2.2.2 Cancellous bone screws
Cancellous bone screws are available in 6.5 mm and 4.0 mm diameters. The 6.5 mm screw has three different thread lengths—16 mm, 32 mm and fully threaded (Fig. F2D).
F2D: Cancellous bone screws are available as fully threaded or in two different thread lengths: 16 mm & 32 mm.
Only the partially threaded cancellous bone screws will produce interfragmentary compression at the fracture site using a single-sized drill bit. If interfragmentary compression is desired then it is important to choose the proper length thread combination so that the threads are located only within the trans cortex/ cancellous fragment (Fig. F2E). Cancellous bone screws should only be used in soft cancellous bone since it may be impossible to remove them from hard cortical bone without breaking them.
Fig. F2E: When inserting a cancellous bone screw, the threads must only engage the trans cortex if it is to act as a lag screw.
Fig. F2F: In this figure, two cancellous bone screws inserted in lag fashion are illustrated.
Only the partially threaded cancellous bone screws will produce interfragmentary compression at the fracture site using a single-sized drill bit.
Cancellous bone screws should only be used in soft cancellous bone .
To insert a large cancellous bone screw a 3.2 mm or 3.6 mm hole (hard cancellous bone) is drilled across the entire bone. A 6.5 mm tap is then used to tap the thread into the bone and measure the length of the screw to be used. The tap does not cut the entire 6.5 mm thread, leaving some uncut bone to be cut by the screw itself at the time of insertion; therefore, the torque necessary to insert a cancellous bone screw will be greater than that of a cortex screw and much greater in hard bone than in soft bone (Fig. F2F).
Screws should be placed perpendicular to the long axis of the bone if weight bearing loads are to be expected.
2.3 Screw position
Screws are designed to provide purchase in bone that will be advantageous in fracture fixation. They are designed to be loaded in tension and not in bending or shear. Interfragmentary compression is greatest when the forces on the surfaces of the fragments are normal (perpendicular to the fracture plane). To accomplish this, the screw must be placed perpendicular to the fracture planes in all directions (Fig. F2G). This means that when a fracture spirals, the screws used to fix this fracture must spiral as well. The loads experienced by the reduced and stabilized fracture will be those imposed by the screws and by the loads associated with use (i.e., postoperative weight bearing). Vector analyses will show that the loads of weight bearing change the resultant forces (loads) through the bone so that simple fixations become far more complex systems when subjected to weight bearing (Fig. F2H). As a rule of thumb, screw placement should be perpendicular to the long axis of the bone if weight bearing loads are to be expected. Screws placed perpendicular to the fracture plane will be subjected to shear forces during weight bearing. A decision on screw placement may represent a compromise if the screw itself will induce large shear forces independent of weight bearing forces. In these cases, the screw should be placed at an angle between perpendiculars drawn to the fracture plane and to the along axis of the bone. In all instances, the screw placement should spiral in the longitudinal plane in concert with the fracture (Fig. F2G).
Normal forces on the fragment surfaces create the greatest compression.
Fig. F2G: To efficiently create interfragmentary compression, the screws must be placed perpendicular to the fracture plane in all three dimensions.
Fig. F2H: Weight bearing loads can result in shear forces at the fracture site and displacement of the fragments if the screws are inserted perpendicular to the fracture plane.
2.4 Plate fixation
A variety of plates are available for use in internal fixation. Recent introductions of new plate designs with new materials have been utilized in many animal species including humans; the most commonly used plate in equine orthopedics, however, continues to be the dynamic compression plate (DCP) fabricated in stainless steel. This is available in two cross-sectional dimensions, the narrow and broad configuration. The narrow DCP has the screws placed in a straight line while the broad plate has the screws placed in a staggered configuration. The sectional properties of the plates, especially around the screw holes, determine their strength and fatigue resistance. The larger cross-sectional dimensions of the broad plate make it the choice for most applications. The dynamic condylar screw (DCS)- and corresponding dynamic hip screw (DHS) plates have correspondingly larger cross-sectional dimensions and would therefore be stronger than the broad DCP. They are only manufactured, however, with the large sliding screw at one end which limits their application in the horse. Choosing the proper length plate for use in any specific situation may represent a dilemma based on soft tissue viability, surgical approach, and configuration of the fracture. While four cortex screws are recommended as a minimum on each side of the fracture with each screw threaded into both cortices (eight cortices) it is best to plate most equine long bone fractures from end to end. Two or more plates are often used in the repair of fractures in the horse. These plates should be placed at right angles to each other to optimize the inertial properties of the fixation. Specific recommendations regarding plate size and number are addressed with individual fractures and arthrodeses as they are presented in later chapters.
DCP = Dynamic compression plate
DCS = Dynamic condylar screw plate
DHS = Dynamic hip screw plate
Self-compressing DCP:
1 Drill 3.2 mm hole 1 cm from fracture site
2 Measure depth
3 Tapping with 4.5 mm
4 Insert first screw loosely, displace plate toward fracture
5 Drill 3.2 mm hole on opposite side using yellow load guide
6 Measure depth
7 Tapping with 4.5 mm
8 Insert second screw
9 Tighten first screw 10. Apply other screws
Drill the first hole approximately 1 cm from the fracture plane.
2.4.1 Plate application
The technique for application of a plate will be described for the dynamic compression plate (DCP) as a self-compressing plate and with the tension device as used in certain circumstances, such as with a dorsal plate arthrodesis of the metacarpal phalangeal joint.
2.4.2 Self-compressing DCP
Video SCDCP