ACL-Restoration using semitendinosus tendon

Bioabsorbable Interference Screw


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ACL-Restoration using semitendinosus or patellar tendon

In recent years surgical restoration of the insufficient or ruptured anterior cruciate ligament using the patient's own semitendinosus or patellar tendon has become increasingly popular ( Fig. finished ACL-Restoration ).

In "high tech" countries like Switzerland or the USA this operation is performed approximately 0.8 times per 1000 inhabitants a year. This amounts to about 4000 cases a year in Switzerland, respectively 200.000 operations a year in the USA.

Interference screws

The use of metallic interference screws for the fixation of patellar tendon grafts in anterior cruciate reconstruction is widely accepted (Kurosaka).

The method of such an operation is described in detail in the section operative technique .

The direct graft fixation achieved with metallic interference screws provides immediate stable fixation and promotes good healing by applying compressive forces to the cancellous bone plugs within the bony tunnels. Early postoperative weight-bearing permits accelerated rehabilitation which assists in decreasing pain and swelling and leads to a faster postoperative rehabilitation (Shelbourne).

Advantages of bioabsorbable screws

So far mostly metallic screws have been utilized to provide initial graft fixation. However possible long term adverse effects of permanently implanted metallic devices have been suggested (Black). To avoid potential metal sensitivity and stress concentrations associated with permanently implanted metal screws, and to allow undistorted postoperative radiological follow-up studies, as well as to avoid the necessity for a possible second operation for metal removal several companies tried to developed bioabsorbable interference screws.

The purpose of these implants is to provide initial fixation strength comparable to commercially available metallic interference screws while allowing eventual resorption and replacement by host bone. Clinical and biomechanical studies demonstrated that metal screws used in cruciate ligament surgery can be successfully replaced by bioabsorbable screws. Biocompatibility and complete degradation of these screws, as well as their substitution by new bone have been confirmed by clinical experience over many years (Johnson, Stahelin).

Function-adapted bioabsorbable screw

Because available bioabsorbable interference screw have several mechanical limitations a new bioabsorbable screw was developed (Stahelin). Design and manufacture of this particular screw, the SYSORB interference screw are in accordance with the special properties of the bioabsorbable material. This lightweight screw (0.7 grams) is completely degraded over a period of 12 to 24 months and substituted by new trabecular bone.

Function-oriented thread profile

The SYSORB interference screw does not have a standard thread, but rather a thread profile which takes into account the material properties of cancellous bone and bioabsorbable screws, as well as the particular architecture of cancellous bone. The thread profile is symmetrical, because the screw is subject to diametrically opposed stress on the side of the bone plug and on the side of the tunnel wall.

Function-adapted screw driver

The six ribs of the SYSORB screwdriver are formed in such a way that when the screw is inserted the torque is distributed along the complete length and no centrifugal or torsional forces are created which could lead to breakage of the screw.

Self-cutting and bone compressing

The excellent load distribution allows the screw to be inserted without any tapping and thus results in optimum compression of the neighbouring bone material.

Good biocompatibility

The favorable load transmission between screwdriver and screw permits this interference screw to be manufactured of the mechanically less robust amorphous polymer polymer poly (D,L-lactide). This material is resorbed better in the body than the highly crystalline poly (L-lactide).

Biodegradation

The degradation of polylactide in the body is effected in stages, namely be hydrolysis and phagocytosis:


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Copyright 1996 Andreas C. Staehelin
Most recent update February 12, 1996