121 - Posterior Femoral Cutaneous Nerve Mapping: A Magnetic Resonance Imaging and Cadaveric Dissection Study

Poster Board Number: 121
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Abstract Body : INTRODUCTION: Adequate understanding and care of cutaneous nerves during preoperative surgical planning is lacking. During proximal hamstring repair surgery, the posterior femoral cutaneous nerve (PFCN) is left vulnerable to iatrogenic injury. Preoperative planning for this surgery utilizes magnetic resonance imaging (MRI) to determine the degree of hamstring injury, but currently does not have the capacity to visualize and map the PFCN, which courses directly over the proximal hamstring attachment site. The aim of this study was to establish a MRI protocol for the characterization of the PFCN.

METHODS: A mixed cohort of non-pathological hips from living volunteers (n = 4; age = 23 ± 0 years; BMI = 24.8 ± 1.3 kg/m^2; m:f = 1:3) and fresh-frozen cadaveric donors (Science Care, USA; n = 6; age = 71 ± 3 years; BMI = 23.1 ± 2.8 kg/m^2; m:f = 2:4) were examined (approved ethics #: 121404). Each hip was imaged in a supine position with 3T MRI (Siemens 3T Prisma) using T1 turbo spin-echo (TSE) and T2 magnetization prepared rapid gradient-echo (MP-RAGE), 3D short inversion time inversion recovery (STIR), and 3D Dual Echo Steady State sequences (DESS). The PFCN was identified, measured, and compared on each MRI sequence using image analysis software (3DSlicer, The Slicer Community). After imaging, the cadaveric specimens were dissected to confirm the PFCN in situ and compare the anatomy with imaging data. Through the gluteal region, the specimen’s skin and subcutaneous fat was denuded with fine dissection to indicate inferior cluneal branches of PFCN. Gluteus maximus was then reflected in the approximate midline to identify the main trunk of PFCN. Fine dissection of the region surrounding the ischial tuberosity was done to identify the perineal branches of the PFCN.

RESULTS: For the living subjects, although anatomical landmarks were clear on imaging data, the PFCN was not identifiable in any imaging sequence. For the cadaveric specimens, the DESS sequence provided adequate visualization of the PFCN, but the T1 sequences were unable to differentiate nerve from muscle. Although fibrous tissue was clear for MP-RAGE, the fascial planes were not identifiable which made it difficult to visualize the PFCN. For DESS, the fascial planes were easily visible, allowing for PFCN identification. The location of the PFCN and its branches in situ were confirmed and validated with cadaveric dissection.

CONCLUSION: The DESS sequence was most suitable to identify and track the PFCN. Because of the usefulness in tissue contrast, DESS has been primarily used for cartilage imaging. Given the clarity of surrounding landmarks, fascial planes, and nerve fibres, DESS should now be further implemented to map the PFCN. These imaging observations were further validated through cadaveric dissection following MRI. To reduce the frequency of PFCN iatrogenic injury, clinicians must be provided with the tools to understand anatomy. Inclusion of these imaging techniques and further refinement of these protocols will help deliver a preoperative plan to avoid iatrogenic PFCN damage during proximal hamstring repair.

SIGNIFICANCE: An adequate understanding of the anatomy of cutaneous nerves is currently lacking during preoperative surgical planning. Establishing MRI protocols of sufficient quality for cutaneous nerve identification and mapping may lead to reduced iatrogenic injury during surgical procedures.