OTA 2005 Posters

Scientific Poster #59 Basic Science

Biomechanical Analysis of Plate and Screw Fixation for Full Crutch Weight Bearing of Humeral Shaft Fractures: Synthetic Bone and Cadaveric Testing

R.V. O'Toole¹; Romney C. Andersen¹; O. Vesnovsky; M. Alexander³;

T. Topoleski²; Jason W. Nascone¹; M.F. Sciadini¹; Clifford Turen¹;

W.A. Eglesder¹ (n-all authors);

¹R. Adams Cowley Shock Trauma Center, University of Maryland Medical Center, Baltimore, Maryland, USA;

²Department of Mechanical Engineering, University of Maryland, Baltimore County, Baltimore, Maryland, USA;

³University of Maryland Medical Center, Baltimore, Maryland, USA

Purpose: Fractures of the humeral shaft are treated operatively to allow full upper extremity weight bearing on crutches when lower extremity weight bearing is restricted. Traditionally, large 4.5-mm LC-DC plates have been used. In contrast, our institution has been using smaller 3.5-mm LCP with locking screws. The purpose of this study was to compare the biomechanical properties of 4.5-mm plates with 3.5-mm plates for weight bearing in humeral shaft fractures.

Our second purpose was to investigate whether there is any advantage in using locking screws instead of nonlocking screws in this clinical scenario.

Methods: We modeled a comminuted midshaft humerus fracture. Synthetic humeri were plated posteriorly over a 1-cm midshaft transverse gap. There were three study groups: 4.5-mm LC-DC plates (nonlocking screws), 3.5-mm LCP (nonlocking screws), and 3.5-mm LCP (locking screws). Stiffness and failure testing was conducted. Ten matched-pairs of fresh-frozen cadaveric humeri were harvested, radiographed, and then had DEXA scans performed to ensure similarity between matched pairs. Two groups were compared: 3.5-mm LCP (nonlocking screws) and 3.5-mm LCP (locking screws), both with a 1-cm midshaft fracture gap. Stiffness, fatigue, and failure testing was performed.

Results: For the synthetic bones, the 4.5-mm LC-DC plate was significantly more stiff in anteroposterior, medial-lateral, axial, and torsional testing than either of the 3.5-mm plate constructs (ANOVA, P< 0.001, n=6 per group). There was no significant difference in stiffness between the 3.5-mm plates with locking and with nonlocking screws. In initial failure testing of the constructs in axial loading as might be experienced with the use of crutches, all three plate constructs failed well above anticipated physiologic loads (failure load for all three constructs >2900N). The cadaveric portion of the study has also demonstrated no advantage of locking screws over nonlocking screws for stiffness or failure testing. The two groups were similar in anteroposterior, medial-lateral, axial, and torsional stiffness and also showed no statistical difference in failure force (n= 10). Fatigue and failure testing has shown both constructs able to withstand strenuous fatigue testing (90,000 cycles, 2Hz, 440N, representing 3 months at 1000 cycles per day), and to fail at forces above anticipated clinical loads.

Conclusions/Significance: This study is the first to analyze plate fixation of humeri with an emphasis on axial loading as is likely experienced in crutch walking. Synthetic bone and cadaveric testing both demonstrate that 3.5-mm plates are probably adequate for full weight bearing and that locking screws appear to offer no obvious biomechanical benefit in this application.