Biomechanical testing of the suspensory apparatus of the phalanx distalis in the claw of beef bulls

The objective of this study was to test mechanically the suspensory apparatus of the third phalanx in the horn capsule. The ultimate stress [N/mm(2)] was determined and the part of the tissue was investigated where the samples failed in tensile testing. Healthy beef bulls without lameness (German Simmentals; age 18-20 months; 527 +/- 25 kg; n = 13) were chosen for this purpose. The claws were prepared in such a way in order to measure the ultimate stress of the suspensory apparatus in the proximal part of the wall segment in three locations (dorsal, abaxial, axial). Furthermore the samples of four bulls were examined histologically after testing. In the dorsal testing position the values for the shoulder and pelvic limb ranged from 2.47-3.13 N/mm(2). In the abaxial position values varied between 4.08-4.87 N/mm(2), in the axial position between 2.27-2.66 N/mm(2). The maximal strength in each position (dorsal, abaxial, axial) in corresponding claws (lateral-medial or front-hind, respectively) did not differ significantly. The abaxial testing position, however, had significantly higher values for the ultimate stress compared to the dorsal and axial positions within one claw. In 57.1 % of all samples the tissue was torn apart at the level of the interdigitating dermal and epidermal papillae (stratum papillare of dermis). In 27.6 % of the testing specimens the tissue was disrupted in the deeper layer of the stratum reticulare. Some samples showed disruption within the horn (11.5 %) or at the attachment of the third phalanx (3.8 %). Histological sections show that the tissue is often torn apart in an uncontrolled way transversely to the interdigitating dermal and epidermal lamellae. If dermis and epidermis are separated as complete lamellae the weakest spot can be found in the epidermal stratum spinosum. If the zone of separation lies deeper it is the connective tissue of the dermal stratum reticulare which is affected. If disruption occurs within the horn the weakest location is the connection between the wall horn and the coronary horn. Separation near the third phalanx can be found at the fibrocartilaginous attachment of the dermis at the bone. There was no correlation between the mean values of ultimate stress and the respective level of disruption. The experimental setup and the choice of material are discussed as well as the findings and their compatibility with the latest theories of the pathogenesis of bovine laminitis.