Does the Nice Knot Offer Less Elongation Than the Modified Prusik Knot? An In Vitro Study in Cadaver Quadriceps Tendons.

BACKGROUND

ACL graft-suture fixation can be constructed with needle or needleless techniques. Needleless techniques have the advantage of decreased injury, preparation time, and cost. The Nice knot is common among upper extremity procedures, and has been shown to have higher load to failure and less elongation compared with other double loop knots; however, there are no studies that have looked at its use for ACL graft-suture construct to determine whether it offers less elongation relative to other needleless techniques.

QUESTIONS/PURPOSES: In a cadaver quadriceps tendon model, we asked: (1) Does the Nice knot have less elongation than the Prusik knot? (2) Does the Nice knot have increased peak load and stiffness compared with the Prusik knot? (3) What were the modes of failure of each knot?

METHODS

Sixteen quadriceps tendon grafts were harvested from 16 cadaver knee specimens. The median (range) age of the donors was 80 years (70 to 96) and included three male and five female donors. Eight grafts were prepared with the Prusik knot and eight with the Nice knot using a braided polyblend suture. The graft-suture constructs were mounted in a materials testing machine and subjected to a tensile loading protocol beginning with pretensioning of three cycles from 0 to 100 N at 1 Hz followed by a constant load of 50 N for 1 minute then cyclic loading of 200 cycles from 50 to 200 N at 1 Hz. The constructs were loaded to failure as the final step of the loading protocol. Elongations of the construct after each loading step, peak load, stiffness, and graft cross-sectional area were compared.

RESULTS

Construct elongations (median [IQR]) for the Nice knot were lower than that of the Prusik knot after pretensioning (4.4 mm [0.8] versus 5.7 mm [1.4]; p = 0.02), preloading (0.6 mm [0.3] versus 1.0 mm [0.3]; p = 0.005), and cyclic loading (7.4 mm [1.4] versus 10.9 mm [2.1]; p = 0.005). Peak load was not different for the Prusik knot construct compared with the Nice knot (334 N [43] versus 312 N [13]; p = 0.08). Stiffness of the Prusik knot construct (103 N/mm [17]) was no different than the Nice knot construct (110 N/mm [13]; p = 0.13). Graft cross-sectional area of the Prusik knot constructs (85 mm2 [35]) were similar to the grafts of the Nice knot constructs (97 mm2 [31]; p = 0.28). Failure mode of the constructs did not differ between groups; it was caused by suture rupture near the knots that secured the free suture ends to the machine and was seen in all 16 tests.

CONCLUSIONS

The results of this biomechanical study show that the Nice knot construct has similar or greater biomechanical properties compared with the Prusik knot in the graft suture construct, although the magnitude of the differences are not likely to the level of clinical importance.

CLINICAL RELEVANCE: The Nice knot offers an attractive alternative option for needleless ACL graft preparation technique. Future studies should consider comparison to established needle techniques such as Krackow or whipstitch and testing in an intraarticular component in an in vivo model.