Biomechanical Boundary Conditions

Ideally, a graft fixation construct should be similar in strength and stiffness to the native human anterior cruciate ligament (ACL). Current fixation techniques demonstrate a wide range of in vitro measured graft fixation loads (200N to 1200N). However, today evidence is still limited regarding real in vivo requirements for initial construct strength.⁸ Noyes et al calculated that the ACL is loaded up to 454N during activities of daily living,⁹ but other authors reported a high amount of good and excellent results with fixation types whose failure load is far below that anticipated value.^10–12 For soft tissue graft fixation using biodegradable interference screws, failure loads of 250N to 800N have been demonstrated depending on screw length and insertion torque.^13–15

Graft fixation generally is divided into anatomical (aperture or joint line fixation), nonanatomical, and semi-anatomical (Fig. 39-1), according to the location of fixation in relation to the joint line. Graft fixation directly at the joint line (site of the native ACL insertion) is called anatomical, whereas an extracortical fixation (e.g., staples, fixation buttons) is called nonanatomical. For example, transfixation devices or tibial interference screws that are not deeply inserted provide an intraosseous and thus a semi-anatomical mounting of the graft.

Fig. 39-1 A, Nonanatomical and indirect femoral and tibial fixation; B, anatomical and direct femoral and semianatomical and direct tibial fixation; C, anatomical and direct femoral and tibial fixation.

This classification is important because the site of graft fixation determines the length of the complete graft fixation construct. The length of the native ACL is between 2.8 and 3.7 cm. Reconstruction of the anteromedial bundle of the ACL results in an intraarticular graft length of only approximately 2.5 to 3.2 cm. Using femoral and tibial nonanatomical fixation devices, the length of the graft fixation construct can easily reach 10 to 15 cm (see Fig. 39-1). In correlation to the distance between the fixation devices, reversible elastic longitudinal deformations of the graft have been demonstrated. This phenomenon also is known as the “bungee-cord effect.” Furthermore, sagittal intratunnel graft motion (reversible) might occur due to anteroposterior translation of the graft during knee flexion and extension using a nonanatomical or semianatomical graft fixation technique (“windshield-wiper effect”).^15,16 Thus a long distance between the femoral and tibial fixation device results in low construct stiffness and graft–tunnel motion.

Additionally, indirect and direct graft fixation should be distinguished (see Fig. 39-1). The concept of indirect graft fixation implicates the use of linkage material between graft and fixation device (e.g., suture loop of Endobutton fixation), whereas direct fixation means anchoring of the graft without any additional material except the fixation device itself (e.g., interference screws, staples, transfixation devices). Longitudinal irreversible graft deformation might occur using indirect fixation techniques due to stretch-out of linkage material (e.g., suture loop of Endobutton fixation) or suture attachment at the graft tissue.^13,17,18

Longitudinal and sagittal graft–tunnel motion inhibits constant graft–tunnel contact at the tunnel entry site and thus compromises bony graft incorporation at the site of the native ACL insertion.^5,19 Furthermore, graft–tunnel motion might lead to graft laceration at the tunnel entry site during dynamic loading²⁰ and is a factor responsible for the development of tunnel enlargement.¹⁶ Although current literature is not consistent concerning the correlation between tunnel enlargement and postoperative knee stability^16,21,22 in revision ACL reconstruction, tunnel enlargement is of clinical importance and should therefore be prevented.

According to these considerations, interference screw fixation offers high construct stiffness and prevents intratunnel graft motion because it allows for direct and anatomical graft fixation at the level of the joint line combined with adequate initial fixation strength.^5,23 Recent clinical data support the belief that clinical outcome can be improved with anatomical joint line fixation using interference screws for hamstring tendon grafts.²⁴

Biological Boundary Conditions

In addition to correct tunnel placement, graft incorporation represents the main factor for long-term survival of an ACL reconstruction and is mainly influenced by the type of fixation. The surgical target is to create biological (and biomechanical) boundary conditions that allow for the restitution of a native ACL insertion site anatomy.⁵ The normal ACL insertion site consists of four zones (so-called direct ligament insertion). The first zone comprises the ligament, the second is characterized by fibrocartilage, the third zone consists of a mineralized cartilage tidemark, and the fourth is where the mineralized cartilage tidemark inserts into the subchondral bone plate.⁵ The design of this complex insertion anatomy allows distribution of longitudinal and shear forces from the ligament into the subchondral bone plate. The development of a direct ligament insertion at the level of the joint line has been demonstrated histologically in an animal model using anatomical soft tissue graft fixation with compression at the tunnel entry site (interference screws).²⁵ In contrast, the use of a nonanatomical fixation technique in the same model showed the development of an indirect type of ligament insertion or only a delayed formation of a direct type of insertion.²⁵ In indirect insertions, the surface of the ligament connects with the periosteum whereas the deeper layers connect to bone via Sharpey fibers (e.g., medial collateral ligament). This is of inferior mechanical competence compared with the direct ligament insertion.⁵

Thus it is reasonable to assume that neutralization of graft-tunnel motions using anatomical and direct interference screw fixation obviously improves osseous graft incorporation by means of the development of a native ACL insertion anatomy.²⁵

Interference Screws

Metallic interference screws initially were developed for fixation of grafts with attached bone blocks. These screws are threaded sharply to achieve good starting conditions for screw insertion and secure graft fixation. The use of this type of interference screw for soft tissue graft fixation might lead to laceration of the graft tissue during screw insertion, especially if high insertion torque is generated. Thus different round-threaded interference screws have been developed for soft tissue graft fixation.^26,27 The first round-threaded metallic interference screw for direct fixation of soft tissue grafts was developed by L. Pinczewski, the round-headed cannulated interference screw (RCI)^10,26 (Fig. 39-2). More recently, biodegradable interference screws have been developed and biomechanically as well as clinically tested for fixation of BPTB and soft tissue grafts.^27,28 Soft tissue graft fixation using biodegradable interference screws was first described by Stähelin and Weiler.²⁹

Fig. 39-2 Different interference screws. A, Early metallic interference screw, sharp threaded; B, Sysorb biodegradable interference screw, round threaded; C, round-headed cannulated interference (RCI) screw, metallic, round threaded; MegaFix biodegradable interference screws, sharp threaded at the tip (D) and round threaded at the body (E).

(A–C, From Weiler A, Hoffmann RF, Sudkamp NP, et al. Replacement of the anterior cruciate ligament. Biomechanical studies for patellar and semitendinosus tendon fixation with a poly[D,L-lactide] interference screw. Unfallchirurg 1999;102:115–123. D and E, By permission of KarlStorz, Tuttlingen, Germany.)

Biodegradable interference screws have been demonstrated to be advantageous compared with metallic screws by means of undistorted radiological imaging, uncompromised revision surgery, and minimized risk of graft laceration. However, one might consider that most currently available biodegradable interference screws do not show complete degradation and subsequent osseous replacement of the former implant site because they consist of slow degrading and high-molecular poly-L-lactide.²⁷ Thus the use of intermediate degrading stereo-co-polymeric materials such as poly-(L-co-D,L-lactide) are preferable.^27,30

The newest screw generations are sharply threaded just at the tip for easy starting conditions of the screw, followed by a blunt threading to prevent tissue laceration (see Fig. 39-2). Biodegradable as well as metallic interference screws are distributed in different sizes (diameter and length). Thus precise matching of graft and tunnel diameters can easily be performed.

Mechanical studies have shown that a tight fit among the screw, the graft, and the tunnel is essential for sufficient fixation strength.^13,31–33 When deciding to oversize a screw to improve fixation strength (particularly at the tibial site), an increased length has been demonstrated to be superior to an increased diameter.^13,34 Furthermore, an oversized screw diameter increases the insertion torque in contrast to a longer screw. We therefore recommend increasing screw length instead of using massively oversized screws in diameter (e.g., Delta screws) to avoid violating the tendon tissue.

In summary, hamstring tendon interference screw fixation offers the following advantages:

Graft Preparation

Graft preparation is one of the essential surgical steps and should be performed carefully. Possible configurations include four-stranded grafts using a doubled semitendinosus and doubled gracilis tendon or the quadrupled semitendinosus tendon. Other possibilities are three- or five-stranded grafts.

Because preservation of the gracilis tendon has been demonstrated to be beneficial^35,36 and due to the fact that anatomical joint line fixation requires only a short graft (at least 7 cm) (Fig. 39-3), the use of a four-stranded semitendinosus tendon graft should be routinely achieved. To gain a sufficient length of the semitendinosus tendon, the tendon can be harvested including the periosteal distal insertion of the tendon. In most cases this results in a tendon length of at least 28 cm. If the semitendinosus tendon is very short (less than 26 cm) or thin, one can additionally harvest the gracilis tendon to create a four- or five-stranded semitendinosus/gracilis tendon graft.

Fig. 39-3 Quadrupled semitendinosus tendon autograft with EndoPearl attached to the femoral end.

The four-stranded graft is prepared with the help of a suture board while arthroscopic preparation of the knee is performed. The proximal and distal endings of the semitendinosus tendon are armed with #2 polyester sutures in a whipstitch fashion (Fig. 39-4). Care has to be taken to pull all slack out of each suture pass. Then the construct should be manually tensioned to allow potential slippage to be taken out of the construct (Fig. 39-5). The tendon is then looped over itself using the so-called “W-technique,” and a polyester passing suture is brought through each loop (Fig. 39-6). The looped tendons are then pulled through a graft sizer (Fig. 39-7). The resulting diameter of the graft is usually between 7 and 9 mm. A tight fit of the graft in the tunnel generally is required to improve fixation strength and graft incorporation. The diameter of the graft is a given value, which needs to be known before tunnel creation. When hybrid fixation (see later discussion) is used, sizing of the graft in increments of 1 mm is sufficient; if interference screw fixation is used solely, we recommend sizing in increments of 0.5 mm³³ to allow for the required tight fit. A marking suture using #0 absorbable suture has to be set 2 cm from the femoral end of the graft to show the surgeon if the graft is inserted deep enough into the femoral tunnel. The side effect of the suture is that good passage of the graft in the tunnel is ensured, and twisting of the graft around the screw during its insertion is prevented (see later discussion; see also Fig. 39-3).

Fig. 39-4 Arming the tendon with #2 polyester sutures in whipstitch fashion.

Fig. 39-5 Manual tensioning of the armed tendon to prevent later slippage of the sutures.

Fig. 39-6 To prepare a quadrupled graft, the tendon is looped over itself using the so-called “W-technique” and a polyester passing suture is laid through each loop.

Fig. 39-7 Measuring the diameter of the graft.

If femoral hybrid fixation using the EndoPearl (Linvatec, Largo, FL) device is desired, the pearl is tied to the femoral end of the graft (see Fig. 39-3). It is important that the knot fixing the pearl to the graft is not placed at the side of the screw, which would result in an increased graft diameter, possibly leading to problems during graft insertion. The tibial end of the graft is sutured in a baseball-stitch technique using #0 absorbable sutures to ease the insertion of the tibial screw and to increase initial tibial graft fixation strength (see Fig. 39-3).

Femoral Interference Screw Fixation

Tunnel Preparation

According to the current literature the femoral tunnel should be drilled in the ten-o’clock position for right knees or in the two–o’clock position for left knees.^37–39 Arnold et al demonstrated that this position cannot or can hardly be achieved when using the conventional transtibial techniques (single incision).⁴⁰ Thus we routinely use the anteromedial portal technique in all ACL reconstruction procedures because it allows for an anatomical lateral tunnel placement.

In the anteromedial tunnel technique, we create the femoral tunnel first in approximately 120 to 130 degrees of flexion (Fig. 39-8). Thus tunnel direction is directed more to the center of the bone and away from the posterior femoral cortex, which prevents posterior breakage of the tunnel wall. As this amount of knee flexion only hardly can be achieved when using a standard leg holder, we routinely use a lateral support at the level of the thigh and put the leg on the operating table in maximum knee flexion (Fig. 39-9).

Fig. 39-8 In the anteromedial tunnel technique, the femoral tunnel is created in approximately 120 to 130 degrees of flexion to prevent posterior breakage of the tunnel wall.

(By permission of KarlStorz, Tuttlingen, Germany.)

Fig. 39-9 Knee held in maximum flexion during creation of the femoral tunnel via the anteromedial portal.

In ACL reconstruction using hamstring tendons, the graft diameter has to be known before tunnel creation to allow for a tight fit between graft and tunnel wall, in contrast to BPTB grafts for which 8- to 10-mm tunnels are routinely used as determined by the size of the harvested bone blocks.

The standard sizing when using interference screw fixation at the femoral site alone is:

Tunnel diameter = Graft diameter = Screw diameter

The length of the screw should be 23 to 25 mm to achieve sufficient initial fixation strength.

Screw Insertion

During femoral screw insertion it is of great importance to control knee flexion to prevent screw divergence, which might result in decreased fixation strength, and to allow for an easy screw start. If the anteromedial portal technique is used, the knee flexion angle has to be exactly the same as it was during tunnel creation. If the transtibial technique is used, one needs to find the desired knee flexion angle for proper screw insertion. In this situation a standardized flexion of 120 degrees might not be appropriate. We therefore recommend palpating the tunnel with a nitinol wire or the tip of the screwdriver prior to screw insertion to ensure parallel screw placement.

In addition to correct direction of screw insertion, other surgical details can ease femoral screw insertion:

• A nitinol guidewire can be used to identify tunnel direction and to secure correct screw placement by insertion of the screw over the wire (cannulated screwdriver necessary).

• Tunnel direction can be verified using the screwdriver. Furthermore, the place of screw insertion can be dilated a little bit when the graft is already inserted using the screwdriver or a small dilator (4 mm) (Fig. 39-10). This technique preconditions the tunnel for screw insertion and correct placement.

• Notching of the femoral tunnel wall using a special instrument (Fig. 39-11) eases screw insertion and secures posterior positioning of the graft in the tunnel.

Fig. 39-10 Dilation of the screw site using a small dilator (4 mm), followed by screw insertion.

Fig. 39-11 Notching of the anterior edge of the femoral tunnel entry to facilitate screw insertion and anterior placement of the screw. A, Schematic picture. B–G, Intraoperative views.

(A, By permission of KarlStorz, Tuttlingen, Germany.)

Concerning the depth of screw insertion into the femoral tunnel, there is a main difference between metallic and biodegradable interference screws. Biodegradable interference screws should be countersunk a few millimeters (2 to 3 mm) below the surface to allow for overgrowth of connective tissue. If metallic interference screws are advanced too deep, later hardware removal (e.g., in case of revision reconstruction) might be complicated. The use of round-headed interference screws on the femoral site, as recommended in earlier years for BPTB graft fixation, is not recommended anymore because a prominent screw head might impinge with the graft in full extension. If a metallic screw head is countersunk, later revision reconstruction might be compromised because screw removal can create large bone defects due to the head of the screw needing to be exposed for removal.

Interference Screw Position

On the femoral site, screw position generally is anterior to the graft to allow anatometric posterior placement of the graft in the tunnel.

Problems in femoral hamstring tendon interference screw fixation might occur if the graft tends to rotate around the screw during its insertion; this might lead to an undesired position of the graft (Fig. 39-12). When using a standard right-threaded screw in right knees, the graft might rotate toward an anterolateral position, resulting in too-anterior graft placement and lateral wall impingement. Thus Pinczewski et al could show a difference of clinical outcome between right and left knees after hamstring tendon interference fit fixation.⁴¹ Subsequently, a reversed-threaded interference screw was developed for use in right knees.⁴¹ However, in a left knee with a conventional screw or in a right knee with a reversed screw, the graft might rotate toward the 12-o’clock position, which might lead to compromised control of knee rotation.^40,42 We therefore recommend preventing screw rotation completely.

Fig. 39-12 Rotation of the graft to an undesired 12-o’clock position in femoral hamstring tendon interference screw fixation in a left knee.

Techniques to minimize the risk of graft rotation during interference screw insertion include the following:

• Tight suturing of the proximal end of the graft (see Fig. 39-3). This decreases graft rotation by stabilizing the bundles of the graft and preventing strands of the graft from being caught by the surface of the screw.

• Hybrid fixation. Femoral hybrid fixation using the EndoPearl device allows undersizing of screw diameter, resulting in decreased insertion torque without compromising fixation strength. Decreased insertion torque minimizes the risk of graft rotation during screw insertion.

• Notching the screw insertion site (see Fig. 39-11).

• Bone wedge technique (Fig. 39-13). The use of this technique prevents the graft from rotation during screw insertion and allows for 360 degrees of bone contact around the graft. At the anterosuperior tunnel aperture, a thin (2 to 4 mm) bone wedge can be detached using a special chisel (Fig. 39-14). The screw is then advanced between the tunnel wall and the bone wedge. Care has to be taken because a very large bone wedge might decrease fixation strength; thus hybrid fixation is recommended.

Fig. 39-13 Bone wedge technique. At the anterosuperior position of the tunnel entry site, a thin bone wedge is detached using a special chisel (A). The screw then is advanced between the tunnel wall and this bone wedge (B–D).

Fig. 39-14 Specially designed chisel for the bone wedge technique.

(By permission of KarlStorz, Tuttlingen, Germany.)

Femoral Hybrid Fixation

An important method that further minimizes graft–tunnel motions and improves initial fixation strength as well as construct stiffness is the so-called hybrid fixation.^31,43 The principle of hybrid fixation is to combine two fixation techniques at one site.³¹ At least one of the devices used should be able to achieve sufficient fixation strength alone. This device is combined with a second technique in order to neutralize its possible biomechanical and biological disadvantages (e.g., graft–tunnel motion) (Table 39-1).

Table 39-1 Possibilities for Hybrid Fixation

Type of Fixation	Possible Methods
Femoral hybrid fixation	Interference screw and EndoPearl Suture button and interference screw Suture button and cancellous bone plug Transfixation and interference screw Transfixation and cancellous bone plug
Tibial hybrid fixation	Interference screw and suture to bony bridge Interference screw and suture button Interference screw and staples Cancellous bone plug and suture button Cancellous bone plug and suture to bony bridge Cancellous bone plug and tying of sutures over screw

On the femoral site, graft fixation generally is more forgiving compared with the tibia from a mechanical point of view. However, disturbed biological incorporation is a problem of femoral fixation due to the higher graft–tunnel motions.²⁵ We therefore recommend aperture fixation on the femur to optimize biological boundary conditions. In order to minimize possible problems of interference screw fixation, such as tissue laceration and graft rotation, femoral hybrid fixation allows for the use of an undersized screw to solve these problems.

For femoral hybrid fixation with interference screws, a biodegradable spherical device, the EndoPearl, has been developed (see Fig. 39-3). The EndoPearl is tied to the femoral end of the graft and achieves an internal locking between the graft and the tip of the interference screw (Fig. 39-15). Thus it increases initial fixation strength, especially in cases of tunnel enlargement (revision reconstruction) or low bone density. Clinically it has been shown to improve knee stability compared with interference screw fixation alone.⁴⁴ If this type of femoral hybrid fixation is used, the femoral tunnel should be created 1 cm deeper than normally recommended (35 mm instead of 25 mm). Additionally the chosen diameter of the screw can be 1 to 2 mm less than the tunnel diameter without compromising fixation strength. This is advantageous because it decreases the risk of tissue laceration (insertion torque) and reduces screw rotation without compromising initial fixation strength.

Fig. 39-15 A, Magnetic resonance image (MRI) demonstrating contact between the EndoPearl and the tip of the interference screw (arrow) to achieve an internal locking. B, MRI demonstrating position of the tibial screw posterior to the graft (arrow).

Thus the standard sizing when using femoral hybrid fixation with the EndoPearl is:

Tunnel diameter = Graft diameter = Screw diameter − 1 mm

As an alternative, tunnel diameter can be increased and screw size chosen according to the graft diameter when hybrid fixation is used. This allows for hamstring tendon interference screw fixation in revision cases as well. As another alternative, a cortical bone plug can be tied to the femoral end of the graft.⁴⁵

Femoral hybrid fixation with interference screws also can be achieved with the additional use of an Endobutton. In these cases, especially if a continuous loop is used, the screw acts only to neutralize graft tunnel motion and prevent synovial inflow. Thus the use of small screws (5 to 6 mm) is sufficient.

Tibial Interference Screw Fixation

Possible Problems during Screw Insertion

A specific problem at the tibial site is the “blind” insertion of the screw. Thus the risk of inserting the screw not parallel to the graft is increased compared with the femoral site. Furthermore, it might easily happen that the screw is not inserted between the graft and the tunnel wall but instead pushes the graft forward during its insertion. To prevent this accidental graft dislocation, the surgeon should hold the holding sutures of the graft tightly during screw insertion. After tibial screw insertion we recommend controlling graft and screw placement by an intratunnel view using the arthroscope (Fig. 39-16).

Fig. 39-16 Intratunnel view to control tibial interference screw and graft position.

Tibial Hybrid Fixation

When interference screw fixation is used for soft tissue grafts, some authors are concerned about a possible fixation slippage at the tibial fixation site even though fixation strength at the femoral site is considered sufficient.^13,18,32,46 The risk of graft slippage on the tibial site can be nicely compensated with oversized screws, or better, by using hybrid fixation on the tibia, especially in females.^34,47

An easy and safe method of tibial hybrid fixation is to tie the holding sutures of the graft over a bony bridge after the tibial screw is inserted (Fig. 39-17). For this a monocortical drill hole is created 2 cm distally of the tibial tunnel exit site. Then one strand of each suture is passed through the hole and tied over the created bony bridge.⁴⁸

Fig. 39-17 Tibial backup fixation by suturing the linkage material over a bony bridge. A, A monocortical hole is prepared 2 cm distally from the tibial tunnel using the screwdriver. B, The cancellous bone is tunneled using a curved clamp. C and D, A suture loop is passed from the distal to the proximal hole using a needle. E–I, One strand of each attached suture is passed through the holes using the suture loop and tied over the created bony bridge. J, The knot should be countersunk into the tibial tunnel to avoid the development of a subcutaneous granuloma. t, Tibial tunnel.

(A–D, By permission of KarlStorz Tuttlingen, Germany.)

In cases with a low insertion torque of the interference screw, the use of a suture button instead of the suture over bony bridge might be beneficial (Fig. 39-18). Manual rotation of the button tightens the linkage material, thus safely preventing graft slippage.

Fig. 39-18 Tibial hybrid fixation using a suture. A–D, The tibial tunnel exit site first has to be prepared using a special instrument. E–G, The sutures are passed through the holes of the fixation button and tied over the button. H and I, Manual rotation of the button using a special instrument tightens the linkage material.

(From Strobel M. Manual of arthroscopic surgery. Berlin, 2001, Springer-Verlag.)