Theme Three
The class nobody checked
How common is this finding in people who feel fine?
The simplest question in this collection, and the one most reliably skipped. Before a finding can be used to explain a symptom, someone has to know how often it turns up in people with no symptom at all. Spine, knee and hip here; shoulder and the pooled age-gradient in the further reading. Four decades, and the same answer every time.
11
Magnetic Resonance Imaging of the Lumbar Spine in People without Back Pain
NEJM · 1994 · Jensen MC, Brant-Zawadzki MN, Obuchowski N, Modic MT, Malkasian D, Ross JS · Blinded cross-sectional study, n=98
What it does: Scans people with no back pain, mixes their images among symptomatic ones, and has neuroradiologists read them blind. 52% had a bulging disc, 27% a protrusion. Only 36% had normal discs at every level.
Epistemological angle: The founding document of the theme. A positive finding and a diagnosis are different objects, and the difference is precisely the base rate that nobody had measured. The blinding matters more than it looks: the radiologists could not know which images came from which group, which forecloses the objection that the abnormalities were read into the scans. What the study establishes is not that imaging is useless but that its output is uninterpretable without a denominator.
My take: Thirty years old, and practice has been slow to shift. The prevalence figure is the lesser lesson; the greater one is how much more than a number it takes to change what we do.
12
Incidental Meniscal Findings on Knee MRI in Middle-Aged and Elderly Persons
NEJM · 2008 · Englund M, Guermazi A, Gale D, Hunter DJ, Aliabadi P, Clancy M, Felson DT · Framingham population cohort, n=991
What it does: Uses a population sample rather than volunteers — people were not selected for having or lacking knee symptoms. Meniscal tear prevalence ran from 19% in women aged 50 to 59 up to 56% in men aged 70 to 90. Of those with a tear, 61% reported no knee pain, aching or stiffness in the previous month.
Epistemological angle: The population sampling is what makes this the strongest entry in the theme. Volunteer studies invite the objection that volunteers are unrepresentative; a community cohort does not. And the number that matters is the 61%, because it converts the abstract point about base rates into a direct statement about diagnostic value: most people carrying this finding are not in pain, so its presence in a patient who is in pain cannot on its own explain the pain. That is a claim about what the finding licenses, not about what it does. A tear may well be contributing something in some people — high prevalence in the well does not refute causation in the ill, any more than the fact that most smokers never develop lung cancer refutes the effect of smoking. What high prevalence destroys is the finding's ability to tell you which people.
My take: Published five years before FIDELITY, in the same journal, by people the field respected. Much of what would have predicted that trial's result was already in print. The evidence was not missing so much as unpersuasive at the time, which is a harder problem than ignorance and not obviously anyone's failing.
13
Prevalence of Femoroacetabular Impingement Imaging Findings in Asymptomatic Volunteers
Arthroscopy · 2015 · Frank JM, Harris JD, Erickson BJ, Slikker W, Bush-Joseph CA, Salata MJ, Nho SJ · Systematic review, 26 studies, 2,114 asymptomatic hips
What it does: Pools imaging studies of hips in people with no hip symptoms, mean age 25. Cam morphology was present in 37% overall — 54.8% in athletes against 23.1% in the general population. Pincer morphology was present in 67%, though the authors note it was poorly and inconsistently defined across studies. Of the seven studies reporting on the labrum, labral injury was found on MRI in 68.1% of asymptomatic hips.
Epistemological angle: The theme's fourth joint, and the one where the lesson is currently live rather than historical. Spine, shoulder and knee learned this in 1994, 1995 and 2008; hip arthroscopy volumes grew through the 2010s on findings whose asymptomatic prevalence was being established at the same time. The pincer figure carries a second lesson, one the authors state plainly: a prevalence estimate inherits the definition used to generate it, and when only four of twenty-six studies defined the deformity explicitly, 67% is a number about the literature as much as about hips. That is a different failure from the one at entries
11 and
12 — there the denominator was unmeasured, here it is measured against a moving numerator.
My take: Two-thirds of pain-free young hips showing labral injury is the same shape of number as Sher's shoulders, arriving twenty years later in a field that had the earlier examples available to it. I would like to think that means we act on it faster this time. The volumes suggest otherwise, and I do not exempt myself from the reasons why.
Also worth reading
Sher JS, Uribe JW, Posada A, Murphy BJ, Zlatkin MB. Abnormal findings on magnetic resonance images of asymptomatic shoulders. J Bone Joint Surg Am 1995;77(1):10–15 · PMID 7822341 · doi:10.2106/00004623-199501000-00002. 34% of pain-free shoulders with a cuff tear, 54% over the age of sixty. Makes the point in the tissue surgeons are most confident about.
Brinjikji W, Luetmer PH, Comstock B, Deyo RA, Jarvik JG et al. Systematic literature review of imaging features of spinal degeneration in asymptomatic populations. AJNR Am J Neuroradiol 2015;36(4):811–816 · PMID 25430861 · doi:10.3174/ajnr.A4173. Establishes that the base rate is a function of age rather than a number: disc degeneration in pain-free people from 37% at twenty to 96% at eighty.
Boden SD, Davis DO, Dina TS, Patronas NJ, Wiesel SW. Abnormal magnetic-resonance scans of the lumbar spine in asymptomatic subjects. J Bone Joint Surg Am 1990;72(3):403–408 · PMID 2312537 · no DOI in record. Anticipates entry 11 by four years.