Femoral Rotation : When the Gold Standard is a bit rubbish

The Gold Standard for femoral component rotation following total knee arthroplasty is a line parallel to the Surgical Epicondylar Axis (SEA) on a three-dimensional CT reconstruction of the knee.

There, I’ve said it. Unfortunately it is a crap Gold Standard. But it’s the best we’ve got. One of the big problems about discussing femoral rotational alignment is that we have a bad Gold Standard. It is difficult to find, not universally accepted and, my suspicion is, that in some occasional cases it may even be completely wrong.

What is the SEA? You are probably all used to talking about the epicondylar axis. Unfortunately we need to be very careful about our definitions. There are two epicondylar axes. The simplest one is the Anatomical Epicondylar Axis (AEA), also frequently called the Transepicondylar axis or even just THE Epicondylar axis. This is a line running from the apex of the lateral epicondyle to the apex of the medial epicondyle. (Figure 1) The Surgical Epicondylar Axis (SEA), is also sometimes called the Transepicondylar axis or the Epicondylar axis (hence the confusion!!). The SEA is a line running from the sulcus on the medial side, which is a small distance (5-10mm) posterior and proximal to the apex of the epicondyle. The theoretical reason that the SEA is anatomically superior is that the MCL attaches to the sulcus on the medial side instead of the apex. Thereby matching the SEA should recreate normal tension in the collateral ligaments. The best summary of many of these landmarks is probably in Jans Victor’s review article from 2009.[1]

Slide2Figure 1. Landmarks for the Surgical Epicondylar Axis . The medial sulcus is often very shallow.

If you are reading anything about the epicondylar axis make sure you clarify which one they are talking about. The AEA is about 3° externally rotated to the SEA (in my study 3.7° SD 0.6° range 2.4° to 4.8°).[2]

So why am I sticking with the SEA? What are we really trying to recreate? The theory on the rotation which we are trying to recreate is to match the axes of rotation around which the mechanical axis of the tibia and/or the patella rotate. There have been several virtual, fluoroscopic, cadaveric and computer aided studies looking at the relationship between the SEA and the flexion – extension axis (FEA) of the tibia (also termed the Functional Flexion Axis, FFA).[3-8] There is a lot less on the relationship with the patella, mainly because it can only be measured dynamically.[9] There is a huge body of work on these concepts which I am breaking down into one sentence with lots of qualifiers.

Generally, on average the FEA is close to perpendicular to the SEA and also to the axis around which the patella rotates.

This is why we measure the SEA. It is the only of those landmarks (FEA/patella axis) which we can measure before and after knee replacement. So it is the only one that we can use to check our results. So what’s wrong with the SEA? Firstly it is probably not perpendicular to the FEA and the axis of rotation of the patella in some people – while, “on average”, it is perpendicular all of the studies show standard deviations of a few degrees. Secondly, it is hard to measure accurately. Several studies have shown that it cannot be accurately found during surgery[10-15]. Others have shown that it is even difficult to find on CT scans. On 2D postoperative scans it is very hard to find in about 10% of cases. On 3D preoperative scans you can find it in about 98%. The medial sulcus is often a pretty flat dip in the bone, (Figure 1) so there is usually a small degree of measurement error, but occasionally up to 3-4°. The accuracy is very dependent on the experience of the person looking at the scans. So if you want to assess postoperative femoral rotational alignment look at the SEA but don’t rely on it as the sole landmark during surgery. If you can get preoperative CT or MRI scans look at the relationship between the SEA and the posterior condyles as a useful guide to rotation. However, check that it has been measured accurately and don’t follow it blindly – after all, it is a “bit rubbish”!

The only other system which looks promising is to base it on the centre of the posterior condyles using either cylinders or spheres. This looks like a better landmark to reproduce the FEA in some papers.[8, 7, 16] Unfortunately, this approach also has its problems. Firstly, you can’t exactly find it during surgery. The radii of curvature of the medal and lateral femoral condyles are frequently not the same (Figure 2). Therefore referencing the surface of the posterior condyles during surgery is not quite the same as looking at 3D cylinder matching techniques on preoperative scans. They are also frequently damaged by arthritis. However there is less difficulty finding and referencing the condyles during surgery than the epicondyles. Secondly, while “on average” it is close to the FEA, there is still a wide individual variation. Thirdly, there is even more variation when comparing the cylinder matching techniques to the axis of rotation of the patella. Finally, and this is the clincher with regard to research, you can’t find it again after you’ve done a knee replacement so you can’t use it to assess postoperative component malrotation.


Figure 2: Epicondylar axes (from “A common reference frame for describing rotation of the distal femur, Victor et al) [17]. Note the different radii of curvature of the posterior condyles which makes an axis between the centre of the spheres different to the posterior condylar line.

You should keep noticing the crucial qualifying phrase “on average”. There is a worrying tendency in the literature to pronounce that because one landmark is “on average” parallel to another that they ARE parallel. Instead they should be describing the range of variation with the full dataset including the numbers of extreme outliers. It seems that the variations are often dismissed as measurement errors. While some of it will be measurement error much of it is likely to be individual anatomical variation which we should be considering.

So what is the “Gold Standard” we need? Unfortunately, due to all the measurement errors and anatomical variations there will never be a perfect, simple, easily identified, bony “Gold Standard” landmark. I feel that the best position in which to insert the femoral component is the position which gives the best functional outcome for each individual patient. Therefore there are a large number of bony, soft-tissue and component factors which will need to be considered in each case. But once we have decided the best combination of parameters we need to figure out how to make it into a practical, reproducible and cost-effective surgical technique. So – that’s what we are going to work towards over the next few articles. Wish me luck…

  1. Victor J. Rotational alignment of the distal femur: a literature review. Orthop Traumatol Surg Res. 2009;95(5):365-72. doi:10.1016/j.otsr.2009.04.011.
  2. Talbot S, Dimitriou P, Radic R, Zordan R, Bartlett J. The sulcus line of the trochlear groove is more accurate than Whiteside’s Line in determining femoral component rotation. Knee Surg Sports Traumatol Arthrosc. 2014:1-11. doi:10.1007/s00167-014-3137-8.
  3. Colle F, Bruni D, Iacono F, Visani A, Zaffagnini S, Marcacci M et al. Changes in the orientation of knee functional flexion axis during passive flexion and extension movements in navigated total knee arthroplasty. Knee Surg Sports Traumatol Arthrosc. 2015:1-9. doi:10.1007/s00167-015-3816-0.
  4. Feng Y, Tsai T-Y, Li J-S, Rubash HE, Li G, Freiberg A. In-vivo analysis of flexion axes of the knee: Femoral condylar motion during dynamic knee flexion. Clin Biomech. 2016;32:102-7. doi:http://dx.doi.org/10.1016/j.clinbiomech.2015.12.006.
  5. Kobayashi H, Akamatsu Y, Kumagai K, Kusayama Y, Aratake M, Saito T. Is the Surgical Epicondylar Axis the Center of Rotation in the Osteoarthritic Knee? The Journal of Arthroplasty. 2015;30(3):479-83. doi:http://dx.doi.org/10.1016/j.arth.2014.10.024.
  6. Matziolis G, Pfiel S, Wassilew G, Boenicke H, Perka C. Kinematic analysis of the flexion axis for correct femoral component placement. Knee Surg Sports Traumatol Arthrosc. 2011;19(9):1504-9. doi:10.1007/s00167-011-1554-5.
  7. Mochizuki T, Sato T, Blaha JD, Tanifuji O, Kobayashi K, Yamagiwa H et al. The clinical epicondylar axis is not the functional flexion axis of the human knee. J Orthop Sci. 2014;19(3):451-6. doi:10.1007/s00776-014-0536-0.
  8. Yin L, Chen K, Guo L, Cheng L, Wang F, Yang L. Identifying the Functional Flexion-extension Axis of the Knee: An In-Vivo Kinematics Study. PLoS ONE. 2015;10(6):e0128877. doi:10.1371/journal.pone.0128877.
  9. Iranpour F, Merican AM, Baena FRY, Cobb JP, Amis AA. Patellofemoral joint kinematics: the circular path of the patella around the trochlear axis. J Orthop Res. 2010;28(5):589-94.
  10. Kinzel V, Ledger M, Shakespeare D. Can the epicondylar axis be defined accurately in total knee arthroplasty? Knee. 2005;12(4):293-6. doi:10.1016/j.knee.2004.09.003.
  11. Moon YW, Seo JG, Lim SJ, Yang JH. Variability in femoral component rotation reference axes measured during navigation-assisted total knee arthroplasty using gap technique. J Arthroplasty. 2010;25(2):238-43. doi:10.1016/j.arth.2008.08.013.
  12. Siston RA, Patel JJ, Goodman SB, Delp SL, Giori NJ. The variability of femoral rotational alignment in total knee arthroplasty. J Bone Joint Surg Am. 2005;87(10):2276-80. doi:10.2106/JBJS.D.02945.
  13. Yau WP, Chiu KY, Tang WM. How precise is the determination of rotational alignment of the femoral prosthesis in total knee arthroplasty: an in vivo study. J Arthroplasty. 2007;22(7):1042-8. doi:10.1016/j.arth.2006.12.043.
  14. Yin L, Chen K, Guo L, Cheng L, Wang F, Yang L. Identifying the Functional Flexion-extension Axis of the Knee: An In-Vivo Kinematics Study. PLoS ONE. 2015;10(6):e0128877. doi:10.1371/journal.pone.0128877.
  15. Lustig Sb, Lavoie Fdr, Selmi TA, Si, Servien E, Neyret P. Relationship between the surgical epicondylar axis and the articular surface of the distal femur: an anatomic study. Knee Surg Sports Traumatol Arthrosc. 2008;16(7):674-82. doi:http://dx.doi.org/10.1007/s00167-008-0551-9.
  16. Eckhoff D, Hogan C, DiMatteo L, Robinson M, Bach J. Difference between the epicondylar and cylindrical axis of the knee. Clin Orthop Relat Res. 2007;461:238-44. doi:10.1097/BLO.0b013e318112416b.
  17. Victor J, Van Doninck D, Labey L, Van Glabbeek F, Parizel P, Bellemans J. A common reference frame for describing rotation of the distal femur: a ct-based kinematic study using cadavers. J Bone Joint Surg Br. 2009;91(5):683-90.

4 thoughts on “Femoral Rotation : When the Gold Standard is a bit rubbish

  1. Thank you, Simon, for this very simple, eye opening article. Every time I think I’ve got femoral component positioning figured,something like this comes along. Looking forwards for the next in this series.


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