Are re-injured ligaments equivalent mechanically to injured ligaments: The role of re-injury severity?

• Published in: Proceedings of the Institution of Mechanical Engineers. Part H, Journal of engineering in medicine Journal of engineering in medicine, 2018-07, Vol.232 (7), p.665-672
• Main Authors: Sevick, Johnathan L, Heard, Bryan J, Lo, Ian KY, Randle, John A, Frank, Cyril B, Shrive, Nigel G, Thornton, Gail M
• Format: Article
• Language: English
• Subjects: Animals; Biomechanical Phenomena; Creep (materials); Cyclic loads; Equivalence; Failure load; Healing; Health risks; Injuries; Injury prevention; Joint and ligament injuries; Knee; Ligaments; Ligaments, Articular - injuries; Ligaments, Articular - physiology; Mechanical Phenomena; Mechanical properties; Mechanical tests; Rabbits; Risk factors; Stiffness; Strain; Wound Healing
• ISSN: 0954-4119; 2041-3033
• DOI: 10.1177/0954411918784088

The consequences of ligament re-injury have received limited attention. Although the mechanical properties of injured ligaments improve over time, these properties are never fully recaptured, rendering these injured ligaments susceptible to re-injury. Previous injury is a significant risk factor for recurrent injury, and this re-injury can result in longer absence from activity than the initial injury. A rabbit medial collateral ligament model was used to compare mechanically re-injured right medial collateral ligaments to injured left medial collateral ligaments. Two groups of different re-injury severity were investigated: ‘minor’ re-injury comparing transection re-injured right medial collateral ligaments to transection injured left medial collateral ligaments; ‘major’ re-injury comparing gap re-injured right medial collateral ligaments to transection injured left medial collateral ligaments. Initial injuries for both groups were right medial collateral ligament transections 1 week before re-injury. After 5–6 weeks of healing, mechanical testing was performed to determine (dimensionally) cross-sectional area; (structurally) medial collateral ligament laxity, failure load, and stiffness; and (materially) cyclic creep strain and failure stress. Because we wanted to evaluate whether the mechanical properties of re-injured ligaments were equivalent or, at least, no worse than injured ligaments, we used equivalence/noninferiority testing. This approach evaluates a research hypothesis of equivalence, rather than difference, and determines whether comparisons are ‘statistically equivalent’, ‘noninferior’, or ‘potentially inferior’. Transection re-injured and gap re-injured ligaments were ‘statistically equivalent’ structurally to transection injured ligaments. Transection re-injured ligaments were ‘noninferior’ both materially and dimensionally to transection injured ligaments. Gap re-injured ligaments were ‘potentially inferior’ both materially and dimensionally to transection injured ligaments. Two differences between the re-injuries, which affect healing, may explain the mechanical outcomes: the presence or lack of healing products and the proximity of ligament ends at the time of re-injury. Our findings suggest that (in the short term) there is a severity of re-injury below which there is no additional disadvantage to the healing process, mechanical behaviour, and resulting potential for re-injury.