Unexpected varus deformity and concomitant metal ion release and MRI findings of modular-neck hip stems: descriptive RSA study in 75 hips with 8 years' follow-up

Abstract

Background and purpose - Modular-neck hip stems have been identified with corrosion-related problems from the neck-stem junction. We report an ongoing varus deformity of modular-neck hip stems with simultaneous metal ion release observed during a study comparing the migration of modular vs. standard hip stems.Patients and methods - We followed 50 patients with modular and 25 with standard neck stems using radiostereometry (RSA). At 5-year follow-up, we noted a compromised integrity of the modular stem with varus deformity in the neck-stem interface. Changes in head-tip distance as well as whole-blood ion concentration and MRI findings were analyzed. The modular stems were followed further up to 8 years.Results - The head-tip distance decreased continuously by 0.15 mm per year resulting in 1.2 (95% CI 1.0-1.4) mm at 8 years for modular stems, while for the standard stems at 5 years, the decrease was 0.09 (CI 0.0-0.2) mm or 0.02 mm/year. For the modular stems, the reduction in head-tip distance correlated to the increase in whole-blood cobalt concentration at 8 years but not to the MRI grading of tissue reactions. At 5 years, cobalt levels were 4.9 µg/L for modular stems and at 8 years 4.8 µg/L, whereas for standard stems this was 1.0 µg/L. After 8 years, 9 of 72 stems had been revised for different reasons, but only 1 with obvious adverse local tissue reaction (ALTR).Interpretation - We present a surprisingly large progressive deformation at the modular neck-stem junction, but so far without a definite clinical problem. Even the femoral head seems to show slight compression onto the taper over time. A high rate of revisions for the modular type of this stem has raised general concerns, and it has been recalled from the market.

Trial registration: ClinicalTrials.gov NCT01512550.

Figure 1.

Standard and modular stem designs used in this study.

Figure 2.

Measurement of the head–tip distance.

Figure 3.

Examples of measurements of total neck length. The figure shows all possible measuring points along the 3 different CCD angles. The point of intersection is defined as the intersection between the CCD 130° line and the longitudinal axis of the stem.

Figure 4.

ABG II modular head (LFit) and neck after revision with corrosion on the neck part engaged in the stem–neck junction.

Figure 5.

Mean values with 95% CI of the reduction in head–tip distance in mm for different follow-up moments in months up to 5 years for the standard design and up to 8 years for the modular version.

Figure 6.

Change in position of the hip head with respect to the postoperative situation in X-direction (perpendicular to the hip–stem axis) and Y-direction (along the hip–stem axis), for 1, 2, 5, and 8 years’ postoperative follow-ups. The ellipse presents the 95% prediction interval of the head position change for each follow-up moment.

Figure 7.

Estimates from our linear regression model showing cobalt whole-blood concentration vs. reduction in head–tip distance at the 8-year follow-up.