PREVIOUS KNEE INJURY OR TRAUMA
High-impact loading knee injuries such as tears of the
meniscus, ligaments, or capsule; joint dislocations; and intra-
articular fractures in young individuals has been previously
linked with a 5.2-fold increased relative risk of developing
subsequent knee OA.
22
The energy initially absorbed by the
joint surfaces at the time of injury has been suggested as an
important predictor of development of knee post-traumatic
OA (PTOA).
64
This PTOA has fundamentally different etiol-
ogy than the primary degenerative OA discussed previously
in this review. The emerging hypothesis related to PTOA is
that the severity of the initial trauma and the subsequent cas-
cade of pathophysiological events such as in
flammation and
chondrocyte senescence, along with any residual joint insta-
bility, incongruity, or alteration in biomechanics, contribute
substantially to the onset and progression of knee OA.
65
The risk of knee PTOA is most likely even higher in
young military Service Members due to the high-energy
nature of most battle
field injuries. In fact, in a recent report
in combat-injured warriors who could not return to duty,
injuries to the knee resulted in post-traumatic knee OA in
every case at an average of 19 ± 10 months after injury.
66
This trajectory of knee PTOA development after combat
injuries appears to be much steeper than the 10 to 15 year
rate previously reported after anterior cruciate ligament rup-
tures or meniscal damage in the general population.
67
–69
Given that most combat-related injuries resulting from high-
energy explosions involve multiple limbs and joints,
66
it is
likely that the concurrent injury to the knee of the intact
limb along with altered joint biomechanics after amputation
could lead to a greater risk of developing PTOA in individ-
uals with traumatic unilateral lower limb amputation. Addi-
tional research to better understand the involvement of
multiple joint tissues and the critical cellular and molecular
events after trauma and injury is needed to develop strate-
gies (e.g., surgical, pharmaceutical, rehabilitative, etc.) to
slow or halt the onset or progression of knee PTOA after
lower limb amputation.
PHYSICAL ACTIVITY LEVEL
Previous
findings concerning the association between exer-
cise, sports participation, and risk of knee OA have been
somewhat perplexing. For instance, regular exercise has
been suggested as a favorable option for maintaining articu-
lar cartilage health.
35
Experimental studies in animals have
also shown that loading of healthy joints through moderate
bouts of running is associated with increased articular carti-
lage thickness, proteoglycan content, and mechanical stiff-
ness of the tissue.
23,24
In addition, recreational- or even
elite-athlete level long-distance running have shown to be
unrelated to accelerated incidence or severity of radiographic
knee OA in the absence of underlying joint disease.
70,71
In
contrast, other studies have associated participation in spe-
ci
fic sports that involve running, jumping, and heavy lifting
with higher incidents of knee OA and an increased rate of
disease progression.
72,73
In general, individuals with unilateral transtibial amputa-
tion, regardless of traumatic or nontraumatic origin of the
initial injury, demonstrate decreased activity levels when
compared to nonamputees.
74
In previous literature, it has
been reported that individuals with lower limb amputation
on average take 1,540 to 3,163 steps per day,
75,76
as com-
pared to healthy adults who ambulate anywhere between
4,000 and 18,000 steps per day.
77
Given that mechanical
loading of the knee joint is inherently linked to mainte-
nance of the articular cartilage, adequate levels of mechani-
cal stimulation are essential for maintaining articular
cartilage tissue homeostasis through balancing solid matrix
synthesis and degeneration.
78
Therefore, lower frequency
of knee joint loading due to diminished activity levels can
lead to the reduction in cartilage tissue resiliency
35
needed
to meet the requirements of higher demanding activities,
such as sports participation or returning to active duty
that may be desired by young Service Members with lower
limb amputations.
It has been suggested that individuals with amputations
who participate in sports and/or regular physical activity report
signi
ficant benefits both physically and psychologically, such
as improved strength, endurance, balance, and improved self-
esteem and QOL.
79
Previous studies have shown that 32 to
60% of individuals with lower limb amputation participate in
some form of sports, whether recreationally or competi-
tively.
80
Currently, whether sports participation in individuals
with lower limb amputation could be a risk factor for onset
and progression of knee OA remains unknown. In a small
study, Melzer et al
1
found no differences in the intact limb
knee OA incidence between individuals with amputations
who did and did not play volleyball. Additional longitudinal
data are needed to better understand how early return to high-
demanding sports activities may contribute to the onset and
progression of knee OA. Additionally, Service Members are
anticipated to perform physical activities beyond level walk-
ing and running, which warrants further investigation of
such activities.
CONCLUSIONS
Available scienti
fic evidence to date supports that young
military Service Members with traumatic, unilateral lower
limb amputations may be at increased risk for developing
knee OA compared to nonamputees. Given the high life
expectancy of young injured military Service Members, devel-
opment of effective rehabilitative programs to prevent or delay
knee OA through early risk factor identi
fication and modifica-
tion is a crucial step in optimizing long-term function and
QOL after traumatic, unilateral limb amputations. Future
development of such programs should span a comprehensive
range. Components should include screening of the intact limb
for prior high-energy trauma and joint pain, managing body
weight, further study of intact side knee joint mechanics, and
MILITARY MEDICINE, Vol. 181, November/December Supplement 2016
42
Development of Knee Osteoarthritis After Unilateral Lower Limb Amputation