Study design <p>Retrospective case series.</p> Objective <p>To evaluate clinical and radiological outcomes following in situ transdiscal L5–S1 fixation (delta fixation) in elderly osteoporotic patients with high-grade spondylolisthesis, with emphasis on functional recovery and changes in sagittal spinopelvic alignment.</p> Methods <p>15 patients aged &gt; 60&#xa0;years (mean age 74.9 ± 14.8&#xa0;years) with Meyerding Grade III–IV L5–S1 spondylolisthesis and osteoporosis (DEXA <i>T</i> score ≤ − 2.5) underwent posterior-only in situ transdiscal fixation and neural decompression. Pre- and postoperative spinopelvic parameters were assessed using standing radiographs and Surgimap® software. Functional outcomes were evaluated using the Oswestry Disability Index (ODI) and the Visual Analog Scale (VAS) for back and leg pain, with a minimum follow-up of 24&#xa0;months.</p> Results <p>All patients demonstrated significant functional improvement. ODI improved from 66.8 ± 7.1 to 37.3 ± 5.1 (<i>p</i> &lt; 0.001), VAS for low back pain from 8.3 ± 1.5 to 2.7 ± 0.5 (<i>p</i> &lt; 0.001), and VAS for leg pain from 4.5 ± 1.7 to 1.2 ± 0.6 (<i>p</i> &lt; 0.001). Among sagittal parameters, significant improvements were observed in thoracic kyphosis (<i>p</i> = 0.006), sagittal vertical axis (<i>p</i> = 0.010), and PI–LL mismatch (<i>p</i> = 0.032). Changes in pelvic incidence, pelvic tilt, lumbar lordosis, and sacral slope were not statistically significant.</p> Conclusions <p>In situ delta fixation with neural decompression offers substantial clinical benefit in elderly osteoporotic patients with high-grade L5–S1 spondylolisthesis. Functional recovery occurred despite limited changes in local spinopelvic sagittal alignment, highlighting that global compensation and neural decompression may suffice for clinical improvement in this high-risk population.</p> <p><i>Level of evidence</i>: Level IV—Case series.</p>