Mechanical properties and microstructural organization of common ulnar collateral ligament grafts: Palmaris longus and gracilis tendons

Abstract

Ulnar collateral ligament (UCL) injuries are becoming increasingly common. The palmaris longus (PL) and gracilis (GR) tendons are the most common grafts used in UCL reconstructions. While clinical studies have demonstrated relatively similar outcomes for either graft, there is little quantitative data describing these grafts from a material perspective, specifically the mechanical and microstructural properties of these tissues and how they respond under dynamic loading. The purpose of this descriptive laboratory study was to quantify and compare the mechanical and microstructural properties of PL and GR tendons. A total of 13 PL and 11 GR cadaveric human tendons were obtained. Each specimen was divided into three subregions and subjected to preconditioning, ramp‐and‐hold stress‐relaxation and ramp‐to‐failure testing. Mechanical parameters were computed for each sample, and a polarized light imaging technique was used to simultaneously evaluate dynamic microstructural properties during testing. The PL had larger toe‐ and linear‐region modulus values than the GR. Within the GR, the distal subregion had stronger collagen alignment than the proximal subregion at the zero, transition and linear portions of the stress‐strain curve. The PL and GR, have similar mechanical properties and similar microstructural alignment under load. The PL graft has similar properties throughout its length whereas the GR properties exhibited slight differences in strength of alignment along its length. The PL and GR exhibit larger moduli values and more strongly/uniformly aligned collagenous microstructure when qualitatively compared to data previously published on the native UCL.

Biomechanical Properties of Knee Medial Collateral Ligament Compared to Palmaris Longus for Ulnar Collateral Ligament Reconstruction

“…2 Stress-strain graph of one of the kMCL samples depicting outcomes measures analyzed, including: toe region Young's modulus (slope of the toe region line), linear region Young's modulus (slope of the linear region line), yield stress, maximum stress, and maximum toughness (the area under the curve) to increase with increasing strain for tendons [6,9-11 16]. The strain used in the present study was 5%, which demonstrated a similar decrease in stress compared to previous findings for both PL and kMCL [6,28].…”

“…From this sample, a 5 × 50 mm measured section was dissected from the mid-substance of the tendon. Previous research has demonstrated there is no significant difference in the material properties of the distal, proximal, and middle sections of the PL tendon [28].…”

“…Furthermore, the average age of patients undergoing UCLR is 19.1 ± 4.1 years, demonstrating a significantly lower age of PL autografts used surgically [17]. The 50 mm mid-substance graft was selected to fit the customized jig to ensure consistency during mechanical testing and as a result we were unable to characterize the entire graft length, although Solon et al previously demonstrated that the distal, mid, and proximal third of the PL tendon demonstrate similar mechanical properties [28]. The uniaxial tensile test performed in this study was subjected to a constant strain rate of 1% per second and may not characterize the tissues for all loading conditions, although strain rate dependence is relatively weak over physiological loading rates for ligaments and soft-tissues [5].…”

Biomechanical Effect of Differential Tensioning on Suture-Augmented Ulnar Collateral Ligament Reconstruction of the Elbow

“…33,34 In contrast, recent microstructural and mechanical characterization of the palmaris graft showed that it had an increased modulus, a higher baseline strength of fiber alignment, and greater change in fiber alignment during tensile loading compared with the native UCL. 36 It is possible that the addition of the suture tape in the augmented reconstruction could offer an additional checkrein to share the load and deformation on the palmaris graft, resulting in a lessened potential dynamic realignment and greater similarity of native collagen architecture dynamics to the native UCL. Future studies examining changes in dynamic collagen realignment with and without suture tape could investigate this potential advantage of the augmented reconstruction procedure.…”

Optical Imaging of Dynamic Collagen Processes in Health and Disease

“…Dynamic changes in microstructural organization can be recorded and analyzed via the creation of collagen fiber alignment maps (Figure 2D) (Quinn and Winkelstein, 2008;Lake et al, 2010;Buckley et al, 2013;Skelley et al, 2015;Shah et al, 2016;Zhang et al, 2016;Barnum et al, 2017;Wu et al, 2018;Smith et al, 2019). These spatial maps of collagen organization then can be correlated with tissue microstructural and mechanical properties to link structure-function relationships (Skelley et al, 2015;Skelley et al, 2016;Wright et al, 2016;Smith et al, 2019;Solon et al, 2021).…”