The goal of this clinical trial is to compare two different types of nerve blocks in patients undergoing surgery for primary Total Knee Arthroplasty. The main question aim to answer is:

- is there a benefit in functional muscle power output of the leg?

Participants will be randomized into either the intervention group or the control group and:

• receive Subsartorial Saphenous Nerve Block (SSNB) + IPACK in intervention group (group 1)
• receive Femoral Nerve Block (FNB) + IPACK in control group (group 2)

Researchers will compare the 2 groups to see if there are differences in :

• functional muscle power output of the leg?
• muscle function, mobility, clinical and radiological results, ROM, pain control (NRS), opioid consumption, length of hospital stay, patient satisfaction, mobility, reduction of costs?

Prospective, randomized, double-blinded, single-center, controlled clinical trial with intervention group (SSNB + IPACK, group 1) and control group (FNB + IPACK, group 2), randomization by sealed envelopes.

The number of participants will be 72 per group, calculated based on a sample size calculator program.

The 3 surgeons will be blinded as well as the patients regarding the nerve block they will receive.

We will explain to the patients how both the blocks will be carried out, that 1 block will be located "higher on the leg" and 1 "lower on the leg". The nerve blocks will be carried out by a well-trained anaesthesiologist following standard procedures preoperative.

The IPACK will be applied by the surgeons intraoperative, following a standard protocol. The used implant for all patients will be the cemented ATTUNE™ Primary Knee System® (DePuySynthes Johnson&Johnson).

Every patient will wear a brace fixed in full extension after surgery for the first 24 hours. Postoperative all patients will receive a standard set of adequate analgesics.

The postoperative rehabilitation program will follow our well-established physiotherapy scheme.

Performed assessments:

• Range of motion (ROM)
• medical history
• physical examinations
• radiologic controls
• adverse events
• falls during hospital stay (standard protocol)
• pain assessment (numeric rating scale)
• Questionnaires (EQ-5D-5L und Oxford Knee Score) will be filled in by the patients
• Functional assessment: Cycle sprint test one a specially instrumented indoor bike (Verve Info Tec PTY LTD) fit with an instrumented crank (InfoCrank Power Meter, Verve Cycling, West Perth, Australia)
• Functional assessment: Manual muscle testing (MMT)
• Functional assessment: Timed up and go test (TUG)

Superiority analysis using a two sample t-test, significance level 0.05, power 80%.

Data will be stored electronically in the data base (RedCap). In case of missing data there will always be attempts to obtain these data afterwards (contacting the patients by phone). Further on statistical analysis is performed.

Dropouts, before the 6 weeks follow-up, will be replaced by recruitment of new subjects.

For quality assurance the sponsor, the Ethics Committee or an independent trial monitor may visit the research sites.

Inclusion criteria:

• age > 18yrs
• primary TKA
• BMI 35kg/m2 or less
• able to give informed consent as documented by signature
• clinical history without any contraindications for the planned intervention

Exclusion criteria:

• age < 18 years
• revision-TKA
• BMI >35kg/m2
• absent contact information
• inability or contraindications to undergo the investigated intervention (TKA, FNB, SSNB), - clinically significant concomitant diseases
• pregnancy
• inability to follow the procedures and follow-up procedures of the study (e.g. due to language problems, psychological disorders, dementia, living abroad, etc.
• withdrawal from the study

randomization by sealed envelopes