We review a novel system of fracture repair – the ‘string of pearls’ or SOP plating system, currently produced by Orthomed.
fig. 1 - SOP-plates come in a range of sizes
This UK company started producing the system for the veterinary market three years ago, and has since gone on to produce kits in a variety of sizes; 2.0, 2.7 and 3.5mm (see fig. 1). The name derives from its appearance; screws are placed into round ‘pearls’ and these are separated by cylindrical internodes, the ‘string’ (fig. 2). What follows is a brief introduction to the system. It’s far from exhaustive but we hope it gives an idea as to their design and application.
The SOP system is a subtle modification to the tried and tested theory of Locking Plates, which themselves carry a number of advantages over conventional plates such as DCPs and reconstruction plates with which we are more familiar. Conventional plates require the screws to squeeze the plate down on to the bone surface in order to achieve a rigid construct. This means that firstly, the bone stock has to be of sufficient quality to prevent the threads stripping when the screw is tightened (no good for osteoporitic bone) and secondly, the plate has to be accurately contoured to the surface it is to be applied to. This can be time consuming and at times, exasperating to get exactly right.
fig.2 - detail of SOP-pate construction
Locking plates overcome these two issues by locking the screw head into the plate itself, thus providing a point of rigid fixation at the screw/plate interface. As the screw is locked to the plate, it does not require the screw threads to pull the plate onto the bone to achieve stability. In this sense the system might be viewed as an ‘internal – external skeletal fixator’! This in turn means the plate does not have to be exactly contoured to the bone as it will sit just off the surface.
So, is this just a short-cut for lazy surgeons? Contouring is still required, but without needing to exactly fit the plate to the surface of the bone, comminuted fractures as well as those involving irregular bone surfaces can be repaired significantly more rapidly, minimising the associated risks of prolonged operative times. In addition, the organising fracture haematoma and periosteal blood supply of the bone can be preserved, allowing for minimally invasive repairs and biological healing. Furthermore, where anatomical reconstruction of the fracture is obligatory (e.g. articular fractures) , the action of tightening the screw to squeeze a conventional plate to the bone may sometimes result in disruption of a perfect reduction.
fig.3 - ventrodorsal radiograph of right ilial shaft/ acetabular fracture in a cat.
fig.4 - lateral view of same fracture
fig.5 - SOP-plate fitted to lateral aspect of ilial shaft extending over dorsal aspect of acetabulum.
It is reasonable to ask: ‘If we’ve already got a locking plate system, why do we need another?’ The first locking plates were modified conventional plates with specially cut screw-holes. However, these require dedicated ‘locking screws’. The SOP uses regular AO screws which are secured via interference fit. Secondly, long, flat plates are easy to contour in one plane, but very difficult to contour in others. The cylindrical internode design, or ‘string’ of the SOP plate allows contouring in multiple planes very easily(see figs 3-6). In addition, the mechanics of the system mean the plate remains very strong even if screw holes are left empty.
fig.6 - lateral view of repair
Is this the answer to all our fracture repair needs? Sadly not, as there are a few limitations inherent to the SOP system. For example, the plate will not allow ‘static loading’ of screws to compress a fracture gap in the way a DCP can (useful to minmise callus formation or help with delayed/ non-unions). Also to
fig.7 - craniocaudal radiograph of plate-rod repair of right femur
be effective in long bone fractures the SOP may require the addition of an intramedullary pin (plate-rod, fig 7). As a result, the SOP’s primary use in long bone fractures is with comminuted fractures as a ‘buttress plate’, i.e. when the fracture cannot be reconstructed and there will be no load sharing through the bone. Some surgeons will prefer to use an ESF in these situations for speed of application and familiarity of use, however, soft-tissue irritation and implant-associated infections are not uncommon where long bones are involved, especially in the larger patient. In our experience, the
fig.8 - mediolateral view of comminuted humeral condylar fracture/ luxation
SOP-plate comes in very handy where a plate is required, but exact contouring is difficult, for example pelvic fractures and the epicondyles of the humerus (see figs. 8-10).
As with all techniques, there is a learning curve and new instrumentation to
fig.9 - intraoperative view of elbow repair; there is a lag-screw visible cranial to the plate
purchase, and some of the conventional attitudes to screw and plates need to be revised when using the SOP. Overall though, it is an invaluable system to have, either as a primary method of repair, or just as importantly as the
fig.10 - post-op radiograph of implants in place
‘Plan B’ when intra-operatively things don’t go quite the way we want them to!
