What are the key recommendations for alignment in total knee arthroplasty (TKA) for an adult patient with end-stage knee arthritis?

Alignment in Total Knee Arthroplasty: Key Points for Resident Presentation

Primary Recommendation

Functional alignment (FA) should be the preferred approach in modern TKA, as it achieves the highest proportion of balanced gaps (96.5%) compared to mechanical alignment (54.7%) or kinematic alignment (66.4%), while requiring significantly fewer soft-tissue releases and improving patient-reported outcomes, particularly for patients with constitutional Type I alignment. 1, 2

Evolution of Alignment Philosophy

Traditional Mechanical Alignment (MA) - The Outdated Gold Standard

• MA positions components perpendicular to the limb mechanical axis with the goal of achieving neutral alignment 1
• Critical flaw: MA fails to account for individual anatomical variations and requires extensive soft-tissue releases in 65% of cases 1
• Patient satisfaction remains problematic at only 80-90%, meaning up to 1 in 5 patients are dissatisfied 3
• MA achieves balanced gaps in only 54.7% of cases across all four gap measurements 2
• The systematic approach of MA ignores constitutional variations in knee phenotypes 4

Kinematic Alignment (KA) - The Anatomic Approach

• KA resurfaces the knee to restore pre-arthritic anatomy, maintaining native femoral joint line obliquity 5
• Tibial resection is adjusted to balance flexion and extension gaps 5
• KA achieves balanced gaps in 66.4% of cases - better than MA but still suboptimal 2
• KA produces the most valgus and internally rotated femoral components and most varus tibial components 2
• Limitation: True KA may reproduce extreme anatomical alignments that could compromise outcomes 5

Restricted Kinematic Alignment - The Safety Compromise

• Represents a compromise between MA and true KA with defined safe zones of alignment 5
• Avoids reproducing extreme anatomical alignments while preserving more native anatomy than MA 5

Functional Alignment (FA) - The Current Best Practice

• FA is an evolution of KA enabled by robotic-assisted technology that manipulates alignment, bone resections, soft-tissue releases, and implant positioning to optimize TKA function for each patient's specific anatomy 5
• FA achieves balanced gaps in 96.5% of cases - significantly superior to both MA and KA 2
• FA requires soft-tissue releases in only 16% of cases versus 65% for MA 1
• FA patients demonstrate higher KOOS Symptoms scores (86.6 vs 82.5) and KOOS Quality of Life scores (76.1 vs 70.7) compared to MA 1
• 94% of FA patients would recommend the procedure versus only 82% of MA patients 1

Assessment of Alignment

Preoperative Imaging Protocol

• Full-length standing hip-to-ankle radiographs are the gold standard for assessing mechanical axis and planning alignment strategy 6, 7
• Standard AP knee radiographs are valid for determining coronal plane alignment at the knee, but only hip-to-ankle views provide accurate mechanical axis information 6, 7
• AP radiographs should be obtained with 10° internal rotation to improve interpretation of varus and valgus alignment 6, 7
• Weight-bearing views are essential as they reflect functional alignment under physiologic load 7

Postoperative Imaging Protocol

• Routine postoperative evaluation consists of standing AP and lateral views plus axial tangential patellofemoral view 6, 7
• Full-length standing radiographs should be obtained to assess overall limb alignment 6
• Annual or biennial weight-bearing radiographs are recommended to detect subclinical wear and subtle alignment changes 6, 7
• After baseline hip-to-ankle radiograph, subsequent follow-up can utilize targeted knee radiographs 6

Constitutional Alignment and Patient Phenotypes

Importance of Phenotype Recognition

• Constitutional alignment classification (Coronal Plane Alignment of the Knee - CPAK) is critical for selecting optimal alignment strategy 1
• For patients with preoperative CPAK Type I alignment, FA demonstrates significantly higher Forgotten Joint Score (71.3 vs 56.8) and KOOS-Quality of Life (76.4 vs 64.2) compared to MA 1
• FA achieves the highest proportion of balanced gaps across all constitutional alignment subgroups 2

Bone Resection Principles

Comparative Resection Depths

• MA resects the least bone from the medial tibial plateau 2
• KA is most bone-preserving for the posteromedial femoral condyle 2
• FA is most bone-preserving for distal femoral resections while achieving superior balance 2
• FA produces the most externally rotated femoral components 2

Role of Enabling Technology

Robotic-Assisted TKA

• Robotic assistance enables precise execution of FA by allowing real-time adjustment of alignment, resections, and soft-tissue balance 5, 3
• Sensor-embedded smart inserts or computer-assisted platforms provide objective load data to confirm intraoperative knee balance 3
• Enabling technology allows surgeons to select desired alignment targets and execute with high precision 3

Common Failure Mechanisms Related to Alignment

Malalignment as a Cause of Revision

• Historically, malalignment was a significant cause of TKA failure, though its percentage has decreased with modern techniques 6
• Malalignment refers to suboptimal alignment of prosthetic components relative to each other and is evaluated on full-length standing radiographs 6
• Aseptic loosening (39.9%) remains the most common late failure mechanism, often related to alignment issues 6

Critical Pitfalls to Avoid

Imaging Errors

• Never rely on standard AP knee radiographs alone for alignment assessment - they cannot accurately measure the mechanical axis 7
• Avoid non-weight-bearing radiographs as they fail to demonstrate true functional alignment under load 7
• Do not obtain radiographs in neutral rotation when assessing varus/valgus alignment 7

Surgical Technique Errors

• Systematic MA approach that ignores individual anatomical variations leads to excessive soft-tissue releases and suboptimal balance 1, 4
• Pursuing neutral mechanical axis in all patients disregards constitutional knee phenotypes 4
• Failure to assess preoperative alignment phenotype prevents optimization of alignment strategy 1

Outcomes and Survivorship

Short to Mid-Term Results

• At 2-year follow-up, FA demonstrates comparable or superior patient-reported outcomes to MA with fewer soft-tissue releases 1
• No significant differences in reoperation rates or implant survivorship between FA and MA at 2 years 1
• Long-term follow-up remains crucial to assess both outcomes and implant survivorship of personalized alignment concepts 5

Patient Satisfaction

• Traditional MA achieves only 80-90% patient satisfaction 3
• FA demonstrates 94% patient recommendation rate versus 82% for MA 1
• Balanced gaps achieved through FA are associated with improved clinical outcomes 3