Knee Alignment Techniques.

The position (alignment) of the new knee replacement components significantly affects the function, motion, and overall feel of the new joint. This makes alignment philosophy an essential part of understanding knee replacement surgery. This article reviews the three major strategies used in knee replacement surgery: mechanical alignment (balance to a straight leg), kinematic alignment (balance to the original anatomy), and functional alignment (balance to the ligament tension).

Below is an anatomic model of the knee. The knee cap (patella), and the associated tendons, cover the front portion of the knee. The femur is the thigh bone; the tibia is the shin bone. The supporting ligaments (MCL and LCL) are seen on the sides. The MCL and LCL stabilize the sides of the knee preventing abnormal sideways motion.

Surgeons typically choose the alignment method to use based on their experience with these techniques, availability of advanced technology (robotic or navigated), implant considerations, and patient-specific factors. Each approach has its advantages and limitations, which we will discuss further.

While the orthopedic community continues to debate the superiority of alignment techniques, studies have shown that mechanical, kinematic, and functional alignment provide comparable durability of knee replacement implants. Across all three methods, implant survivorship remains high, with no increased risk of loosening or mechanical failure attributable to the alignment philosophy. Also, further studies are starting to show trends of improvements in patient outcomes with kinematic and functional alignment.

Before we discuss these three techniques, it’s helpful to know that knee anatomy varies widely from person to person. About 30% of people are naturally bowlegged, 15% are knock-kneed, and the remaining 55% are relatively straight. The orientation of the joint line also differs: in some individuals it is parallel to the floor, while in others it slopes downward. Knees vary in their rotational alignment. There’s also a range of slope angles at the top tibia (typically between 3-10 degrees). Lastly, there are even variants in some patients’ natural ligament laxities. These differences matter because, when each of these factors combine, they create a uniqueness to each person’s knee.

Mechanical Alignment: The Traditional Standard

Mechanical alignment, formalized in 1970, became the first widely adopted alignment strategy. The mechanical axis can be thought of like an imaginary line drawn from the center of the hip down to the ankle. The goal of a mechanically aligned knee is to place the knee right in the middle of that line. That will create a straight leg and a joint line that is parallel to the floor. To achieve this, the component of the femur (thigh bone) is aligned to the center of the hip and the component of the tibia (shin bone) is aligned to the center of the ankle.

When first introduced, it was found to be the most reproducible method with very successful outcomes. This method remained the gold standard for decades.

This alignment technique produces a standardized, reproducible result regardless of the patient’s natural anatomy before arthritis was developed. However, this method will frequently create ligament imbalances due to bone reactions that ignore anatomy. The surgeon will then need to perform selective ligament releases to try to improve the balance of those knees. In some cases, a completely natural ligament balance cannot be fully achieved with this method.

Mechanical alignment can be performed using specially designed instruments or robotic/computer-assisted technology. Both methods allow precise bone cuts and component positioning. The need for intra-operative ligament releases is a result of the alignment philosophy itself, not by whether the procedure is instrumented or robotic.

Kinematic Alignment: Restoring the Knee’s Native Alignment

Kinematic alignment takes a different approach and is intended to restore the patient’s pre-arthritic anatomy rather than placing the leg into a standardized “neutral” position. The goal is to align the components, so the knee moves around its original three-dimensional axes.

This surgery also benefits from protection of the soft tissue envelope (ligaments and tendons), including the poster cruciate ligament (PCL). In surgery, the only alteration of the soft tissue envelop comes after capsular releases from gentle manipulation of contractures. By respecting the original joint line orientation and limb alignment, surgeons find that the ligaments remain naturally balanced without the need for releases.

Kinematic alignment can be performed very effectively with specially designed instruments or with robotic/navigated technologies. The use of robotic assistance may be particularly helpful in patients with significant bone loss, prior ligament injuries, or in especially more complex surgeries.

Kinematically aligned knees are not necessarily intended to appear straight after surgery; instead, they often reflect the natural variety of leg angles seen in the general population. Kinematic alignment works best with modern, anatomically designed implants; like those that provide a relatively constrained ball and socket motion on the inside of the knee, and a looser space (especially in flexion) on the outside of the knee.

There are also some modified versions of the kinematic alignment technique that combine a hybrid of anatomical and functional approaches. For example, some techniques may resurface the tibia anatomically and then balance the femur functionally (inverse kinematic alignment), while others do the opposite, resurfacing the femur anatomically and then balancing the tibia to the soft tissue envelope. The specific technique a surgeon uses to achieve balance depends on their experience and preferences, allowing them to tailor the procedure to each patient’s unique anatomy.

Functional Alignment: Balancing Ligaments During Surgery

Functional alignment focuses on achieving a well-balanced knee by assessing the ligament and the soft tissue envelope tension of the knee during surgery. This alignment method has become much more popularized with the increased use of robotic and navigated technologies. Rather than relying on fixed mechanical axes or pre-arthritic measurements, the surgeon evaluates ligament tension intraoperatively.

With the use of robotics, or navigation, the surgeon will first manipulate the knee to identify the balance pattern of the ligaments. The computer system will provide a balance graph of the looseness or tightness of the soft tissue envelope at different ranges of motion. The surgeon will then virtually plan unique positions of the femoral and tibial components to correct these imbalances in the knee.

In some cases, the surgeon needs to account for confounding factors that may affect the final outcome, such as bone loss, bone spurs and capsular contractions. This method prioritizes balanced tension throughout the range of motion, rather than trying to re-create the original knee anatomy. Typically, the surgeon will try to create stability (equal balance on both sides of the knee) at full extension, and may allow some degree of looseness on the outside of the knee (lateral) in flexion.

The goal is a stable, well-functioning knee that respects the patient’s intra-operative soft-tissue constraints. Its effectiveness depends on surgeon skill and careful intraoperative assessments. The method for creating the new alignment of the knee and achieving stability may vary among surgeons.

In conclusion, there are multiple ways that a surgeon can position your new knee. You can play an active role by researching and seeking out surgeons who specialize in approaches that align with your preferences. Studies indicate these methods often yield comparable long-term outcomes in pain relief, daily function, and implant longevity, though some patients experience a more natural knee feel and faster recovery with personalized techniques. Be sure to talk openly with your surgeon about your lifestyle and goals, as ongoing advancements in technology and research are making procedures more customized and effective for everyone.

References:

Boutros and colleagues(2025): Kinematic versus Mechanically Aligned Total Knee Arthroplasty: A Meta-analysis of Randomized Controlled Trials. Researchers combined results from 21 trials to compare kinematic alignment with mechanical alignment. Kinematic alignment yields clinical outcomes at least equivalent to mechanical alignment, with small but statistically significant improvements in early function, joint awareness, and patient satisfaction, without increased risk of complications or revision.

Gusho and colleagues(2025): No Superiority of Total Knee Arthroplasty Alignment Philosophies: A Network Meta-Analysis Comparing Mechanical, Anatomical, Kinematic, Restricted Kinematic, and Functional Alignment Among Randomized Controlled Trials. This review looked at over 1,400 patients from many studies comparing different ways to align the knee during surgery. There were no significant differences noted in reported outcomes or complications.

Young and colleagues(2025): Functional Versus Mechanical Alignment in Total Knee Arthroplasty: A Randomized Controlled Trial. Functional alignment may provide improved outcomes for a specific subgroup of patients

Personalized alignment in total knee arthroplasty(2021): Current Concepts