OTA 2008 Posters

Scientific Poster #35 Basic Science OTA-2008

Biomechanical Effect of Insertion Orientation on Trochanteric Entry Nails

Daniel S. Horwitz, MD (a-Stryker Howmedica Osteonics);
Kathryn Schabel, MD (a-Stryker Howmedica Osteonics);
Thomas F. Higgins, MD (a-Stryker Howmedica Osteonics);
Erik Kubiak, MD (a-Stryker Howmedica Osteonics);
University of Utah, Salt Lake City, Utah, USA

Objectives: Techniques for performing intramedullary nailing of femur fractures continue to evolve, and the relative ease of use of trochanteric entry nails has led to broadened indications and popularity in use for diaphyseal fractures. The history of trochanteric entry nails leads to concern about hoop stress created by placing an unforgiving implant that is straight in the anteroposterior plane into the curved, intact proximal femur, and subsequent catastrophic failure. The purpose of this biomechanical study was to analyze trochanteric nail orientation, diameter, and cortical thickness as they relate to catastrophic failure of the proximal femur .

Methods: 10 intact cadaver femora were reamed and instrumented with trochanteric entry intramedullary nails (Stryker Howmedica) of progressive diameter until either a 15-mm diameter nail was inserted or catastrophic failure occurred. Each femur started with a 9-mm nail reamed to 11 mm, and continued with 11-mm, 13-mm, and 15-mm nails, each overreamed by 2 mm. Each nail was inserted 25 cm in two orientations, first with the anterior bow oriented medially (jig anterior), and then with the bow anterior (jig parallel to the floor). Insertion was performed by a single experienced surgeon. Visual catastrophic failure was noted, and strain gauges were used to confirm catastrophic failure.

Results: Two femurs fractured at 11 mm, five fractured at 13 mm, and three tolerated 15- mm nails with no fracture. 6 of 10 fractured with the bow of the nail apex anterior and 1 of 10 with apex medial orientation (jig vertical) while inserting a 13-mm nail. These data indicate inserting trochanteric nails with bow anterior orientation has a higher probability of causing intraoperative fracture (P = 0.027, Fisher exact test). At the time of fracture, a ratio of cortical thickness to canal diameter of 0.5 (± 1 standard deviation) was observed and the intact femurs had significantly higher cortical thickness to canal diameter ratios (P = 0.002). The ratios of cortical thickness to medullary canal diameter were significantly different between the femora that fractured and those that did not (P = 0.002).

Conclusion: Orienting the anterior bow medially during trochanteric entry nailing of femur fractures by lifting the jig can decrease the chance of intraoperative proximal femur fracture. Preoperative measurements of femoral cortical thickness and canal diameter are useful measures for determining optimal nail size and predicting risk of fracture.

If noted, the author indicates something of value received. The codes are identified as a-research or institutional support; b-miscellaneous funding; c-royalties; d-stock options; e-consultant or employee; n-no conflicts disclosed, and *disclosure not available at time of printing.

• The FDA has not cleared this drug and/or medical device for the use described in this presentation   (i.e., the drug or medical device is being discussed for an “off label” use).  ◆FDA information not available at time of printing. Δ OTA Grant.