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Effects of Body Weight Restriction on the Development and Progression of Spontaneous Osteoarthritis in Guinea Pigs.

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Hartley albino guinea pigs develop spontaneous
osteoarthritis (OA) of the knee joint. A study was done
to determine the importance of body weight in the
pathogenesis of this disease. Two groups of 20 male
guinea pigs each were maintained on the same diets. The
control group was allowed ad libitum feed consumption
and the other group was restricted to 30-35 gm of feed
per day. Ten animals from each group were killed at 9
months of age to evaluate histologic features of the knee
joints. The severity of the OA lesions was reduced by
40%, in conjunction with a 28% decrease in body
weight, in the diet-restricted group. The remaining
animals were killed at 18 months of age. Those in the
diet-restricted group had a 56% reduction in severity of
lesions, with a 29% decrease in body weight. These
results indicate that body mass in guinea pigs, as in
humans, is an important predisposing factor for the
development of spontaneous OA of the knee.
occurs in Hartley albino guinea pigs (9,lO). The disease progression is similar to that in other laboratory
animals that develop OA of the knee joint (11-13), and
the lesions resemble those of medial compartment
knee OA in humans (14). The cause or causes of this
naturally occurring OA in guinea pigs are unknown.
However, laboratory-raised guinea pigs on ad libitum
feeding regimens typically become quite obese with
increasing age (10). The incidence and severity of the
degenerative changes in their knee joints also increase
with age. We hypothesized that obesity might be an
important predisposing factor in the development of
spontaneous OA in guinea pigs and questioned
whether diet restriction of sufficient magnitude to
significantly reduce body weight compared with that of
ad libitum-fed controls, yet maintain good health,
would influence disease progression. Our findings are
presented here.
Osteoarthritis (OA) affects nearly 10% of the
population over the age of 60, with knee OA being one
of the most common causes of pain and disability in
the elderly (1). Various predisposing factors including
conformational abnormalities, knee injury, and obesity have been associated with the disease (2-7). A
body mass index >30 has been strongly associated
with bilateral knee OA (8).
Spontaneous OA of the knee joint commonly
From Lilly Research Laboratories, Eli Lilly and Company,
Indianapolis, Indiana.
Alison M. Bendele, DVM, PhD; James F. Hulman.
Address reprint requests to Alison M. Bendele, DVM,
PhD, Lilly Research Laboratories, Indianapolis, IN 46285.
Submitted for publication February 26, 1991; accepted in
revised form April 11, 1991.
Arthritis and Rheumatism, Vol. 34, No. 9 (September 1991)
Experimental animals. Forty 30-day-old male
Hartley albino guinea pigs from Charles River (Wilmington, MA) were housed individually in 38 x 34 x
18-cm wire cages. The animals were assigned to control (n = 20) and diet-restricted (n = 20) groups. Food
(Purina Guinea Pig Chow #5025; Ralston Purina,
Richmond, IN) and water were available for ad libitum
consumption by the control animals from the beginning to the end of the study. Guinea pigs in the
diet-restricted group were fed 30 gm of food (Purina
Guinea Pig Chow #5025) from day 42 to day 180, and
then 35 gm for the remainder of study. The body
weights of all guinea pigs were recorded at approximately monthly intervals.
Animals were killed by exposure to carbon
dioxide. All guinea pigs were necropsied to determine
their general condition and to evaluate abdominal fat
stores. The knee joints and any other tissues showing
gross abnormalities were collected and fixed in formalin for histologic evaluation. Use of animals for this
study was in accordance with United States Department of Agriculture guidelines for humane care.
Study design. Ten control and 10 diet-restricted
guinea pigs were killed at age 9 months for histopathologic evaluation of knee joints. The remaining animals
continued on their respective diets until 18 months of
age, at which time they were killed and the knees were
collected for microscopic examination.
Histopathologic evaluation of knee joints. Soft
tissue, excluding the medial and lateral joint capsule,
but including the patellar tendon, was removed from
the right and left femorotibial joints. The tissues were
placed in 10% neutral buffered formalin for 24 hours.
Subsequent decalcification for 2 weeks in a SurgiPath
Decalcifier I (SurgiPath Medical Industries, Grayslake, IL) allowed the joints to be cut frontally into 2
approximately equal portions, which were returned to
the decalcifier for an additional 2 4 4 8 hours.
Paraffin-embedded whole joint sections were
prepared and stained with hematoxylin and eosin
(6p-thick sections) or with toluidine blue (8p-thick
sections). Step sections were cut at 150-2OOp intervals, so that representative samples from all areas of
the articular surface could be evaluated. Both knee
joints from all animals in each group were examined
microscopically, without knowledge of the animals’
group origin. Each joint was assigned a total score on
the basis of the most severe lesion observed in the
various step sections.
Hispathologic alterations on the medial tibial
plateaus and femoral condyles were graded as minimal, mild, moderate, marked, or severe and assigned
numerical scores of 1, 2, 3, 4, or 5 , respectively.
Normal joints were given a score of 0. Minimal
changes consisted of small foci of chondrocyte degeneration and chondrocyte loss in the superficial layer of
the articular cartilage. Decreased toluidine blue staining of the matrix and surface disruption also were
present. Mild lesions were of a similar type, but
extended into the upper portion of the middle layer of
the cartilage. Moderate changes generally affected
most of the cartilage surface in the histologic section
and extended well into the lower portion of the middle
layer. Marked degeneration extended well into the
deep zone but not to the tidemark. Severe lesions were
characterized by full-thickness (to the level of the
tidemark) loss of chondrocytes and proteoglycan.
Quantitation of the involved area was determined by
multiplying the degeneration score by 1, 2, or 3,
representing one-third, two-thirds, or total surface
involvement. In addition, if small, medium, or large
chondrophytes or osteophytes were present, 1, 2, or 3
points was accordingly added to the depth X area
score. A synovitis/synovial reaction score for each
joint was also included; this score was based on the
severity of proliferative changes in the synovial membrane (minimal = 1, mild = 2, moderate = 3, or
marked = 4).
Survivability, body weights, and clinical observations. Three guinea pigs died during the course of the
study. One in the diet-restricted group died at 10
months of age, and 2 in the ad libitum-fed group died
at age 13 and 17 months. One had bronchopneumonia;
the others had histologic evidence of chronic nephropathy, which probably contributed to their deaths. The
remaining animals of both groups were clinically normal throughout the study. Eighteen-month-old animals
in the diet-restricted group exhibited increased spontaneous activity compared with control guinea pigs,
which were fairly inactive. No difference in activity
levels was observed at 9 months.
The mean body weight ( t S E M ) of l-month-old
animals at the start of the study was 437 ? 27 gm. By
the time the guinea pigs were 2 months of age, the
mean body weight of the diet-restricted animals was
significantly different from that of the controls (Figure
1). The difference remained significant for the duration
of the study. At the termination of the study, when
animals were 18 months old, the difference in body
weight was -375 gm. Although their length and girth
were not measured, the animals of both groups generally were of similar length. However, there were
prominent between-group differences in abdominal
and thoracic girth. Abdominal fat stores were 50%
greater in the control group. At necropsy, animals in
the diet-restricted group had full range of motion of
their knee joints, whereas those fed ad libitum had up
to a 70% reduction in capacity to extend the knee joint.
Histopathologic findings. All control guinea pigs
killed at 9 months of age had mild to marked degeneration of the medial tibial cartilage, with the majority
being mild or moderate. Eighteen of the 20 knees had
small or medium-sized tibial osteophytes. Eleven
of 1 guinea pig in the diet-restricted group. The knee
joints of the remaining 9 animals had no evidence of
femoral degenerative changes. Femoral osteophytes
were not present in any guinea pigs of the dietrestricted group. Minimal to mild synovitis occurred in
only 5 of 20 joints, making this parameter significantly
different from the value in controls (Table 1).
With all aspects of the lesion taken into consideration, there was a 40% decrease in bilateral lesion
severity (total animal score) in 9-month-old dietrestricted guinea pigs, which weighed 29% less than
the ad libitum-fed controls. Within the various groups,
there was no obvious correlation between body weight
and lesion severity. However, the weight differences
between animals in each group were not striking at this
time point (Figure 1). No differences in maturity or
morphology of the tibial physis were observed in
9-month-old animals of either group.
All 18-month-old ad libitum-fed guinea pigs had
moderate to severe degeneration of the medial tibial
cartilage, with medium to large tibial osteophytes
(Figure 2A). Likewise, all animals had some femoral
cartilage degeneration, ranging in severity from mild to
severe. Medium-sized femoral osteophytes occurred
in 1 of 9 guinea pigs. (Data from the 17-month-old
animal that died are included in these analyses.) Synovial scores for all 18 knees ranged from 1 (1 knee) to
4. Those with scores of 3 (6 knees) and 4 (3 knees) had
marked thickening of the synovium as a result of
papillary proliferation, mononuclear inflammatory cell
infiltration, and fibroplasia. Most of the animals in this
group had moderate to marked sclerosis of the tibial
and femoral subchondral bone. Within this group,
body weights ranged from 1,150 gm to 1,508 gm
1000 -
700600 -
5001 Y
I .
Figure 1. Mean (fSEM) body weight of diet-restricted and ad
libitum-fed guinea pigs (see Materials and Methods for details).
There were significant differences ( P 5 0.05) at all times from age 2
months to age 18 months.
knees had minimal to moderate femoral cartilage degeneration, and 2 had small femoral chondrophytes.
Minimal to mild synovitis occurred in 13 of the 20
joints. Tibial cartilage degeneration was observed in
both knees of all diet-restricted animals. However, the
severity of the lesion was significantly less than that in
the ad libitum-fed controls (Table 1). Small to mediumsized tibial osteophytes were present in only 9 of the 20
knees, making the tibial osteophyte score significantly
different from that of the controls. Mild degeneration
of the medial femoral cartilage occurred in both knees
Table 1. Body weight and histologic scores in 9-month-old and 18-month-old diet-restricted and ad libitum-fed male guinea Dies*
9-month-old animals
Body weight (gm)
Tibial cartilage degeneration score
Tibial osteophyte score
Femoral cartilage degeneration score
Femoral osteophyte score
Synovial score
Total joint score
Total animal score
18-month-old animals
(n = 10)
Ad libitum-fed
(n = 10)
(n = 9)
Ad libitum-fed
(n = 9)
803 2 18t
5.60 f 0.48$
0.55 2 0.1%
0.60 f 0.41
0.00 2 0.00
0.30 f 0.13$
7.05 f 1.05t
14.10 f 2.99$
1,126 2 23
7.50 f 0.46
1.15 2 0.13
2.10 f 0.53
0.10 f 0.07
0.80 2 0.16
11.60 2 1.09
23.40 2 3.10
919 f 217
8.00 2 0.48"
1.33 f 0.21t
1.56 2 0.63t
0.00 f 0.00
0.78 f 0.15t
11.67 f 1.127
23.33 f 2.93t
1,296 f 44
11S O f 0.47
2.69 f 0.12
9.81 f 0.63
0.13 f 0.13
2.63 5 0.22
26.75 f 1.20
53.50 f 2.92
* Scores were derived as described in Materials and Methods. One animal in the diet-restricted group died at age 10 months; 2 animals in the
ad libitum-fed group died at age 13 months and 17 months, respectively (data from the 17-month-oldguinea pig are included with those for the
18-month-old animals). Values are the mean 2 SEM.
t P 5 0.001 versus ad libitum-fed controls, by Dunnett's 2-tailed t-test.
$ P 5 0.05 versus ad libitum-fed controls, by Dunnett's 2-tailed t-test.
Figure 2. Photomicrographs of toluidine blue-stained sections of the medial aspect of the femorotibial joint from 18-month-old guinea pigs. A,
Control guinea pig, allowed to feed ad libitum. There are areas of full-thickness cartilage degeneration, extending focally to subchondral bone,
on both the tibia and the femur (F) (small arrows). Marked meniscal degeneration is evident, and cartilaginous debris is present in the joint
space. The synovium is markedly thickened (large arrow) due to fibrosis and papillary proliferation of synoviocytes. A large chondrophyte is
evident on the femur, and a medium-sized osteophyte projects from the tibia. Both the tibia and the femur show sclerosis of the subchondral
bone. B, Experimental guinea pig, subjected to dietary restriction as described in Materials and Methods. There are lesions of minimal-to-mild
cartilage degeneration extending over the tibial surface (small arrow); a small tibial osteophyte is also present. The femoral cartilage is intact,
but there is minimal loss of staining, indicating proteoglycan loss, at the far axial aspect (arrowhead). The synovium is histologically normal
(large arrow). (Original magnification x 40.)
(Figure 1). There was a significant correlation ( P 5
0.05) between body weight and lesion severity, as
determined by linear regression analysis.
Joints of the 18-month-old diet-restricted guinea
pigs resembled those of the 9-month-old ad libitum-fed
animals with respect to the severity of the OA lesion
(Figure 2B). Tibia1 cartilage degeneration was significantly less than that in the 18-month-old ad libitum-fed
controls (Table 1). Of the 18 knees evaluated, 3 had no
tibial osteophytes, 8 had small osteophytes, 5 had
medium osteophytes, and only 2 had large osteophytes. Femoral cartilage degeneration and synovitis
scores also were significantly decreased as compared
with controls (Table 1). The total animal scores, which
were indicative of bilateral lesion severity, were decreased by 56% in conjunction with a 29% reduction in
body weight. No differences in maturity or morphology of the tibial physis were observed in 18-month-old
animals in either group.
Osteoarthritis of the knee joint commonly occurs both in humans and in guinea pigs, and the
prevalence increases with age (1,lO). The role of
obesity in the pathogenesis of knee OA in humans has
been investigated. Results of epidemiologic studies
indicate that the additional mechanical stress resulting
from obesity is an important risk factor for the development of OA (4,5,7,8). The importance of other
factors associated with increased fat deposition is not
clear (14). Our previous findings have indicated that
the earliest lesions of spontaneous OA in guinea pigs
consistently occur when the animals are approximately 3 months old and weigh 70CL800 gm, which
suggests that age-related or weight-related factors are
important in the pathogenesis (9,lO). Results of the
study reported here demonstrate the importance of
increased body weight as a risk factor for the development of OA in guinea pigs. Animals maintained on
restricted diets had evidence of beneficial effects as
early as 9 months of age. The beneficial effects of
continuous dietary restriction were even more obvious
at 18 months of age, and knees from these animals
were histologically similar 1.0 those of the 9-month-old
animals. Diet restriction, with the resulting maintainance of relatively low body weight, significantly retarded the development of this naturally occurring
degenerative process.
Eighteen-month-old guinea pigs in the dietrestricted group exhibited increased spontaneous ac-
tivity compared with that of the 18-month-old ad
libitum-fed controls. However, it seems unlikely that
the increased mobility was of major importance in
decreasing the lesion severity, since differences in
mobility were not observed in the 9-month-old dietrestricted animals, which also had amelioration of the
disease. Also, other studies have shown that sciatic
nerve transection, a procedure which results in severely restricted weight-bearing and joint use, essentially eliminates the degenerative process (Bendele
AM: unpublished observations).
The importance of biomechanical factors in
causing or predisposing to the development of OA is
well documented (1-3,6,7). Occupational stresses result in OA developing in joints that are chronically
subjected to overuse or abnormal use. Angular limb
deformities result in OA lesions in chronically stressed
areas ofjoints. In the case of naturally occurring OA in
animals, a biomechanical derangement in the form of
patellar luxation has been identified as the cause of OA
in STWIN and STIUORT mice (12,15). Surgical correction of the patellar displacement prevents lesion
development. Primary or secondary varus or valgus
defects have been identified in other mouse strains that
develop spontaneous OA (16).
Conformational abnormalities have not been
described in guinea pigs or other animal species that
develop spontaneous OA of the knee (9-1 1). However,
the localization of the degenerative changes to the
medial aspect of the knee in the guinea pig suggests
that abnormalities associated with load-bearing are important in the pathogenesis of OA. The early lesions of
spontaneous OA in guinea pigs start in the area of the
tibial plateau that is not covered by the meniscus and
resemble the lesions commonly seen in a similar location
in human knees (14). In humans, 75% of the total load
passes through the medial aspect of the knee joint (17). If
load-bearing in the guinea pig is similar to that in humans
with respect to medial versus lateral predominance, it
seems logical that increased load as a result of increased
body weight would exacerbate the natural tendency for
this area of the joint to degenerate. Results of our study
suggest that this may be the case, since maintaining a
lower body weight profoundly influenced the natural
progression of the disease. Further studies of this model
may be useful in determining what, if any, level of
histologic improvement in the knee joints of older guinea
pigs with established lesions can be achieved by weight
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development, pigs, guinea, effect, progressive, spontaneous, restrictions, weight, osteoarthritis, body
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