Effect of hydroxyapatite coating on the radio-clinical results of a grit-blasted titanium alloy femoral taper. A case-control study of 198 cementless primary total hip arthroplasty with the Alloclassic™ system.

BACKGROUND: The effect of hydroxyapatite (HA) coating on the fixation of a cementless femoral stem is discussed, in particular in cases of primary fixation with geometrically stable components. Therefore, we performed a comparative retrospective study of a series of Alloclassic-SL™ stems to: 1) present the long-term results and 2) evaluate the contribution, if any, of proximal HA coating.

HYPOTHESES: Long-term cementless press-fit ("flat wedge-shaped") fixation is reliable and HA coating only improves the radiological results of the proximal bone-prosthesis interface.

MATERIALS AND METHODS: One hundred and ninety-eight Alloclassic total hip arthroplasties were performed in 179 patients, mean age 66 years old (22-85), including 105 with proximal HA coating and 93 with the original grit-blast coating. One hundred and ninety-three hips were analyzed after a mean follow-up of 9.8 years (1-24 years).

RESULTS: Results were excellent or good in 184 hips (95%) with no significant difference between the 2 groups (Merle d'Aubigné≥16 for 89/92 (98%) without HA compared to 95/101 (94%) with HA P=0.59). Radiographic signs of stable osseointegration were observed in 173 hips (90% of the cases). HA coating significantly improved the radiographic results of the proximal bone-implant interface: (42/92 (46%) of the stems without HA had proximal radiolucencies in zones 1 and 7 compared to 4/101 (4%) with HA (P=0.0001)). Polyethylene wear>0.1 mm/year was observed in 6 hips (3%) including 1/101 (1%) in the group with HA versus 5/92 (5.4%) without HA (P=0.17). One intra-operative femoral fracture occurred and there were 9 dislocations in the first 3 postoperative months (4.5%). The main cause of revision surgery was recurrent dislocation (11/17 cases). The "revision per-100 observed-femoral component years" was 0.10 in both groups and survival for aseptic loosening of the stem was 100% (95% CI=73.2% to 100%) at 20 years.

CONCLUSION: This study shows that secondary fixation by osseointegration of a straight standard grit-blasted titanium alloy non-anatomical implant is reliable. Possible proximal fibrous encapsulation, which is reduced by HA coating, but especially conventional polyethylene wear, were the main limitations of this system.

LEVEL OF EVIDENCE: III retrospective case-control study.