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.
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