Accuracy of clinical isometry and preload testing during anterior cruciate ligament reconstruction

To determine the effect of suture elongation on the accuracy of tension isometers, an anatomic specimen was used to measure peak loads on suture materials placed in extreme nonisometric positions. Accuracy and reproducibility were determined for three tension isometers, and the influence of suture elongation was determined for one of them. In one device, reproducible readings ranged from -12.3% to + 23.6% of mean static loads recorded by hydraulic testing equipment; average error approached 39.1%. Values tended to be overestimated if readings ranged as high as 32.5% over specific excursion ranges. Selected suture materials were subjected to these loads, using hydraulic testing equipment. Stainless steel exhibited extremely low levels of elongation but only at large diameters. Ethibond had similar low levels of elongation but only at large diameters. Monofilament nylon exhibited significant elongation that could cause erroneous readings and nonisometric placement. The measurement tolerances of a selected tension isometer should be considered by the surgeon performing anterior cruciate ligament (ACL) substitution. Several readings, both loading and unloading, can be used to determine preload and isometry. Material properties of the selected suture and the accuracy of the tension isometer should be considered when interpreting readings from isometric positioners. This brings into question the ability of currently available tension isometers to deliver readings within prescribed tolerances for either isometry of preload testing.