Muscle MRI Fat Fraction Before Surgery to Predict Strength Recovery After Foot-Drop Surgery in Charcot-Marie-Tooth Disease

Description recommendations and covers background, design, exposure, predictor assessment, outcomes, follow-up, sample size, statistical analysis plan, data management and ethics.

1. Background and Rationale Charcot-Marie-Tooth disease (CMT) constitutes the most prevalent inherited peripheral neuropathy, with an estimated prevalence of 1:2,500-1:5,000. Among genetically defined adult cases, CMT1A (PMP22 duplication), CMT1X (GJB1 pathogenic variants) and CMT2A (MFN2 pathogenic variants) collectively account for the majority.

   Distal lower-limb weakness-most conspicuously impairment of ankle dorsiflexion-gives rise to foot drop, progressive flexible cavovarus deformity, plantar pain and callus formation, recurrent ankle sprains and falls, and diminished walking capacity and health-related quality of life. In large patient-reported surveys, foot and ankle weakness (99.7%) and impaired balance (98.6%) emerge as the most prevalent and most impactful symptoms (Johnson et al., 2014).

   Where conservative management fails, individualized combined surgery-comprising soft-tissue balancing (tendon transfers, musculotendinous lengthenings, capsular and ligamentous procedures) with or without corrective osteotomies-constitutes the mainstay of treatment.

   Given the frequent and often severe degeneration of the tibialis anterior and peroneal musculature, transfer of the tibialis posterior (TP) tendon through the interosseous membrane to the dorsum of the foot represents the principal procedure for restoring active dorsiflexion; three-dimensional gait analysis has shown significant gains in swing-phase dorsiflexion consistent with active muscular substitution rather than a merely tenodic effect (Dreher et al., 2014).

   Postoperative outcomes, however, remain heterogeneous. The transferred donor muscle is itself subject to the same neurogenic fatty degeneration that affects the surrounding musculature, and no objective preoperative variable is currently available to inform the surgeon as to how the TP (or co-transferred flexors) will perform after transfer.

   Muscle fatty infiltration can be quantified objectively by means of chemical-shift-encoded magnetic resonance imaging (MRI). Multi-echo Dixon techniques generate pixel-wise proton density fat fraction (PDFF) maps, show high reproducibility (interobserver ICC > 0.8), and detect fatty replacement in CMT muscles that appear unremarkable on semi-quantitative grading.

   Calf-muscle fat fraction correlates with clinical severity and functional scores and exhibits genotype-specific distribution patterns (Morrow et al., 2016; Kim et al., 2019; Sun et al., 2023).

   We therefore hypothesize that preoperative Dixon-derived fat fraction of the donor muscles-principally the TP-predicts postoperative recovery of ankle dorsiflexion strength and may serve as a practical imaging biomarker for surgical decision-making.

2. Design, Setting and Population The study is designed as a prospective, multicenter, single-cohort observational study with five-year follow-up and will be conducted at four tertiary foot-and-ankle centers in China (one coordinating center and three participating sites), each maintaining a partner neuromuscular clinic in neurology and having access to 3.0-T MRI.

   Eligible patients scheduled for CMT foot surgery incorporating TP transfer will be screened consecutively over a 36-month accrual window, and the recruitment status will be updated on ClinicalTrials.gov. The index limb-the first foot to undergo surgery-constitutes the unit of analysis; contralateral surgery performed during follow-up will be recorded as a concomitant intervention, and the second limb will be excluded from analysis.

   For registration purposes the design is specified as follows: Study Type, Observational; Observational Model, Cohort; Time Perspective, Prospective; Patient Registry, No; Sampling Method, Non-Probability Sample. The study population comprises adults with genetically confirmed CMT1A, CMT1X or CMT2A who are scheduled to undergo individualized surgery including TP transfer.

3. Exposure: Individualized Combined Surgery Including Tibialis Posterior Tendon Transfer All participants will receive routine care, with the operative strategy selected by the treating surgeon according to deformity severity; the study does not assign interventions. The index procedure must include TP tendon transfer for augmentation of dorsiflexion.

   The standard technique entails harvest of the TP through a medial approach with maximal tendon length preserved and routing through a generously sized posterolateral window in the interosseous membrane (direct anteriorization) or through the tibialis anterior sheath.

   The tendon is inserted into the middle cuneiform or the base of the second metatarsal (or the lateral cuneiform) by means of a bone tunnel or interference/anchor fixation, and is tensioned with the ankle in neutral dorsiflexion and the forefoot in neutral alignment.

   Whether the transfer is split or full, together with its route, insertion site, tension and fixation method, is documented in a standardized operative record.

   All adjunct procedures are recorded individually:

   peroneus longus-to-brevis transfer or tenodesis; flexor digitorum/hallucis longus transfer (to the peroneus brevis, the TP stump or the dorsum); Achilles tendon lengthening (percutaneous or open, when resting equinus > 10 degrees); plantar fascia release; first metatarsal dorsiflexion (closing-wedge) osteotomy; modified Jones procedure (extensor hallucis longus transfer to the first metatarsal neck with interphalangeal fusion); lesser claw-toe correction (MTP release, PIP fusion, flexor-to-extensor transfer); cuboid or calcaneal (Dwyer) osteotomy; and midfoot osteotomies. Fixed hindfoot deformity necessitating arthrodesis constitutes an exclusion criterion (see Eligibility). Postoperative management is standardized across sites: a below-knee cast for 4-6 weeks with graded weight-bearing, followed by a removable boot and a protocolized, uniform progressive rehabilitation program (range-of-motion exercises, then strengthening of the dorsiflexors and evertors, and subsequently gait retraining); any deviation from the protocol is documented.

4. Predictor Assessment: Preoperative Dixon MRI Fat Fraction Imaging is performed within 1 month (30 days) before surgery, preferably at 3.0 T (1.5 T acceptable), using a knee or flex coil with the participant supine and the foot in the neutral position.

   The acquisition protocol comprises an axial chemical-shift-encoded Dixon sequence-preferably a multi-echo three-dimensional gradient-echo acquisition with T2* correction, with two-point Dixon acceptable-extending from the fibular head to the distal tibial plafond so as to encompass the maximal cross-sectional area of all target muscle bellies, with contiguous 3-4 mm slices; a vendor-reconstructed PDFF map is generated (FF = fat signal / (fat + water) × 100%).

   Site-specific acquisition parameters are logged; a standardized imaging manual is disseminated to all centers, and monthly agarose-oil phantom scans are used to monitor cross-site FF calibration.

   The following 11 muscles of the index limb are quantified: tibialis anterior (TA), extensor digitorum longus (EDL), extensor hallucis longus (EHL), tibialis posterior (TP - the primary donor muscle), flexor digitorum longus (FDL), flexor hallucis longus (FHL), peroneus brevis (PB), peroneus longus (PL), medial gastrocnemius (MG), lateral gastrocnemius (LG) and soleus (SOL).

   For the region-of-interest (ROI) methodology, a whole-muscle ROI is delineated on the PDFF map at the slice of maximal muscle cross-section (with boundaries verified on the fat and water images), deliberately excluding fascia, intramuscular tendon, major fat septa and neurovascular structures; the measurement is repeated on two contiguous slices, and the three-slice mean constitutes the muscle FF. Muscle cross-sectional area (cm²) is recorded concomitantly.

   All muscles are measured independently by two readers-a musculoskeletal radiologist and a trained orthopaedic researcher-both blinded to the clinical data; the mean of their values is used in the analyses. A randomly selected 20% subset is re-measured to quantify inter- and intra-reader reliability (target ICC ≥ 0.85), with discrepancies resolved by consensus adjudication.

   Three derived indices are computed: the donor composite FF (a cross-sectional-area-weighted mean of the muscles actually transferred), the dorsiflexor FF (mean of TA, EDL and EHL) and the triceps surae FF (mean of MG, LG and SOL).

5. Outcome Measures The primary outcome is the change from baseline in maximal voluntary isometric ankle dorsiflexion strength, measured with a hand-held dynamometer (HHD) and normalized to body weight (N/kg), at 3 and 6 months and at 1, 2, 3, 4 and 5 years after surgery; 12 months constitutes the principal endpoint of interest.

The HHD protocol is administered with the participant seated, the knee flexed to 90 degrees and the ankle in the neutral position, the padded device being applied to the dorsal forefoot; a make-test of 3-5 seconds is performed three times with 30-second inter-trial rest, and the mean of the three trials is recorded.

This protocol shows intra- and inter-rater ICCs of 0.94 and 0.96, respectively, with an MDC95 of 0.77 N/kg (Kimura et al., 2018). For each participant the same calibrated device and the same assessor are used, and assessors remain blinded to the MRI-derived FF results.

The secondary outcomes comprise functional performance tests, ankle range of motion, validated clinical scales and a ten-domain symptom battery covering the most prevalent manifestations of CMT:

1-minute walk distance (1MWD; the distance covered in 1 minute at habitual pace on a level 20-m course with the usual orthosis; the 1-minute walk test shows good test-retest reliability and strong concurrent validity with the 6-minute walk test, the conventional endurance measure in CMT); 10-meter walk test (10MWT; comfortable speed, m/s, mean of 2 trials, 2-m acceleration/deceleration zones, usual device); active and passive ankle dorsiflexion and plantarflexion range of motion (universal goniometer with bony landmarks; passive dorsiflexion with knee flexed 45 degrees and extended; passive plantarflexion prone; mean of 3, degrees); CMT Examination Score version 2 (CMTESv2; 7 items scored 0-4 - sensory symptoms, motor symptoms legs/arms, pinprick with Neurotip, vibration with Rydel-Seiffer graduated 128-Hz tuning fork at the great toe/medial malleolus/tibial tuberosity, MRC leg strength, arm strength of first dorsal interosseous/abductor pollicis brevis; total 0-28, higher = more severe); Walk-12 (12 items 0-3, total 0-36, higher = worse perceived walking ability); Foot and Ankle Disability Index (FADI, 0-100, and FADI-Sport; higher = better); SF-12 physical and mental component summaries (norm-based, higher = better); symptom battery, domain 1 - foot-ankle weakness (HHD, N/kg; MRC grading); domain 2 - balance impairment and falls (timed up-and-go, seconds; single-leg stance, seconds, maximum 30; number of falls in the past 6 months); domain 3 - sensory loss (Rydel-Seiffer vibration 0-8 at the great toe and medial malleolus; 10-g Semmes-Weinstein monofilament at 3 plantar sites, yes/no); domain 4 - foot-ankle pain (11-point numeric rating scale 0-10 for average pain during the past week at rest, at night and during activity); domain 5 - fatigue (Fatigue Severity Scale, 9 items, 1-7, mean); domain 6 - muscle cramps (4-week diary: episodes per week, distribution, night cramps yes/no; approximately 80% of individuals with CMT report cramps at least weekly); domain 7 - hand weakness and fine-motor impairment (Jamar grip strength, kg, handle 2, mean of 3; 9-hole peg test, seconds); domain 8 - deformity burden (count and map of plantar/dorsal calluses and fissures; weight-bearing radiographic angles at baseline and 12 months: Meary angle, Hibbs angle, calcaneal pitch); domain 9 - orthosis (AFO) dependence (type, wear hours/day and percentage of walking episodes from a 7-day diary); domain 10 - emotional distress (PHQ-4 anxiety/depression screen, 0-12). All symptom instruments are administered in an identical manner at every visit.

An exploratory outcome, assessed within an optional sub-study, is the change in FF of the transferred donor muscle(s) at 12 months on repeat Dixon MRI, obtained to characterize post-transfer donor-muscle remodeling.

Safety outcomes comprise the incidence, through 60 months, of surgical and study-related complications: wound dehiscence or infection (superficial or deep), sural or other nerve injury, deep vein thrombosis, complex regional pain syndrome, hardware irritation, over- or under-correction necessitating revision, and reoperation; each event is graded as mild, moderate or severe and is captured at every visit.

6. Follow-up Schedule At baseline (within 1 month before surgery) the following are obtained: written informed consent, demographic data, verification of the genetic report, the full clinical assessment battery, weight-bearing radiographs (standard care), Dixon MRI and a standardized surgical planning form.

Postoperative visits occur at 3 months (± 2 weeks), 6 months (± 1 month), 12 months (± 1 month) and 24, 36, 48 and 60 months (± 2 months) and comprise the full assessment battery, with MRI confined to the optional 12-month sub-study. Missed visits are rescheduled within the designated windows, and the reasons for non-attendance are documented.

7. Sample Size Estimation The primary analysis is a multivariable linear regression of ΔDF at 12 months on preoperative TP-PDFF, adjusted for seven covariates (age, sex, genotype, disease duration, baseline MVIC-DF, baseline CMTESv2 and number of adjunct procedures), giving eight predictors in total.

Assuming a medium-to-large effect (f² = 0.20; full-model R² ≈ 0.17), consistent with published calf-muscle FF-strength correlations of r ≈ 0.5-0.7, and specifying α = 0.05 (two-sided) with 80% power, G*Power 3.1 (F test; linear multiple regression, fixed model; R² deviation from zero) requires 82 complete cases.

Allowing for attrition of up to 20% over five years, 100 participants will be enrolled. With 80 analyzable participants the study retains ≈80% power for the eight-predictor model at f² ≈ 0.20-0.21 and >90% power to detect a correlation of r ≥ 0.35 between TP-PDFF and ΔDF at 12 months (96% at r = 0.40; Fisher z-test).

Enrollment of 100 is also expected to yield genotype subsets of approximately 25-70 (CMT1A) and ≥ 10 each (CMT1X, CMT2A), sufficient for descriptive subgroup analyses.

8. Statistical Analysis Plan Analysis sets: the primary analysis set (PAS) comprises participants who undergo TP-transfer surgery and have a valid baseline TP-PDFF and a 12-month MVIC-DF assessment; the full analysis set (FAS) comprises all operated participants with at least one postoperative assessment. Sensitivity analyses are performed on the FAS and within each genotype.

Descriptive statistics: continuous variables are summarized as means ± SD or medians with IQRs, and categorical variables as frequencies (%); baseline characteristics are tabulated by genotype. The distributions of TP-PDFF and ΔDF are examined by the Shapiro-Wilk test and Q-Q plots, with transformation or rank-based methods applied as appropriate.

The primary correlation analysis uses the Pearson coefficient for preoperative TP-PDFF versus ΔDF (N/kg) at each postoperative time point (principal: 12 months); the Spearman rank correlation serves as a sensitivity analysis, and scatter plots with fitted regression lines are presented.

The primary regression regresses ΔDF at 12 months on TP-PDFF plus the seven prespecified covariates, reporting unstandardized coefficients with 95% CIs, standardized coefficients and semipartial R²; multicollinearity is screened (VIF \< 5) with residual diagnostics. Analogous models are fitted at 24 and 60 months and for the donor composite FF.

A linear mixed-effects model of MVIC-DF across all visits, incorporating a time (categorical) × TP-PDFF interaction with random intercepts and slopes and an unstructured residual covariance estimated by REML, characterizes the differential recovery trajectory by TP-PDFF level.

Clinically meaningful recovery is defined as ΔDF ≥ 0.77 N/kg at 12 months (MDC95; Kimura et al., 2018). Logistic regression including TP-PDFF and the covariates is fitted; discrimination is quantified by the AUC with bootstrap optimism-corrected internal validation (1,000 resamples; target AUC ≥ 0.75), together with a calibration plot and sensitivity and specificity at the Youden index.

Mixed-effects models provide valid inference under a missing-at-random assumption. Multivariable regressions use multiple imputation by chained equations (m = 20), with all outcomes and covariates in the imputation model; complete-case and δ-adjusted tipping-point analyses serve as sensitivity analyses.

The primary hypothesis is tested once, two-sided, at α = 0.05. Secondary and subgroup analyses are supportive or exploratory and are reported as estimates with 95% CIs and nominal p-values without formal adjustment; Holm-adjusted values are reported descriptively within outcome families.

Prespecified subgroups are defined by genotype (CMT1A, CMT1X, CMT2A), deformity rigidity (fixed vs flexible), concomitant Achilles lengthening (yes vs no) and baseline TP-PDFF tertiles, with interaction terms tested.

Safety outcomes are summarized as incidences with Clopper-Pearson 95% CIs. Analyses are performed in R (≥4.3; lme4, mice, pROC) and/or SAS 9.4. The SAP is finalized and version-controlled before the first database lock; analysis code and the de-identified dataset are archived.

9. Data Management and Quality Control Data will be captured in electronic case report forms (REDCap) incorporating range and logic checks; source data verification will be performed for a randomly selected 20% sample, and central monitoring of imaging and outcome data will be undertaken.

Assessors at each site will undergo training and certification (HHD, goniometry, walk tests and CMTESv2) based on a combined instruction video, with annual re-certification. An imaging core laboratory will be established at the coordinating center, and the DSMB will review safety data annually.

10. Ethics and Dissemination The study will be conducted in accordance with the Declaration of Helsinki and applicable Chinese regulations. Each site will obtain institutional review board approval, and written informed consent will be obtained before any study-specific procedure.

The study will be registered on ClinicalTrials.gov and mirrored on ChiCTR before the enrollment of the first participant, and participant data will be de-identified and handled in accordance with the Personal Information Protection Law of the People's Republic of China.

Participants will receive no monetary payment but will be reimbursed for travel expenses in accordance with local policy. Aggregate results will be posted on ClinicalTrials.gov within 12 months of the primary completion date and submitted for publication in peer-reviewed journals.

Several limitations warrant acknowledgment: the absence of a concurrent control group, whereby within-patient change will be interpreted against published 12-month natural-history data indicating stable walk-test performance in CMT.

Further limitations include heterogeneity of adjunct procedures-mitigated by standardized rehabilitation, comprehensive documentation of all procedures and prespecified covariate adjustment-and inter-vendor variability in MRI, mitigated by phantom-based calibration and a shared imaging manual.