Complications of trochanteric fracture fixation
View/ Open
Date
04/07/2015Author
Goffin, Jerome Moon Ho
Metadata
Abstract
SHS vs. Gamma Nail
Using FE analysis, the behaviour of the Gamma nail and the SHS was
compared in an osteoporotic bone model for the fixation of three- and fourpart
trochanteric fractures (31-A2 in the AO classification, types IV and V in
Evans’ classification). The size of the medial fragment was varied based on
clinical data and the case of a fractured greater trochanter was also considered.
Results show that for Evans’ type V stabilized with a Gamma nail and for
Evans’ types IV and V with the SHS, cancellous bone around the lag screw is
susceptible to yielding, thus indicating a risk of cut-out. The volume of bone
susceptible to yielding increases with an increase in size of the medial fragment.
On the other hand, Evans’ type IV with a Gamma nail is not predicted to cut
out. These findings suggest that future clinical trials investigating fixation
of unstable proximal fractures should include the size of the medial fragment
and the integrity of the greater trochanter as covariables and be powered to
evaluate whether intramedullary devices are superior to SHSs for Evans’ type
IV fractures and inferior/equivalent for type V.
PFNA
Because of the contradictory body of evidence related to the potential benefits
of helical blades in trochanteric fracture fixation, the e↵ect of bone compaction
resulting from the insertion of a PFNA was evaluated.
A subject-specific computational model of a trochanteric fracture (31-A2 in
the AO classification) was developed with lack of medial support and bone
density was varied to account for variability in bone properties amongst hip
fracture patients.
Results show that for a bone density corresponding to 100% of the bone density
of the cadaveric femur, there does not seem to be any advantage in using a
PFNA with respect to the risk of blade cut-out. On the other hand, in a
more osteoporotic femoral head characterized by a density corresponding to
75% of the initial bone density, local bone compaction around the helical blade
provides additional bone purchase, thereby decreasing the risk of cut-out, as
quantified by the volume of bone susceptible to yielding.
These findings indicate benefits of using a PFNA over an intramedullary nail
with a conventional lag screw and suggest that any clinical trial reporting
surgical outcomes regarding the use of helical blades should include a measure
of the femoral head bone density as a covariable.
TAD and Screw Position
Using a simple mathematical formulation, the relationship between the position
of the lag screw tip (relevant to both intramedullary and extramedullary
devices) and the concept of tip-apex distance (TAD) was derived. TAD is
widely used in operating theaters as a surgical guideline in relation to the fixation
of trochanteric fractures and in clinical studies as a predictor of lag screw
cut-out. In order to visualize better this concept, the locus of points having
the same TAD was plotted and TAD variations as the location of the lag screw
tip was varied were described. Findings show that TAD should be adjusted
for the size of the femoral head (a variable which varies a lot according to
the sex of the patient) and that no correlation exists between TAD and bone
morphometry indices obtained from μCT data (BV/TV and Tb.Th).
Therefore, these results seem to suggest that TAD lacks mechanical justification
and that predictors which are based on mechanical properties, such as
bone density, should be investigated further.
Cut-out and Screw Position
The biomechanical performance of a CT scan-based three-part trochanteric
fracture model (31-A2 in the AO classification) stabilized with a SHS was
compared for nine di↵erent positions of the lag screw (3 x 3 arrangement,
from anterior to posterior and from inferior to superior).
Results show that the volume of bone susceptible to yielding in the head and
neck region is the lowest for inferior positions and increases as the lag screw
is moved superiorly. Overall, for this specific subject, the models less likely
to lead to cut-out are the ones corresponding to inferior middle and inferior
posterior positions of the lag screw. In this study, TAD was anti-correlated
with the risk of cut-out, as quantified by the volume of bone susceptible to
yielding, which suggests that a TAD > 25 mm cannot be considered to be an
accurate predictor of lag screw cut-out.
Further clinical studies investigating lag screw cut-out should attempt to find
more reliable predictors of cut-out that should better reflect the biomechanics
and subject-specificity of the femoral head.
Plasticity Formulations
Using an FE model of unstable trochanteric fracture stabilized with a SHS,
the benefits of two plasticity-based formulations, Drucker-Prager and crushable
foam, were evaluated and compared to the commonly used linear elastic model
of trabecular bone in order to predict relative risk of lag screw cut-out for
five distinct load cases. The crushable foam plasticity formulation leads to a
much greater strain localization, in comparison to the other two models, with
large plastic strains in a localized region. The plastic zone predicted with
Drucker-Prager is relatively more di↵use. Linear elasticity associated with a
minimum principal strain criterion provides the smallest volume of elements
susceptible to yielding for all loading modes. The region likely to undergo
plastic deformation, as predicted by the linear elastic model, is similar to that
obtained from plasticity-based formulations, which indicates that this simple
criterion provides an adequate estimate of the risk of cut-out.