title: Restoration of strength despite low stress and abnormal imaging after Achilles injury.
abstract: OBJECTIVE
  To determine the usefulness of clinical imaging in predicting the mechanical properties of rabbit Achilles tendons after acute injury.
  
  
  METHODS
  We created a 2 x 7-mm full-thickness central tendon defect in one Achilles tendon of healthy rabbits. Rabbits in groups of 10 were killed immediately and 4 and 8 wk after surgery (n = 30). We then performed magnetic resonance (MR) imaging, ultrasonography (US), bone mineral densitometry (BMD), and mechanical testing to failure using a dual-cryofixation assembly on experimental and contralateral tendons. The main outcome measures included tendon dimensions, optical density (OD) of T1-weighted, proton density (PD), and T2-weighted MR sequences, US focal abnormalities, BMD of the calcaneus, and stress and peak load to failure.
  
  
  RESULTS
  On MR imaging and US, all dimensions of the injured tendons after 2 wk and more were greater than those of the contralateral tendons (P < 0.05). The mean T1-weighted OD was greater at 4 wk (256 +/- 53) and 8 wk (184 +/- 24) than immediately after surgery (149 +/- 15). Mechanical stress was markedly lower in the experimental than in the contralateral tendons at both 4 wk (39 +/- 9 vs 77 +/- 16 N x mm(-2)) and 8 wk (58 +/- 6 vs 94 +/- 26 N x mm(-2); P < 0.05). Mean peak load to failure was significantly lower immediately after surgery (332 +/- 128 N) than at 4 and 8 wk (712 +/- 106 and 836 +/- 90 N, respectively). Both high T1-weighted OD (r = -0.73) and PD OD (r = -0.69) correlated with lower mechanical stress (P < 0.05). In the experimental tendons, higher T1-weighted OD correlated with lower peak load (r = -0.46; P < 0.05).
  
  
  CONCLUSIONS
  Normal peak loads 4 wk after injury were withstood by an enlarged tendon of lower stress. These findings support progressive physical loading 4 wk after an Achilles tendon injury. T1-weighted OD constituted a marker of tendon mechanical recovery.
authors.affiliation: Bone and Joint Research Laboratory, University of Ottawa, Ottawa, Ontario, Canada. gtrudel@ottawahospital.on.ca
authors.forename: Guy
authors.initials: G
authors.lastname: Trudel
authors.forename: Geoffrey P
authors.initials: GP
authors.lastname: Doherty
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Koike
authors.forename: Nanthan
authors.initials: N
authors.lastname: Ramachandran
authors.forename: Martin
authors.initials: M
authors.lastname: Lecompte
authors.forename: Laurent
authors.initials: L
authors.lastname: Dinh
authors.forename: Hans K
authors.initials: HK
authors.lastname: Uhthoff
mesh_terms: D000125:Achilles Tendon / Q000000981:diagnostic imaging / Q000293:injuries* / Q000601:surgery; D000818:Animals; D001696:Biomechanical Phenomena; D005260:Female; D008279:Magnetic Resonance Imaging; D011792:Quebec; D011817:Rabbits; D013314:Stress, Mechanical; D013312:Stress, Physiological / Q000502:physiology*; D052256:Tendinopathy / Q000175:diagnosis* / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery; D014463:Ultrasonography