Anatomical variations in the length and structure of the sleeves or “roots” of cervical nerves have meaningful implications for surgical approaches and neuroanatomy. The sleeves refer to the coverings of nerve roots as they exit the spinal cord and canal, which are relevant in several regions, notably in the ansa cervicalis (a nerve loop in the neck) and the dorsal root entry zones of the cervical spinal nerves.
Cervical Nerve Sleeve Length: Key Data
• Dorsal Root Entry Zone (DREZ) Lengths:
The average length of the dorsal root entry zone (sleeve) varies by cervical level:
• C5: 13.0±1.6mm
• C6: 13.8±0.5mm
• C7: 12.8±1.5mm
• C8: 12.0±0.8mm
• Shorter at higher and lower levels such as C1 (1.0±0.0mm) and C2 (5.8±0.5mm).
• Phrenic Nerve Rootlet Lengths:
Mean lengths measured for unembalmed cadavers:
• C3: 3.9±2.4cm
• C4: 3.6±2.6cm
• C5: 0.5±0.8cm
This shows great variability and highlights the significance of rootlet length in the formation of nerves such as the phrenic nerve.
Anatomical Variations
• Ansa Cervicalis:
The ansa cervicalis demonstrates notable variability in the length and structure of its nerve roots (or sleeves). The inferior root formation varies considerably, with possible contributions from C1 to C4, and can present as double, triple, or even quadruple formations, or may be absent entirely. The superior root is generally more consistent and symmetric.
• In one study, the classic single inferior root form was seen in just 27% of cases, with double or more roots making up the rest.
• The overall length and number of branches may differ considerably, and multiple or bifurcated root configurations have been documented.
Clinical Significance
• Variation impacts surgical approaches—especially during neck surgeries, as the variability in length, number, and origin of these nerve sleeves/roots can affect both identification and preservation of nerves.
• Morphometric studies provide benchmarks: C5 and C6 consistently have the longest root entry zones, posing specific challenges in interventions such as dorsal root entry zone (DREZ) lesioning, nerve repair, and decompression
The width of the lower cervical vertebrae (C5–C7) increases caudally (from top to bottom of the cervical spine):
• C5 width: 20.80±1.87mm
• C6 width: 21.74±3.77mm
• C7 width: 22.59±3.55mm
Some sources report slightly different but similar values, generally ranging:
• C5: 20–26mm
• C6: 21–25mm
• C7: 22–27mm
These dimensions refer to the transverse diameter (side-to-side) of the vertebral body, taken from anatomical studies and CT-based measurements. The data show gradual widening from C5 to C7, with C7 usually being the broadest of the typical cervical vertebrae.
These measurements provide essential reference points for surgery, prosthesis design, and radiological assessment. Some variation exists between populations and measurement techniques.