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Research Papers

# Quantifying Ligament Cross-Sectional Area via Molding and Casting

[+] Author and Article Information
Kelly H. Schmidt

RR&D Center of Excellence for Limb Loss Prevention and Prosthetic Engineering, VA Puget Sound Heath Care System, Seattle, WA 98108; Department of Mechanical Engineering, University of Washington, Seattle, WA 98195

William R. Ledoux1

RR&D Center of Excellence for Limb Loss Prevention and Prosthetic Engineering, VA Puget Sound Heath Care System, Seattle, WA 98108; Department of Mechanical Engineering and Department of Orthopedics and Sports Medicine, University of Washington, Seattle, WA 98195wrledoux@u.washington.edu

1

Corresponding author.

J Biomech Eng 132(9), 091012 (Sep 01, 2010) (6 pages) doi:10.1115/1.4001881 History: Received July 14, 2009; Revised May 25, 2010; Posted May 27, 2010; Published September 01, 2010; Online September 01, 2010

## Abstract

Ligament cross-sectional areas are difficult to determine because ligaments are soft tissues, can be very short, and may be deep between bones. However, accurate measurements are required for determining the material properties from mechanical testing. Many techniques have been tried, but most suffer from one or more of the following: tissue deformation, tissue destruction, submersion of the tissue in saline, the need for a clear line of site, the inability to detect concavities, or poorly defined cross-sectional perimeters. Molding techniques have been used but have been limited by material issues such as large shrinkages, the inability to capture small detail, or the need to destroy the mold to remove the ligament. In this study, we developed a suitable molding and casting technique without systematic shrinkage that could accurately capture the odd shapes and concavities of foot and ankle ligaments with small clearances between bones. Metal rods of 1.62 mm, 2.90 mm, 3.18 mm, and 9.43 mm in diameter were molded using a liquid silicone rubber and cast with polyurethane. The effect of cutting the mold for specimen removal was investigated, and similar tests were done in the presence of saline. Image analysis software was used to determine the cross-sectional areas from photographs of cut castings. In addition, four different ligaments (each $n=5$) were dissected, molded, and cast. The cross-sectional area of each ligament was obtained. The maximum difference in area for all cases was 2.00%, with the majority being less than 1.00%; the overall root mean square error was $0.334 mm2$ or 0.97%. Neither cutting the mold for specimen removal nor the presence of saline affected the cross-sectional area of the castings. Various representative foot and ankle ligaments were also molded and cast to capture fine detail of the ligament midsubstance including concavities. We have developed a method of measuring ligament cross-sectional area that can overcome the limitations of other area measurement techniques, while accounting for the complicated anatomy of the bones of the foot. The method was validated using metal rods of known diameters, and a representative set foot ligaments $(N=20)$ was analyzed.

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## Figures

Figure 1

Molding preparation and procedure: (a) potting bony ends in PMMA, (b) suspended specimen with tongue depressor attached ready for molding, (c) finished mold, and (d) ligament removal with scalpel and retractor

Figure 2

Casting of the calcaneofibular ligament from a single foot. Left is the lateral view, and right is the medial view: (a) fibular insertion, (b) calcaneal insertion, (c) trabecular bone of calcaneus exposed with bone saw, and (d) ligament midsubstance showing fiber orientation.

Figure 3

Ligament cross sections from castings for all 20 ligaments tested in this study. Cross sections taken from the center of each ligament. Images are not all in the same anatomical orientation because left and right feet were used. Nevertheless, the broad range of cross-sectional areas and shapes are demonstrated.

Figure 4

Example with overhanging bones and small clearances: (a) interosseous 4th metatarsal—5th metatarsal (IM4M5) bone-ligament-bone specimen with overhanging bone, (b) mold of IM4M5 showing cross section of ligament, (c) casting of IM4M5 specimen, and (d) cross section of IM4M5 casting. The gap between the bones (see arrow) of the ligament specimen (a) and the casting (c) differ partly due to a slightly different camera angle. The white polyurethane filling the gap is actually very thin and disintegrated when the cross section was cut. A line marks where the cross section was cut on the casting (c). The cross sections of (b) and (d) differ slightly because the cross section of (d) was cut on a single plane, while (b) is a 2D representation of a 3D object, which includes shape changes throughout the length of the ligament.

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