Direct arthroscopic cartilage assessment remains the gold standard. It is recommended by the International Cartilage Repair Society (ICRS) to systematically assess cartilage status during arthroscopy but this examination is highly subjective, poorly reproducible, time-consuming and lacks precision. US has shown good potential for cartilage evaluation but is limited in extra-articular conditions. It is also difficult to manually maintain a perfect perpendicularity between the ultrasound beam and the curved surface of the cartilage. Therefore, we have developed a navigated intra-articular US probe (NIAUS). The NIAUS probe could contribute to a more exhaustive and direct intra-articular evaluation of cartilage integrity. Navigation enables control of the US echo pulse perpendicularity and its localisation relative to the joint. Our objectives were (1) to evaluate automatic cartilage thickness measurement with the NIAUS probe in comparison to high definition MRI on cartilage samples, (2) to generate a real-time 3D map of the thickness parameter on samples, and (3) to demonstrate the feasibility of a full NIAUS probe cartilage scan on a specimen distal femur in arthroscopic conditions.

The NIAUS probe is a 4.5mm probe consisting of a 64 element linear array transducer with a central frequency of 13 MHz and a motorised head. The NIAUS probe is navigated. The rotating US head position is controlled by navigation in order to enable constant perpendicular acquisition of cartilage. The NIAUS probe thickness measurement (1) was evaluated on bone and cartilage samples of 9 tibial plateaus. The cartilage thickness was measured via automatic segmentation. Each sample was also scanned in a high resolution MRI (4,7 Tesla) and cartilage thickness was semi-automatically extracted for comparison. During NIAUS scan, (2) a visual 3D map was generated. Finally (3), we scanned two distal femurs with the NIAUS probe in arthroscopic navigated conditions on one specimen and a 3D map of the distal femur thickness was generated in real time.

NIAUS thickness measurement (1) absolute error compared to MRI for 9 plateaus ranged from 0.15mm to 0.32mm in median, p25=0.07 and 0.18, p75=0.28 and 0.5 respectively. 3D maps of the sample cartilage thickness (2) were generated in real time during the NIAUS scan. The cadaveric procedure (3) was conducted without incident via the two anterior portals and a 3D map of the distal femurs cartilage thickness was generated.

A precise US arthroscopic grading and scoring of cartilage during surgery could help for better standardisation, prediction of results and making “live” decisions. Our in vitro experiment shows good results compared to MRI for NAIUS cartilage thickness measurement, and our cadaveric study demonstrate the feasibility of a NIAUS scan in arthroscopic conditions. Our results are encouraging and a clinical trial is currently being designed for preliminary in vivo NIAUS evaluations of cartilage thickness compared to MRI.

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.