Biomechanische Untersuchungen am Fesselgelenk (Articulatio metacarpophalangea) des Pferdes

The fetlock joint (articulatio metacarpophalangea) plays an
exceptional role within the limb, as it is the only joint which is
in hyperextension in the standing horse. It is therefore exposed to
special loading situations compared to other joints. The objective
of this study is to describe in a detailed way the morphological
parameters of loading in the equine fetlock joint
(metacarpophalangeal articulation). 45 specimens of 26 horses of
different breeds and sizes, male and female were examined. Their
age ranged from foal to 30 years. Using Computertomographic
Osteoabsorptiometry (CT-OAM) and Chondrocrassometry (CT-CCM) the
distribution of the subchondral bone density and the cartilage
thickness were determined topographically in the 3D reconstruction
of the joint. Generally the subchondral bone density increases in
all bones forming the fetlock joint with increasing age of the
horses. Up to the age of 2 years no significant increase in the
subchondral bone density can be stated. From the age of 3 years
onward a distinct increase can be measured until it comes to
stagnation in horses older than 13 years of age. In most cases the
areas with homogeneous subchondral bone density extend within the
articular surface. In the dorso-distal area of the trochlea a broad
transversal band of high bone density can be observed, which at the
angle of stance corresponds to a respectively dense part of the
phalangeal articular surface. The dorsal and palmar area at the
edge of the trochlea, as well as the palmar edge of the fovea
articularis show low bone density. Altough the lateral edges of the
trochlea and the fovea articularis show a higher density. The
method of split lines showed no definite preferential orientation
of the collagen fibres in the subchondral bone of the fetlock
joint. Therefore it is not possible to draw any conclusions
concerning physiological incongruence of this articulation. In
animals from 3 months to 2 years of age a change in the trochlear
profile can be observed from a cylindrical condyle to a concave -
convex shape in abaxial direction. This could be related to the
results described above. Surfaces of the trochlea (palmar area) and
the sesamoid bones articulating in the standing horse show a higher
subchondral density. In contrast to the subchondral bone density
the cartilage thickness generally decreases in older horses. After
the period of maximum growth in horses the decrease of cartilage
thickness stagnates at the age of 3 to 4 years. Articular surfaces
in the trochlea, the proximal phalanx and the sesamoid bones which
are in contact during stance show low cartilage thickness. It seems
that areas which are loaded more statically way are covered by a
thinner cartilage layer while areas loaded more dynamically are
covered by a thicker cartilage layer. Generally speaking the
tendency towards a negative correlation between subchondral bone
density and cartilage thickness can be observed in the fetlock
joint of the horse. An exception to this rule is the vertex area in
the sagittal groove, where both a high density and a high cartilage
thickness can be found.