This prospective observational study aims to determine the factors affecting the success of unilateral spinal anesthesia in patients undergoing orthopedic lower extremity surgery. Unilateral spinal anesthesia is preferred to minimize unwanted effects such as hypotension and to provide more stable hemodynamic conditions, especially in vulnerable patient populations.

Unilateral spinal anesthesia is defined as achieving sensory and motor block predominantly on the operated side without significant contralateral spread. However, the factors influencing the success of this technique have not been fully clarified.

This study will evaluate various patient-related and procedure-related factors, including age, sex, body mass index, patient positioning, needle type and orientation, injection technique and local anesthetic dose. The primary outcome is the success of unilateral spinal anesthesia, defined as the absence of contralateral block.

The results of this study are expected to contribute to improving clinical practice by identifying major and minor factors affecting block success and optimizing anesthesia techniques.

This prospective observational study is designed to investigate the factors influencing the success of unilateral spinal anesthesia in patients undergoing lower extremity orthopedic surgery.

Spinal anesthesia is widely used in lower extremity procedures; however, it may be associated with hemodynamic instability such as hypotension. Unilateral spinal anesthesia has been proposed as a technique to limit sympathetic blockade and maintain more stable hemodynamic conditions. Despite its advantages, the determinants of successful unilateral block remain unclear.

Patients aged between 18 and 65 years with American Society of Anesthesiologists (ASA) physical status I-III who are scheduled for elective lower extremity surgery will be included in the study after providing written informed consent. This study will be conducted in a single tertiary care center.

No intervention will be performed by the investigator. All anesthesia procedures will be carried out according to routine clinical practice. The investigator will record patient demographics, including age, sex, height, weight, and body mass index.

Procedure-related variables such as patient positioning, spinal needle type, needle orientation, injection speed, level of injection, local anesthetic type and dose will be recorded.

Block characteristics will be assessed intraoperatively, including sensory and motor block distribution. The primary outcome is defined as successful unilateral spinal anesthesia, characterized by the absence of contralateral sensory block. Secondary outcomes include factors associated with block success and variability in block characteristics.

Data will be collected using standardized case report forms and analyzed to identify factors influencing the success of unilateral spinal anesthesia. The results of this study are expected to contribute to optimizing anesthesia techniques and improving patient safety.

Inclusion Criteria:

• Patients aged 18-65 years
• ASA physical status I-III
• Undergoing elective unilateral lower extremity surgery under spinal anesthesia
• Provided written informed consent

Exclusion Criteria:

• ASA IV patients
• Known allergy to local anesthetics
• Contraindications to spinal anesthesia
• Decompensated congestive heart failure
• Hemodynamic instability (including sepsis, septic shock, or need for inotropic support)
• Pregnancy
• Patients with vertebral anomalies (e.g., scoliosis, vertebral fractures) preventing proper positioning

Adult patients aged 18-65 years with ASA physical status I-III undergoing elective unilateral lower extremity surgery under spinal anesthesia in a single tertiary care center.

Individual participant data will not be shared due to patient confidentiality and institutional data protection policies