Message Number: YG5532 | New FHL Archives Search
From: RRC
Date: 2001-07-15 18:56:00 UTC
Subject: Bob C: Ferret Skeleton: Pelvic Limb

Skeleton femoris: (Synonyms: pelvic limb, femoral appendage, leg
skeleton, crus bones).

Attached to the sacrum are the two pelvic bones, the os coxae (= aitch
bones, innominate bones, hip bones, pelvic bones). Each ox coxa is
composed of one minor and three major bones, all fused together to
support the weight of the animal while leaping, landing, running, and
just plain getting into trouble. The ilium (= ilial bone, haunch bone,
hook bone, os ilium) is the more cranial of the four bones, and is
attached to the sacrum at the sacroiliac joint, which is supported by
ligaments and rough bony surfaces, but allows some movement between the
sacrum and the os coxae. The ilium is fused to the ischium (= ischial
bone, pin bone, os ischii) in the acetabular region, which is where you
can find the fused remnants of the acetabular bone (= os acetabulii).
The acetabular bone, only found in immature individuals, is roughly
triangular in shape, and makes up the central floor of the acetabulum.
Fused to both the ilium and the ischium is the pubis (= pubic bone, os
pubis). The pubis is roughly "C"-shaped, and when fused to the other
bones forming the os coxa, creates a large opening, or hole, called the
obturator foramen. The medial aspect of the pubis is straight and
roughened where a cartilaginous joint is formed with the pubis on the
opposite side (in roughly 10% of ferrets or less, the pubic joint will
fuse, forming a hard, immobile joint. I have found this most common in
older, whole males). The acetabulum is the cup-shaped socket which holds
the head of the femur in place, and is formed by the union of the four
bones mentioned. In ferrets, these are delicate bones, and can display a
wide range of pathology. Fractures are fairly common and usually occur
in the pubis and/or ischium. Occasionally, one or both of the os coxae
can permanently fuse to the sacrum, which interferes with mobility and
the ferret's ability to tightly curl up to keep warm. The acetabulum
displays at least some minor pathology in 80% or more of the ferret
skeletons I've studied, usually at least some reactive bone or
arthritis. Commonly, degenerative joint changes are present within the
acetabular cup, especially opposite the notch. Minor infections are not
common, but not rare either. In older female pet ferrets, especially
those who have had long term adrenal disease, the amount of osteoporosis
in the os coxae can be staggering; in several instances, one or both os
coxae have fragmented during normal skeleton preparation. Most arthritis
seems centered around the acetabulum, but I have seen minor amounts of
arthritis and reactive bone near the sacroiliac joint. As already
mentioned, on occasion, sometimes the two pubic bones fuse together. In
four individuals, I have seen false sockets where the head of the femur
has slipped out of the acetabulum and formed a new cup on the ilium; 1
was a feral ferret from New Zealand, and 3 were pet ferrets. In one of
the pet ferrets with the false acetabulum, the head of the femur had
broken off at the neck and fused to the ilium, forming the edge of the
new cup. In two cases, I found bone masses consistent with cancers.
(Synonyms: pelvis = hip bone, pelvic girdle, cingulum membri pelvini).

Male ferrets have a heterotrophic bone called the os penis. It is a
slender, grooved bone with a hook at one end and an inflated base at the
other. The base is embedded in a tendon which passes under the pelvis
and is attached to the caudal edge at the joint of the two pubic bones.
The grove on the bone supports the urethra, maintaining patency during
coitus. The hook, about the same shape as the end of a crochet hook,
supports the glans penis and provides a hard substrate used to stimulate
the nerve rich portion of the female ferret's vagina, which helps induce
ovulation (odor and neck biting also help). The hook also is the cause
of many a male ferret's (and owner's) fright as it is occasionally
caught on wire cages or lips of hard, sharp surfaces, such as bucket
rims. When this happens, the entire penis can be pulled out of the
sheath, and damage to various tissues, the os penis, or the supporting
tendon can occur. Fractures to the os penis are more common than you
might wish to think; I have seen 5 healed fractures in approximately
350 skeletons. In each case, the fracture was well healed, meaning the
ferret survived long after the accident. The dangers associated with a
fractured os penis are damage to the nerve or blood vessels supplying
the penis, damage to the urethra, or damage to the spongy tissue itself.
The os penis is one of the few bones that get larger as the ferret gets
older, which provides a reliable method of aging based on the size of
the base, but neutering makes the os penis smaller, so appropriate
adjustments have to be made. Once, I saw reactive bone tissue on the
ventral edge of the os penis. On rare occasions (less than 5%), female
ferrets have an irregular bony os clitoris. I have only seen a few os
clitorae and none possessed pathology. (Synonyms os penis = baculum, os
genitale, L'os pénien, penis bone, Penisknochen; os clitoris = clitoral bone)

The femur in ferrets is fairly straight, has a large head and a huge
trochanter for the insertion of the hip muscles. There is a lateral
fabella present behind the lateral condyle, and in a few skeletons, I've
found a smaller medial fabella. The patella is robust, as would be
expected in such a heavily muscled animal. The femur is one of the most
commonly damaged and pathological bones I've seen in ferrets, having a
12%+ injury rate in the skeletons I've studied. Fractures are common and
seem to occur most often on one of three areas; the neck, the proximal
shaft, and those involving one or both condyles. In a third of the
fractures involving the neck of the femur, a fracture to the diaphyseal
shaft was also present. Condylar fractures tended to occur most often to
the medial condyle. Arthritis was present in 80% or more of femurs from
ferrets older than 3 years of age, and centered on the articular edges
of the caput (head) and condyles. In some cases, reactive bone was found
on the head and neck, but was more common in the area of the condyle. In
one case, the ferret sustained a spiral fracture to the proximal
diaphyseal shaft which never healed properly, forming a functional false
joint. In this individual, there was an extreme amount of reactive bone
tissue present, suggesting a chronic infection. In four individuals, the
femoral head slipped out of the acetabulum and formed a false socket on
the ilium. On two femurs, a bone mass as present, suggesting cancer.
(Synonyms: femur = os femoris, roundbone, thigh bone, leg bone; patella
= knee cap, os patellae; fabella = condylar sesamoids, lateral/medial
femoral sesamoids, os sesamoideum m. gastrocnemii).

The tibia is the longest bone in the ferret, with a large, flatish
"head" composed of the medial and lateral condyles and a large, pointed
intercondylar area separating the two. There is a notch which accepts
the head of the fibula, a long, thin bone which attaches to the tibia at
two points; proximally at the lateral condyle and distally with the
lateral aspect of the medial malleous. Tibial fractures are common, but
not as common as femoral fractures. Most fractures are low on the tibal
diaphysis, near the ankle, and might be caused by being stepped on;
however, the same sort of injury may be cause by falls. In most cases
where the tibia is fractured, the fibula is also. In two cases, the
fibula fused to the tibia at the site of fracture. Arthritis is fairly
common in older ferrets, especially along the edges of the condyles.
Some minor arthritis on the medial malleous has been found in older
ferrets. In one case, the lateral malleous of the fibula was fused to
the medial malleous of the tibia, with lots of reactive bone tissue
present. I suspected a massive infection, but couldn't rule out a
possible old fracture. I have seen 3 cases of bone tumors or cancers on
the medial upper diaphysis of the tibia. (Synonyms: tibia = shin bone,
os tibiae; fibula = brooch bone; pin bone, os fibulae).

The ferret has seven tarsal bones, arranged like most other carnivores
(PROXIMAL ROW: ASTRAGALUS = talus, ankle bone, knucklebone, os tali,
tibial tarsal bone [NOTE: talus is historically reserved for primates,
but modern anatomists are moving away from astragalus]; CALCANEUS = heel
bone, calcaneum, os calcis, fibula tarsal bone. MEDIAL ROW: NAVICULAR =
os naviculare, central tarsal, centrale, os tarsi centrale, scaphoid.
DISTAL ROW: MEDIAL CUNEIFORM = T1, os tarsale I, os cuneiforme mediale,
1st tarsal bone; INTERMEDIATE CUNEIFORM = T2, os tarsale II, os
cuneiforme intermedium, middle cuneiform, 2nd tarsal bone; LATERAL
CUNEIFORM = T3, os tarsale III, os cuneiforme laterale, 3rd tarsal bone;
CUBOID = T4, os tarsale IV, os cuboideum, 4th tarsal bone). The
cuneiforms are variable in shape, but the astragalus and calcaneus are
distinctive enough for species identification, with the cuboid and
navicular slightly less so. I have seen six fractures of the ferret
talus; all resulted in moderate to extensive fusion, with at least minor
formation of reactive bone tissue. Sometimes two cuneiforms are
naturally fused together. Signs of arthritis on the bones are rare,
except in the calcaneus and astragalus, where it is fairly common in
older ferrets. Sometimes short bone spurs form within the Achilles
tendon at the point of the hock (the caudal part of the tuber calcanei).
I have never seen a bone mass in the talus. I suspect the large number
of talus fractures are due to being stepped on, being caught in doors,
or being trapped within narrow crevices. (Synonyms: tarsal bones = ossa
tarsi, ankle bones, tarsus, hock).

Like in the manus (hand), the ferret has five metatarsals in their pes
(foot). The metatarsals are numbered from 1 to 5, starting at the big
toe (1st metatarsal, MT1, os metatarsale I). Metatarsals seem to be
injured frequently, based on healed fractures and reactive bone. In
several cases, I have seen two or more metacarpals fused together
midshaft. In one case, four metacarpals were fused at the proximal end.
A plantar sesamoid is generally present, although I once found two in a
single individual. There are two small sesamoids between each metatarsal
and first phalanx, which are rarely fused to the metatarsal or the 1st
phalanx. Signs of infection are uncommon. I have never seen a cancerous
growth. (Synonyms: metatarsus = foot bones, ossa metatarsalia, pastern
bones, rays).

The phalanges of the foot follow those of the hand with no major
difference. The reader is referred to the description of the thoracic
skeleton, bearing in mind subtle differences exist between the pes and
the manus.

Overall, I would say the most common pathologies follow those for the
thoracic skeleton, and are (in order) healed fractures, arthritis,
reactive bone, bone masses.

Bob C