Cross-Sectional Area Measurement Techniques of Soft Tissue: A Literature Review

Abstract

Evaluation of the biomechanical properties of soft tissues by measuring the stress-strain relationships has been the focus of numerous investigations. The accuracy of stress depends, in part, upon the determination of the cross-sectional area (CSA). However, the complex geometry and pliability of soft tissues, especially ligaments and tendons, make it difficult to obtain accurate CSA, and the development of CSA measurement methods of soft tissues continues. Early attempts to determine the CSA of soft tissues include gravimetric method, geometric approximation technique, area micrometer method, and microtomy technique. Since 1990, a series of new methods have emerged, including medical imaging techniques (e.g. magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound imaging (USI)), laser techniques (e.g. the laser micrometer method, the linear laser scanner (LLS) technique, and the laser reflection system (LRS) method), molding techniques, and three-dimensional (3D) scanning techniques.

Keywords: Area measurement; Biomechanics; Cross-sectional area; Soft tissues.

Fig. 1

A precision apparatus used to measure the thickness of tendon designed by Woo et.al., in which a vertical rod supported by a double cantilever spring system was placed on the specimen. Then a minimal vertical force was applied on the tendon by the rod and the spindle of the micrometer was then lowered to contact the sharp point on the top of the vertical rod. An electrical circuit was used to indicate contact ⁸ .

Fig. 2

(A) Constant area micrometer designed by Ellis which consists of torque measuring thimble (A), spindle (B), plug (C), and side blocks (D). The specimen was placed between blocks (D) and beneath the blade portion of the plug (C). The thimble of the micrometer head was turned to bring the spindle into contact with the plug until the thimble torque reached its prescribed value. The thimble was then released and the micrometer read ²⁴ . (B) Oval‐shaped slot area micrometer designed by Toritsuka. The graft was placed into an oval‐shaped slot 6 mm in width and a constant pressure of 0.12 MPa was applied with an attached spring. The sliding caliper was read and then the CSA was calculated ⁴⁹ .

Fig. 3

Application of microtomy technique on anterior cruciate ligament (ACL) cross‐sectional area measurement. (A) Slice plane of ACL cross‐section. (B) ACL cross‐sectional area measurement using Image J software. The surface of ACL is stained by colored ink for enhanced contrast. Ruler provided calibration data ⁵¹ .

Fig. 4

Magnetic resonance image (MRI) image for hamstring tendon cross‐sectional area (CSA) measurement. (A) Axial view of the right knee at the widest point of the medial femoral condyle. (B) Magnified view at the same level showing the semitendinosus (ST) and gracilis (GR) tendons. (C) Demonstration of the use of the region‐of‐interest tool to trace the cross‐sectional area of the ST tendon ⁴¹ .

Fig. 5

In‐vivo ultrasound imaging technique for cross‐sectional area (CSA) measurement of semitendinosus and gracilis tendon. Upper graph: axial T1‐weighted sequence of a left knee magnified four times. The semitendinosus tendon (ST) and gracilis tendon (GT) CSA are displayed. Lower panel: Ultrasound images and cross‐sectional area of the ST and gracilis tendon calculated with the ellipse tool (white dotted lines) of the ultrasound device ⁷⁰ .

Fig. 6

(A) Schematic drawings of a specially designed in vitro ultrasound cross‐sectional area (CSA) measurement device. The inside of the device is filled with a saline solution. Both ends of the specimen are held by clamps. A weight is then connected to the string to prevent the specimen from sagging. (B) Ultrasonogram of a specimen and its CSA, which is assumed to be the sum of the pixels in the enclosed area ³⁵ .

Fig. 7

Computed tomography (CT) image of left thigh (A) and that treated with ImageJ (B)78.

Fig. 8

Silicone rubber/polymethylmethacrylate (PMMA) molding technique measuring the cross‐sectional area (CSA) of foot ligament: (A) interosseous 4th metatarsal‐5th metatarsal (IM4M5) bone‐ligament‐bone specimen; (B) silicon rubber mold of IM4M5 showing cross‐section of ligament; and (C) PMMA casting of IM4M5 specimen ²⁷ . Alginate molding technique measuring equine CSA of equine superficial digital flexor tendon (SDFT): (D) transverse section through the alginate mold of SDFT (E) a photograph of a transverse section through the tendon used to make the mold at the same level ³⁶ .

Fig. 9

Schematic drawing of laser micrometer system and the clamp assembly. The specimen profile widths are measured by a laser telemetric system, which is a microprocessor‐controlled device that sends collimated scanning laser beams from the transmitter to the receiver. When an object is placed within the laser beams, a precise shadow is cast onto the receiver. This information is then transmitted to a controller where the width of the object is reported. The specimen's profile width (PW) and a reference distance (RD, from the top of the profile to the upper edge of the laser beams) are measured at each angular increment as the specimen is rotated through 180° ³⁹ .

Fig. 10

The reconstructed cross‐sectional shapes of porcine anterior cruciate ligament (ACL) obtained using laser micrometer (middle) are shown together with the corresponding histological sections (right). ³⁸

Fig. 11

(A) The schematic of charge‐coupled device (CCD) laser reflectance system, in which a CCD laser displacement sensor was mounted onto a rotary motion table. The frame has a threaded shaft connected to two arms that hold the specimen and allows for vertical translation relative to the CCD laser reflectance system. (B) A diagram depicting an overhead view of the CCD laser reflectance system with a biological specimen over the center of rotation (COR) of the system. “R” represents the total radius of the system, “d” represents the distance to the surface of the specimen, and “r” represents the inner radius of the specimen with its respective angle (y) ⁹⁰ .

Fig. 12

Photograph of the laser reflectance system installed on an Instron 8872 servohydraulic testing machine to allow cross‐sectional area measurement during tensile Testing ⁴⁰ .

Fig. 13

(A) Lateral view and (B) top view of linear laser scanner. DX and DY are the distance between lasers and the offset imposed to avoid mutual interference. DL1 and DL2 are the distances measured by each laser between itself and the specimen surface ⁹⁵ .

Fig. 14

Freehand three‐dimensional ultrasound (FUS) reconstruction of the Achilles tendon (AT). Three‐dimensional ultrasound image reconstructions were created in Stradwin using manually digitized cross‐sections (dotted white lines within inset) of the tendon segmented at approximately 10‐mm to 15‐mm intervals along the length of the tendon ¹⁰¹ .