Current Concepts
Material Properties and Composition of Soft-Tissue Fixation

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Abstract

Surgical interference screws and suture anchors for attaching soft tissue, such as ligaments and tendons, to bone are routinely used in arthroscopic surgery and sports medicine. Interference screw fixation provides a press fit between bone, graft/tendon, and screw and is frequently used to attach replacement ligaments in tunnels drilled for anterior and posterior cruciate ligament reconstruction. Suture anchors are used in surgical procedures wherein it is necessary for a surgeon to attach (tie) tissue to the surface of the bone, for example, during joint reconstruction and ligament repair or replacement. The composition of these implants ranges from metals to polymers and composites. Typically, because of the relatively large amount of torque that must be applied during insertion, these screws are constructed from metal. However, interference screws and suture anchors have also been constructed from bioabsorbable polymers and composites. The ideal material would (1) provide adequate mechanical fixation, (2) completely degrade once no longer needed, and (3) be completely replaced by bone. Because no material has been shown to be superior for all applications, the surgeon must weigh the advantages and disadvantages of each to evaluate the optimum material for a given application and patient. The purpose of this article is to present a comprehensive review of the commercially available interference screws and suture anchors, with an emphasis on implant composition, interaction, and design. This article provides the orthopaedic surgeon with a background on biomaterials, specifically those used in interference screws and suture anchors. Because there is no material that is perfect for all surgical situations, this review can be used to make educated decisions on a case-by-case basis.

Section snippets

Metals

Most bone anchors currently approved for clinical use are made of metal.1 The 2 most commonly used metals are stainless steel and titanium. Potential problems associated with metallic implants include migration and magnetic resonance imaging artifact. Metal anchors also remain permanently embedded in the bone, thereby limiting options for anchor placement during subsequent surgery. Metals are shiny or lustrous solid materials that are malleable and ductile yet very durable. The shiny and

Polymers

Polymer-based absorbable implants were first used in the early 1960s when American Cyanamid (Wayne, NJ) developed Dexon, a polyglycol material that was used as a resorbable suturing material.11 In the 1970s polymers for use as biomaterials in orthopaedics became popular because of the demand for biocompatibility and nontoxicity. Furthermore, because of the difficulties in imaging and revision surgery that metal implants posed, polymers were investigated. Polymers do not interfere with

Biocomposites

Biocomposites have the same advantages of polymers, such as ease of postoperative imaging16, 52 and revision surgery, with the added benefit of bone formation within the screw. A composite consists of 2 different materials, and those used in interference screws and suture anchors (Table 2) consist of a ceramic and polymer. All available implants consist of a bioabsorbable polymer and bioactive ceramic, but PEEK-ceramic composites are currently being investigated.50 Studies have shown good

Screw Geometry

Although attention must be given to material composition and biomechanical suitability, it is important to consider the screw geometry. This is especially important when comparing different studies, because some have suggested that screw geometry is a more important determinant of pullout strength than the type of material. Several studies have stressed the importance of various aspects of screw geometry, such as thread diameter,15, 76 core diameter, screw length,77 gap size,77, 78 buttress

Commercially Available Products

Companies producing interference screws and suture anchors were identified by contacting the operating room coordinators at local hospitals, checking articles for references to orthopaedic companies, and obtaining product pamphlets from exhibitors at the American Academy of Orthopaedic Surgeons annual meeting in February 2009. Each company was contacted, and information on available products was obtained from each company's Web site and sales representatives.

Implant Review

The review of commercially available implants found 33 metal suture anchors and 16 metal interference screws (Table 3). All of the available metal interference screws and 73% of suture anchors (24) are made of titanium alloys.

Table 1 outlines the available bioabsorbable and biostable polymers. For bioabsorbable polymers, pure PLLA was the most common in both interference screws (14 [74%]) and suture anchors (22 [55%]). The second most common material was PLGA for interference screws (3 [16%]),

Conclusions

Interference screws and suture anchors are commonly used for fixation of soft tissue to bone in arthroscopic surgery and sports medicine. Interference screws tightly sandwich the graft/tendon between the screw and the bone, whereas suture anchors attach soft tissue to an implant embedded in bone. Many implants are made from metals; however, the advancement of new polymers and composites has made the use of these materials more common (Table 5). In biceps tenodesis and ACL reconstruction,

Acknowledgment

The authors thank Arthrex for research support. They also thank the Arthrex, Stryker, Smith & Nephew, Biomet, ConMed Linvatec, Mitek, Zimmer, and ArthroCare sales representatives for assistance with product literature.

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