Introduction
The metal on metal implants was introduced without the proper stepwise introduction. The ASR resurfacing hip arthroplasty (RHA) withdrawn due to high clinical failure rates and the large diameter head THA (LDH-THA) are also widely abandoned. Early (2 year) radiostereommetry studies does not support early instability as cause of failure but more likely metal wear products. A possible advantage may be maintenance of bone mineral density (BMD).
We present 5 year prospective follow up from a randomized series, aiming to report changes from baseline and to investigate links between implant micromotion, Cr & Co ions and BMD.
Patients/Materials & Methods
Patients eligible for an artificial hip were randomized to RHA, Biomet LDH-THA or standard Biometric THA. 19, 17 and 15 patients completed 5 year follow-up. All followed with BMD of the femur, acetabulum and for RHA the collum. RHA and THA with whole blood Co and Co. LDH-THA only at 5 year. RHA had marker based RSA of both components, cup only for LDH-THA. Translations were compiled to total translation (TT= √(x2+y2+z2)). Data were collected at baseline, 8 weeks, 6 months, 1, 2 and 5 years.
Statistical tests: ANCOVA for TT movement, Spearman’s correlation for BMD, Cr, Co and BMI to TT at 5 years
Results
RSA: The 5 year median (25%to75%) RHA cup translations were X=-0.00(-0.49 to 0.19) Y=0.15(-0.03 to 0.20), z=0.24(-0.42 to 0.37) and TT 0.58 (0.16 to 1.82) mm. For the LDH-THA X=-0.33(-0.90 to 0.20) Y=0.28(0.02 to 0.54), z=0.43(-1.12 to -0.19) and TT 1.06 (0.97 to 1.72) mm. The TT was statistically different (p<0.05) for the two cups. The RHA femoral component moved X=0.37(0.21 to 0.56) Y=0.02(-0.07 to 0.11), z=-0.01(-0.07 to 0.26) and TT 0.48 (0.29 to 0.60) mm at 5 years. There was no TT movement from year 2.
The mean (SD) acetabular BMD was diminished to 93(90- 97)% for RHA and 97(93- 99.9)% for THA, but LDH-THA maintained 99(95-103)%. Overall femoral BMD was unchanged at 5 years for all interventions, but both stemmed implants lost 17% at the calcar.
Median (25%to75%) whole-blood Cr peaked in the LDH-THA group with 1.7 (0.9 to 3.1) followed by RHA 1.2 (0.8 to 5.0) and THA with 0.5 (0.4 to 0.7)ppb.
For Co the highest levels were found in RHA with 1.6(0.8 to 4.7) followed by LDH-THA 1.2 (0.7-1.7) and THA 0.2 (0.2 to 0.6) ppb.
The only correlations above +/-0.3 for TT were the RHA femoral component with a correlation of 0.47 to BMI, 0.30 to Co and Cr. The ASR cup conversely had a negative correlation of -0.60 to BMI and again, the LDH-THA cup had a negative correlation of -0.37 to Cr.
Discussion/Conclusion
In contrast to registered revision rates, we found significantly larger movement for the Biomet cup than the ASR cup. The metal ion levels were similar. The LDH-THA cup maintained the acetabular BMD best at 5 years, but the difference was small, we are limited by small numbers and the correlations between TT and the covariates showed no clear pattern.