Supplementary table 1:

Outline of the structured introduction of Neurosurgery residents to the most common neurosurgical procedures at the Department of Neurosurgery at the Cantonal Hospital St. Gallen, Switzerland. Note that with more experience, more procedures of increasing complexity are added in a stepwise manner. Before residents are allowed to perform higher-level surgery, permission by the head of department and proof of regular training in the microsurgical skills laboratory are required. During every surgical procedure residents are assisted by qualified consultants to guarantee high quality patient care during each procedure. The resident may perform further surgical procedures not explicitly mentioned here at the discretion of the consultant assisting the case.

(*=after having assisted in three procedures; +=after introduction by senior resident; #=after having assisted in thirty procedures; ∧=after having assisted in five procedures; ∨=after having assisted in ten procedures; °=after having assisted in 30 procedures and having performed 20 procedures for lumbar disc herniation – only one level, patient < 80 years, no significant co-morbidity, no relevant spinal malalignment; ⌘=after having assisted in 15 procedures; %=after having performed ten non-complex lumbar decompression surgeries)

Completed months of training / Surgical procedure
2 / - Burr hole trepanation for chronic subdural hematoma*
- External ventricular drainage*
- Intracranial pressure (ICP)-monitoring probes+
6 / - Interlaminar fenestration, microsurgical sequestrectomy and microdiscectomy for mediolateral lumbar disc herniation#
- Peripheral nerve and/or muscle biopsy∧
12 / - Isthmotomy, microsurgical sequestrectomy and microdiscectomy for lateral / intraforaminal lumbar disc herniation (except L5-S1)*
- Diagnostic infiltrations (e.g. lumbar nerve root, facet joint, intervertebral disc, iliosacral joint, medial branch block, sacral epidural infiltration)∨
- Cryorhizotomy∨
18 / - Interlaminar fenestration, microsurgical decompression and undercutting for lumbar spinal canal stenosis°
- Craniotomy for intracranial lesion (approach)∨
- Ventriculo-peritoneal shunt∧
24 / - Isthmotomy (as mentioned above) including L5-S1
- Re-do surgery for lumbar disc herniation
- Anterior cervical discectomy for cervical disc herniation / cervical spinal canal stenosis⌘
- Lumbar decompression / laminectomy for “complex” lumbar spinal canal stenosis (> 1 segment, relevant spinal malalignment)%
30 / - Lumbar spondylodesis (e.g. posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF))∨
36 / - Craniotomy with surgery of an intracranial lesion∨