DIAG IM3 - Page 66

15Internal 3 Dental Equipment Fixation & Instrumentation Implant Designs and Specifications www.vetinst.com SELF-TAPPING OR NON SELF TAPPING ? For many years, veterinary orthopaedics have been complicated by the use of two incompatible plating systems: AO/ ASIF type cortical screws plus compression type plates, and Sherman type screws plus Sherman and Venables type plates. The two systems differed both in their metallurgy and their strength. All our implants are now made from stainless steel of the latest human specification, exceeding DIN 17443 and ISO 5832/1 (all are versions of stainless 316LV). This is the same specification material used by other manufacturers. In addition, where international standards have been applied to screw threads e.g. 3.5 cortical screws (DIN 58812) our implants meet these standards. It is now safe to use implants from a variety of sources provided they meet the above specifications. All implants made in Europe must be compatible with each other. Round hole plates may be used with cortical screws. Only corticocancellous screws with spherical heads can be used to achieve compression using compression plates. There is a significant trend towards the use of self tapping screws over the traditional pre-tapped type. This trend is occurring in both veterinary and human orthopaedics. It takes roughly twice as long to drill, tap and insert a traditional screw compared with a self tapper. Any saving in time reduces morbidity and saves money. These savings are most pronounced in lengthy procedures involving long plates and multiple screws. Self-Tapping Cortical Screws After many years of being advised by AO that self-tapping screws were unacceptable, it now seems that they are fine and are supplied as selftapping cortical screws. The pull out resistance is unaffected provided the self-tapping flutes sit outside the bone. In our view long holes in hard bone should be pre-tapped e.g. a trans-condylar screw in the mature humerus or a position screw through the calcaneus.Veterinary Instrumentation’s cortical kits are now available with self-tapping cortical screws or regular cortical screws which require a tap. Self-tapping screws are particularly suited to the proximal tibia where cortices are thin. Most self tapping screws are therefore used in the TPLO and TTO procedures. Self tappers are les s suitable for use in hard cortical bone or dense bone such as the calcaneous or the distal humerus. This is not surprising in that the short self tapping flutes on the screw are less efficient at cutting threads than a purpose designed tap. Once the short cutting flutes on the screw tip are full of bone debris the tapping becomes inefficient, increasing the risk of splits. Implant Codes To minimise confusion regarding ordering, we have given our implants codes which accurately describe the item in question. For example: C2706052 is a compression plate for 2.7 screws which has 6 holes and is 52mm long. It is suggested that as a new set up, the surgeon opts for a self tapping kit. However, taps should also be purchased and used in appropriate situations. e.g. calcaneous, distal humerus and distal 1/3 of the tibia. The thread form of both screws is identical and the self-tapping threads will follow a pre-tapped pilot hole. To further assist in decision making and ordering, templates are printed on pages 349 to 376 which show the exact shape of all the plates available. Each outline is described by its own code. Acrylic overlays of these templates for use during fracture planning with radiographs are available free of charge from Veterinary Instrumentation. Which screw? Hexagonal S elf-tapping or needs a tap To acheive the same pull out resistance as standard screws it is important that the self tapping flutes sit outside the trans cortex. AO type Self Tapping Cortical. All have a CSST prefix followed by the diameter and length of screw, e.g. CSST2718 is an 18mm 2.7 self-tapping cortical screw. The self tapping effect of the screw may be achieved in different ways. AO type screw, cortical or cancellous Prefix is CS followed by the diameter and length of screw, e.g. CS3520 is a 20mm 3.5 cortical screw. Cortical self tappers have 1-3 self tapping flutes cut into the tip of the screw to cut a thread as the screw is inserted. HEAD TYPE Cancellous screws have a ‘pigtail’ tip which does not cut through cancellous bone but rather expands and eases the screw through, minimising damage to the fragile bone. There is one important consequence to this. In the proximal tibia during the TPLO procedure, if the pilot hole for the longest screw penetrates the proximal fibula and a full length cancellous screw is selected then the ‘pigtail’ will not be able to penetrate through what is effectively three trans cortical layers and will jam resulting in either a loose screw or possibly the head shearing off. Either 2.0 or 2.7 (7/64”) Sherman type self-tapping screw. All have ST prefix followed by the length of screw, e.g. ST2012 is a 12mm 2.0 self-tapping screw. Slotted S ingle slot or cross head (Philips type) 3.5 (9/64”) Sherman type self-tapping screw. Prefix is ST35 followed by the length of screw, e.g. ST3519 is a 19mm 3.5 self-tapping screw. 50