Wednesday, January 26, 2011

KILAS RISET: Pressure Gradient and Subsurface Shear Stress on the Neuropathic Forefoot


Supriadi 
STIK Muhammadiyah Pontianak



Introduction
Stresses within the neuropathic foot’s tissues can be estimated by pressure distributions and may provide information regarding the potential for skin breakdown.
Foot ulceration is a common and costly problem in people with diabetes mellitus (DM) and peripheral neuropathy (PN).
High peak plantar pressure coupled with a lack of protective sensation from sensory PN is a known contributing factor to the development of these plantar foot ulcers (Caselli et al., 2002; Pitei et al., 1999; Veves et al., 1992). 
As people with DMPN and high peak plantar pressure walk, the soft tissues of their feet are subjected to repetitive stresses which have been associated with the location of skin breakdown (Boulton et al., 1983).
The peak pressure gradient is defined as the spatial change in plantar pressure around the location of the peak plantar pressure (Mueller et al., 2005). 
This peak pressure gradient is associated with peak plantar pressure in the forefoot of people with DMPN and a history of a foot ulcer (r = 0.59), but peak pressure gradient appears to provide additional information as an indicator of plantar skin injury (Mueller et al., 2005).
More recently, Zou et al. described the peak maximum shear stress within the subsurface soft tissues of the foot (Zou et al., 2007). 
The maximum shear stress plays a key role in the mechanical failure criteria (Hills et al., 1993) and may predict tissue trauma and breakdown by estimating internal stresses. Finite element models of the foot have been used to predict stresses within the neuropathic foot (Actis et al., 2004; Chen et al., 2001; Gefen, 2003, Goske).


The purposes:
Determine the magnitude of peak plantar pressure, pressure time integral, peak pressure gradient, and peak maximum shear stress and determine the association of these variables with one another.
Research design and methods:
Sample: 16 healthy people without DMPN (CON), 16 people with DMPN and no history of skin breakdown and 22 individual with DMPN + U.
Device: 
Sensation was tested using the 5.07 Semmes Weinstein monofilaments and a Bio-Thesiometer (Bio Medical Instrument, Newbury, OH). All subjects were unable to sense the 5.07 Semmes Weinstein monofilament on at least two sites on the plantar foot as described elsewhere Sensation also was quantified with a Bio-Thesiometer. 
Plantar pressure assessment: Plantar pressures were collected during walking using F-scan system (Tekscan, Boston, MA).  Each F-scan sensor contains 960 sensing locations or pixels. Each pixel is 5.08 X 5.08 mm.
Determination of plantar pressure variables: The PPP and PPG were determined using custom software. The PPG then was determine in a define area (3 X 3 F-scan sensor pixels (231.2 mm2)) around the PPP by calculating the highest changes in pressure (PG) from one node (half pixel apart) to the next according to rows and columns and by diagonal.
Determining the PMSS; 3-D, principal, and shear stresses were first calculated for the subsurface tissues. The maximum shear stress was then calculated in the forefoot at 8 different depths from 0 to 10 mm, and the peak maximum shear stress was identified as the greatest maximum shear stress value (Zou et al., 2007).

Statistical analysis:
One-way analysis of variance with post-hoc t-tests with Bonferroni correction were used to test for differences between the three groups for age, body mass index, glycated hemoglobin, VPT, and each of the pressure variables in the forefoot (PPP, PTI, PPG, PMSS, and depth of PMSS).
Pearson correlation coefficients were used to examine the association of these pressure variables with one another for all 54 subjects. Criterion for significance was set at p = 0.05.

Results:
The PPP (p < 0.05), PPG (p < 0.0009), PMSS (p < 0.05), and Depth of PMSS (p < 0.0006) were different between groups. Post-hoc testing revealed differences between CON and DMPN+U groups for PPP (increase of 26.1% relative to CON, p <0.03), PPG (increase of 88.3% relative to CON, p <0.0004), PMSS (increase of 28.6% relative to CON, p < 0.02), and depth of PMSS (decrease of 25.4% relative to CON, p < 0.0002). 
CON and DMPN groups for depth of PMSS (decrease of 13.0% relative to CON, p < 0.03). DMPN and DMPN+U groups for PPG (increase of 44% relative to DMPN, p < 0.02). No differences were noted for PTI. A strong association was noted for: PMSS and PPP (r = 0.97; p < 0.0001), PMSS and PPG (r = 0.79; p < 0.0001), PPG and PPP (r = 0.84; p < 0.0001), and depth of PMSS and PPG (r = -0.56; p < 0.0001) A relatively lower association was noted for depth of PMSS and PPP (r = -0.27; p = 0.05).


Kesimpulannya:   
Hasil penelitian ini memberikan dukungan lebih lanjut bagi pentingnya  menentukan puncak tekanan gradien dan puncak tegangan geser maksimum ketika menilai pasien dengan DMPN dan puncak tekanan plantar  yang tinggi.

Referensi:
Lott, Donovan.J. et al. Pressure Gradient and Subsurface Shear Stress on the Neuropathic Forefoot Journal of Clini. Biomech. 2008: 23 (3): 342–348.

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