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My name is Luke Barba from Biology in Materials Science, and this is my sandbox.

Design

Modern artificial ligaments see the most use in anterior cruciate ligament reconstruction. Artificial ligaments seek to mimic or exceed the performance of the native ACL. (Ref 6) The mechanical performance of an artificial ligament is based on abrasion resistance, withstanding flexural and rotational fatigue, (Ref 1) and preventing graft slippage or rupture. (Ref 9) Biocompatibility is critical to the performance of the artificial ligament in vivo. (Ref 4) Biocompatibility is related to new tissue ingrowth, (Ref 3) fibroblast migration, osseointegration of bone, reduction of inflammation, preventing scar tissue infiltration, and improving hydrophilicity. (Ref 4) Tissue ingrowth and fibroblast migration improves the mechanical strength of the artificial ligament, (Ref 3) and osseointegration with the surrounding bone reduces the likelihood of graft slippage. (Ref 9) Artificial ligaments seek to minimize inflammation and scar tissue infiltration because they hinder the mechanical strength and can cause graft rupture. (Ref 4) The artificial ligaments strive to be hydrophilic because hydrophobicity can trigger the host's natural response to foreign bodies. (Ref 4)

The Ligament Advanced Reinforcement (LARS) artificial ligaments are commonly used today. They are made of polyethylene terephthalate (PET). (Ref 4) They consist of an intraosseous and an intra-articular portion. The intraosseous section consists of longitudinal fibers bounded by a knitted transverse structure. This knitted structure prevents deformation and resists abrasion. (Ref 6,2) The intra-articular portion is made of longitudinal fibers pretwisted at a $90^{\circ }$ angle. This section resists fatigue and promotes tissue ingrowth. (Ref 6) Leeds Keio ligaments consist of a polyester mesh structure. It seeks to mimic the mechanical properties of the native ACL. The porous nature of the ligament promotes tissue ingrowth which improves mechanical properties. (Ref 6) The PGA Dacron artificial graft consists of 75% braided biodegradable polyglycolic acid and 25% permanent Dacron thread. (Ref 2) The Kennedy LAD artificial ligament is made of polypropylene ribbons. It promotes tissue ingrowth and the progressive transfer of load onto the new ligament. (Ref 3)

Ligament Advanced Reinforcement System (LARS) artificial ligament. The two end sections are the intraosseous portions and center portion is the intra-articular region.

The native ACL of a human has a tensile strength between 1760 N and 2622 N, (Ref 4) and an elongation at failure of approximately 10%. (Ref 3) The mechanical properties of the native ACL vary throughout the human population. The strength of a child's ACL is much greater than an adult's. (Ref 3) PGA Dacron artificial ligaments have an ultimate tensile strength of 3600 N and a mean ultimate elongation of approximately 20%. (Ref 3) Kennedy LAD ligaments have a tensile strength at failure of 1700 N and a stiffness of 50 N/mm. (Ref 3) Leeds-Keio artificial ligaments have an ultimate tensile strength of 2000 N and a stiffness of 270 N/mm after tissue ingrowth. (Ref 3) LARS artificial ligaments have varying mechanical properties depending on the amount of fibers used. A 60 gauge LARS ligament has an ultimate tensile strength of 2500 N while a 120 gauge ligament has a tensile strength of 5600 N. (Ref 6, 14) The strength of LARS and Leeds-Keio ligaments are heavily dependent on the degree of tissue ingrowth. The ingrown tissue improves viscoelastic properties and reduces friction. (Ref 6)

Coatings have been added to artificial ligaments to improve their biocompatibility. 58S bioglass and hydroxyapatite coatings improve osseointegration and cellular activity (Ref 4) when deposited onto PET ligaments using the soaking method. (Ref 1, 4) Hydroxypropyl cellulose surface treatments have been shown to improve osseointegration for PET ligaments. (Ref 1) Uncoated PET is hydrophobic, so coatings are added to improve the hydrophilicity. (Ref 4) Hyaluronic acid coatings reduce hydrophobicity and have been shown to reduce scar tissue formation and inflammation. (Ref 4) Hyaluronic acid and chitosan composite coatings are deposited onto artificial ligament surfaces by the layer-by-layer technique, and they have been shown to enhance new bone formation at the ligament interface. (Ref 9) The chitosan is used to reduce hydrophobicity and improve osseointegration and mineral deposition, and the hyaluronic acid promotes cell differentiation and growth. (Ref 9) Poly(sodium styrene sulfonate) coatings have been shown to improve knee functionality and mimicry of the native ACL. (Ref 1, 12)