Do Biomechanical Tests Adequately Portray Clinical Loading Scenarios?
1
Ricci, W M; 2Cartner, J L; 2Whitten, S A; 2Day, M A
Washington University Medical Center, St. Louis, MO
2
Smith & Nephew, Inc., Memphis, TN
[email protected]
1
Proximal femur (23%) and foot and ankle studies (23%)
comprised the majority of studies, followed by distal tibia (18%),
distal femur (12%), proximal tibia (10%), mid-shaft tibia (7%),
pelvic (5%), and femoral shaft (2%) studies. Pelvic studies utilized
the lowest body weight justifications (668 ± 45 N), whereas
proximal femur studies implemented the highest body weight loads
(739 ± 93 N). The most commonly cited mass associated with
body weight was 70 kg, but this was only denoted in 12% of the
articles.
When compared to the report from the United States
Department of Health and Human Services Centers for Disease
Control (CDC) and Prevention and National Center for Health
Statistics, the biomechanical studies found in this search
underestimated mean body weight. The CDC reports a mean body
weight for US females across all ethnicities as 74 kg (727 N) and
reports a mean body weight for US males across all ethnicities as
86 kg (847 N), for an averaged total across both groups of 80 kg
(787 N). A difference of nearly 160 N between the loading level
cited for testing justifications and the average US citizen was seen.
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INTRODUCTION:
Bridging the gap between basic biomechanical research and
clinical orthopaedic care must be approached with caution.
Loading scenarios used in biomechanical testing are often limited
to constrained monotonic axial compressive loads that lead to
failure modes that are not similar to those witnessed clinically.
Clinical loading is highly unlikely to be so simple. Recently, use
of multiplanar repetitive loads (simultaneous cyclical axial
compressive bending and torsional loading) have been shown to
better simulate clinical failure of fixation constructs than the more
often utilized constrained monotonic axial compressive loads.
Outcome parameters of biomechanical tests such as stiffness
and fatigue survival are highly dependent upon the chosen load
force and are, at best, just estimates of clinical behavior.
Furthermore, the loading forces utilized in published reports are
inconsistent. The purpose of this investigation was to analyze the
loading scenarios utilized in published reports of biomechanical
fracture fixation studies and to provide a systematic categorization
methodology such that future studies would have a basis for
choosing an appropriate loading scheme for comparative analyses.
METHODS:
We searched the following medical journals for publications
dated 2000-2009 that included a reference to biomechanical testing
and utilized ‘body weight’ (keyword) as an orthopaedic loading
scenario: Clinical Biomechanics, Clinical Orthopaedics and
Related Research, Injury, Journal of Biomechanics, Journal of
Bone and Joint Surgery – American, Journal of Bone and Joint
Surgery – British, Journal of Orthopaedic Research, Journal of
Orthopaedic Trauma, Journal of Trauma. The number of hits per
journal was recorded and the percentage of included articles was
calculated. Only lower extremity, weight-bearing anatomical
regions were included. Likewise, any publications that did not
specifically pertain to orthopaedic traumatology were excluded.
Included articles were categorized based upon weight bearing
protocol, implant type (Nails, Plates and Screws, and External
Fixation), and anatomical region. Fisher’s f-tests for variance,
followed by two-tailed Students’ t-tests at 95% confidence were
utilized to determine differences between means where
appropriate. Results from the literature search were compared to
reported body weights published by the United States Department
of Health and Human Services Centers for Disease Control and
Prevention and National Center for Health Statistics [1].
RESULTS:
With the given search criteria, 5,289 publications were idenified
as potentially applicable. Of these, only 56 articles (1%) met all
inclusion conditions and were included in analyses. The majority
of these articles (78%) cited full body weight and utilized this
justification as a load basis for biomechanical testing. Other
publications mentioned half body weight (9%), partial weightbearing (9%), crutch assisted weight-bearing (3%), and toe-touch
only weight-bearing (1%), and referenced associated load
justifications respectively.
A mean of 629 ± 224 N (range: 31 – 980 N) was chosen as a
biomechanical test load, indicating minimal consensus among the
included articles. Of all articles utilizing full body weight
justifications, a mean of 722 N was reported, but the range spanned
over 370 N (Figure 1).
Nails (46%) and plates and screws (42%) were evenly cited,
whereas there were few external fixation (12%) biomechanical
tests. The body weight values cited indicated no statistical
difference between implant type (p = 0.24 - 0.77, depending on
implant type), although external fixation studies routinely cited
lower body weights.
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550 - 599 600 - 649 650 - 699 700 - 749 750 - 799 800 - 849 850 - 899 900 - 949
Body Weight Ranges (N)
Figure 1. Histogram of those articles in the medical literature
referencing some quantity for full body weight as load justification
for a biomechanical test.
DISCUSSION:
Consistent loading parameters among biomechanical tests are
not well documented. Engineering analyses of multifactorial
physiological realities are limited by our choice of test parameters.
Biomechanical testing results should always support clinical
experience, and in this study we observed a discrepancy between
biomechanical test parameters, which may ultimately influence our
standard of care. The guidelines for surgeon decision making may
not always be plausibly ascertained from the biomechanical
literature. Researchers and clinicians should evaluate each
situation with caution in order to provide the best solution to both
mechanical simulations and patient weight-bearing protocol, and a
recommended body weight justification may be higher than what
was previously thought. A loading scheme of 722 – 787 N may be
most appropriate based on the literature search and reported CDC
findings of this study.
REFERENCES:
[1] McDowell MA, et al. Anthropometric Reference Data for
Children and Adults: U.S. Population, 1999-2002. Advance Data
from Vital and Health Statistics 361. United States Department of
Health and Human Services Centers for Disease Control and
Prevention and National Center for Health Statistics. July 7, 2005.
Poster No. 1738 • 56th Annual Meeting of the Orthopaedic Research Society