Biomechanical testing of the LCP--how can stability in locked internal fixators be controlled?
Karl Stoffel, Ulrich Dieter, Gwidon Stachowiak, André Gächter & Markus Kuster
New plating techniques, such as non-contact plates, have been
introduced in acknowledgment of the importance of biological factors
in internal fixation. Knowledge of the fixation stability provided
by these new plates is very limited and clarification is still
necessary to determine how the mechanical stability, e.g. fracture
motion, and the risk of implant failure can best be controlled. The
results of a study based on in vitro experiments with composite bone
cylinders and finite element analysis using the Locking Compression
Plate (LCP) for diaphyseal fractures are presented and
recommendations for clinical practice are given. Several factors
were shown to influence stability both in compression and torsion.
Axial stiffness and torsional rigidity was mainly influenced by the
working length, e.g. the distance of the first screw to the fracture
site. By omitting one screw hole on either side of the fracture, the
construct became almost twice as flexible in both compression and
torsion. The number of screws also significantly affected the
stability, however, more than three screws per fragment did little
to increase axial stiffness; nor did four screws increase torsional
rigidity. The position of the third screw in the fragment
significantly affected axial stiffness, but not torsional rigidity.
The closer an additional screw is positioned towards the fracture
gap, the stiffer the construct becomes under compression. The
rigidity under torsional load was determined by the number of screws
only. Another factor affecting construct stability was the distance
of the plate to the bone. Increasing this distance resulted in
decreased construct stability. Finally, a shorter plate with an
equal number of screws caused a reduction in axial stiffness but not
in torsional rigidity. Static compression tests showed that
increasing the working length, e.g. omitting the screws immediately
adjacent to the fracture on both sides, significantly diminished the
load causing plastic deformation of the plate. If bone contact was
not present at the fracture site due to comminution, a greater
working length also led to earlier failure in dynamic loading tests.
For simple fractures with a small fracture gap and bone contact
under dynamic load, the number of cycles until failure was greater
than one million for all tested constructs. Plate failures
invariably occurred through the DCP hole where the highest von Mises
stresses were found in the finite element analysis (FEA). This
stress was reduced in constructions with bone contact by increasing
the bridging length. On the other hand, additional screws increased
the implant stress since higher loads were needed to achieve bone
contact. Based on the present results, the following clinical
recommendations can be made for the locked internal fixator in
bridging technique as part of a minimally invasive percutaneous
osteosynthesis (MIPO): for fractures of the lower extremity, two or
three screws on either side of the fracture should be sufficient.
For fractures of the humerus or forearm, three to four screws on
either side should be used as rotational forces predominate in these
bones. In simple fractures with a small interfragmentary gap, one or
two holes should be omitted on each side of the fracture to initiate
spontaneous fracture healing, including the generation of callus
formations. In fractures with a large fracture gap such as
comminuted fractures, we advise placement of the innermost screws as
close as practicable to the fracture. Furthermore, the distance
between the plate and the bone ought to be kept small and long
plates should be used to provide sufficient axial stiffness.
Stoffel K, Dieter U, Stachowiak G, Gächter A, kuster m.
Biomechanical testing of the LCP--how can stability in locked
internal fixators be controlled?. Injury 2003; 34 Suppl 2:B11-9.
journal paper/review (English)
date of publishing
Injury (34 Suppl 2)
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