| 477) A 39-year-old woman presents to clinic with a history of long-standing right leg pain. She complains of “achy” pain at the hip radiating down her femur to the knee, which is exacerbated on exertion, especially with jogging more so than climbing stairs, but also present during rest. She takes acetaminophen for it, which helps her pain. Her past medical history is significant for systemic lupus erythematosus diagnosed at age of 20, for which she has been taking on and off oral prednisone. She does not smoke cigarettes but he drinks alcohol 2 or 3 times per week. She denies any drug use. Physical examination is significant for limited range of motion of the right hip, especially with external rotation. A plain x-ray shows a dulling of the femoral head but no other pathology. What is the next step in diagnosis? | |
| A. Bone scan | |
| B. CT scan | |
| C. MRI scan | |
| D. PET scan | |
| E. Ultrasound | |
Filed under: USMLE STEP III QUESTION BANK, USMLE Test Prep | Tagged: aRCHER ORTHOPEDICS, USMLE STEP 3 ORTHOPEDICS |
Archer USMLE Step 3 Blog 
c
B
This is a beautiful question. It is very well thought and it capitalizes on the myriad of beliefs and opinions on what should be the ideal investigation if plain x-ray is normal.
What is the diagnosis? Most likely, steroid induced osteo-necrosis of the hip joint (medical) or just, avascular necrosis of the hip stage 1 (surgical).
What are the differentials?
An infection, a tumor, osteoarthritis or fracture involving the hip joint or a soft tissue tear around the hip joint
How do we investigate that further?
ANS: MRI scan (C)
Why?
Bone scan (or Bone scintigraphy) can detect a cold spot popularly called ‘the dough-nut sign’. This sign signifies a non-reactive area of bone tissue. The problem here is that a non reactive bone is not necessarily dead. Some tumors do not cause bone reaction and can actually stimulate the osteoclasts to present as ‘cold spots’.
Also, this lady has been on prednisolone for years, can this be a silent abscess. Cold abscess can really mimic ‘the doughnut sign’. This test is not specific.
PET Scan (SPECT) is more sensitive to detect cold spots. It is a functional test not a visual (structural) test. This test is not specific for the same reason mentioned above.
Ultrasound is used by rheumatologists and pediatric orthopedic surgeons as a diagnostic tool. It can easily visualize superficial aberrations around the hip joint in neonates and adults. It can also visualize osteoporosis in extremities (having minimal soft tissue over them). It does not penetrate intraosseous lesions very efficiently. Ultrasound is not as sensitive as DEXA in detecting bone density changes. Power doppler can detect joint cartilage erosions and doppler can also detect vascular changes in bone tumors. But in general an ultrasound detect morphological changes around the joint outside the bones. Emergency physicians have used it to detect fractures but this test is not ideal for investigating osteonecrosis.
CT scan is a serial x-ray that will be helpful for follow up. It will be normal (right now) when the plain x-ray is normal. But it will become helpful in distinguishing the stages of osteonecrosis as the disease progress. And if low pulsed ultrasound therapy is used to treat this condition, it will also detect improvement. This test can not visualize osteonecrosis when there is a normal plain x-ray – like in this case.
MRI (Option C) is very accurate in visualizing dead bone, abscess and living tissue. MRI can literally draw a line between dead bone and living bone. And then conveniently color the dead bone as black. Abscesses are color white and circled. That’s what I call distinction.
References:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3480825/
http://orthoinfo.aaos.org/topic.cfm?topic=a00216
Click to access 2009Bone_Scan_All.pdf
http://www.arthritisandpainclinic.com/ultrasound.html
Click to access Corrales.pdf
http://www.rad.washington.edu/academics/academic-sections/msk/teaching-materials/online-musculoskeletal-radiology-book/osteonecrosis (this shows MRI image)
http://emedicine.medscape.com/article/336829-overview (this shows an abscess using MRI)
Click to access 113.pdf
https://en.wikipedia.org/wiki/Positron_emission_tomography
http://www.radiologyinfo.org/en/info.cfm?pg=bone-scan
http://www.radiologyinfo.org/en/info.cfm?pg=bone-scan#limitations
http://www.radiologyinfo.org/en/info.cfm?pg=dexa
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3732915/
http://pediatrics.aappublications.org/content/105/4/896.full
Modified AMA reference style.
[P.S – some researchers are convinced that Bone scan is better than MRI. I am curious to find out the view of Dr Archer.]
Disclaimer – I have read many articles and journals for this post, I do not discredit any author. I acknowledge & thank every author who contributed to the information I have provided. In the modified AMA reference style, I duely acknowledged all authorships and copyrights. I have not listed authors by name).
Thank you.
any pt with chronic steriod use sickle cell disease and prothrombotic conditions can lead to avascular necrosis of head of femur
diagnosis is by MRI
there are few indications of MRI in rheumatology :
1) avascular necrosis
2) osteomyelitis
3) diagnosis of ankylosing spondylitis