When a drill bit is used to drill a hole in bone the tip of the bit creates heat due to friction. High temperatures in bone (>54°C) may occur causing protein coagulation and bone necrosis. Temperature generation is inversely related to drilling rate when sharp drill bits are used. Increased pressure on the drill bit will increase the cutting rate and reduce bone heat generation but the introduction of bending to the drill bit by a surgeon pressing on the drilling machine may lead to drill bit breakage. Cooling of the drill bit is impractical since it has been shown that more than 500 ml/min of saline are necessary to adequately cool the bone [1]. Temperature control is possible however by using saline as a lubricant in the drill hole to decrease friction at the point of drilling. Much of the frictional heat generated during the drilling process is incorporated into the swath material. Periodic removal of this bony material from the flutes of the drill bit during the drilling cycle will decrease heat buildup and allow for further swath material accumulation. Impaction of swath material in the drill bit's flutes will decrease cutting rates of the drill bit since there will be nowhere for the cut bone to go. Saline should be supplied as a lubricant to the drill point at the time of drilling and can be placed into the drill hole when the bit is removed periodically for cleaning. Drill bits are designed to circulate fluid for lubrication. The fluid descends via the lands and ascends with the swath material (Fig. F2A).
Pretapping insures a good interface between bone and screw.
Pretapping of drill holes prior to screw insertion insures a good interface between the bone and screw. It also permits the screw to be inserted with less torque. Using saline for lubrication increases the ease of hole tapping and screw insertion as well. Screws without tapping flutes on their tip can easily be removed and reinserted during surgery without danger of cross threading the hole in the bone. Special care must be taken when using self-tapping screws in this regard. In equine bone self-tapping screws may not work well because the flutes of the tap may fill up before the entire cortex is penetrated, leading to imperfectly threaded holes and heat generation.
Fig. F2A: Lubricating fluid circulates along a drill bit by descending behind the lands and ascending with the swath material.
Avoid the use of self-tapping screws in thick cortices.
2.2 Screw types
There are two basic types of screws. The cortex screw is fully threaded, has relatively fine threads and is used in cortical bone and dense cancellous bone. It is the screw that is used most commonly in equine orthopedics. The cancellous bone screw is partially threaded and has a larger coarser thread. It is used in soft cancellous bone and may be used as a substitute for a cortex screw if the cortex screw has stripped the threads in the bone during insertion. Both screws are available in a wide variety of sizes and special large 5.5 mm cortex screws have been developed with the horse in mind.
2.2.1 Cortex screws
Fig. F2B: A fully threaded cortex screw is made to act in lag fashion by overdrilling the cis cortex and cutting threads only in the trans cortex.
A cortex screw is fully threaded and can be used as a fixation position screw with threads holding in both cortices to attach a plate to a bone. It can also serve as an interfragmentary screw that compresses two fragments together by drilling a glide hole in the near cortex (cis cortex) while providing a threaded hole in the far cortex (trans cortex) (Fig. F2B). Cortex screws are available in a large range of sizes with the 3.5 mm, 4.5 mm, and 5.5 mm diameters used most commonly in the horse. Testing of the various screw sizes has been carried out and the results show that in general screw strength is related to its core diameter [2]. Therefore, increased strength comes with larger screw diameters. When soft bone is encountered, the large-diameter screw threads hold better. In general, equine bone is so dense that cancellous bone screws are rarely needed. Therefore when dealing with a stripped screw hole it would be better to replace the stripped screw with a cortex screw of a larger diameter rather than substituting it with a cancellous bone screw.
Cortex screws used as lag screws require a glide hole.
Larger-diameter screws provide better holding power in soft bone.
To use a cortex screw as a lag screw it is necessary to use a large drill bit, equivalent to the outside diameter of the screw thread, to drill the glide hole through the cis cortex while a smaller drill bit, equivalent to the approximate core diameter of the screw, is needed to drill the smaller threaded hole into the trans cortex. For each cortex screw size, drill bits of the proper diameter are available to drill both the glide and threaded holes; see table (Fig. T2A).
Fig. T2A
The techniques for insertion of lag screws are shown in Video P1LAGSCR and in the animation Video DBASICS. After reduction of the fragments using bone clamps, K-wires, or some other device, a large glide hole is drilled through the cis cortex using a drill guide to prevent drill bit wobble and to protect the overlying soft tissues. This hole must be drilled across the fracture plane which may include cancellous bone as well as cortical bone, especially near the metaphyses. A drill insert is then placed into the glide hole and pushed across the fracture plane. This guide will center the thread hole precisely. The thread hole is drilled through the trans cortex using this drill guide along with the appropriate diameter drill bit. The drill bit and drill guide are then removed and the hole is countersunk to provide a seat for the head of the screw. It is important to turn the countersink tool a full 360° in a clockwise direction to avoid any ridges that would be created by simply oscillating the instrument back and forth. The countersink depression should only be deep enough to support the head of the screw and prevent its bending during tightening. Next the hole is measured with the depth gauge to determine the proper screw length; then it is tapped. Tapping of the thread hole is accomplished by inserting the tap through the tap sleeve into the glide hole and cutting threads in the trans cortex. The tap should be advanced into the bone by using two half turns forward and then one quarter turn back to keep the bone cuttings clear of the cutting edges. If the tap starts to bind, cutting should stop so that the tap can be removed and cleaned before continuing. The tap must be maintained free of all swath material in its longitudinal grooves in order to cut satisfactorily. Finally, a screw of the proper diameter and length is chosen and inserted using the screwdriver. As mentioned above, saline used as a lubricant will decrease the torque occasioned by tapping and screw insertion.
The glide hole should extend just beyond the fracture plane.
Rotate the countersink through a full 360°.
Power tapping is convenient, but can quickly lead to breakage or stripping.
Video P1LAGSCR: Techniques for insertion of lag screws.
Video DBASICS: Introduction to drill basics.
Although power drilling is recommended for drilling holes in bone, power tapping is reserved for applications where many screws are to be used in an internal fixation, as with a long plate or a double plating procedure. Special attention should be paid to directing the tap in the same plane as the drill hole. If the tap misses the hole in the trans cortex it may jam or break or the cis cortex threads may be stripped. When inexperienced