The problem
At 15 years, the survival of these implants is greater than 96% and at 25 years they
exceed 82%[1] however, this good survival is obscured by a high percentage of patients, around
20%, who continue with pain and other discomfort nonspecific in the knee over time.[2]
Dissatisfaction after a Total Knee Arthroplasty (TKA) is a phenomenon well documented
in the literature.[3]
Based on a review of the Swedish Registry, Dunbar et al reported that 17% of patients
were dissatisfied with the TKA result.[4] Baker et al. re-evaluated data from the National Register of England and Wales:
71% of patients reported improvement in knee symptoms, but only 22% rated the results
as “excellent”[5].
In the search for solutions to improve these results, new assistive technologies for
the implant have been proposed (from navigation to robotics). Although this is a multifactorial
problem, lately an emphasis has been placed on alignment as a modifiable cause of
this problem.[6]
There is controversy regarding the most appropriate type of alignment in TKA.[6]
The balanced knee concept has no clear definition and can be achieved in more than
one way. There are 2 classic techniques described for this: The measured resection
(MR) and the gap balancing of the femur and tibia.[7]
In MR, anatomical references are used to place the implant. Bone cuts are made regardless
of the ligamentous situation and are based on the transepicodyle axis, the anteroposterior
femoral axis or Whiteside axis and the posterior axis of the condyles. The surgeon
follows the guidelines of the instruments, trying to make the distal and posterior
femoral bone resections similar; ligaments later adapt through soft tissue releases.
In the gap balancing (GB), distraction systems are used to define the best position
of the arthroplasty and subsequently bone cuts and ligamentous releases are adapted
to it.
But regardless of the technique used, the goal for decades has been the same, to obtain
a mechanically aligned knee.[8]
The classic is still mechanical alignment, but this approach ignores the anatomy of
the native joint and the relationship between the origin and insertion of the soft
tissues.
What's new: kinematic, functional, anatomical alignment etc.
The mechanical alignment looks for a knee that respects the mechanical axis of the
lower limb, in which the mechanical axles of the femur and tibia form an angle of
180° between them.[8] The problem is that this axis is natively neutral in only a certain percentage of
the population, which is why it has been questioned as a parameter to be followed.
Constitutional varus has been described in up to 30% of men and 17% of women.[9]
There is no doubt about the excellent results that have been achieved with mechanically
aligned TKA.[8] But new studies have generated more interest in a new concept of alignment; kinematic
alignment. Two randomized clinical trials and other multicenter studies showed that
patients treated with kinematic alignment reported a significantly better difference
from mechanical alignment in pain relief, function, flexion, and more normal knee
sensation, with implant survival of about 2, 3 and 6 years.[10]
[11]
[12]
The kinematic alignment seeks to reconstruct the axis of the limb prior to the implantation
of the Arthroplasty (pre-arthrosic axis),[8] and has 3 objectives[7]: Restore the native tibiofemoral articular surface, restore the native alignment
of the lower limb and restore the native laxity of the knee.
Riviere et al defined 5 implant alignment methods[13]: Mechanical, tight mechanical, anatomical, kinematic, and restricted kinematics.
Recently Oussedick et al also defined functional alignment.[14] In their study, they argue that mechanical alignment ignores the importance of soft
tissue. Kinematic alignment, on the other hand, respects the soft tissues, but ignores
the mechanical environment, which is why it proposes functional alignment as a hybrid
technique that respects the soft tissue tension, but at the same time the mechanical
environment of the limb.
In our opinion, it is in this line that the further development and evolution of alignment
in knee prosthesis will be considered.
Functionally aligned TKA shows a way to achieve specific patient kinematics, manipulating
bone resections and slightly modifying the position of components to limit the need
for periarticular soft tissue release.[15] This is accomplished with advanced technical aids, such as robotic surgery or navigation.
The additional precision achieved with these techniques allows a non-neutral lower
extremity alignment to be achieved more reproducibly.[16] With functional alignment by robotic surgery, the gaps can be balanced by changing
the direction of the components in all 3 planes. This positioning can be individualized
to the patient's knee, maintaining safe alignment limits, which considers a safe range
of 0 ± 3° of coronal alignment.[14]
This hybrid model that provides a balanced knee but remains within the safe limits
of mechanical alignment could have promising results,[14] reducing the morbidity associated with the great release of soft tissues that sometimes
requires mechanical alignment and being able to achieve better patient satisfaction
without compromising the longevity of the implant.[15]
The literature published so far comparing mechanical versus kinematic alignment shows
similar results in the short term, with some studies describing a positive difference
for kinematic alignment.[8] It is possible that functional alignment is the method that leads to the best results,
but the literature is still scarce, so the optimal goal of alignment remains an open
question, whose answer should continue to be investigated.
