Biomechanics and histology of bovine claw suspensory tissue in early acute laminitis.

Weakening of the suspensory tissue supporting the pedal bone is the central issue in the theory of acute bovine laminitis, but this aspect has never been tested. The objective of this study was to investigate the effect of laminitis on the suspensory tissue. The hypothesis was that clinical and histological signs of acute laminitis are associated with decreased strength of the suspensory tissue of the bovine claw. Nonpregnant dairy heifers (n=10) received oral oligofructose overload (17 g/kg of body weight) and were killed 24 (n=4) and 72h (n=6) after overload. Control heifers (n=6) received tap water and were killed at 72 or 96h. Clinical, orthopedic, and histological examinations were carried out to confirm the occurrence of laminitis. After euthanasia, 2 adjacent tissue samples including the horn wall, lamellar layer, dermis, and pedal bone were cut from the dorso-abaxial aspect of each claw. Tissue samples were kept on ice until mounted on a mechanical testing frame, fixed by horn and bone, and loaded to failure. A stress displacement curve was generated and measurements of physiological support (force needed to displace 1mm beyond first resistance) and maximal support (force needed to break the tissue) were recorded. Heifers treated with oligofructose developed clinical signs consistent with ruminal and systemic acidosis after treatment as well as acute laminitis, characterized by weight shifting (35% of observations vs. 6% in controls), moderate lameness (100 vs. 17%, score of 3 out of 5 at 72h), and reaction to hoof testing (30 and 50% at 48 and 72h, respectively, vs. 0% in controls). Histological examination of claws from heifers killed 72h after overload showed changes consistent with acute laminitis, including stretched lamellae, wider basal cells with low chromatin density, and a thick, wavy, and blurry appearance of the basement membrane. Biomechanical results showed no effect of oligofructose overload on physiological support of the suspensory tissue at 24 and 72h after overload; in contrast, overload increased maximal support of the tissue 72h after overload. Herd of origin and location of the sample had large effects on both physiological support and maximal support (herd=547 N/cm(2); location=531 N/cm(2)) of claw suspensory tissue (herd=260 N/cm(2); location=327 N/cm(2)). Despite clinical and histological signs of laminitis, no weakening of the suspensory tissue of the bovine claw was detected at 24 and 72h after oligofructose overload. Herd factors appeared to be important for claw suspensory tissue strength.