Background

More than 100,000 anterior cruciate ligament (ACL) reconstructions are estimated to be performed annually in the United States.^1,2 There has been a tremendous amount of research on both graft selection and fixation methods.^3–5 The increased use of soft tissue grafts and the concern regarding soft tissue interference screw fixation (e.g., graft pullout, slippage, damage) have led to the development and use of femoral cross-pin fixation in ACL reconstruction. One successful technique for cross-pin fixation is the TransFix ACL reconstruction technique (Arthrex, Naples, FL).

Unlike interference screw fixation, which is dependent upon screw geometry, bone density, and interface gap, the fixation strength of the TransFix is limited only by the strength of the graft and the device itself (size, geometry, and material composition). The mode of failure of the TransFix pin during biomechanical studies has consistently been bending and breakage, unlike the evidence of graft slippage for interference screw constructs.

Biomechanical and Clinical Results

The Arthrex TransFix implant is made of titanium, and the Bio-TransFix implant is made of poly-L-lactic acid (PLLA), as shown in Fig. 36-1. In vivo, the Bio-TransFix implant hydrolyzes into lactic acid, which is then metabolized into CO₂ and H₂O. A proprietary degradation study of the Bio-TransFix demonstrated that it does not lose shear strength through 52 weeks.

Fig. 36-1 The Arthrex TransFix implant is made of titanium, and the Bio-TransFix implant is made of poly-L-lactic acid (PLLA).

Several studies have shown that the TransFix and Bio-TransFix have significantly better structural properties for maximum load, stiffness, strength, and slippage of soft tissue grafts as compared with interference screw fixation and other cross-pin fixation techniques. Also, because of the fixation technique, the potential for tunnel widening is significantly decreased and the strength of the graft is compromised much less than that from interference screw fixation.

Fabbriciani et al used the TransFix system in conjunction with fresh, ovine, doubled Achilles tendons and ovine femurs.⁵ Cyclical loading comparisons of bioabsorbable and metal RCI screws (Smith & Nephew, Andover, MA), the LINX HT (Mitek, Norwood, MA), and the TransFix implant showed significantly lower mean values of graft elongation for the TransFix construct (1.5 ± 0.1 mm) over 1000 cycles. The maximum load to failure (LTF) for the TransFix was 890N ± 175N, which, unlike the other devices in the study, is comparable to that of the intact ovine ACL (725N ± 77N).

Becker et al showed that the stiffness of the TransFix construct (184 N/mm) approximates the stiffness of the human ACL (242 N/mm, as reported by Woo et al⁶) and provides significantly greater ultimate strength than interference screw fixation.⁷ This study compared three fixation methods using a porcine femur model: (1) TransFix fixation of a quadruple tendon, (2) 8- × 20-mm biodegradable interference screw fixation of a quadruple tendon, and (3) 8- × 20- mm titanium screw fixation of a patellar tendon–bone graft using a porcine femur model. Interference screw fixation of the patellar tendon and quadruple tendon resisted only 59% and 37%, respectively, of the pullout strength of the TransFix (1303N ± 282N). The TransFix had significantly less construct displacement during cyclical loading than the interference screw/quadruple graft construct.

Ahmad et al demonstrated that interference screw fixation and the Rigidfix cross-pin technique were inferior to the Bio-TransFix and the Endobutton for graft slippage during cyclical loading and ultimate LTF.⁸ After 1000 cycles, the graft displacement for the Bio-TransFix was 1.13 mm compared with greater than 5 mm for the interference screw and Rigidfix. This study also showed significantly greater LTF of the Bio-TransFix (746N ± 119N) as compared with the interference screw technique (539N ± 114N).

As these and other studies have demonstrated, the use of the TransFix system offers considerable advantages compared with other femoral fixation systems in terms of yield load, stiffness, and deformation and elongation under cyclical loading. These results offer stable fixation of the graft during the postoperative period, before graft healing has occurred. The inherent rigidity of the TransFix limits graft-tunnel motion during physiological loading. Intratunnel motion has been associated with tunnel widening.⁹ Fauno and Kaalund reported a significant reduction in tunnel widening in the femur when TransFix was used compared with Endobutton fixation at 1-year follow-up for a prospective randomized study.¹⁰ Unlike interference screw techniques, in which the graft is squeezed and possibly damaged during screw insertion, graft strength is maximized with the TransFix technique.

In 1998, Wolf¹¹ reported his initial results with the TransFix fixation. Eighty-eight percent of patients at follow-up described their postoperative knees to be normal or near normal. The 27 patients had a mean KT-1000 side-to-side laxity difference of 1.5 mm at follow-up. (Ahmad reports that greater than 5 mm may be considered clinical failure.⁸)

Recently, Harilainen et al¹² showed no significant difference in IKDC scores at 2-year follow-up for TransFix cross-pin fixation versus metal screw fixation. In this controlled prospective randomized study, 85% of the TransFix group and 73% of the screw patients were in the IKDC A or B categories.

Surgical Technique

The TransFix technique requires that a 3-mm drill pin be passed from lateral to medial. Although no neurovascular complications have been reported, theoretically the medial (and lateral) neurovascular structures are at risk. The author’s lab has shown that a defined “safe zone” exists in which a distal femoral cross-pin can be reliably placed without damaging the local neurovascular structures.¹³ In this anatomical cadaveric study, the absolute neurovascular safe zone during cross-pin guidewire placement is from +20 degrees (0 degrees equals “parallel to the floor” line) and –40 degrees (lowering the guide more posteriorly) (Fig. 36-2).

Fig. 36-2 The absolute neurovascular “safe zone” during cross-pin guidewire placement is from +20 degrees (0 degrees equals “parallel to the floor” line) to –40 degrees (lowering the guide more posteriorly).

The TransFix technique is designed for soft tissue grafts such as hamstring autograft or tibialis tendon allograft. The author prefers tibialis tendon allograft and has performed more than 300 TransFix ACL reconstructions with this particular graft. A stepwise approach is as follows:

Fig. 36-3 Prepare the tibialis graft with a running or Krackow locking stitch, and size it to the nearest-millimeter diameter.

Tip 1: Make sure the graft runs easily through the selected diameter.

One advantage of a tibialis graft is that the surgeon can select or trim the graft to the desired size. The completed graft is placed in a moist sponge that has been soaked in antibiotic solution.

Fig. 36-4 Complete the tibial tunnel through a small anterior medial tibial incision using a posterior cruciate ligament referencing guide.

Tip 2: The knee flexion angle must be maintained until the TransFix is implanted.

Tip 3: The TTG should easily be placed in the over-the-top position. If not, the tibial tunnel may be too anterior. A Beath pin is drilled through the distal cortex but not through the skin.

Fig. 36-5 Remove the Beath pin. Insert the matched TransFix tunnel hook through the tibial tunnel, and position it in the femoral socket.

Tip 4: Do not overtighten the drill guide on the lateral cortex. This causes the sleeve to skive along the metaphysis and throw off the alignment of the guide. The sleeve should rest lightly on the bone.

Tip 5: Do not push hard on the TransFix guide pin when drilling. This can cause the trocar tip to skive along the metaphysis and throw off the aim. The TransFix guide pin is threaded and will ease across the femur.

Tip 6: The 3-mm guide pin should easily pass back and forth, ensuring a smooth passage through the tunnel hook. Pass the guidewire back and forth several times manually until it glides easily.

Fig. 36-6 Drill the 5-mm broach with a stop collar over the 3-mm guidewire.

Fig. 36-7 Pass and deliver the nitinol wire out of the tibial tunnel. Pass the selected graft in a retrograde fashion.

Tip 7: The nitinol wire should glide back and forth very easily after seating the graft proximally. The kink in the wire from graft passage should be pulled medially to prevent capturing the implant on insertion. If the graft–tunnel interface is too tight, the graft will not be seated proximally. This will result in the nitinol wire breaking on implant insertion. The knee flexion angle at the time of reaming the femoral socket must be maintained.

Tip 8: A blunt probe can be used to push the graft up the femoral tunnel to assist in seating the graft completely in the femoral socket.

Tip 9 (the most critical): The implant should be advanced manually along the same direction as the nitinol wire. An assistant should confirm smooth glide of nitinol wire throughout the insertion. Hand-inserting the implant as far as possible allows for more surgeon/assistant feedback. If resistance to glide is noted with the nitinol wire, then the surgeon should immediately confirm proper implant orientation.

Fig. 36-8 Seat the implant at the appropriate depth (match the calibration line to the reamer) with gentle taps using a mallet only after smooth passage of the wire is confirmed.

If orifice fixation is desired, this can be achieved with the addition of a cancellous bone block¹⁵ or 20-mm femoral retroscrew (Arthrex) placed distal to the TransFix. Ishibashi et al demonstrated that proximal fixation resulted in reduced anteroposterior translation compared with more distal fixation.¹⁶ Anatomical fixation close to the joint line results in increased knee stability and graft isometry. Fixation of the graft in the tunnel by an interference screw or bone block also may mitigate synovial fluid infiltration into the tunnel.

Troubleshooting and Common Problems

The tunnel hook jig cannot be passed into the tibial/femoral socket: Confirm the sizes of the tunnel and tunnel hook. Always verify that the knee flexion angle is maintained after drilling the femoral socket. This ensures that no resistance will be encountered while passing the graft from the tibial to the femoral socket.

The graft will not seat properly in the femoral socket: If the graft cannot be pushed up into the femoral socket with a blunt probe while the assistant pulls the nitinol wire, then the surgeon should resize the graft. The graft diameter is likely to be too large. Do not soak the graft in saline after sizing; often swelling can increase graft diameter. The graft can be trimmed, or in extreme cases, the femoral tunnel can be reamed up one size. Always make sure that the knee flexion angle is maintained after drilling the femoral socket.

The nitinol wire does not glide very easily: Again, always make sure that the knee flexion angle is maintained after drilling the femoral socket. The graft is not seated proximally (see earlier).

The TransFix implant is “trapping wire”: Confirm that the implant insertion angle is identical to that of the nitinol wire. The graft may not be seated proximally (see earlier).

The nitinol wire breaks: This typically happens early in the insertion phase. Broken ends of wire can easily be removed from both medial and lateral directions. In general, this is because the graft is not seated enough proximally or the implant insertion angle is different from that of the guide pin/nitinol wire. Resize the graft, and reconfirm the correct angle of implant insertion. A portion of the wire will be left in the femur/implant if the wire breaks after the implant is seated. This situation is usually not a problem if the graft is secure, but it will be obvious on radiographs.

The graft is severed upon implant insertion: The threads on the TransFix implant can injure the graft if it is countersunk in bone too far. Use another allograft if available, or suture the graft together (tubularize) and convert to using Endobutton fixation or soft tissue interference screw fixation.

Conclusion

In summary, TransFix cross-pin fixation offers favorable strength and stiffness values as well as excellent early clinical results.