Absolute vs Relative Risk Reduction: Number Needed to Treat in Hand Surgery

I recently read Overdosed America: The Broken Promise of American Medicine by John Abramson.  The story of the COX-2 inhibitor, Vioxx, is one of the prime examples used in the book to show how pharmaceutical marketing and publication bias can result in disastrous outcomes for the public.  However, the part of the book that I found most interesting was when the author started looking at medications from a number needed to treat (NNT) standpoint.  NNT is a statistical measure used to determine the number of patients who need to be treated to prevent one additional bad outcome.  An example from the book is Fosamax.  Women with a mean age of 68 and a T score -2.5 or were prescribed Fosamax for four years had a 56% less likely to have a hip fracture than women not taking Fosamax. It is important to understand that the 56% less fractures is the relative risk reduction (RRR).  The control group had a 0.5% chance of having a hip fracture and treatment group a 0.2% chance of having a hip fracture.  The absolute risk reduction (ARR) is only 0.3%.  Preventing hip fractures in elderly patients is clearly an important public health goal, but based on NNT calculations, 81 women with osteoporosis would need to take Fosamax for 4 years (at a cost of over $300,000) to prevent one hip fracture.

It made me think about the number needed to treat for hand surgery.  For example, how many patients would need to undergo nerve conduction studies to prevent one patient from having an incorrect diagnosis and undergoing a surgery that will not help?  How many patients would need to undergo open reduction and internal fixation of an extra-articular distal radius fracture to prevent a future surgery for malunion correction or a poor outcome due to ulnar impaction?  How many patients need to be treated with volar locked plating to prevent one complication related to percutaneous pinning?  Especially for treatment options such as volar plating, the large cost difference between the two groups can quickly make saving one complication very expensive.

For example, assume that the complication rate for volar locked plating is 10% and the complication rate for percutaneous pinning is 20% (I am just making these numbers up for illustration).  The NNT is 10.  If locked plates “cost” an average of $1,500 per surgery and K-wires cost an average of $100 per surgery, then it costs $14,000 to prevent one complication associated with the use of percutaneous pins.  Not all complications are equal.  A pin tract infection after percutaneous pinning or stiffness after volar locked plating may not require a return to trip to the OR.  If we were to look at another illustrative example looking at the major complication rate, the numbers might look different.  If the major complication rate after percutaneous pinning is 4% and the major complication rate after volar locked plating is 2% (again, just picking numbers for illustrative purposes), then the relative risk reduction sounds amazing.  The headlines would read, “volar locked plating reduces major complications by 50% over percutaneous pinning!”.  However, the relative risk reduction is only 2%, meaning the NNT is 50, “costing” $60,000 to prevent one major complication related to percutaneous pinning.

As usual, in hand surgery, we need more data. Unlike the Fosamax trial referenced above that had over 8,500 randomized subjects, most trials in hand surgery have fewer than 100 subjects.  While there are many factors related to these small numbers, it still makes it difficult to draw powerful conclusions about out diagnostic and treatment modalities.  But as physicians, I think that we have to be able to critically look at the data and take things like the NNT into consideration.



  1. Abramson DJ. Overdosed America. Harper Collins; 2013.
  2. Cummings SR. Effect of Alendronate on Risk of Fracture in Women With Low Bone Density but Without Vertebral Fractures<SUBTITLE>Results From the Fracture Intervention Trial</SUBTITLE>. JAMA : the journal of the American Medical Association. 1998;280(24):2077. doi:10.1001/jama.280.24.2077.

Article written by:

John Fowler is an Assistant Professor in the Department of Orthopaedics at the University of Pittsburgh. He completed his residency in Orthopaedics at Temple University Hospital in Philadelphia and a hand fellowship at the University of Pittsburgh. His research passion is the use of musculoskeletal ultrasound. When not working, he tries to spend as much time as possible with his amazing wife Amy and three children: Alexis, Kyle and Eliana.

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