A prospective randomized study comparing electrochemically deposited hydroxyapatite and plasma-sprayed hydroxyapatite on titanium stems

Background and purpose

Plasma-sprayed hydroxyapatite (HA) is a successful coating for fixation of uncemented femoral stems. There may be alternative coatings with advantages in bone remodeling and transport of bone-active substances. We investigated whether an electrochemically deposited hydroxyapatite, Bonemaster (BM), might be a safe alternative in total hip arthroplasty. Our hypothesis was that the new coating would not be inferior to the conventional one.

Patients and methods

50 patients (55 hips) were included. The stem was tapered and porous-coated proximally. On top of the porous coating was either HA or BM. Patients were evaluated postoperatively and after 3, 6, 12, and 24 months to measure fixation by radiostereometric analysis (RSA), bone mineral density by dual-energy X-ray absorptiometry (DXA), and conventional radiography. Clinical evaluation was performed with Harris hip score and Oxford hip score, both preoperatively and after 2 years.

Results

After 2 years, the stems had subsided 0.25 (HA) and 0.28 (BM) mm and there were no statistically significant differences between the groups in any direction, regarding both migration and rotation. The BM group retained significantly more bone than the HA group in Gruen zone 1 during the first 2 years. The Harris and Oxford hip scores were similar in both groups.

Interpretation

Electrochemically deposited hydroxyapatite on an uncemented stem does not appear to be inferior to plasma-sprayed HA regarding clinical and radiological results, bone remodeling, and micromotion after 2 years follow-up.Aseptic loosening is the most frequent complication of total hip arthroplasty (THA) (Havelin et al. 2000). The long-term survival is thought to depend partly on bone loss or osteolysis in the proximal femur after insertion. Plasma-sprayed hydroxyapatite (HA) coatings appear to give effective fixation in the femur (Hallan et al. 2007). Alternative coatings may, however, influence bone remodeling around the prosthesis and may function as a carrier of bone-active substances.Bonemaster (BM) (Bonemaster is a registered trademark of Biomet Europe) is an electrochemically deposited hydroxyapatite (EDHA) coating (Rößler et al. 2002). This technique makes it possible to add biological substrates such as antibiotics or adhesion peptides to the coating and still keep the coating very thin compared to plasma-sprayed HA.The thickness of a hydroxyapatite coating is a compromise between the mechanical properties and dissolution of the coating. A thinner coating minimizes the potential of particle shedding during insertion. Fewer particles in the joint mean less third-body wear and less periprosthetic osteolysis (Peters et al. 1992, Shanbhag et al. 1994, Campbell et al. 1995, McKellop et al. 1995). Thinner coatings also lower the risk of HA delamination and preserve the porosity of the underlying metallic coating of the implant. The irregular implant surface increases the surface area, providing a greater contact and ingrowth area (Sewing et al. 2002). EDHA, as in Bonemaster, forms a needle-like porous structure (Rößler et al. 2001) and enhances early-stage fixation between implant and bone (Ban et al. 1997).We designed a prospective randomized trial to compare conventional plasma-sprayed HA with electrochemically deposited HA after insertion of an uncemented femoral stem. This is the first clinical trial with the Bonemaster coating. We hypothesized that implants with Bonemaster would achieve the same degree of stability and bone remodeling, and the same clinical outcome as implants with traditionally plasma-sprayed HA.