Introduction

This chapter explains why the fixation properties of high stiffness and resisting slippage are the two critical factors in restoring anterior laxity to a knee reconstructed with an anterior cruciate ligament (ACL) graft. Definitions and examples of high- and low-stiffness fixation are provided, along with evidence that high-stiffness cortical fixation placed away from the joint line restores anterior laxity as well as intratunnel fixation with an interference screw, even though the graft is a few centimeters longer. Resisting slippage, the most important fixation property, goes hand-in-hand with stiffness. A fixation device that resists slippage also provides high stiffness; however, the reverse relationship does not hold true. A discussion is included of the extensive biological and mechanical advantages of the use of high-stiffness, slippage-resistant cortical fixation over intratunnel fixation, including a more rapid, stronger, and stiffer tendon–tunnel biological bond; a stronger, stiffer, and more slippage-resistant mechanical fixation; and less tunnel widening. A rationale for selecting a soft tissue fixation device is presented. The surgeon reading this chapter should find these basic engineering principles useful when designing his or her own ACL reconstruction technique.

Fixation Stiffness and Slippage: Critical Factors in Restoring Anterior Laxity

An ACL graft construct is composed of the ACL graft and the two fixation devices at either end (Fig. 29-1). Anterior laxity in the knee of the athlete returning to sport is determined by the stiffness of the ACL graft construct and the length of the ACL graft, not by the strength of the graft and the tension in the graft.^1–4

Fig. 29-1 An anterior cruciate ligament (ACL) graft construct is composed of the ACL graft and the fixation devices used on the femur and tibia. The anterior laxity is determined by the stiffness of the ACL graft construct and the ability of the fixation devices to prevent slippage of the graft during cyclical exercise and early exercise. Intratunnel devices such as the interference screw and IntraFix are stiff but slip readily under cyclical load. Examples of high-stiffness fixation devices that also resist slippage and are used in the femur include the EZLoc and Bone Mulch Screw with bone graft. Examples of high-stiffness fixation that resist slippage and are used at the distal end of the tibial tunnel include the WasherLoc, with or without bone dowel.

The stiffness of the ACL graft construct determines anterior laxity, the firmness of the Lachman test, and the quality of the “endpoint.” The stiffness of the ACL graft construct is increased more by the use of stiffer fixation devices than by the use of a shorter graft. A short graft with elastic fixation does not restore anterior laxity as well as a longer graft with stiffer fixation.^5,6 The concept that shortening the graft increases the stiffness of the ACL graft construct, as proposed by some studies,^7–9 is incorrect when high-stiffness, slippage-resistant cortical fixation devices are used.⁴ The most effective way to construct a stiff ACL graft and restore a firm Lachman test is to use high-stiffness, slippage-resistant cortical fixation devices on both ends of the graft.^1,3,5,6

A determinant of anterior laxity that is as equally determinant as the stiffness of the ACL graft construct is maintenance of the length of the ACL graft set at the time of fixation. Maintaining the initial set length of the ACL graft after implantation requires the use of fixation devices that resist slippage during cyclical loading and exercise. Fortunately, there are fixation devices that do resist slippage and provide high stiffness; however, they are applied at the distal end of the tunnels and engage cortical bone, which is 30 times stronger than cancellous bone.^5,10–12 The use of intratunnel devices such as the interference screw and IntraFix (DePuy Mitek, Norwood, MA) do not resist slippage, and the ACL graft readily elongates under cyclical load.^5,13–15 Therefore the use of high-stiffness, slippage-resistant cortical fixation that purchases cortical bone is the best strategy for maintaining the initial length of the ACL graft and compensating for the obligatory loss in tension from cyclical movement of the knee and exercise (Fig. 29-2).^2,3

Fig. 29-2 The loss of intraarticular graft tension (IAT) and increase in anterior laxity (maximal anterior translation) with low-stiffness double staple fixation is shown after a series of treatments designed to simulate exercise. All the tension in the graft was lost after cyclical loading of the knee 20 times to a maximum tensile load in the graft of 170N.

Definition and Examples of High- and Low-Stiffness Fixation

High-stiffness fixation can be arbitrarily defined as a fixation stiffness that is greater than 400 N/mm when tested in young human bone. The WasherLoc with bone dowel (stiffness of 565 N/mm), the WasherLoc alone (506 N/mm), and tandem screws and washers placed on the cortex distal to the tibial tunnel (414 N/mm) all provide stiffness greater than 400 N/mm in young human bone (see Fig. 29-1). On the femoral side, the EZLoc (infinite stiffness) and Bone Mulch Screw (575 N/mm) are high-stiffness fixations because they purchase cortical bone, which is 30 times stronger than cancellous bone. Each of these high-stiffness cortical fixation devices resists slippage well during cyclical loading.^{5,6,11,16–20}

In contrast, low-stiffness fixation provides less than 400 N/mm in young human bone (Fig. 29-3). The interference screw (340 N/mm), double staples placed in the cortex distal to the tibial tunnel (174 N/mm), sutures tied to a post (70 N/mm), closed loop Endobutton (79 N/mm), IntraFix (49 N/mm), and sutures tied to an Endobutton (25 N/mm) are all examples of fixation devices that provide low stiffness. None of these low-stiffness intratunnel and cortical fixation devices resists slippage well during cyclical loading.*

Fig. 29-3 Examples of low-stiffness cortical fixation that poorly resist slippage include the double staples (belt buckle) and sutures tied to a post. These fixation devices, along with the intratunnel devices such as the interference screw and IntraFix, slip readily under cyclical loading, and their use with aggressive rehabilitation should be approached cautiously.

Comment About Intratunnel Fixation with an Interference Screw

The location of fixation of an ACL graft with respect to the joint line has been a topic of debate.^17,22–24 Proponents of fixation of an ACL graft at the level of the joint line with an interference screw base their opinion on two studies.^7,9 The seminal study showed that anterior laxity was better restored with intratunnel fixation with an interference screw than with double staples applied distal to the tibial tunnel in porcine knees.⁷ A subsequent study showed that anterior laxity was better restored with intratunnel fixation with an interference screw than with sutures tied to a post in human knees.⁹ The explanation offered for the better restoration of anterior laxity with intratunnel fixation with an interference screw versus cortical fixation with double staples and sutures tied to a post was that intratunnel fixation shortens the effective length of the graft, which increases the stiffness of the knee.^7–9

More recent studies have questioned the use of double staples, sutures tied to a post, and porcine knees to study the effect of the level of fixation on anterior laxity and knee stiffness. Double staples (174 N/mm) and sutures tied to a post (70 N/mm) are low-stiffness fixation devices (i.e., <200 N/mm), whereas distal tibial fixation devices such as tandem screws and washers (414 N/mm), WasherLoc (506 N/mm), and WasherLoc and bone dowel (565 N/mm) are high-stiffness fixation devices (i.e., >400 N/mm).^5,17