Summary
Unintentional over-tightening of orthopaedic screws resulting in loss of screw purchase
(“screw stripping”) is a potential complication during fracture fixation. This report
describes the microstructural and radiographic effects on bone caused by stripping
a 2.7 mm orthopaedic screw, and subsequent replacement in these stripped screw sites
with either a 3.5 mm cortical or a 4.0 mm cancellous orthopaedic screws in both tapped
and untapped holes. Over-tightening of a 2.7 mm screw resulted in shearing of the
bone directly engaged by the screw threads. Additional fractures occurred in surrounding
bone, primarily superficial subperiosteal saucer fractures in the cis cortex and to
a lesser degree, in the subendosteum of the trans cortex. Bone damage was consistently
more severe in the cis cortex. Salvaging stripped screw sites by insertion of a 3.5
mm or 4.0 mm screw resulted in additional fracturing and displacement, which was more
severe in the untapped sites. Decreased holding power of screws placed in stripped
screw sites could be attributed to collateral structural bone damage occurring during
screw stripping and replacement screw application.
Over-tightening of orthopaedic screws until mechanical failure (“screw stripping”)
results in damage to cortical bone which extends beyond the outer thread diameter
of the screw. Damage, most often subperiosteal saucer fractures, was greater in the
cis cortex. The subperiosteal saucer fractures decreased the functional length of
screw engagement of the larger replacement screws in the cortical bone, which may
account for decreases in expected holding power.
Keywords
Canine - screws - screw stripping - fracture repair
