This prospective clinical study aimed to evaluate the effectiveness of an imageless navigation system in total hip arthroplasty (THA) via the direct anterior approach (DAA) on a traction table for reducing functional limb length discrepancy (LLD) compared with conventional techniques. Seventy-two patients with advanced hip osteoarthritis, avascular necrosis, or developmental dysplasia of the hip undergoing THA between March 1, 2021 and September 30, 2021 were included. Functional LLD was assessed using a preoperative block test. Participants were assigned to the navigation or conventional group based on patient preference. Outcomes included operative parameters, intraoperative fluoroscopy time, limb length measurements, and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores.

This single-center prospective comparative study was conducted at the Joint Reconstruction Center of China Medical University Hospital between March 1, 2021 and September 30, 2021. Eligible participants were adults diagnosed with advanced hip osteoarthritis, avascular necrosis, or developmental dysplasia of the hip requiring THA. Exclusion criteria included prior hip joint infection, severe LLD requiring additional reconstructive procedures, significant spinal pathology affecting spinopelvic balance, or radiographs inadequate for measurement.

All procedures were performed via the direct anterior approach (DAA) on a traction table by the principal surgeon. Functional LLD was assessed preoperatively using a block test, with wooden blocks (5 mm increments) placed under the heel until the patient reported equal leg lengths.

Patients in the navigation group underwent THA with an imageless navigation system (Stryker OrthoMap Versatile Hip Navigation) to record intraoperative limb length change and acetabular cup positioning; the conventional group underwent standard THA without navigation.

Primary outcome was the patients' expectation gap (PEG), defined as the difference between preoperative functional LLD and postoperative radiographic limb length change. Secondary outcomes included intraoperative fluoroscopy time, WOMAC scores at baseline, 3 weeks, and 3 months postoperatively, and the proportion of outliers (PEG > 10 mm).

The study was approved by the Research Ethics Committee of China Medical University Hospital (CMUH112-REC1-149). Written informed consent was obtained from all participants, and the study adhered to the Declaration of Helsinki.

Inclusion Criteria:

• Clinical diagnosis of advanced hip osteoarthritis (OA) warranting total hip arthroplasty (THA)
• Clinical diagnosis of avascular necrosis (AVN) of the femoral head warranting total hip arthroplasty (THA)
• Clinical diagnosis of developmental dysplasia of the hip (DDH) warranting total hip arthroplasty (THA)

Exclusion Criteria:

• Documented history of prior hip joint infection
• Severe limb length discrepancy requiring supplementary surgical interventions
• Significant spinal pathology affecting spinopelvic balance
• Radiographs in which the lesser trochanters and teardrops cannot be clearly defined