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Anterior Cruciate Ligament (ACL) Tunnel Placement: A Radiographic Comparison Between Navigated Versus Manual ACL Reconstruction

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Abstract

Anterior cruciate ligament (ACL) reconstructions have a high rate of inaccurate tunnel placements, even when performed by experienced surgeons. Computer-assisted surgery may assist in eliminating inconsistent graft tunnels by increasing precision and providing navigation feedback of the surgical field. This study aimed to compare the accuracy of tunnel placements between computer-navigated and manual ACL reconstructions using radiography. Significant differences between the two methods were found, with navigated tibial tunnels placed more anterior in the anatomic ACL footprint since impingement could be precisely calculated. Both methods produced acceptable accuracy in the hands of an experienced orthopedic surgeon. The precision of radiographic evaluation was less than that of computer navigation.

The incidence of anterior cruciate ligament (ACL) reconstruction has increased over recent years reflecting the growing participation in sports activities by the general population. Some estimate as many as 175,000 ACL reconstructions are performed annually in the United States, at a cost of more than $1 billion.^1,2 At the same time, the number of surgeons performing ACL surgery have increased. In fact, 80% of all ACL surgeries are performed by surgeons who perform <20 ACL reconstructions per year.³ This fact is attributable, in part, to the estimates of the rate of revision surgery, which are as high as 40% (range: 10%-40%), potentially resulting in more than 35,000 revisions a year.⁴

Studies demonstrate that the majority of revisions are related to technical errors, primarily in tunnel placement.⁵ Historically, the most common error in ACL reconstruction is a femoral tunnel that is too anterior.⁶ Any malposition of the tunnels in the femur and tibia is shown to significantly influence the outcome of ACL reconstruction procedures.^7,8 For example, an excessively posterior placement of the graft on the tibial side may lead to impingement on the posterior cruciate ligament (PCL), resulting in loss of flexion and graft stretching. An excessively anterior placement of the graft on the tibia can impinge on the intercondylar notch.⁹ Impingement can lead to persistent effusion, failure to regain full extension and, eventually, graft failure.¹⁰ The net result of inappropriate tunnel placement is clinically poor outcomes, including poor Lysholm and International Knee Documentation Committee scores, increased laxity, synovitis, pain, higher rates of graft failure and, ultimately, revision.^5,6,11-15 Accurate positioning of these tunnels can minimize graft stretching, decrease the risk of impingement and avoid complications such as graft failure.^16,17

Despite this knowledge, great variability exists in tunnel placement, resulting in high rates of revision ACL surgery.^18-22 Unfortunately, proper placement of tunnels remains difficult with current arthroscopic techniques, even when performed by surgeons experienced in ACL reconstruction.^18-22

As a result of the difficulties encountered in traditional arthroscopic methods, one of the goals of computer-assisted orthopedic surgery is to eliminate inconsistent placement of graft tunnels during ACL reconstruction by increasing the precision of the procedures. Computer-assisted surgery benefits surgeons by providing feedback on the instruments and bony anatomy position inside the surgical field.^4,23 The OrthoPilot (B. Braun Aesculap, Tuttlingen, Germany), used in this study, is an image-free, wireless system that does not require preoperative computed tomography (CT), radiographic data, or intraoperative fluoroscopy. Using infrared cameras and minimally invasive markers, the precise location of instruments in three-dimensional space can be established. The cameras can track the position of the instruments to within <1 mm and <1° in combination with a computer system.

Although experience with computer-assisted technique is limited, several surgeons report enhanced accuracy in tunnel placement.^8,21,23-28

This study sought to compare the accuracy of tunnel placement between navigated and manual ACL reconstruction using plain radiographs.

Materials and Methods

Forty-one consecutive, manual, single-bundle ACL reconstruction procedures, followed by 42 navigated single-bundle ACL reconstruction procedures using the OrthoPilot system, were performed over a 1-year period by the senior author. Postoperative plain anteroposterior (AP) and lateral radiographs were analyzed using the method described by Khalfayan et al¹¹ to determine tunnel placement.Complete data were available for 34 navigated reconstructions and 31 reconstructions.

Radiographs were considered adequate if the overlap of the posterior condyles was >90% laterally. The measurement Z_f describes femoral tunnel placement on a lateral radiograph. The measurement is expressed as a percentage of the distance along Blumensaat’s line from anterior to posterior. The measurement Z_t describes tibial tunnel placement on a lateral radiograph and is expressed as a percentage of the anterior –to posterior length of the tibial plateau. The measurement X_t describes tibial tunnel location on an AP radiograph, expressed as a percentage of the width of the tibial plateau from lateral to medial. The measurement X_f describes femoral tunnel placement on an AP radiograph and is a percentage of the medial to lateral distance along a line drawn tangential to the femoral condyles; the measurement reflects the position of the femoral tunnel in relation to the width of the tibial plateau.

A single surgeon trained to make the measurement and masked to the use of computer-assisted surgery versus manual techniques read all radiographs.

The Northwestern University Feinberg School of Medicine Institutional Review Board approved this study.

Results

The radiograph results show navigated and manual ACL reconstruction placed tibial and femoral tunnels in acceptable positions (Table). The navigated values for Z_f averaged 73.2% (73% of the length of Blumensaat’s line from anterior to posterior on the femur), with an SD of 7.6%. The no-navigated value for Z_f was 77.1%, with an SD of 10%. The navigated value for Z_t averaged 40.4% (40% of the length of tibia on the lateral from anterior to posterior), with an SD of 8.3%. The non-navigated value for Z_t was 46.5%, with an SD of 7.8%.

Data from the Table show that computer assisted surgery and manual results are within the tunnel positions correlated with positive clinical results, because the guidelines set by Khalfayan et al¹¹ are 50% to 65% for X_t, 50% to 70% for X_f, 20% to 40% for Z, and 60% for Z_f. The average tunnel positions from Khalfayan et al¹¹ are 71% for Z_f, 26% for Z_t, 57% for X, and 61% for X_f.

Discussion

The improved accuracy of computer assisted surgery in tunnel placement is documented in the literature.^8,21,23-28 Eichhorn²⁵ compared postoperative radiographs of 300 manual versus computer assisted surgical ACL reconstruction and found significantly improved accuracy and tunnel position with the computer-assisted surgical technique.²⁵ Specifically, manual ACL reconstructions had tibial tunnels that were too posterior and femoral tunnels that were too vertical. Plaweski et al²⁶ conducted a randomized, controlled trial assessing the accuracy of navigated techniques. His study confirmed improved accuracy and consistency of tibial tunnel position.²⁶ Computer-aided navigation systems can provide enhanced precision in tunnel placement and may reduce the rate of revision ACL surgery. In this study, the authors investigated the accuracy of tunnel placement using plain radiography.

In comparing the accuracy of computer assisted surgery versus manual ACL techniques in the hands of a single experienced surgeon, the results show that both techniques achieve tunnel positions correlated with positive clinical results. Navigated tibial tunnel placement was more anterior in the anatomic ACL footprint. This was due to the tunnel being safely placed more anterior without risk of impingement. The authors did not see a difference in standard deviation values between the two groups.

Radiographic evaluation was substantially less accurate than the computer navigation measurements. Actual differences in tunnel position were most likely minimized by the relative lack of precision in precise measurement of tunnel placement, which is a conclusion supported by other studies in the literature.^21,27,28 Picard et al²¹ sought to compare the accuracy in tunnel placement as performed with a traditional arthroscopic versus a computer assisted technique in 20 identical foam knees.²¹ He found the standard equipment resulted in a femoral tunnel that was off by 4.2±1.8 mm and a tibial tunnel that was off by 4.9±2.3 mm from the ideal location. Despite the finding, Picard e al²¹ did not see a statistically significant difference between the two surgical techniques in tunnel placement using plain radiography as the basis for accuracy comparison.²¹ He concluded that radiographic views were inadequate to assess tunnel placement, because they could not establish the alignment in three dimensions. In another study, Hoser et al²⁷ evaluated postoperative femoral tunnel placement using both plain radiography and CT. He was unable to measure femoral tunnel position using plain radiography in 92.2% of all knees on the AP view and in 21.6% on lateral radiography. He concluded that plain radiography is not sufficient for the evaluation of femoral tunnel placement because of poor tunnel visibility and inaccurate tunnel projection. Cole et al²⁸ studied the accuracy of a lateral radiography in assessing ACL femoral tunnel position.He found that a malpositioned tunnel could not be distinguished reliably from an anatomically placed tunnel with a single-plane lateral radiograph. An additional problem could be encountered with the use of bioabsorbable implants. Metallic implants can help in the detection of the bone tunnels on radiography. With increasing use of bioabsorbable interference screws, however, the authors assessing tunnel position using plain radiography will become more difficult than in the past.

Conclusion

Significant differences between navigated and non-navigated ACL reconstructions were identified, despite deficiencies and difficulties in the use of radiography for the evaluation of tunnel placement. Navigation resulted in more anterior anatomical placement of the tibial tunnel. Both methods produced acceptable tunnel placement in the hands of an experienced surgeon. Radiographic evaluation was less accurate than computer navigation, and other imaging techniques (CT or magnetic resonance imaging) may be helpful in evaluating the accuracy of tunnel placement.

References

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Authors

Drs Koh, Koo, Leonard, and Kodali are from the Feinberg School of Medicine, Northwestern University, Chicago, Ill.