Implant preloading in extension reduces spring length change in dynamic intraligamentary stabilization: a biomechanical study on passive kinematics of the knee
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Dynamic intraligamentary stabilization (DIS) is a primary repair technique for acute anterior cruciate ligament (ACL) tears. For internal bracing of the sutured ACL, a metal spring with 8 mm maximum length change is preloaded with 60–80 N and fixed to a high-strength polyethylene braid. The bulky tibial hardware results in bone loss and may cause local discomfort with the necessity of hardware removal. The technique has been previously investigated biomechanically; however, the amount of spring shortening during movement of the knee joint is unknown. Spring shortening is a crucial measure, because it defines the necessary dimensions of the spring and, therefore, the overall size of the implant.
Seven Thiel-fixated human cadaveric knee joints were subjected to passive range of motion (flexion/extension, internal/external rotation in 90° flexion, and varus/valgus stress in 0° and 20° flexion) and stability tests (Lachman/KT-1000 testing in 0°, 15°, 30°, 60°, and 90° flexion) in the ACL-intact, ACL-transected, and DIS-repaired state. Kinematic data of femur, tibia, and implant spring were recorded with an optical measurement system (Optotrak) and the positions of the bone tunnels were assessed by computed tomography. Length change of bone tunnel distance as a surrogate for spring shortening was then computed from kinematic data. Tunnel positioning in a circular zone with r = 5 mm was simulated to account for surgical precision and its influence on length change was assessed.
Over all range of motion and stability tests, spring shortening was highest (5.0 ± 0.2 mm) during varus stress in 0° knee flexion. During flexion/extension, spring shortening was always highest in full extension (3.8 ± 0.3 mm) for all specimens and all simulations of bone tunnels. Tunnel distance shortening was highest (0.15 mm/°) for posterior femoral and posterior tibial tunnel positioning and lowest (0.03 mm/°) for anterior femoral and anterior tibial tunnel positioning.
During passive flexion/extension, the highest spring shortening was consistently measured in full extension with a continuous decrease towards flexion. If preloading of the spring is performed in extension, the spring can be downsized to incorporate a maximum length change of 5 mm resulting in a smaller implant with less bone sacrifice and, therefore, improved conditions in case of revision surgery.
KeywordsACL repair Anterior tibial translation Dynamic intraligamentary stabilization Knee kinematics
Anterior cruciate ligament
Anterior tibial translation
Dynamic intraligamentary stabilization
Range of motion
Alexander Bürki is acknowledged for his excellent support during biomechanical testing.
JH co-designed the study, composed the manuscript concept, and wrote the manuscript. BV conducted the biomechanical tests, performed all statistical analyses, and edited the complete manuscript. CK and JH operated all cases and helped editing the final draft version of the manuscript. DD supervised the study and helped editing the final draft version of the manuscript. PhZ co-designed the study, supervised it, and edited the complete manuscript. All authors read and approved the final manuscript.
The study was funded and implants were provided by Mathys Ltd., Bettlach, Switzerland.
Compliance with ethical standards
Conflict of interest
DD is a member of Mathys company. All other authors declare that they have no conflicts of interest.
Ethical approval and Informed consent
Four Thiel-fixated entire and intact human cadaveric bodies were obtained with informed consent from the Institute of Anatomy of the University of Bern.
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