Biomechanical Testing of Double Hemitenotomy for Equine Deep Digital Flexor Tendon Lengthening

Abstract

Deep digital flexor (DDF) tenotomy is performed in horses with laminitis to diminish the force of the DDF on the third phalanx (P3) which causes P3 rotation due to loss of laminar integrity counteracting the DDF pull. DDF tenotomy results in a guarded prognosis for athletic use. Consequently, a surgical procedure for DDFT lengthening rather than complete transection may be advantageous and allow for better recovery of tendon function compared to traditional tenotomy. The objective of this study was to develop a double hemitenotomy (DHT) technique to lengthen the equine DDFT. Models were used to 1) evaluate the DDFT tensile strength following DHT; 2) evaluate the DDFT lengthening following DHT compared to complete tenotomy (CT) and; 3) compare the reduction in DDFT load following DHT to CT. We hypothesized that DHT would result in less than 3 mm lengthening of the DDFT and at least half the load reduction of CT. The study was divided into two parts using cadaveric models. In part 1, cadaveric DDFT pairs (n=30) were utilized. Two hemitenotomies were created in one tendon while the other served as an intact control. Monotonic tensile load was applied to a maximum of 500 kg using an Instron Universal Testing Machine. Tendon lengthening, load reduction with hemitenotomy and load at failure were recorded by the Instron. In part 2, cadaveric forelimb pairs (n=16) were subjected to DHT followed by CT under monotonic compressive load applied by the Instron. Interactions between independent and dependent variables were assessed with statistical tests including the Wilcoxon Signed Ranks test and Friedman test. DHT resulted in significant DDFT lengthening and load reduction in both isolated tendons and intact cadaveric forelimbs. Less lengthening was achieved with DHT compared to CT (P=0.008). No difference in load reduction between DHT and CT was observed (P=1). A smaller load reduction was observed following the first hemitenotomy incision when compared to the second hemitenotomy incision (P=0.022). Isolated DHT tendons failed at a tensile load of 195 kg compared with none of the intact tendons (P=0.0001). None of the tendons failed in part 2. In conclusion, DHT was comparable to CT in load reduction. DHT reduced tensile strength, but load at failure was similar to or exceeded the estimated load on the DDFT in vivo at stance. DHT may be a useful alternative for surgical management of horses with laminitis, but in vivo studies are needed to confirm these findings.