Kinematic or Anatomic Alignment Techniques in Primary Total Knee Arthroplasty
Ashton H. Goldman
Mark W. Pagnano
Key Concepts
Kinematic (anatomic) alignment is a true measured resection technique whereby the general goal is to restore the limb to its prearthritic alignment and maintain the native joint line position and inclination throughout the arc of motion. Some believe this minimizes the need for soft tissue releases and creates a more natural feeling knee.
The senior author’s fundamental premise is that, within some reasonable boundaries, priority should be given to placing the femoral component such that it mimics the 3-dimensional position of the native femur. Within boundaries of ±2 or 3° of varus the tibia is then positioned to accommodate that femoral position and achieve ligamentous balance in both flexion and extension.
Anatomic alignment is individualized for each patient, and although it can be performed simply and pragmatically, it is not a cookbook approach. Definitively it is not just leaving varus knees in varus or cutting every tibia in slight varus; instead, it is a deliberate attempt to incrementally adjust the femoral and tibial cuts to mimic the individual patient’s anatomy without straying great distances from established norms of alignment.
In our practice, kinematic/anatomic principles are applied only to varus and neutral knees. It is the senior author’s opinion that many valgus knees have inherent bony deformities that make mechanical alignment a better choice for the valgus knee.
Studies show that neutral mechanical alignment ±3° is a safe boundary in terms of long-term durability. Deliberately aiming for a target beyond ±3° is not recommended, so the technique should have clear boundaries.
It is the senior author’s contention that, although modern cemented implants seem to tolerate some mild deviations from mechanical alignment, at some point there is a mechanical limit because we are using biomaterials, including metal, plastic, and bone cement. For that reason, we have established 2 to 3° of deviation as the reasonable boundary within which we will adjust component position.
In the typical varus knee, standard mechanical alignment techniques routinely overresect the lateral tibial plateau in both flexion and extension relative to normal anatomy (Figure 52.1). As a consequence, additional external rotation of the femoral component (typically 3°) is routinely needed to balance the knee in flexion and the distal femur is cut in less valgus to balance in
extension. The combination of relative overresection and additional external rotation elevates the medial joint line in both flexion and extension (thereby risking midflexion instability) and decreases posterior femoral offset (limiting ultimate range of motion). Anatomic alignment principles seek to limit those systematic deviations from native anatomy and produce a better balanced total knee arthroplasty without the routine need for medial ligamentous releases (Figure 52.2)
Figure 52.1 ▪ An illustration demonstrates the relative overresection of the proximal lateral tibia and the alteration of the distal femoral joint line that is routinely forced when following mechanical axis alignment principles as compared with the native anatomy of the knee.
Figure 52.2 ▪ An illustration demonstrates the typical native anatomy of the knee with the proximal tibial joint surface in slight varus and the distal femur in valgus. The values represented in the figure are mean values; surgeons should be aware of the range of individual variability. Note that contemporary kinematic or anatomic alignment techniques typically account for that individual variability rather than preselecting an average, mean, or generic target.
With this technique, because the tibia is not relatively overresected laterally in flexion, the appropriate femoral rotation is most often 0° relative to the posterior condylar axis. Zero degrees femoral rotation will mimic the native femur.
Before attempting anatomic alignment, a surgeon must be confident in his/her ability to make precise bone cuts in slight varus/valgus or should use surgical navigation or robotics to achieve that precision.
Sterile Instruments and Implants
Routine knee retractors
Wide and narrow rongeurs for osteophyte removal
Total knee system with following qualities:
Favorable trochlear design
Ability to make fine varus/valgus adjustment
Posterior referencing for both femoral sizing and rotation
Surgical Approaches
Medial parapatellar, midvastus, and subvastus approach are all appropriate for this technique.
Preoperative Planning
The senior author has both a simple, pragmatic planning process and a more complicated, mechanistic approach.
A full-length standing radiograph of the lower limbs is preferred in all cases; is optional for the simple, pragmatic planning process; and is required for the more complicated, mechanistic planning approach.
Simple, Pragmatic Planning
For knees with preoperative varus deformity or neutral alignment:
On the anteroposterior (AP) radiograph the distal femoral valgus angle is measured by defining the angle between the distal femoral subchondral bone and the center of the femoral canal. Typically, this will measure about 5 or 6° but is variable. The distal femoral resection angle is set to match this measured angle, but if the angle exceeds 8°, we pragmatically dial that back to 7 or 8° (Figure 52.3)
On the AP radiograph the proximal tibial varus inclination is measured by defining the proximal tibial subchondral bone laterally and medially and the long axis of the tibia. If there is some medial bone loss, then an estimate of the original joint inclination can typically still be made by referencing unworn regions or looking at the contralateral knee (Figure 52.4)Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
