Introduction

Graft selection, graft position, fixation, and postoperative rehabilitation are clearly implicated in the success and failure of anterior cruciate ligament (ACL) reconstruction. Tensioning of the ACL is another important factor in providing immediate and long-term stability in the reconstructed patient. Jones¹ stated that tension applied to the ACL graft at the time of surgery should be enough to eliminate an anterior drawer sign but still allow a full range of motion. Excessive initial tension can lead to graft failure, fixation failure, loss of knee motion, excessively reduced anterior laxity, and cartilage degeneration.^2–6 Lewis et al⁶ introduced the term overcorrected to describe the phenomenon in which the tibia was positioned posterior and externally rotated relative to the femur. Andersen and Jorgensen⁷ used a prosthetic ligament to study the consequence of overcorrected ACL reconstructions. These kinematic alterations result in increased graft forces at all flexion angles, increased forces in the posterior cruciate ligament (PCL), and alteration of the normal roll–glide mechanism during knee motion. Melby et al² found that graft tension–related posterior tibial subluxation resulted in an increase in quadriceps force needed to achieve full knee extension and may lead to an extensor lag if quadriceps atrophy is present.

As more sophisticated models of ACL tensioning have developed, efforts have focused on the effect of tension on the graft itself. The terminology can be confusing; terms such as preloading, pretensioning, preconditioning, and initial tension have been used. For this text, pretensioning refers to any loading of the graft that is performed before the graft is pulled into the femoral and tibial bone tunnels. Preconditioning refers to the loading of the graft that is performed once the graft has been fixed within one of the tunnels (usually the femoral tunnel). The term initial tension refers to the tension in the graft after fixation within both tunnels.

Initial tension affects the biology of the graft and is a time-dependent interplay of host incorporation and biomechanical forces. A number of factors affect graft tension, including initial tension (pretensioning and preconditioning), ligament length, graft isometry, tissue type, and fixation type (Fig. 54-1). For the purposes of this chapter, we will review the consequences of overtensioning the ACL graft both histologically and functionally, explore the factors in low-tension (bone–tendon–bone [BTB]) and high-tension (soft tissue) grafts, identify the ideal knee fixation angle, and provide suggestions for identifying tensioning problems and possible solutions. Historically, much debate has existed in the literature regarding these issues, with relatively few clinical studies focusing on graft tension; however, some consensus has been identified in recent years to lend clarity to this confusing topic.

Fig. 54-1 Factors affecting ultimate graft tension.

Native Anterior Cruciate Ligament Tension

The goal of ACL reconstruction is to restore normal knee kinematics. Success requires a basic understanding of the forces affecting the normal ACL. Markolf et al⁸ and Wascher et al⁹ studied a series of loading experiments on cadaver specimens by isolating the bone plug that contained the ligament’s tibial insertion and attaching a load transducer. Passive extension of the knee generated forces in the ligament only during the last 10 degrees of extension, reaching 50 to 240N with hyperextension. Internal tibial torque generated greater forces in the ligament than did external tibial torque and increased as the knee was extended. The greatest forces (133–370N) were generated when 10 N/m of internal tibial torque was applied in hyperextension. A varus moment of 15 N/m generated forces of 94 to 177N at full extension, and a similar valgus moment generated a mean force of 56N independent of knee flexion.

Active knee motion near extension substantially increases forces across the ACL. Paulos et al¹⁰ reported that lengthening of the ACL was observed during active extension from 40 degrees of knee flexion. Active quadriceps contraction imparts significant strain on the ACL. Arms et al¹¹ showed that over the first 45 degrees of flexion, the quadriceps increased strain, whereas it decreased strain at flexion angles greater than 60 degrees. Beynnon et al¹² studied the forces on the anteromedial bundle of various exercises. Isometric quadriceps contraction at 15 degrees of knee flexion elicited the highest peak strain (4.4%), followed by open chain flexion-extension with a 45N weight (3.8%) and a Lachman test (3.7%). Isometric quadriceps contraction at 30 degrees contributed significantly less strain (2.7%) to the ACL.

More recent investigation has focused on the importance of both the anteromedial and posterolateral bundles in ACL function. Sakane et al¹³ examined in situ forces in nine human ACLs in response to applied anterior tibial loads at knee flexion angles of 0 to 90 degrees. Their results showed the magnitude of ACL force to be maximal with anterior tibial loads applied at 15 degrees of knee flexion, which differs slightly from Markolf’s earlier conclusions. The magnitude of force in the posterolateral bundle was larger than that in the anteromedial bundle at knee flexion angles between 0 and 45 degrees, reaching a maximum of 75N at 15 degrees of knee flexion under an anterior tibial load of 110N. The magnitude of force on the anteromedial bundle, in contrast, remained constant independently of flexion angle. They concluded that the magnitude of force in the posterolateral bundle was significantly affected by knee flexion angle and anterior tibial load and was the major restraint to anteroposterior (AP) instability in early flexion.

No graft currently in use matches the complex anatomy of the normal ACL. Woo et al¹⁴ compared anteromedial ACL reconstructions using hamstring and patellar tendon reconstructions versus the native ACL and found significant laxity to a combined rotational load involving internal rotation and valgus tibial torque at 15 and 30 degrees of flexion. This highlights the importance of the posterolateral bundle to knee stability in early knee flexion and has led some to question the anteromedial reconstructions and revisit the idea of anatomical double-bundle reconstructions.

Basic Science and Graft Histology

Evidence from animal experiments lends credence to a “window” of acceptable ACL graft tension. Graft tension that is too high can eventually lead to greater laxity and poorer results than knees fixed under low graft tension. Conversely, other animal studies demonstrate that deliberately de-tensioned grafts lose strength to a greater extent than the normally tensioned ACL. Although a stimulus is essential for the orientation of newly formed collagen during the remodeling phase,¹⁵ basic research studying the effect of graft tension cautions that high initial tension may be detrimental to the remodeling process. Yoshiya et al¹⁶ found that patellar tendon reconstructions in dogs exposed to high initial graft tension of 39N showed focal degeneration within the graft and replacement of the normal parallel arrangement of collagen fibrils by a myxoid, extracellular matrix. Microangiography demonstrated improved vascularity when the initial tension was 1N rather than 39N. Laxity measurements of the two different preloads showed increased stability of the highly tensioned graft at time zero. However, at 3 months, laxity between the two groups was similar.

The use of allograft tissue is gaining popularity in ligament reconstruction. The substantial decrease in graft strength during initial phases of healing in frozen allograft tissue relates to the inflammatory stages associated with revascularization, not the effects of freezing itself.¹⁷ Jackson et al¹⁸ studied the biomechanical outcomes of devitalized ACL at 0, 6, and 26 weeks after treatment with a freeze probe. At 6 weeks a significant reduction in maximum load to failure was observed. However, at 26 weeks, no differences were noted between frozen and contralateral controls relative to laxity, load to failure, stiffness, or modulus of elasticity.

Katsuragi et al¹⁹ and Mikami et al²⁰ studied nonphysiologically high initial tension after freezing ACL ligaments in dogs. After applying a freeze-thaw treatment to both ACLs, they applied initial tension of 20N to the test group and compared it with the physiologically tensioned contralateral extremity. The tensile strength in the highly tensioned knee decreased significantly at 6 and 12 weeks. Histologically, the collagen fibers in the highly tensioned knees were coarser and disoriented. In the midsubstance of the ACL, the physiological specimens had a spindle-shaped nucleus; however, the highly tensioned ACL had signs of degenerative changes (Fig. 54-2). Overtensioning causes significant degenerative changes in native, autologous BTB, and freeze-thaw–treated ACL. Over time, the mechanical properties of the overtensioned ACL graft deteriorate compared with physiological tension.

Fig. 54-2 A, Physiologically tensioned anterior cruciate ligament (ACL) at 6 weeks (magnification × 20). B, Highly tensioned ACL at 6 weeks (×20). C, Core sample from a physiologically tensioned ACL at 12 weeks (×100). D, Core sample from a highly tensioned ACL at 12 weeks (×100). E, Midsubstance sample from a physiologically tensioned ACL at 12 weeks (×100). F, Midsubstance sample from a highly tensioned ACL at 12 weeks (×100).

From Katsuragi R, Yasuda K, Tsujino J, et al. The effect of nonphysiologically high initial tension on the mechanical properties of in situ frozen anterior cruciate ligament in a canine model. Am J Sports Med 2000;28:47–56.

Graft-Specific Tensioning

Stiffness and elasticity vary among autograft tissues. Burks and Leland⁴ determined that the graft tension needed to restore normal anterior laxity is tissue specific. The material (stiffness) and geometrical (size and length) properties of the graft influence the amount of tension that needs to be applied. In cadaveric knees, they reconstructed the ACL using bone–patellar tendon–bone (BPTB), doubled semitendinosus, or iliotibial band grafts. They tensioned each graft to match translation of an applied 20-pound load (89N) using the KT-1000. The BPTB graft returned the knee to its preoperative condition with a mean of 3.6 pounds (16.2N); doubled semitendinosus graft, 8.5 pounds (38.3N), and iliotibial band graft, 13.6 pounds (61.2N). Due to the characteristics of BTB graft superstructure and fixation, they have been generalized as low-tension grafts, whereas the mechanical properties of soft tissue grafts have necessitated high tension for graft fixation. We will review some of the clinical evidence supporting this.

High-Tension Grafts

Multiple outcome studies have indicated that hamstring tendon autografts are associated with higher postoperative laxity measurements than patellar tendon autografts, with clinical instability rates ranging from 10% to 30%.^26,27 Instability is defined as a 3-mm side-to-side difference in anterior tibial translation compared with the contralateral intact knee during manual maximum translation. Clinical instability rates ranging from 10% to 30% have been reported following reconstruction with hamstring tendon autografts. Soft tissue grafts present unique challenges. Namely, the length of the tendons are longer, more friction exists within the tunnels, and bone–bone healing is lacking. Considering the properties of soft tissue grafts, most surgeons reconstruct using higher tensions compared with BTB grafts.²⁵ Burks and Leland⁴ suggested tension two to three times that required in BTB reconstructions to restore normal laxity patterns. Boylan et al²⁸ applied three different loads in a cadaveric model: 68N, 45N, and 23N at 30 degrees of flexion. KT-1000 measurements of anterior translation demonstrated that the 68N load restored stability, whereas the knees loaded with 45N and 23N had significantly more laxity.

The clinical evidence offers some clarity to the issue. In a prospective randomized study, Yasuda et al²⁹ followed the clinical results at 2 years in patients who received doubled semitendinosus/gracilis (2xST/Gr) grafts tensioned at 80N, 40N, or 20N. The average side-to-side difference in anterior laxity in the 80N group was significantly less than that in the 20N or 40N group. There were no complications, all patients regained maximum extension, and no difference in subjective outcomes were noted. Kim et al³⁰ randomly allocated 48 patients to three groups in which three different tensions—8 kg (78N), 12 kg (117N), or 15 kg (147N)—were applied to a penta-semitendinosus graft. Postoperatively, the average side-to-side differences in anterior laxity were 1.3, 2.1, and 2.4 mm, respectively, at a minimum of 1 year. There were no significant differences among the groups in subjective clinical results, anterior laxity, or knee extensor strength.

As outlined earlier, overcorrecting the joint can lead to deleterious effects on kinematics of the graft and loss of motion. Furthermore, the increase in cartilage compression that occurs with overconstraint and loss of knee motion may lead to a greater degree of degeneration over time. Vachtsevanos et al³¹ looked at the relationship of soft tissue tensioning and return of postoperative range of motion. They conducted a prospective randomized study using 61 patients reconstructed with autogenous 2xST/Gr. Patients were divided into two groups (15 pounds [68N] and 20 pounds [88N]) and followed in the postoperative period for an average of 15 months. The average side-to-side difference in anterior laxity was 1.7 ± 1.5 mm (68N) and 2.8 ± 2.0 mm (88N), which was significant (Fig. 54-3). Additionally, they found a significant delay in regaining extension in the 88N group (Fig. 54-4). Based on the evidence presented, it is reasonable to assume that an initial tension of 70N in soft tissue grafts provides consistent return of motion and clinical stability and that substantially greater tension does not improve results.

Fig. 54-3 Side-to-side difference in 15 and 20 pounds of initial tension measured in millimeters with KT-1000 at 20-pound anterior drawer at an average of 15 months’ follow-up. Average side-to-side difference in the 15-pound group was 1.7 ± 1.5 mm; in the 20-pound group, 2.8 ± 2.0 mm (P = 0.047).

Fig. 54-4 Time to achieve 90 and 120 degrees of flexion for 15- and 20-pound groups (P = 0.018).

Stress Relaxation, Preconditioning, and Pretensioning

Due to the viscoelastic characteristic of graft tissue, the initial tension of ACL grafts is finite in nature. Beynnon et al³² reported that initial tension applied to the BTB graft at the time of fixation immediately decreased by creep elongation. Additionally, the percentage of stress relaxation is independent of the peak load applied to the graft.³³ The data are similar for soft tissue grafts. Höher et al³⁴ demonstrated a loss of 45% of graft tension in quadrupled hamstring grafts within 30 minutes due to stress relaxation. In vitro studies have been performed to determine the influence of cyclical loading on graft tension following ACL reconstruction. For studies performed with hamstring tendon grafts, bovine extensor tendon grafts, and patellar tendon grafts, cyclical loading reduced graft tension by 50% or more.^28,35,36 Numazaki et al³⁷ in a porcine ex vivo study fixed patellar tendon grafts with initial tensions of 20N, 80N, and 140N. A cyclical force–relaxation test was performed for 5000 cycles until the graft was stretched by 2 mm. The average peak load values after cycling were 105N, 157N, and 205N, respectively. Considering the in situ force of the ACL at full extension (16–87N),⁸ the authors concluded there was no benefit in tensioning the patellar tendon graft greater than 20N. For soft tissue grafts under the same initial loads, the average peak load values after cycling were 27N, 41N, and 39N. Increasing initial tension from 20 to 80N presented a significant increase in peak load after cycling; however, tensioning to 140N did not confer any benefit.

Attempts have been made to minimize graft viscoelasticity after reconstruction; pretensioning and preconditioning of the soft tissue grafts before final fixation have been recommended. To review, pretensioning refers to any loading of the graft that is performed before the graft is pulled into the knee (graft preparation board) (Fig. 54-5). Preconditioning refers to loading of the graft that is performed once the graft has been fixed in one of the tunnels (usually the femoral tunnel). Preconditioning may be further divided into cyclical and isometric preconditioning. The former denotes a load applied in multiple consecutive bouts (cycles), and the latter denotes a constant load applied on the graft before the final fixation of the graft.

Fig. 54-5 Example of pretensioning doubled semitendinosus/gracilis graft using Acufex Graftmaster board.

Despite the common use of pretensioning in graft preparation, there is little clinical evidence to support its effectiveness. Howard et al³⁸ noted a 10% increase in length of patellar tendon grafts after applying an 89N load for 15 minutes. They concluded that without pretensioning, significant postimplantation graft creep will occur. Several pretensioning techniques have been developed for soft tissue grafts. Tension of 15 to 20 pounds (44–88N) for 10 to 20 minutes has been suggested for pretensioning hamstring tendon grafts. The initially set tension gradually decreases over time as the graft is stretched and the sutures are tightened around the tendon bundle. Accordingly, it would be misleading to claim that the graft was tensioned with a constant load during the entire time. However, it is reasonable to assume that pretensioning does eliminate some creep within the suture–graft interface and allow the surgeon to provide a more consistent initial tension, and it certainly improves the ease of graft preparation. There is no obvious utility in pretensioning BTB grafts.

More sophisticated studies looking at the effectiveness of preconditioning over time are inconclusive. Nurmi et al³⁹ tested the effectiveness of preconditioning by assigning anterior tibialis grafts to three groups: control, cyclical preconditioned, and isometric preconditioned. The residual graft tension was then recorded immediately after the application of an initial graft tension of 80N and fixation into tibia with an interference screw. Immediately after screw insertion, the residual (AT) graft tensions were 79N ± 19N, 100N ± 17N, and 102N ± 15N, respectively. However, after 1 hour, a 60% decrease occurred in the initially set tension. Ejerhed et al⁴⁰ prospectively randomized 53 patients undergoing patellar tendon autograft into two groups. One group of patients underwent isometric preconditioning by passive stretching at a constant load of 39N for 10 minutes immediately prior to implantation. The other group underwent no preconditioning before the implantation of the graft. At an average of 25 months, no significant objective or subjective differences were noted. To date, there is no clinical evidence proving that pretensioning or preconditioning of grafts will overcome the viscoelastic nature of graft tissue and improve clinical outcomes. However, cyclical preconditioning does allow the surgeon to identify the force–flexion curve of a specific graft before final fixation.

Knee Fixation Angle

When considering the ideal knee flexion angle at fixation, certain assumptions must be made. Namely, the femoral and tibial tunnels are placed near the native ACL origin and insertion. Excessively anterior placement of the femoral tunnel causes significant increases in ACL tension in flexion and laxity near extension. We know from kinematic studies that the forces generated in the graft as well as in the intact ACL are greatest at full extension and minimal between 30 and 60 degrees of knee flexion.⁴¹ Cadaveric studies by Bylski-Austrow et al⁵ concluded that a greater increase in force (50–115N) and greater posterior shifts in tibial position were produced by changing the flexion angle at tensioning from 0 to 30 degrees, rather than doubling the initial tension from 20 to 40N (15–35N). Gertel et al⁴² measured ACL graft force using BTB in cadaveric knees and concluded that graft forces are greater when the initial tension was applied at 30 degrees of flexion rather than at 0 degrees. This has led most surgeons to fix their grafts between 10 and 30 degrees of flexion.²⁵

Asahina et al⁴³ conducted one of the few clinical studies examining the importance of knee flexion angle at graft fixation. Using the modified Macintosh technique, 19 patients were fixed with the knees at full extension, whereas 25 patients were fixed at 30 degrees of flexion. A 70N force measured with a spring balance was used for both groups. At an average of 38 months, the range of motion in the extension group was significantly better. However, the stability of the knees and arthroscopic appearance of the grafts were significantly worse. This led the authors to suggest that graft fixation at 30 degrees of flexion is more effective in restoring stability and sustaining graft viability.

Ultimately, there is no set number for the ideal knee flexion angle at graft fixation that can be effective in every reconstruction. Clearly, fixation in the arc of 10 to 30 degrees imparts a significant force to the ACL graft, and fixation at greater than 60 degrees can create difficulty in regaining maximal extension postoperatively. The surgeon must look at the behavior of the graft during cyclical preconditioning to determine the ideal knee flexion angle for fixation, which will be discussed in the following section.

Tensioning Devices and Strategies

As mentioned previously, every graft produces its own individual force–flexion curve based primarily on the position of tunnel placement. After the graft is fixed in the femoral tunnel, the knee should be taken through a range of motion with tension on the tibial end of the graft (cyclical preconditioning). Based on graft length changes, which are assessed by motion of the graft within the tunnel, the point where the graft will be slack and where graft tension increases should be identified. Ideally, an isometric placed graft will show no significant motion and could be tensioned at any point in the flexion arc.⁴⁴ Clinically, 2 to 3 mm of elongation is acceptable as long as the pattern of deviation throughout the range of motion reproduces that of the native ACL. Namely, in well-placed tunnels, this inflexion point occurs near extension as the graft begins to shorten in the tunnel.

Cunningham et al⁴⁵ studied the results of 13 orthopaedic sports medicine physicians who were asked to tension a soft tissue graft as they would in surgery and then maximally. They found that graft tensioning was highly variable and questioned whether tension should be more accurately measured and controlled intraoperatively. Commercially available ACL tensioning devices can be applied to the tibial end of the graft (Figs. 54-6 and 54-7). These allow the surgeon to dial in the set amount of tension, eliminating the need for an assistant. To date, no clinical studies have identified more consistent or improved outcomes using these devices.

Fig. 54-6 Example of a preconditioning device (prototype).

Fig. 54-7 Example of another preconditioning device: the Tie Tensioner (DePuy-Mitek, Johnson & Johnson Gateway).

Although an obvious controversy exists concerning the ideal pretensioning, preconditioning, and initial tensioning of ACL grafts, consensus does prevail. In low-tension grafts (BTB), pretensioning imparts little benefit and the graft should be fixed in the range of 20N. For high-tension grafts (semitendinosus), pretensioning may improve the immediate viscoelastic behavior of the graft and the graft should be fixed at no greater than 80N. Preconditioning of both graft types is advised to characterize the inflexion point of the individual graft before final fixation. With well-placed tunnels, this point should be near full extension (10–30 degrees), at which time the desired initial tension can be set.