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Trends ORTHO TRIBUNE | APRIl 20108 Mini-implants Any form of skeletal anchorage, including miniscrews, is by defi- nition an implant: “An implant is an artificial material implanted into the body, which is to remain there either permanently or for an extended period.” More than 30 different terms for orthodontic screws are used in the international literature. The most common of these are mini-implant and miniscrew, while the terms minipin or pin are preferred when speaking to patients. At present, there are more than 30 manufacturers of miniscrew sys- tems (Fig. 5a–h). The number of screws per system ranges from two to 154 different types. In order to assist practitioners in selecting such devices according to their practice’s needs, the most important decision-making criteria for choosing implant systems are discussed below. Material All miniscrews are made from pure titanium or from an alloy of titanium with aluminium or vanadium. The biocompatibility of such materials, the metal surface of which is in direct contact with the bone, has been firmly estab- lished.11–14 Osseointegration Brånemark was the first to define the concept of osseo- integration, which he described as “a direct functional and structural link between living bone tissue and the surface of a force-absorbing implant.”15–17 Several authors, such as Costa and Maino, view anchoring a minis- crew not as osseointegration, but as a skeletal resistance block.18,19 In the opinion of Cope and Bumann, mini- screws are anchored by mechanical stabilisation and not by osseointe- gration.20,21 Diameter of the miniscrew The diameter of the miniscrews on the market varies between 1.2 and 2.3 mm. Diameter specifications of a screw normally refer to its outer diameter, i.e., the size of the shaft, including the thread. For secure and primarily mechanical anchorage, a certain amount of bone is required around the screw. To date there have been no studies on the amount of bone actually required; the information available suggests 0.5 to 2 mm. At an interradicular level, the amount of space available prescribes the maximum diameter of the screw. Poggio et al.22 , Schnelle et al.23 , and Costa et al.24–25 provide some suggestions as to the verti- cal space required, i.e., the space between the enamel/cement inter- face and the mucogingival line. These investigations clearly indi- cate that the diameter of a mini- screw should not exceed 1.6 mm. It should be noted that the sta- bility of a miniscrew in the bone depends on its diameter and not on its length.26–27 length of the miniscrew The length of the miniscrews on the market varies between 5 and 14 mm. Length specifications of a miniscrew usually refer to the shaft, i.e., the threaded section. Like the diameter, the length of the screw selected depends on the amount of bone available. Depending on the region, the total thickness of the bone is between 4 and 16 mm.28 The length of a screw is of sec- ondary importance to the diameter when it comes to secure anchor- age, as mentioned above. Various studies have shown that it is the thickness of the cortical section that plays a more important role.29–31 As far as the distribution of force over the body of the screw is concerned, FEM analyses have shown that the load is applied only in the region of the cortical bone.32–33 When selecting the length of the screw, the depth of the gingiva must also be taken into account, with an average layer depth of 1.25 mm. Thus, the ratio between the length of the head (the part of the screw outside the bone) and the length of the threaded section (the part of the screw inside the bone) should be at least 1:1. Poggio et al.22 recommend lengths of 6 to 8 mm. Costa24,25 suggests miniscrews with a length of between 6 and 10 mm. Based on these stud- ies, it would appear that it’s not nec- essary to use longer screws. This has been confirmed by numerous clinical studies. Easy identification of length and diameter through colour-coding of the screws can be accomplished by means of anodisation, using for example, Ortho easy (FORESTA- DENT). AD AD Fig. 6: The stress resistance (fracture level in Ncm) depends on the diameter of the miniscrew (according to Kyung, modification by the authors). Figs. 7a, 7a: Interradicular X-ray image showing spatial ratios. f OT page 6 7a. 7b.