Monday, January 10, 2011

Metal ion analysis post hip resurfacing: guidance for surgeons- published in 2010

[Before you look at these comments, remember, the resurfacing procedure by DePuy was not approved in the US.  I thought this information was important because the metal is similar and the guidance provided leads me to believe that not much is known about the measuring the metal in patients from this hip surgery regardless of whether the surgery was a total hip or a resurfaced hip. I surely could be wrong but this is my hunch.]

1)      “The systemic effects of increases in metal ions are not fully understood but have been reported to include cerebral damage.  It may be that pathological changes, whether local or systemic, do not manifest for some years.  It is for this reason that efforts should be made to quantify the expected metal ion exposure.”

2)      Guidelines recommend close follow up of patients with Cr or Co levels in serum or whole blood samples measured to be greater than 7ug/L for bilateral patients.  [I understand it to be 4 for unilateral patients.]

3)      Questions that are unanswered:
a.       What are the normal blood/serum ion concentrations for patients? [in this case they are looking at bilateral and unilateral MOM hip joints.]
b.      What is the relationship between whole blood and serum concentrations of Co and Cr?
c.       Which blood fraction is the most reliable to be used for measurement?

[Keep in mind, this article was published this year and these questions are being asked!  It makes me somewhat less confident that there is any real clarity around the following questions: (1) what are the ranges of toxicity, (2) What procedure should be used to measure the toxicity?]

4)      This study had two groups of patients and a total of 450 blood samples were sent for whole blood and serum metal ion analysis.

5)      They concluded that while previous studies found that at low metal levels, whole blood and serum concentrations did not correlate sufficiently well to be used interchangeably and therefore thought that whole blood (vs serum) represented the best test for systemic exposure; this test found that at higher metal concentration levels however, the relationship between whole blood and serum could be used interchangeably. These authors recommend using serum levels to measure these concentrations. [So now the question becomes, what is a low level and a high level such that you can use these two measures interchangeably?]

6)      These levels are very well correlated with the size of the femur head.  The larger the head, the less the metal ion concentration.  The three groups of head size they reviewed:
a.       Small: less than or equal to 46
b.      Medium: 47-49
c.       Large: greater than or equal to 50

7)      This study suggested that serum Co samples provide the most reliable indicator of bearing performance. [Remember, we are looking at resurfacing information here not total hip replacements and we are looking at both bilateral and unilateral results.]

8)      This study further noted that Serum Co has also been shown to change dynamically in response to patient activity. [The angle of hip placement will also come into play here.]

[We have to get some better understanding of the ranges of normal and toxic, albeit, the information is not very definitive on this subject re metal toxicity from the hips.  As I read other studies, I will print the information.  My femur size was mid range-49- but I had abnormally high levels of CR.]

My interpretation of this information may not be right on the mark because I have no background in this area but the high level concepts gleaned from this study are likely fine.  The key takeaway point is that the measurement of these levels does not appear to be an exact science.

My questions now: (1) How do we know what the long term effects are if we can’t measure the current effects?  (2) What levels really indicate the replacement of the hip if one is not symptomatic with consistent pain every day?

Connie

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