Reliability of ultrasound in evaluating the plantar skin and fat pad of the foot in the setting of diabetes

Reliability of ultrasound in evaluating the plantar skin and fat pad of the foot in the setting of diabetes

REVIEWED BY | Sheri Anne Rae ASA SIG: Musculoskeletal

REFERENCE | Authors: Morrison T, Jones S, Causgy RS, Thoirs K, PLoS ONE 16(9): e0257790

WHY THE STUDY WAS PERFORMED

In people with diabetes mellitus, the plantar foot soft tissue can undergo structural changes including reduced thickness or elasticity impairing the foot’s ability to resist and absorb mechanical loads and causing the foot to become vulnerable to ulceration. Amputation is a serious sequela in diabetics with foot ulcerations. Ultrasound may be used to assess the plantar soft tissue in the feet of people with diabetes who are at increased risk of pathological changes. The aim of this study was to determine the intra and interobserver reliability of soft tissue characteristic assessments using ultrasound and to determine the intraobserver reliability of plantar soft tissue measurements performed on unloaded and compressed ultrasound images obtained at foot sites with known risk of tissue injury in people with diabetes mellitus.

HOW THE STUDY WAS PERFORMED

A convenience sample of 15 participants with (N = 6) and without diabetes (N = 9) was recruited for the repeated measures design study. Ultrasound images were taken in both unloaded and compressed states at four plantar foot sites known for increased peak plantar pressure and frequency of ulceration:

• heel

• lateral sesamoid of the hallux

• second sub-metatarsal head

• third sub-metatarsal head.

At each of the four plantar foot sites, thickness measurements were undertaken by one experienced sonographer twice to control the test-retest environment at discrete tissue layers (Figure 1) including:

• skin

• superficial subcutaneous (microchamber)

• horizontal fibrous band

• deep subcutaneous (macrochamber)

• total soft tissue depth.

Figure 1. Ultrasound image demonstrating the plantar heel with the transducer in the longitudinal plane and measurement locations. Asterisk indicates gel stand-off

“Ultrasound measurement of the unloaded total soft tissue depth is the most sensitive for detecting small change and thus is the preferred measurement method.”

For the soft tissue characteristic assessments, an a priori sample size of 22 participants with (intraobserver N = 13, interobserver N = 9) and without (intraobserver N = 9, interobserver N = 13) diabetes was recruited. Ultrasound tissue characteristics of the previously noted plantar foot sites in an unloaded state were assessed for each anatomical soft tissue layer. The echogenicity and the distinctness of the anatomical layers were compared with a reference set of images from a healthy young adult foot and assessed with the dichotomous variables same or not same. For intraobserver testing, one experienced sonographer independently performed tissue characterisation assessments and scored the images. For the interobserver testing, two experienced sonographers independently analysed and scored the images.

WHAT THE STUDY FOUND

Intraobserver reliability was achieved with measurements of the total soft tissue depth in an unloaded state at all anatomical sites in the test and retest trials. Relative intraobserver reliability was also achieved for unloaded state measurements of the deep subcutaneous (macrochamber) layer. However, the unloaded total soft tissue depth measurements were more sensitive for detecting small changes than the unloaded deep subcutaneous (macrochamber) measurements. Absolute reliability for measurements of the deep subcutaneous (macrochamber) layer and total soft tissue depth was always higher in the unloaded state when compared to the compressed state. Reduced reliability for measurements made in the compressed state may be attributable to variations in the manual pressure applied through the handheld transducer causing the transducer to slip from the central measurement position. Plantar skin thickness measurements demonstrated intraobserver reliability only when made at the site where the skin was thickest – at the heel pad. Measurement of the superficial subcutaneous (microchamber) layer and the horizontal fibrous bands was not consistently reliable across any of the anatomical sites. Surface undulations, poor differentiation of the anatomical layers and the relative thinness of the skin, superficial subcutaneous (microchamber) layer and horizontal fibrous band may make calliper placement difficult and susceptible to measurement error. While intraobserver agreement was demonstrated for some of the tissue characteristic assessments (heel skin, lateral sesamoid fat pad and the second and third sub-metatarsal heads skin and fat pad), acceptable interobserver agreement was not demonstrated for any of the tissue characteristic assessments.

RELEVANCE TO CLINICAL PRACTICE

Regular clinical risk assessment may not necessarily capture the subclinical manifestations (such as tissue degeneration, stiffening and atrophy) that can proceed ulcer development. Ultrasound can be used to depict these characteristics and has potential for a further positive impact in assessing the risk of plantar foot ulceration in people with diabetes mellitus. Ultrasound measurement of the unloaded total soft tissue depth is the most sensitive for detecting small change, and thus is the preferred measurement method for detecting differences between groups and changes in individuals across repeated measurements. Serial ultrasound soft tissue characterisation assessments of plantar foot soft tissue on the same patient should be made by the same observer where possible.