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Sources of Variation in Flexor Tendon Repair Stiffness Measurements
Patrick J. Schimoler, MS1; Aakash Chauhan, MD, MBA2; Pierre-Marc François, MS3; Alexander Kharlamov, MD, PhD2; Edward Birdsong, MD2; Mark C. Miller, PhD2
1University of Pittsburgh, Pittsburgh, PA; 2Allegheny General Hospital, Pittsburgh, PA; 3ENS Cachan, Cachan, France

Introduction: There is a large variation in reported flexor tendon repair stiffnesses within the same technique and a lack of uniformity in stiffness calculations. This study evaluates the importance of included tendon length and gap presence in repair stiffness calculation.
Materials and Methods: Seven cadaveric flexor digitorum profundus tendons were transected in Zone II and repaired with a 4-strand cross-stitch cruciate technique (4-0 braided suture, no epitendinous stitch). A tensile testing machine displaced the tendon ends at 0.5mm/s. Cameras tracked markers attached in a 50mm line, centered on the transection site. Gap formation was determined from video. Ten regions, 4 to 40mm in length and centered at the injury site, were formed to ensure consistent comparison of regions between specimens. Stiffness was calculated as the slope of the linear portion of the force-displacement curve for all ten regions both before and after gapping. Repeated measures ANOVA determined whether region length or gap formation significantly affected stiffness. Significant results were compared with Tukey's Method (95% confidence interval).
Results: Pre-gap repair stiffness was significantly larger than post-gap repair stiffness (p<0.01). Repair stiffness calculated over 4mm was significantly larger than repair stiffness calculated over 32mm or more (p=0.036). Pre-gap repair stiffness calculated over 4mm was significantly larger than pre-gap repair stiffness calculated over 12mm or more and all post-gap lengths (p=0.041).
Conclusions: Prior to gap formation when bunching was present, measurements showed infinite stiffness at the transection site where the transected ends stayed in contact and from there, decreasing stiffness up to the repair zone edge (Figure 1). From the repair zone edge outwards, the pre-gap stiffness was nearly constant because the undamaged tendon's stiffness outside the repair zone was much higher than the repair stiffness. Post-gap stiffness was nearly constant across all regions because the tendon within the repair zone was stress free and not bunched, remaining in a fixed configuration while the underlying suture stretched with increasing load. The tendon outside the repair zone was much stiffer than the suture within, so its deformation was negligible compared to the suture causing markers to translate uniformly and stiffness to be constant. Repair stiffness can be misleading; studies should describe the length over which the stiffness is calculated and the presence of repair bunching or gap formation.

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