Anterior cruciate ligament graft fixation first in anterior and posterior cruciate ligament reconstruction best restores knee kinematics
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To evaluate the effect of different graft fixation sequences in one-stage anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) reconstruction on (1) knee biomechanics and (2) tibiofemoral alignment.
Twelve porcine knees were used in this study. Five fixation sequences were performed (angle indicating knee flexion): (a) PCL at 30° and ACL at 30°, (b) PCL at 90° and ACL at 30°, (c) ACL at 30° and PCL at 30°, (d) ACL at 30° and PCL at 90°, and (e) ACL and PCL simultaneous fixation at 30°. Anterior and posterior tibial translation was measured under an 89 N load. A 3-D digitizer was used to measure the change in anteroposterior (AP) tibiofemoral position.
None of the graft fixation sequences restored the AP laxity of the intact knee, and there are minimal differences in the in situ tissue forces in the ACL and PCL grafts. The reconstructions with fixation of the PCL graft first resulted in a significantly larger change in AP tibiofemoral position from the intact knee at 60° and 90° of knee flexion than the reconstructions with fixation of the ACL graft first (p < 0.05).
Fixation of the ACL graft at 30° of knee flexion followed by fixation of the PCL graft can best restore the tibiofemoral position of the intact knee. This study has clinical relevance in regard to the effect of graft fixation sequence on the position of the tibia relative to the femur in one-stage ACL and PCL reconstruction.
KeywordsOne-stage ACL and PCL reconstruction Multiligamentous injuries Porcine
Multiligamentous knee injuries are relatively rare but debilitating with 0.001–0.013% of orthopaedic injuries suggested by the literature [16, 18, 30, 43, 48]. This kind of injury is usually associated with sports accidents or vehicular trauma resulting in knee dislocation or subluxation and requires surgical treatment [8, 37]. Combined anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) injuries have been reported to be equivalent to knee dislocations regarding severity of ligamentous injury and mechanism of injury , where the ACL is the primary restraint to anterior tibial translation (ATT) and the PCL is the primary restraint to posterior tibial translation (PTT) [2, 7]. In a recent analysis of 106 patients with multiligamentous knee injuries, 70% of the patients involved combined disruption of the ACL and PCL .
One-stage reconstruction of the ACL and PCL is the most common procedure for patients with multiligamentous knee injuries . To the best of our knowledge, there is no consensus regarding the best protocol for the sequence of graft fixation in this procedure. Many authors recommend PCL graft fixation first [11, 12, 13, 20, 26, 33, 42, 44, 50, 53], but others support initial ACL graft fixation followed by PCL graft fixation . Some authors recommended simultaneous tensioning of the ACL and PCL grafts [17, 26, 28]. Kim et al.  found higher functional scores achieved when the grafts were tensioned simultaneously, compared with patients treated with a PCL-first approach.
Differences in the graft fixation sequences may alter the position of the tibia on the femur and lead to tibiofemoral malalignment. This malalignment may cause abnormal in situ tissue forces on the grafts, resulting in graft loosening or abnormal knee kinematics [20, 53]. In addition, it may also cause increased tibiofemoral contact pressure, which may result in early osteoarthritis .
To our knowledge, there is one biomechanical study regarding one-stage ACL and PCL reconstruction. The aim of that study was to determine the optimal graft tension required on the ACL and PCL during both cruciate reconstructions to restore anteroposterior (AP) laxity and ACL graft tissue force close to native cruciate. However, the effect of graft fixation sequence was not evaluated .
The purpose of this study is to evaluate the effect of different graft fixation sequences in one-stage ACL and PCL reconstruction on (1) knee biomechanics and (2) tibiofemoral alignment in the knee without external loading. It was hypothesized that the best sequence to restore intact knee kinematics and tibiofemoral position would be primary fixation of the ACL graft at full extension in the porcine knee and subsequent PCL graft fixation at 90° of knee flexion. This study is the first to evaluate the effect of graft fixation sequence in one-stage ACL and PCL reconstruction on knee biomechanics.
Materials and methods
Twelve (n = 12) fresh-frozen adult porcine hind limbs were used in this study. The specimens were disarticulated at the hip joint, and the skin and subcutaneous tissue were removed leaving the knee joint capsule intact. The specimens were stored at −20 °C and thawed at room temperature the night before testing [32, 49]. Before testing, the femur and the tibia were sectioned approximately 10 cm from the joint line and potted in epoxy for mounting in aluminium clamps. During testing, the specimens were kept moist with saline solution.
The knees were tested using a robotic testing system consisting of a robotic manipulator (CASPAR Staubli, Orto MAQUET) and a universal force/moment sensor (UFS; Model 4015; JR3 Inc.) having a force and moment accuracy ±0.2 N and ±0.1 Nm. The robotic system has positional control in 6 degrees of freedom (DOF) with a repeatability of motion within ±0.02 mm at each joint. A computer program controlled the displacements of the forces/moments in all 6 DOF and performed data acquisition. The passive path of the intact knee from 30°, which is full extension in the porcine knee , to 90° of flexion was determined by the robotic testing system in 0.5° increments , which was done by minimizing all the forces and moments applied to the joint at each increment .
Each knee was tested at the following knee states: (1) intact ACL and PCL, (2) ACL or PCL sectioned (in a randomized order), (3) both ACL and PCL sectioned, and (4) one-stage anatomic single bundle (SB) ACL and PCL reconstruction. The following five fixation sequences were performed for each cruciate reconstruction (angle indicating knee flexion): (a) PCL fixation at 30° and then ACL fixation at 30° (PCL 30°/ACL 30°), (b) PCL fixation at 90° and then ACL fixation at 30° (PCL 90°/ACL 30°), (c) ACL fixation at 30° and then PCL fixation at 30° (ACL 30°/PCL 30°), (d) ACL fixation at 30° and then PCL fixation at 90° (ACL 30°/PCL 90°), and (e) ACL and PCL simultaneous fixation at 30° (Sim. ACL 30°/PCL 30°). The fixation order was randomized to remove order effects from the data, such as tunnel widening and visco-elastic effects of the graft.
The knees were tested under 89.0 N anterior tibial (AT) and posterior tibial (PT) loads at 30°, 60°, and 90° flexion [21, 22, 31], where the force of 89.0 N was equivalent to the force used by the KT-1000 arthrometer [10, 52]. Since the passive path serves as the reference position to which external loads are applied in order to collect kinematics data, the passive path was determined for the intact knee as well as for each reconstruction. The ATT and PTT of the knee at each flexion angle were calculated by comparing the tibial AP positions before and after each loading condition . Repeated testing after the removal of the ACL, PCL, and grafts allowed the in situ tissue force to be determined by the principle of superposition [27, 38, 40], whereby the change in force is measured for the same passive path and knee motion during applied loads.
The robotic testing system utilized a Cartesian coordinate system that defines axes in the AP, mediolateral, and proximodistal directions of the tibia based on Fujie et al. . A 3-dimensional digitizer (Faro Arm Platinum; Faro Technologies Inc.), with an accuracy of 0.025 mm according to the manufacturer, was used to measure the relative tibial position for each reconstruction at each knee flexion angle. For this, registration screws placed on the tibia and the femur were digitized at different flexion angles similar to the methods of Wang et al. . On the femur, the point at the centre of the epicondylar axis was used. The centre of the epicondylar axis was calculated by the location of the lateral epicondyle and the origin of the MCL on the femur . On the tibia, a point on the end effector was used. The relationship between the point on the tibia and the centre of the epicondylar axis on the femur allowed the change in tibiofemoral position from the intact knee to be determined in the AP direction of the tibia.
Before testing, diagnostic arthroscopy was performed to confirm the intact cruciates, menisci, and articular cartilage. First, the intact knee was tested, and then in a randomized order either the ACL or PCL was resected with a blade, punch, and shaver, and the knee was tested. Afterwards, the remaining ligament was resected, and the ACL- and PCL-deficient knee was retested. Finally, one-stage anatomic SB ACL and PCL reconstruction was performed.
Soft tissue grafts were harvested from bovine extensor digitorum communis tendons . All tendons were folded into two stranded grafts and trimmed to an 8 mm diameter. The looped side of the graft was tethered to an extra-cortical button, and the other end was whip-stitched with a No. 5 polyester suture. For both grafts, the femoral side was fixed using the extra-cortical button. Both the ACL and PCL grafts were fixed at 40 N of tension (Meira Corp.) , and the tibial side of each graft was fixed by wrapping the strands of grafts around two screws on the tibial bone clamp. The order in which the ACL and PCL grafts were fixed was determined by the fixation sequence of the reconstruction. During simultaneous ACL and PCL graft fixation, both the ACL and PCL grafts were tensioned simultaneously followed by the grafts being clamped separately in a randomized order. IRB approval was not required at the University of Pittsburgh, Pittsburgh, PA, for this study, as all animal tissues were commercially acquired.
Statistical analysis of the differences in AP translation, in situ tissue force, and change in AP tibiofemoral from the intact knee was performed using a one-factor repeated measures analysis of variance (ANOVA) with knee state as the factor, followed post hoc analysis using a Bonferroni correction since multiple contrasts were made. Statistical significance was set at p < 0.05, and all statistical analysis was performed using SPSS (v. 24; SPSS Inc.). Results are reported as mean ± SD. To estimate the number of samples needed in this study, an a priori power analysis was performed (v. 188.8.131.52; G*power) using the significance level of 0.05, a power of 0.80, and being able to detect a difference in ATT of 2.5 mm, based on previous data , which resulted in n = 8 samples.
The most important finding of this study is that the tibiofemoral position is closest to the intact knee when the ACL is fixed first in one-stage ACL and PCL reconstruction. Also, this study found that none of the reconstructions restored the anterior and posterior stability of the intact knee. In the setting of one-stage reconstruction of ACL and PCL injuries, different fixation protocols have been used: fixation of the PCL graft first, fixation of the ACL graft first, and fixation of the ACL and PCL graft being fixed simultaneously [5, 11, 13, 26]. Restoration of knee biomechanics and tibiofemoral position close to the intact knee are aims of the reconstruction. These parameters could be affected by different graft fixation sequences. Achievement of better knee stability could be the reason that most authors suggest priority of PCL graft fixation. While the current study revealed that no reconstruction restored knee stability, it did reveal that fixation of the PCL graft first could lead to less AP translation at 30°, 60°, and 90° of knee flexion compared to the ACL graft being fixed first (in a few cases).
Concerns about PCL graft fixation arise when we evaluate tibiofemoral alignment after one-stage ACL and PCL reconstructions. The present study revealed better tibiofemoral alignment after the ACL graft was fixed first. Many authors have emphasized the importance of restoring normal tibiofemoral alignment during one-stage cruciate reconstructions, although this can be difficult to accomplish [17, 44]. Shifting of the tibia relative to the femur may cause abnormal graft in situ tissue forces, resulting in loosening of the graft or abnormal knee kinematics [20, 53]. It also may cause increased contact pressure on the knee cartilage and meniscus, resulting in early osteoarthritis .
To the best of our knowledge, there is no consensus on the graft tension, knee flexion angle, and the order of graft fixation in one-stage ACL and PCL reconstruction. We believe that in the setting of one-stage ACL and PCL reconstruction, the fixation sequence is an important factor that needs to be considered. The order of graft fixation is the key in determining the position of the tibia relative to the femur. If the PCL graft is fixed first, the tibia will move forward and will be fixed anteriorly. On the contrary, if the ACL graft is fixed first, the tibia will move backward and be fixed posteriorly (Fig. 5). Fixation of the second graft may not make a significant change in the tibiofemoral alignment since the first graft has already determined the tibial position.
The present study considered different knee flexion angles and fixation sequences at the time of graft fixation in order to evaluate tibiofemoral alignment and knee biomechanics. In terms of the ACL- and PCL-deficient knee, when the PCL graft was fixed first, regardless of the knee flexion angle, the tibia moved anteriorly. When the ACL graft was fixed first or the ACL and PCL grafts were fixed simultaneously at 30° of knee flexion (full extension in the porcine knee), the position of the tibia changed significantly less in the AP direction compared to reconstructions with fixation of the PCL graft first at 60° and 90° of knee flexion.
In the present study, the ACL graft tissue force was not restored to the intact value under AT loading except at 60° of knee flexion; however, when the ACL was fixed first, the ACL graft tissue force was higher. Conversely, under PT loading some reconstructions did restore the intact PCL tissue force and the graft tissue force was generally higher when the PCL was fixed first.
Accordingly, the results of this study support the hypothesis of fixation of the ACL graft first, at 30° knee flexion (full extension in porcine knee), followed by the PCL at 90° of knee flexion in that tibial position and ACL graft tissue force were best restored. In addition, the results of biomechanical test regarding AP laxity, tibiofemoral alignment, and in situ tissue forces in the ACL and PCL grafts indicate that there is no significant difference between fixation of the PCL graft at 30° or 90° of knee flexion when the ACL is fixed first at 30° of knee flexion.
Questions concerning the graft-tensioning protocol of both grafts still remain unresolved . In a biomechanical study, Markolf et al.  could not find a graft-tensioning protocol that restored AP laxity and graft tissue forces in both grafts compared to the intact state. Most of the studies about one-stage ACL and PCL reconstructions did not describe the graft tension of their reconstructions [5, 9, 17, 20, 26, 28, 33, 34, 42, 44, 50, 53]. This study also found that no combined reconstruction totally restored tibiofemoral position, AP laxity, and graft tissue forces; however, ACL first fixation best restored tibial position and there were only minor differences between the reconstructions in the other factors.
There are several limitations in this study. The porcine knee was used as a model [15, 19, 23, 24] to explore the effects of the fixation sequence in one-stage ACL and PCL reconstruction, and the results might offer future direction in a human study. Many possible graft fixation protocols can be used during one-stage ACL and PCL reconstruction, and this study focused on knee flexion angle at fixation and graft fixation sequence. Further studies involving ACL and PCL graft tension could aid in further restoring knee biomechanics after this injury. This study has clinical relevance in that it offers insight into the fixation sequence effect on tibiofemoral alignment during one-stage ACL and PCL reconstruction.
This study revealed that fixation of the ACL graft first at 30° knee flexion (full extension in the porcine knee) followed by fixation of the PCL graft at a low or high knee flexion angle can restore the tibiofemoral alignment of the intact knee and that these two reconstructions have no significant differences regarding AP stability, tibiofemoral alignment, or in situ tissue forces in the ACL and PCL grafts.
Compliance with ethical standards
Conflict of interest
The authors declared that they have no conflicts of interest in the authorship and publication of this contribution.
This study was funded by the Department of Orthopedic Surgery at the University of Pittsburgh.
No ethical approval was required from IACUC.
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