New insights into intrinsic foot muscle morphology and composition using ultra-high-field (7-Tesla) magnetic resonance imaging

A Novel Intrinsic Foot Muscle Strength Dynamometer Demonstrates Moderate-To-Excellent Reliability and Validity

Background

Intrinsic foot muscle (IFM) weakness can result in reduced foot function, making it crucial for clinicians to track IFM strength changes accurately. However, assessing IFM strength can be challenging for clinicians, as there is no clinically applicable direct measure of IFM strength that has been shown to be reliable and valid with the foot on the ground.

Purpose

The purpose was to investigate the intra-rater and inter-rater reliability of a novel, budget-friendly IFM dynamometer and determine its agreement with a handheld dynamometer (HHD). The researchers also examined correlations of foot morphology and activity level to IFM strength.

Study design

Descriptive Laboratory Study.

Methods

Two assessors measured IFM strength of 34 healthy volunteers (4 male, 30 female; age=21.14±2.57, height=164.66 ±7.62 cm, mass=64.45±11.93 kg) on two occasions 6.62±0.78 days apart with the novel dynamometer to assess intra- and inter-rater reliability. The HHD was used to measure IFM in the first session in order to assess validity.

Results

For the novel dynamometer, intra- and inter-rater reliability was moderate-to-excellent (ICC = 0.73 - 0.95), and the majority of the strength tests were within the 95% limits of agreement with the HHD. Wider foot morphology and a higher number of days walking over the prior seven days had small but significant correlations with IFM strength (dominant foot r = 0.34, non-dominant foot r = 0.39; r = -0.33, -0.39 respectively).

Conclusion

This novel IFM dynamometer is a budget-friendly ($75) tool that was shown to be reliable and valid in a healthy population.

Levels of evidence

Level 3©The Author(s).

Intrinsic foot muscle size and associations with strength, pain and foot-related disability in people with midfoot osteoarthritis

BACKGROUND

To compare intrinsic foot muscle size between people with and without symptomatic midfoot osteoarthritis, and examine the association between muscle size and strength, pain and foot-related disability.

METHODS

Twenty-three participants with symptomatic midfoot osteoarthritis and 23 age, sex and BMI matched controls were included. Intrinsic foot muscle cross-sectional area was measured using MRI. Hand-held dynamometry was used to assess foot and ankle muscle strength, and foot-related pain and disability was measured using Manchester Foot Pain & Disability Index.

FINDINGS

Small and non-statistically significant differences were found in intrinsic foot muscle cross-sectional area between the two groups (effect sizes 0.15-0.26, p > 0.05). Muscle strength was reduced in the midfoot osteoarthritis group, with differences of 12-33% (effect sizes 0.47-1.2). In the control group, moderate positive associations) existed between foot muscle cross-sectional area and lesser digits flexor strength (r = 0.5 to 0.7, p < 0.05). Conversely, in the midfoot osteoarthritis group, negligible positive associations were found (r < 0.3, p > 0.05). Associations between foot muscle cross-sectional with and pain and disability scores in the midfoot osteoarthritis group were negligible (r < -0.3, p > 0.05).

INTERPRETATION

Despite reductions in maximal isometric muscle strength, midfoot osteoarthritis does not appear to be associated with reduced intrinsic foot muscle cross-sectional area measured by MRI. Muscle compositional or neural factors may explain the reductions in muscle strength and variation in symptoms in people with midfoot osteoarthritis and should be investigated.

7 T Musculoskeletal MRI: Fundamentals and Clinical Implementation

ABSTRACT

This review summarizes the current state-of-the-art of musculoskeletal 7 T magnetic resonance imaging (MRI), the associated technological challenges, and gives an overview of current and future clinical applications of 1 H-based 7 T MRI. The higher signal-to-noise ratio at 7 T is predominantly used for increased spatial resolution and thus the visualization of anatomical details or subtle lesions rather than to accelerate the sequences. For musculoskeletal MRI, turbo spin echo pulse sequences are particularly useful, but with altered relaxation times, B1 inhomogeneity, and increased artifacts at 7 T; specific absorption rate limitation issues quickly arise for turbo spin echo pulse sequences. The development of dedicated pulse sequence techniques in the last 2 decades and the increasing availability of specialized coils now facilitate several clinical musculoskeletal applications. 7 T MRI is performed in vivo in a wide range of applications for the knee joint and other anatomical areas, such as ultra-high-resolution nerve imaging or bone trabecular microarchitecture imaging. So far, however, it has not been shown systematically whether the higher field strength compared with the established 3 T MRI systems translates into clinical advantages, such as an early-stage identification of tissue damage allowing for preventive therapy or an influence on treatment decisions and patient outcome. At the moment, results tend to suggest that 7 T MRI will be reserved for answering specific, targeted musculoskeletal questions rather than for a broad application, as is the case for 3 T MRI. Future data regarding the implementation of clinical use cases are expected to clarify if 7 T musculoskeletal MRI applications with higher diagnostic accuracy result in patient benefits compared with MRI at lower field strengths.

Improving the measurement of intrinsic foot muscle morphology and composition from high-field (7T) magnetic resonance imaging

Magnetic resonance imaging (MRI) can be used to quantify intrinsic foot muscle morphology and composition. Due to the high spatial resolution required to adequately capture the architecturally complex anatomy, manual segmentation is time consuming and not clinically feasible. The aim of this study was to evaluate if a reduced number of MRI slices can be used to accurately estimate intrinsic foot muscle volume and composition. A three-dimensional 2-point Dixon sequence of the whole foot was acquired at 7-Tesla for thirteen asymptomatic individuals and twenty individuals with plantar heel pain. Slice intervals of 2, 3, 5, 10, 15 and 30 were used to calculate alternative muscle volume and composition, and were compared to reference values calculated from every available slice. Agreement between methods was assessed by calculating mean differences and 95% limits of agreement, and inspection of Bland -Altman plots. In both groups, slice intervals of 2, 3 and 5 provided excellent precision for all muscles (measurement error < 1%). Larger slice intervals of 10, 15 and 30 provided excellent precision for some muscles, but for other muscles (e.g. small forefoot muscles), error was up to 7.3%. Bland-Altman plots showed no systematic measurement bias. This study provides a quantitative basis for selecting a reduced number of slices to measure intrinsic foot muscle volume and composition from MRI. A slice interval of 10 may provide a balance between efficiency (36 mins vs. 6 h) and accuracy (error < 2.4%) across all intrinsic foot muscles in asymptomatic individuals and those with plantar heel pain.

Fat Is Consistently Present within the Plantar Muscular Space of the Human Foot—An Anatomical Study

Background and Objectives: The foot comprises of active contractile and passive connective tissue components, which help maintain stability and facilitate movement during gait. The role of age- or pathology-related degeneration and the presence of fat within muscles in foot function and pain remains unclear. The existence of fat has to date not been quantified or compared between individuals according to age, sex, side or subregion. Materials and Methods: 18 cadaveric feet (mean age 79 years) were sectioned sagittally and photographed bilaterally. Fat in the plantar muscular space of the foot (PMSF) was quantified through the previously validated manual fat quantification method, which involved observing photographs of each section and identifying regions using OsiriX. Fat volume and percentage was calculated using a modified Cavalieri’s method. Results: All feet had fat located within the PMSF, averaging 25.8% (range, 16.5–39.4%) of the total PMSF volume. The presence of fat was further confirmed with plastination and confocal microscopy. Conclusions: These findings suggest that fat within the PMSF is a consistent but highly variable finding in elderly cohorts. Fat within the foot muscles may need to be considered a norm when comparing healthy and non-healthy subjects, and for therapeutic interventions to the foot. Further work is required to understand in detail the morphological and mechanical presence of fat in the foot, and compare these findings with pathological cohorts, such as sarcopenia. Additionally, future work should investigate if fat may compensate for the degeneration of the intrinsic muscles of the foot, with implications for both the use of orthotics and pain management.

Imaging of the Pes Cavus Deformity

Direct-type cavus foot deformities are most commonly encountered and are primarily sagittal plane deformities. Direct deformities should be delineated from rarer triplane pes cavovarus deformities. The lateral weight-bearing radiograph is the cornerstone of imaging evaluation of direct pes cavus foot deformity. The apex of Meary talo-first metatarsal angle on the lateral radiograph represents the pinnacle of the cavus deformity and assists in subclassification of the deformity. With routine application, ancillary radiographic imaging techniques, such as the modified Saltzman view or the modified Coleman block test, can give valuable insight into deformity assessment and surgical planning.