Strength Evaluation of Proximal Femora with a Lateral Defect
After Explantation of Orthopaedic Devices
1Russell, TA; +2Cartner, JL; 2Zheng, Y1Campbell Clinic, Memphis,
TN, 2Smith & Nephew, Inc., Memphis, TN
[email protected]
INTRODUCTION:Compression hip screws (CHS) and Blade Plates have
been
successful in treating intertrochanteric hip fractures. CHS may
remainthe preferred treatment for stable intertrochanteric
fractures [1]. Axial and rotational stability is important for
adequate fixation. A new device,InterTAN CHS, which marries CHS
with two integrated interlocking screws, provides resistance to
axial and rotational loading while maintaining reduction and
strength (Figure 1a). The implant, however, leaves a larger
explanted defect in the femur as compared to predicate devices. The
purpose of this study was to evaluate the structural strength
effect of a lateral defect created during implantation of this new
device using both finite element analysis (FEA) and
biomechanicalcadaveric testing.
METHODS:Finite Element Analyses
Three-dimensional CAD models cortex-only femora were reworked to
form lateral defects simulating screw insertions per the
corresponding surgical technique for InterTAN CHS, Classic CHS
plate, and the TC-100 Blade Plate, respectively. A ten-node
modified quadratic tetrahedron element FEA model was set up in
ABAQUS/CAE Version 6.6-1. Linear elastic isotropic material
properties were assign to the bone models.
FEA analyses were restricted to the proximal region of the
implant at a distance equal to 101.6 mm (4 inches) below the tip of
the lesser trochanter. Models were fixed distally with all
translational degrees of freedom being constrained. A unit load was
applied to the femoral head through its center in and orientation
of 10 degrees lateral and 10 degrees posterior.
(a) (b)Figure 1: Intertan CHS (a) device, and (b) test
set-up.
Biomechanical Cadaveric TestingSeven cadaveric femora were
implanted with an Intertan CHS device.
The matched contralateral side was implanted with either a
Classic CHS Plate (n = 3) or a TC-100 Blade Plate (n = 4) (Smith
& Nephew, Inc., Memphis, TN). Next, the implants were removed,
leaving a lateral defect corresponding to the shape of the implant.
The proximal femur was resected distally and potted at a distance
equal to 101.6 mm (4 inches) below the tip of the lesser
trochanter.
Each femur was offset 10º in flexion and 10º in adduction [2]
and loaded through a hemispherical polyethylene cup attached to a
multidirectional bearing plate (Figure 1b). Each specimen was
loaded at a constant rate until failure. Failure was defined as a
decrease in load on the load versus displacement curve. The failure
location and peak load were noted.
RESULTS:Finite Element Analyses
FEA stress distributions among the three implants differed the
most at the lateral defect region (Figure 2). The peak stress of
the new Intertan CHS device was shown to be 44% lower than CHS and
68% lower than the Blade Plate for 10/10 axial compressive bending.
These finding indicate that for these testing conditions, the
proximal screw holes in the Intertan CHS system does not affect the
structural strength of the bone.
(a) (b) (c)Figure 2: Stress distributions for a 10/10
orientation of the (a) Intertan CHS, (b) CHS, and (c) Blade Plate
devices.
Biomechanical Cadaveric TestingUltimate load-to-failure values
in the 10/10 axial compressive
bending were not affected by the explantation lateral defect,
when the Intertan CHS was compared to the Classic CHS Plate or the
TC-100 Blade Plate. A two-tailed Student’s paired t-test indicated
no statistically significant difference between Intertan CHS and
contralateral implant type (p = 0.27 at α = 0.05 when compared to
the Classic CHS Plate, and p = 0.16 at α = 0.05 when compared to
the TC-100 Blade Plate). DISCUSSION:
The size of lateral defect left from explantation of the
Intertan CHSdevice did not detrimentally influence the overall bone
substrate mechanical strength when tested in a computational gait
model or in a biomechanical cadaveric model as compared to the CHS
and Blade Plate. The new Intertan CHS device functions to resist
rotational instability and should be considered clinically
acceptable when gait cycle loading is considered.
REFERENCES:[1] Baumgaertner et al. Intramedullary Versus
Extramedullary
Fixation for the Treatment of Intertrochanteric Hip Fractures.
CORR 348:87-94, 1998.
[2] ISO 7206-8. Implants for surgery -- Partial and total hip
joint prostheses -- Part 8: Methods of determining endurance
performance of stemmed femoral components.
Poster No. 797 • 55th Annual Meeting of the Orthopaedic Research
Society
