The suspensory apparatus of the distal phalanx in normal horses.

This research investigates the structure and function of the suspensory apparatus of the distal phalanx (SADP) in normal horses, and uncovers its position and integration with other parts of the hoof.

Research Objectives and Methods

• This study aims to examine the shape, function, and microanatomy of the SADP in horses, which has significant functional and clinical importance.
• The research was conducted using descriptive gross and microanatomy methodologies.
• The researchers used the feet of six normal Standardbred horses for their study.
• The feet were cut into blocks across traditional perpendicular transverse axes and also along the operational axes of the SADP.
• These sections were then decalcified and processed for staining techniques such as haematoxylin and eosin, Jones’ periodic acid silver methenamine, and Masson’s trichrome stains.

Key Findings

• The study found that the presentation of SADP collagen bundles varied depending on the angle of the sectioning. SADP collagen bundles came across as irregular and unstructured in traditional midline toe transverse plane sections.
• However, when sections were made transversely along planes (70° and 30°), aligned with the long axis of the SADP, collagen bundles were organized in linear rows. The alignment demonstrated the bundles’ continuity, stretching from their origin on the distal phalanx’s parietal ridges, right through to the secondary epidermal lamellar’s basement membrane.
• At the parietal ridge interface, the collagen bundles fused into smaller, strongly silver-staining structures. These penetrated the cortical bone and merged with adjacent osteons.
• The study indicated variations in the collagen bundles’ arrangement in proximal and distal sagittal sections.
• In proximal sagittal sections, the bundles were linear and tilted at 70° to the ground surface while distal sagittal sections displayed the collagen bundles in a multi-angled, wheel-spoke configuration centred around the distal phalanx’s apex.

Conclusions and Implications

• Sectioning along functional axes unveiled the true suspensory nature of the SADP. The study demonstrated how it connects the parietal surface to the lamellar hoof wall.
• The research found that SADP/distal phalanx insertions have penetrating fibres that extend through the chondral-apophyseal interface, reaching up to and in between distal phalanx osteons.
• The researchers concluded that if lamellar measurements are made along the longer, functional midline 70° transverse plane, it would more accurately portray the lamellae’s suspensory function compared to measurements from sections perpendicular to the dorsal aspect of the distal phalanx.
• The study suggests that understanding the pathophysiology of laminitis – a common and painful condition affecting the hooves of horses – as degeneration of the SADP, should inform future therapeutic strategies effectively.