Large Animal Surgery Resident Purdue University College of Veterinary Medicine West Lafayette, Indiana
To date no studies have investigated biomechanical properties of the tension band created by screw fixation of proximal sesamoid bones (PSB) as used in arthrodesis of equine metacarpophalangeal joints. The aim of this study was to compare biomechanical properties of two screw sizes (4.5-mm and 5.5-mm cortex screws) inserted into PSBs using lag technique. Five pairs of forelimbs were collected from adult horses euthanized for reasons unrelated to orthopedic disease. Following screw insertion, limbs were placed in a materials testing system and single cycle to failure was performed under controlled axial loading. Mean stiffness (kN/mm) and failure load (kN) was compared between groups. All constructs failed by transverse fracture of PSBs through the screw holes. There was no significant difference in construct stiffness between 4.5-mm (0.343 ± 0.288 kN/mm) and 5.5-mm (0.293 ± 0.102 kN/mm) groups (p = 0.723). The mean failure load was similar between groups (2.93 ± 0.441 kN for 4.5-mm and 2.891 ± 0.52 kN for 5.5-mm screws, p = 0.910). Limitations of this study include a small sample size and a lack of cyclic testing and bone density measurements. Results of this study show that in this mode of loading, screw size is not a critical determinant of tension band strength. The suspensory apparatus and PSBs are the weakest part of the construct when lag technique of the PSB is used in metacarpophalangeal joint arthrodesis. This supports reported successful use of both screw sizes in clinical cases.
