Holding Power of Orthopedic Screws in Equine Third Metacarpal and Metatarsal Bones: Part II. Adult Horse Bone

Abstract

Comparison was made of the holding power of 5.5 and 4.5 mm cortical orthopedic screws inserted into third metacarpal and metatarsal cadaver bones from 3‐ and 8‐year‐old horses. The tensile strength of these screws was tested mechanically. In nine comparative trials of these screws, 5.5 mm screws pulled out of bone in five trials at an average of 116.0 kg tensile force and broke in four trials at an average of 1383.2 kg. A 4.5 mm screw pulled out of bone at 834.5 kg in one trial, and screws broke at an average of 849.2 kg in eight trials. The larger 5.5 mm screw required a significantly greater (p = 0.022) pullout force than the mean force at 4.5 mm screw breakage. Fixation failure was due to screw breakage or bone shear, with 5.5 mm screws occasionally creating bone fragmentation during pullout. The average tensile breaking strengths of the 5.5 mm screws (1391.4 kg) and 4.5 mm screws (832.7 kg) determined mechanically were similar to forces at screw breakage during pullout testing in bone. Since the 5.5 mm screws have greater holding power and tensile strength than 4.5 mm screws, the use of the 5.5 mm screw in fracture repair in adult horses is recommended.

Screw Fixation in Lag Fashion of Equine Cadaveric Metacarpal and Metatarsal Condylar Bone Specimens: A Biomechanical Comparison of Shaft and Cortex Screws

“…Previous studies compared pull‐out rather than shear strength. Yovich et al 8 reported that 5.5‐mm cortex screws pulled out from equine bones at a tensile force 33% greater than that for 4.5‐mm cortex screws. By contrast, Kirpensteijn et al 10 and Blikslager et al 12 reported no significant differences in the holding power of 4.5‐mm and 5.5‐mm cortex screws in calf bones.…”

“…During recovery from anesthesia and within the first few weeks after fixation, these fractures are exposed to large loads (1–4× body weight), occasionally resulting in screw or bone failure and loss of reduction 5,6 . Initially, 4.5‐mm cortex screws were used; later 5.5‐mm cortex screws were advocated because of their superior strength 7,8 . The pullout strength of 5.5‐mm cortex screws is greater than that of 4.5‐mm cortex screws, but their relative performance in physiologic shear loading has not been compared 7 .…”

Pullout Properties of 3.5‐mm AO/ASIF Self‐Tapping and Cortex Screws in a Uniform Synthetic Material and in Canine Bone

“…Uniaxial pullout testing measures holding power by applying tension along the longitudinal axis of the screw, 18 and is a standard method for evaluating the efficacy of a bone screw. Pullout strength of bone screws has been shown to be most dependent on material properties of bone (bone mineral density, cortical thickness, geometry, age), major diameter of the screw, thread pitch, and difference between major and minor screw diameter, and less dependent on screw material 3,10,14,15,18–21 …”

Calcified tissue structure in the distal condyle of the third metacarpal bone in young Thoroughbred horses

“…Screw number, size, type and position have a substantial influence on fracture healing. In equine Mc3, 5.5 mm diameter cortical bone screws had a greater pullout and breaking strength than 4.5 mm screws (Yovich, Turner and Smith 1985b). A more recent study recommended the use of shafted 4.5 or 5.5 mm screws due to greater shear strength than fully-threaded screws, with no difference found between 4.5 and 5.5 mm cortical screws' shear strength (Rahm, Ito and Auer 2000).…”