A NEW TOOL TO OBJECTIVE ASSESSMENT OF ROTATIONAL INSTABILITY AND ACL/PCL DEFICIENT KNEES INSIDE MAGNETIC RESONANCE IMAGING OR CT SCAN.
Porto Knee Testing Device
ACL EVALUATION

THE MEASUREMENT IS PERFORMED USING A PERPENDICULAR LINE TO TIBIAL SLOPE CROSSING THE MOST POSTERIOR POINT OF THE TIBIAL PLATEAU AND ITS DISTANCE TO A PARALLEL LINE CROSSING THE MOST POSTERIOR POINT OF THE FEMORAL CONDYLE.

This process is repeated with and without pressure for medial and lateral compartments identifying the same points as bony landmarks (FIG. 3).

Porto Knee Testing Device Ligaments

FIG. 3



For the ACL it can also be measured evaluating the difference in each of the two points of measurement is calculated between the two sets (with and without pressure) obtaining the anterior translation, in millimeters, for medial and lateral tibial plateaus: MPT - Medial plateau translation; LPT - Lateral plateau translation (FIG. 4).
Similar procedure is performed for healthy and injured knees.

Besides MPT and LPT, it was also calculated the difference between LPT and MPT (LPT-MPT) which reflects the increased translation of lateral tibia condyle associated to rotational instability.
The registered differences between injured and healthy knees for MPT, LPT and LPT-MPT can be used to assess the difference in behavior between stable and ACL-deficient knees (Dif. MPT, Dif. LPT and Dif. LPT-MPT) (FIG. 4).

It is also possible to use horizontal cuts in MRI to measure the angle between the line of the posterior cortex of the tibia and femur.

The influence of ACL in joint stability under torsional load is not yet fully understood16. Several papers concerning clinical outcomes of double-bundle ACL reconstruction have been reported but “measuring” its influence in improving clinical results has proven to be a difficult task. A prospective comparable study of single- and bi-socket reconstruction by Hamada et al found no difference in clinical outcomes17. Similar (or not significantly different) outcomes regarding stability were described by Muneta et al18 and Yasuda et al 19,20. On a meta-analysis of randomized control trials no clinically significant differences were found on KT-1000 nor pivot-shift test4. However no reproducible method capable of quantifying rotational laxity was considered. We must recognize that proper measurement tools will guide our surgical practice. This has probably been the case of ACL reconstruction once the use of anterior translation measurements as primary means to assess the outcome of ACL surgery might have led to subsequent development of effective techniques of controlling anterior translation but not of rotational instability, particularly during high demand activities21-23.

Porto Knee Testing Device Ligaments

FIG. 4



We believe that the ideal tool should provide simultaneous anatomic and functional evaluation, be reliable and reproducible, without significant increase in costs. Recognizing that the application of combined internal rotation and valgus torques to the knee can more precisely recreates the anterolateral subluxation that occurs in knee joint during the pivot shift test24 we believe that foot rotation should not be restricted while applying translational force. Ongoing study is now comparing results of foot in neutral without rotational restriction (as herein described) with forced maximum external and internal rotation during anteroposterior load transmission using PKTD®.

Okazaki et al25 quantified the anterolateral rotational laxity  of ACL-deficient knees by pivot shift test in open MRI. Using the same method, Slocum's anterolateral rotatory instability test in open magnetic resonance imaging, Tashiro et al26 demonstrated that side-to-side differences of anterolateral tibial translation correlates with clinical grade of pivot-shift test and stress radiography but not with KT-2000 arthrometry. The cutoff value was established as 3.0 mm which can be considered in line with the results herein described (cutpoint for Dif. LPT-MPT of 3.5mm). Using the aforementioned method, Izawa et al27 reported better rotatory stability of anatomic double-bundle reconstruction comparing to single bundle. However open MRI devices are not routinely available in a majority of knee surgery centers and despite intraobserver and interobserver reproducibility having been reported it requires availability of a trained surgeon capable of executing pivot shift test during MRI. On the other hand it has also been demonstrated that mechanized pivot-shift achieves greater accuracy than manual testing28.

Several other methods have been proposed5,29 aiming to quantify rotational laxity in an objective and reliable method including robotics30, navigation (computer-assisted)31, radiostereometric analysis32, stress radiographies33 or several arthrometers5,8,11,34-37 reflecting the growing consensus around the need to describe objectively rotational behavior of knee joint for diagnostic purposes, detecting risk factors or controlling surgical outcomes.