The Effects of the Femoral Component on Cortical Bone Shape Following Total Hip Arthroplasty
+Rosenbaum, T G; +Ashrafi, S; +Lester, K; +**Bloebaum, R D
+Bone and Joint Research Lab, SLC VA Health Care System, **Dept of Orthopedics, University of Utah, Salt Lake City, UT
INTRODUCTION: Femoral components may affect the magnitude
and direction of loads placed on the proximal femur. Altered loads may
result in stress-shielding, which can lead to bone resorption and
mechanical implant loosening. Previously, finite element analysis
models have qualitatively predicted in vivo cortical bone shape changes
such as stress shielding and point effect1, 2, and quantitively predicted
bone density changes due to the femoral component3. Bilateral human
femur studies have also quantified bone mineral changes between the
implanted and unimplanted femurs4, 5 but only qualitatively describe
changes in cortical bone shape5. To improve predictions based on
implant design, studies should demonstrate the bone response due to the
presence of the implant. After an extensive literature review, this study
appears to be the first to quantify cortical bone shape differences
between unimplanted and implanted human femur pairs from the same
donors. Since the bilateral femurs experienced similar in situ conditions,
cortical bone shape represented how the bone adapted to the magnitude
and direction of the loads6. The objective of this study was to quantify
cortical bone shape differences in order to deduce how the bone
responded to the implant in an IRB approved protocol. Shape
differences were quantified by comparing area, centroid, polar moment
of area, principal moments of area, location of the principal axes (Imax,
Imin), and inner/outer cortical diameter along Imax and Imin. The null
hypothesis tested was that there would be no significant difference in the
parameters of geometric shape between matched femoral pairs.
METHODS: The five femur pairs (implant in situ time, 59 ± 46
months) were embedded in PMMA and sectioned at 25% (most
proximal), 45%, 65%, and 85% (referred to as levels 1-4, respectively)
along the length of the femoral component (Alloclassic design). The
unimplanted femur was sectioned at the same levels. The 3-mm thick
sections were oriented according to anatomical regions.
Microradiographs were taken and digitally scanned. Cortical bone
measurements and calibrations were performed in Scion Image (NIH).
A macro was used that calculated area, centroid (x, y), polar moment of
area (Ip), maximum and minimum second moments of area (Imax,
Imin), and location of principal axes (Phi1, Phi2). Phi1 was the location
of Imax (minor axis), and Phi2 was the location of Imin (major axis).
Moments of area provide information on the cross-section’s resistance to
bending or torsional loads. Polar moment of area (J) was calculated
from Imax and Imin (J = Imax + Imin). Inner and outer cortical bone
diameter was measured along Imax/Imin in Scion Image. The position
of the Imax and Imin on the implanted femur was the axes selected for
measuring both unimplanted and implanted cortical bone diameters.
Statistical analysis included a two-way repeated measures analysis of
variance (ANOVA) with Huynh-Feldt corrected degrees of freedom,
with level (1, 2, 3, 4) as one repeated measurement and content
(unimplanted or implant) as another repeated measurement. A
significance of α ≤ 0.05 (p-value ≤ 0.05) and β ≤ 0.20 (Power =1-β,
Power ≥ 0.80) was established.
RESULTS: The mean area for the implanted specimens (293 +
158)mm2 was significantly less than the unimplanted specimens (424 +
108)mm2 (p = 0.016). Three patients (3, 4, and 5) had larger decreases
in cortical area for the implanted femur (Table 1). Activity score
(function, deformity, and motion) from the Harris Hip Score (HHS) was
inversely correlated (r2 = 0.82, p = 0.034) to bone loss (Table 1). Pain,
the other HHS element, was 44 for all the patients. No correlations (r2 <
0.2) existed between % bone loss and implant in situ time or patient
weight.
Table 1 – Percent bone loss between the cortical bone area (mm2) of the
unimplanted (Unimpl) and implanted (Impl) femur of each patient. As
the activity score decreased, % bone loss increased.
Patient #
Unimpl
Impl
% Bone Loss
Activity
1
450 + 38
410 + 73
9%
56
2
591 + 20
500 + 79
15%
56
3
394 + 61
233 + 43
41%
37
4
281 + 16
148 + 43
47%
46
5
370 + 28
137 + 29
63%
18
Centroid x was not significantly affected by the implant (p = 0.061).
In centroid y, trends were observed between levels. Ip and Imin means
were higher in the unimplanted than the implanted (p < 0.05), but not
Imax (p = 0.096). The location of the principal axes varied between
patients and between levels.
The outer diameter along Imax did not significantly change between
unimplanted and implanted (p = 0.998); however, the Imax inner
diameter of the implanted is significantly larger than the unimplanted (p
= 0.030). Along Imin, the outer diameter (p = 0.005) and inner diameter
(p = 0.003) significantly increased in the implanted as compared to the
unimplanted (Table 2 and Figure 1). The mean percent differences
between the unimplanted and implanted were larger for the inner
diameter along Imax (20%) than the inner diameter along Imin (16%)
showing net resorption along Imax.
Table 2 – Cortical bone inner and outer diameters of the unimplanted
and implanted along Imax and Imin of implanted femur (mean + std). P
values and % differences are between unimplanted and implanted of
each diameter measurement.
Dia. (mm)
Unimplanted
Implanted
p value
% Diff
Imax(outer)
29.8 + 4.8
29.8 + 3.9
0.998
0%
Imax(inner)
19.1 + 6.1
23.8 + 6.6
0.030
20%
Imin(outer)
31.8 + 6.8
33.4 + 7.2
0.005
5%
Imin(inner)
20.3 + 8.7
24.1 + 9.6
0.003
16%
Figure 1 – Cortical bone of patient 5 (level 4) showing principal axes.
Anterior
Anterior
Imin
Imax
Imin
Medial
Unimplanted (Left)
Imax
Medial
Implanted (Right)
DISCUSSION: The reduction in cortical area observed in the
implanted femur has usually been attributed to stress-shielding2.
Resorption in the implanted femur agrees with Wolff’s Law and Frost’s
Mechanostat Theory6 in that bone adapted to lower strains in the
implanted femur because the minimum effective strain was not
maintained. Decreased activity scores correlated to increased bone loss
between femur pairs; therefore, bone loss cannot be attributed to implant
design alone. This relationship requires further investigation.
Although the direction and amount the principal axes rotated between
unimplanted and implanted was variable, resorption along the Imax and
cortical expansion along the Imin were the mechanisms for the Imax
shifts between unimplanted and implanted. This is because Imax can
only shift if bone resorbs/deposits or diameter expands/erodes in a new
location. Since the mean % differences between the unimplanted and
implanted were larger for the inner diameter along Imax (20%) than the
inner diameter along Imin (16%), resorption concentrated around the
Imax axis. The data also showed cortical expansion along Imin
contributed to Imax shifts. However, the level of contribution of
resorption versus cortical expansion requires further investigation. By
understanding why the cortical bone changed in the implanted femur, we
may equate the relationship of bone adaptation, implant design, and
patient activity to methods of maintaining bone stock.
REFERENCES: 1Doblare M. and Garcia J.M. J Biomech. 35, 1
(2001). 2Weinen H. et al. J Biomech 33, 809 (2000). 3Kerner et al. J
Biomech. 32, 695 (1999). 4Sychterz C. et al. Clin Orthop, 389, 218
(2001). 5Bugbee W., Sychterz C., and Engh C. South Med J. 89, 1036
(1996). 6Frost H. M. Anat Rec. 219, 1 (1987)
50th Annual Meeting of the Orthopaedic Research Society
Poster No: 1337