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Detection and prevalence of variant sciatic nerve anatomy in relation to the piriformis muscle on MRI
Vanja Varenika1, Amelie M Lutz2, Christopher F Beaulieu2
1Department of Radiology and Biomedical Imaging, University of California, San Francisco, 505 Parnassus Avenue, San Francisco, CA, 94143, USA. vanja.varenika@ucsf.edu.
Objective:
To determine whether known variant anatomical relationships between the sciatic nerve and piriformis muscle can be identified on routine MRI studies of the hip and to establish their imaging prevalence.
Methods:
Hip MRI studies acquired over a period of 4 years at two medical centers underwent retrospective interpretation. Anatomical relationship between the sciatic nerve and the piriformis muscle was categorized according to the Beaton and Anson classification system. The presence of a split sciatic nerve at the level of the ischial tuberosity was also recorded.
Results:
A total of 755 consecutive scans were reviewed. Conventional anatomy (type I), in which an undivided sciatic nerve passes below the piriformis muscle, was identified in 87% of cases. The remaining 13% of cases demonstrated a type II pattern in which one division of the sciatic nerve passes through the piriformis whereas the second passes below. Only two other instances of variant anatomy were identified (both type III). Most variant cases were associated with a split sciatic nerve at the level of the ischial tuberosity (73 out of 111, 65.8%). By contrast, only 6% of cases demonstrated a split sciatic nerve at this level in the context of otherwise conventional anatomy.
Conclusion:
Anatomical variations of the sciatic nerve course in relation to the piriformis muscle are frequently identified on routine MRI of the hips, occurring in 12-20% of scans reviewed. Almost all variants identified were type II. The ability to recognize variant sciatic nerve courses on MRI may prove useful in optimal treatment planning.
Insights
Anatomical variations of the sciatic nerve and piriformis muscle are common on hip MRI scans. Recognizing these sciatic nerve variations can aid in optimal treatment planning.
Area of Science:
- Radiology
- Anatomy
- Orthopedics
Background:
- The sciatic nerve and piriformis muscle relationship can vary anatomically.
- These variations may impact clinical presentations and treatment.
Purpose of the Study:
- To identify known variant anatomical relationships between the sciatic nerve and piriformis muscle on routine hip MRI.
- To determine the prevalence of these variations in imaging.
Main Methods:
- Retrospective review of 755 hip MRI studies over 4 years from two centers.
- Categorization of sciatic nerve-piriformis muscle relationships using the Beaton and Anson classification.
- Recording of sciatic nerve splitting at the ischial tuberosity.
Main Results:
- Conventional anatomy (Type I) observed in 87% of cases.
- Variant anatomy (Type II) found in 13% of cases, with one sciatic nerve division passing through the piriformis.
- Most variants (65.8%) were associated with a split sciatic nerve at the ischial tuberosity.
Conclusions:
- Anatomical variations of the sciatic nerve course relative to the piriformis muscle are frequent on hip MRI (12-20%).
- Type II variations were the most common identified.
- Recognizing these sciatic nerve variants on MRI is crucial for effective treatment planning.
