Document Title: Wallace-AJSM-Jan02.shtml
Article Title: Creep Behavior of a Rabbit Model of Ligament Laxity after Electrothermal Shrinkage In Vivo
Authors: Andrew L. Wallace, MD, PhD, FRACS(Orth), Robert M. Hollinshead, MD, FRCSC and Cyril B. Frank, MD, FRCSC
Publication: American Journal of Sports Medicine, Baltimore, Maryland
Date: January-February 2002
Volume 30, Number 1, pages 98-102
Keywords: thermal shrinkage, knee, ACL, electrothermal, tissue biomechanics.

(Figures included. Reference-denoting numbers appear in the same point size as the document text.)

Comments: This is an interesting article from the viewpoint of fundamental research in the realm of thermal shrinkage. Note that because thermal shrinkage is also used for shoulders, this article initially discusses this application...although the study entailed thermal shrinkage of the MCLs of rabbits, and thus is very much knee-relevant. The article provides grounds for serious concern regarding the use of thermal shrinkage for knee ligaments, specifically with regards to stretching-out of tissue and failure of the tissue to fully regain the tensile strength that is the hallmark of normal collagen. Wallace notes that the "risk of early failure or development of recurrent laxity in the first few weeks after thermal shrinkage is significant", and recommends further research with regards to rehabilitation and mobilization. Careful reading of this article (along with subsequent pondering and critical analysis) reveals that the issue of low-load viscoelastic behaviour, hence creep (manifested as stretching-out) of the treated ligament, remains a concern long after the first few weeks mentioned in the foregoing quote. Additionally, please keep in mind that the knee of a rabbit is not exposed to proportionally the same physiological loadings as the human knee. Consider that the human knee is situated at the interface of two lever arms (tibia and femur) that are proportionally much longer than their counterparts in the rabbit; furthermore, humans walk on only two legs and yet rabbits have four legs at their disposal. These factors mean that the thermal-shrinkage-related worries raised by this study are greatly amplified when human knees are being considered.

ABSTRACT

Deformation of capsular tissue under constant load (creep) may lead to recurrent laxity after thermal shrinkage for shoulder instability. We investigated the effects of thermal shrinkage in a rabbit model in which the tibial insertion of the medial collateral ligament was elevated and shifted toward the joint line to create abnormal laxity. On the right side, radiofrequency electrothermal energy was applied to the shifted ligament, while the left side served as a control. Length, laxity, mass, cross-sectional area, water content, and creep behavior of the ligament were assessed at 0 (N = 8), 3 (N = 7), and 12 (N = 6) weeks postoperatively. Laxity was reduced with thermal treatment (0.65 ± 0.31 compared with 3.33 ± 0.25 mm). After 3 weeks, ligament mass, area, and water content were significantly increased in the thermally treated group compared with the untreated controls. At 12 weeks, cyclic creep strain remained greater than that in controls (1.25% ± 0.65% compared with 0.93% ± 0.22%). Although thermal shrinkage reduced laxity, there was increased potential to creep and failure at low physiologic stresses. These findings suggest that loading of thermally treated tissues should be carefully controlled during the early phase of rehabilitation after surgery.

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