Purpose: To determine the usefulness of clinical imaging in predicting the mechanical properties of rabbit Achilles tendons after acute injury.
Methods: We created a 2 × 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·mm−2) and 8 wk (58 ± 6 vs 94 ± 26 N·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.