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Trends ORTHO TRIBUNE | MAy 20106 Measuring of the thickness of the mucous membrane A pointed sensor with an attached rubber ring is used to measure the thickness of the gingival tissue in the direction of insertion (Fig. 6). This information may be useful when determining the final length of the screw and possibly when inserting the miniscrew. When choosing the length, the bone repository and the thickness of the mucous membrane in the direction of insertion play a role; in the retromolar section of the lower jaw and in the palate, the thickness of the mucous membrane is often more than 2 mm. The part of the miniscrew inside the bone must be at least as long as the part outside the bone. The vari- ous dimensions must be taken into account. The thickness of the bone in the direction of insertion determines the required length of the minis- crew: • bonethicknessgreaterthan10mm: miniscrews with a length of up to 10 mm are to be used; • bonethicknesslessthan10mmand greater than 7 mm: miniscrews withalengthof8mmor6mmareto be used; and • bone thickness less than 6 mm: miniscrews cannot be used. The following guidelines aid in selecting the length: • in the buccal region of the upper jaw: 8 mm or 10 mm; • in the palatinal region (depending on the region): 6, 8 or 10 mm; and • in the lower jaw: usually 6 mm or 8 mm. Determination of the type of thread Self-cutting miniscrews require pre-drilling (also known as pilot drilling) appropriate to the length and diameter of the screw, as well as to the quality of the bone. A self- tapping miniscrew will find its own way into the bone and requires no pre-drilling (Figs. 7a, 7b). Bone is more or less elastic depending on site, age and struc- ture. However, the screw diameter, the thickness of the cortical bone and the hardness of the bone at the insertion site limit the extent to which this method can be used. Without pre-drilling, the bone will be strongly compressed during insertion and thus suffer a related tension stress. This may result in the cracking of the bone around the insertion site. When the screw is screwed into the bone, it is subjected to high loads. Depending on the bone qual- ity, the resistance against insertion and the continuity of the rotational movement, high torsional forces can result. In regions with thick cortical bone and a much looser bone structure (e.g. the upper jaw), the use of self- tapping screws is recommended. In regions where the cortical bone is thick and the bone structure is dense (e.g. the anterior lower jaw) both self-cutting and self-tapping screws may be used, in each case following perforation of the com- pact bone. Transgingival penetration The miniscrew must penetrate through gingival tissue, which must thus be perforated during insertion. Two methods are used for the per- foration of the gingival tissue: a) excision of the gingival tissue; or b) direct insertion of the screw through the gingival tissue. There are currently no published studies that investigate the effect of these two methods on post-opera- tive problems, histological effects and/or the loss rate of miniscrew. Preparation of the bone site Protection of the bone is an impor- tant aspect. Insertion without pre- drilling results in tensional stress within the bone, which may lead to post-operative complications. Particularly in the case of crestal- ly placed screws, bone displacement may result in a severe expansion of the periosteum. The thickness of the cortical bone, especially in the lower jaw, can have a significant effect on the torque of the screw. To ensure that the screw is not overloaded during inser- tion, the compact bone of the anterior lower jaw should be per- forated by pre-drilling as mentioned earlier. Pre-drilling should be done at a maximum of 1.500 rpm–1 , using a short pilot drill and water-cooling to reduce the risk of damaging the root (Figs. 8a, 8b). Insertion of the miniscrew The miniscrew must be removed from its sterile packag- ing (Fig. 9) or the work rack (Figs. 10a–d) without contamination. The thread of the screw may not be touched. The screw should be inserted at a constant rotational speed (at approximately 30 rpm–1 ) and with as uniform a torque as possible. Manual insertion Manufacturers supply various Fig. 9: Sterile miniscrew supplied in pin-holder (tomas-pin, DENTAURUM). Figs. 10a–d: Preparation of the work rack and removal of the blades. Figs. 11a–f: Preparation of the instruments and insertion of two miniscrews into the palate by machine. Fig. 12: Linking of the miniscrew to the orthodontic appliance. f OT page 5 g OT page 8 Fig. 10a Fig. 10c Fig. 11a Fig. 11c Fig. 11e Fig. 11b Fig. 11d Fig. 11f Fig. 10d Fig. 10b