Our 2.0mm stacked hole locking plates can be used either with our 2.0mm locking screws or with 2.0mm cortical/cortical self-tap screws. In common with all locking plate constructs, when used with locking screws, precise contouring to the bone surface is not required. Leaving a small gap between plate and bone helps minimise damage to the periosteal blood supply. Closely positioned holes (6mm between centres) makes these plates very useful for smaller fragments.
Standard DCP or Round Hole Plates are held into position by being trapped between the head of the screw and the bone. Tightening the screw pulls the bone up to the plate. When the screw is tight the threads of the screw pull against the bone holding the plate in position. The situation with locking screws and plates is very different. Although the threads of the screw shaft engage the bone, the interface between bone and screw is not related to attachment of the plate. In locking plates and screws it is the interface between the screw head and the plate which attaches the plate to the screw. As the screw is tightened the bone maintains its position relative to the plate, it is not drawn up to it. The screw head engages with the plate. This has a number of implications:
Contouring of Plates
With standard DCPs the plate must be contoured exactly. When the screws are tightened the bone fragments will be pulled towards the plate and assume the contour of the plate. If the plate is not contoured correctly the screws will pull the fragments out of alignment. Using a locking plate and screws once the plate is applied the bone fragments will be held in position relative to one another as is the case with external fixation. Indeed locking plates are sometimes referred to as ‘internal, external fixation’. Locking plates do not therefore require contouring in the same way as DCP type plates. This has particular relevance in the TPLO procedure where contouring the plate is very important and very time consuming. If a Standard Locking TPLO Plate fits all TPLO osteotomies there are significant savings in terms of time and morbidity.
Screw Angulation and Numbers
In order to fit and lock into the plate the screw must be inserted at a fixed angle relative to the hole in the plate. Drill Guides are provided which screw into the locking holes to ensure that this happens. It is not always desirable that the screws are at 90 degrees to the plate, particularly close to joints. The angle of the screw is dictated by the plate not the surgeon. The rigid attachment of the screw to the plate gives any locking construct a high degree of angular rigidity compared to a DCP construct where a spherical screw head in an oval hole results in a relatively flexible construct. This means in any given situation that to achieve the same rigidity as a DCP construct a locking construct needs fewer screws. Again this has particular implications in terms of time and morbidity. It also offsets, to a degree, the higher costs of locking screws and plates.
Screw Diameter and Strength
The increased angular rigidity places greater stresses onto the screw. Because we are not relying on the threads of the screw in the bone to
pull the plate to the bone they do not need to be as coarse as standard Cortical Screws. They need, only, to hold their position in the bone.
Thus for the same outside diameter of screw we can increase the core diameter without increasing the likelihood of bone/ screw interface failure. Increasing the diameter of the core hugely affects the AMI (resistance to bending) of the screw. The AMI of standard 3.5 cortical screws as used in SOP™ is 1.6, the AMI of standard locking screw is 2.6. A SOP™ screw is far more likely to fail than is a standard locking screw. Screw failure is the typical mode of failure of SOP™ constructs. Standard Cortical Screws are not designed for use as Locking Screws.
Minimally Invasive Plate Osteosynthesis (MIPO)
MIPO aims to interfere with the patient’s natural response to fracture as little as possible while establishing and maintaining stabilisation of the fracture. Locking plates do not need close contouring to the bone, indeed it is not necessary to have contact between the plate and the bone. The periosteum is therefore preserved and the fracture is not disturbed. Additionally the angular stability of locking screws coupled with the need for fewer screws are desirable features in any MIPO procedure. Locking Screws were developed for osteoporitic bone where Standard DCPs and Screws tend to strip out. The bone screw interface is under far less stress using locking technology. This is an important quality in juvenile bone also. Veterinary MIPO candidates tend to be young.