its consideration in the armamentarium of constructs available
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for soft-tissue ACL reconstruction.
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Type of Study: Experimental study Level 4
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Key Words: anterior cruciate ligament, fixation, pull out strength, hamstring, Tape locking screw, TLS
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56 57 58 INTRODUCTION
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Anterior cruciate ligament (ACL) injury is a common condition presenting to the orthopedic clinic. As the primary
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stabilizer against anterior translation of the tibia on the femur and a significant contributor to resistance of torsional
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and valgus stresses at the knee, ACL deficiency frequently results in considerable morbidity. With an annual
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incidence of 200,000 ACL reconstructions in the U.S. and the lifetime burden of ACL injuries treated with
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reconstruction estimated to be $7.6 billion annually, the economic impact to society is significant [1-3]. ACL
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rupture is a season-ending injury requiring months of rehabilitation and activity modification in even the most elite
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athletes. Surgical treatment is recommended as the treatment of choice for return to sport [4-6].
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Although bone-patellar tendon-bone autograft has been reported to result in a more statically stable knee, functional
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and subjective patient outcomes appear to be similar compared to quadruple hamstring tendon autograft [7]. While
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each method of reconstruction has its own advantages, quadruple hamstring tendon autograft has become a popular
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reconstructive modality among sports surgeons as a result of its exceptional tensile strength and decreased donor-site
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morbidity in comparison to bone-patellar tendon-bone autograft[8].The weak link in soft tissue ACL reconstruction,
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however, is its initial fixation, which has been shown to be critical for early, aggressive rehabilitation [9-10]. The
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maximum force applied across the ACL during activities of daily living is approximately 450N [11]. The immediate
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strength of fixation post-operatively must reliably withstand the stresses of rehabilitation for a minimum of 3
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monthsin order to allow for adequate biological fixation of tendons at the tunnel entrance [12].The process of
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remodeling, known as “ligamentization,” of the autograft continues for at least 2 years postoperatively [13-15].In
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comparison to bone-patellar tendon-bone autograft, soft tissue ACL fixation carries ahigher risk of graft-fixation
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construct elongation and potential for tunnel widening and laxity, a complication attributed to strength of initial
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fixation and slower bone-tendon healing[10, 16-19].Mild loss of flexion strength is also a recognized consequence
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of the harvesting of two hamstring tendons compared with mild loss of extension strength, range of motion and
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increased donor-site morbidity with bone-patellar tendon-bone autograft[7, 8, 20-21].
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A variety of soft-tissue ACL fixation systems are available and vary significantly with regards to their fixation
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quality [16, 22]. The majority of these constructs employ either a suspension method with far cortical fixation or an
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interference-type fixation, with each technique carrying its own advantages and disadvantages. A novel hybrid ACL
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reconstruction technique utilizing a suspension-type construct in conjunction with an interference screw ispresented.
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Known as the Tape Locking Screw (TLS ™) construct (TLS, FH Orthopedics, Heimsbrunn, France), this construct
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suspends a quadruple hamstring graft from polyethylene terephthalate tape and secures the tape with an interference
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screw inserted at the cortex rather than into the bone tunnel from within the knee (Figure 1). This techniquehas
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demonstrated good initial clinical results, capitalizes on the benefits of both interference-type and far cortical
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fixationand addresses many of the aforementioned concerns with soft-tissue ACL reconstruction [23, 24].
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The Tape Locking Screw technique has several potential advantages over the majority of soft-tissue ACL
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fixation systems. Femoral and tibial fixation is achieved with an interference screw with polyethylene terephthalate
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(TLS) strip, which is attached to a closed autograft tendon loop (Figure 1
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). Because the interference screw is not placed through the graft itself for interference fixation, the length of graft
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needed is shorter, allowing for the construction of a quadruple hamstring graft from a single harvested tendon. This
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leaves additional tendons available for other reconstructive uses, reduces donor-site morbidity and minimizes the
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reduction of flexion strength seen in the harvesting of two hamstring tendons. Moreover, the TLS strip itself
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interacts better with interference screw fixation compared to a graft, conforming to the tunnel to theoretically help
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reduce slippage at its interface with the bone [25]. The construct is pretensioned at 50kg for 2 minutes prior to
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insertion to allow flattening at the tape-tendon junction and prevent lengthening post-operatively, with excellent
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maintenance of length stiffness [25]. Additionally, because the screw fixes the tape close to the graft, the shorter
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working length of the suspension construct within the tunnel results in less elasticity than is seen with standard
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suspension systems and may reduce tunnel widening caused by bungee and windshield wiper effects [17]. Fixation
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through the TLS strips also results in shorter bone tunnels, which are 10 mm (femur) and 15 mm (tibia) in length
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and 4.5 mm wide, maximizing bone stock. The shorter portion of graft within the tunnel raises concerns regarding
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its ultimate strength and healing. There have been several studies, however, demonstrating that healing of the graft
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at the junction where it enters the joint is of primary importance by 3-4 months post-operatively [26]. Furthermore,
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animal models have shown no kinematic or biomechanical differences, including pullout strength, when utilizing a
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shorter intratunnel graft length [27-28]. Another benefit unique to the tape locking screw construct is that the graft
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is press-fit into the bone tunnels and has 360 degrees of contact. The TLS is the only system that provides this type
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of fixation, allowing for direct tendon to bone healing through the entire circumference of the graft.
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As a relatively new construct, there is a paucity of literature evaluating the TLS fixation system’s biomechanical
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strength. The TLS designers have previously published on its pullout strength in a cadaveric femoral head model,
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but both the method of testing and the use of femoral heads rather than distal femora presents potential for
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confounding [25]. Given the advantages of the TLS system and the lack of available data, this biomechanical study
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seeks to evaluate the pullout strength and method of failure of this novel fixation method. No tendon graft was
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utilized in this study since the entire fixation within the bone is provided by the tape/screw construct and because the
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goal of the study was to determine failure load of the construct in isolation. Addition of tendon would not affect
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pullout strength of the construct, but would introduce an additional variable with potential failure occurring at the
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graft rather than at the fixation mechanism, which was the focus of this study.Both a human cadaveric femur model
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and a porcine femur model were used. Because our human cadaveric femora were elderly, we chose to include a
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porcine model to help simulate increased bone density, a model that has been utilized regularly in ACL fixation
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testing described in the literature [29].Our hypothesis was that pullout strength would be similar to or better than
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existing constructs for soft-tissue ACL fixation.
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MATERIALS AND METHODS
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Human distal femora were stored frozen and thawed overnight in room temperature in preparation for the procedure.
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The day of testing, distal femora were harvested from cadaver specimens and denuded of all soft tissue. Once the
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distal femora were prepared, testing was immediately carried out without refreezing.The specimens were maintained
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moistened with physiologic saline during the entirety of the testing process and all testing was performed at room
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temperature of 21 degreesCelsius. All applicable international, national, and institutional guidelines for the care and
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use of animals were followed.
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The TLS fixation system was implanted into twenty-two fresh frozen human distal femora (50 – 89 years old)
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randomized using a random number generator to 10x20mm titanium or PEEK screws (Figure 2). Fixation was
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performedutilizing the specific technique and instruments provided by TLS by a single sports fellowship trained
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orthopedic surgeon.4.5mm femoral bone tunnels weredrilled at the native ACL footprint and directed to the 11
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o’clock position on right femurs and 1 o’clock position in left femurs.A 10mm retrograde reamer was used to widen
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tunnels for the 10mm tunnel length closest to the ACL origin. A tap was then used to prepare the cortical opening of
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each tunnel for insertion of the screw. Finally, the tape was passed through the tunnel and secured with a screw at
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the cortical opening of the tunnel. After implantation, a single cycle load to failure test was performed utilizing an
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electromechanical testing system (Test Resources, Shakopee, MN) (Figure 3). A tensile force was applied directly to
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the loop of exposed tape(to which graft would typically be attached)with traction applied in line with the femurat a
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loading rate of 5 mm/min until failure was observed.The response of the specimen was recorded andyield load
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(defined by the point on the curve where the slope first clearly decreased) was calculated from the resultant load-
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displacement curve formulated utilizing Test Resources software (Test Resources, Shakopee, MN). All
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biomechanical testing was performed by an engineer independent of TLS without any conflict of interests related to
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the testing.Specimens were then explanted and scrutinized for mode of failure. Identical biomechanical tests were
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performed on sixteen porcine distal femora, harvested from skeletally mature Yorkshirepigs obtained at a local
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slaughterhouse.
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RESULTS
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Results are summarized in Table 1 and Figure 4. In the human model, the mean pullout strength was 523 ± 269 N
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with the PEEK screw and 578 ± 245 N with the titanium screw. In the porcine femur model, mean strength was 616
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± 177 Nwith PEEK, 584 ± 245 N with titanium. There was no statistical difference among the four groups, including
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when individually comparing PEEK vs. titanium and human vs. porcine. Tape slippage at the screw bone interface
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(ie. tape pulling out around the screw) was the mode of failure in all cases and all groups tested.The screw remained
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in place and there was no tearing of the tape itself. There was no evidence of fracture as a cause for failure.
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DISCUSSION
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The most important finding of this study is that the TLS construct demonstrates excellent pullout strength which
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affirms our hypothesis. While no graft was utilized in this study, the construct itself is the anchoring point of the
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graft. Thus, utilizing the construct alone tested fixation strength while eliminating potential confounding by failure
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through the hamstring graft. On secondary data analysis, it was also noted that there was no statistical difference in
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pullout strength between the PEEK and titanium groups.
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A 2006 experimental study in porcine femora demonstrated increased pullout strength when hybridizing a
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bioabsorbableinterference screw(BioRCI, Smith & Nephew Endoscopy, Andover, MA) at the notch between the
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graft and bone, with far cortical fixationutilizing the EndoButton CL (Smith & Nephew Endoscopy, Andover, MA
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[30]. However, there is currently limited data available on the biomechanical strength of the TLS construct, a
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fixation method that is hybrid by design. The only human study to date evaluating pullout strength of the TLS
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construct was performed by its designers. In their human cadaveric study, the mean pullout strength was determined
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to be an impressive 1742 +-397 N [25]. However, the construct was implanted in femoral heads rather than distal
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femora. Furthermore, the femoral heads were enclosed in a box such that during pullout testing, a reactive force
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resisting pullout was applied to the femoral head around the tunnel. This potentially increases pressure within the
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bone and confounds the results. We believe that our method of testing yields a more reliable representation of the
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construct’s strength of fixation. Our results demonstrate pullout strengths similar to those reported in the literature
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for soft-tissue ACL fixation, including several constructs evaluated by Kousa, et al in their porcine model study
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[Table 2].
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Limitations
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This study has somelimitations. The strength of fixation of the construct is highly dependent on the quality of the
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bone [31-32]. Given that we used elderly cadaver bone and did not obtain bone mineral density measures, the bone
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quality will be significantly reduced compared with that of the young patients undergoing ACL reconstructions.
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However, if the results demonstrate good fixation in low quality bone, it can be extrapolated that the fixation will be
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improved with better quality bone. We also utilized porcine bone in order to help simulate increased bone density, a
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model that has been utilized regularly in ACL fixation testing described in the literature [29].Also, the direction of
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traction was in line with the femur, rather than in line with the bone tunnel, which would have been the “worst
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possible” scenario for pullout strength testing. Finally, solely load to failure was assessed in this study – of future
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interest would be the evaluation of cyclic loading as well fatigue testing. Notwithstanding these limitations, this
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study contributes valuable data to surgeons considering tape locking screw fixation. It is also the first independent
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evaluation of the TLS construct as well as the first to test it in distal femora and in twospecies of bone.
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CONCLUSIONS
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In the setting of soft-tissue ACL reconstruction, initial graft fixation is of utmost importance. The available
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constructs for soft-tissue ACL fixation abound, therefore it is critical to biomechanically evaluate the fixation
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strength of emerging products. The TLS system is unique and demonstrates several advantages over other
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constructs, most notably the utilization of a shorter graft, shorter intratunnel suspension working length,
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maximization of bone stock, and 360-degree press-fit graft-bone contactto allow direct circumferential healing.. Our
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results, in addition to the discussed advantages of the TLS system, support its consideration in the armamentarium of
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constructs available for soft-tissue ACL reconstruction.
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Fig 1. Diagram of the TLS construct. Reproduced, with permission from FournituresHospitalieres Orthopedics
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(TLS, FH Orthopedics, Heimsbrunn, France).
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Fig2. Clinical images the potted human cadaveric distal femur with the TLS construct implanted. The pot is
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attached to the electromechanical device and a carabiner system, through which a pullout force is applied, is hooked
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to the TLS tape loop.
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Fig 3.The electromechanical testing system with the specimen in place and ready to test. A dual load-cell system is
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incorporated into the chain to ensure consistent results.
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Fig 4. Graphic representation of average pullout strength in Newtons per group tested.
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TABLES
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Table 1. Mean pullout strength, sample size and 95% confidence intervals for each of the study groups.
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Table 2. Yield strength as reported by Kousa, et al for several constructs tested in porcine femora [10].
Table 1
Mean Pullout Strength (N) Sample Size (#) 95% Confidence Interval (N)
Human PEEK 523+-269
RESULTS Human Porcine Titanium PEEK 578+-245 616+-177
Porcine Titanium 584+-245
11 342-704
11 413-743
10 481-686
6 430-802
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Mean Yield Load (N) Sample Size (#)
Kousa, et al Yield Load Results for Several Fixation Devices Implanted in Porcine Femora EndoButton Bone Mulch RigidFix BioScrew RCI Screw SmartScrew CL Screw ACL 1086 +- 185 1112 +- 295 868 +- 171 589 +- 204 546 +- 174 794 +-152 10
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Table 2
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FUNDING AND CONFLICTS OF INTEREST
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Donations of constructs for testing as well as funding support was provided by FH Orthopaedics. Minn Saing, MD
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is also a consultant for FH Orthopaedics. There are no other conflicts of interests or funding sources to report.
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S1877-0568(17)30049-X http://dx.doi.org/doi:10.1016/j.otsr.2017.01.006 OTSR 1689
To appear in: Received date: Revised date: Accepted date:
11-8-2016 11-1-2017 19-1-2017
Please cite this article as: Ayzenberg M, Arango D, Gershkovich GE, Samuel PS, Saing M, Pullout Strength of a Novel Hybrid Fixation Technique (Tape Locking Screw TM ) in Soft-Tissue ACL Reconstruction:A Biomechanical Study in Human and Porcine Bone, Orthopaedics and Traumatology: Surgery and Research (2017), http://dx.doi.org/10.1016/j.otsr.2017.01.006 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.
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Original article
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Pullout Strength of a Novel Hybrid Fixation Technique (Tape Locking Screw ™) in Soft-Tissue ACL Reconstruction:A Biomechanical Study in Human and Porcine Bone
4 Mark Ayzenberg,Dillon Arango, Grigory E Gershkovich, Praveen S Samuel,Minn Saing
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Einstein Medical Center 5501 Old York Road Willowcrest Building 4th Floor Philadelphia, PA 19141
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Corresponding author t:
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[email protected]
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ABSTRACT
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Introduction
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A novel hybrid anterior cruciate ligament(ACL) reconstruction technique known as Tape Locking Screw™ (TLS)is
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gaining popularity. Utilizing a suspension-type construct in conjunction with an interference screw,thistechnique has
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demonstrated successful initial clinical results with the use of quadruple hamstring graft. However, there is currently
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limited data available on the biomechanical strength of this fixation. This study investigates the pullout strength of
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theconstructin human distal femora as well as in a porcine model.The construct is tested in isolation, without the use
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of any graft. We hypothesized that the pullout strength of this construct would be similar to or better than current
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fixation systems available.
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Materials and Methods
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The Tape Locking Screwhybrid fixation system was implanted into twenty-two fresh frozen human distal femora
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(50 – 89 years old) randomized to 10x20mm titanium or polyether ether ketone (PEEK) screws by a single sports
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fellowship trained orthopedic surgeon.Given that the graft is secured to polyethylene terephthalate tape within the
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construct, the construct was implanted without any graft in order to isolate the device for biomechanical testing.
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After implantation, a tensile force was applied directly to the loop of tape at a loading rate of 5 mm/min using an
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electromechanical testing system. The failure load was calculated from the resultant load-displacement curve.
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Specimens were then visually examined for mode of failure. Similar biomechanical tests were performed on sixteen
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porcine femora.
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Results
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In the human model, the mean pullout strength was 523 ± 269 N with the PEEK screw and578 ± 245 N with the
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titanium screw. In the porcine femur model, mean strength was 616 ± 177 Nwith PEEK, 584 ± 245 N with titanium.
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There was no statistically significant difference in failure loads between these four groups. Tape slippage at the
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screw bone interface was the primary mode of failure in all the groups tested.
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Discussion
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Our results demonstratethat the hybrid technique provides excellent pullout strength in comparison to other soft-
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tissue ACL fixation methods, with tape slippage being the mode of failure in all specimens tested. This data, in
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addition to the advantages of the TLS system, support its consideration in the armamentarium of constructs available
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for soft-tissue ACL reconstruction.
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Type of Study: Experimental study Level 4
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Key Words: anterior cruciate ligament, fixation, pull out strength, hamstring, Tape locking screw, TLS
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56 57 58 INTRODUCTION
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Anterior cruciate ligament (ACL) injury is a common condition presenting to the orthopedic clinic. As the primary
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stabilizer against anterior translation of the tibia on the femur and a significant contributor to resistance of torsional
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and valgus stresses at the knee, ACL deficiency frequently results in considerable morbidity. With an annual
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incidence of 200,000 ACL reconstructions in the U.S. and the lifetime burden of ACL injuries treated with
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reconstruction estimated to be $7.6 billion annually, the economic impact to society is significant [1-3]. ACL
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rupture is a season-ending injury requiring months of rehabilitation and activity modification in even the most elite
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athletes. Surgical treatment is recommended as the treatment of choice for return to sport [4-6].
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Although bone-patellar tendon-bone autograft has been reported to result in a more statically stable knee, functional
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and subjective patient outcomes appear to be similar compared to quadruple hamstring tendon autograft [7]. While
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each method of reconstruction has its own advantages, quadruple hamstring tendon autograft has become a popular
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reconstructive modality among sports surgeons as a result of its exceptional tensile strength and decreased donor-site
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morbidity in comparison to bone-patellar tendon-bone autograft[8].The weak link in soft tissue ACL reconstruction,
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however, is its initial fixation, which has been shown to be critical for early, aggressive rehabilitation [9-10]. The
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maximum force applied across the ACL during activities of daily living is approximately 450N [11]. The immediate
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strength of fixation post-operatively must reliably withstand the stresses of rehabilitation for a minimum of 3
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monthsin order to allow for adequate biological fixation of tendons at the tunnel entrance [12].The process of
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remodeling, known as “ligamentization,” of the autograft continues for at least 2 years postoperatively [13-15].In
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comparison to bone-patellar tendon-bone autograft, soft tissue ACL fixation carries ahigher risk of graft-fixation
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construct elongation and potential for tunnel widening and laxity, a complication attributed to strength of initial
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fixation and slower bone-tendon healing[10, 16-19].Mild loss of flexion strength is also a recognized consequence
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of the harvesting of two hamstring tendons compared with mild loss of extension strength, range of motion and
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increased donor-site morbidity with bone-patellar tendon-bone autograft[7, 8, 20-21].
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A variety of soft-tissue ACL fixation systems are available and vary significantly with regards to their fixation
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quality [16, 22]. The majority of these constructs employ either a suspension method with far cortical fixation or an
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interference-type fixation, with each technique carrying its own advantages and disadvantages. A novel hybrid ACL
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reconstruction technique utilizing a suspension-type construct in conjunction with an interference screw ispresented.
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Known as the Tape Locking Screw (TLS ™) construct (TLS, FH Orthopedics, Heimsbrunn, France), this construct
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suspends a quadruple hamstring graft from polyethylene terephthalate tape and secures the tape with an interference
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screw inserted at the cortex rather than into the bone tunnel from within the knee (Figure 1). This techniquehas
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demonstrated good initial clinical results, capitalizes on the benefits of both interference-type and far cortical
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fixationand addresses many of the aforementioned concerns with soft-tissue ACL reconstruction [23, 24].
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The Tape Locking Screw technique has several potential advantages over the majority of soft-tissue ACL
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fixation systems. Femoral and tibial fixation is achieved with an interference screw with polyethylene terephthalate
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(TLS) strip, which is attached to a closed autograft tendon loop (Figure 1
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). Because the interference screw is not placed through the graft itself for interference fixation, the length of graft
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needed is shorter, allowing for the construction of a quadruple hamstring graft from a single harvested tendon. This
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leaves additional tendons available for other reconstructive uses, reduces donor-site morbidity and minimizes the
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reduction of flexion strength seen in the harvesting of two hamstring tendons. Moreover, the TLS strip itself
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interacts better with interference screw fixation compared to a graft, conforming to the tunnel to theoretically help
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reduce slippage at its interface with the bone [25]. The construct is pretensioned at 50kg for 2 minutes prior to
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insertion to allow flattening at the tape-tendon junction and prevent lengthening post-operatively, with excellent
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maintenance of length stiffness [25]. Additionally, because the screw fixes the tape close to the graft, the shorter
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working length of the suspension construct within the tunnel results in less elasticity than is seen with standard
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suspension systems and may reduce tunnel widening caused by bungee and windshield wiper effects [17]. Fixation
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through the TLS strips also results in shorter bone tunnels, which are 10 mm (femur) and 15 mm (tibia) in length
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and 4.5 mm wide, maximizing bone stock. The shorter portion of graft within the tunnel raises concerns regarding
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its ultimate strength and healing. There have been several studies, however, demonstrating that healing of the graft
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at the junction where it enters the joint is of primary importance by 3-4 months post-operatively [26]. Furthermore,
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animal models have shown no kinematic or biomechanical differences, including pullout strength, when utilizing a
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shorter intratunnel graft length [27-28]. Another benefit unique to the tape locking screw construct is that the graft
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is press-fit into the bone tunnels and has 360 degrees of contact. The TLS is the only system that provides this type
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of fixation, allowing for direct tendon to bone healing through the entire circumference of the graft.
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As a relatively new construct, there is a paucity of literature evaluating the TLS fixation system’s biomechanical
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strength. The TLS designers have previously published on its pullout strength in a cadaveric femoral head model,
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but both the method of testing and the use of femoral heads rather than distal femora presents potential for
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confounding [25]. Given the advantages of the TLS system and the lack of available data, this biomechanical study
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seeks to evaluate the pullout strength and method of failure of this novel fixation method. No tendon graft was
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utilized in this study since the entire fixation within the bone is provided by the tape/screw construct and because the
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goal of the study was to determine failure load of the construct in isolation. Addition of tendon would not affect
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pullout strength of the construct, but would introduce an additional variable with potential failure occurring at the
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graft rather than at the fixation mechanism, which was the focus of this study.Both a human cadaveric femur model
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and a porcine femur model were used. Because our human cadaveric femora were elderly, we chose to include a
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porcine model to help simulate increased bone density, a model that has been utilized regularly in ACL fixation
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testing described in the literature [29].Our hypothesis was that pullout strength would be similar to or better than
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existing constructs for soft-tissue ACL fixation.
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MATERIALS AND METHODS
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Human distal femora were stored frozen and thawed overnight in room temperature in preparation for the procedure.
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The day of testing, distal femora were harvested from cadaver specimens and denuded of all soft tissue. Once the
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distal femora were prepared, testing was immediately carried out without refreezing.The specimens were maintained
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moistened with physiologic saline during the entirety of the testing process and all testing was performed at room
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temperature of 21 degreesCelsius. All applicable international, national, and institutional guidelines for the care and
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use of animals were followed.
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The TLS fixation system was implanted into twenty-two fresh frozen human distal femora (50 – 89 years old)
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randomized using a random number generator to 10x20mm titanium or PEEK screws (Figure 2). Fixation was
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performedutilizing the specific technique and instruments provided by TLS by a single sports fellowship trained
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orthopedic surgeon.4.5mm femoral bone tunnels weredrilled at the native ACL footprint and directed to the 11
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o’clock position on right femurs and 1 o’clock position in left femurs.A 10mm retrograde reamer was used to widen
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tunnels for the 10mm tunnel length closest to the ACL origin. A tap was then used to prepare the cortical opening of
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each tunnel for insertion of the screw. Finally, the tape was passed through the tunnel and secured with a screw at
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the cortical opening of the tunnel. After implantation, a single cycle load to failure test was performed utilizing an
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electromechanical testing system (Test Resources, Shakopee, MN) (Figure 3). A tensile force was applied directly to
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the loop of exposed tape(to which graft would typically be attached)with traction applied in line with the femurat a
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loading rate of 5 mm/min until failure was observed.The response of the specimen was recorded andyield load
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(defined by the point on the curve where the slope first clearly decreased) was calculated from the resultant load-
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displacement curve formulated utilizing Test Resources software (Test Resources, Shakopee, MN). All
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biomechanical testing was performed by an engineer independent of TLS without any conflict of interests related to
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the testing.Specimens were then explanted and scrutinized for mode of failure. Identical biomechanical tests were
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performed on sixteen porcine distal femora, harvested from skeletally mature Yorkshirepigs obtained at a local
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slaughterhouse.
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RESULTS
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Results are summarized in Table 1 and Figure 4. In the human model, the mean pullout strength was 523 ± 269 N
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with the PEEK screw and 578 ± 245 N with the titanium screw. In the porcine femur model, mean strength was 616
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± 177 Nwith PEEK, 584 ± 245 N with titanium. There was no statistical difference among the four groups, including
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when individually comparing PEEK vs. titanium and human vs. porcine. Tape slippage at the screw bone interface
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(ie. tape pulling out around the screw) was the mode of failure in all cases and all groups tested.The screw remained
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in place and there was no tearing of the tape itself. There was no evidence of fracture as a cause for failure.
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DISCUSSION
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The most important finding of this study is that the TLS construct demonstrates excellent pullout strength which
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affirms our hypothesis. While no graft was utilized in this study, the construct itself is the anchoring point of the
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graft. Thus, utilizing the construct alone tested fixation strength while eliminating potential confounding by failure
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through the hamstring graft. On secondary data analysis, it was also noted that there was no statistical difference in
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pullout strength between the PEEK and titanium groups.
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A 2006 experimental study in porcine femora demonstrated increased pullout strength when hybridizing a
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bioabsorbableinterference screw(BioRCI, Smith & Nephew Endoscopy, Andover, MA) at the notch between the
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graft and bone, with far cortical fixationutilizing the EndoButton CL (Smith & Nephew Endoscopy, Andover, MA
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[30]. However, there is currently limited data available on the biomechanical strength of the TLS construct, a
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fixation method that is hybrid by design. The only human study to date evaluating pullout strength of the TLS
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construct was performed by its designers. In their human cadaveric study, the mean pullout strength was determined
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to be an impressive 1742 +-397 N [25]. However, the construct was implanted in femoral heads rather than distal
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femora. Furthermore, the femoral heads were enclosed in a box such that during pullout testing, a reactive force
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resisting pullout was applied to the femoral head around the tunnel. This potentially increases pressure within the
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bone and confounds the results. We believe that our method of testing yields a more reliable representation of the
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construct’s strength of fixation. Our results demonstrate pullout strengths similar to those reported in the literature
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for soft-tissue ACL fixation, including several constructs evaluated by Kousa, et al in their porcine model study
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[Table 2].
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Limitations
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This study has somelimitations. The strength of fixation of the construct is highly dependent on the quality of the
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bone [31-32]. Given that we used elderly cadaver bone and did not obtain bone mineral density measures, the bone
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quality will be significantly reduced compared with that of the young patients undergoing ACL reconstructions.
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However, if the results demonstrate good fixation in low quality bone, it can be extrapolated that the fixation will be
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improved with better quality bone. We also utilized porcine bone in order to help simulate increased bone density, a
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model that has been utilized regularly in ACL fixation testing described in the literature [29].Also, the direction of
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traction was in line with the femur, rather than in line with the bone tunnel, which would have been the “worst
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possible” scenario for pullout strength testing. Finally, solely load to failure was assessed in this study – of future
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interest would be the evaluation of cyclic loading as well fatigue testing. Notwithstanding these limitations, this
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study contributes valuable data to surgeons considering tape locking screw fixation. It is also the first independent
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evaluation of the TLS construct as well as the first to test it in distal femora and in twospecies of bone.
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CONCLUSIONS
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In the setting of soft-tissue ACL reconstruction, initial graft fixation is of utmost importance. The available
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constructs for soft-tissue ACL fixation abound, therefore it is critical to biomechanically evaluate the fixation
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strength of emerging products. The TLS system is unique and demonstrates several advantages over other
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constructs, most notably the utilization of a shorter graft, shorter intratunnel suspension working length,
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maximization of bone stock, and 360-degree press-fit graft-bone contactto allow direct circumferential healing.. Our
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results, in addition to the discussed advantages of the TLS system, support its consideration in the armamentarium of
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constructs available for soft-tissue ACL reconstruction.
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Fig 1. Diagram of the TLS construct. Reproduced, with permission from FournituresHospitalieres Orthopedics
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(TLS, FH Orthopedics, Heimsbrunn, France).
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Fig2. Clinical images the potted human cadaveric distal femur with the TLS construct implanted. The pot is
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attached to the electromechanical device and a carabiner system, through which a pullout force is applied, is hooked
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to the TLS tape loop.
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Fig 3.The electromechanical testing system with the specimen in place and ready to test. A dual load-cell system is
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incorporated into the chain to ensure consistent results.
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Fig 4. Graphic representation of average pullout strength in Newtons per group tested.
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TABLES
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Table 1. Mean pullout strength, sample size and 95% confidence intervals for each of the study groups.
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Table 2. Yield strength as reported by Kousa, et al for several constructs tested in porcine femora [10].
Table 1
Mean Pullout Strength (N) Sample Size (#) 95% Confidence Interval (N)
Human PEEK 523+-269
RESULTS Human Porcine Titanium PEEK 578+-245 616+-177
Porcine Titanium 584+-245
11 342-704
11 413-743
10 481-686
6 430-802
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Mean Yield Load (N) Sample Size (#)
Kousa, et al Yield Load Results for Several Fixation Devices Implanted in Porcine Femora EndoButton Bone Mulch RigidFix BioScrew RCI Screw SmartScrew CL Screw ACL 1086 +- 185 1112 +- 295 868 +- 171 589 +- 204 546 +- 174 794 +-152 10
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Table 2
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FUNDING AND CONFLICTS OF INTEREST
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Donations of constructs for testing as well as funding support was provided by FH Orthopaedics. Minn Saing, MD
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is also a consultant for FH Orthopaedics. There are no other conflicts of interests or funding sources to report.
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