Abstract
Several implants have a proven track record of durability and function in patients over many years. As manufacturers' patents expire it is understandable that cheaper generic copies would be considered. There is currently no established, independent method of determining design equivalence between generic and branded orthopaedic implants.
We acquired 10 boxed, as manufactured components consisting of the generic OptiStem XTR model (n=5) and branded Exeter (n=5) femoral stems. Two examiners were blinded to the implant design and independently measured the mass, volume, trunnion surface topography, roughness, trunnion cone angle, CCD angle and femoral offset using peer-reviewed methods. We then compared the stems using these parameters.
We found that the OptiStems (1) were lighter (p<0.001) (2) had a rougher trunnion surface (p<0.001) with a greater spacing and depth of the machined threads (p<0.001), (3) had greater trunnion cone angles (p=0.007) and (4) a smaller radius at the top of the trunnion (p=0.007). There was no difference for stem volume (p=0.643), CCD angle (p=0.788) or offset (p=0.993).
This study is the first independent investigation of the equivalence of a generic orthopaedic implant to its branded design. We found a clear difference in trunnion roughness, trunnion cone angle and radius, and implant mass when comparing the two generic and branded stem designs. All implants require standard regulatory processes to be followed. It does not appear feasible that generic implants can be manufactured to predictability guarantee the same performance as generic drugs.
We found a number of physical differences between the generic and branded implants. Whilst both designs are likely to work in clinical practice, they are different.
