The method of choice for treating shaft fractures has been internal fixation with a bone plate. Fracture healing time is described by Brinker et al. (1997), and ranges from 1-14 months, depending on the age of the dog. Plating systems such as the conventional semitubular plate, the DCP (Dynamic Compression Plate, Allgöwer et al., 197 1) or the newer LC-DCP (Low Contact-Dynamic Compression Plate((Perren et al., 1983, Gautier et al., 198)) have established themselves as implants. Clinical experience and animal experiments have shown that bone plates cause structural changes ' in the form of cortical pressure necrosis and result in decreased mechanical quality of the underlying bone (Eitel et al., 1988, 1989, 1990a, 1990b; Peterhofen, 1989). Microangiographic experiments conducted on the canine radius by Rhinelander (1968) showed that pressure between the bone plate and bone result in avascularity of the underlying bone. The phenomenon of pressure necrosis beneath the plate (Perren et al., 1983) led to the development of plating systems with a reduced contact area such as the ZESPOL-system (Ramatowsli, 1979), the LC-DCP (Perren et al., 1988) and the PC-Fix (Perren and Buchanan, 1995). Radiographic documentation of various phases of fracture healing with different plate contact areas (Mönch, 1988; Remiger et al., 1994, Perren, 1991) demonstrate that a minimal contact area results in early, stable fracture healing and a more homogenic bone structure (van Frank Hassnoot, 1995; Baumgaertel et al., 1991; Heitemeyer et al., 1991, Claudi and Oedekoen). A monocortical plating system that is angularly stable and does not contact the underlying bone (comparable to an internal fixator) optimizes the biological aspects of fracture healing, and is a promising alternative that allows undisturbed callus formation under the plate. The NoN-Contact-Plate (NCP) is a bone plate constructed from stainless steel. So far, the NCP has been available as a 1.2 cm wide, 0.4 cm thick bone plate in various lengths and with different numbers of plate holes, for use with 4.5 mm screws. A negative cortical thread is cut into each plate hole and is compatible with 4.5 mm self-tapping screws. A high, positive conical thread is cut into the necks of the screws, which only engage the threads of the plate hole. The thread is higher in the neck of the screw (but the pitch is the same as that of the shaft) and distances the plate from the bone when the screws are tightened. Engagement of the screw with the plate enables the construction to be angularly stable. A 2.5mm spiral bit, two threaded drill sleeves, a depth gauge and a screwdriver are required for installing the NCP plate. The drill sleeves are screwed into the plate hole, followed by pre-drilling a hole with a cortical drill-bit. The drill sleeves are then removed, the depth of the canal is measured" and 4.5 mm self tapping cortical screws are tightened until the head of the screw engages the plate and the shaft is anchored in one cortex of the bone. The holes are prepared and filled on alternating sides of the fracture line.
Animals
We have used the NCP in patients with fractures of the tibia, radius and ulna, humerus, and with corrective osteotomies and femur pseudoarthrosis. The NCP has been used in 80 patients: fractures of the humerus, femur, tibia/fibula fractures, a femur pseudoarthrosis and a corrective osteotomy of the radius/ulna. The course of treatment was documented from implantation to removal of the implant with post-removal controls. With the exception of a corrective osteotomy performed on a Doberman, all of the patients suffered traumatic fractures incurred in traffic accidents, during play, being kicked by horses or falls. All of the patients were surgically treated 24 to 48 hours after the fracture occurred.
Internal fixation
Attention was given to the usual points of fracture management such as: vascularity of the fracture and maintaining the fracture hematoma. The fractures were treated with a NCP secured with 4.5mm self-tapping monocortical screws. Six to twelve screws were implanted in the bone, depending on the length of the limb. The patients received 25mg/KG Cephalexin (Cefaseptin) IV, before and after operative repair. The skin sutures were removed 10 days post-operative. Radiographic and clinical follow-up examinations were conducted in short time spans (circa 14 days), because a novel method was being implemented. The owners were counseled to limit the activity of the dogs (leash walks, cage rest) for 5 weeks following surgery.
Results
The postoperative condition of the patients was unremarkable. The limbs surgically treated tentatively bore weight 1 day after surgery and were weightbearing 4 days post operative. Callus formation was visible on radiographs made 20 days after surgery. The fracture line was not radiographically evident 21-28 day after surgery. All of the dogs developed a visible callus bridge during healing, which was more distinct in the trans cortical face and did not encompass the plate. All of the patients had vital tissue under the plate, especially the periosteum. Radiographic monitoring showed that the longest healing time was 4 months, and that the shortest healing time was 1 month. The average healing time was 2.5 months, which is half of the time needed for healing following internal fixation with the DCP (Brinker et al.,1988). Compared to conventional bone plates, the vitality of the periosteum was preserved. In every instance the tissue underneath the plate was vital and showed undisturbed circulation. The NCP is secured in one face of the cortical bone and without contacting the bone, compared to more established plates that are secured in both cortical faces and have epiperiostal contact to the bone. It can be assumed that the NCP allows for the microinstability that is necessary for biological aspects of fracture healing. Due to these results, we believe that the NCP is the most suitable plate for repairing canine fractures of the diaphysis. The NCP offers advantages compared to more conventional plates. The healing time is substantially decreased due to secondary fracture healing. The biological aspects of fracture healing are improved, perfusion of the fracture fragments, fracture hematoma and periosteum are maintained. A reduction of bone quality underneath the plate does not occur. The variation in screw length is reduced, and eliminates the need for keeping a stock of screws of widely varying lengths, as is the case with bicortical plating systems. By engaging the threads of the screw with the threads of the bone plate, an absolutely stable internal fixation is achieved. After pre-drilling with a 2.5 mm spiral drill, the self-tapping screws can be anchored in the compact bone near to the plate.
References
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