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The use of metallic interference screws for the fixation of patellar tendon grafts in anterior cruciate reconstruction is widely accepted. The direct graft fixation achieved with metallic interference screws provides immediate stable fixation and promotes good osseous integration by applying compressive forces to the cancellous bone plugs within the bony tunnels (Kurosaka). Early postoperative weight-bearing permits aggressive rehabilitation which assists in decreasing pain and swelling. Furthermore the postoperative recovery is accelerated (Shelbourne). However possible long-term adverse effects of permanently implanted metallic devices have been suggested (Black).
In an effort to avoid potential metal sensitivity and stress concentrations associated with permanently implanted metal screws, to allow undistorted postoperative radiological follow-up studies (Shellock) and to avoid the necessity for a possible second operation for metal removal, several companies (Acufex, Arthrex, Depuy, Linvatec, Physis, Instrument Makar, SYNOS) have developed bioabsorbable interference screws. The purpose of these implants is to provide initial fixation comparable to commercially available metallic interference screws while allowing eventual resorption and replacement by host bone.
Since 1992 bioabsorbable interference screws for the fixation of B-Pt-B grafts have been used in more than 300 patients undergoing anterior cruciate ligament reconstruction. To date there has been neither fixation failure nor signs which might be indicative of an adverse reaction to the bioabsorbable material. Sequential MRI studies show uneventful incorporation of the autograft as well as disappearance of the bioabsorbable screw (Stahelin).
In standard applications such as the internal fixation of fractures, bone screws are inserted across the boundary of two pieces of material in order to pull one piece toward the other. The full pulling force acts on the small crossectional area of the screw core. For this kind of use, the superior tensile strength of metallic implants outweighs other potential advantages of using bioabsorbable material.
It is not surprising that the performance of bioabsorbable screws is similar to that of metallic screws (Kousa). The stress which acts on an interference screw is entirely different. Interference screws are inserted along the boundary between two different materials. The interference screw is inserted along the bone plug between the wall of the plug and the wall of the hole, thus securing the bone plug against pullout-forces. In this case, the force is not concentrated on the small crossectional area of the screw, but is distributed along the full length of the screw. A tensile strength of the screw material close to the strength of the surrounding bone material is sufficient. An additional effect of the interference screw is that it wedges the plug inside the hole. For this effect, it is the compressional strength of the screw material, which counts. Even bioabsorbable screws have a higher compressional strength than the surrounding bone.
A major disadvantage of many of the earliest designed bioabsorbable screws has been that they break easily during insertion (Barber, Brown). Most of the available screws are direct copies of their metallic counterparts with triangular or hexagonal shaped screwdriver recesses. Turning the screwdriver induces considerable tensile forces on the surrounding screw material, which in metallic screws seldom poses a problem.
Adapting the design of the SYSORB -screw to be used as an interference screw has overcome this weakness and virtually eliminated screw breakage during insertion. (Stahelin).
Because of its higher mechanical strength which is particularly important during screw insertion when stresses are at a peak the most frequently used bioabsorbable polymer has become poly (L-lactide) (PLLA)(Barber). However it has recently been seen that the degradation time of crystalline PLLA implants in humans is considerable longer than had been anticipated (Böstman).
Bioabsorbable crystalline polymers degrade inhomogenously. The break down products consist of slowly degrading insoluble crystallites that may give rise to late tissue reactions, and possibly accumulate in the lymph nodes (Verheyen). Thus for medical applications one should favor the use of amorphous bioabsorbable polymers (Andriano).
Poly (DL-lactide) (PDLLA) is a similar material. It is mechanically less robust, but employment more advantageous, because it is amorphous and degrades completely within two years. The new design of the SYSORB interference screw provides better torque transmissions and lower tensile forces. Therefore this screw can be manufactured from the biologically superior PDLLA without the danger of screw breakage during insertion.
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Copyright © 1996 Andreas C. Staehelin
Most recent update February 12, 1996