Novel evaluation of bone reaction/growth around full hydroxyapatite-coated stem using digital tomosynthesis imaging: confirmation of very early bone growth around the stem

Full hydroxyapatite (HA) coated implants are placed into the bone marrow cavity by compressing cancellous bone. The Full HA coated compaction stem, embedded in compressed cancellous bone, is known to form an osteogenic bridge with the endosteum. The aim of this study was to trace and characterize the changes occurring at the interface between compressed cancellous bone and the full HA coated compaction stem during the early postoperative period. There were 37 of 48 hips that underwent total hip arthroplasty (THA) with full HA coated compaction stem (POLARSTEM™, Smith and Nephew, Baar, Switzerland) since January 2020. 37 hips were observed for bone reaction around the implant using tomosynthesis images and simple radiographs at 1 week, 6 weeks, 3 months, 6 months, 1 year and 2 years follow up. Digital tomosynthesis (DTS) with reduced metal artifact (T-smart®, SHIMADZU, Kyoto, Japan) was used for this study. Spot welds in cancellous bone were observed on DTS images and analyzed using image processing software (Image J) for temporal comparison. At 6 months post-operatively, radiographs revealed multiple spot welds (MSWs) surrounding full HA coated compaction stem in 27.8% of patients. DTS images detected MSWs in 36.4% of cases at 6 weeks and in 100.0% at 3 months postoperatively. All patients had an excellent clinical progress and did not have thigh pain. Tomosynthesis images demonstrated MSWs suggestive of bone-bridge formation between the full HA coated compaction stem and the bone starting in the early postoperative period (6 weeks). These findings support the good clinical outcomes reported, including low implant loosening, minimal subsidence, reduced stress shielding, and low incidence of thigh pain. The osteointegration process was observed around the full HA coated compaction stem at 6 weeks postoperatively, suggesting that early fixation between the implant and the compressed cancellous bone had been achieved.