Abstract
Objective To quantify the amount of the screw head thread and the plate hole thread connection
in two 3.5 mm locking plates: Locking Compression Plate (LCP) and Polyaxial Locking
System (PLS).
Materials and Methods A micro - CT scan of a screw head - plate hole connection was performed pre- and
post destructive tests. Tests were performed on bone surrogates in a fracture gap
model. The 3.5 LCP and 3.5 PLS plates, with 3 perpendicular screws per segment were
used in a destructive static test. The 3.5 PLS plates with mono- and polyaxial screws
were compared in a cyclic fatigue tests in two orthogonal directions. Pre - and post
- test scan datasets were compared. Each dataset was converted into serial images
depicting sections cut orthogonally to locking screw axis. The amount of engagement
was detected through automated image postprocessing.
Results The mean amount of the thread connection for the LCP was 28.85% before and 18.55%
after destructive static test. The mean amount of the connection for the PLS was 16.20%
before and 14.55% after destructive static test. When inserted monoaxially, the mean
amount of the connection for the PLS screws was 14.4% before and 19.24% after destructive
cyclic test. The mean amount of the connection for the polyaxial inserted PLS screws
when loaded against plate thickness was 2.99% before and 2.08% after destructive cyclic
test. The mean amount of the connection for the polyaxial inserted PLS screws when
loaded against plate width was 3.36% before and 3.93% after destructive cyclic test.
The 3D visualization of the thread connection showed that the initial interface points
between screw head and plate hole are different for both LCP and PLS after the destructive
testing. Depending on the type of applied force, there was either loss or increase
of the contact.
Clinical Relevance Micro-CT offers news possibilities in locking implant investigation. It might be
helpful in better understanding the nature of locking mechanism and prediction of
possible mode of failure in different systems.
Keywords
micro-ct - locking screw - locking plate - polyaxial - LCP
