Effect of contact stress in bones of the distal interphalangeal joint on microscopic changes in articular cartilage and ligaments.

This study examines how contact pressure affects the microscopic composition of parts of horse joints, and suggests that changes in the contact stress of these parts could lead to lameness in horses.

Objectives and Methods

The research aimed to study the adaptive responses of various sections of a horse’s distal interphalangeal (DIP) joint, particularly components like articular cartilage and distal sesamoidean impar ligament (DSIL), to contact stress. The researchers also investigated the deep digital flexor tendon (DDFT).

• To examine these components, the scientists included specimens from 21 horses. They inserted a pressure-sensitive film between the articular surfaces of the DIP joint and subjected the digit to a load to record stress patterns.
• Data gathered was used to form finite element models (FEM), simulating the pressures and stresses inside the joint. Histological analysis, i.e., microscopic examination, of the navicular bone, distal phalanx, and distal attachments of the DSIL and DDFT was also conducted.

Findings

Going through the pressure-sensitive film records, the scientists discerned significant increases in contact area and load pressure when the joints were dorsiflexed, particularly between the distal phalanx and navicular bone, as well as between the middle phalanx and navicular bone.

• The FEM analysis indicated the presence of compressive and shear stresses at the joint’s locations.
• The histologic examination revealed depletion of proteoglycans, essential constituents of cartilage, in the articular cartilage of older horses (ranging from 7 to 27 years old). This evidence corroborates age as a factor in horses’ osteoarthritis.
• Additionally, the presence of numerous tidemarks (lines that demarcate the calcified from the non-calcified cartilage) in articular cartilage was noted. The number of tidemarks was especially high among clinically normal older horses and tended to be even more in horses with navicular syndrome. This might indicate a progressive degenerative process.
• A progressive increase in proteoglycans (basically, resilience-conferring molecules) was found in the DSIL and DDFT of clinically normal horses.

Conclusions

The study concludes that maximum stress on the equine navicular bone and adjoining joints surfaces when in dorsiflexion. This increased stress load could trigger microscopic transformations, such as the advance of tidemarks and loss of proteoglycans in the articular cartilage or increased proteoglycan production in the DSIL and DDFT. The research suggests these changes are likely adaptive responses to stress, but when they progress, they may lead to lameness in horses.