predictive of OA radiographic progression.
40
However, mea-
sures useful in predicting progression of existing OA may dif-
fer from those associated with initiation of OA.
40
Previous investigations of walking in individuals with
transtibial amputations (mean age ranging 41.2
–56.3 years old)
have reported 33 to 56% greater peak KAM on the intact
limb than on the prosthetic limb, depending on walking speed
and the type of prosthetic foot used in the study.
13,41
–44
While
this is greater relative to an asymmetry of ~10% in non-
amputees,
45
the intact limb may not necessarily be overloaded
in direct comparison to the limb of a speed-matched non-
amputee. For example, Royer et al
44
found a 56% greater
peak KAM on the intact limb relative to the prosthetic side in
individuals with unilateral transtibial amputations, associated
with a 45% greater tibial plateau bone mineral density on the
intact limb, relative to the prosthetic side. However, neither
peak KAM magnitude nor the bone mineral density for the
intact limb were signi
ficantly different from speed- and age-
matched nonamputees.
44
In contrast, a number of other studies
have instead found mechanical differences in the sagittal
plane, reporting 48% greater peak external knee extension
moments on the intact limb, relative to the prosthetic side.
12
–14
Higher-level analysis of net GRFs can also lend insight
into pathomechanics of knee OA in individuals with limb
amputation. In general,
findings from collected literature sup-
port net overloading of the intact side relative to the prosthetic
side, as well as relative to speed-matched nonamputees. For
example, persons with unilateral transtibial amputation have a
greater intact-limb peak vertical GRF during loading response
relative to nonamputees, by as much as 4 to 10% during
walking
46,47
and 35 to 45% during running.
48
Knee
flexion
angle and external knee extension moments, which are associ-
ated with GRF overloading and elevated axial knee joint com-
pression,
49
have also been found greater in the intact limb of
individuals with unilateral lower limb amputations compared
to nonamputees.
14,47
Such net GRF differences may or may
not lead to differences at each of the proximal joints,
depending on concurrent kinematics and muscle activity. One
study accounting for the latter factors found 23% greater
peaks in axial knee joint total compression force on the intact
limb relative to the prosthetic side, and 9% greater relative to
nonamputee limbs.
11
Prosthetic device mechanical properties have also proven
to have a signi
ficant effect on mechanics of the proximal
intact limb. Brie
fly, findings indicate that prosthetic foot stiff-
ness and energy return properties can effect intact limb early
stance GRFs, with as much as a 7% of body weight increase
in peak vertical GRF when using a solid ankle cushion heel
foot versus an energy storage and return foot.
50,51
Energy
storage and return feet can also reduce intact limb peak KAM
by as much as 13% versus a solid ankle cushion heel foot,
41
while an active prosthetic foot that provides timed, active pro-
pulsion near prosthetic limb push-off can decrease the mag-
nitude of intact limb peak KAM by as much as 26%.
43
Currently, there is a clear need to ascertain whether such
prosthetic advancements are associated with a reduced inci-
dence of knee joint pain and early OA in the intact limb of
patients with unilateral amputation.
MUSCLE WEAKNESS
Lower limb muscle weakness is a hallmark impairment for
primary knee OA in older adults. In general, muscular strength
is critical for maintaining proper dynamic joint function as
muscles aid in shock absorption and proper force transfer
across the joint.
52
To this end, quadriceps muscle weakness
has been suggested as a strong risk factor for primary knee
OA.
6
Several mechanical theories have been previously
suggested for the potential relationship between quadriceps
muscle weakness and structural knee OA development and
progression. For instance, it has been suggested that quadri-
ceps muscle plays a joint protective role as a shock absorber
to dampen the rate of knee loading such as decreasing the
heel strike transient during the loading response phase of
gait.
53
In individuals with lower limb amputation, prior stud-
ies have shown a signi
ficant decrease in quadriceps strength
for the prosthetic limb when compared to the intact limb.
45,54
Quadriceps atrophy has also been noted on the prosthetic side
in comparison to the intact limb.
55
Comparisons of individ-
uals with unilateral transtibial amputations to a control group
have also demonstrated that those with amputations have
weaker quadriceps bilaterally compared to nonamputees, which
were highly correlated with increased rates of vertical impact
loading of the lower limb during gait.
45,54
While the quadriceps remain the major focus of research
efforts examining the role of muscle weakness in pathogenesis
of primary knee OA, a new body of evidence is emerging to
suggest that hip muscle weakness may also be a risk factor
for knee OA.
56,57
As the hip shares a common segment (i.e.,
the femur) with the knee, adequate hip muscle performance
is necessary to provide dynamic proximal stability for
maintaining appropriate knee joint mechanics during weight
bearing.
58
For example, Chang et al
56
reported that a greater
hip abductor strength is associated with a reduced likelihood
of medial compartment knee OA progression. Other studies,
however, have shown that strengthening of the hip muscula-
ture can lead to signi
ficant improvements in pain and function
despite virtually no change in KAM in older adults with pri-
mary knee OA.
59,60
Strength testing in individuals with unilat-
eral transfemoral amputations, traumatic and nontraumatic,
has shown that isometric hip abduction strength of the ampu-
tated limb is 47 to 54% lower compared to the intact limb
and 35 to 65% lower compared to nonamputees.
61,62
Con-
versely, Nadollek et al
63
found no difference in elderly indi-
viduals with unilateral transtibial amputations between the
prosthetic and intact limbs; however, comparisons to hip
strength in a control group were not made. Additional
research is needed to provide a clearer picture of the preva-
lence and extent of lower limb muscle weakness and its
potential relationship with development and progression of
knee OA after traumatic lower limb amputation.
MILITARY MEDICINE, Vol. 181, November/December Supplement 2016
41
Development of Knee Osteoarthritis After Unilateral Lower Limb Amputation