Pilot study

A pilot study sought to determine a reliable method of quantifying soft tissue thickness between the surface of an APFO and bones overlying the medial longitudinal arch (related to the effects of orthotic arch height). The pilot did not examine the reliability of quantifying soft tissue thickness under the calcaneus, because with its superficially location, shape and surrounding tissue properties, it is easily identifiable and has demonstrated good reliability (Rome, 1998; Telfer et al., 2014).

7.3.2.1Orthosis

The SalfordinsoleTM (Salfordinsole Health Care Ltd, UK) was chosen as an example APFO

(Majumdar et al., 2013) but like most orthotic products it is impenetrable to ultrasound signals due to the presence of air in its materials. To study its effect on foot tissues an exact copy of

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the APFO was made in a rigid plastic sonographic material (Northplex®_{). To create these} copies, positive plaster of paris moulds of the orthotic were created from a milled EVA version of the SalfordinsoleTM. A 3 mm Northplex sheet was subsequently heat moulded and vacuum

formed over the SalfordinsoleTM positive model. Northplex® allows ultrasound signals to pass

through its structure and is almost incompressible in sheet form. It remains very rigid when moulded into an APFO shape, being similar to a polypropylene style foot orthotic (Figure 7.3).

Figure 7.3 Photo of Northplex® orthotic used during the plot study testing

7.3.2.2Ultrasound and Scanning Platform

A MyLab 70 Xvision ultrasound machine and 13MHz linear array transducer (Type, LA523, Esoate Europe, United Kingdom) was used to image plantar soft tissues on top of the orthotic. Measures of soft tissue thickness were obtained in the arch. The navicular was assumed to represent the peak in the medial arch height and correspond to peak orthotic arch height. A plateau on the plantar surface of the navicular was used as an internal bony reference for measures of arch tissue thickness (Figure 7.4).

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This landmark was imaged in the frontal plane and lateral to the navicular tuberosity by 1/3 of the navicular width. Frontal plane images of transverse tissue thickness, between the bony reference landmark and skin were recorded through the Northplex® insole with the subject stood on top. A platform incorporating a 50 mm x 120 mm opening (Figure 7.5) was used to position the ultrasound transducer under the orthotic/foot at the and arch scanning site.

Figure 7.5 Scanning platform used to enable US imaging of the plantar soft tissues under the arch through the Northplex® insole.

7.3.2.3Ultrasound scanning protocol

As outlined, a 4 – 13 MHz linear array probe was used to scan the navicular plateau through the Northplex insole. This probe was selected because its frequency enabled sufficient resolution of the navicular plateau at depths ranging between 25-40mm through the Northplex orthosis. A higher frequency transducer would provide greater resolution of soft tissue structures situated between skin and bone, however for this study the goal was to achieve the most detailed visualisation of the hyperechoic rim in bone forming the plantar surface of the navicular plateau, which was achieved by the selected probe. Furthermore, this probe was selected because its footprint area (50mm x 8mm) was sufficient to visualise the anatomy under the midfoot but also because the dimensions of the probe enabled ease of use whilst scanning through the scanning platform.

To ensure the probe appropriately contacted the plantar surface of the orthotic a standoff pad was used. This has also been shown to decrease the generation of intense echoes in superficial regions positioned in close proximity to a probe (Biller 1998), thus is particularly applicable

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when scanning through the Northplex orthotic therefore assisting in visualisation of the navicular plateau. The acoustic impedance (Z) of Northplex is not known however, it is thought to be similar to that of polypropylene, a comparable thermoplastic material that has an acoustic impedance of 2.36 (106 _{Rayls) (Selfridge, 1985).}

Before data were collected, a grid was drawn on the subject’s medial midfoot. This grid consisted of perpendicular lines positioned 5mm apart. Once the grid was drawn, a freehand scan was performed on each participant. This enabled identification of the scanning site for the navicular plateau, which was subsequently marked on the skin with reference to the grid. Northplex® is transparent, thus markings at the skins surface could be viewed from the underside of the orthotic whilst the subject stood on top thus facilitated placement of the ultrasound probe when scanning.

During scanning the probe was positioned at the scanning site (transversely) under the orthotic and the target area (navicular plateau) was positioned on the top/centre of the screen. As described previously, for this study the primary interest was characterising tissue thickness between the navicular plateau and skin, and not the specific architecture of the soft tissue structures. A higher frequency probe may have enabled more detailed analysis of these structures however a higher frequency with shorter wavelengths would decrease the depth of the view due to higher attenuation of ultrasound waves within the tissues. The depth settings were adjusted by approximately 10mm deeper than the target region of interest. The gain of the ultrasound machine was also adjusted to alter the grey scale imaging of the navicular plateau with a view to improving the clarity of the hyperechoic rim in bone forming the plantar surface of the navicular plateau. The depth and gain settings were consistent for the same subject across all conditions. During data collection the probe was held in a perpendicular position relative to the target region with the assistance of a spirit level.

Each participant stood with their right foot on the orthotic which was secured over the platform aperture. Participants stood on one leg and used hand rails to prevent sway. To improve the extent to which this static assessment might replicate soft tissue compression in walking, each subject was fitted with a vest weighted by 5 % of their own body weight. This weight was a compromise between what was tolerable during testing and trying to increase loading to the equivalent of body weight, since forces passing through the foot exceed body weight twice during stance (Richards, 2008). The same examiner (the PhD candidate) recorded three images

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of tissue thickness for each subject and the probe was removed between each scan. To quantify intra-rater reliability the same examiner took all scans on two separate occasions twenty four hours apart.

7.3.2.4Pilot study data analysis

Image J software was used to quantify tissue thickness through the Northplex® orthosis with the assessor blinded to image and day of scan. This software has excellent inter-rater reliability when measuring soft tissues from ultrasound images (McCreesh et al., 2011). Day 1 and day 2 mean values of tissue thickness were derived from the three images taken for each subject. Intraclass correlation coefficients (ICC; 3,1) were calculated using SPSS (Version 19, SPSS Inc., Chicago, IL) to assess day-to-day reliability of the measurements. Excellent reliability was defined as an ICC > 0.75 (Fleiss, 1999).

7.3.2.5Pilot study results

The intra-rater reliability for quantifying tissue thickness between the skin and navicular plateau through a Northplex® orthosis was excellent, with an ICC 3,1 of 0.980, 95% CI [0.92 0.99]. Day 1 and day 2 individual measurements of tissue thickness are shown in Figure 7.6. A mean (SD) value of 27.7 mm (SD 4.2 mm) was recorded for day 1 testing and a mean of 27.3 mm (SD 3.8 mm) was found for day 2.

Figure 7.6 Day 1 and day 2 values of arch tissue thickness (mm) measured through the Northplex® insole. 0 5 10 15 20 25 30 35 40 1 2 3 4 5 6 7 8 9 10 Ti ssue Thi ckn e ss (mm) Subject No Day1 Day2

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The results of the pilot showed that using ultrasound to quantify tissue thickness in the arch of the foot through a Northplex® orthosis has excellent intra-rater reliability.