**2. Biomechanics**

After a meniscal tear, the effectiveness of meniscal repair strictly relies on the tissue quality and defect location with respect to the vascular supply. Tears in the vascularized "red" peripheral zone are more likely to heal, while the more common lesions in the avascular "white" zone have poor healing potential [16, 17]. When the majority of the meniscus is not salvageable, a meniscectomy is usually performed. It has been well documented that meniscectomy increases the risk of degenerative joint disease of the knee. For example, Persson et al. demonstrated that in almost 2,500 patients followed for more than 20 years, the risk of developing arthritis after a partial meniscectomy was almost 6 times higher than the standard population, with a 17% absolute risk [18].

Structurally, it has been demonstrated that meniscal allografts should be frozen, not sterilized using chemicals or radiation. It the first published series of isolated meniscal allografts, Milakowski et al. found that graft processing methods were vital to the success of the procedure [19]. He reported that lyophilized grafts lead to inferior results compared to fresh frozen grafts. While clinical series have not shown benefit of cryopreserved over fresh frozen, basic science studies have shown slightly better mechanic properties with cryopreserved, with a higher elastic modulus and point of rupture [20].

While meniscus allograft transplantation appropriately addresses a prior total or subtotal meniscectomy, an allograft is not a solution for the treatment of a partial meniscus defect. The Collagen Meniscus Implant (Ivy Sports Medicine, Germany) is a porous biologic scaffold. It consists of 97% type I collagen purified from bovine Achilles tendon while the remaining portion is composed of glycosaminoglycan (GAG). The specific size of the scaffold's micropores are controlled to increase the fibrocartilage maturation while avoiding pseudo-capsule formation and foreign body reaction [21].

The second scaffold type consists of a synthetic polyurethane-based material composed of flexible segments made from polycaprolactone 80% and stiff segments made from urethane 20% (Actifit; Orteq Sports Medicine, London, UK). The scaffold slowly biodegrades, with an estimated decomposition time of 4 to 6 years. The implant itself is also highly porous to allow for sufficient ingrowth [22]. Both the Actifit and the CMI implants come in separate configurations for medial or lateral meniscus defects.
