|
           
| |
 
The Whys and
"What-for" of
Leg Conformation
Karen
I. Timm, DVM, PhD
Correct
leg conformation optimizes bone and joint health throughout the life of the
animal and therefore is important to its well-being. An alpaca with crooked legs
has a much greater chance of developing early "wear-and-tear"
pathology of the bones and joints, such as joint pain and arthritis, than one
with correctly aligned legs.
Wear-and-tear pathology is painful, and of course no one
wants an animal to be in constant pain from sore joints. But incorrect leg
conformation can also decrease the animal’s effective life-span. Animals that
find it painful to move will often not eat well and have poorer-quality fiber;
males in particular may have decreased fertility. Hind leg problems may prevent
a male from breeding because it is too uncomfortable to kush to breed the
female. When choosing or evaluating alpacas, certainly features like micron
count are important, but leg conformation as close to ideal as possible should
be a primary consideration.
The long-term importance of good leg conformation was
reflected in ARI’s initial importation screening standards. Based upon input
from owners, veterinarians, and other experts in the field, ARI established a
set of minimum conformational
standards in 1995. As with all of the standards, the objective was to establish
functionally relevant, objective criteria. It is important to note that
initially the veterinarians used these standards on an
"all-or-nothing" basis. For example, the maximum allowable angulation
for "knock knees" of the front legs was set at 15 degrees. While many
owners would view an animal with a 10-degree angulation as having
marginal-to-bad forelegs, it would have passed the vet screening. In contrast,
the animal with a 20-degree angulation would have failed.
The veterinary examination has continued as a
"pass-or-reject" system. But in response to owner requests for better
overall leg conformation, the minimum standards for imported animals were raised
in the phenotypic examination. Consequently, those animals whose legs were
somewhat crooked but not badly enough to be rejected by the veterinary exam
could fail to qualify through the phenotypic exam.
Terminology
It is important that veterinarians, owners, and importers
start with a similar understanding of what constitutes normal leg conformation,
how it is evaluated, and the consequences of skeletal defects.
First let’s review some terminology used in anatomy. Look
at the front leg in Figure 1.
Actually, pure anatomists don’t call this a leg since it is technically an
arm. It may also be called a front limb or forelimb. The bones, starting at the
shoulder and working to the ground, are the scapula, humerus, radius, and ulna;
then there are two rows of carpal bones, metacarpals 3 and 4, sesamoids, and
phalanges 1, 2, 3 of digits 3 and 4. Alpacas have two "fingers," the
third and fourth digits, corresponding to the third and fourth fingers of the
human, the middle and ring fingers.
 
Front Limb
Rear Limb
The diarthrodial (highly movable) joints of the limb are the
combination of the ends of bones, with their cartilage covering, joined to each
other by connective tissue of joint capsule and ligaments containing synovial
fluid. Ligaments are tough connective tissue bands that run across joints from
bone to bone and provide major support for the joints. The joints contain
synovial fluid, a viscous, slimy substance that cushions and lubricates the
joint.
Muscles and tendons cross the joints so that the pull of
muscles changes the angle of the joint. Flexion makes the angle smaller;
extension increases it. When you curl your forearm up to your body, for example,
you are flexing your elbow, wrist, and finger joints. When you drop your arm to
your side the joints are extending.
The joints of the forelimb are the shoulder, elbow, carpal,
fetlock, and interphalangeal. The carpus, or knee, is equivalent to the human
wrist.
The rear leg (truly a leg) is made up of the os coxae (ilium,
ischium, pubis), femur, patella, tibia, and fibula, three rows of tarsal bones,
metatarsals 3 and 4, sesamoids, and phalanges 1, 2 and 3 of digits 3 and 4 (toes
3 and 4). The joints of the hind limb are the sacroiliac, hip, stifle, hock,
fetlock, and interphalangeal. The stifle is the equivalent of the human knee and
the hock the ankle.
 The
alpaca with correct leg conformation has straight legs when viewed from the
front or back (Figure 2).
In other words, when the front limb is observed, a plumb line should drop
straight from the shoulder, through the knee and fetlock, and between the two
toes. The toes should point forward. When the hind limb is observed from behind,
a plumb line should drop in a similar manner, from the hip through the hock and
fetlock and between the back of the two toes. The toes should likewise point
forward. When the animal is viewed from the side, the shoulder, elbow, and
stifle should have some angulation (Figure
1). The carpal region should
be straight (180 degrees) while the fetlock should have an angle of
approximately 190 degrees (Figure
3).
Describing the range of angulation is easy if you understand
a couple of anatomic precepts. First, angulation of the joints is defined as the
range of motion possible during flexion. Thus, the fetlock angle is defined in
relation to the metacarpus and the possible range of motion. A line dropped from
the tuber ischiadicum should touch the back of the hock, and the metatarsus
should be vertical. Normal angulation of the hock should be approximately 140
degrees.
 
Front Limb
Rear Limb
Anyone who has worked with horses and observed their conformation would call
alpacas post legged (legs too straight). Alpacas definitely have straighter legs
than horses, but they should not have a truly post-legged conformation. Animals
that are post legged will have less cushion to their gaits and are prone to
patellar and stifle problems.
The legs should also move freely and evenly. They should
travel in essentially straight lines with the forward plane of movement. While
alpacas have a relatively narrow chest and pelvis and move with their legs
underneath them, they should not "single track"—that is, they should
not look as if they are walking a tightrope. The distance side to side between
foot falls should match the breadth of the individual animal. In other words,
the column of support provided by the correctly aligned alpaca leg should be a
straight line in relation to the animal’s line of travel.
Alpacas are quadrapedal "cursors" (runners) that
move forward in a straight line, designed so that they can travel far or fast on
their four legs. Dogs, cats, and horses are also quadrapedal cursors. In
general, this group of animals includes predators and medium-to-large-size
herbivores.
Camelids are able to move distances as necessary and can run
to escape predators. The astounding stamina and speed of young male Patagonian
guanacos as they chase each other at continuously high speeds across the
landscape are just one example of the camelid’s highly developed cursorial
movement. Guanacos are also capable of jumping off of large rock outcroppings,
an ability that is also characteristic of quadrapedal cursors.
The basic design of quadrapedal cursors maximizes each stride
for speed, endurance, or a combination of the two. The length of the individual
stride times the rate of the stride results in speed. Endurance involves speed
coupled with economy of effort, which results from a combination of body shape,
the ways body parts move, and the mass of body parts. Cursorial animals have
relatively longer legs in relation to the body, and the muscle mass is carried
closer to the body. Often the distal segment—wrist and hand or ankle and
foot—is longer than those of other animals with different functions, and even
the portion from elbow to carpus or stifle to hock is relatively longer.
Within the camelids the camels have optimized these features
even more than the South American varieties ( Figure
4).
The "hand" and
"foot" is lighter because there are fewer digits. The alpaca retains
only digits 3 and 4 on which to walk, compared to the full mammalian complement
of five digits.
Foot posture also contributes to lengthening stride. The
bear, a plantigrade animal, walks on the entire hand and foot and has relatively
short limbs compared to the alpaca, a digitigrade, which means it walks on nail
and pad under part of its toes.
The joints of quadrapedal cursors also allow movement in
essentially only the forward-to-backward plane, thereby simplifying motion and
conserving energy. To illustrate this point, compare the movement in the human
shoulder or wrist to that of the alpaca. The human shoulder and wrist each
rotate essentially 360 degrees, while the alpaca shoulder and wrist (carpus)
move in one plane.
Evaluating Leg Conformation
 The
best way to evaluate the alpaca’s legs is to observe the animal moving in a
calm, relaxed manner, either loose or on a very loose lead. The animal is walked
directly away from and toward the examiner and is observed in a side view as
well. But theory and reality sometimes clash during evaluations of importations.
While the animals are loose, they frequently are nervous. Moreover, most animals
do not like to have their legs palpated, making it difficult to evaluate legs in
a "hands-on" manner.
The exam can be an entertaining undertaking in a narrow
alleyway with curious alpacas on either side of the loose animal. Animals that
are scared and tense will often crouch, which results in a sickle- and
cow-hocked appearance, so it is very important that the animals are as relaxed
as possible.
An animal will pass or fail the importation screening
depending on the veterinarian’s observations of the limbs. In the forelimb,
defects in alignment most commonly occur in the knee. Knock knees (carpus valgus)
are the most common defect. Knock knees are a bending at the carpus toward the
midline ( Figure 5).
A less commonly occurring defect of the knee is "calf knees," a
condition in which the limbs actually bend backward at the knee
(Figure
6).
The two common defects of the
fetlock are weak pasterns and cocked ankle. In weak pasterns, the fetlock
sometimes sags so much that it hits the ground when the animal walks. Cocked
ankle means that the pastern actually points somewhat backward (Figure
7).
 In
the hind limb the most common problems are sickle hocks, too much angle to the
hock when viewed from the side (Figure
6), and cow hocks, where the
hocks point toward the midline (Figure
5). Sickle hocks and cow hocks
usually occur together. The extremely post legged animal and the animal with
patellar luxations are also considered to have hind-limb defects.
Screening criteria for rejection by the veterinarians include
the following:
Forelimb
• Knock knees, angle greater than 15 degrees ( Figure
5)
• Calf knees, angle less than 165 degrees (preferably stated, greater than
195 degrees) ( Figure 6)
• Cocked ankle, angle greater than 90 degrees (preferably stated, less than
180 degrees) ( Figure 7)
• Down in fetlock or weak pasterns, less than 30 degrees (preferably
stated, greater than 240 degrees) ( Figure
7)
 Hind
limb
• Cow hocks, angle greater than 10 degrees toward midline
• Sickle hocks, angle less than 125 degrees ( Figure
6)
• Fetlock, same as with forelimb ( Figure
7)
Other leg conformation faults, recorded as "Other
Defects" on the veterinary screening form, may result in rejection of an
animal. Most commonly the animals rejected in this category move in an extremely
base-narrow manner or have other defects in movement that indicate pathology in
joints that are not visible to the veterinarian.
Since the 1996 changes to the screening criteria, leg
conformation is also evaluated by the phenotypic screeners. Often there is
discussion between the veterinarians and phenotypic screeners to increase
consistency in their evaluations.
The phenotypic screeners deduct points for slight and
moderate defects. The most common faults noted have been slight-to-moderate
sickle hock, slight-to-moderate cow hock, and different gradients of knock
knees. Points are also deducted for weak pasterns, a relatively common
occurrence. An animal with moderate knock knees and moderate cow and sickle
hocks, for example, would lose 21 points and would not be imported.
Always keep in mind that the animals are being evaluated as
they are presented and as they appear at the time of evaluation. Later injuries
are certainly possible so that animals may exhibit leg problems after
importation.
The Fiber Factor
 From
the veterinarian’s perspective, the ideal animal for evaluation is as naked as
a cue ball—there should be no fiber hiding the animal. Unfortunately, this is
seldom the case: most animals have moderate-to-heavy coats during the screening
evaluation. The quantity and distribution of fiber in particular can make
evaluation of the limbs very difficult. It is hard to see the limbs of a suri
with fiber dragging the ground. One of the biggest problems this poses is that
limbs that are actually straight can appear crooked (Figure
8).
One indication of normal leg
conformation on the heavily fibered animal would be that the toes point forward.
In general, the toes will point outwards with abnormal angulation of the limbs (Figure
5), so if the toes are
pointing forward, the legs are most likely straight. Calf-kneed animals can be
the exception. In this case the toes point forward even though the knees are
back from the normal.
If there are doubts about an animal’s legs, they should be
wrapped to compress the fiber. However, it is important not to wrap tightly, to
avoid supporting joints that may be weak. In some cases, legs may need to be
shorn to be able to truly evaluate an animal.
More Than Science
Though precise angles are specified, evaluation of limbs is
not a pure science. It requires the screener to develop over time and through
observation of many animals knowledge of what the correct leg conformation of
alpacas should be and the skill to identify it.
As with the entire screening process, evaluation of limbs has
been an evolving process. The current standards provide a higher minimum bar and
have resulted in the importation of more structurally sound animals than was
previously the case.
Copyright © 1998 by Karen I. Timm.
About the Author
Since 1983 Karen I. Timm, DVM, PhD, has taught anatomy and
small animal medicine in the College of Veterinary Medicine at Oregon State
University and in the last three years has participated in many importation
screenings for ARI. She holds a PhD in anatomy and DVM from the University of
California, Davis, and has worked as a mixed animal practitioner. She is
coauthor, with Brad Smith, DVM, PhD, and Patrick O. Long, DVM, of Llama
and Alpaca Neonatal Care.
|
