Effect of femoral stem surface coating on clinical and radiographic outcomes of cementless primary total hip arthroplasty: a patient-matched retrospective study

Abstract

Purpose: This study aims to determine whether changing the stem coating grants superior outcomes at a minimum follow-up of five years.

Methods: Retrospective review of a consecutive series of primary total hip arthroplasties (THAs) operated by direct anterior approach between 01/01/2013 and 31/12/2014. Two stems were compared, which were identical except for their surface coating; "the Original stem" was fully coated with hydroxyapatite (HA), while "the ProxCoat stem" was proximally coated with plasma-sprayed titanium and HA. Matching was performed. Clinical assessment included modified Harris hip score (mHHS), Oxford hip score (OHS), and forgotten joint score (FJS). Radiographic assessment evaluated alignment, subsidence, pedestal formation, heterotopic ossification, radiolucent lines ≥ 2 mm, spot welds, cortical hypertrophy, and osteolysis.

Results: 232 hips received the Original stem and 167 the ProxCoat stem, from which respectively five hips (2.2%) and no hips (0%) underwent revision. Matching identified two groups of 91 patients, with comparable patient demographics. At > five years follow-up, there were no differences in OHS (16 ± 6 vs 15 ± 5; p = 0.075) nor FJS (81 ± 26 vs 84 ± 22; p = 0.521), but there were differences in mHHS (89 ± 15 vs 92 ± 12; p = 0.042). There were no differences in alignment, subsidence, pedestal formation, heterotopic ossification, cortical hypertrophy, and osteolysis. There were differences in prevalence of proximal radiolucent lines (12% vs 0%; p < 0.001) and distal spot welds (24% vs 54%; p < 0.001).

Conclusion: At a minimum follow-up of five years, this study on matched patients undergoing primary THA found that ProxCoat stems results in significantly fewer radiolucent lines, more spot welds, and less revisions than Original stems, thus suggesting better bone ingrowth.

Keywords: Cementless stem; Clinical outcomes; Radiographic outcomes; Surface coating; Total hip arthroplasty; Total hip replacement.

Fig. 1

AMIStem-H (referred to as Original stem) (Medacta, Switzerland) first undergoes a sandblasting process to roughen the entire surface of the stem to 2.5–6 μm; then, it is fully coated with an 80 μm layer of HA

Fig. 2

AMIStem ProxCoat (referred to as ProxCoat stem) (Medacta, Switzerland) first undergoes a sandblasting process to roughen the entire surface of the stem to 2.5–6 μm; then, the proximal two-thirds of the stem are coated with a 300 μm layer of MectaGrip (unalloyed titanium with a pore size of 100–350 μm) through Air Plasma Spray (APS) technology, and finally, the proximal two-thirds of the stem are coated with an 80 μm layer of HA

Fig. 3

Measurements on anteroposterior radiographs of pre-operative femoral anatomic parameters: canal flare index (CFI = Ai/Ei), cortical thickness index (CTI = (Ee-Ei)/Ee), canal bone ratio (CBR = Ei/Ee), canal calcar ratio (CCR = Ei/Bi), and morphologic cortical index (MCI = Be/Di)

Fig. 4

Measurements on anteroposterior radiographs of post-operative femoral canal fill ratio (CFR), which was calculated by dividing the femoral stem width by the endosteal diameter width at five levels with the lesser trochanter (LT) as reference point: (i) 2 cm above the tip of the LT (As/Ai), (ii) at the level of the tip of the LT (Bs/Bi), (iii) 2 cm below the tip of the LT (Cs/Ci), (iv) 7 cm below the tip of the LT (Ds/Di), and (v) 10 cm below the tip of the LT (Es/Ei)

Fig. 5

Flowchart describing the selection of patients for the study. During the study period, a total of 232 hips (220 patients) received the Original stem and 167 hips (160 patients) received the ProxCoat stem. Propensity score matching resulted in two groups of 91 patients each, with comparable patient demographics and pre-operative femoral morphology