title: Low-energy impact of human cartilage: predictors for microcracking the network of collagen.
abstract: We aimed to determine the minimum mechanical impact to cause microstructural damage in the network of collagen (microcracking) within human cartilage and hypothesized that energies below 0.1 J or 1 mJ/mm3would suffice.
  
  
  
  We completed 108 low-energy impact tests (0.05, 0.07, or 0.09 J; 0.75 or 1.0 m/s2) using healthy cartilage specimens from six male donors (30.2 ± 8.8 yrs old). Before and after impact we acquired, imaging the second harmonic generation (SHG), ten images from each specimen (50 μm depth, 5 μm step size), resulting in 2160 images. We quantified both the presence and morphology of microcracks. We then correlated test parameters (predictors) impact energy/energy dissipation density, nominal stress/stress rate, and strain/strain rate to microcracking and tested for significance. Where predictors significantly correlated with microstructural outcomes we fitted binary logistic regression plots with 95% confidence intervals (CIs).
  
  
  
  No specimens presented visible damage following impact. We found that impact energy/energy dissipation density and nominal stress/stress rate were significant (P < 0.05) predictors of microcracking while both strain and strain rate were not. In our test configuration, an impact energy density of 2.93 mJ/mm3, an energy dissipation density of 1.68 mJ/mm3, a nominal stress of 4.18 MPa, and a nominal stress rate of 689 MPa/s all corresponded to a 50% probability of microcracking in the network of collagen.
  
  
  
  An impact energy density of 1.0 mJ/mm3corresponded to a ∼20% probability of microcracking. Such changes may initiate a degenerative cascade leading to post-traumatic osteoarthritis.
authors.affiliation: University of Connecticut, Department of Biomedical Engineering, Storrs, CT, USA.
authors.forename: B
authors.initials: B
authors.lastname: Kaleem
authors.affiliation: Department of Mechanical Engineering, University of Connecticut, Storrs, CT, USA.
authors.forename: F
authors.initials: F
authors.lastname: Maier
authors.affiliation: University of Connecticut Health Center, Orthopedic Surgery, Farmington, CT, USA.
authors.forename: H
authors.initials: H
authors.lastname: Drissi
authors.affiliation: University of Connecticut, Department of Biomedical Engineering, Storrs, CT, USA; Department of Mechanical Engineering, University of Connecticut, Storrs, CT, USA. Electronic address: dmpierce@engr.uconn.edu.
authors.forename: D M
authors.initials: DM
authors.lastname: Pierce
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D002358:Cartilage, Articular / Q000293:injuries* / Q000378:metabolism / Q000473:pathology; D003094:Collagen / Q000378:metabolism*; D006801:Humans; D016015:Logistic Models; D008297:Male; D000073798:Second Harmonic Generation Microscopy; D013314:Stress, Mechanical; D055815:Young Adult
chemical_list: D003094:Collagen
keywords: Bone; Energy dissipation; Human articular cartilage; Impact energy; Mechanical injuries; Stress