The "Shamrock method" is a new ultrasound-guided lumbar plexus block. The aim of the study is to determine the minimum effective volume of ropivacaine 0.5% required to produce an effective lumbar plexus block with this method. Using the Dixon and Massey up-and-down method study design, 30 patients scheduled for elective total hip and knee arthroplasty will be included in the study.

Inclusion Criteria:

• orthopaedic patients scheduled for elective hip and knee arthoroplasty
• ASA 1 - 2 or stable ASA 3
• age between 18 and 70 years
• good ability to communicate and to cooperate

Exclusion Criteria:

• ASA > 3 or unstable ASA 3
• BMI > 32
• chronicle pain
• nephrological diseases or nephropathy
• known contraindications against the use of local anaesthetics
• pregnancy