Men's Osteoporosis Support Group

Normal knee joint space

BMC Musculoskelet Disord. 2008 Sep 8;9(1):119. Minimum joint space width and tibial cartilage morphology in the knees of healthy individuals; A cross-sectional study. Beattie KA and others. PMID: 18778479. (Note: the full text of this article is available free by clicking the link on the top right side of the PubMed page).

This study has a nexus with osteoporosis in that the investigators were checking to see if it was possible to use the same method that investigators use to determine osteoporosis when evaluating knees for osteoarthritis (OA). To determine if an individual has osteoporosis a DXA bone density test is done and the results are compared to the young healthy individuals' scores, the T-score. If the individual either has a history of fracture or has a T-score of equal to or less than -2.5 Standard Deviations from the norm for the femoral neck or the lumbar spine, that is diagnosed as osteoporosis. See this Update for more information on this topic. So the investigators wanted to know if they could compare current knee joint space to some normal value and then determine an individual had knee osteoarthritis. Or would they have to follow each individual over time to verify loss of joint space.

I also have an interest in this topic because I recently had an arthroscopic procedure done on my left knee due to a torn meniscus. Prior to this procedure I had X-rays done which showed near normal joint space and minimal evidence of OA. I had pain and swelling which necessitated the MRI and arthroscopic procedure. There are several ways to take knee X-rays and there is some controversy as to the best way to get ideal results. Some have suggested weight-bearing and others non-weight-bearing X-rays. Which is the right one and could the Beattie and others study provide help deciding that? I suspect that other men with osteoporosis have their share of knee problems and might benefit from more information on this topic.

The authors used fixed-flexion knee x-rays in this study. They describe a method they use in their full article, and here is a device that is manufactured specifically for this procedure. Additionally a special peripheral MRI was done on each knee. A similar device is shown here. They took X-rays and MRIs of the knees of healthy male and female individuals ranging in age from the twenties to the sixties to see if minimum joint space width (mJSW) changed with age in these healthy individuals.

They found in the 73 female and 47 male participants that mJSW did not significantly decrease with increasing decade (p>0.05) in either sex. Females had a mean (SD) medial mJSW of 4.8 (0.7) mm compared to males with corresponding larger value of 5.7 (0.8) mm. The authors conclude, "Results suggest there may be no need to differentiate a t-score and a z-score in OA diagnosis because cartilage thickness and JSW remain constant throughout life in the absence of OA."

This was a pilot study not involving enough participants to make definitive conclusions. But the results are suggestive that comparing an individual's current mJSW from a fixed-flexion knee x-ray to this study's gender-specific results would give fairly accurate reading regarding whether an individual had OA, and to what degree he or she had it. Thus, a male's mJSW should be about 5.7 mm, no matter what his age.

Editor's comments. Regarding whether to have a weight-bearing X-ray taken, they note one study that found mJSW was 17% and 19% greater when non-weight bearing X-rays were done in females and males, respectively. So, if you had a non-weight-bearing X-ray of the knee, you either have to subtract about 20% of the joint space, or, preferably, have a weight-bearing X-ray taken to get accurate results.

If we are to expect good results from comparing our knee X-rays to this study, they would also need to be done with the fixed-flexion method. This allows repeat X-rays to be compared to previous ones, or individual X-rays to be compared to the norms described in this study. My recollection isn't perfect regarding how my X-rays were done, but I don't recall a device similar to what is described in the article or the one pictured online. I know if I have future X-rays I'll ask for the fixed-flexion method, which is also weight-bearing, so it covers both potential problem areas.

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