The most commonly used landmark to set femoral rotation is the posterior condyles. They are easy to find and easy to use. There is very little measurement error. There is very little interobserver error using them during surgery. From Femoral Rotation : When the Gold Standard is a bit rubbish you’ll see that there is a relationship between the posterior condyles (PCL, Figure 1) and the flexion extension axis (FEA) of the knee. All good then. Lets just use them and get on with it…
Figure 1: Landmarks (Victor, 2009)
Unfortunately for all of us, that’s not going to work – at least not for every knee.
The simplest argument against the posterior condyles is bone loss. They are frequently affected by osteoarthritis. In pretty much every valgus knee and a lot of varus knees there is posterior condylar cartilage loss. This is pretty easy to account for – just add a couple of millimetres or a degree or two of rotation to one side. But when the bone starts getting eroded it gets harder to judge. (Figure 2)
Figure 2: Severe bone loss on condyles. PCL is 2.4° externally rotated to SEA.
The next problem is that the posterior condyles are not always perpendicular to the FEA (or the SEA, or the axis of rotation of the patella, or the trochlear groove). There is a high degree of individual anatomical variation (even without arthritic bone loss). The most commonly accepted version of this anatomical variation is lateral femoral condyle hypoplasia which is commonly associated with valgus knee deformity. It leads to a relative internal rotation of the posterior condyles to other landmarks such as the SEA and trochlear groove. Unfortunately it seems that lateral condyle hypoplasia is only one of the anatomical variations. There may be an overall association with valgus alignment but internal and external rotation of the posterior condyles relative to the SEA or the trochlear groove can occur unpredictably in any knee regardless of coronal alignment.[2-5] For example, here is a valgus knee with a prominent (? Hyperplastic) lateral femoral condyle leading to an externally rotated PCL. (Figure 3)Figure 3: Valgus knee with large lateral femoral condyle.
Whenever we are looking at the consistency of a landmark we need to look at what we are comparing it to. As discussed inFemoral Rotation : When the Gold Standard is a bit rubbish our best bet clinically is a CT scan of the Surgical Epicondylar Axis (SEA), though in laboratory situations comparison to the axis of rotation of the tibia or patella is also valid.
Based on the comparison the SEA, on average the posterior condylar line (PCL) is 2° to 3° internally rotated. Once again the key point is the phrase “on average”. In comparison to the SEA all the studies give quite a wide range, generally around 10 ° to 12° with standard deviations of 2° to 2.7°[6-8, 4]. My own CT studies recorded the PCL as -2.1° (internally rotated), SD 2.0°, range -7.7° to +3.6° in normal knees and -2.3°, SD 2.5°, range -8.7° to +4.1° in osteoarthritic knees.
The only conclusion to draw is that a line across the posterior aspect of the posterior condyles does not have a consistent relationship to the SEA. (One answer to this is to use preoperative 3D CT scans to produce cylinders and spheres to find the centre of rotation of the posterior condyles and use the centres of these spheres as a rotational landmark. This produces a landmark which is externally rotated to the PCL as the radii of the medial and lateral femoral condyles are different. I’ll discuss this approach and kinematic alignment in later articles.)
This leads to the next point. Many surgeons who aim for mechanical axis alignment will externally rotate their component 3° from the PCL (=PC+3). This is to compensate for the average medial proximal tibial angle (MPTA) which is 3° varus. (Figure 4) This is once again relying on an average value, so it doesn’t suit a lot of people. The MPTA varies from a 2° valgus to about 8° varus . Measuring the MPTA and correcting by this angle doesn’t work either – there is an association between increasing MPTA and increasing internal rotation of the PCL – but it isn’t a direct relationship. So blindly following the PC+3 doesn’t make a lot of sense, and yet some implant sets only have this as a rotational option. Again, this is based on average values instead of individual measurements.Figure 4: Mechanical axis alignment. Creating a tibial perpendicular to the mechanical axis of the tibia. More bone is taken off the lateral tibial plateau than medial, therefore the femur is externally rotated to take more bone off the medial femoral condyle than lateral.
So how do we measure anatomical variations? One approach has been to get a preoperative CT scan or MRI and measure the angle between the PCL and SEA. Then during surgery with conventional instruments use a sizing guide which allows variable angles from the posterior condyles and dial in the measured angle. Michaut et al used this technique to get 77% of cases within 2° of the SEA. This is an improvement over using the PC+3, but still nowhere near perfect. It can be inaccurate on 2D slices so consider using 3D reconstructions (Osirix software is probably the easiest if you can’t get your implant company to do preoperative plans for you). Obviously it requires a scan – which not everyone wants to get. It also doesn’t take into account variations in the anatomy of the trochlear groove relative to the posterior condyles (more on that later).
So, if you like the posterior condyles, be careful – they may be easy to use but they are often wrong. Get a sizing guide you can adjust. Adjust for bone and cartilage loss. Compare you PC+3 to other landmarks, in particular the trochlear groove – which is the next story…
- Luyckx T, Zambianchi F, Catani F, Bellemans J, Victor J. Coronal alignment is a predictor of the rotational geometry of the distal femur in the osteo-arthritic knee. Knee Surg Sports Traumatol Arthrosc. 2013;21(10):2331-7. doi:10.1007/s00167-012-2306-x.
- Jones C, Nawaz Z, Hassan A, White S, Khaleel A. The variability in the external rotation axis of the distal femur: an MRI-based anatomical study. European Journal of Orthopaedic Surgery & Traumatology. 2016;26(2):199-203. doi:10.1007/s00590-015-1719-x.
- Chao TW, Geraghty L, Dimitriou P, Talbot S. Averaging rotational landmarks during total knee arthroplasty reduces component malrotation caused by femoral asymmetry. J Orthop Surg. 2017;12(1):74. doi:10.1186/s13018-017-0575-2.
- Thienpont E, Schwab P-E, Paternostre F, Koch P. Rotational alignment of the distal femur: anthropometric measurements with CT-based patient-specific instruments planning show high variability of the posterior condylar angle. Knee Surg Sports Traumatol Arthrosc. 2014;22(12):2995-3002. doi:10.1007/s00167-014-3086-2.
- Amaranath JE, Moopanar TR, Sorial RM. Defining distal femoral anatomy for rotational alignment in total knee arthroplasty: a magnetic resonance imaging-based study. ANZ J Surg. 2014;84(11):852-5. doi:10.1111/ans.12708.
- Griffin FM, Insall JN, Scuderi GR. The posterior condylar angle in osteoarthritic knees. J Arthroplasty. 1998;13(7):812-5.
- Nagamine R, Miura H, Inoue Y, Urabe K, Matsuda S, Okamoto Y et al. Reliability of the anteroposterior axis and the posterior condylar axis for determining rotational alignment of the femoral component in total knee arthroplasty. J Orthop Sci. 1998;3(4):194-8.
- Poilvache PL, Insall JN, Scuderi GR, Font-Rodriguez DE. Rotational landmarks and sizing of the distal femur in total knee arthroplasty. Clin Orthop. 1996(331):35-46.
- Newman Cea. Femoral Rotational Asymmetry is a common anatomical variant. Submitted 2017. 2017.
- 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. 2015;23(11):3306-16. doi:10.1007/s00167-014-3137-8.
- Hayasaka Sea. Femoral Rotational Asymmetry is linked to proximal tibial varus. 2017 (submitted).