# Total Knee Arthroplasty: Alignment, Balance, and Implant Selection

## Introduction

Total knee arthroplasty (TKA) is a highly effective procedure for end-stage knee arthritis, with over 700,000 procedures performed annually in the United States. Despite high overall satisfaction rates, approximately 15 to 20% of patients remain dissatisfied after surgery. Success depends on achieving appropriate alignment, balanced soft tissue tension, and selection of an implant suited to the patient's anatomy and demands.

## Indications and Contraindications

Primary indications include osteoarthritis, rheumatoid arthritis, and post-traumatic arthritis with failed conservative management. Radiographic findings include joint space narrowing, subchondral sclerosis, osteophytes, and deformity. Contraindications include active infection, severe peripheral vascular disease, a nonfunctional extensor mechanism, and neuropathic (Charcot) arthropathy.

## Alignment Philosophy

### Mechanical Alignment

Mechanical alignment is the traditional gold standard, aiming to restore the mechanical axis to a neutral line from the hip center to the ankle center. The femoral component is placed in 5 to 7 degrees of valgus relative to the anatomic axis, and the tibial component is cut perpendicular to the tibial mechanical axis. The goal is a neutral hip-knee-ankle (HKA) angle of 180 degrees, with soft tissue releases performed to balance the knee in a rectangular flexion and extension gap.

### Kinematic Alignment

Kinematic alignment aims to restore the native joint line and the patient's constitutional alignment. Bone cuts are made to resurface the articular surfaces at their native thickness, potentially reducing the need for soft tissue releases. Proponents argue for improved patient satisfaction and more natural knee kinematics, though there are concerns about placing components outside traditional safe zones for alignment, which may risk early failure.

### Robotic-Assisted and Navigation-Assisted Alignment

Computer navigation and robotic platforms (MAKO, ROSA, CORI) improve the precision of bone cuts and component positioning, reducing outliers compared to conventional instrumentation. Whether long-term clinical superiority over conventional techniques exists remains debated. These technologies are useful for complex deformities and for implementing kinematic alignment protocols.

## Soft Tissue Balancing

### Principles

The goal is to achieve equal and symmetric medial and lateral tension in both flexion and extension. A balanced knee has a rectangular gap in both full extension and 90 degrees of flexion. Imbalance leads to instability, accelerated wear, stiffness, or pain.

### Gap Balancing Technique

Bone cuts and soft tissue releases are performed to create equal flexion and extension gaps. Femoral component rotation is determined by the balanced flexion gap rather than by bony landmarks. This directly addresses soft tissue tension but is dependent on accurate tibial cut and ligament balance.

### Measured Resection Technique

Bone cuts are made referenced from anatomic landmarks: the posterior condylar axis, transepicondylar axis, and Whiteside line (anteroposterior axis). Femoral component rotation is set at approximately 3 degrees of external rotation from the posterior condylar axis, with soft tissue releases performed secondarily to balance the knee. This provides reproducible bony landmarks independent of ligament tension during cuts.

### Common Soft Tissue Releases

For varus deformity, sequential release of the deep medial collateral ligament, posteromedial capsule, pes anserinus, and superficial MCL is performed. For valgus deformity, release of the iliotibial band, lateral capsule, popliteus, and lateral collateral ligament is performed cautiously. For flexion contracture, posterior capsule release, removal of posterior osteophytes, and distal femoral recut (2 mm) are performed.

## Implant Design and Selection

| Design | PCL Status | Constraint Level | Key Advantages | Key Indications |
|--------|-----------|-----------------|----------------|-----------------|
| Cruciate-Retaining (CR) | Preserved | Least constrained | More natural kinematics; bone preservation | Intact, competent PCL; balanced knee |
| Posterior-Stabilized (PS) | Sacrificed | Moderate (cam-post) | Reliable rollback; easier balancing | PCL-deficient; valgus knee; prior patellectomy |
| Medial Congruent/Pivot | Variable | Moderate | Medial stability; lateral mobility | Normal knee kinematics replication |
| Varus-Valgus Constrained (VVC) | Sacrificed | High | Handles ligament insufficiency | Moderate ligament deficiency; revision |
| Rotating Hinge | Sacrificed | Highest | Maximal stability | Severe instability; massive bone loss; tumor |

### Cruciate-Retaining (CR)

The CR design preserves the posterior cruciate ligament (PCL), relying on it for femoral rollback and flexion kinematics. It requires a competent PCL and adequate balance, and may provide more natural proprioception.

### Posterior-Stabilized (PS)

The PS design substitutes the PCL with a post-and-cam mechanism, providing predictable femoral rollback and simplifying balancing. It is indicated when the PCL is incompetent, contracted, or when significant deformity requires extensive releases. Risks include post-cam impingement and post fracture, though both are rare.

### Medial Congruent / Medial Pivot

This design replicates the ball-and-socket medial compartment with lateral compartment mobility, providing inherent rotational stability without a post-and-cam mechanism. Growing clinical data supports favorable outcomes.

### Constrained Designs

Varus-valgus constrained (VVC) designs are used for ligamentous insufficiency or significant bone loss. Rotating hinge designs are used for severe instability, complex revision scenarios, or tumor reconstruction. Increased constraint transfers stress to the bone-implant interface, raising the risk of loosening.

## Patellar Considerations

Patellar resurfacing is standard practice in North America, though the decision remains debated internationally. Resurfaced patellae demonstrate lower rates of anterior knee pain and reoperation in most registry data. Overstuffing the patellofemoral joint should be avoided, aiming for a composite thickness equal to or slightly less than the native patella. Patellar tracking is assessed with the no-thumbs test before closure.

## Complications Related to Alignment and Balance

Instability is the most common reason for early revision TKA, resulting from imbalance in flexion, extension, or both. Stiffness may result from overstuffing the joint, component malrotation, or inadequate rehabilitation. Malalignment outliers beyond 3 degrees from neutral correlate with higher revision rates in some studies. Component malrotation, particularly internal rotation of the femoral or tibial component, causes patellar maltracking, lateral release necessity, and pain.

## Clinical Pearls

Coronal alignment within 3 degrees of the neutral mechanical axis using conventional techniques is associated with the best long-term survivorship, though kinematic alignment is a viable alternative with emerging supportive data. A well-balanced knee is more important than any single alignment target, as instability remains the leading cause of early TKA revision. Femoral component rotation is critical for both patellar tracking and flexion gap balance; multiple references (transepicondylar axis, Whiteside line, posterior condylar axis) should be used and cross-checked. The choice between CR and PS designs should be based on PCL integrity, deformity severity, and surgeon experience rather than dogma.

## References
1. Ritter MA, Davis KE, Meding JB, et al. The effect of alignment and BMI on failure of total knee replacement. *J Bone Joint Surg Am*. 2011;93(17):1588-1596.
2. Howell SM, Papadopoulos S, Kuznik KT, Hull ML. Accurate alignment and high function after kinematically aligned TKA performed with generic instruments. *Knee Surg Sports Traumatol Arthrosc*. 2013;21(10):2271-2280.
3. Kayani B, Konan S, Tahmassebi J, et al. Robotic-arm assisted total knee arthroplasty is associated with improved early functional recovery. *Bone Joint J*. 2018;100-B(7):930-937.
4. Abdel MP, Oussedik S, Parratte S, et al. Coronal alignment in total knee replacement: historical review, contemporary analysis, and future direction. *Bone Joint J*. 2014;96-B(7):857-862.