Introduction

The fixation of soft tissue grafts and the four-bundle hamstring grafts have been the weak point of the anterior cruciate ligament (ACL) hamstring graft fixation. Kousa et al¹ and Brand et al² have published laboratory studies to provide some guidance in selecting the strongest fixation device. On the femoral side the cross-pins and closed-loop Endobutton are more than double the strength needed for activities of daily living (1000N versus 450N). The tibial side remains the weak part of the construct. The IntraFix device (DePuy Mitek, Norwood, MA) has been reported to be around 1000N, and the staples on the cortex approximately 500N. Harvey et al³ demonstrated that screws with cortical fixation were almost twice the strength of screws that were placed in the cancellous bone portion of the tunnel. This presentation will describe the technique of using a cortical cancellous screw fixation for the four-bundle hamstring graft.

Fixation of the Graft in the Tibial Tunnel

The four-bundle graft is pulled up through the tibial tunnel with the leader sutures attached to the Endobutton (Fig. 45-1). The graft is pulled 3 cm into the femoral tunnel. The Endobutton is flipped on the periosteal surface of the femur. The graft is pulled distally so that approximately 1 cm of graft is protruding out of the tibial tunnel.

Fig. 45-1 The four-bundle hamstring graft.

The sutures of the graft are attached to the tensioner. Approximately 50N of tension is applied to the semitendinosus graft and 30N of tension to the gracilis graft. The knee is cycled for 12 cycles, and the tension usually drops off in both grafts. This is reapplied and cycled again until the tension remains static.

The flexible guidewire is inserted into the middle of the four bundles, and the XtraLok BioScrew (Conmed/Linvatec, Largo, FL) is inserted up the middle of the four bundles (Fig. 45-2). The stability of the knee is measured at the end of the fixation and compared with the preoperative manual maximum measurement of the opposite knee.

Fig. 45-2 The tension is applied to each of the grafts, and the screw is inserted up the middle.

Results

We have performed a randomized clinical trial comparing the XtraLok BioScrew and IntraFix tibial fixation device. The results were given as a podium presentation at the Arthrosopy Association of North America (AANA) meeting in Vancouver in May 2005. There were no significant differences in the functional outcome or in the mechanical stability as measured by the KT-1000 arthrometer. The initial results did show a trend to lower KT values at the 1-year follow up. The BioScrew cortical fixation combined with the tensioner has given excellent and reproducible clinical results in hamstring soft tissue graft fixation.

XtraLok Tips and Troubleshooting

The XtraLok tibial bioabsorbable screw comes in two lengths: 35 and 40 mm. It is available in 8-, 9-, 10-, and 11-mm sizes.

The theory of the screw design is centered on the biomechanical principles that a centrally placed, longer, and larger screw with cortical fixation improves the pullout strength of hamstring grafts on the tibial side. The screw is tapered from 8 mm at the insertion tip to 9 mm at the cortical end. It is recommended to use one size larger than the tibial tunnel. For example, if the tunnel is 7 or 7.5 mm, then the author uses an 8- × 40-mm screw. The screw is designed to have the maximum purchase on the cortex. The screw should only be placed level with the cortex and not beyond. The nitinol guidewire is placed up the middle of the four bundles of hamstring graft. These are tensioned and separated by the mechanical SE tensioner (Conmed/Linvatec). The screw must be started with considerable axial load. Once the screw is inserted about halfway, the guidewire should be removed. This avoids pushing the guidewire into the joint or having the screw bind on the wire and break it. It is important to stop about one turn short of the screw being flush at the cortex. The depth of the screw is palpated with a finger, and after the graft is cut off, the screw can be inserted another turn if necessary. It is important not to insert the screw beyond the cortex as it loses 50% of its pullout if situated in only cancellous bone. The screw cannot be reversed, and when this happens, it should be advanced to the internal aperture to obtain proximal cortical fixation.