Summary

The ultimate load of donor quadriceps tendon after graft harvest was reduced by 53.0%, equivalent to 2.3% reduction of ultimate load for each 1% of reduction of cross-sectional area, when compared to the intact contralateral tendon.

Abstract

Introduction

Graft selection is an important consideration in anterior cruciate ligament (ACL) reconstruction (ACLR). Quadriceps tendon (QT) autograft is gaining popularity as ACL graft. Native QT size varies and can be measured by magnetic resonance imaging and ultrasound. Harvesting of a quadriceps tendon autograft leads to a reduction in donor tendon tissue which may potentially result in tendon rupture. Furthermore, quadriceps muscle weakness after ACLR can have significant implications for clinical outcomes. The aim of this study is to evaluate graft size-depended strength reduction of donor QTs and the effect of the ratio graft size to donor QT size on the failure strength of the remaining donor tendon.

Methods

Institutional approval was received for this study (Corid ID 1186). Within pairs of cadaveric extensor mechanisms, each specimen was randomly assigned to either 1) intact QT or 2) donor QT group. A partial thickness soft tissue graft was harvested from the donor QTs, aiming for a graft size of 5 mm x 10 mm x 70 mm. Donor QT before harvest, autograft and contralateral intact QT were laser scanned (Faro Arm, Faro Inc.) to measure their cross-sectional area (CSA). Due to tendon size variation, each graft represented a different percentage of the tendon’s initial CSA. The patella was potted in a resin block, which was clamped to the machine base. The proximal end of the tendon was secured by a freeze clamp. All tendons were cycled 20 times (20 to 50 N), preloaded with 10 N, and finally loaded to failure in an axial testing machine at a rate of 10 mm/min. Tissue elongation was measured with a digital image correlation system (DIC, Correlated Solutions, Inc.).

Results

A total of 5 pairs of human extensor mechanisms (mean age 55.6 ± 12.2 years) were tested. Failure mode for all donor QTs was mid-substance failure, and failure at or close to the bony insertion in all intact QTs. The donor tendons’ native CSA (234.6 ± 29.6 mm2) was reduced by 23.0 ± 4.01% after graft harvest. The intact QT tendons failed under higher ultimate load than the donor tendons (5461 ± 1212.4 vs 2562.74 ± 638.6 N, respectively; P<0.001) with a mean reduction of 53.0% when compared to the intact contralateral QT. The mean ratio of reduction in QT CSA to reduction in ultimate load was 0.44±0.09, equivalent to an average of 2.3% reduction of ultimate load for each 1% reduction of tendon CSA.

Conclusion

The main finding of this study is that the ultimate load of donor QTs was reduced by 53.0% and that 1% of reduction of tendon CSA resulted in a 2.3% reduction of the QT’s ultimate load compared to the intact contralateral QT. Limitations of this cadaveric study include the sample size, the assumption of paired tendons having similar biomechanical properties and the inability to account for postoperative in-vivo tissue adjustments. These findings could help explain prolonged quadriceps weakness after ACLR and help determining how large a QT autograft can be harvested, to reduce the risk of complications.