BIOMECHANICAL PERFORMANCE OF AN ANGLE STABLE PLATE SYSTEM AT THE DISTALHUMERUS

Abstract

Introduction: Because of strong loads acting in the elbow joint, intraarticular fractures with a methaphyseal comminuted fracture site at the distal humerus demand a lot from the osteosynthetic care.

Ambiguities arise concerning to the anatomic position of the implants and the resulting mechanic performance.

Aim of this study was the comparison of three anatomic variations of one angle stable plate system as to their mechanic stability.

Material and Methods: As a fracture model an AO C 2.3-fracture on an artificial bone (4th Gen. Sawbone) was simulated via double osteotomy in sagittal and transversal plane. The fractures were equipped with a prototypical version of the Variax Elbow System (Stryker) in the variations 90° (lat+post), 90° (med+post) and 180° (med+lat).

A physiological load distribution (Capitulum Humeri 60%, Trochlea humeri 40%) could be guaranteed for by a therefore designed test set up. In three test series, the load to failure (static), the system rigidity (static) and the median fatigue limit (dynamic) were determined. The tests were conducted under 75° flexion and 5° extension and the relative displacements were recorded.

Results: In extension, the 180° (med+lat) alternative achieved the highest load to failure (2959 N), stiffness (1126 N ± 127 N) and median fatigue limit (1046 N ± 46 N) followed by the 90° (lat+post) alternative.

Great differences could be stated with the 180° (me d+lat) alternative in extension in comparison to the flexion (p< 0,05): under flexion the failure already appeared at 1077N and the stiffness reduced to 116 N ± 10 N. The highest stiffness (202 N ±19 N) under flexion load could be determined for 90° (med+post). As to stiffness, the 90° (lat+post) alt ernative lay in between. Decreases of fracture gaps due to a failure of screw bone interface and a bending of plates could be determined as failure patterns in case of static load. Under dynamic load especially fatigue fractures occurred at the implant system in terms of broken plates and screws.

Conclusion: In vivo the highest loads occur at the distal humerus in extension direction which can best be transferred, in static as well as in dynamic regard, by a 180° alternative. An alternative to that is the 90° (lat+post) variation due to its advantageous me chanic performance under static and dynamic extension load. But still the nature of fracture with size and position of the fragments useable remains decisive for the choice of an osteosynthesis.

The mechanic superiority of the 180° alternative (minimized gap displacement and high stiffness of the system respectively) in extension direction in comparison to a 90° alternative can be explained by the 90° position of the plates and hence reduced moment of inertia. Less stiffness under flexion direction arises from the long levers, which cause high bending moments.