Chapter 47 Hamstring Anterior Cruciate Ligament Reconstruction with IntraFix Tibial Fastener

Introduction

The optimal initial graft fixation technique for hamstring tendon anterior cruciate ligament (ACL) grafts remains controversial.¹^–⁶ Biomechanical studies have demonstrated that cross-pin and Endobutton-CL femoral fixation techniques provide excellent initial fixation properties.^7,⁸ However, tibial fixation of hamstring tendon ACL grafts has been more problematic. This is primarily due to the lower bone mineral density of the proximal tibia and the fact that tibial fixation devices must resist tension applied parallel to the axis of the tibial bone tunnel.^2,⁹^–¹¹ Extratunnel tibial fixation techniques that anchor to the tibial cortex can provide secure initial fixation; however, the implants are often prominent and cause local skin irritation and pain, requiring a second operation for removal.¹² Intratunnel tibial fixation using interference screws eliminates the problem of prominent hardware, but the single interference screw technique has been shown to have somewhat low initial fixation strength and increased slippage under cyclical loading.^2,^4,^9,¹³

The IntraFix tibial fastener was designed with two goals in mind, one mechanical and one biological. The first goal was to achieve more rigid intratunnel fixation of soft tissue grafts and eliminate or decrease the need for supplemental tibial fixation. The second goal was to maximize bony integration of the soft tissue graft strands into the bone tunnel wall. To achieve these goals, the device was designed with an expandable, four-channel, ridged, 30-mm polyethylene sheath and a tapered Delrin expansion screw. The four channels individually capture and grip each of the four strands of the hamstring tendon graft into separate compartments and directly compress each of the graft strands against cancellous bone. We performed cyclical and single load to failure (LTF) tests comparing the plastic IntraFix and bioabsorbable interference screws in paired young to middle-aged human cadaver tibiae with human doubled gracilis and semitendinosus grafts (DGSTs) (Table 47-1). The plastic IntraFix demonstrated a mean ultimate failure load of 800N and stiffness of 200 N/mm, which was significantly higher than interference screw fixation. In an independent biomechanical study comparing commonly used hamstring tendon graft tibial fixation devices, Kousa et al¹⁴ demonstrated that the IntraFix had the highest LTF (1309N) and stiffness (267 N/mm) and the least amount of slippage (1.5 mm) after cyclical loading.

Table 47-1 Sizing Scheme for Bio-IntraFix

Graft Diameter	Drill Tunnel	Screw Size
7 mm	8.5 mm	6–8 mm
8 mm	9.0 mm	6–8 mm
9 mm	10.0 mm	7–9 mm
10 mm	11.0 mm	8–10 mm

Following the successful clinical introduction of the nonabsorbable IntraFix tibial fastener, a Bio-IntraFix composed of poly-L-lactic acid/tricalcium phosphate (PLLA/TCP) was developed to satisfy the desire of some surgeons for a bioabsorbable device. Testing of the Bio-IntraFix using DGST and paired human cadaveric tibiae (mean age 63 ± 10 years) was performed with the line of force applied parallel to the axis of the tibial tunnel. Elongation was measured using a video camera system to determine displacement of contrast markers attached to the graft and bone. The graft–Bio-IntraFix–bone complex was cyclically loaded between 50N and 200N at a rate of 0.5 Hz for 1000 cycles (33 min, 20 sec). One thousand cycles approximates 1 week of postoperative intermittent passive motion. The mean ultimate failure load and linear stiffness for the Bio-IntraFix were 643 ± 152N and 325 ± 111 N/mm, respectively. Elongation after 1000 cycles was 2.28 ± 1.19 mm. The failure load of the Bio-IntraFix was found to be superior to that of the Delta screw (Arthrex, Naples, FL), and the stiffness was two times that of the Delta Screw. During cyclical loading, three of six Delta screws failed compared with only one of six DGST grafts fixed with the Bio-IntraFix.

The second goal of this design was to maximize the amount of contact between graft tendons and bone. Fixation outside tunnels and from suspension devices results in a loose fibrous attachment between the tunnel wall and the graft, with little if any bony ingrowth into the graft(Fig. 47-1). In contrast, direct compression of tendon to bone by interference screws within the tunnel leads to bony ingrowth including Sharpey fiber formation.¹⁵ However, a single interference screw inserted next to a bundled four-stranded graft definitely leaves a considerable portion of the tunnel filled by the screw and some of the tendons without bony contact. In contrast, the IntraFix has the potential for more extensive bone–graft integration because each strand is pressed against bone and the entire tunnel wall is in contact with graft. A limited histological study performed on the IntraFix in sheep demonstrated early bony integration (Fig. 47-2). Thus extrapolation of the interference screw data to this device seems justified. Further evidence of extensive bony integration when using the IntraFix and Bio-IntraFix comes from direct examination of the tibial tunnel many months after reconstruction. Fig. 47-3 shows an example of the appearance of the tibial tunnel 1 year after ACL reconstruction; it was obtained during a revision case after the sheath had been removed and the arthroscope inserted into the tibial tunnel. It shows a firm surface, apparent integration of the sutured tendons into the bone tunnel wall, capillary ingrowth, and no loose fibrous tissue.

Fig. 47-1 Transaxial magnetic resonance image of the tibia in an anterior cruciate ligament (ACL) reconstructed patient showing a cannulated screw anteriorly and a graft bundle posteriorly. Note the very limited contact of the graft bundles with the surface of the bone tunnel.

Fig. 47-2 Histology 12 weeks after reconstruction of the anterior cruciate ligament (ACL) in a sheep using autogenous extensor tendons and tibial fixation with IntraFix, seen at the bottom right of slide. Note Sharpey fiber formation (linear strands) and new bone ingrowth (dark blue) into tendon (light blue).

Fig. 47-3 Appearance of the tibial tunnel using an arthroscope during revision surgery following removal of the IntraFix device. Note the apparent integration of tendons/sutures 360 degrees around the tunnel and imprint of the sheath’s ridges.

Surgical Technique

Graft Preparation

Preparation of the tendons is facilitated by the use of a graft preparation board. The two tendons are cut to a total length of 20 to 21 cm, and the opposite ends of the tendons are whipstitched for 4 to 5 cm using a #2 nonabsorbable suture. This length of tendon graft allows for 25 mm of the DGST tendon graft to be inserted into the femoral tunnel and typically results in a significant length of suture-reinforced tendon within the tibial tunnel and a short (1-cm) length of the tendons extending outside the tibial tunnel. If more of the DGST tendon graft is inserted into the femoral tunnel, or if the tibial tunnel is longer than 40 to 45 mm, then the total length of the two tendons should be increased accordingly. This is important because suture-reinforced tendon constructs have been shown to increase pullout strength by 30% to 40% in our laboratory biomechanical tests.

Use with Allografts

If a soft tissue allograft such as a tibialis tendon is used, we prefer to divide each end of the allograft in two for a distance of 5 cm and then to whipstitch each strand so that a four-stranded construct comparable to a DGST is created. The IntraFix four-chambered sheath accommodates and provides more uniform compression with a four-strand graft preparation compared with a two-stranded graft. The graft construct is then placed on a tensioning board, cinching the whipstitched sutures and removing creep from the graft construct. Removing creep from the graft–suture construct is particularly important if supplemental fixation is required.

Tibial Tunnel

Our preferred method for performing endoscopic ACL reconstruction is the transtibial tunnel technique. The transtibial technique allows a longer femoral tunnel to be drilled compared with drilling the femoral tunnel through the anteromedial portal and also allows cross-pins to be used for the femoral fixation. Another advantage of the transtibial technique is that the femoral tunnel does not have to be drilled with the knee in hyperflexion, which constricts fluid inflow and limits visualization in the notch. The disadvantage of the transtibial tunnel technique is that it provides more limited access to the sidewall of the lateral femoral condyle compared with drilling the femoral tunnel through the anteromedial portal.

The tibial tunnel must be carefully oriented in both the sagittal and coronal planes for several reasons. Due to the large cross-sectional area of four-strand hamstring tendon grafts, sagittal placement of the tibial tunnel is especially critical.¹⁶ The tibial tunnel position in the sagittal plane determines whether the ACL graft impinges against the roof of the intercondylar notch in full knee extension.^2,¹⁶^–¹⁹ Roof impingement is associated with effusions, loss of extension, anterior knee pain, quadriceps weakness, and increased anterior laxity. Coronal plane orientation is the primary determinant of placement of the femoral tunnel along the side wall of the intercondylar notch and, to some degree, of the length of the femoral tunnel. A more medial starting position on the tibia allows the femoral tunnel to be drilled closer to the 10- or 2-o’clock position along the sidewall. A femoral tunnel at the 10-o’clock (right knee) or 2-o’clock position (left knee) is important because a single-bundle ACL graft positioned at these locations in the intercondylar notch is more effective at resisting combined rotatory loads than one placed at the 11-o’clock position. Biomechanical studies have demonstrated little difference in coupled anterior tibial translation between this graft and a double-bundle hamstring ACL reconstruction at low degrees of flexion.²⁰

In our surgical technique, a tibial tunnel length of 35 to 45 mm is optimal because this will accommodate the entire 30-mm IntraFix or Bio-IntraFix with no chance of the device protruding into the joint. In general, setting the variable angle tibial aimer between 45 and 55 degrees will allow these tibial tunnel lengths to be achieved. The guidelines of Jackson and Gasser,²¹ Howell,¹⁵ and Simmons et al²² are used for intraarticular placement of the tibial guide pin. If necessary, the tibial guide pin position can be checked by intraoperative radiographs or fluoroscopy with the knee in maximum extension.

Tunnel Sizing

When using the plastic IntraFix, the diameter of the tibial tunnel should equal the diameter of the suture-reinforced end of the graft. When using the Bio-IntraFix, the tibial tunnel should be drilled 0.5 to 1.0 mm larger than the diameter of the suture-reinforced end of the graft because the Bio-IntraFix sheath does not compress or flow during screw insertion. Biomechanical testing of this oversized scheme showed no loss of fixation strength for the Bio-IntraFix compared with tunnels sized to the same diameter as the graft. Half-millimeter–sized drill bits can be used to make this sizing more precise. The tibial tunnel should be drilled with a fluted drill to prevent anterior drift of the tunnel as the proximal cortex is breeched.

After drilling the tibial tunnel, it is important to clear soft tissue from around the edges of the tibial tunnel using an electrocautery pencil and a Cobb periosteal elevator for several reasons. First, a clear view is necessary to ensure that the tab of the IntraFix sheath is flush with the tibial cortex and that the sheath is fully inserted. Second, a clear view helps ensure that the screw is neither over- nor under-inserted into the tunnel. Finally, clearing of soft tissue also improves the ability to see and trim excess tendon and sheath at the end of the case so that there is no prominence that might later irritate the patient.

Femoral Tunnel and Graft Fixation

Because the IntraFix tibial fastener can be used with any femoral fixation technique, the choice of the femoral fixation is based on the surgeon’s preference. However, we prefer cross-pins or the Endobutton-CL because these fixation techniques have been shown to be strong and stiff and to have the least amount of elongation under cyclical loading.^7,⁸ More importantly, these two femoral fixation techniques permit equal tensioning of all four graft strands. This is an important goal because, as shown by Hamner et al,²³ it is necessary to equally tension all four strands of a DGST graft to maximize initial graft strength and stiffness. An equally tensioned DGST graft was stronger and stiffer than a 10-mm, central-third patellar tendon autograft. However, when no attempt was made to equally tension all four graft strands, the ultimate failure load and stiffness of the DGST graft were not statistically different from that of a doubled semitendinosus tendon graft alone. Thus failure to equally tension all four graft strands of a DGST graft negated any contribution from the doubled gracilis tendon graft.