This report briefly compares the attributes of the as-cast cobalt chromium microstructure to the as-cast heattreated microstructure highlighting what aspects make it better suited for resurfacing devices. It also compares the wear performance of the resurfacings with wear rates from journal arti...
SESM 3028 BIOMATERIALS COURSEWORK 2023
Microstructure
As-cast cobalt chromium microstructure with large carbides can be better suited for resurfacing devices than as-cast
heat-treated microstructure with diffused carbide particulates. The large carbides present in as-cast cobalt
chromium provided mechanical stability to the matrix, prevented adhesive wear by fracturing, closed abrasive
scratches, and formed wear-resistant asperities that protected the softer matrix. The large carbides also self polishes
when dealing with scratches during steady state wear.
In contrast, as-cast heat-treated cobalt chromium microstructure with diffused carbide particulates is not as wear-
resistant due to the unstable particulate carbides that can easily detach from the matrix, making it mechanically
unstable. This increases three-body abrasion and surface fatigue [1].
Figure 1 shows a scanning electron micrograph of Figure 2 shows a scanning electron micrograph of a hot
as cast CoCrMo microstructure with blocky isostatic pressed and solution annealed as-cast CoCrMo
carbides [1]. microstructure with particulate carbides [1].
Analysis of cases
The wear performace of the three Biomet ReCap metal on metal (MOM) resurfacings were:
Case 100018 Case 100075 Case 100107
Cup volumetric wear rate (mm3/yr) 0.35 0.56 3.46
Femoral head volumetric wear rate(mm3/yr) 0.03 0.82 8.55
Table 1 shows the wear performance of the three Biomet MOM resurfacings.
Looking at Table 1, Case 100018 exhibits the lowest volumetric wear rate for both the cup and femoral head, while
Case 100107 had the highest volumetric wear rate for both components. The wear rates for Case 100075 were
moderate, with higher wear on the femoral head than on the cup.
A Biomet ReCap MOM study conducted by Newcastle University on six failed joints found a mean volumetric wear
rate of 6.1 mm3/year over a mean in-vivo time of 6.7 years. This wear rate is higher than what is predicted to cause
wear-related failures [2]. Another study found that a volumetric wear rate above 2.3 mm 3/year can lead to wear
related failures [3]. Case 100107, with a very high wear rate, could have fallen into this category.
Another study done by the Hamburg University of Technology on nine large MOM Biomet Magnums found an average
volumetric wear rate of 1.079 mm3/year for the head and 0.6507 mm3/year for the cup. The highest volumetric wear
rates for both the cup and head were for the same patient which were 5.5078 mm3/year and 4.8047 mm3/year
respectively. The size of the head ranged from 46 to 56 mm and the bearings were in situ for a median of 3.4 years.
There has been a record on strong trends in lower revision rates with larger head sizes in the National Joint Registry
[4]. This data correlates with the bigger head size having a lower volumetric wear rate in this study.
An analysis done on 78 patients with failed ASR (Articular Surface Replacement) hip resurfacings by DePuy had 85 hips
revised for Adverse Reaction to Metal Debris (ARMD). Volumetric analysis showed a highly variable wear rate with a
median of 9 mm3/year with a range of 1.1-999.7 mm3/year. From the samples, the femoral head had more wear than
the cup [5]. Case 100075 and 100107 correspond to having more wear on the femoral head as well.
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