Rationale

Computer assistance for precision navigation has been increasingly common in everyday applications such as the global positioning system (GPS) for drivers and sailors and has spread into surgical applications such as total knee replacement, pedicle screw placement, stereotactic brain surgery, and otolaryngology. In orthopaedic surgery, computer-assisted navigation has repeatedly been demonstrated to improve accuracy of total knee replacement components, not only in reducing outliers but also in correcting consistent repeated errors made by experienced surgeons.^9,10 Similarly, improved accuracy in the placement of total hip components has also been demonstrated.¹¹ Clinical outcomes have been shown to be comparable between navigated and nonnavigated groups.^2,5

Need for Precision in Tunnel Placement

Clinical outcomes in ACL reconstructed patients are significantly related to accurate tunnel placement. Although there may not be a clear consensus on where tunnels should be placed, ample evidence exists that certain tunnel positions will result in mechanical problems with the graft and/or produce inappropriate kinematics. Multiple authors have indicated that incorrect tunnel placement can result in pain, laxity, synovitis, loss of range of motion, graft impingement, and graft failure.^12–23 In longer-term follow-up, errors in tunnel placement result in an increased risk of arthritis.¹⁸ Although shorter-term studies may not demonstrate substantial differences, there remains a significant risk of arthritis following ACL reconstruction, and this is likely to related in part to tunnel placement.

Current Accuracy without Navigation

Multiple authors have recommended techniques and anatomical landmarks for accurate tunnel placement; however, few studies have been performed to assess the accuracy of surgeons in reproducibly creating accurate tunnels. Part of the difficulty in assessment is the difficulty in accurately assessing intraarticular distance with the monocular, angled arthroscope and limited ability to place measuring devices in the joint in appropriate orientation. In addition, it is difficult to assess isometry or the projection of the intercondylar notch or other sources of impingement in the knee.

Clinically, accuracy of ACL reconstruction techniques can be assessed by the number of revision ACL reconstructions performed each year. Recent reports suggest that approximately 10% to 20% of all cases are revised.^24,25 The vast majority of the failures are related to technical errors, specifically tunnel placement.^24–26 The most common error is excessive anterior femoral tunnel placement, which can decrease rotational stability and may result in a graft that is lax in extension and tight in flexion.^22,24 Among experienced surgeons, it has been noted that the tibial tunnel can be placed too far posterior in order to avoid notch impingement.^2–4 This can result in posterior cruciate ligament (PCL) impingement with the knee in flexion and subsequent loss of knee flexion or strain on the graft. In addition, the graft will tend to be more vertically oriented and contribute less rotational stability.²⁴

Several studies have been performed under various conditions to assess the accuracy of ACL tunnel placement. The Pittsburgh group evaluated tunnel placement by two experienced ACL surgeons in 20 foam knee models using standard arthroscopic guides. Actual tibial tunnel placement was a mean of 4.9 mm from the ideal tunnel site. Actual femoral tunnel placement was a mean of 4.2 mm from the ideal tunnel site. These differences were believed to be significant.⁶

Another study from the same group demonstrated the variability of tunnel placement by surgeons with 100 to 3500 cases of experience. Two fellows and two experienced surgeons each drilled 10 tunnels in foam knees. Tibial placement by experienced surgeon 1 varied by 2 mm; experienced surgeon 2, 3.4 mm; fellow 1, 2.1 mm; and fellow 2, 2.4 mm. On the femoral side, variability was less for experienced surgeons: experienced surgeon 1, 2.3 mm; experienced surgeon 2, 3.0 mm; fellow 1, 4.5 mm; and fellow 2, 4.1 mm. Clearly, substantial variability was observed.²⁷

Surgeon accuracy in tunnel placement has also been evaluated in cadavers.²⁸ In an advanced arthroscopy course, instructors placed tunnels in 24 specimens. The tunnel placement was then evaluated. Fifty percent (12/24) of the femoral tunnels and 25% (6/24) of the tibial tunnels were “unacceptable.” Similar results have been anecdotally noted by instructors at other training courses.

Evaluation of tunnel placement in vivo has also been performed in several centers. Harner recently reported on a series of 30 patients in which the tibial guide pin placement was evaluated by the use of intraoperative fluoroscopy.²⁹ Tibial pins were placed using standard arthroscopic landmarks: namely, 7 mm anterior to the PCL, the medial tibial eminence, the anterior horn of the lateral meniscus, and the center of the visualized ACL tibial footprint. After reviewing the pin placement, Harner believed that it was necessary to reposition the pin 43% of the time. Typically, the tendency was for the experienced ACL surgeon to place the tibial tunnel too posterior (13/14 cases). In addition, repositioning of the pin was as frequent in the last 10 cases (5/10) as in the first 10 cases (5/10).

Similar results were found for a series of 24 patients in which tunnel position was evaluated postoperatively by radiographs.³⁰ Two experienced ACL surgeons performed ACL reconstructions and recorded their perceptions of femoral and tibial tunnel placement. These were then correlated with actual tunnel placement by a blinded observer. The femoral tunnel demonstrated excellent (perfect) correlation coefficient (R² = 1) on the anteroposterior (AP) radiograph (medial-lateral placement) between perceived and actual position. Good correlation (R² = 0.55) was found for the lateral radiograph (AP position). However, the ability of the surgeons to describe medial-lateral tibial tunnel position was poor (R² = 0.14), and the true AP position of the tibial tunnel had no correlation (R² = 0.07, P = 0.36) to the surgeons’ perception. The authors concluded that four tunnels (12.5%) “were in very different positions than that expected by the surgeon.”

Other authors have noted that radiographic analysis of tunnel placement demonstrated too-posterior placement of the tibial tunnel and a relatively vertically oriented (the 11- or 1-o’clock position) femoral tunnel using standard arthroscopic instrumentation.^2,4

The evidence suggests that there is room for improvement in the accuracy of ACL tunnel placement, even among the more experienced surgeons who typically participated in these studies. Accuracy among less experienced surgeons would likely be lower.

Techniques of Computer-Assisted Navigation

Essential elements of computer-assisted navigation for ACL reconstruction include the ability to register and accurately track the relative positions of the tibia and femur as well as the intraarticular landmarks that guide correct tunnel placement. This can be accomplished by several methods, but most current solutions involve markers on rigid bodies attached to the tibia and femur with pins or screws. These are tracked intraoperatively by use of a binocular infrared camera attached to a computer that can calculate the relative position of the femur and tibia to less than 1 mm and less than 1 degree of precision¹