ObjectiveTo investigate the application and effectiveness of the biological type acetabular cup (diameter ＜44 mm) in adult Crowe type IV developmental dysplasia of the hip (DDH). MethodsBetween April 2001 and August 2013, biological type acetabular cup was used in total hip arthroplasty for the treatment of Crowe type IV DDH in 16 cases (20 hips). There were 3 males and 13 females, aged 31-69 years (mean, 49 years). Unilateral hip was involved in 12 cases, and bilateral hips in 4 cases. The patients showed pain of the hip joint and inequality of lower limb (shortening of affected limb 1.8-6.0 cm in length, 3.5 cm on average). Acetabular deformity, the relationship and the severity of femoral head dislocation were comfirmed on the X-ray films. The preoperative Harris score was 34.0±6.9. ResultsAll patients achieved healing of incision by first intention, with no complication of infection or neurovascular injury. Sixteen cases were followed up 4-12 years (mean, 7.5 years). At 2 weeks after operation, dislocation occurred in 2 cases, and were fixed with plaster for 3 weeks after reduction of the hip. Postoperative X-ray films showed complete reduction of femoral head; the average acetabular coverage of the cup of the weight-bearing area was 98.5% (range, 98.2%-99.1%). The cup from the Ranawat triangle was 4.6-7.0 mm (mean, 5.8 mm) in medial shifting, and was 4.5-7.9 mm (mean, 6.2 mm) in elevation, it located at cup lateral surface area inside the iliopectineal line and the Kohler line (＜40%); the cup abduction angle was (45±5)°, and the anteversion angle was (10±5)°. The other patients had no prosthesis loosening except 1 patient having extensive acetabular prosthesis loosening because of acetabular osteolysis at 12 years after operation. The hip Harris score was significantly improved to 85.0±7.5 at 1 year after operation (t=14.34, P=0.01). ConclusionThe acetabular grinding process to retain enough bone combined with a small cup of biological prosthesis treating adult Crowe type IV DDH has the advantages of satisfactory coverage and initial acetabular fixation, so good early and mid-term effectiveness can be obtained.