Funktionell-anatomische Untersuchungen an den Zehengelenken (Articulationes interphalangeae) der Schultergliedmaße des Pferdes

Functional anatomical investigations on the interphalangeal joints
of the forelimb in the horse. The proximal and distal
interphalangeal joints are of particular importance in sporting
horses, as degenerative diseases or fractures lead to a loss in
performance. It was the objective of this study to investigate
several biomechanical properties of the coffin and pastern joints.
Forty three specimens of 30 male and female horses of different
breeds and sizes were examined. Their age ranged between 3 months
and 30 years. Using computertomographic osteoabsorptiometry
(CT-OAM), the distribution of the subchondral bone density was
determined topographically in three-dimensional reconstructions of
the distal and proximal interphalangeal joints. The specimens were
divided into 6 age groups. Following exarticulation the joints were
evaluated for chondral lesions. These were documented by using a
template of the respective articular surface and were summed up
graphically in order to demonstrate the most common locations for
chondral damage. The size of the articular surfaces was determined
in order to investigate a relationship between the subchondral bone
density and the size of the joints. Five specimens (coffin joint
and pastern joint) were examined with regard to split lines in the
articular cartilage. The articular cartilage was pierced at regular
intervals (2 mm) at 90° angles with needles that had previously
been dipped into ink. The subchondral plate of four specimens
(proximal and distal interphalangeal joint), after maceration and
demineralisation, was investigated with regard to subchondral split
lines in the same way as described above. The CT-OAM shows a
relatively low subchondral bone density in the first age group
(horses up to 9 months). In the following age groups the bone
density generally increased, varying with the age of the horse. The
maximum bone density is found in the palmar areas of the coffin
joint and the pastern joint. As far as the coffin joint is
concerned, this is due to weight bearing and the tension of the
deep digital flexor tendon, which causes a compressive loading of
the navicular bone. The latter is pressed against the distal
articular area of the middle phalanx. In the proximal
interphalangeal joint, the suspensory apparatus, in addition to
weight bearing, has an enhancing effect on the subchondral bone
density. The subchondral bone density is significantly higher on
the distal articular surface of the proximal phalanx than on the
distal articular surface of the middle phalanx. One of the reasons
being that the trochlea of the metacarpophalangeal bone is about
20% smaller than the trochlea of the proximal interphalangeal bone.
This means that the same weight has to be carried by a smaller
area; thus increasing the relative load of the articular surface
and resulting in a higher subchondral bone density. The orientation
of the cartilaginous and subchondral split lines is quite similar.
In the proximal interphalangeal joint the split line pattern has a
clear sagittal preference. In principle this applies to the coffin
joint, too. In the distal articular surface of the middle phalanx,
however, split lines are oriented in a transverse direction at the
palmar aspect of the trochlea. This indicates transverse strain of
the trochlea in this area. The orientation of the cartilaginous and
subchondral split lines is interpreted as an expression of
functional adaptation to tensile stress. Cartilage lesions are seen
in areas with a high subchondral bone density as well as in areas
with a relatively low subchondral bone density. The lesions in
areas with a high subchondral bone density are caused by permanent
high mechanical loading. In contrast lesions in areas with a
relatively low subchondral bone density are attributed to peak
loads of a short duration. The investigations on the subchondral
bone density could significantly deepen the understanding of the
biomechanical properties of the interphalangeal joints. Long-term
strain could be visualised by the study of the split lines. This
gives evidence of a physiological incongruence of the
interphalangeal joints. It is, however, necessary to investigate
the contact surfaces and contact pressures in vitro in order to
evaluate the articular surfaces with regard to physiological
incongruence.