Why Enabling Technology Is Critical for Personalized Knee Alignment

Total knee arthroplasty (TKA) is one of the most common joint replacement surgeries, with around 2.6 million procedures performed annually worldwide. The number of primary TKA procedures is projected to grow by 85% by 2030 to 1.26 million procedures per year in the US alone^[1] .

Most patients undergoing TKA surgery report satisfactory results. However, a significant proportion — around 20% — remain unsatisfied with their outcomes following surgery. Comparisons with total hip replacement show that the Forgotten Joint Score for TKA is significantly lower and that fewer patients feel their knee feels “natural” after surgery (56% vs 22%)^[2],^[3] .

Improving functional outcomes, decreasing pain, and enhancing patient satisfaction following TKA has become a key focus of ongoing research and technological advancements. Substantial improvements over the past two decades in implant designs, surgical techniques, and postoperative rehabilitation programs have improved implant survival rates. However, they have not decreased dissatisfaction among patients^[4],^[5],^[6],^[7].

Several clinical studies have shown that malalignment has a significant impact on implant failure rates, and it is a one of the main causes of pain and instability^[8],^[9],^[10] . Until now, the optimal alignment for TKA has remained an unanswered question in the orthopaedic community. Mechanical alignment (MA) remains the most commonly used technique but has become seen as one of the potential causes of patient dissatisfaction rates and residual symptoms^[11]. Personalized alignment has emerged as a growing trend. There is  a consensus on the necessity to use enabling technologies when opting for this approach.

Mechanical alignment with a neutral hip-knee-ankle (HKA) angle of 180° has long been the gold standard in TKA. Uncomplicated MA in TKA has demonstrated long-term implant survival, but functional outcomes are variable^[12].

In 2011, Bellemans et al. demonstrated that every patient's anatomy is unique, with a wide range of hip-knee-angle variations across the population. Restoring neutral alignment may not be the best option for patients who have always had a natural varus^[13] . For this reason, the concept of MA and its 'one size fits all' approach has become the subject of debate in the orthopaedic community. MA is limited in its ability to replicate the anatomy and kinematics of a patient's natural knee^[14] .

Therefore, new personalized alignment methodologies have been introduced with the aim of improving functionality for patients via closer reproduction of their native knee alignment. Methodologies such as adjusted mechanical, kinematic, and restricted kinematic alignment are gaining broader acceptance.

Each of the three alignment philosophies requires a different technique, as summarized below. (Sources: Lustig et al., Winnock et al. )

Adjusted mechanical alignment: 

• Adapts the conventional MA technique to under-correct the constitutional frontal deformity (varus or valgus) to a maximum of 3°
• Implant positioning adjustment is made on the femoral side; tibial implant stays mechanically aligned

Kinematic alignment:

• The aim is knee resurfacing with the restoration of the pre-arthritic anatomy and preservation of the soft-tissue envelope
• Full anatomic positioning of the TKA implants along three kinematic axes and joint lines of the native joint

Restricted kinematic: 

• Considered a hybrid option between MA and kinematic alignment for patients with substantial deformities
• The algorithm involves modifying bone cuts to a 'safe range.'

While no conclusive clinical studies have demonstrated the superiority of one planning philosophy over another, a common consensus is that consistently reproducing a personalized alignment is difficult. The use of enabling technologies, such as advanced imaging and 3D planning software, personalized guides, navigation, or robotics is considered critical to success^[15],^[16],^[17].

Conventional instruments were originally designed and developed to execute mechanical alignment. A lack of proper assessment tools to find the most suitable alignment for each patient makes following a personalized alignment philosophy challenging. Limited insights into the 3D anatomy of the knee during surgery will further increase the amount of variability or outliers^15,16,17 .

In addition, this may result in dangerous outcomes when a desired personalized alignment is situated at the boundaries of the safe zone. For instance, a +/- 5 degree variability on a patient with a 3-degree varus, which is corrected to a 3-degree varus, may result in an 8 degree varus.

Therefore, it is even more important to reproduce the alignment precisely when opting for a personalized alignment approach.

Precision is the ability to have consistent results without outliers. Accuracy is the capacity to achieve a predefined target. In this context, the target is a perfect alignment for each patient. As such, both accuracy and precision are necessary to achieve the optimal alignment for each patient consistently.

To determine the right target, i.e., the right alignment for each patient, outliers need to be reduced as they can cause indecisiveness when the results overlap. To address this, studies must be performed to compare the outcomes for different alignment philosophies using precise enabling technology.

3D planning provides an in-depth, three-dimensional assessment of the patient's anatomy. This includes the patient's wear patterns and alignment angles. According to clinical judgment, this information can then be used to optimize the position of the tibial and femoral components in the coronal and sagittal plane. The use of enabling technologies, such as personalized guides, allows surgeons to transfer these planned alignment angles into the operating theater with precision.

Over the past 12 years, studies have demonstrated that a surgical plan is more reproducible, i.e., with fewer outliers in different planes, using 3D planning and personalized guides in comparison to standard instruments^[18],^[19] . In addition, personalized guides are a cost-efficient solution and easily accessible to any surgeon wanting to enhance accuracy and precision regardless of their preferred alignment philosophy.