Cyclic tensile tests of Shetland pony superficial digital flexor tendons (SDFTs) with an optimized cryo-clamp combined with biplanar high-speed fluoroscopy

Abstract

Background: Long-term cyclic tensile testing with equine palmar/plantar tendons have not yet been performed due to problems in fixing equine tendons securely and loading them cyclically. It is well established that the biomechanical response of tendons varies during cyclic loading over time. The aim of this study was to develop a clamping device that enables repetitive cyclic tensile testing of equine superficial digital flexor tendon for at least 60 loading cycles and for 5 min.

Results: A novel cryo-clamp was developed and built. Healthy and collagenase-treated pony SDFTs were mounted in the custom-made cryo-clamp for the proximal tendon end and a special clamping device for the short pastern bone (os coronale). Simultaneously with tensile testing, we used a biplanar high-speed fluoroscopy system (FluoKin) to track tendon movement. The FluoKin system was additionally validated in precision measurements. During the cyclic tensile tests of the SDFTs, the average maximal force measured was 325 N and 953 N for a length variation of 2 and 4 % respectively. The resulting stress averaged 16 MPa and 48 MPa respectively, while the modulus of elasticity was 828 MPa and 1212 MPa respectively. Length variation of the metacarpal region was, on average, 4.87 % higher after incubation with collagenase. The precision of the FluoKin tracking was 0.0377 mm, defined as the standard deviation of pairwise intermarker distances embedded in rigid bodies. The systems accuracy was 0.0287 mm, which is the difference between the machined and mean measured distance.

Conclusion: In this study, a good performing clamping technique for equine tendons under repetitive cyclic loading conditions is described. The presented cryo-clamps were tested up to 50 min duration and up to the machine maximal capacity of 10 kN. With the possibility of repetitive loading a stabilization of the time-force-curve and changes of hysteresis and creep became obvious after a dozen cycles, which underlines the necessity of repetitive cyclical testing. Furthermore, biplanar high-speed fluoroscopy seems an appropriate and highly precise measurement tool for analysis of tendon behaviour under repetitive load in equine SDFTs.

Keywords: Cryo-clamp; Equine; Horse; Strain; Superficial digital flexor tendon (SDFT); XROMM.

Fig. 1

Loading and unloading patterns of Shetland pony SDFT. a Typical force-time-diagram of repetitive cyclic tensile testing at 4% strain with initial decrease of force and plateau phase., b Stress-time-curves normalized over 1 s, b (I) creep effect in the 1st cycle at 2 and 4% strain, b (II) creep effect in the 1st cycle at 6% strain in untreated SDFT (blue) and with collagenase (orange) during the 60th cycle, b (III) creep effect during the 1st to 60th cycle in untreated SDFT at 6% strain, c Hysteresis curves during “walk” (black) vs. “trot” (red) during the 60th cycle, d Hysteresis curves in untreated SDFT (blue) and with collagenase (orange) during the 60th cycle, e Hysteresis curves at 4% strain (“trot”) during the 1st to 10th cycle and at 6% strain during the 60th to 90th cycle in untreated SDFT

Fig. 2

Measuring strain of the SDFT with FluoKin. Measuring strain within the 60th loading/unloading cycle on the basis of changing intermarker distances of implanted tantalum beads in the SDFT detected with FluoKin. a Spatial displacement of the proximal and distal tantalum bead at walk (black) and trot (red), b Mean difference between two tantalum beads during load cycles mimicking different gaits (error bars reflecting its maximum and minimum), dark grey: “gallop”, white: “walk”, grey: “trot”

Fig. 3

Measuring precision of the FluoKin gait lab. a Test sheet (70 mm × 60 mm × 4 mm to 2 mm falling edge from the largest to the smallest beads, front view (with imprinted distance [mm] for each pair of beads) and side view), b Resolution of the FluoKin system, images of the two image intensifier/camera systems without magnification, c Precision measurements with tantalum beads (0.8 mm and 1.0 mm) in an aluminium sheet and a forelimb (tendon and bone); *significant difference (p < 0.019)

Fig. 4

Mounting and cryogenic clamping devices for cyclic testing with pony SDFT. a mounting device for the short pastern bone (os coronale) with b technical drawing including a schematic of the specimen, c cryo-clamp, opened and with copper filling tube, d SDFT in the cryo-clamp mounted in the u-shaped connector, e technical drawing of one cryo-jaw, side view, f technical drawing of one cryo-jaw, top view

Fig. 5

Cryogenic clamping device for pony DDFT or SDFT of horses (30 × 55 × 80 mm). a Front view with copper filling tube, b Inside front view, c Front view with coated filling tube and Teflon funnel. We used a custom foam pipe insulation to keep the clamping device cold for a noticeably longer time

Fig. 6

Schematic design for the testing protocol of each group