There are various techniques for securing the soft tissue to the bony tunnel in ACL reconstruction. Each one has strengths and weaknesses. Pinczewski pioneered the use of the RCI interference fit metal screw for soft tissue fixation. The use of a similar type of bioabsorbable screw that was used in bone tendon bone fixation was a natural evolution. To overcome the weak fixation in poor quality bone, the use of a round pearl, made of PLLA or bone, was developed. This improved the pullout strength by 50%. The Endo-button, popularized by Tom Rosenberg, was improved with the use of a continuous polyester tape. This made the fixation stronger and avoided the problems of tying a secure knot in the tape. The cross-pin fixation has proven to be the strongest, but has a significant fiddle factor to loop the tendons around the post. The Arthrex technique is the easiest to use. Weiler, Caborn, and colleagues have summarized the current concepts of soft tissue fixation.
The estimates of the force on the normal ACL during activities of daily living are as follows:
Level walking: 169N Ascending stairs: 67N Descending stairs: 445N Ascending ramp: 27N Descending ramp: 93N
It is commonly quoted that a person needs more than 445N pullout strength of the device just to handle the activities of daily living. However, Shelbourne has reported good results with the patellar tendon graft fixed by tying the leader sutures over periosteal buttons (Ethicon, J&J, Boston, MA). This form of fixation has a low failure strength, but is clinically successful.
The gold standard of the interference fit screw fixation of the bone tendon bone, 350 to 750N, has been used to compare the soft tissue fixation.
The pullout strengths also vary from tibia to the femur. The femoral pullout is higher because the tunnel is angled to the graft and the pull is against the screw that is placed endoscopically. In the tibial tunnel, the graft pulls away from the screw in the direct line of the tunnel.
The initial fixation points were at a distance from the normal anatomical fixation of the ACL. The trend has been to move the fixation closer to the internal aperture of the tunnel. This shortening of the intra-articular length has improved the stiffness of the graft.
The pullout strength of bioabsorbable screw can vary widely depending on its composition. The screw fixation has also been shown to be bone quality dependent. These considerations should be taken into account when choosing a femoral fixation device for soft tissue grafts.
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