Introduction
The anatomy of the ACL is complex, with a multitude of small fascicular bundles twisting around each other during knee flexion [
15], commonly simplified into two functional bundles: anteromedial (AM) and posterolateral (PL) based on their tibial attachment sites [
13]. These contribute to knee stability in a specific pattern, depending on the knee flexion angle [
1,
11,
39]. This pattern reflects the structure of the native ACL, functioning as groups of fibres which lengthen and slacken across the range of motion [
16,
39].
The optimal method of ACL reconstruction is yet to be determined. Biomechanical [
4,
25,
33,
34] and clinical [
19,
21,
26,
42,
52] papers have demonstrated superior restoration of native knee kinematics or improved function after double-bundle (DB) surgery than single-bundle (SB) surgery; however, equivocal results have also been reported [
14,
23,
24,
30,
32,
47,
48]. Double-bundle surgery is more complex, imposing greater technical demand, longer operative time, the possibility of tunnel convergence and notch impingement, and greater expense [
3,
7,
8]. A DB reconstruction is defined in this paper as having two grafts, each with separate bone sockets at each end, forming a four-socket construct.
The femoral attachment site has the most effect on ACL graft isometry and tension with knee flexion [
16,
27,
41], so a simplified graft construct which covers most of the femoral ACL attachment is attractive. Therefore, the purpose of this study was to examine the relative merits of a three-socket (3S) construct, with a V-configuration graft located into a single tibial tunnel and double femoral ACL graft tunnels [
49]. Unlike previous work, the present study evaluated ACL reconstructions using cortical suspensory fixation, enabling different tensioning angles for each graft bundle. It was hypothesised that ACL reconstruction using three- and four-socket techniques would more closely restore native knee kinematics compared to anatomic single-bundle two-socket (SB) surgery, whilst 3S surgery would restore native knee kinematics to a similar extent as four-socket DB surgery. Although there have been many previous studies of ACL graft constructs, there has not been a study which has compared the relative biomechanical ability of each of the three constructs to restore native knee laxity.
Discussion
The most important finding of this study is that, contrary to the hypothesis, a bifurcating graft with three bone sockets (3S) did not restore native knee laxities significantly more closely to normal than a single-bundle (SB) ACL reconstruction. Laxities after 3S and DB surgery were comparable, with the DB reconstruction providing significantly less laxity than SB surgery under the coupled moments of the simulated pivot shift. Tensioning the ACL grafts to match a quantified Lachman test of the intact knee also offered restoration of intact laxity of IR and the SPS for all reconstructions.
This is the only biomechanical study comparing the kinematics of anatomic single- and multi-bundle ACL reconstructions with total suspensory fixation. This adjustable fixation allowed a laxity-matching graft tensioning protocol to be used, akin to minimising the side-to-side difference clinically; it allowed the knees to be restored close to intact behaviour with all three graft configurations. Previous biomechanical studies have applied a range of pre-determined graft tensions with varied results, often reporting significantly more laxity than the native knee [
2,
17,
51], especially during IR and the SPS. This emphasises the value of quantifying the laxity of the ‘intact’ contralateral knee per-operatively, and subsequent graft tensioning, towards optimising the control of knee rotation at time zero.
Some studies have previously evaluated three-socket grafts of different graft type, configuration or fixation to that of the present study [
23,
35,
47,
48]. Yagi et al. [
47] reported that three-tunnel surgery more closely restored intact anterior laxity than an SB reconstruction at 0° and 30° and in situ graft forces from 0° to 60°; both reconstructions failed to restore native anterior restraint from 0° to 60° of flexion. Conversely, Yamamoto et al. [
48] found no significant difference in anterior laxity between three-tunnel and laterally placed SB except at higher angles of flexion where SB was superior. Petersen et al. [
35] compared a similar three-tunnel construct to DB surgery, reporting significantly less anterior laxity at 0° and 30° for the latter. In contrast, the present study found no significant difference between 3S and DB surgery at any angle of flexion; only the SB at 60° had significantly greater laxity. Kim et al. [
23] compared the kinematics of four anatomic quadriceps ACL grafts: three-bundle SB with two femoral and one tibial sockets, three-bundle SB with two tibial and one femoral sockets, and DB reconstruction. All multi-bundle reconstructions were similar to the intact with significantly less anterior laxity than SB at 60° and 90°, whilst no difference was seen between the SB and deficient states at these flexion angles. In the present study, the SB was tensioned at 30° rather than in extension; this may have contributed to an improved performance at greater angles of knee flexion.
A number of studies have reported coupled tibial displacements in response to SPS loading. Kim et al. [
23] found no differences in ATT between any knee state under simulated pivot shift, but found significantly lower forces in the SB construct than the intact ACL from 0° to 30° and significantly higher forces in DB and one-femur/two-tibia tunnel constructs at 15° and 30°, respectively. In contrast, this study found significant increases of coupled motion between the intact and ACL-deficient states at extension and early flexion, similar to previous work [
14,
48]. Yagi et al. [
47] found significantly less coupled ATT during the SPS after two-femur/two-tibia reconstruction than SB surgery, but it remained significantly more than the intact state. Petersen et al. [
35] reported a similar superiority of DB over a two-femur/one-tibia graft, with significantly less ATT at 0° and 30° whilst also remaining significantly more lax than the intact state. Similarly to Yamamoto et al. [
48], this study found that anatomic SB and a two-femur/one-tibia graft (3S) were able to restore ATT to values similar to the intact state. DB surgery had a tendency to over-constrain the joint in extension, which has been reported previously [
25].
The present study further examined the IR component of the simulated pivot shift, and all reconstructions were comparable to the intact state. In contrast to Kondo et al. [
25], significant increases in IR were seen between the intact and ACL-deficient states at all flexion angles. This remained true for SB and 3S, whilst DB surgery maintained significantly less laxity than the deficient state at 0° and 15°, as well as the SB state from 0° to 30°, near the angle at which the pivot shift occurs [
12]. These findings were consistent with previous work suggesting that SB and two-femur/one-tibia 3S grafts produced sufficient control at 15°, but became less efficacious with increased knee flexion [
48].
There are several limitations of the present study. Firstly, the specimens were 66 ± 8 years old, higher than the patient group who experience ACL rupture, but comparable to similar cadaveric studies [
24,
43]. The clinical pivot shift is a dynamic examination through a range of motion. We were unable to replicate this using a single robotic manipulator, and the combined moments were imposed at a static flexion angle, therefore this and other studies [
14,
23,
47] have not mimicked the in vivo kinematics but only the coupled laxities. These results represent time-zero data and do not account for ACL graft and other soft tissue changes during rehabilitation. This may be particularly relevant in the context of the initially over-constrained knee that may settle to a clinically effective reconstruction. Finally, there was no muscle loading, so these results reflect passive restraints, as in clinical laxity testing. The advantages of the study design, however, included the ability to perform three ACL reconstructions in each knee, thus allowing the within-specimen repeated-measures statistical analysis to discern their relative performances without inter-specimen variability.
The clinical relevance of this work relates to the choice of ACL reconstruction, and how that choice is affected by differences in the resulting laxity of the knee, among the SB, 3S, and DB constructs in this study. This study suggests equivalence for SB, 3S, and DB ACL reconstructions in their control of tibial anterior translation and rotation. This supports the use of a correctly positioned SB graft; while there was a consistent trend for reduced laxity with 3S over SB, it was not significant during any single comparison. Yasuda et al. [
50] reported significantly better control of ATT and the pivot shift 2 years after DB compared to SB surgery. In keeping with the present study, the two-femur/one-tibia graft produced comparable laxity to the DB but not significantly less than the SB, despite a reported increase in stability with complete attachment site restoration [
40]. The three-socket 3S surgery is less complicated than DB surgery yet produced comparable results; it preserves bone stock and spares gracilis, which is beneficial in the context of multi-ligament injury. This study suggests that it could be considered as an alternative to DB surgery. The literature reports a wide variety of graft tensions during ACL reconstruction [
2,
17,
51], and although tensioning devices are available, no definitive clinical benefit has been reported [
38]. This study suggests that quantifying the Lachman test of the uninjured knee and using this as a tensioning guide closely restores native knee laxity.