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Comparison of muscle fiber directions between different levator ani muscle subdivisions: in vivo MRI measurements in women

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Abstract

Introduction and hypothesis

This study describes a technique to quantify muscle fascicle directions in the levator ani (LA) and tests the null hypothesis that the in vivo fascicle directions for each LA subdivision subtend the same parasagittal angle relative to a horizontal reference axis.

Methods

Visible muscle fascicle direction in the each of the three LA muscle subdivisions, the pubovisceral (PVM; synonymous with pubococcygeal), puborectal (PRM), and iliococcygeal (ICM) muscles, as well as the external anal sphincter (EAS), were measured on 3-T sagittal MRI images in a convenience sample of 14 healthy women in whom muscle fascicles were visible. Mean ± standard deviation (SD) angle values relative to the horizontal were calculated for each muscle subdivision. Repeated measures ANOVA and post-hoc paired t tests were used to compare muscle groups.

Results

Pubovisceral muscle fiber inclination was 41 ± 8.0°, PRM was −19 ± 10.1°, ICM was 33 ± 8.8°, and EAS was −43 ± 6.4°. These fascicle directions were statistically different (p < 0.001). Pairwise comparisons among levator subdivisions showed angle differences of 60° between PVM and PRM, and 52° between ICM and PRM. An 84° difference existed between PVM and EAS. The smallest angle difference between levator divisions was between PVM and ICM 8°. The difference between PRM and EAS was 24°. All pairwise comparisons were significant (p < 0.001).

Conclusions

The null hypothesis that muscle fascicle inclinations are similar in the three subdivisions of the levator ani and the external anal sphincter was rejected. The largest difference in levator subdivision inclination, 60°, was found between the PVM and PRM.

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Notes

  1. In the rest of this paper we use the term muscle “fiber” instead of muscle fascicle because the term muscle “fiber” is more commonly used in the clinical literature. Given the assumption that the direction of the fiber is collinear with its fascicle as visualized on MRI this seems acceptable.

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Acknowledgements

We gratefully acknowledge the intellectual contribution of Catherine Brandon, MD, Musculoskeletal Division, Department of Radiology, University of Michigan, and her assistance in obtaining the high-quality MR images for this study, and Prof. Burkhardt Seifert, Division of Biostatistics, University of Zurich, for his contribution to the data analysis.

Institutional review board approval

IRB University of Michigan 2005-0011 (EMRLD) and IRB 1999-0395.

Funding

EMRLD (Evaluating Maternal Recovery from Labor & Delivery) funded by NIH ORWH SCOR P50 HD044406, OPAL II funded by RO1 HD 38665; grant to CB from the Swiss National Science Foundation.

Conflicts of interest

None.

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Authors and Affiliations

  1. Department of Obstetrics and Gynecology, University of Michigan, Ann Arbor, MI, USA

    Cornelia Betschart, Janis M. Miller & John O. L. DeLancey

  2. Department of Obstetrics and Gynecology, University of Zurich, Frauenklinikstrasse 10, 8091, Zurich, Switzerland

    Cornelia Betschart

  3. Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, USA

    Jinyong Kim & James A. Ashton-Miller

  4. School of Nursing, University of Michigan, Ann Arbor, MI, USA

    Janis M. Miller

Authors
  1. Cornelia Betschart
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  2. Jinyong Kim
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  3. Janis M. Miller
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  4. James A. Ashton-Miller
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  5. John O. L. DeLancey
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Corresponding author

Correspondence to Cornelia Betschart.

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Betschart, C., Kim, J., Miller, J.M. et al. Comparison of muscle fiber directions between different levator ani muscle subdivisions: in vivo MRI measurements in women. Int Urogynecol J 25, 1263–1268 (2014). https://doi.org/10.1007/s00192-014-2395-9

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  • DOI: https://doi.org/10.1007/s00192-014-2395-9

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