underlying disease that could be used for initiation of early
prevention and intervention strategies.
OBESITY
Obesity is a well-documented individual risk factor for pri-
mary knee OA in older adults. To this end, a three-fold
increase in risk of future knee OA development has been
previously reported for young men between the ages of 20
and 29 years with BMI values between 24.7 and 37.6 kg/m
2
compared to their leaner counterparts with BMI values
between 15.6 and 22.8 kg/m
2
.
19
Epidemiologic studies have
previously demonstrated that obesity is linked to both the
development and progression of knee OA
20
; however, there
is considerable debate about how obesity contributes to the
onset and progression of the disease. Potential mechanisms
for the contribution of obesity to knee OA include (1) a gen-
eralized negative metabolic environment re
flecting a sys-
temic in
flammatory response to the products secreted by the
adipose tissues;
30,31
(2) increased cumulative compressive
loads experienced by the joint due to a greater body mass;
32
or (3) a combination of both metabolic and biomechanical
factors. Currently, evidence in support of the metabolic
explanation of the link between obesity and knee OA are
mixed. Although some authors have suggested that meta-
bolic factors associated with obesity contribute to the patho-
genesis of knee OA,
30,31
others have not supported this
premise.
33
On the other hand, the biomechanical theory
explaining the potential link between knee OA and obesity
is well supported by the contention that excessive body mass
increases the loads placed on the knee joint.
32
For example,
it has been reported that a weight increase of 1 kg can result
in a four-fold (4 kg) increase in compressive knee joint loads
per step during activities of daily living.
32
However, the
potentially deleterious effects of greater joint loads due to an
increase in body mass may be countered by the lower activity
level, slower preferred walking speed, and less number of
steps taken per day by individuals with higher body mass,
therefore reducing the total knee joint loading exposure.
Clinical observations suggest that individuals with traumatic
lower limb amputation are at increased risk for weight gain
and obesity. Kurdibaylo
34
reported higher fat content in body
mass for individuals with transtibial (21%) and transfemoral
amputations (23%) compared to age-matched controls (13%).
Norvell et al
3
also reported signi
ficantly higher average body
weight and BMI for individuals with unilateral transtibial and
transfemoral amputations compared to control subjects greater
than 40 years of age. Younger individuals with amputations,
in particular, are at high risk of obesity progression within
their
first year status post amputation, mostly as a result of a
sedentary lifestyle immediately after amputation but before
prosthesis
fitting.
34
Given that military standards for recruit-
ment commonly exclude overweight volunteers, increased
risk of obesity after lower limb amputation is most likely a
consequence of the limb loss, which provides a great oppor-
tunity for initiation of early weight management strategies.
ABNORMAL KNEE JOINT MECHANICS
Knee joint mechanics experienced over time create a cus-
tomary joint loading history that helps to precondition the
tissue to withstand repeated mechanical demands without
sustaining injury.
35
However, joint damage may occur when
the mechanical environment is signi
ficantly altered, such that
new patterns of cartilage stresses\strains outside a habituated
envelope result.
35
This may be a concern after a traumatic,
unilateral lower limb amputation, where the intact limb is
challenged by increased demands for body support and for-
ward progression. Several key reviews within the past
decade have summarized the speci
fic mechanical factors
which in
fluence the onset and progression of compartment
knee OA in the general population
9,36,37
and after lower
limb amputations.
38,39
In both populations, the external knee
adduction moment (KAM) has been the most frequently
used surrogate measure for medial knee joint loading related
to knee OA. The KAM may be roughly estimated by multi-
plying the magnitude of the ground reaction force (GRF) in
the frontal plane with the lever arm distance between the
line of action of the GRF and the knee joint axis of rotation
(Fig. 1). Individuals with knee OA characteristically demon-
strate elevated peak KAM during walking that are strongly
associated with severity of medial compartment knee OA,
which is 10 times more prevalent in the general nonamputee
population than lateral compartment knee OA.
9,36
Further-
more, for patients with existing medial knee OA, KAM
magnitude at baseline has strong associations with baseline
knee pain severity, and was shown to be longitudinally
FIGURE 1.
Schematic representation of intact limb knee adduction
moment (KAM) during initial limb loading. Ground reaction force (GRF)
magnitude, along with the perpendicular distance (white dotted line)
between the GRF line of action and the frontal plane knee center of rotation
approximates KAM magnitude, which is highly determinate of compressive
load within the medial knee compartment.
MILITARY MEDICINE, Vol. 181, November/December Supplement 2016
40
Development of Knee Osteoarthritis After Unilateral Lower Limb Amputation