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in OI patients.
9-12
A study by Englebert et al. found
that muscle strength differed significantly between
types I, III and IV OI, in concordance with the rela-
tive severity of each disease. Type I OI patients were
observed to have largely normal strength, except for
periarticular hip muscles, which demonstrated dimin-
ished strength. Type III OI patients had significantly
decreased muscle strength, which was especially appar-
ent in the lower extremities. Type IV patients had
decreased muscle strength in the proximal muscles of
both upper and lower extremities. Interestingly, the hip
joints of patients with OI tended to exhibit extensor-
flexor muscular imbalance. In fact, increased imbalance
appeared to be associated with decreased muscular
strength. Authors of this study suggested that under-
development of extensor muscles may be due to the
fact that patients with severe OI are born with severely
bowed legs and experience retarded motor milestones.
In any discussion on this topic, it will be important
to acknowledge the difficulty of separating primary
muscle involvement from secondary effects such as
decreased muscle development due to bone weakness
or recent fracture. This may be of importance when
evaluating intercostal muscle strength in the presence
of marked thoracic asymmetry. Children with OI also
tended to have more normal mean muscle strength in
the upper extremities as compared to the lower extremi-
ties. It was suggested that this may be due to higher
fracture frequency in the lower extremities, which may
result in weakness secondary to muscle disuse.
9
Treatment of muscle weakness in pediatric OI
patients should be centered on encouraging patients to
improve functional self-sufficiency through compen-
satory strategies or other means.
13
However, research
has suggested that medical treatment may contrib-
ute to strength improvement. A recent study dem-
onstrated that grip strength improves rapidly with
bisphosphonate treatment.
14
Unfortunately, it is dif-
ficult to demonstrate that this effect occurs indepen-
dently of other factors. The authors hypothesized
that this rapid improvement in muscle strength may
be due to a similarly rapid reduction of bone pain
caused by bisphosphonate treatment,
15
which could
improve perceived strength. Similar improvements
in muscle strength were observed in the aforemen-
tioned patient who presented with muscle weakness
and was treated with pamidronate infusions.
1
Little
attention has been focused on either the evaluation or
treatment of decreased muscle strength in adults with
OI. Of 1524 adults responding to the Osteogenesis
Imperfecta Registry questionnaire, 572 (37.5%)
reported “weak muscle in arms/legs” and 413 (27.1%)
reported experiencing “muscle cramps” (Osteogenesis
Imperfecta Registry, Kennedy Krieger Institute, 2013).
In adults, decreased muscle strength may be the result
of impaired physical fitness or non-weight bearing
in more severe cases. Of interest, no association with
low muscle mass has been reported secondary to low
dietary intakes of vitamin D in non-OI osteoporosis.
16
However, this has not been assessed in OI children or
adults.
TENDON AND LIGAMENT
Ligament weakness or instability contributes to
joint hypermobility observed in a reported two-thirds
of patients with OI.
17
However, joint laxity is usually
less severe than in patients with classic Ehlers-Danlos
syndrome (EDS). In fact, patients with features of
both OI and EDS who exhibit N-terminal mutations in
COL1(α1) are reported to have both OI and joint laxity
typical of EDS.
18
Alterations in ligament and tendon are undoubtedly
responsible for the development of scoliois, spondylo-
listhesis, ankle laxity and flatfoot. Nevertheless there
is little research into the behavior of ligament and ten-
don in patients with OI. There have been numerous iso-
lated cases of tendon rupture reported in patients with
OI. Reports have included ruptures of the patellar ten-
don,
19-21
Achilles tendon,
22,23
quadriceps tendon
24
and
the flexor tendon of the palm.
25
Further, there have been
several reports of avulsion fractures associated with
the patellar
22,26
and triceps tendons.
27
These isolated
cases have not led to significant research into the pre-
cise etiology of tendon ruptures in OI patients, though
it should be noted that the reported cases have a fairly
high proportion of either simultaneous or subsequent
bilateral pathology.
Surgical repair of tendon ruptures and avulsion
fractures in OI patients generally parallels treatment
of similar pathologies in patients without underlying
conditions. However, with avulsion fractures, bone
strength and density must be considered when select-
ing treatment strategies to prevent risk of further frac-
tures. These considerations are best exemplified in a
case report that described treatment of chronic disrup-
tion of the patellar tendon in a patient with OI.
20
The
procedure used a technique described by Dejour et al.,
28
who proposed harvesting a contralateral autograft of
quadriceps tendon, a section of patella, middle third of
patellar ligament and a section of tibial bone. However,
in this particular case, it was deemed that the unique
OI pathology increased fracture risk in the contralateral
knee, while also resulting in a low-quality autografted
tendon. Thus, allograft was used.
Tendon pathology has been investigated in the
murine OI model. Tail tendons from
oim
/
oim
mice
appear to be weaker because of smaller cross-sectional
areas
29
and deficient fibrillar collagen molecule packing,
