Optimal primary total knee arthroplasty (TKA). Femoral component rotation

Optimal component alignment is crucial
for successful outcome of primary total knee arthroplasty (TKA). Femoral
component rotation is one of the most important factors in TKA, as rotational
misalignment affects the flexion stability as well as tibiofemoral and
patellofemoral kinematics. Flexion malalignment is a known cause of pain, stiffness, patellar and flexion instability 1-8.

In the classical mechanical alignment
concept, the femur must be implanted parallel to the surgical transepicondylar
axis (STEA) 9. The surgical TEA is thought to best approximate the
flexion/extension axis of the knee, however it can be difficult to palpate and
reference intraoperatively 10-14. The surgical epicondylar axis and posterior
condylar axis form the posterior condylar angle (PCA), which  is on average of 3 degrees of external
rotation. Although many factors such as gender, condylar hypoplasia, and
coronal alignment can disturb the rotation of the distal femur and change the
angle between the posterior condylar line (PCL) and transepicondylar axis. Many
studies have demonstrated that the angle between PCL and surgical TEA may range
from 3° of
internal rotation to 10° of
external rotation 13, 33, 34, 64.

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Aglietti et al. studied preoperative
knee CT scans, and developed a simplified formula for the relation between
distal femur rotation and frontal alignment of the knee which increases the PCA
by 1° per 10° of
coronal deformity increments from varus to valgus, resulting for instance in 2° external rotation for a 20° varus knee and 5° external rotation for a 20° valgus knee. Based on this concept and
without the use of the preoperative CT scan, rotational accuracy is within ± 2° of TEA in 80% of the cases 65. We
can accept, that standard external rotation for about 80% of the knees are 3° for varus knees and 5° for valgus knees 21.

The rationale for the TEA method is
derived from the observation that the normal tibial joint line is between 3° and 5° of varus relative to the long axis of
the tibia. If the tibial resection is made in 3° of varus, an equal symmetrical
posterior condylar resection will result in a rectangular flexion gap. If the
tibial resection is 90° to
the long axis of the tibia, 3° to 5° of
external rotation will be necessary in order to recreate a rectangular gap
29.

Determination of the surgical TEA is
known to be difficult; therefore most systems use the default posterior
reference and PCL landmark to determine the femoral component ER. Hungerford
15 introduced the concept of 3 degrees of external rotation (ER) relatively
to the posterior condylar line (PCL) for the femoral component.

Femoral
component rotation may be determined by several techniques; these include gap
balancing techniques, dependent on ligament tension, and currently used by most
surgeons measured resection techniques based on anatomical landmarks, including

Alternative methods exist that involve
patient-specific instrumentation (PSI) and computer navigation.

Measured resection – “femur first” or “bony
landmarks” technique

The measured resection method was
developed by Hungerford for use in cruciate-retaining TKA 15. In this
technique, the bone resections are performed according to the bony landmarks
followed by soft tissue balancing, in which the femoral component is aligned
with respect to the epicodylar line which best 
approximates the flexion/extention gap. This approach involves
referencing the rotation off several possible bony landmarks summarized in
Figure 1. No gold standard for rotational reference has yet been generally
accepted.

 

Posterior
condylar line
(PCL)

The posterior condylar line is defined
as the tangent line
to the most posterior part of the femoral condyles withe the femur viewed along
its mechanical axis 22. Referencing the femoral rotation off the posterior
condylar line can be used in most current systems and is relatively easy.
Unfortunately, it has been postulated that this reference may be unreliable and
give rise to many errors, such as in the case of posterior condylar defects in
varus and valgus knees 17. The PCL is in relative internal rotation to the
femoral rotation due to the larger size of the posteromedial femoral condyle.
Victor reported a literature review analysis of different studies describing
the angular relationship between the different axis of the distal femur in the
axial plane. The posterior condylar line has a mean internal rotation of 3
degrees relative to the surgical TEA, 5 degrees relative to the anatomical TEA,
and 4 degrees relative to the trochlear AP axis, respectively 21. In the
valgus knee, the PCL internal rotation tendency is even greater because of the
hypoplastic lateral femoral condyle 23. Three degrees of femoral component
rotation is the value routinely used in varus, and five degrees – in valgus
malalignment, respectively 27. For every 1 mm of asymmetry in condylar
cartilage loss, the femoral rotation measurement with the use of PCL changes by
1 degree 16. PCL can be in the range of 1 to 9 degrees of internal rotation
relatively to the anatomical TEA 24. The PCL referencing should be therefore
used with caution.

 

 

Surgical
transepicondylar axis (STEA) and anatomical transepicondylar axis (ATEA).

The transepicondylar axis reference
method is reliable as the TEA approximates the flexion-extension axis of the
knee. Placing the femoral component parallel to the TEA allows to obtain a
rectangular resection gap in over 90% of cases. In some cases the TEA is easier
to identify intraoperatively compared to the identification of the posterior
condylar angle or the posterior condylar line especially in revision cases.
However, the primary disadvantage of this technique is the difficulty in
defining the TEA in obese patients. The epicondylar eminences are often poorly
visualized and can be overlied by the collateral ligaments, everted patella,
and particularly by fat tissue. There are studies suggesting that over 50% of
malalignment errors are due to difficulty in identifying the epicondylar
eminences 19. In the Yoshino et al. study of 48 patients with osteoarthritis,
the medial sulcus could only be determined in 30% of the knees. The difficulty
in locating the sulcus increased with the severity of arthritic changes 28.
Removal of the soft tissue improves the identification of the condyles,
assessment of the TEA and reduces the risk of  femoral component malrotation.

Two tranepicondylar lines can be
identified, the anatomical (ATEA) and the surgical (STEA) .

The TEA is classically defined as a
transverse line drawn between the most prominent points on the epicondyles and
is also known as the anatomical epicondylar axis (AEA) 24. According to a
study by Akagi, the angle between ATEA and PCL was found to be 6.8° on preoperative CT scans 2.

The STEA refers to a line drawn from
the medial sulcus to the lateral epicondyle. It is a secondary anatomical axis,
useful for determination of rotational orientation of the femoral component
when the posterior condylar surfaces cannot be used. Berger et al. used
surgical transepicondylar axis (STEA) to determine the posterior condylar angle
subtended as the angle between this axis and the PCL. Measurement of the
posterior condylar angle referenced from the surgical epicondylar axis yielded
a mean posterior condylar angle of 3.5 degrees (+/- 1.2 degrees) of internal
rotation in males, and a mean posterior condylar angle of 0.3 degrees (+/- 1.2
degrees) of internal rotation in females. 22. The surgical epicondylar axis provides
a visual rotational alignment reference during primary arthroplasty and may
improve femoral component alignment in revision.

An angle between ATEA and STEA was
reported by Yoshino et al. to be 3.2°±1°, with ATEA being more externally
rotated 28.  Another study by Victor
reported that the mean angle between the anatomical and surgical TEA is 2
degrees 21.

 

 

Trochlear anterior-posterior axis
(TAPA) – Whiteside’s line (WL) and the sulcus line (SL)

The trochlear anterior-posterior axis
– Whiteside’s line, is a line connecting the deepest point of the trochlea to
the center of the intercondylar notch 23. The anteroposterior axis indicates
the direction of the trochlea in healthy knees and is perpendicular to the ATEA
25. Femoral component rotation is oriented perpendicular to the TAP axis. This
reference is reliable and can be applied in patients with distorted condylar
anatomy – such as condylar hypoplasia or defect. TAPA is less reliable than TEA
in the valgus knee and in trochlear dysplasia 26. The line perpendicular to
the AP axis is externally rotated by 3.5° relative to the PCL in normal knees.
The internal rotation angle of the line perpendicular to the anteroposterior
axis relative to the epicondylar axis is 0.1° ± 3.3° (medial femorotibial arthritis), 1.3° ± 3.3° (patellofemoral arthritis), and 2.3° ± 3.1° (normal knees) 32.

The main disadvantage of this
reference lies in the difficulty of defining the trochlea AP axis in trochlear
dysplasia and in patients with destructive arthritis of the anterior
compartment 18, and with significant varus or valgus deformity 26,32. The
AP line is  variable and therefore its
isolated use to determine the femoral component rotation in patients with
destructive arthritis may result in malrotation of the femoral component and
should not be used as a single landmark 21 but complementary to other
reference axis.

An alternative to determination of the
TAPA is the sulcus line reference. In this technique, the trochlear groove is
perceived as a three-dimensional structure; multiple points in the trochlear
groove (forming usually an arc) are connected then reoriented to achieve a
straight line along the coronal aspect of the trochlear groove. This technique
is believed to reduce the parallax error compared to TAPA because there is only
one true coronal alignment axis. The TAPA relies on the accuracy of
determination of the anterior point in the proximal section of the trochlea;
this point is frequently affected by osteoarthritis and even though both axes
reference the trochlear groove, the SL has geometrical advantages that make it
a more accurate landmark 61. Accuracy of the SL approach was measured using
postoperative CT scans. Chao et al. in their study compared the SL to SEA and
PCL which showed a mean 0.7° of
internal rotation (5.5 ° internal to 4.6° external rotation) and a mean of 1.6° of external rotation (7.6° internal rotation and 9.3° of external rotation) respectively
62.

 

Summary of “bony landmarks” technique

There is no consensus yet as to which
is the best rotational reference method for proper femoral component rotation
and to which all other parameters can be compared. Each landmark is affected by
various factors as mentioned previously. To increase accuracy, it is
recommended to cross-check at least two landmarks and use multiple references
whenever possible to reduce errors.

 

Gap balancing or “tibia first” technique

Another technique of determining the
femoral component rotation is the gap balancing method, relying on ligament
balancing to establish a symmetrical and rectangular flexion and extension gaps
prior to definite bone resection and component placement. Spacers of different
types are used to achieve correct ligament tension. These devices rely on force
applied manually by the surgeon or  may
include various sensors and tensor tools.

This technique is possible in knees
with moderate degenerative changes and small deformities not requiring
extensive soft tissue release. The extension gap is balanced first with
appropriate medial release in varus knees. After balancing the knee in
extension, it is flexed to 90° and
some form of tension measuring device is applied across the medial and lateral
compartments. The femoral component is then rotated in order to achieve flexion
gap symmetry. When this method is used, 90% of knees are implanted in 5° of external rotation relative to the
posterior femoral condylar line 29,30. Boldt et al. found that with this
technique the posterior condylar angle was within 3° of the surgical TEA in 90% of knees
31. The flexion gap balancing method is characterized by excellent
reliability with the prerequisition of intact collateral ligaments. The
technique does not involve identification of anatomic landmarks and closely
approximates the knee flexion axis. For a rectangular flexion gap to be
created, it is paramount to perform an accurate proximal tibial cut.

 

Summary of “gap balancing” technique

There is no gold standard for
assessing the resection quality. Various devices have been developed for this
purpose, e.g.  spacer blocks, laminar
spreaders, and tension jigs. In a study comparing balancing methods: the gap
balancing, AP trochlear axis, and TEA, Katz et al. demonstrated that the gap
balancing method may be superior in accuracy and reliability; this possibly
results from the fact, that the technique does not involve identification of
poorly visible bone landmarks 20.

 

Computer-assisted navigation

          Computer-assisted
navigation was introduced to supplement TKA surgery with the potential to
improve positioning and alignment of TKA components. Several meta-analyses
35-37 have demonstrated that although the average coronal plane alignment
after computer-assisted navigation TKA was not different from conventional TKA,
the variability in the outcome was reduced. A number of studies showed that
navigation-assisted TKA improves alignment in TKA more predictably than
conventional jig-based surgery, while decreasing blood loss and enabling faster
post-operative recovery 35, 38-41. There is conflicting evidence as to
whether computer navigation improves the accuracy of component rotation. The
technology proved to be effective in reducing outliers in the coronal and
sagittal planes, but to has failed to improve the rotational alignment. To
date, navigation has not provided an efficient solution for optimizing the
rotational alignment of femoral component 42-49. Siston et al. also showed
that navigation systems that rely on directly digitizing the femoral
epicondyles to establish alignment axis did not provide a more reliable means
of establishing femoral rotational alignment than traditional techniques did.

Patient-specific instrumentation

New technologies are continually
emerging in arthroplasty. Patient-specific instrumentation (PSI) systems are
interactive computer planning tools that use preoperative imaging techniques
such as computed tomography (CT), magnetic resonance imaging (MRI) and
full-length radiography with rapid prototyping technology for preoperative determination
of bony resections and implant sizing. Computer-generated models are used to
manufacture disposable cutting blocks that are thought to help the surgeon
reproduce the preoperative plan during surgery. Such systems aim to improve
three-dimensional implant positioning while reducing overall costs of
instrumentation and implants 52-53. The
literature is inconclusive in terms of superiority of PSI over conventional
instrumentation (CI). There are only a few studies reporting the accuracy of
femoral component rotation using PSI 54,55.  Other
studies report that PSI does not improve femoral rotation in TKA 56, 57.  Fu et al. (58) conducted a
meta-analysis and found no obvious statistical difference between PSI and CI in
the postoperative mechanical limb axis or femoral component placement. However,
in other studies PSI was found to be effective in significantly reducing
outliers in femoral component rotation 54, 55. As with all technologies, PSI has its
disadvantages, including delay in surgery and considerable costs for
preoperative scans and manufacture of cutting guides, radiation exposure
associated with CT prototyping (59,60), as well as the learning curve.

Summary

          Functional
outcomes of TKA, both short and long-term, are highly dependent on correct
rotational alignment of prosthetic components. There are many studies
discussing individual advantages and potential problems with methods used for
referencing the rotation alignment. No gold standard has been universally
agreed upon to date; therefore surgeons should familiarize themselves with a
variety of references and methods to establish the correct femoral component
rotation. To reduce the rate of femoral component  malrotation, cross-checking of at least two
references should be performed during the TKA procedure.