For a long time, we have known that the stiffness of the ACL (Anterior Cruciate Ligament) reconstructions evolved during time after the surgery [5,17-19]. It passes through a first stage during the per-surgery phase where it is very resistant [6,7,20]. The graft can indeed support up to twice as much loads as a native ACL (2000N). Then, this resistance decreases during the second stage, the necrosis stage (it has been evaluated that the ACL resistance during this stage is at about 40% compared to the native ACL=approximately 800N). After this stage, the ACL tendon graft transforms itself into a neoligament (6 months after the surgery, the resistance of this neoligament is at about 80% compared to a native ACL). The anchor points of the graft in the tunnels should also not be forgotten. These are indeed much less resistant compared to the graft just after the surgery [15,16].
The work of Bienvenutti , Markolf [3,4,21,22], Noyes [6,7], Butler [14,23] have shown that the Lachman test (Flexion of the knee between 15 and 25° as Torg has shown) has a high sensibility for detecting ACL ruptures because the ACL was the primary brake while applying anterior tibial translation (> 80%) in this articular range but this test lacked objectivity. The arrival of the first laximeters (Daniel , with KT 1000, Staubli  with Telos), now more than thirty years ago, allowed for the first time to objectify these tests done on ACL ruptures (in pre surgery) but analyzing the graft’s stiffness in particular during the post-surgery phase with accuracy was not possible. The KT 1000 and the Telos were pioneers in this field of study as they allowed analysis to be done with different forces applied on the tibia but they were either not precise enough or to invasive [25-45]. The GNRB® (Figure 1), which is a new motorized and computerized compliance-metry device created during the years 2000 (company Genourob®, France), see study of Robert , allowed measures to be much more precise regarding the compliance of the ACL following knee injuries (additionally to the evaluation of the differential of displacement at a certain force applied commonly known as laxity). Figure 1:
GNRB Motorized device: a new motorized and computerized laximeter.
Nowadays, it is the only international study to have been done on the state of the resistance of the ACL reconstruction at day 0 (10 minutes after the surgery) until the full healing of it (more than two years after the surgery). During the tests, the data that is saved comprises several displacement measurements while applying different forces on the tibia: force/displacement allows the tracing of compliance curves (inverse of the stiffness or rigidity=1/R).
Maximal resistance of the graft and its anchor points during time were of course taken into account during the post-surgery phase. Thus, a GNRB test was done during the surgery (at the end of the surgical operation), the 1st month and the 2nd month with 100 N max applied (much less than the peak force that the ACL can sustain while walking, 350 N according to Nagura ) but enough to evaluate the state and behavior of the ACL graft while under stress.
During the surgical procedure, the graft has a resistance which is much higher than the normal ACL (around 4000N depending on the sample instead of 2000N for a normal native ACL) and the anchor points that are used today have resistances varying from 800 to more than 1000N. Therefore, the force applied during the GNRB test (100 N) is ten times lower than the lowest rupture thresholds, the ones that are associated to the anchor points! One month into the rehabilitation phase, the same force (100N) was applied by the GNRB but this time, the lowest rupture thresholds were rather located on the ACL graft itself (around 800N) and on the anchor points of the semitendinosus-gracilis (That is different when patellar tendon surgery is performed because of the presence of bone tissue at the two endpoint of the graft allowing healing in the femoral and tibial tunnels to be faster). Consequently, there is no way of modifying the ACL graft’s stiffness while doing GNRB tests. If that were the case, walking would be forbidden to the patient because the forces applied would be much higher (peak force > 300 N) than the GNRB tests at 100 N.
A study has been realized to follow the evolution of the resistance of ACL grafts over time. Tests were done per-surgery and gave objective results. The data collected allowed the creation of a database composed of the initial resistances that the ACL grafts provided just after the surgery (The patient were still under anesthesia=no muscular activity was therefore present and after the GNRB device can detect muscular activity during test ).
Another important fact is that this study has been realized by the same operator for each of the two surgical techniques (same surgeon that applied the same surgical procedure (semitendinosus-gracilis for one and patellar tendon technique for the other). All patients were diagnosed with isolated ACL rupture. This permitted concluding whether there were different final resistances with the same surgical procedure. The operator of the GNRB® was also the same experienced operator for both of the surgical techniques that were done (the tightening force used to maintain the patella against the femur were the same for both knees, likewise concerning the positioning of the patient’s knee). A recent study done at the Biomechanical department of the University of Lyon 1 has validated the high reproducibility of the GNRB® device with more than 10000 tests that were run whilst respecting different standards.
The starting point was to see if by applying different sollicitations after the surgery, the same stiffness or flexibility results were found on the graft one year later (with an initial resistance almost identical at day 0 like we will see later). After one year, the ACL graft has already strongly evolved generating a neoligament but we know that the ACL reconstruction process is not fully achieved. At this time, we can highlight the fact that the ACL reconstruction process is already very advanced and accordingly, at this state, the compliance does not modify a lot afterwards (see Table 1 and 5 Y after surgery).
Operated patients using Hamstring Tendon Technique.
The aim of this study was to demonstrate the evolution of the stiffness curves of these ACL reconstructions over time using none-invasive equipment in order to draw reliable conclusions from it. The interest we found in doing this was that it proved how the compliance curves of the ACL graft evolved over time after surgery (we supposed this but could not verify if it was true). Also, analyzing the curves can bring interesting information to the surgeon one month after the surgery. He/she can indeed see if the curves behave properly or not (biomechanic of the graft). We managed to prove that the evolution of these so-called compliance curves was different when patients were subjected to an aggressive rehab program. Finally, the surgeons can detect bad evolution of the graft’s compliance (divergent curves) and recommend the PTs to choose an adequate personalized rehabilitation program for the patient.