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Does the use of Fluoroscopy and Isometry during Anterior Cruciate
Ligament (ACL) Reconstruction Affect Surgical Decision Making?
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Objective: Poor results following ACL reconstruction are often due to inaccurate graft
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placement. Numerous strategies have been advocated to improve accuracy and
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consistency of tunnel positioning, including computer-assisted navigation. Less
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expensive alternatives, such as intra-operative fluoroscopy and isometry, have also been
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advocated for confirming guide pin placement prior to reaming the femoral tunnel. It is
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unknown how often these techniques cause surgeons to change the location of their
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femoral tunnel at the time of surgery. We undertook this study to determine how often
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this approach results in re-positioning of the guide pin prior to final graft placement. We
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hypothesized that a lower level of surgeon experience would lead to a higher frequency
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of re-positioning compared to a higher level of experience.
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Design: Prospective, case series.
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Setting: Institutional
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Patients: Intra-operative data were gathered prospectively from 413 consecutive,
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primary ACL reconstructions performed by the sports medicine group at our institution.
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Of the 413 procedures enrolled in this study, 407 were available for analysis. Six
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procedures were excluded because the tension isometer was unavailable during the
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procedure.
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Interventions: Isometry and fluoroscopy were utilized in all cases to aid in the accurate
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placement of the femoral tunnel. If the femoral pin was changed due to the results of
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isometry or fluoroscopy, this was recorded. The percentage of cases involving a change
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in the femoral pin due to the use of these techniques was calculated. This percentage was
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also calculated separately for cases performed by a staff surgeon (fellowship trained
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sports medicine staff) as well as less experienced surgeons (current sports medicine
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fellows).
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Main Outcome Measurements: The main outcome measurement was whether the
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femoral pin was changed.
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Results: Of the 407 procedures available for review, 62 (15%) of them involved a
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change in femoral pin position secondary to information provided by intra-operative
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isometry or fluoroscopy. In the procedures performed by more experienced surgeons the
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pin was changed in 40 of 253 (16%) cases while in those performed by less experienced
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surgeons it was changed in 22 of 154 (14%) cases.
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Conclusions: The intra-operative use of isometry and fluoroscopy during ACL
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reconstruction led to changes in the femoral tunnel placement 15% of the time. The
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influence of these instruments on intra-operative decision making does not seem to
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diminish with surgical experience.
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Key Words: Guide wire, graft placement, femoral tunnel.
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INTRODUCTION
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Correct graft placement is the principal tenet of successful Anterior Cruciate Ligament
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(ACL) reconstruction. Numerous studies have demonstrated that inaccurate placement of
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either the tibial or femoral tunnel can lead to a poor outcome (1-4). Many different
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techniques have been employed by surgeons intra-operatively to ensure correct tunnel
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placement. These techniques include careful visual inspection using known landmarks,
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intra-operative fluoroscopy, isometry and more recently the use of computer assisted
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navigation (5-7). Two of these techniques, intra-operative fluoroscopy and isometry
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measurement, are used routinely at our institution and are the focus of this paper.
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A well placed ACL graft will demonstrate favorable “isometric” behavior throughout
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knee range of motion. If the graft tightens during extension and loosens during flexion or
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its length remains constant throughout a range of motion, this is considered favorable
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(8,9). An incorrectly positioned femoral tunnel alters this relationship in a predictable
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manner. An anteriorly placed femoral tunnel will result in a graft which tightens during
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knee flexion and loosens during extension. This is undesirable and may result in graft
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failure due to excessive tension or a “captured” knee. A femoral tunnel placed too far
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posteriorly may violate the posterior cortex of the femur thereby compromising graft
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fixation. Isometry and fluoroscopy can be used to aid in correct placement of the femoral
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tunnel and avoid these surgical pitfalls. Once the tibial tunnel is established, a guide pin
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is placed into the position of the proposed femoral tunnel. A lateral fluoroscan image can
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be taken to ensure that the posterior cortex is not in danger of being violated when the
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tunnel is reamed.
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The routine use of isometry and intra-operative fluoroscopy is standard for ACL
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reconstructions performed at our institution. While anecdotally these tools seem
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valuable, their influence on surgical decision making has never been established or
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quantified. The aim of this study is to determine the influence of isometry and
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fluoroscopy on intra-operative decision making. Specifically, we seek to determine how
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often femoral tunnel placement is changed based on the findings of intra-operative
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isometry and fluoroscopy. Our second goal is to determine whether the influence of
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these techniques on surgical decision making is dependant upon the experience level of
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the operating surgeon.
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MATERIALS AND METHODS
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Approval of the Internal Review Board was obtained prior to the commencement of this
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study. Four-hundred and thirteen consecutive, primary ACL reconstructions were
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enrolled in this study. All procedures were performed using an arthroscopically assisted
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single incision technique with either bone-patellar tendon-bone or quadrupled hamstring
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autograft. After graft harvest, a tibial tunnel was established in a standard fashion.
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Landmarks for tibial tunnel placement were the posterior aspect of the anterior horn of
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the lateral meniscus and the posterior cruciate ligament. The tunnel was placed in the
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posterior aspect of the ACL footprint. Attention was then turned to the femoral notch. A
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notchplasty was performed as deemed necessary by the operating surgeon. A beath pin
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was then inserted into the potential femoral tunnel site using a trans-tibial technique and a
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6 or 7mm over the top guide (Smith & Nephew, Andover, MA). A suture was then
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placed though the eyelet of the beath pin and connected to a tension isometer
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(MedMetric, San Diego, CA). The isometry profile was then determined with a constant
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load applied to the suture as has been previously described (10). A lateral fluoroscopic
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image was obtained to determine the position of the beath pin in the sagittal plane. At
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this point the surgeon made a decision to either accept the current femoral tunnel position
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or to reposition the beath pin. The beath pin was moved anteriorly if it appeared that the
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posterior wall would be compromised during reaming. It was moved posteriorly if it
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appeared to be so far anterior that it was in a non-anatomic position or causing a non-
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desirable isometry profile. A non-desirable isometry profile was one that demonstrated
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loosening in extension and tightening in flexion. If the beath pin was repositioned this
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was recorded.
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The percentage of cases involving a change in the femoral pin due to the use of these
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techniques was calculated. This percentage was also calculated separately for cases
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performed by staff surgeons (fellowship trained sports medicine staff) as well as less
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experienced surgeons (current sports medicine fellows).
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All lateral fluoroscan images were taken with a standard technique which involves
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obtaining an image where the posterior femoral condyles are superimposed. These
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images were subsequently reviewed by a single experienced sports medicine surgeion
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and the Femoral Guide Pin Distance (FPD) calculated as described by Harner et al (11)
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(Figure 1).
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RESULTS
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Of the 413 procedures enrolled in this study, 407 were available for analysis. Six
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procedures were excluded because the tension isometer was unavailable during the
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procedure. Staff surgeons performed 253 (62%) procedures while fellows performed the
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remaining 154 (38%). Sixty-two (15.23%) of 407 procedures involved a change in
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femoral pin position. In the procedures performed by more experienced surgeons the pin
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was changed in 40 of 253 (15.81%) cases while in those performed by less experienced
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surgeons it was changed in 22 of 154 (14.29%) cases. The change in FPD between the
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initial pin position and final pin position was -2.48% (σ=10) indicating the femoral tunnel
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was moved posteriorly 2.48%.
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DISCUSSION
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Correct tunnel placement is crucial to the success of ACL reconstructions. Any
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technique which helps the surgeon to avoid the unfavorable isometric properties of an
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anteriorly placed graft is worth considering. Likewise, any technique which helps avoid
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the graft fixation issues associated with disruption of the posterior femoral cortex is also
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useful. This study demonstrates that the routine use of isometry and fluoroscopy
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influenced the placement of the femoral tunnel 15.23% of the time.
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The design of this study makes it impossible to determine if the use of these techniques
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has any influence on the actual clinical outcome of ACL reconstructions. There is,
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however, value in knowing whether the use of isometry and fluoroscopy affect surgical
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decision making. The use of these tools is associated with increased operative time as
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well as radiation exposure. If they do not influence the surgeon, then their routine use
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would be hard to justify. In this study we demonstrate that they do indeed influence the
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surgeon approximately once in every six to seven cases. The overall femoral tunnel
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change was -2.48 % as defined by FPD. This indicates that on average the femoral tunnel
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was repositioned more posteriorly. Current knowledge of ACL function and anatomy
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suggest that a graft placed as posteriorly as possible with an isometry profile that tightens
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in extension is desirable (7-9,12,13). With this concept in mind, it is unlikely that the
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repositioning of the pin would be detrimental to clinical outcome.
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It has been suggested that the use of isometry in the hands of a surgeon experienced in
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ACL reconstruction has little value (14). In this study we do not find isometry and
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fluoroscopy to be any less influential in the hands of more experienced surgeons
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compared to fellows.
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This study has several important limitations. The procedures were performed by many
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different surgeons who may be influenced differently by isometry and fluoroscopy data.
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This weakness is also a strength, as this makes our findings more applicable to the entire
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population of surgeons performing ACL reconstructions. It is also possible that the
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surgeons’ awareness that his/her decision to reposition the femoral pin was being
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documented may actually have influenced his/her decision to do so. Finally, we did not
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record whether the femoral pin repositioning was based on the fluoroscan or isometry
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data. This would have provided another important piece of information. Due to the fact
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that the tunnel was most often positioned posteriorly, it is likely that it was the isometry
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data that was more often influential.
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In this study we find that intra-operative isometry and fluoroscopy influence the
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surgeons’ decision making approximately 15% of the time. A more experienced surgeon
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is just as likely to be influenced by these instruments as a less experienced surgeon. In
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cases where the femoral pin was repositioned, it was usually moved posteriorly.
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FIGURE LEGEND
Figure 1: The Femoral Guide Pin Distance (FPD): calculated as a ratio with the
entire length of the intercondlyar notch in the sagittal plane (Blumensaat’s line) as the
denominator (AC) and the distance between the center of the guide pin and the
anterior extent of the intercondylar notch as the numerator (BC).
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