Loop Changes After Knee Flexion-Extension Movement in a Cadaveric Anterior Cruciate Reconstruction Model
Introduction
Anterior cruciate ligament (ACL) reconstruction relies heavily on secure femoral fixation to ensure graft stability during postoperative rehabilitation. Femoral suspension fixation devices, including fixed-loop devices (FLDs) and adjustable-loop devices (ALDs), are widely used for this purpose. FLDs, such as the Endobutton, offer high failure loads and reliability but face limitations in cases with short femoral tunnels due to their minimum loop length of 15 mm. This constraint necessitates additional bone tunnel length to accommodate plate flipping, potentially leading to the “bungee effect,” which compromises graft-to-bone healing. ALDs, exemplified by the Ultrabutton, address these limitations through an adjustable one-way locking mechanism that allows precise loop length customization. However, concerns persist regarding potential loop loosening under cyclic loading, particularly during early postoperative knee flexion-extension movements. This study compared loop length changes between ALDs and FLDs under simulated postoperative stresses in a cadaveric model to evaluate their reliability.
Methods
Specimen Preparation and Surgical Protocol
The study utilized cadaveric knee joints approved by the Department of Human Anatomy and Embryology at Peking University Health Science Center. The native ACL was arthroscopically removed, and standardized bone tunnels were drilled. The femoral tunnel, positioned at the center of the ACL’s direct insertion site, measured 6 mm in diameter and 40 mm in length. A tibial tunnel was created using a 55° angled guide, targeting the anteromedial aspect of the native ACL footprint. Polyester tape, folded to approximate a 6 mm graft diameter, served as the ACL substitute.
Group Allocation and Fixation Techniques
Knees were divided into two groups based on femoral fixation:
- FLD Group (n = 10): Endobutton devices with loop lengths ranging from 15–30 mm were used. The loop length was measured pre-fixation using calipers, with the distance from the button plate to the loop end recorded [Figure 1]. After femoral fixation, the tibial end was tensioned at 80 N for 15 seconds and secured to a staple under full knee extension.
- ALD Group (n = 12): Ultrabutton devices were adjusted to a predefined loop length before fixation. Post-tensioning and tibial fixation mirrored the FLD protocol.
Cyclic Loading Protocol
Both groups underwent 30 cycles of passive knee flexion-extension (0°–120°) to simulate early postoperative rehabilitation. Loop lengths were remeasured immediately after cycling. In the ALD group, loop lengths were reassessed 24 hours post-fixation to evaluate delayed retraction.
Data Collection and Analysis
Loop length changes were calculated as the difference between pre- and post-cycling measurements. Statistical analysis employed independent t-tests using SPSS 21.0, with significance set at P < 0.05.
Results
Immediate Loop Length Changes
The FLD group exhibited a mean loop elongation of 1.83 ± 0.77 mm (range: 0.6–2.9 mm), while the ALD group showed 1.64 ± 0.75 mm (range: 0.5–3.0 mm). No significant difference was observed between groups (t = 0.579, P = 0.569).
Delayed Retraction in ALDs
Twenty-four hours post-fixation, ALD loops retracted by 0.29 ± 0.33 mm (range: 0–1.1 mm). Three specimens displayed no retraction, suggesting stabilization of initial elongation.
Discussion
Biomechanical Performance of ALDs vs. FLDs
This study demonstrated comparable loop elongation between ALDs and FLDs under cyclic knee motion, supporting the hypothesis that ALDs do not exhibit inferior stability. The absence of significant differences challenges previous biomechanical studies reporting greater ALD elongation, which often used non-physiological models or isolated device testing. The cadaveric model here replicated clinical conditions, with graft tensioning at 80 N and fixation under full extension aligning with intraoperative practices.
Mechanisms of Loop Elongation
Loop elongation in both devices likely arises from material creep under tensile stress. FLDs, despite their fixed design, showed elongation comparable to ALDs, indicating inherent viscoelastic properties of the sutures or tape. In ALDs, initial elongation included contributions from suture slippage and creep, but the 24-hour retraction (≤1.1 mm) suggested partial recovery from creep, leaving residual elongation attributable to slippage. This highlights the importance of intraoperative re-tensioning, particularly for ALDs with short initial loops (e.g., 12 mm in one specimen), where elongation exceeded 3 mm.
Clinical Implications
ALDs offer advantages in cases with short femoral tunnels (<30 mm), where FLDs are impractical. By eliminating the need for excess tunnel length to flip the button plate, ALDs reduce the "bungee effect," potentially enhancing graft integration. The minimal retraction observed 24 hours post-fixation supports ALD reliability, though surgeons should account for initial elongation during tensioning.
Limitations
The study’s reliance on caliper measurements introduced potential human error, though standardized protocols minimized variability. Small sample sizes limited subgroup analyses, such as correlations between initial loop length and elongation. Future studies could incorporate electronic sensors for real-time loop monitoring and assess long-term biomechanical effects in vivo.
Conclusion
Under simulated postoperative knee flexion-extension stresses, ALDs and FLDs exhibited equivalent loop elongation, affirming the reliability of adjustable-loop devices in ACL reconstruction. ALDs provide a viable alternative to FLDs, particularly in cases requiring short femoral tunnels or precise graft-tunnel matching. Surgeons should consider intraoperative re-tensioning of ALDs to mitigate initial elongation, ensuring optimal graft stability during early rehabilitation.
doi.org/10.1097/CM9.0000000000000907
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