Abstract
Objectives:
The purpose of anatomic anterior cruciate ligament (ACL) reconstruction (ACLR) is to replicate the patient’s normal anatomy and restore physiological kinematics. Graft selection is an important consideration, with quadriceps tendon (QT) gaining popularity as ACL autograft. Previous studies have shown very good clinical results with the use of QT autograft. Aiming for an individualized approach includes choosing an adequate graft size which aligns with the morphological and patient-specific characteristics of each individual. Native QT size varies and can be measured by magnetic resonance imaging (MRI) and ultrasound. Harvesting a central quadriceps tendon autograft leads to a reduction in tendon strength which may potentially result in tendon rupture. Furthermore, quadriceps muscle weakness being among the most common postoperative complications after ACLR can have significant implications for clinical outcomes. Therefore, a thorough understanding of graft size-depended strength reduction is important for decision making in clinical practice. The aim of this study is to evaluate 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. The extensor mechanisms from fresh frozen cadaveric knee pairs were harvested. Within pairs, each specimen was randomly assigned to either 1) intact QT or 2) donor QT group. For each pair, donor QT after harvest and contralateral intact QT were tested. In the donor QT group, a partial thickness soft tissue graft was harvested from the center of the QT, aiming for a graft size of 5 mm x 10 mm x 70 mm. Prior to the testing of the tissues, the donor QT before harvest, resected graft and contralateral intact QT were laser scanned (Faro Arm, Faro Inc.) to measure their cross-sectional area (CSA). Due to size variation of the initial tendons’ CSA, each graft represented a different percentage of the tendon’s initial CSA. The patella was potted in resin blocks. For tensile testing, the resin block was clamped to the machine base and the proximal end of the tendon was secured by a freeze clamp at 80mm tendon length. 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. The tissues were marked with optical markers to measure elongation with a digital image correlation system (DIC, Correlated Solutions, Inc.). The intact tendon served as control to the donor tendon of the same individual.
Results:
A total of 10 paired human extensor mechanisms from 5 individuals (mean age 55.6 ± 12.2 years) were tested. Failure mode for all donor QTs was mid-substance failure, one of which also experiencing failure at the bone insertion. Failure mode in the five intact QTs was bone insertion failure, with one intact QT specimen also demonstrating an additional mid-substance rupture. The CSA of the donor tendons before harvest (234.6 ± 29.6 mm2) was reduced 23.0% on average after graft harvest. The intact QT tendons failed under higher ultimate load than the donor tendons (5461 ± 1212.4 N vs. 2562.74 ± 638.6 N, respectively; P<0.001) (Table 1). Ultimate load was significantly reduced in the donor QT when compared to the intact paired QT of each individual with a mean reduction of ultimate load of 53.0%. 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 the tendon strength per 1% reduction of tendon CSA.
Conclusions:
The main finding of this study is that the ultimate load of donor QTs was reduced by 53.0% and that each 1% of reduction of tendon CSA resulted in a 2.3% reduction of the QT’s ultimate load compared to the intact contralateral QT. These findings could help explain prolonged quadriceps weakness after ACL reconstruction, especially in athletes with decreased quadriceps muscle strength. However, this cadaveric study does not account for postoperative tissue adjustments, such as healing, in-vivo. Ultimate loads and CSA measurements for the intact QT were comparable to previous publications. Limitations of this cadaveric study include the small sample size and the assumption of paired tendons having similar biomechanical properties. Future studies should investigate how factors, such as graft length, age, and sex, influence graft-size dependent reduction of donor tendon strength. This data can help determine how large a QT autograft can be harvested to optimize individualized graft selection in ACLR and reduce the risk of complications.