Dimensional changes of the tibial nerve and tarsal tunnel in different ankle joint positions in asymptomatic subjects

Key points

1. 01Tarsal tunnel anterior-posterior distance increased by 0.78 mm in plantarflexion (p < .001) and decreased by 0.37 mm in dorsiflexion (p = .027) compared with neutral
2. 02Tibial nerve cross-sectional area decreased significantly from plantarflexion to dorsiflexion (p = .035)
3. 03Tibial nerve became rounder in plantarflexion (flattening ratio fell from 3.64 to 2.85) and flatter in dorsiflexion (flattening ratio rose to 4.08)
4. 04Effect sizes for differences between plantarflexion and dorsiflexion were large (Cohen's d >= 0.80) for tunnel APD, nerve APD, nerve TD, and flattening ratio
5. 05Ultrasound intra-rater reliability was excellent across all measurements (ICC 0.91 to 0.99)

How it was conducted

Design

Cross-sectional observational study

Participants

13 healthy volunteers, 18 ankles (8 ankles excluded due to prior pathology); mean age 33.2 years, mean BMI 21.1 kg/m2; 69% female

Ankle positions tested

Anatomical (0 degrees), 30 degrees plantarflexion, 30 degrees dorsiflexion - measured with standard goniometer, non-weight-bearing

Imaging

13-MHz linear ultrasound probe at the malleolar-calcaneal axis; measurements by one examiner with >10 years musculoskeletal ultrasound experience

Outcomes

Tarsal tunnel anterior-posterior distance (APD); tibial nerve APD, transverse distance (TD), cross-sectional area (CSA by ellipsoid formula), and flattening ratio (TD:APD)

Statistical analysis

Friedman test with Wilcoxon post hoc comparisons; Cohen's d effect sizes; ICC for reliability

What they found

• Tarsal tunnel APD at anatomical position: 11.31 +/- 2.18 mm; increased to 12.09 +/- 2.31 mm in plantarflexion (change +0.78 +/- 0.65 mm, p < .001, effect size d = 1.20); decreased to 10.95 +/- 2.15 mm in dorsiflexion (change -0.36 +/- 0.64 mm, p = .029, effect size d = -0.55)
• From plantarflexion to dorsiflexion, tarsal tunnel APD decreased by 1.14 +/- 0.80 mm (p < .001, effect size d = 1.43)
• Tibial nerve CSA: 11.05 mm2 (median, IQR 9.5-14.1) at neutral, 12.97 mm2 (9.9-15.3) in plantarflexion, 11.55 mm2 (9.7-15.5) in dorsiflexion; CSA change from plantarflexion to dorsiflexion was not statistically significant (p = .085, effect size d = 0.40)
• Tibial nerve flattening ratio: 3.74 +/- 0.76 at neutral, decreased to 2.93 +/- 0.70 in plantarflexion (p < .001, d = -1.12), increased to 4.08 +/- 0.61 in dorsiflexion; plantarflexion to dorsiflexion difference -1.15 +/- 0.78 (p < .001, d = -1.48)
• Tibial nerve APD: from plantarflexion to dorsiflexion, APD increased by 0.53 +/- 0.59 mm (p = .002, d = 0.89)
• Tibial nerve TD: from plantarflexion to dorsiflexion, TD decreased by 0.80 mm (median IQR -1.4 to -0.5, p < .001, d = -1.42)
• Intra-rater reliability: ICC 0.91 to 0.99 for all measurements

Limitations

• Very small sample of 18 ankles in 13 subjects limits generalizability and statistical power
• Tarsal tunnel transverse width could not be measured because it exceeded the ultrasound probe length, so total tunnel cross-sectional area was not assessed
• Measurements taken only at the proximal tarsal tunnel; longitudinal nerve glide and distal branch changes were not evaluated
• Ankle plantarflexion and dorsiflexion are triplanar motions and accompanying midfoot/rearfoot eversion was not controlled; interrater reliability was not assessed

Why it matters

For patients

For patients with tarsal tunnel syndrome, ankle position matters: plantarflexion may open the tunnel and relieve nerve compression, while dorsiflexion tends to narrow it and may worsen symptoms.

For clinicians

These ultrasound findings support the biomechanical rationale for the dorsiflexion-eversion provocation test and suggest that positioning the ankle in mid-range plantarflexion could reduce tibial nerve compression; however, the small healthy-subject sample means clinical application requires further study.

For readers

This is a small pilot-level study in healthy volunteers only; it does not establish what happens in patients with tarsal tunnel syndrome or at extreme ankle angles, and the findings should be interpreted as hypothesis-generating rather than definitive.