Static and fatigue strength of a fixation device transducer for measuring anterior cruciate ligament graft tension

To determine which exercises do not overload the graft-fixation complex dur ing intensive rehabilitation from reconstructive surgery of the anterior cr uciate ligament (ACL), it would be useful to measure ACL graft lends during rehabilitative activities in vivo in humans. A previous paper by Ventura e t al. (1998) reported on the design of an implantable transducer integrated into a femoral fixation device and demonstrated that the transducer could be calibrated to measure graft lends to better than 10 percent full-scale e rror in cadaveric knees. By measuring both the static and fatigue strengths of the transducer, the purpose of the present study was to determine wheth er the transducer could be safely implanted in humans without risk of struc tural failure. Eight devices were loaded to failure statically. Additionall y seven devices were tested using the lip-and-down method to estimate the m edian fatigue strength at a life of 225,000 cycles. The average ultimate st rength was 1856 +/- 74N and the median fatigue strength was 441 N at a life of 225,000 cycles. The maximum graft load during normal daily activities i s estimated to be 500 N and the 225,000 cycle life corresponds to that of t he average healthy individual during a 12-week period. Considering that pat ients who have had all ACL reconstruction are less ambulatory than normal i mmediately following surgery and that biologic incorporation of the graft s hould be well developed by 12 weeks thus decreasing the land transmitted to the fixation device, the FDT can be safely implanted in humans without und ue risk of structural failure.