The mechanics of internal fixation of fractures of the distal femur: a comparison of the condylar screw (DCS) with the condylar plate (CP).

Distal femoral fractures are rare and usually complex. Mostly, they are fixed with the Dynamic Condylar Screw (DCS) or the 95 degrees condylar plate (CP). The simplicity of applying the DCS compared with the CP led us to investigate whether any possible mechanical deficiencies of the CP would detract from its technical advantages, thus limiting the indications for its use in the treatment of fractures of the distal femur. An in vitro investigation was carried out to measure the stability of a Y-osteotomy (with and without medial metaphyseal bone defect) stabilized either with the CP or the DCS. 8 pairs of human cadaveric femora classified according to their bone density were used. CP and DCS were applied to 1 bone in each pair by means of three lag screws (anterior, posterior and through the plate). Physiological loading was simulated and measurements were taken at the level of the osteotomy in the frontal and sagittal planes in order to assess rotational instability and the amount of gap opening in the vertical branch of the osteotomy. There was no relevant difference in the mechanical properties of the two fixations for fractures without medial defect, even if the stability of the fixation was reduced by removing the distal screw. Furthermore, interfragmental movement was minimal. In the frontal plane, simulated closure resulted in closure of the medial branch of the osteotomy in every case without any opening of the vertical branch of the osteotomy. In the sagittal plane, the closure of all branches of the osteotomy was confirmed for 11 bones and a rotation of the condyle was observed in 5 bones (3 CP, 2 DCS). Removing the distal lag screw did not increase the instability. Even in osteoporotic bones, the DCS provided the same stability as the CP. For simple Y-osteotomies, the CP did not offer any technical or mechanical advantages. The stability in the frontal plane however was significantly reduced in osteotomies with medial defect. The amplitude of interfragmental movement on all bones fixed by the CP, except for 1 pair, was greater than those fixed by the DCS. The absence of the anterior lag screw did not reduce stability. However, the absence of the lag screw within the implant considerably weakend the fixation--more so for the CP than for the DCS. Instability reached a maximum without any lag screw at all, which again was more pronounced for the CP than for the DCS. The Dynamic Condylar Screw (DCS) must be regarded as the implant of choice both technically and mechanically even in osteoporotic bones, but the distal condylar block must be at least 4 cm in length.