OroSTIFF: Face-referenced measurement of perioral stiffness in health and disease

Abstract

A new device and automated measurement technology known as OroSTIFF is described to characterize non-participatory perioral stiffness in healthy adults for eventual application to patients with orofacial movement disorders associated with neuromotor disease, traumatic injury, or congenital clefts of the upper lip. Previous studies of perioral biomechanics required head stabilization for extended periods of time during measurement, which precluded sampling patients with involuntary body/head movements (dyskinesias), or pediatric subjects. The OroSTIFF device is face-referenced and avoids the complications associated with head-restraint. Supporting data of non-participatory perioral tissue stiffness using OroSTIFF are included from 10 male and 10 female healthy subjects. The OroSTIFF device incorporates a pneumatic glass air cylinder actuator instrumented for pressure, and an integrated subminiature displacement sensor to encode lip aperture. Perioral electromyograms were simultaneously sampled to confirm passive muscle state for the superior and inferior divisions of the orbicularis oris muscles. Perioral stiffness, derived as a quotient from resultant force (DeltaF) and interangle span (DeltaX), was modeled with multilevel regression techniques. Real-time calculation of the perioral stiffness function demonstrated a significant quadratic relation between imposed interangle stretch and resultant force. This stiffness growth function also differed significantly between males and females. This study demonstrates the OroSTIFF 'proof-of-concept' for cost-effective non-invasive stimulus generation and derivation of perioral stiffness in a group of healthy unrestrained adults, and a case study to illustrate the dose-dependent effects of Levodopa on perioral stiffness in an individual with advanced Parkinson's disease who exhibited marked dyskinesia and rigidity.

Figure 1

Instrumentation and configuration to assess the force-displacement relation (stiffness) for the perioral tissues. The equal-arm scissor cantilever and stainless steel lip saddles (hooks) positioned for imposing increases in interangle span and automated stiffness sampling. A lip span adjustor accommodates to individual differences in lip aperture and used to set initial position (L₀ + 15 mm). The stainless steel 0.020” shimstock chin anchor articulated on a needle bearing and was positioned over the subject’s mental symphysis for vertical stabilization. InVivo Metrics 4 mm Ag/AgCl surface electrodes placed over orbicularis oris superior (OOS) and orbicularis oris inferior (OOI) muscle recording sites.

Figure 2

A schematic block diagram of the OroSTIFF system. The device is equipped with a subminiature differential variable reluctance transducer to measure displacement of the lips, and a pressure transducer to derive the reactive force associated with perioral tissue recoil. A fixed pneumatic resistive load, in the form of 30 gauge blunt tip cannula, permits the moveable cantilevers of the OroSTIFF device to return to its initial position [L₀ + 15 mm] following each imposed stretch trial.

Figure 3

Typical force-span hysteresis curve sampled from a normal adult subject with graphic insert showing span-time (black line) and force-time (dashed line) plots. Point A: preload condition of the OroSTIFF device on a subject's face; B: onset of interangle stretch phase ; C: peak interangle stretch; the recoil phase (D–E) during which stiffness is calculated (ΔForce/ΔSpan). Regions from A–E and just after C reflect bending (elasticity) of the OroSTIFF device without change in S-DVRT position output.

Figure 4

Regression functions for male (dotted black line) and female subjects (solid gray line). $$\text{Male regression equation}:{\widehat{Y}}_{\mathit{\text{ij}}}=.04682-{.00442}^{*}{\mathit{\text{Span}}}_{\mathit{\text{ij}}}+.00059{\mathit{\text{Span}}}_{\mathit{\text{ij}}}^2$$ $$\text{Female regression equation}:{\widehat{Y}}_{\mathit{\text{ij}}}=.04583-{.00396}^{*}{\mathit{\text{Span}}}_{\mathit{\text{ij}}}+.00054{\mathit{\text{Span}}}_{\mathit{\text{ij}}}^2$$

Figure 5a & 5b

The distribution of mean and standard error of EMG RMS values (µV) for upper lip (5a) and lower lip (5b) recording sites for all participants during the ‘face-relaxed’ non-participatory conditions. Eighty-five observations at each interangle span except at 25 mm, where there were only 3 observations.

Figure 5a & 5b

The distribution of mean and standard error of EMG RMS values (µV) for upper lip (5a) and lower lip (5b) recording sites for all participants during the ‘face-relaxed’ non-participatory conditions. Eighty-five observations at each interangle span except at 25 mm, where there were only 3 observations.

Figure 6

Perioral stiffness data and quadratic functions for a 68-year old male with Parkinson’s disease in the ON (with anti-PD meds) and OFF (without meds) conditions plotted relative to an age- and sex-matched healthy control.