Date of Award

January 2014

Document Type

Thesis

Degree Name

Medical Doctor (MD)

Department

Medicine

First Advisor

Seth D. Dodds

Subject Area(s)

Surgery, Biomechanics

Abstract

The aim of this in vitro study was to develop an understanding of the instability associated with scaphoid waist fractures, and secondly, to evaluate scaphoid fracture fixation with two commonly utilized headless compression screw designs. We evaluated inter-fragmentary motion, rather than compression alone, in scaphoids maintained within their native wrist joint following two fracture patterns and after internal fixation with a fully-threaded and a partially threaded screw design. Eight fresh frozen cadaveric upper extremity specimens were obtained and dissected of their superficial soft tissues from forearm to metacarpal head. Each specimen's scaphoid was subjected to a transverse waist fracture, followed by a 3mm volar based wedge osteotomy. The wrists were tested with intact scaphoids, with the two fracture patterns, and following internal fixation with both screw types. The positions of the scaphoid fragments were tracked in real time using an optical motion-tracking system during wrist flexion and extension under physiologic loading. Transverse waist fractures resulted in significant inter-fragmentary rotational motion along the scaphoid central axis, while the addition of volar bone loss was associated with increased flexion/ extension and radial/ulnar deviation rotational motion. Internal fixation with both screw types provided marked reduction in inter-fragmentary motion when compared to the un-fixed simulated volar-wedge fracture, with up to a 92% restriction in rotational motion in the flexion/ extension scaphoid rotational axis. In the pronation/ supination rotational axis, the fully-threaded screw yielded less inter-fragmentary motion with maximal loading when compared to the partially-threaded compression screw (1.28 degrees vs. 1.62 degrees, respectively; p = 0.005). Both screw designs allowed for no more than 0.88mm in translational motion in a single plane following fixation. Significant inter-fragmentary instability was noted following scaphoid waist simulated fractures with and without volar bone loss. Internal fixation with a headless, variable pitch compression screw provides biomechanical stability in a simulated scaphoid fracture model with bone loss. While the fully-threaded screw may have a biomechanical advantage over the partially-threaded screw, the absolute difference in inter-fragmentary motion between the designs is relatively small.

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