Measured give rise to many errors, such as in

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.

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Posterior condylar line

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. The mean angle between the anatomical and surgical TEA is 2
degrees 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.

 

 

Anatomical
transepicondylar axis

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. An angle between
aTEA and sTEA was reported by Yoshino et al. to be 3.2°±1°, with aTEA being more externally
rotated 28.  

 

 

Surgical transepicondylar axis

The sTEA runs 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 epicondylar
axis (SEA)fo 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.

 

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. The TEA is easier to
identify intraoperatively than the posterior condylar angle or the PCL, 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 sTEA and eliminates femoral component malrotation.

 

 

Trochlear AP axis and the sulcus line
(SL)

The trochlear anterior-posterior axis
– Whiteside’s line – TAP axis 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. The TAP
axis reference is reliable and can be applied in patients with distorted
condylar anatomy – such as condylar hypoplasia or defect. ATA 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 degrees relative to
the PCA 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 highly 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 Victor.

An alternative to determination of the
TAP 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 APA because there is only
one true coronal alignment axis. The APA 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 62 . Chao et al. in their study compared the SL to SEA and PCL which
showed a mean 0.7 degrees of internal rotation (5.5 internal to 4.6 external
rotation) and a mean of 1.6  degrees of
external rotation (7.6  degrees internal
rotation and 9.3 degrees of external rotation) respectively.

 

 

 

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.