General Session: Tumor, Trauma, Infection, Image

Presented by: R. O'Toole - View Audio/Video Presentation (Members Only)

Author(s):

E. Jazini(1), N. Klocke(2), O. Tannous(1), H. Johal(1), J. Hao(2), D. Gelb(1), J. Nascone(1), R. O'Toole(1), B. Bucklen(2), S. Ludwig(3)

(1) University of Maryland, Department of Orthopaedics at Shock Trauma, Baltimore, MD, United States
(2) Musculoskeletal Education and Research Center (MERC) , A Division of Globus Medical, Inc., Audubon, PA, United States
(3) University of Maryland, Baltimore, MD, United States

Abstract

Purpose: Large forces during normal pelvic ring motion (i.e. bending forward during bilateral stance) often demand trans-sacral screws to improve fixation following fracture. However, no biomechanical studies have evaluated fracture stability with one versus two screws, the latter of which cannot always be placed clinically. Therefore, this study aimed to compare differences in biomechanical stability observed when one versus two trans-iliac, trans-sacral screws were placed for minimally and highly comminuted zone II, class C-type sacral fractures with anterior pubis symphysis disruption.

Methods: Fresh-frozen cadaveric lumbopelvic specimens (n=5) were instrumented using one trans-sacral screw (T1), followed by two trans-sacral screws (T2) after fracture gap creation. A zone II vertical fracture was modeled first by using an oscillating saw through the right sacral foramen, then by using rongeurs to enlarge the fracture to a sizable gap (10mm). All tested constructs had an anterior pubis symphysis plate attached, once the pubis symphysis had been resected with a scalpel. Once modeled, immediate postoperative biomechanical stabilization during bilateral stance was simulated by flexion-extension (FE) and axial rotation (AR) with a "floating hip model." Pure bending moments (≤8Nm) were applied at L4; the right hemipelvis was permitted 5 degrees-of-freedom, and the left was fully constrained. Relative Eulerian motion between left and right iliac crests for pelvic ring motion, across the intended sacral fracture zone, and between L4 and the constrained hemipelvis (lumbopelvic), was assessed with the Optotrak motion capture system. Motions were normalized to each specimen's individual intact range of motion and then were averaged; paired student t-tests revealed significant differences (p≤0.05) between the two clinically pertinent presentations of these fracture types.

Results: Overall, average normalized motion was reduced by the use of a second trans-sacral screw. Sacral fracture zone stability was not significantly reduced with two screws (FE=233.9±202.2%, AR=259.5±126.0%) versus one screw (FE=348.5±298.3%, AR=347.6±227.6%, p>0.05). Non-significant differences between the numbers of screws used were found for pelvic ring stability (T1: FE=134.5±94.0%, AR=77.1±45.6%; T2: FE=114.1±98.1%, AR=90.9±76.4%, p>0.05) and overall lumbopelvic motion (T1: FE=100.1±9.2%, AR=149.8±85.9%; T2: FE=108.2±25.6%, AR=143.8±83.6%); only during axial rotation did these values approach significance (p=0.07) for lumbopelvic motion.

Conclusion: Two-screw methods showed improved biomechanical rigidity; however, this study was limited by the number of specimens examined. Across the sacral fracture zone in both FE and AR, a trend toward increased stability with two screws was noted. Yet, although the pelvic ring was the most stabilized region of interest of the three investigated, overall stability remained statistically equivalent across both constructs and motion types, suggesting that compression provided by even one trans-sacral screw across the comminution supplies stabilization.