A quantitative analysis of optimum design for rigid ankle foot orthoses: The effect of thickness and reinforcement design on stiffness

BACKGROUND

An ankle foot orthosis (AFO) which is prescribed to be rigid should only deform a small amount to achieve its clinical goals. Material thickness and the design of reinforcing features can significantly affect AFO rigidity, but their selection remains based on anecdotal evidence.

OBJECTIVES

To quantify the effect of these parameters on AFO stiffness and to set the basis for quantitative guidelines for the design optimisation of rigid AFOs.

STUDY DESIGN

Experimental and computational study.

METHODS

A polypropylene AFO was produced according to UK standard practice and its stiffness was experimentally measured for 30Nm of dorsiflexion. Its geometry and mechanical characteristics were utilised to create a finite element (FE) model of a typical AFO prescribed to be rigid. Following validation, the model was used to quantify the effect of material thickness and reinforcement design (i.e., reinforcement placement, length) on stiffness. A final set of AFO samples was produced to experimentally confirm key findings.

RESULTS AND CONCLUSIONS

For a specific AFO geometry and loading magnitude, there is a thickness threshold below which the AFO cannot effectively resist flexion and buckles. FE modelling showed that stiffness is maximised when reinforcements are placed at the anterior-most position possible. This key finding was also experimentally confirmed. The stiffness of an AFO reinforced according to standard practice with lateral and medial ribbing was 4.4 ± 0.1 Nm/degree. Instructing the orthotic technician to move the ribbings anteriorly increased stiffness by 22%. Further stiffening is achieved by ensuring the reinforcements extend from the footplate to at least two-thirds of the AFO's total height.

The enhanced paper grip test can substantially improve community screening for the risk of falling

BACKGROUND

Lower-limb strength measures can enhance falls risk assessment but due to the lack of clinically applicable methods, such measures are not included in current screening. The enhanced paper grip test (EPGT) is a simple-to-use and cost-effective test that could fill this gap. However, its outcome measure (EPGT force) has not yet been directly linked to the risk of falling.

RESEARCH QUESTION

Is the EPGT a good candidate for falls risk screening in older people in the community?

METHODS

Seventy-one older people living independently in the community were recruited for this prospective observational study (median age 69 y, range 65y-79y). Lower-limb and whole-body strength were assessed at baseline using the EPGT and a standardised hand-grip method respectively. Incident falls were recorded monthly for a year through follow-up telephone conversations. The capacity of individual strength measures to predict falls and to enhance an established falls risk assessment tool (FRAT) commonly used by UK's national health service (NHS) was assessed using binomial logistic regression. The analysis was repeated for the subset of participants without history of falling at baseline (prediction of first-ever falls).

RESULTS

Increased EPGT force and increased symmetry in strength between limbs were significantly associated with reduced risk of falling. Compared to the NHS-FRAT, the EPGT correctly classified more people (73% vs 69%), it achieved higher sensitivity (56% vs 26%) and higher negative predictive value (76% vs 68%). Complementing the NHS-FRAT with the EPGT produced a more comprehensive model that correctly classified 91% of participants and achieved 98% specificity, 81% sensitivity, 89% negative and 96% positive predictive value. Replacing the EPGT with hand-grip strength consistently undermined prediction accuracy. The EPGT remained highly accurate when focused on the prediction of first-ever falls.

SIGNIFICANCE

The EPGT can substantially enhance falls screening in the community. These results can also inform effective personalised strength exercise interventions.

Reliability and Validity of Shore Hardness in Plantar Soft Tissue Biomechanics

Shore hardness (SH) is a cost-effective and easy-to-use method to assess soft tissue biomechanics. Its use for the plantar soft tissue could enhance the clinical management of conditions such as diabetic foot complications, but its validity and reliability remain unclear. Twenty healthy adults were recruited for this study. Validity and reliability were assessed across six different plantar sites. The validity was assessed against shear wave (SW) elastography (the gold standard). SH was measured by two examiners to assess inter-rater reliability. Testing was repeated following a test/retest study design to assess intra-rater reliability. SH was significantly correlated with SW speed measured in the skin or in the microchamber layer of the first metatarsal head (MetHead), third MetHead and rearfoot. Intraclass correlation coefficients and Bland-Altman plots of limits of agreement indicated satisfactory levels of reliability for these sites. No significant correlation between SH and SW elastography was found for the hallux, 5th MetHead or midfoot. Reliability for these sites was also compromised. SH is a valid and reliable measurement for plantar soft tissue biomechanics in the first MetHead, the third MetHead and the rearfoot. Our results do not support the use of SH for the hallux, 5th MetHead or midfoot.

An exploration of the mechanistic link between the enhanced paper grip test and the risk of falling

The enhanced paper grip test (EPGT) offers an easy-to-use measure of hallux plantar-flexion strength that does not need expensive specialised equipment. Literature suggests that it could be a useful screening tool to assess the risk of falling in older people. However, research on a specific mechanistic link to the risk of falling is lacking. It is hypothesised here that muscle weakening (assessed by the EPGT) is indicative of impaired ability to recover balance after a slip or a trip. To get an initial assessment of validity of the above hypothesis, the EPGT is compared against an established lab-based measure of lower-limb strength that is capable of assessing a person's ability to recover balance after a slip or a trip: maximum isometric leg press push-off force (leg press force). A gender-balanced sample of twenty people (median age=34 y) was recruited. Two different but equaly valid techniques of administering the EPGT were included regarding whether the participants' ankle was supported by the examiner or not. Results for the two EPGT techniques differed susbtantialy but they were both significantly associated with leg press force and therefore linked to better ability to maintain balance after a slip or a trip. The "ankle not held" EPGT technique was more strongly correlated to leg press force (r(18) = 0.652, p = 0.002) than the "ankle held" (r(18) = 0.623, p = 0.003) and appears to be the more favourable technique to administer the EPGT. These findings offer new insight on a potential mechanistic link between the EPGT and the risk of falling and support its optimal use in future research involving older people.

Effective and clinically relevant optimisation of cushioning stiffness to maximise the offloading capacity of diabetic footwear

INTRODUCTION

Optimising the cushioning stiffness of diabetic footwear/orthoses can significantly enhance their offloading capacity. This study explores whether optimum cushioning stiffness can be predicted using simple demographic and anthropometric parameters.

METHODS

Sixty-nine adults with diabetes and loss of protective sensation in their feet were recruited for this cross-sectional observational study. In-shoe plantar pressure was measured using Pedar® for a neutral diabetic shoe (baseline) and after adding cushioning footbeds of varying stiffness. The cushioning stiffness that achieved maximum offloading was identified for each participant. The link between optimum cushioning stiffness and plantar loading or demographic/anthropometric parameters was assessed using multinomial regression.

RESULTS

People with higher baseline plantar loading required stiffer cushioning materials for maximum offloading. Using sex, age, weight, height, and shoe-size as covariates correctly predicted the cushioning stiffness that minimised peak pressure across the entire foot, or specifically in the metatarsal heads, midfoot and heel regions in 70%, 72%, 83% and 66% of participants respectively.

CONCLUSIONS

Increased plantar loading is associated with the need for stiffer cushioning materials for maximum offloading. Patient-specific optimum cushioning stiffness can be predicted using five simple demographic/anthropometric parameters. These results open the way for methods to optimise cushioning stiffness as part of clinical practice.

Screening for the loss of protective sensation in people without a history of diabetic foot ulceration: validation of two simple tests in India

The ability of the Ipswich touch test (IpTT) and Vibratip^TM to detect loss of protective sensation (LOPS) was tested against a neurothesiometer in an outpatient diabetic population without a history for ulceration. Our results support the use of the IpTT as a screening tool for LOPS, but not of Vibratip^TM.

Increased exposure to loading is associated with decreased plantar soft tissue hardness in people with diabetes and neuropathy

AIMS

Literature indicates that altered plantar loading in people with diabetes could trigger changes in plantar soft tissue biomechanics which, in turn, could affect the risk for ulceration. To stimulate more research in this area, this study uses in vivo testing to investigate the link between plantar loading and tissue hardness.

METHODS

Tissue hardness and plantar pressure distribution were measured for six plantar areas in 39 people with diabetes and peripheral neuropathy.

RESULTS

Spearman correlation analysis revealed that increased pressure time integral at the 1st metatarsal-head region (r = -0.354, n = 39, P = 0.027) or at the heel (r = -0.378, n = 39, P = 0.018) was associated with reduced hardness in the same regions. After accounting for confounding parameters, generalised estimating equations analysis also showed that 10% increase in pressure time integral at the heel was associated with ≈ 1 unit reduction in hardness in the same region.

CONCLUSIONS

For the first time, this study reveals that people with diabetes and neuropathy who tend to load their feet more heavily also tend to have plantar soft tissues with lower hardness. The observed difference in tissue hardness is likely to affect the tissue's vulnerability to overload injury. More research will be needed to explore the implications of the observed association for the risk of ulceration.

A novel concept for low-cost non-electronic detection of overloading in the foot during activities of daily living

Identifying areas in the sole of the foot which are routinely overloaded during daily living is extremely important for the management of the diabetic foot. This work showcases the feasibility of reliably detecting overloading using a low-cost non-electronic technique. This technique uses thin-wall structures that change their properties differently when they are repeatedly loaded above or below a tuneable threshold. Flexible hexagonal thin-wall structures were produced using three-dimensional printing, and their mechanical behaviour was assessed before and after repetitive loading at different magnitudes. These structures had an elastic mechanical behaviour until a critical pressure (P _crit = 252 kPa ± 17 kPa) beyond which they buckled. Assessing changes in stiffness after simulated use enabled the accurate detection of whether a sample was loaded above or below P _crit (sensitivity = 100%, specificity = 100%), with the overloaded samples becoming significantly softer. No specific P _crit value was targeted in this study. However, finite-element modelling showed that P _crit can be easily raised or lowered, through simple geometrical modifications, to become aligned with established thresholds for overloading (e.g. 200 kPa) or to assess overloading thresholds on a patient-specific basis. Although further research is needed, the results of this study indicate that clinically relevant overloading could indeed be reliably detected without the use of complex electronic in-shoe sensors.

Comparative study of the strength characteristics of a novel wood-plastic composite and commonly used synthetic casting materials

BACKGROUND

Woodcast® is a wood-plastic composite casting material that becomes pliable and self-adhesive when heated to 65 °C and returns to being weightbearing as it cools down. The present study aims to test whether this novel non-toxic casting material is strong enough for clinical use by comparing its strength against materials that are already used in weightbearing casting applications such as total contact casts.

METHODS

The strength of Woodcast® samples was compared against the strength of two commonly used synthetic casting materials (Delta-Cast®, OrthoTape). The effect of environmental factors such as cooling, prolonged heating and exposure to water was also assessed.

FINDINGS

The results of this study indicated that Woodcast® is stronger than the synthetic casting materials in compression but weaker in tension. The flexural strength of Woodcast® was 14.24 MPa (±1.25 MPa) while the respective strength of Delta-Cast® and OrthoTape was 18.96 MPa (±7.46 MPa) and 12.93 MPa (±1.93 MPa). Independent samples t-test indicated that the difference between Woodcast® and the other two materials was not statistically significant (P > .05). Woodcast® recovered 90% and 78% of its tensile or flexural strength respectively after 15 min of cooling at ambient temperature and its strength was not reduced by prolonged heating. On average, exposure to water reduced the flexural strength of Delta-Cast® by 6% and of OrthoTape by 44%. The strength of Woodcast® was not affected by exposure to water.

INTERPRETATION

The comparison between Woodcast® and commonly used synthetic casting materials indicated that Woodcast® is indeed strong enough to be safely used in weightbearing casting applications.

The relationship between hallux grip force and balance in people with diabetes

BACKGROUND

Diabetes accelerates the decline in muscle strength in older people and substantially increases the risk for fall and injury. Weakening of lower extremity muscles, in particular, is a strong predictor for falls, but currently there is no established method for its assessment in clinics. The paper grip test (PGT) offers a qualitative assessment of hallux plantar flexor strength and its usefulness for predicting falls has been demonstrated in non-diabetic populations.

RESEARCH QUESTION

The aim of this study is to test whether the PGT can be used for a quantitative assessment of lower-extremity strength and to investigate its relationship with isometric muscle strength and balance in people with diabetes and peripheral neuropathy.

METHODS

Isometric muscle strength of all muscle groups of the foot-ankle was assessed using a dynamometer in sixty-nine people with diabetes and neuropathy. Postural sway and the gripping force exerted by the participants during the PGT was measured for the same participants using a plantar pressure assessment system. These measurements were repeated in regular intervals for 18 months in a longitudinal observational cohort study.

RESULTS

Cross-sectional analysis of baseline data showed that people who failed the PGT swayed more. Analysis of longitudinal data showed that increasing hallux grip force is significantly associated with reduced postural sway. No significant association was found between dynamometry-based measurements of strength and postural sway. Hallux grip force was significantly correlated to the strength of all muscle groups of the foot-ankle complex.

SIGNIFICANCE

These results indicate that hallux grip force can assess the strength of the foot-ankle muscles and could potentially be used to identify people at risk of falling. This sets the basis for the development of new screening protocols to assess weakening of the muscles of the foot-ankle and to enhance risk assessment for falls in people with diabetes and peripheral neuropathy.

The relationship between the mechanical properties of heel-pad and common clinical measures associated with foot ulcers in patients with diabetes

AIM

The present study aims at investigating the correlation between the mechanical properties of the heel-pad of people with type-2 diabetes and the clinical parameters used to monitor their health and ulceration risk.

METHODS

A new device for the in-vivo testing of plantar soft tissues was built and pilot-tested. This device consists of an ultrasound probe connected in series with a dynamometer. Loading is applied manually using a ball-screw actuator. A total of 35 volunteers with type-2 diabetes were recruited and the thickness, stiffness of their heel-pads as well as the energy absorbed during loading were assessed. The participants with diabetes also underwent blood tests and measurements of Ankle Brachial Index and Vibration Perception Threshold.

RESULTS

Pearson correlation analysis revealed strong correlations between triglycerides and heel-pad stiffness (r=0.675, N=27, p<0.001) and between triglycerides and energy (r=-0.598, N=27, p=0.002). A correlation of medium strength was found between Fasting Blood Sugar (FBS) and stiffness (r=0.408, N=29, p=0.043).

CONCLUSIONS

People with type-2 diabetes and high levels of triglycerides and FBS are more likely to have stiffer heel-pads. Increased stiffness could limit the tissues' ability to evenly distribute loads making them more vulnerable to trauma and ulceration.