Dr. Normand G. Ducharme
Upper airway obstruction is a common cause of poor performance in horses. Left laryngeal hemiplegia (ILH or roaring) has an estimated prevalence of 3 to 8%. In this disease, the left half of the larynx (voice-box) becomes paralyzed. When the horse inhales, the arytenoid cartilage (flapper) and attached vocal cord on the left side are sucked inward and obstruct the flow of air. The resulting airflow limitation is the cause of poor performance in racehorses, while abnormal noise production, with or without airflow limitation, interferes with show horses' performance or score.
Laryngoplasty, or “tie-back” surgery is currently the preferred surgical treatment for laryngeal hemiplegia, whereby, the paralyzed left arytenoid cartilage is sutured in an open position to restore airflow. This surgical procedure is successful in only about 60% of racehorses, a relatively low success rate because the flapper often fails to stay in position after surgery. In fact a recent study comprising 200 horses documented that even in successful cases the flapper loosened such that on average only 50% of the opening was left 6 weeks after surgery. Flapper loosening will clearly affect the airflow, however we currently do not know how large the airway must be in order to achieve normal air flow. Indeed, a lesser degree of flapper opening may achieve the same airflow characteristics, while requiring lower forces on the suture.
The information to be gained by this study should allow us to improve the current treatment and, more importantly perhaps, promote the innovation of new treatment. On the goal of improving the current treatment we believe several factors may explain flapper loosening after surgery. First, the suture is being placed between two pieces of cartilage, an inherently weak tissue, and one mechanism of failure is the suture cutting through the cartilage. Second, the suture-cartilage repair must withstand at least three forces, the magnitude of which we do not know: (1) the constant force due to gravity, (2) the intermittent collapsing forces due to swallowing and coughing, and (3) the forces associated with changing airway pressure during exercise.
What do we know about the forces that are placed on the flapper? Our previous studies have shown that the force required to overcome gravity and to fully (100%) open the flapper is 20-25 N. Cadaveric models have also shown that the force on the flapper fluctuates by a mere 3.0 N (in resting force +/- 1.5 N) when exposed to the cyclic changes in windpipe pressure seen during exercise. The forces fluctuation during exercise therefore represents only about 6 to 7.5% of the resting force. We know that the sutures currently used in tie-back surgery can themselves withstand nearly 250 N and the suture-cartilage interaction about 100 to 150N. This leads us to conclude that factors other than gravity or exercise must be important in flapper loosening. This seems even more likely when one considers that failure occurs during the first 6 weeks after surgery when the horse is resting.
We therefore propose to study the forces due to swallowing and coughing in the immediate post surgery period. If we can quantify these forces we could design better suited suture-cartilage interactions that are either strong enough to meet the force requirements, or that allow for collapse during swallowing rather than resisting the forces. Such dynamic implants would potentially allow for collapse during swallowing but not during breathing, and thereby reduce suture cut-through of the cartilage. How do we propose to measure the forces? We will use custom designed and manufactured transducers to measure the force on the tie-back suture. First the transducers will be evaluated using single suture strands loaded cyclically in a materials testing machine. Calibration and measures of repeatability and inherent accuracy will be performed during this step. Subsequently, they will be tested in a cadaveric model that simulates the airway pressure and flow of exercising horses. This will ensure that the transducers provide reliable data, comparable to data collected using more conventional means. This step will allow us to target an appropriate range of forces and to ensure that the transducers function accurately in a dynamic environment. The in vitro model is not capable of recreating swallowing and coughing, therefore the transducers will finally be applied to laryngoplasty sutures in live horses, as described below.
Therefore in the final step, we propose to quantify the real-life forces on the flapper due to coughing and swallowing. Having validated our transducers (during in vitro testing, see above), we will place transducers in horses with tie-backs targeted at the optimal degree of opening. We will then quantify the forces (gravitational, swallowing, coughing, and perhaps other unknown forces for example due to head position or movement) that are applied on the voice box and suture after surgery. In addition we will measure in a 24 hour period how often these forces are applied.
These studies represent an opportunity to acquire data which will help us to take significant steps forward in the management of a very common condition.