Original Article

Complex Distal Humerus Fractures—Comparison of Polyaxial Locking and Nonlocking Screw Configurations—A Preliminary Biomechanical Study

Hungerer, Sven MD^*; Wipf, Felix MSc^‡; von Oldenburg, Geert MSc^‡; Augat, Peter PhD^*; Penzkofer, Rainer PhD^*

^*Institute of Biomechanics, Trauma Center Murnau, Murnau, Germany and Paracelsus Medical University, Salzburg, Austria; and

^‡Stryker Osteosynthesis, Biomechanics Groups, Schönkirchen, Germany and Selzach, Switzerland.

Reprints: Sven Hungerer, MD, Institute of Biomechanics, Trauma Center Murnau, Küntscher Strasse 8, 82418 Murnau, Germany (e-mail: [email protected]).

F. Wipf is an employee of Stryker Osteosynthesis, Switzerland, and G. von Oldenburg is an employee of Stryker Osteosynthesis, Germany, and both receive regular salary. P. Augat receives or has received research support and consultant honorariums from the following companies: Stryker Osteosynthesis, Smith & Nephew, Aesculap, and Arthrex. The Institute of Biomechanics, Murnau, received financial funding for the implants and research work from Stryker Osteosynthesis, Germany, for the study. For the remaining authors, no conflicts of interest were declared.

Accepted May 22, 2013

Journal of Orthopaedic Trauma 28(3):p 130-136, March 2014. | DOI: 10.1097/BOT.0b013e31829d19a4

Abstract

Objective:

The investigation hypothesized that in current anatomical precontoured plates, angular stability plays only a minor role for the efficacy of the osteosynthesis at the distal humerus.

Methods:

An AO C2.3 fracture model was simulated and osteosynthesis performed with plates positioned in parallel. System rigidity and median fatigue limit were analyzed in artificial bones and the cycles to failure in cadaver specimens. Loads were applied in anterior–posterior direction (75° flexion) and axial direction (5° flexion). Four composite bone groups were investigated as follows: (1) 2.7 mm polyaxial locking screws, (2) 3.5 mm polyaxial locking screws, (3) 3.5 mm polyaxial locking screws and a gap bridging screw, and (4) 2.7 mm nonlocking screws. Two cadaver groups were investigated with 3.5 mm diameter polyaxial locking (5) versus nonlocking screws (6).

Results:

There were no differences in stiffness found between the locking versus nonlocking constructs in artificial (1) versus (4) and in cadaver bones (5) versus (6). The larger screw diameter of 3.5 mm in combination with a gap bridging screw significantly increased construct stiffness by 25% (3). The median fatigue limit was significantly increased using larger screw diameters (2) and a gap bridging screw (3). In cadaver bones, the polyaxial locking screws constructs (5) resisted higher peak loads and more cycles until failure compared with nonlocking constructs (6).

Conclusions:

System stiffness increases with larger screw diameters and becomes significant with additional gap bridging screws in artificial bones. The use of polyaxial locking screws in anatomical adapted plates becomes more important in poor bone quality.