Introduction Due to complex anatomical considerations, results of cementless-stem total hip arthroplasty (THA) in hip fusion remain controversial compared with conventional THA. We therefore aimed to analyse 3D anatomy of the fused hip based on pre-operative computed tomography (CT) scans, functional outcomes based on the Harris Hip Score (HHS) and the Hip Disability and Osteoarthritis Outcome Score (HOOS), modifications of hip anatomy based on post-operative standardised radiography, and determine complication rate and long-term survival based on Kaplan-Meier analysis. Methods We retrospectively studied 23 hips that underwent conversion of a fused hip to THA using a 3D CT scan-based custom titanium (Ti alloy hypoxyapetite (HA)-coated stem. The mean follow-up was 15 (9-22) years. Femur anteversion ranged from -29 degrees to 80 degrees. Results HHS improved from 59 to 89 points and average range of motion (ROM) was 88 degrees for flexion. Back pain decreased in 62%, and knee pain decreased in 42%. The mean post-operative leg-length discrepancy was 7.8 mm. No intra-operative complications occurred. One aseptic stem loosening for mechanical failure was observed. The Kaplan-Meier survivorship considering revision for any reasons as the end point was 95.6% [95% confidence interval (CI) 92.4-98.8] at 15 years. Conclusion Custom cementless stems may be an interesting solution to address the particular anatomical teatimes of hip fusion at the time of THA, with an excellent survival rate at 15 years.