Tibial graft fixation with residual tension controlled in anterior cruciate ligament reconstruction

Introduction

In anterior cruciate ligament (ACL) reconstruction, it is generally accepted that the graft should be fixed under adequate tension for successful outcomes (1-3). An animal study has shown that it takes 6 to 12 weeks for bone-patellar tendon-bone (BTB) or soft-tissue grafts to be firmly incorporated into the bone tunnel (4). Therefore, the graft must be rigidly secured with reliable fixation devices throughout this period.

There are several differences between femoral and tibial fixation. First, the bone mineral density of the proximal tibial metaphysis is lower than that of the distal femoral metaphysis (5). Consequently, when using an interference screw (IFS), the failure load of tibial fixation is lower than that of femoral fixation (6), often necessitating additional backup fixation such as pullout sutures around a screw post (7-9). Second, the orientation of the ACL graft aligns with the longitudinal axis of the tibial tunnel in knee extension, when graft tension peaks, resulting in greater load on tibial fixation (10,11). Finally, and most importantly, tibial fixation must be performed under controlled tension. Therefore, tibial fixation devices must not only ensure rigid fixation but also provide properly-controlled tension to be maintained after the graft fixation. However, while most of the previous biomechanical studies on tibial fixation have focused on the fixation strength, there are few reports on the tension controllability or the tensioning techniques to leave the desired tension to the graft after completing fixation.

Various tibial fixation techniques/devices have been employed, including pullout suturing over a button or around a screw post, with the Double Spike Plate (DSP; Meira Corporation, Nagoya, Japan) (Figure 1), a staple, or an IFS (5-22) (Table 1). In pullout suture fixation systems, fixation strength depends on several factors: the strength of the graft-suture connection, the suture material itself, and the suture-device interface (27,28). These pullout fixation techniques not only lead to less stiffness of the construct due to longer interfixation distance (29), but also to loss of graft tension due to further load relaxation of the construct after the graft fixation (23). However, only the DSP allows for maintaining the intended graft tension even after its fixation, as it is designed to complete fixation after load relaxation of the construct by in situ pre-tensioning (12).

Figure 1 DSP system. (A) The DSP is a small titanium plate featuring three holes and two spikes on its underside. (B) During graft fixation, the graft sutures are tied to the top hole of the DSP. A predetermined amount of tension is then applied distally and maintained for 3–5 minutes using a suture passed through the bottom hole. Once adequate pretension is applied, the spikes are hammered into the bone to secure initial fixation, and the procedure is completed by inserting a screw through the central hole (C). A postoperative radiograph demonstrates the proper placement of the DSP. This image is published with the patient’s consent. DSP, Double Spike Plate.

The IFS offers a shorter interfixation distance but exhibits inconsistent fixation strength in the proximal tibia, where bone mineral density is relatively low (5,6,10,11). Furthermore, IFS fixation is less reliable for soft-tissue grafts compared with BTB grafts (24). While graft tension after IFS fixation remains unclear (23), the IFS is the most commonly used device for BTB graft fixation in the tibia (30). Recently, the Bone Plug Tensioning and Fixation (BTF) system (Smith & Nephew Endoscopy, Inc., Andover, MA, USA) (Figure 2) has been introduced as an innovative solution to achieve screw fixation of a BTB graft to the tibia with the intended tension maintained even after the graft fixation. The BTF system consists of a spike button (SB) and a pullout screw (PS). The SB is hammered into the distal aperture of the tibial tunnel, then the PS is inserted through the central hole of the SB into the longitudinal hole of the bone plug located within the tunnel. As the PS advances through the longitudinal hole of the bone plug, it connects the bone plug to the SB on the tibial tunnel aperture and then it pulls the bone plug with increased tension, leading to simultaneous tensioning and fixation of the graft. Thus, the BTF system allows for a shorter interfixation distance, rigid screw fixation, and adjustable graft tension during/after its fixation (25). Consequently, the BTF system has the potential to overcome the limitations of existing tibial fixation devices.

Figure 2 BTF system. (A) The BTF system consists of two primary components: an SB and a PS. First, the SB is hammered into the distal aperture of the tibial tunnel. The PS is then inserted through the central hole of the SB into the bone plug positioned within the tibial tunnel. As the PS advances through the longitudinal hole of the bone plug, it connects the bone plug to the SB and pulls it distally. This distal traction both tensions and secures the graft simultaneously. (B) A postoperative radiograph shows the correct positioning of the BTF system. (C) A postoperative computed tomography scan confirms proper alignment of the SB and PS within the tibial tunnel. This image is published with the patient’s consent. BTF, Bone Plug Tensioning and Fixation; PS, pullout screw; SB, spike button.

At the time of using BTB graft, BTF system can be used only when the distal end of the bone plug remains in the tibial tunnel. If the distal end comes out of the tibial tunnel entrance due to the longer tendinous portion, the BTF system is not applicable. In this case, a pullout suture fixation with the DSP and a screw is applied.

This paper describes the mechanical properties of both the BTF system and DSP and the technique for tibial graft fixation under controlled tension using those devices.

Mechanical performance of BTF system and DSP

The tension controllability and fixation strength were compared between BTF system and IFS using porcine tibiae and BTB grafts. To assess tension controllability, the bone plug was fixed in the tibial tunnel using either the BTF system or the IFS with an initial tension of either 9.8 N or 19.6 N, and then the residual graft tension was recorded. A cyclic loading test between 10 and 250 N was subsequently performed for 1,000 cycles, followed by a tension-to-failure test. It was shown that the mean residual tension for the BTF group was significantly higher than that for the IFS group at the initial tension levels of 9.8 N or 19.6 N. In other words, the mean difference between the initial and residual tension for the BTF group was significantly smaller than with IFS fixation. Thus, the BTF system offers graft fixation at the tension closer to the predetermined value (Figure 3). Furthermore, displacement of the bone plug after the cyclic loading was significantly smaller with BTF than with IFS fixation. In terms of stiffness of the fixation, the BTF system demonstrated significantly greater stiffness than the IFS, while no significant difference was observed in maximum failure load between the two fixation methods (Figure 4). Overall, the BTF system allows graft fixation at a tension closer to the initially-applied tension and provides more rigid fixation than the IFS (25).

Figure 3 Graft tension over time during the fixation process. The initial tension is set at 20 N. a: screw insertion begins. b: the screw advances into the bone plug, gradually transferring tension to the graft. c: fixation is completed, after which the tensioning suture is cut, marking the final residual graft tension. BTF, Bone Plug Tensioning and Fixation; IFS, interference screw.

Figure 4 Examples of load-to-failure tests. (A) BTF system. The maximum failure load was 922.5 N, stiffness was 655.7 N/mm, and the mode of failure was bone plug breakage. (B) IFS fixation. The maximum failure load was 837.6 N, stiffness was 400.0 N/mm, and the mode of failure was bone plug pullout. BTF, Bone Plug Tensioning and Fixation; IFS, interference screw.

The tension controllability of pullout suture fixation with the DSP and a screw was evaluated using porcine tibiae and bovine tendons. The grafts were fixed using DSP and a screw with an initial tension of either 49 N or 98 N, and then the residual graft tension was recorded. The residual tension at 5 minutes later was 49±10 N for the 49 N group and 100±7 N for the 98 N group. Thus, DSP also allows graft fixation at a tension close to the initially-applied tension (12).

Proper initial tension

Graft tension during ACL reconstruction has a significant impact on postoperative outcomes (31-37). Yoshiya et al. (31) performed ACL reconstruction using patellar tendon grafts in a dog model, applying initial tensions of either 1 N or 39 N. Three months postoperatively, the grafts fixed at 39 N exhibited poor vascularity, focal myxoid degeneration, and irregular collagen fiber alignment compared with those fixed at 1 N. Similarly, Fleming et al. (32) studied the effect of initial graft tension on tibiofemoral compressive forces using cadaveric knees reconstructed with patellar tendon grafts. They found that tibiofemoral compressive force in full knee extension was highest at an initial tension of 50 N, followed by 25 N, and was lowest in ACL-intact knees, indicating that higher initial tension increases compressive force. Mae et al. (33) also demonstrated, in a cadaveric study with hamstring grafts, that increasing initial tension caused posterolateral displacement of the tibia with external and valgus rotation relative to the femur. Clinical studies have corroborated these findings (34-37). Taketomi et al. (34,36,37) examined the effects of varying initial graft tension on femorotibial and patellofemoral alignment following anatomic ACL reconstruction using a BTB graft. The graft was fixed either with maximum manual force (higher tension) or at 80 N (lower tension) at full knee extension. Computed tomography (CT) performed one week postoperatively revealed that higher initial tension caused excessive external rotation of the tibia relative to the femur (36), as well as lateral shift and tilt of the patella (37). Moreover, CT imaging at one year postoperatively demonstrated that greater initial tension led to increased tunnel widening at the femoral aperture (34). These findings indicate that excessive initial graft tension should be avoided, as it may result in abnormal kinematics and structural complications. Conversely, insufficient initial tension leads to graft laxity and functional insufficiency (38). To determine optimal graft tension, the concept of laxity match pretension (LMP)—the graft tension required to restore normal anterior laxity—has been proposed (33,38-44). LMP varies depending on the surgical technique and graft type. Previous studies reported that for anatomic rectangular tunnel ACL reconstruction using a BTB graft, the mean LMP values were 8.6 N at 15° of knee flexion (42) and 1.6 N at 20° of flexion (43). For double-bundle and triple-bundle ACL reconstructions with hamstring grafts, the values were 11.2 N and 6.8 N, respectively, at 30° of flexion (44). Considering load relaxation after fixation, our clinical practice currently applies initial graft tensions of 10–20 N at 20° of knee flexion for anatomic rectangular tunnel ACL reconstruction using a BTB graft (45,46). Similarly, for anatomic double- and triple-bundle ACL reconstructions with hamstring grafts, each graft is fixed with an initial tension of 5–10 N; total tension of 10–20 N (45,47).

In situ pre-tensioning and fixation under proper tension to be maintained

First of all, for graft fixation with residual tension controlled, meaning that the tension equivalent to the initially-applied tension remains in graft after fixation, in situ pre-tensioning to remove the creep of the construct should be completed, followed by fixation with the tension maintained. Regarding the pre-tensioning, the method of applying graft pretension prior to fixation significantly influences postoperative stability and reproducibility (48,49). When graft tension is applied manually without the use of a tension meter, studies have shown substantial inter- and intra-examiner variability in residual tension, resulting in poor reproducibility (50,51). Even when a tensioner is used to monitor the initial tension, the residual tension after fixation can differ depending on whether the tensioner is manually held or mounted on the tibia. Thompson et al. (26) reported that pre-tensioning with a manually held tensioner does not generate joint reaction forces because the graft is mechanically disconnected from the tibia during the procedure. Consequently, once the graft is fixed, both joint reaction load and substantial load relaxation occur, making it difficult to achieve the intended residual graft tension. Conversely, pre-tensioning using a tensioner mounted on the tibia produces a joint reaction load before fixation. This load is distributed as a compressive force on the articular surface and a posteriorly directed force on the tibia, which shifts the tibia posteriorly.

Therefore, pre-tensioning with a tibia-mounted tensioner facilitates greater load relaxation than that with a manually-held tensioner before graft fixation. To enable the former pre-tensioning, a tensioning boot secured to the calf using a bandage which is equipped with tensioners was developed (Figure 5). Mae et al. (52) investigated the effectiveness of this device in anatomic double-bundle ACL reconstruction using hamstring grafts in cadaveric knees. The grafts were fixed with the DSP system using three different tensioning techniques: (I) manual tensioning, (II) tensioning boot with flexion-extension motion, and (III) tensioning boot with repetitive manual strong pulls. Their results showed that the tensioning boot technique with repetitive pulls achieved residual tension that most closely approximated the intended initial tension applied to the graft (26).

Figure 5 In situ pre-tensioning. (A) A tensioning boot equipped with tensioners. (B) A tensioning boot is secured to the calf using a bandage. The suture emerging from the caudal side is connected to a tensioner mounted on the boot. While monitoring graft tension, the tensioning suture is repeatedly pulled to promote load relaxation in the graft construct. Once the measured tension stabilizes at the predetermined value, graft fixation is performed using either the DSP system or the BTF system. This image is published with the patient’s consent. BTF, Bone Plug Tensioning and Fixation; DSP, Double Spike Plate.

Based on these findings, our anatomic ACL reconstruction technique employs in situ pre-tensioning using the tensioning boot with repetitive strong manual pulls 10 times before graft fixation. The graft is fixed under controlled tension—10–20 N based on LMP—at 20° of knee flexion using either the DSP or the BTF system. Clinical outcomes from this method have demonstrated high reproducibility and excellent stability. Two years after anatomic triple-bundle ACL reconstruction using hamstring tendon autografts, the mean side-to-side difference in anterior laxity measured with a KT-1000 knee arthrometer under maximum manual force was 0.7±0.7 mm, with 100% of patients showing values between −1 and 2 mm (47). Similarly, 2 years after anatomic rectangular tunnel ACL reconstruction using BTB autografts, the mean difference was 0.2±0.9 mm, with 95.1% of patients falling within the same range (46). These findings confirm that tibial fixation under controlled tension using the DSP or BTF system achieves highly reproducible outcomes with minimal outliers.

There is one “graft tension-controllable” device available: SE Graft Tensioner (Linvatec Corp, Largo, FL, USA) to be installed on the tibia using two pins. The clinical study reported that pre-tensioning using this device does not improve clinical outcomes or knee stability after single bundle ACL reconstruction with hamstring tendon graft with IFS tibial fixation when compared with manual tensioning (53,54). Experimentally, Thompson et al. measured the residual tension after the IFS fixation for single bundle hamstring graft using either this tensioning device or manual tensioning in cadaveric knees. They reported there was no difference in residual tension between the two tensioning techniques (26). We assume this tensioning device should be utilized with the tension-controllable fixation devices, such as DSP or BTF.

There remains the limitation on the tension-controlled graft fixation described here. The biomechanical studies on the fixations with DSP or BTF using the tensioning boot technique focused on the residual graft tension for only 3–5 minutes after finalizing fixation. Thus, it remains unclear how much the initial tension is maintained afterward in daily life or rehabilitation. According to the previous study of the anatomic triple-bundle ACL reconstruction with hamstring tendon graft using the same graft fixation procedure as described in this paper, the tibiofemoral relationship 3 weeks after surgery remained over-constrained, as shown in residual posterior displacement of the tibia (35,55,56). Similarly, when the BTB graft is fixed with this fixation procedure, the residual graft tension of BTB graft could be assumed to be maintained for at least a few weeks in vivo. This suggests that 5-minute pre-tensioning is enough to almost completely remove creep in the construct and to maintain the residual graft tension for weeks.

Conclusions

To achieve tibial fixation under controlled tension, in situ pre-tensioning to remove the creep of the construct should be completed, followed by fixation with the tension maintained. At the time of pre-tensioning, graft tension should be monitored using a tensioner mounted on the tibia, such as with a tensioning boot device. In situ pre-tensioning with repetitive manual pulls should be performed to cause maximal load relaxation before fixation. The graft is then fixed at an adequate initial tension based on LMP: using pullout suture technique with the DSP for soft tissue grafts including hamstring tendon grafts, or the BTF system for BTB grafts.

Acknowledgments

None.

Footnote

Peer Review File: Available at https://aoj.amegroups.com/article/view/10.21037/aoj-2025-1-100/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://aoj.amegroups.com/article/view/10.21037/aoj-2025-1-100/coif). The authors have no conflicts of interest to declare.

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