Purpose of the study: Anterior cruciate ligament (ACL) navigation systems are based on two underlying principles: «statistical» anatomic position and isometric anatomic (anatomometric) positioning. The purpose of this study was to demonstrate that an anatometric positioning of the transplant can be achieved, in other words, that the transplant can be positioned in the original anatomic air of the ligament insertion while preserving an optimal isometry without notch impingement. This study was also conducted to compare conventional systems with a computer-assisted system.

Material and methods: This study was conducted on thawed fresh-frozen cadaver knee specimens with > 120° flexion. The computer-assisted protocol for ACL surgery was applied to ten knee specimens. The original anatomic insertions of the ACL were dissected then inserted at the appropriate points into the computer display. The tibial and femoral insertion points of two classical aiming devices were recorded. These points were compared with the original anatomic insertion.

Results: For the tibia: classical aiming methods proposed a point of insertion posterior to the anatomic insertion for eight knees and within the frontiers of the anatomic insertion for two, in line with the anterior border of the posterior cruciate ligament. The computer-designated point of insertion for the tibial fixation was always within the anterior third of the ACL insertion, generally medially. For the femur, the transition (or isometric) line ran across the anatomic femoral insertion in all knees. It was observed that in all cases, the surgeon could choose an anatomic insertion with lesser anisometry by situating the insertion in the distal part of this line: for nine knees, the computer-designated femoral point was anatomic and with lesser anisometry. The Acufex aiming device produced better anisometry (my=4 mm) than the Arthrex device (my=6 mm) but with a less favorable anisometry curve.

Discussion: The notion of anatometry is compatible with computer-assisted surgery. This study demonstrated that the computer-designated tibial point of insertion is more anterior and medial than the conventional aiming points. This is a potential choice if the absence of a notch impingement can be visualized: Howel described a manual fluoroscopic method. In our opinion, at the present time, optimal choice of the femoral point to achieve the desired anisometric curve is strictly operator-dependent.

Purpose: Anterior cruciate ligament plasty requires an anatomic and isometric implantation avoiding all notch conflict. This requires appropriate position of the bone holes. Recent studies have shown that hole placement is a key problem. In order to attempt to solve this problem, we examined the possibility of imaging-free navigation.

Material and methods: We elaborated a navigation system based on the bone morphing a concept where a static model of the knee is displayed on the screen. The system uses a 3D optic localiser which records the relative positions of five rigid bodies equipped with reflectors fixed on the femur, the tibia, the palper, the femoral aiming devise and the tibial aiming device. The arthroscopic operative technique is based on bone morphing. The operator navigates from the tibial articular hole drawn as a circle around the point T for which the computer maps on the notch the corresponding femoral isometry. On this isometry map, the surgeon navigates to the femoral articular hole drawn as a circle around the point F. The transplant is then fixed in place. The computer searches for a possible transplant-notch conflict and indicates where notch plasty would be necessary. The system was evaluated by comparing the points T and F indicated by the conventional method and by the computer. We compared the frequency of notch plasty with conventional and navigation surgery.

Results: The navigation system was used for 50 knees. The navigated T points were more anterior and more medial than those indicated by the conventional technique. With the conventional method, the anisometry of the central fibre can vary 3 to 13 mm for a given knee, depending on the F point determined. The computer optimises this point. There were less than 5% notch plasties with the navigation method and more than 50% with the conventional method.

Discussion: Bone morphing allows the operator to navigate in the knee, monitoring the operation on the screen model. The computer helps optimise bore hole position but does not indicate the exact position, which is determined by the operator. The computer can provide real time information helping the surgeon determine the ideal hole position in comparison with the conventional method.

Purpose: No conventional surgical technique for ligament reconstruction can be used in all cases to achieve ideal position of the transplant. Navigation systems without visualisation of the anterior cruciate ligament should meet the requirements. This is an operative strategy based on one or more computer assisted procedures enabling ligament reconstruction without the need for conventional pre- per, or postoperative imaging. The principle is based at the present time on the use of a station (computer, localisers, display screen, command pedal) used for processing data (spatial measurements and positioning) delivered by markers fixed on rigid bodies and tools (palpation, aiming tools).

Material and methods: This study was conducted on ten cadaver knees. Each knee was instrumented with the station. Joint kinetics were recorded with and without the ACL and after harvesting the transplant: patellar ligament and hamstring ligaments. Bone morphing was used to draw the tibial and femoral surfaces. Two types of aiming tools were tested by recording the data points issuing from the tibial output and the femoral input. The position of the femoral and tibial holes was determined to achieve the smallest anisometry and absence of notch conflict. Isometric zones were compared with the anatomic zones of the ACL. We also compared the position of the transplant determined by the computer and that determined according to the methods of conventional arthroscopy. An x-ray of each knee was obtained to compare with data in the literature concerning the advised position of the femoral and tibial holes with that established by the computer navigation system. Each knee was tested with KT1000 before and after surgery.

Results: The precision of bone morphing was 0.1 mm. Anisometric curves were compatible with drilling holes calibrated to the size of the implant in four knees. The operator used the navigation system to determine the point of the femoral hole in six knees. The system then calculated the point of the tibial hole automatically eliminating the risk of notch conflict. The anisometric values were less than 2 mm; the distance roof of the notch/anterior border of the transplant was calculated as a function of the radius of the transplant (3.5–5 mm). The position of the tibial hole given by the computer system was always more medial than that given by the tibial aiming tools. The position of the femoral tunnel was always more anterior than that given by the femoral aiming tools. The postoperative KT1000 values were identical to the preoperative values.

Discussion: Navigation without visualisation of the ACL is able to position the ACL in an isometric plane or better in an “anatomometric” plane, to inscribe the joint orifice of the tibial hole on the projection of the anterior arch of the notch on the tibial surface, to draw in real time the isometric femoral map on the notch in order to centre the joint orifice of the tibial hole as well as the corresponding laxity map, to indicate on the femoral notch the point which will be the centre of the joint orifice of the femoral hole, to draw the isometric curve of a given fibre and its corresponding laxity map, and to detect and allow the treatment of any transplant-notch conflict.