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 fix ed 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 inve stigate whether any possible mechanical deficiencies of the CP would detrac t 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-o steotomy (with and without medial metaphyseal bone defect) stabilized eithe r with the CT or the DCS. 8 pairs of human cadaveric femora classified acco rding 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 th e plate). Physiological loading was simulated and measurements were taken a t 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 vertica l branch of the osteotomy. There was no relevant difference in the mechanical properties of the two fi xations for fractures without medial defect, even if the stability of the f ixation was reduced by removing the distal screw Furthermore, inter-fragmen tal movement was minimal. Ln 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). Re-moving th e distal lag screw did not increase the instability. Even in osteoporotic b ones, 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 re duced in osteotomies with medial defect. The amplitude of inter-fragmental 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 reduc e stability. However, the absence of the lag screw within the implant consi derably weakend the fixation - more so for the CP than for the DCS. Instabi lity reached a maximum without any lag screw at all, which again was more p ronounced 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 dista l condylar block must be at least 4 cm in length.