This study aims to determine whether isobaric ropivacaine is more suitable than isobaric bupivacaine for spinal anesthesia in outpatient total hip arthroplasty (THA). The study will evaluate whether ropivacaine allows faster recovery of sensory and motor function while still providing adequate surgical anesthesia and postoperative pain control. Additional aims are to assess the effects of the anesthetic technique on same-day discharge rates, postoperative recovery, urinary retention, opioid consumption, and patient satisfaction after outpatient THA.

Patients undergoing outpatient total hip arthroplasty (THA) under spinal anesthesia require an anesthetic technique that provides adequate surgical anesthesia while allowing rapid postoperative recovery and same-day discharge. Bupivacaine is currently the most commonly used spinal anesthetic for hip arthroplasty, but its relatively long duration of action may delay recovery of motor and sensory function, prolong post-anesthesia care unit (PACU) stay, and increase the risk of urinary retention. Ropivacaine is a long-acting amide-type local anesthetic with a shorter duration of action and a potentially more favorable safety profile, including lower cardiotoxicity and reduced hemodynamic effects. This study aims to determine whether isobaric ropivacaine is more suitable than isobaric bupivacaine for spinal anesthesia in outpatient THA and to identify the optimal dose of ropivacaine for this surgical setting.

Altogether 176 patients scheduled for elective outpatient unilateral total hip arthroplasty at TYKS ORTO Surgical Hospital will be enrolled in this prospective, randomized, double-blind, controlled phase IV clinical trial. Eligible participants will be between 18 and 80 years of age, with a height of 160-186 cm, ASA physical status I-II, and body mass index below 35 kg/m², in accordance with local outpatient surgery criteria. Written informed consent will be obtained from all participants prior to study inclusion.

Patients will be randomized into two study groups to receive either intrathecal isobaric ropivacaine or intrathecal isobaric bupivacaine for spinal anesthesia. Both participants and investigators will remain blinded to group allocation throughout the study period. Spinal anesthesia will be performed according to standard clinical practice. Patient monitoring and perioperative care will follow institutional protocols routinely used for outpatient joint replacement surgery.

Baseline measurements including heart rate, blood pressure, and oxygen saturation will be recorded before administration of spinal anesthesia. After intrathecal injection, patients will undergo continuous monitoring including blood pressure, pulse, electrocardiography (ECG), oxygen saturation, and assessment of sensory block height. Measurements will initially be recorded every five minutes during the intraoperative period and subsequently at longer intervals after transfer to the ward.

The primary outcome of the study is the time to full recovery of sensory and motor function following spinal anesthesia. Secondary outcome measures include the proportion of patients discharged home as planned on the day of surgery, duration of PACU stay and total hospital stay, postoperative pain intensity measured using the Numerical Rating Scale (NRS; minimum 0, maximum 10, higher scores indicating more severe pain), incidence of urinary retention, time required to achieve adequate surgical anesthesia, adequacy of anesthesia during surgery, incidence of postoperative nausea and vomiting (PONV), postoperative opioid consumption, and patient satisfaction with care and recovery.

Postoperative pain management will follow standard institutional protocols. Pain intensity will be assessed regularly in the PACU and surgical ward using the NRS scale. Additional postoperative opioid analgesics may be administered as clinically indicated. Follow-up telephone interviews will be conducted on postoperative days 2 and 3 to assess recovery at home, pain management, discharge-related challenges, and patient satisfaction.

Bupivacaine and ropivacaine are both widely used amide-type local anesthetics for spinal anesthesia in orthopedic surgery. Their mechanism of action is based on reversible blockade of sodium channels in nerve cells, thereby inhibiting nerve impulse conduction. Ropivacaine has been associated with less cardiotoxicity, reduced central nervous system toxicity, and shorter duration of motor blockade compared with bupivacaine, which may improve suitability for outpatient surgery.

Both study medications are routinely used in clinical practice at TYKS ORTO Surgical Hospital and throughout the Nordic countries. Serious adverse effects are rare and are typically associated with overdose or unintentional intravascular administration. Common effects related to spinal anesthesia include transient hypotension due to sympathetic blockade, urinary retention, post-spinal headache, and temporary back pain. Severe complications such as infection or nerve injury are extremely uncommon. The study specifically aims to evaluate whether shorter-duration spinal anesthesia may reduce delayed recovery and urinary retention while maintaining adequate surgical conditions and postoperative analgesia.

All participant data will be handled confidentially. Personal identifiers will be removed from study documents and replaced with study identification numbers. The code key linking participant identities to study data will be stored separately in a password-protected file accessible only to the research team. Paper records will be maintained in locked facilities at Turku University Hospital, and no identifiable data will be shared outside the study team.

Inclusion Criteria:

• 18 - 80 years of age
• Scheduled for a primary elective total hip arthroplasty
• Patient must be a candidate for same day discharge
• Informed consent understood and signed
• Patient is 160 - 186 cm tall

Exclusion Criteria:

• Scheduled bilateral THA
• Contraindication to or failed spinal anesthesia
• Allergy or contraindication to NSAIDs (e.g. chronic kidney disease)
• Diabetes requiring insulin medication
• Preoperative chronic use of illegal substances
• Renal insufficiency (eGFR < 60)
• Unstable psychiatric conditions
• Severe neurological disorder
• Cognitive deficiencies preventing informed consent
• Language barrier preventing completion of study
• Pregnancy or breastfeeding
• Chronic pain syndrome

Individual participant data (IPD) will not be shared because the study data contain sensitive personal health information collected from a relatively small patient population at a single study center. Due to the limited sample size and detailed clinical information, there is a risk that participants could be indirectly identifiable even after de-identification. Data sharing is therefore restricted in order to protect participant privacy and confidentiality in accordance with applicable data protection legislation, institutional policies, and the informed consent provided by study participants.