TOA
Comparison of Load to Failure and Calcar Strain for Two Press-Fit
Femoral Stem Implantation Techniques in Matched Cadaver
Femora During Simulated Impaction
1
Kurtz ,
2
Murphy ,
3
Timmerman ,
3
Nambu ,
3
Brownhill ,
3
Roark ,
3
Moseley
William
Stephen
Irina
Satya
James
Michael
Jon
1) Tennessee Orthopedic Alliance, Nashville, TN 2) Center for Computer Assisted and Reconstructive Surgery – New England Baptist
Hospital, Boston, MA 3) Wright Medical Technology, Inc, Arlington, TN
INTRODUCTION
MATERIALS AND METHODS (continued)
RESULTS (continued)
A tight press-fit condition, coupled with the
wedge-like geometry of a hip stem (Figure 1),
puts substantial hoop stresses into the
proximal femur when the stem is impacted
into prepared bone.
Depending on the degree of press-fit and
the size and quality of the patient’s bone, this
can lead to intra-operative fracture of the
femur.
A recently developed technique for less
invasive implantation utilizes a modified
approach to prepare the femoral canal in-situ
in which the femoral head is not removed until
after the stem has been implanted.
• Twenty four hours prior to testing, specimens were
thawed and cut off six inches below the distal tip of
the stem and potted vertically inside a 3” diameter
aluminum cylinder 3” high using dental acrylic.
• Two hours prior to testing , two rosette strain
gauges (Omega, gage L = 1 mm) were adhered to
the calcar region. (Figure 2)
• For femurs prepared with the traditional technique
one gage was placed below on the medial calcar,
5mm below the resection line and the other was
placed at the same level and 10mm anterior to the
first gage.(1)
• Distances from the gages to the lesser trochanter
were measured on the traditionally prepared femur
and used to place the gages on the SUPERCAP®
femur. (Figure 3)
• The construct was secured in a servohydraulic load
frame (Mini-Bionix , MTS Inc.) using machine
clamps.
• A custom ram was used to apply load to the
proximal surface of the hip stem at a rate of 25
mm/sec.
• The ram was offset 5mm proximally from the
superior flat of the stem to simulate impact loading.
• Load, displacement, and strain data were captured
at a rate of 200 Hz.
• The maximum load to failure, principle strains for
the medial and antero-medial gage locations, and
failure mode were determined for each femur.
• Maximum bone stress was calculated from the peak
von-Mises strain using a value of 17.5 GPa for the
elastic modulus of cortical bone.
• The maximum load and corresponding
principle stresses are summarized in Table 1.
• The mean difference in peak force between
traditional and SUPERCAP® groups was
1,660N (p = 0.01, two sided t-test, and p =
0.027, Wilcoxon signed rank test).
To compare the load to failure and calcar
stress and strain between traditional femoral
preparation and implantation and the less
invasive technique using matched pairs of
cadaver femora.
MATERIALS AND METHODS
• Ten matched-pair cadaveric femurs were
imaged to screen for signs of severe
osteoarthritis, deformity, tumors or existing
proximal hardware.
• Specimen pairs were randomly assigned to
either the traditional or less-invasive
SUPERCAP® technique.
• Femurs were clamped into a custom frame,
reamed and progressively broached following
the surgical protocols for both techniques.
• The femoral head was resected for the
traditional technique prior to femur
preparation, but was left in place for the lessinvasive technique.
• Fluoroscopy was used to verify final broach
size and alignment prior to impaction of the
modular, plasma-sprayed stem (PROFEMUR®
Z, Wright Medical Technology, Arlington, TN).
• The same size stem was used for both sides of
each matched pair.
• Following implantation, the specimens were
wrapped, enclosed in plastic bags, and frozen.
http://www.wmt.com
Figure 2: Specimen prepared using
the traditional surgical technique.
Strain gages on the medial and
antero-medial surfaces are shown.
Mean
Peak
Force: N
Peak
Force
Range: N
Mean
Peak
Strain:
SUPERCAP® Traditional
Delta
8,479
(3,793)
6,813
(3,532)
1660
(1620)
4,568 to
16,988
1,940 to
13,923
-471 to
3,961
3,451
(2,790)
2,451
(1,895)
748
(2,212)
DISCUSSION
3500
Stress (Mpa)
PURPOSE
Property
The force required to fracture the femur
was higher for the SUPERCAP® group, which
suggests that removal of the femoral head
increases the chance of an intraoperative
fracture.
In general, the peak principle strain
correlated with the peak force measurements
(r = 0.573), but the differences in strain
between experimental and control groups
were not statistically significant (p>0.05).
Figure 3: Specimen prepared using
All constructs fractured with vertical splits
the SUPERCAP® surgical technique.
initiating from the proximal end of the stem.
With the traditional technique, fractures
RESULTS
occurred more often on the medial or antero• In general the peak load to failure was greater for the larger size stems. (Figure 4)
medial area, with the split initiating at the
• Seven of the pairs had higher peak loads for the SUPERCAP® femur (delta range: 116 – 3961N),
resection line.
three had higher peak loads for the control technique (delta range: 45 – 470N)
The fractures in the SUPERCAP® group were
5000
90
more variable in location, with only one
Traditional
4500
80
fracture
initiating
antero-medially
and
none
on
SuperCap
4000
70
the medial side.
Force (N)
Figure 1: PROFEMUR® Z modular, plasmasprayed stem component
Table 1: Average Peak Loads and Strain
3000
2500
2000
1500
1000
CONCLUSIONS
60
50
40
30
20
500
10
0
0
1
2
3
4
5
6
7
Displacement (mm)
8
9
10
0
Traditional
SuperCap®
• Figure 4: (Left) A force vs displacement plot of a typical specimen pair. Sudden drops in
force are indicative of bone fracture. (Right) The maximum stress recorded by the strain
gages in the calcar region of the proximal femur.
The use of a less invasive, in-situ technique for
implantation of the femoral stem resulted in a
clinically and statistically significant increase in
the force required to fracture the femur. This
implies that the in-situ technique should
reduce the chances of intraoperative fractures
due to stem impaction.
REFERENCES
1.
Elias JJ et al, “Medial cortex strain distribution during
noncemented total hip arthroplasty” Clin Orthop Rel
Res 2000 (370):250-8.