Summary
This study determined microdamage associated with external fixation pin insertion
and short-term cyclical loading, using an ex vivo ovine tibial model. Orthofix tapered blunt-tipped 3.5/4.5 mm fixation pins and Apex®
self-drilling self-tapping fixation pins, 4 mm in diameter, were used. After insertion,
the constructs were either loaded in cantilever bending or not loaded. Constructs
were then bulk-stained in basic fuchsin, and calcified sections were made. The sections
were reviewed qualitatively and the microcrack surface density (Cr.S.Dn, μm/mm2) was quantified at the pin-bone interface. The pattern and quantity of microdamage
induced was significantly influenced by fixation pin design and cortical region within
the cisor transcortex, but not short-term cyclical loading. Overall, Cr.S.Dn was significantly
increased with use of the Orthofix fixation pin (P < 0.01). Cr.S.Dn was also increased
in the medial cis-cortex, compared with the lateral transcortex (P < 0.05). Diffuse
damage within the medial cis-cortex was higher with the Orthofix pin. In contrast,
the Howmedica Apex® fixation pin caused fracture of the periosteal region of the lateral
transcortex, but relatively little microdamage within the medial cis-cortex. In this
model as a consequence of pin insertion, fixation pin design had significant specific
damage effects on both the medial cis-cortex and the lateral trans-cortex
Keywords
Microdamage - external fixation pin - cyclical loading