Use of an Additional Nonlocking Screw in Olecranon Fracture Osteosynthesis Changes Failure Mechanism

Abstract

Hardware-related complications can occur when plate fixation is used to stabilize osteoporotic fractures involving the olecranon. The use of an additional nonlocking screw, placed retrograde into the proximal fracture segment, may improve stability under load. The purpose of this study was to conduct a biomechanical comparison of olecranon repair constructs with and without this additional retrograde screw. Nine matched pairs of elderly fresh-frozen cadaveric upper extremities were used. Two-part olecranon fractures were modeled, and fracture stabilization was performed. Olecranon plates were implanted either with the standard surgical technique (CTRL) or with an additional retrograde screw (EXPT). Dynamic extensions of increasingly loaded forearms were performed, and comparisons of sustained cycles, maximum load, and total work were made. Relative motion of bone segments was tracked, and modes of failure were assessed. Seventy-eight percent of specimens from the CTRL group failed due to relative fragment displacement exceeding 3 mm, while 78% of EXPT specimens failed due to instantaneous catastrophic failure. There were no significant differences in terms of number of survived cycles, maximum load, or work performed between the groups. The addition of a retrograde screw in this plating technique changes the failure mode from fracture displacement to catastrophic failure. The use of a 3.5-mm retrograde screw in the relatively small proximal ulnar fragment should be avoided, but screws with a smaller diameter may still have potential to improve fixation. Further biomechanical and clinical research is necessary to improve strategies for plate fixation of olecranon fractures in the elderly population. [Orthopedics. 2019; 42(1):e74-e80.].

Figure 1:

Fluoroscopic images of reconstructions with a standard repair method (A) and use of an additional retrograde screw (arrow) (B). The additional screw is directed out of axis of the intended vector for a locking screw and instead goes into the proximal fracture segment.

Figure 2:

Photographs of the biomechanical testing setup. Displacement of a steel cable attached to the triceps tendon creates a downward force, indicated by the arrow (A). The application of the force results in elbow extension (B).

Figure 3:

Schematic showing the steps taken to calculate diastasis. Marker clusters are attached to each body, and coordinate systems are assigned to the clusters (A). Virtual coordinate systems are created, relative to the proximal (Prox.) and distal (Dist.) ulna fragments, and placed directly on top of each other (B). Relative motion between the ulnar fragments moves the virtual coordinate systems, and the distance between them is calculated (C).

Figure 4:

Plots of measured diastasis showing steadily increasing diastasis between segments beyond 3 mm, which was a typical failure mechanism of the standard surgical technique group (A), and secure fixation of the ulnar fragments followed by catastrophic failure, which was the typical failure mode for the additional retrograde screw group (B). The arrow indicates the last cycle that was counted before failure.

Figure 5:

Survival plot showing the ultimate applied loads and failure mechanisms for each specimen (A). The means (solid lines) ± 1 standard deviation (clouds) of the average diastasis are shown as a function of applied load for each group. Standard deviations ceased to exist when only 1 specimen was remaining (B). Abbreviations: CTRL, standard surgical technique group; EXPT, additional retrograde screw group.

Figure 6:

Box and whisker plots showing the interquartile distribution of cycles sustained before failure (A), maximum load sustained by the triceps tendon before failure (B), and work performed against gravity before failure (C). There were no significant differences between groups. Abbreviations: CTRL, standard surgical technique group; EXPT, additional retrograde screw group.