A variety of operative techniques have been used in treating cats with fractures of the long bones with similar diameters such as the humerus, femur and tibia. Bone plating has been the method of choice in treating radial fractures, as the radius is curved, has a narrow oval shaped marrow canal and a neighboring axial joint. Bone plates, external fixation, marrow nails and paracortical fixation have been employed in the treatment of bony shaft fractures in cats.
The aim of this study was to develop a titanium interlocking-nail that can be placed with minimal equipment and is appropriate for treating shaft fractures of the long bones in cats. The nail diameter should be uniform, or only slightly vary so that it can be used in treating humerus, femur and tibia fractures. The implant should only vary in length, to accommodate the differences in bone length between small and large cats.
To accomplish this goal, it was necessary to obtain data concerning the length, extent, narrowest points and widest points of the free marrow cavities as well as the extent of the spongiosa cavities of the feline humerus, femur and tibia.
Biomechanical testing was used to compare the stability of the interlocking-nail to a more traditional DCP-plate that is often used in treating femoral fractures in cats. The stability of the constructs was evaluated in a controlled fashion, using both right and left femurs. The femur was selected for biomechanical testing as it is the most common long bone fractured in cats.
Preliminary results showed that the intact femur could withstand an average of 3 Nm of torque, which was more than the fracture-model constructs (DCP 1.16 Nm; interlocking-nail 1,3) could bear. Deformation compensation was reached at 10 degrees of rotation for intact femurs, at which point a fracture ensued, most commonly in the form of an oblique fracture.
The femur/implant constructs were exposed to extreme torsional stresses, up to five cycles of rotation up to 30 degrees. Implant failure, screw failure or bony failure was not detected in femur/implant constructs (DCP and interlocking nail.)
Clear differences between the constructs were noted during torsion studies. In contrast to the DCP/ femur constructs that had regular hysteresis curves, the interlocking-nail / femur constructs had irregular hysteresis curves, especially during cycles 1 and 2.
Torsional stiffness is defined as the linear portion (slope of the regression) of the curve generated when torque (in Nm) is plotted against rotation (angel in °). This is a measure for evaluating the stability of a construct and the biomechanical influence of the interlocking-nail or plates is stabilizing fractur sites. The constructs used in this study (DCP and Interlocking-nail) did not significantly differ in respect to torsional stiffness (p>0,05). The cycle did, however, influence torsional stiffness.
In this study, the titanium nail and nail/femur constructs demonstrated an increased mean stiffness when respectively compared to the DCP and DCP femur constructs (p<0,05). The LD F2 analysis showed a clear effect.
During the concluding evaluation of the testing materials, no macroscopic damage to the bone in the region of the screws was detected. Neither the DCP steel plate nor the titanium nails used in this study showed signs of damage or deformation in the region of the osteotomy site. The titanium interlocking-nail developed in this study is best suited for diaphyseal fractures of the humerus, femur, and tibia.
1. Leo Brunnberg und Helmut Waibl Versorgung von Humerusfrakturen bei der Katze Kleintierpraxis, 47, 473-481 (2002)
2. Leo Brunnberg, Franck Forterre, Stephan Kaiser, Michael Burger und Helmut Waibl Unterschenkelfrakturen bei der Katze
3. Stephan M. Perren Evolution of the internal Fixation of long bone Fractures
4. J. Bone Joint Surg (Br) 2002; 84-B: 1093-110