COMPARATIVE BIOMECHANICAL STUDY OF 3 TYPE S OF OSTEOSYNTHESIS USED ONSUPRACONDYLAR AND INTERCONDYLAR FRACTURES OF THE DISTAL END OF THE HUMERUS IN ADULT

Purpose of the study Supra and intercondylar fractures are the most co mmon fractures of the distal end of the humerus in adult. An osteosynt hesis consisting of a plate is the treatment of choice. But location a nd type of plate always remain open for debate. The authors present th e results of an in vitro biomechanical study, which compared the stiff ness of three types of osteosynthesis commonly used in these fractures . Material and methods The devices were on one hand the premolded late ral plate of Lecestre and Dupont (Howmedica(R)) used alone or in conju nction with a medial 1/3 tubular plate of the AO group, and on the oth er hand a posterior plate, of which we are developping a new model, th e Lambda plate (Protek(R)) ''Y''-shaped, monoblock, flat and molded on the humerus during operation. The study compared these different meth ods of fixation on fresh human humeri. In a first part, the posterior plate was compared to the single lateral one; in a second part, the, p osterior plate was compared to the coupled lateral and medial plates. The three plates were made of identical material. Both studies used ei ght pairs of bones with supra and intercondylar fractures realized by sawing. Each bane of a pair was fixed with one of the two types of dev ice. Each humerus underwent different loading forces; sagittal bending (anterior and posterior) and torsion. The displacements were recorded using a calibrated measuring device coupled to two displacement senso rs. The stiffness was calculated on force/displacement curves. Results In anterior bending, the Lambda posterior fixation was significantly stiffer than the Single lateral one (p < 0.05) (239 +/- 109 versus 129 +/- 65 N/mm), and was not significantly different of the bilateral fi xation (229 +/- 93 versus 224 +/- 108 N/mm). In posterior bending, the mean stiffness of the Lambda fixation was not significantly different to that of the lateral fixation (91 +/- 27 versus 91 +/- 52 N/mm), an d less than that of the bilateral one (130 +/- 39 Versus 170 +/- 70 N/ mm), but not significantly. In torsion the mean stiffness of the Lambd a fixation was superior to that of the lateral one (146.75 +/- 50.66 v ersus 119.75 +/- 58.8 Nm/rad), and bilateral one (233.31 +/- 107.47 ve rsus 212.31 +/- 113.55 Nm/rad), but again not significantly. Discussio n The ideal osteosynthesis for the fractures of the humeral distal doe sn't exist, because the bane undergoes antero-posterior and posteroant erior cyclical forces during elbow flexion. Therefore the best device should be placed on both sides of the bone, but anatomical reasons mak e this location impossible. According to our study, we think the ''les s worst'' device is the posterior one using the Lambda plate. Its syme trical design allows a best loading distribution on the two columns. I ts thickness compensates for its posterior location and the short inte rval between two holes allows to put many screws (4 to 6) into the epi physis. All supra and intercondylar fractures, comminuted or not, can be treated with this material. The single lateral device isn't stiff e nough. Its stiffness mainly depends on the orientation of the oblique screw in the medial column. But the design of this column doesn't alwa ys allow for an optimal location of the screw, which is the reason of several failures. The bilateral device gives a stiffness, comparable t o the posterior one, but doesn't allow as many screws as the Lambda pl ate in the epiphysis. Therefore, it can't be used in very distal fract ures. Conclusion In spite of the progress of the material and the accu racy of the indications, the treatment of supra and intercondylar frac tures of the distal end of the humerus is always a difficult problem. The devices have to be as stable as possible to allow an early motion. The best one should be placed on both sides of the distal humerus, be cause of the sagittal cyclic forces it undergoes, but this location is anatomically impossible. Our study concludes that the device using th e Lambda plate is stiffer than the single lateral one and equivalent t o the bilateral one. This plate isn't a new concept, but its innovatio n lies in its physical specifications (thickness, short interval betwe en two holes), which allow for all supra and intercondylar fractures v ery stiff devices.