Description
- The Effect of Cyclic Loading on Material Properties of the Porcine ACL
- Unclear why ACL tears occur during “routine” pivoting
- Unclear if repetitive loading contributes to ACL tears
- Investigation of ACL strain behavior, energy dissipation, and load-to-failure under cyclic loading
- Knees from 40 Yorkshire breed pigs (1-4 mo, ~ 150/lb 70 kilo)
- ACL and menisci preserved
- 4 groups: 0,100, 250, 500 cycles
- load-to-failure testing
- Groups were compared with two morphometric indices
- Notch Width Index (NWI) and Notch Shape Index (NSI).
- NWI quantifies knee structural width by comparing Intercondylar Notch Width (ICW) to Epicondyle Width (EW).
- NSI evaluates overall knee geometric configuration by dividing ICW by Intercondylar Notch Height (ICH).
- Mechanical testing – Cyclic load servo-hydraulic MTS
- Tibia aligned 20o w load sensor
- Initial preload of 1 ~ 5 N
- Cyclic loading (100, 250, and 500 cycles) at 0.5 Hz.
- Ground reaction force (GRF) 1.5 - 2.0 x BW (mean 300 N).
- Load to failure testing
- unidirectional tensile loading at 1 mm/sec until rupture.
- Load and displacement data captured at frequency of 100 Hz.
- Ultimate force was measured directly from the load-displacement curves.
- Post-ultimate failure yielding was not considered.
- Anatomical evaluation conducted post-test
- Result – cyclic test
- Hysteresis loop pattern shifts from left to right with increasing cycles
- Strain energy substantially increases until the 50th cycle then levels off
- The initial cycle demonstrates a markedly higher dissipation rate (43.17±17.06 N/mm)
- more significant than subsequent cycles (p < 0.05)
Result – load-to-failure test
- Control =1034 ± 308 kN
- 100 cycles = 840 ± 255.14 kN (-19%) (NS)
- 250 cycles = 795.2 ± 233.7 kN (- 23%) (p < 0.05)
- 500 cycles = 764.2 ± 173.9 kN (-26%) (p < 0.05)
- predominant failure mode fiber tearing at tibia
discussion and limitation
- Cyclic loading ↆ energy dissipation in ACL
- Significantly ↆ ability to handle load
- Load to failure ↆ 23% @250 cycles; ↆ 26% @ 500
- Limited data on cyclic loading – Wotjys 3x and 4x BW failures in 5/10, 8/10 specimens (AJSM 2013)
- in vivo cyclic loading ↆ muscle activation, neuromuscular function, ↑anterior tibial translation
- 10 min passive cyclic loading (.1 H, 200 N) ↆ muscle activation, 51.7% spasms on EMG (Sbriccoli AJSM 2005)
- 37.7% ↆ biceps femoris; 20.7% ↑ATT (Nuccio Phy Ther Sp 2018)
- Repetitive subfailure cycling changes load to failure
- This + fatigue and abnormal neuromuscular activation of agonists may decrease threshold for ACL tear
- Limitations:
- Porcine knee model, not human (but young, healthy, consistent specimens)
- The apparatus, directly affixed to the bone, does not directly mimic normal knee function
