Computer-Assisted Navigation for Anterior Cruciate Ligament Reconstruction

Computer-assisted navigation for anterior cruciate ligament (ACL) reconstruction can increase precision in tunnel placement and also provide valuable outcome information, such as rotational stability. This is accomplished by registering anatomical landmarks and tracking the location of instruments and the tibia and femur in virtual three-dimensional space on the computer. Values such as the location of instruments and measures of impingement and isometry, as well as the location of the femoral and tibial tunnels, are calculated and shown to the operating surgeon in real time. Computer-assisted navigation has been demonstrated in several studies to improve accuracy and decrease laxity of the ACL-reconstructed joint ; the clinical outcomes at this time have not been dramatically different.

Rationale

Computer assistance for precision navigation has been increasingly common in everyday applications such as the global positioning system for drivers 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 the accuracy of total knee replacement components, not only in reducing outliers but also in correcting consistent repeated errors made by experienced surgeons. Similarly, improved accuracy in the placement of total hip components has also been demonstrated. Clinical outcomes have been shown to be comparable or superior for navigated groups.

Need for Precision in Tunnel Placement

Clinical outcomes in ACL-reconstructed patients are significantly related to accurate tunnel placement. Ample evidence exists that certain tunnel positions will result in mechanical problems with the graft and/or produce inappropriate kinematics. Incorrect tunnel placement can result in pain, laxity, synovitis, loss of range of motion, graft impingement, and graft failure. Up to 70%–80% of the complications in ACL reconstruction surgery were a result of malpositioned tunnels. In longer-term follow-up, errors in tunnel placement result in an increased risk of arthritis.

Need for Precision in Tunnel Placement

Clinical outcomes in ACL-reconstructed patients are significantly related to accurate tunnel placement. Ample evidence exists that certain tunnel positions will result in mechanical problems with the graft and/or produce inappropriate kinematics. Incorrect tunnel placement can result in pain, laxity, synovitis, loss of range of motion, graft impingement, and graft failure. Up to 70%–80% of the complications in ACL reconstruction surgery were a result of malpositioned tunnels. In longer-term follow-up, errors in tunnel placement result in an increased risk of arthritis.

Current Accuracy without Navigation

Multiple authors have recommended techniques for tunnel placement; however, the few published studies assessing surgeon accuracy have shown surprisingly poor results. Approximately 10%–20% of all cases are revised, typically related to tunnel placement. 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. Another common error is a posterior tibial tunnel, resulting 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.

The accuracy of ACL tunnel placement has been evaluated in knee models, cadavers, and patients. The researchers from the University of Pittsburgh evaluated tunnel placement by two experienced ACL surgeons in foam knee models using typical arthroscopic guides. Tibial tunnel placement was a mean of 4.9 mm from the ideal tunnel site, and the femoral tunnel was a mean of 4.2 mm from the ideal. This group repeated the study with fellows and experienced surgeons; the tibial tunnel had 2–3.4 mm of average error, and the femoral side 2–4.5 mm of average error.

In a cadaver study performed at an advanced arthroscopy course, instructors placed tunnels in 24 specimens; 50% (12/24) of the femoral tunnels and 25% (6/24) of the tibial tunnels were “unacceptable”.

Clinical studies have also shown variable placement when arthroscopically placed tunnels are analyzed radiographically. Cha et al. reported a series of 30 patients where arthroscopically placed tibial guide pins were evaluated by the use of intraoperative fluoroscopy. The pin had to be repositioned 43% of the time. Typically, the tendency was to place the tibial tunnel too posterior (13/14 cases). Interestingly, there was no improvement in accuracy over time; the pin was repositioned 50% of the time in the last 10 cases.

Similar results were found for a series of 24 patients in which tunnel position was evaluated postoperatively by radiographs. Two experienced ACL surgeons performed ACL reconstructions, and tunnel placement was correlated with postoperative x-rays. Surgeons had poor ( R ²= 0.14) correlation for mediolateral tibial tunnel position, and no correlation ( R ²= 0.07, P = .36) to the true anteroposterior (AP) position of the tibial tunnel. The authors found that 12.5% of tunnels “were in very different positions than that expected by the surgeon.”

Other authors have noted that radiographical 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.

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

Computer-assisted navigation for ACL reconstruction involves accurately tracking the relative positions of the tibia and femur, as well as the intra-articular landmarks that guide correct tunnel placement. Markers on rigid bodies attached to the tibia and femur are tracked intraoperatively by an infrared camera to less than 1 mm and less than 1 degree of error ( Fig. 57.1 ). Some systems require the use of preoperative computed tomography scans or intraoperative fluoroscopy (Brainlab, Westchester, Illinois). Other systems are image free, such as the OrthoPilot (Aesculap, Center Valley, Pennsylvania). Most navigation protocols follow a similar progression of registration of intra-articular and extra-articular landmarks. The following description is of the workflow of the OrthoPilot (Aesculap), which functions to record kinematic and anatomical data and calculates critical values of concern to the surgeon.

The navigation camera and display screen ( Fig. 57.2 ) are set up opposite to the operative side of the patient, and next to the arthroscopy tower or screen. Before the placement of the trackers, the ACL stump is removed and a notchplasty is performed, if desired, and intra-articular pathologies, such as meniscal or chondral injuries, are addressed. The graft is harvested and prepared.

The optical trackers for ACL reconstruction are then attached by either Kirschner wires (K wires; see Fig. 57.1 ) or threaded pins to the tibia and femur. We prefer to use trackers that can be attached by K-wire fixation, avoiding the potential stress riser of a larger threaded pin. The K wires are placed through small stab incisions onto the anterior tibia and medial epicondyle of the femur, minimizing quadriceps morbidity and avoiding interference with instruments.

A passive tracking system using light reflected from trackers is used because there is less weight and no cords, unlike an active LED system. After attachment of the trackers to the femur and tibia, tibial extra-articular landmarks are registered by palpation with a pointer ( Fig. 57.3 ). Kinematic evaluation of knee motion is then performed by recording the relative positions of the femur and tibia in full extension and flexion. Registration and kinematic testing take approximately 90 seconds.

Femorotibial laxity is then assessed by the surgeon at a chosen degree of flexion, usually 30, and also during a pivot-shift maneuver. Anterior/posterior translation in millimeters and degrees of internal and external rotation are recorded ( Fig. 57.4 ). This quantitative rotational measurement is typically not possible without computer assistance.

After the laxity measurement, arthroscopic intra-articular landmarks are registered with the pointer, similar to palpation with a probe ( Fig. 57.5 ). These include the PCL, lateral meniscus, medial tibial spine, anterior and posterior margin of the intercondylar notch, and femoral ACL origin. Accurate measurements of the length of the Blumensaat line provide feedback to the surgeon about the true location of the posterior notch.

At this point the system allows either the femoral or tibial side tunnels to be created first. A tibial guide with reflective markers is then used to place the tibial guide pin with respect to the PCL, the intracondylar notch, and other intra-articular and extra-articular landmarks ( Fig. 57.6 ), to ensure selected tunnel angles and avoid impingement. Double-bundle tunnels can be placed to avoid overlap. The proposed femoral tunnel location is then palpated ( Fig. 57.7 ) to provide the position of the tunnel with respect to the posterior femoral cortex and clockface orientation. Graft isometry and impingement are also displayed for each bundle. Finally, after securing the graft, the final measurements of AP translation and internal and external rotation are obtained under both fixed angle and pivot-shift tests ( Fig. 57.8 ).