TKA ALIGNMENT OPTIONS
Mechanical vs Kinematic vs Restricted Kinematic | Patient-Specific Alignment | Evolution of TKA Philosophy
ALIGNMENT PHILOSOPHIES
Critical Must-Knows
- Mechanical alignment targets neutral 0° ± 3° hip-knee-ankle axis with perpendicular cuts - gold standard for 40+ years
- Kinematic alignment restores native joint line obliquity and ligament isometry - gaining popularity since 2014
- Restricted kinematic alignment combines KA principles with safety boundaries (HKA 0° ± 3°) to avoid extreme outliers
- 20% of patients report dissatisfaction with MA-TKA despite correct alignment - driver for alternative techniques
- No Level 1 evidence shows long-term superiority of any alignment philosophy - decision based on surgeon experience and patient anatomy
Examiner's Pearls
- "Mechanical axis: line from femoral head center to ankle center - MA-TKA aims for 0° deviation
- "Kinematic axis: reproduces native joint line orientation - typically 3° varus in constitutional alignment
- "Safe zone debate: Restricted KA limits alignment to HKA 0° ± 3° to avoid catastrophic outliers
- "Forgotten joint score higher with KA in short-term studies - less clear at 5+ years
Critical TKA Alignment Exam Points
Mechanical Alignment Principles
Neutral mechanical axis 0° ± 3°. Perpendicular cuts to mechanical axis. Femur 5-6° valgus cut (relative to anatomic axis), tibia 0° perpendicular. Ignores constitutional alignment. Traditional gold standard since 1970s.
Kinematic Alignment Philosophy
Restore native joint line obliquity. Co-level resection matching worn cartilage + implant thickness. Maintains constitutional alignment (typically 3° varus). Preserves ligament isometry. Higher patient satisfaction in short-term.
Alignment Measurement
Hip-Knee-Ankle (HKA) angle on standing long-leg radiograph. Mechanical axis: femoral head center to ankle center. Anatomic axis: femoral shaft midline. Difference approximately 5-6° valgus (anatomic axis is more varus).
Safety Boundaries
Restricted KA: HKA 0° ± 3°. Avoids extreme varus (greater than 3°) or valgus (greater than 3°). Component position limits: femur 5° varus to 10° valgus, tibia 5° varus to 5° valgus. Prevents edge loading and catastrophic failure.
Quick Decision Guide: Choosing Alignment Philosophy
| Patient Scenario | Constitutional Alignment | Recommended Approach | Key Consideration |
|---|---|---|---|
| Neutral pre-arthritic alignment, mild-moderate OA | HKA 0° ± 2° on contralateral knee | Mechanical Alignment (MA) | Traditional approach, proven long-term outcomes |
| Constitutional varus (3-5°), symmetric wear | HKA 3-5° varus bilaterally | Restricted Kinematic Alignment | Restore native alignment within safe HKA 0° ± 3° |
| Extreme deformity (greater than 10° varus/valgus) | Severe bone loss, ligament incompetence | Adjusted Mechanical Alignment | May need constrained implants, cannot safely restore KA |
| Young, active patient seeking natural feel | Well-preserved ligaments, minimal deformity | True Kinematic Alignment (if experienced) | Requires advanced technique, no long-term data beyond 10 years |
PERPENDICULARMechanical Alignment (MA-TKA) Principles
Memory Hook:MA-TKA is PERPENDICULAR - every cut is 90° to mechanical axis, creating uniform neutral alignment regardless of patient's native anatomy!
RESTOREKinematic Alignment (KA-TKA) Principles
Memory Hook:KA-TKA aims to RESTORE the knee's native anatomy - the joint line obliquity and ligament tension that existed before arthritis!
SAFERestricted Kinematic Alignment Boundaries
Memory Hook:Restricted KA keeps alignment SAFE - within 3° of neutral to avoid catastrophic edge loading and early polyethylene wear!
Overview and Evolution of TKA Alignment
Historical Context
The debate over alignment in total knee arthroplasty represents one of the most significant philosophical shifts in orthopedic surgery over the past decade. For over 40 years, mechanical alignment was the undisputed gold standard, based on the principle that a neutral mechanical axis (0° ± 3°) would distribute loads evenly across medial and lateral compartments, maximizing implant longevity.
Why the Paradigm Shift?
Despite excellent survivorship with mechanical alignment (greater than 95% at 15 years), 20% of patients report dissatisfaction with their TKA. Common complaints include feeling the knee is "not natural," inability to forget the joint, and persistent anterior knee pain. These outcomes drove the search for alternative alignment philosophies.
Constitutional Alignment Concept
Native Knee Anatomy
Average HKA angle: 1.3° varus (range 3° varus to 3° valgus), Joint line obliquity: Distal femur 3° valgus, proximal tibia 3° varus, Natural ligament tension: Isometric throughout range of motion, Individual variation: Wide spectrum of "normal" alignment
MA-TKA Alteration
Changes joint line: Creates horizontal joint line in most patients, Alters ligament lengths: Requires releases to balance gaps, Modifies kinematics: Changes patellofemoral and tibiofemoral tracking, One-size-fits-all: Ignores individual constitutional variation
The Three Main Philosophies
Alignment Philosophy Evolution
Goal: Neutral 0° ± 3° hip-knee-ankle axis with perpendicular cuts to mechanical axis. Technique: Distal femur cut 5-7° valgus (relative to anatomic axis), proximal tibia cut perpendicular to mechanical axis (0°). Soft tissue balancing via ligament releases. Outcomes: Excellent survivorship (greater than 95% at 15 years), but 20% patient dissatisfaction.
Goal: Restore pre-arthritic joint line obliquity and ligament isometry. Technique: Co-level resections matching worn cartilage thickness + implant thickness. Preserve constitutional alignment (typically 3° varus). Minimal ligament releases. Outcomes: Higher forgotten joint scores and patient satisfaction short-term, limited data beyond 10 years.
Goal: Kinematic principles within safe boundaries (HKA 0° ± 3°). Technique: Start with kinematic approach, but cap alignment at 3° varus or 3° valgus. Adjust component position if exceeding safe zone. Outcomes: Emerging evidence suggests similar satisfaction to KA with potentially safer boundaries. Compromise approach.
Anatomy and Biomechanics
Anatomical Axes and Measurements
| Axis Type | Definition | Normal Value | Clinical Significance |
|---|---|---|---|
| Mechanical Axis (MA) | Line from femoral head center to ankle center | 0° ± 2° (neutral to slight varus) | MA-TKA target - used for load distribution assessment |
| Anatomical Axis | Midline of femoral or tibial shaft | Femur: 5-7° valgus to mechanical axis | Intramedullary alignment guides reference this axis |
| Hip-Knee-Ankle (HKA) | Angle between femoral and tibial mechanical axes | 1.3° varus average (range 3° varus to 3° valgus) | Primary alignment measurement - defines constitutional alignment |
| Joint Line Obliquity | Angle of distal femur and proximal tibia surfaces | Distal femur 3° valgus, proximal tibia 3° varus | KA-TKA aims to restore this native obliquity |
Biomechanical Load Distribution
MA-TKA Load Pattern
Equal 50/50 distribution across medial and lateral compartments.
- Theoretical advantage: Uniform polyethylene wear
- Perpendicular cuts create horizontal joint line
- Requires ligament releases to achieve balanced gaps
- May alter native kinematics and ligament tension
KA-TKA Load Pattern
Reproduces native load distribution (typically 60/40 medial/lateral).
- Matches pre-arthritic loading pattern
- Oblique joint line maintains constitutional alignment
- Preserves native ligament lengths and tension
- Concern: Potential for asymmetric polyethylene wear
Edge Loading Risk
Extreme varus or valgus alignment (greater than 5°) concentrates loads on polyethylene rim, leading to accelerated wear, deformation, and potential catastrophic failure. This is the primary concern with unrestricted kinematic alignment and drives the "safe zone" concept in restricted KA.
Ligament Considerations
Ligament Behavior Across Alignment Philosophies
Changes ligament lengths due to altered joint line orientation. Medial collateral ligament (MCL) typically lengthened in varus knees corrected to neutral. Lateral collateral ligament (LCL) lengthened in valgus knees. Requires systematic ligament releases to achieve rectangular extension gap and parallel flexion gap.
Maintains native ligament lengths by restoring joint line obliquity. MCL and LCL retain isometry throughout range of motion. Minimal to no ligament releases required. Theoretically improves proprioception and patient satisfaction.
Preserves ligaments when possible, but releases if needed to stay within HKA 0° ± 3° safe zone. Hybrid approach accepting some ligament modification to avoid extreme alignment.
Classification of Alignment Philosophies
Traditional Mechanical Alignment (MA-TKA)
Core Principles
Neutral mechanical axis: Create 0° hip-knee-ankle alignment ± 3° tolerance. This distributes loads equally (50/50) across medial and lateral compartments.
Perpendicular cuts: Distal femur and proximal tibia cuts made 90° to their respective mechanical axes, creating horizontal joint line.
Gap balancing: Rectangular extension gap and parallel flexion gap achieved through systematic ligament releases.
Surgical Technique
| Bone Cut | Reference | Target | Technical Detail |
|---|---|---|---|
| Distal femur | Intramedullary rod (anatomic axis) | 5-7° valgus from anatomic axis | 9mm typical resection (8-10mm range) |
| Proximal tibia | Extramedullary alignment rod | 0° perpendicular to mechanical axis | 8-10mm resection from least worn side |
| Posterior femur | Anteroposterior (AP) axis | 3° external rotation from posterior condylar axis | Whiteside's line or epicondylar axis alternative |
| Femoral rotation | Transepicondylar axis preferred | Parallel to flexion gap in balanced knee | Avoid internal rotation - causes patella maltracking |
Soft Tissue Balancing in MA-TKA
Extension Gap Balancing
Goal: Rectangular gap, equal medial and lateral.
- Tight medial: Release superficial MCL off proximal tibia
- Tight lateral: Release iliotibial band (ITB), popliteus
- Severe varus: May need MCL "pie-crusting" or posteromedial release
- Severe valgus: LCL, popliteus, lateral head gastrocnemius release
Flexion Gap Balancing
Goal: Parallel to extension gap, equal medial and lateral.
- Tight flexion gap: Downsize femur or reduce distal femoral resection
- Loose flexion gap: Thicker polyethylene insert
- Asymmetric flexion gap: Adjust femoral component rotation
- Severe imbalance: Consider posterior stabilized or constrained implant
Advantages and Disadvantages
| Aspect | Advantages | Disadvantages |
|---|---|---|
| Evidence Base | Excellent long-term data (greater than 95% survivorship at 15 years) | 20% patient dissatisfaction despite correct alignment |
| Technique | Well-established, reproducible, taught universally | Requires systematic ligament releases, alters native anatomy |
| Patient Satisfaction | Predictable mechanical outcomes | Lower forgotten joint scores, less natural feeling |
| Implant Survival | Proven longevity, wide acceptable alignment range | No clear advantage over KA in randomized trials to 5 years |
Understanding mechanical alignment remains essential for all TKA surgeons - it is the foundation from which alternative techniques have evolved.
Clinical Assessment and Pre-operative Planning
Patient Evaluation for Alignment Choice
History Elements
Constitutional alignment: Review old radiographs if available, Contralateral knee: Is other knee arthritic? What alignment?, Activity level: High-demand patient may benefit from KA "natural feel", Expectations: Discuss forgotten joint concept vs implant longevity, Prior surgery: Previous osteotomy, fracture - may limit options
Examination Findings
Deformity magnitude: Mild (less than 5°), moderate (5-10°), severe (greater than 10°), Deformity correctability: Varus/valgus stress - fixed vs correctable?, Ligament competence: Medial and lateral collateral integrity, Patellofemoral tracking: Maltracking may influence rotational alignment, Range of motion: Severe flexion contracture may limit KA options
Radiographic Planning
Imaging Protocol for Alignment Planning
Both legs if possible to assess constitutional alignment of contralateral knee. Measure hip-knee-ankle (HKA) angle. Assess for extra-articular deformity (femoral or tibial shaft bowing). Determine mechanical axis deviation (MAD) - distance from knee center to mechanical axis line.
Weight-bearing full-extension view. Measure joint line convergence angle (JLCA). Assess medial and lateral joint space - estimate cartilage wear. Identify osteophytes and bone loss. Measure tibial varus/valgus angle.
Assess posterior slope of native tibia. Identify patella height (Insall-Salvati ratio). Check for posterior femoral condylar wear. Assess flexion contracture on lateral view.
Assess patellofemoral tracking and tilt. Identify patella subluxation or dysplasia. Influences decision on femoral component rotation and alignment.
Decision Algorithm for Alignment Philosophy
| Patient Scenario | Constitutional Alignment | Recommended Philosophy | Rationale |
|---|---|---|---|
| Ideal candidate | Neutral HKA 0-2°, mild arthritis | Mechanical Alignment | Traditional approach works well - proven longevity |
| Constitutional varus | Bilateral 3° varus, no previous trauma | Restricted Kinematic Alignment | Restore native alignment within HKA 0° ± 3° safe zone |
| Young, active patient | Any alignment, high expectations for natural feel | Kinematic Alignment (if experienced surgeon) | Maximize patient satisfaction and forgotten joint |
| Severe deformity | HKA greater than 10° varus or valgus, bone loss | Adjusted Mechanical Alignment | Safety concern with KA - may need constrained implant |
| Ligament incompetence | MCL or LCL deficiency, varus/valgus instability | Mechanical Alignment with constrained implant | Cannot achieve stability with KA - need implant constraint |
| Post-traumatic arthritis | Prior fracture, malunion, retained hardware | Adjusted Mechanical Alignment | Anatomy already altered - difficult to define native alignment |
Alignment Philosophy Is Not One-Size-Fits-All
The choice of alignment philosophy must be individualized based on patient anatomy, surgeon experience, and patient expectations. No single approach is universally superior. Beware the surgeon who uses only one technique for all patients - this ignores the spectrum of knee pathology.
Investigations
Radiographic Assessment for Alignment Planning
Imaging Protocol for Alignment Planning
Both legs if possible to assess constitutional alignment of contralateral knee. Measure hip-knee-ankle (HKA) angle. Assess for extra-articular deformity (femoral or tibial shaft bowing). Determine mechanical axis deviation (MAD) - distance from knee center to mechanical axis line.
Weight-bearing full-extension view. Measure joint line convergence angle (JLCA). Assess medial and lateral joint space - estimate cartilage wear. Identify osteophytes and bone loss. Measure tibial varus/valgus angle.
Assess posterior slope of native tibia. Identify patella height (Insall-Salvati ratio). Check for posterior femoral condylar wear. Assess flexion contracture on lateral view.
Assess patellofemoral tracking and tilt. Identify patella subluxation or dysplasia. Influences decision on femoral component rotation and alignment.
Advanced Imaging for Kinematic Alignment
CT-Based Planning
Three-dimensional reconstruction: CT scan of entire lower limb from hip to ankle. Software reconstructs anatomy and identifies pre-arthritic joint line. Custom patient-specific cutting guides manufactured. Allows reproducible kinematic alignment without intraoperative caliper technique.
MRI-Based Planning
Cartilage mapping: MRI sequences can visualize remaining cartilage thickness on medial and lateral compartments. Helps predict bone resection amounts for kinematic co-level technique. May identify subchondral bone edema suggesting overload pattern.
Understanding pre-operative imaging is critical for alignment planning - the choice between MA, KA, or restricted KA often depends on constitutional alignment measured on long-leg radiographs.
Management Algorithm
Decision Algorithm for Alignment Philosophy Selection
| Patient Scenario | Constitutional Alignment | Recommended Philosophy | Rationale |
|---|---|---|---|
| Ideal candidate | Neutral HKA 0-2°, mild arthritis | Mechanical Alignment | Traditional approach works well - proven longevity |
| Constitutional varus | Bilateral 3° varus, no previous trauma | Restricted Kinematic Alignment | Restore native alignment within HKA 0° ± 3° safe zone |
| Young, active patient | Any alignment, high expectations for natural feel | Kinematic Alignment (if experienced surgeon) | Maximize patient satisfaction and forgotten joint |
| Severe deformity | HKA greater than 10° varus or valgus, bone loss | Adjusted Mechanical Alignment | Safety concern with KA - may need constrained implant |
| Ligament incompetence | MCL or LCL deficiency, varus/valgus instability | Mechanical Alignment with constrained implant | Cannot achieve stability with KA - need implant constraint |
| Post-traumatic arthritis | Prior fracture, malunion, retained hardware | Adjusted Mechanical Alignment | Anatomy already altered - difficult to define native alignment |
Alignment Philosophy Is Not One-Size-Fits-All
The choice of alignment philosophy must be individualized based on patient anatomy, surgeon experience, and patient expectations. No single approach is universally superior. Beware the surgeon who uses only one technique for all patients - this ignores the spectrum of knee pathology.
Understanding patient-specific factors is critical for alignment philosophy selection.
Surgical Technique by Alignment Type
Mechanical Alignment Surgical Steps
MA-TKA Step-by-Step
Intramedullary alignment: Enter femoral canal at intercondylar notch anterior to PCL insertion. Choose rod diameter 1mm smaller than canal width. Set distal femoral cutting block at 5-7° valgus from anatomic axis (rod is along anatomic axis). Typical resection 9mm from least worn condyle (measure with cutting block caliper). Verify cut is perpendicular to mechanical axis if using navigation.
Extramedullary alignment: Place alignment rod along tibial crest, aiming at center of ankle (talus dome). Adjust medial-lateral to center of tibial spines. Set cutting block at 0° perpendicular to mechanical axis (confirm with ankle alignment). Typical resection 8-10mm from least worn plateau. Verify posterior slope matches native (typically 3-5°).
Rectangular gap goal: Place spacer blocks in extension gap. Measure medial and lateral gap heights with tensiometer or calipers. Goal is equal medial and lateral tension. If asymmetric, perform ligament releases: Tight medial in varus knee - release superficial MCL from proximal tibia incrementally. Tight lateral in valgus knee - release ITB, popliteus, lateral head gastrocnemius.
3° external rotation from posterior condylar axis (Whiteside's line parallel). Alternatives: Transepicondylar axis (most accurate), or parallel to cut tibial surface with balanced flexion gap. Avoid internal rotation - causes patella maltracking and anterior knee pain. Set anterior-posterior (AP) dimension based on bone cuts and implant size.
Parallel to extension gap goal: Place femoral trial and assess flexion gap at 90° flexion. Should be equal to extension gap height and equal medial-lateral. If flexion gap tight: Downsize femoral component or reduce distal femoral resection. If flexion gap loose: Thicker polyethylene insert. If asymmetric flexion gap: Adjust femoral rotation.
Verify stability through full range of motion with trial components. Check patella tracking - should centralize without lateral tilt or subluxation. Confirm mechanical axis passes through center of knee (navigation or alignment rod). Insert definitive components with cement technique per implant design.
Common Ligament Releases in MA-TKA
| Deformity | Tight Side | Release Sequence | Endpoint |
|---|---|---|---|
| Varus deformity (tight medial) | Medial compartment tight | 1. Osteophytes. 2. Deep MCL (posterior capsule). 3. Superficial MCL (pie-crust). 4. Semimembranosus | Equal medial-lateral tension in extension |
| Valgus deformity (tight lateral) | Lateral compartment tight | 1. Osteophytes. 2. ITB. 3. Popliteus. 4. Lateral head gastrocnemius. 5. LCL (rare) | Equal medial-lateral tension in extension |
| Flexion contracture | Posterior capsule tight | 1. Osteophytes. 2. Posterior capsule release. 3. Increase distal femoral resection (last resort) | Full extension achieved with balanced gaps |
Understanding mechanical alignment technique remains the foundation - even surgeons using alternative techniques must know MA-TKA principles.
Complications and Concerns by Alignment Type
| Complication | MA-TKA Risk | KA-TKA Risk | Prevention Strategy |
|---|---|---|---|
| Polyethylene wear | Symmetric 50/50 loading - predictable wear pattern | Asymmetric loading in varus knees - long-term wear unknown | Restricted KA limits extreme alignment - stay within HKA 0° ± 3° |
| Ligament imbalance | Requires releases to achieve rectangular gaps - risk of over-release | Minimal releases - preserves native tension, but may leave asymmetry | Careful gap assessment and incremental releases in MA |
| Patient dissatisfaction | 20% report knee does not feel natural despite correct alignment | Higher forgotten joint scores - more natural feeling | Set realistic expectations pre-operatively regardless of technique |
| Instability (mid-flexion) | Ligament over-release common - mid-flexion laxity | Preserved ligament lengths - theoretically less mid-flexion instability | Avoid aggressive releases - consider thicker insert if unstable |
| Patella maltracking | Altered Q-angle and trochlear orientation with neutral alignment | Preserved native trochlear groove orientation - less maltracking risk | Femoral rotation critical in both techniques - avoid internal rotation |
| Edge loading (extreme outlier) | MA safe zone 0° ± 3° prevents edge loading in vast majority | Unrestricted KA may create HKA greater than 5° - edge loading risk | Restricted KA with HKA 0° ± 3° boundary mitigates this concern |
| Aseptic loosening | Traditional concern with MA malalignment greater than 3° | Unknown long-term risk with constitutional varus alignment | Long-term registry data needed - currently limited to 10 years KA |
Long-Term Survivorship Data Gap
Mechanical alignment has 40+ years of registry data showing excellent survivorship (greater than 95% at 15 years). Kinematic alignment has only 10 years maximum follow-up, with most studies under 5 years. The true test of KA will be 15-20 year survivorship - whether asymmetric loading and constitutional varus alignment lead to accelerated polyethylene wear or aseptic loosening. This uncertainty must be disclosed to patients when choosing KA.
Failure Patterns
MA-TKA Failure Modes
Malalignment greater than 3°: Increased aseptic loosening risk, Ligament over-release: Instability, mid-flexion laxity, Patella maltracking: Altered Q-angle, anterior knee pain, Patient dissatisfaction: "Unnatural" feel despite correct alignment, Stiffness: Aggressive gap balancing, scar tissue
KA-TKA Theoretical Risks
Edge loading: Extreme varus (greater than 5°) concentrates loads on medial rim, Accelerated wear: Asymmetric loading pattern - long-term unknown, Aseptic loosening: Constitutional varus may overload medial bone-implant interface, Technique error: Incorrect caliper measurement - fails to restore joint line, Severe deformity: Cannot achieve stability without releases
Postoperative Care and Follow-Up
Rehabilitation Protocol (Common to All Alignment Types)
TKA Rehabilitation Timeline
Mobilization: Same-day or day 1 mobilization with physiotherapy. Weight-bearing as tolerated with walking aid. DVT prophylaxis: Aspirin 100mg daily (Australian standard) or LMWH if high risk. Pain management: Multimodal analgesia (paracetamol, NSAIDs, opioids as needed). Range of motion: Passive and active-assisted exercises commenced.
Discharge: Typically day 2-4 depending on mobility and social support. Goals: Walk 50+ meters, negotiate stairs safely, ROM 0-90°. Exercises: Quadriceps strengthening, knee flexion exercises, gait re-education. Follow-up: 2-week wound check (suture/staple removal).
ROM goal: 0-110° by 6 weeks. Strengthening: Progressive resistance exercises, stationary bike. Walking: Wean off walking aid by 4-6 weeks. Return to driving: Typically 4-6 weeks (right knee TKA).
ROM goal: 0-120° by 12 weeks (may plateau at 115-120° - acceptable). Activity: Return to low-impact activities (golf, swimming, cycling). Work: Return to sedentary work by 6-12 weeks depending on demands.
Functional improvement: Continues up to 12 months post-op (pain, stiffness, function). Activity: Return to higher-impact activities (tennis, skiing) by 6-12 months if desired and cleared. Follow-up: 6-week, 3-month, 12-month clinical and radiographic review.
Alignment-Specific Considerations
MA-TKA Rehabilitation
Standard protocol: Same as above - no specific modifications, Ligament releases: May have more stiffness if extensive medial/lateral releases performed, ROM focus: Emphasize early flexion exercises if posterior capsule released, Stability: Test for mid-flexion laxity if over-released - may need brace
KA-TKA Rehabilitation
Faster ROM: Often achieve ROM goals earlier due to preserved ligament lengths, Less stiffness: Typically less post-op stiffness compared to MA (fewer releases), Proprioception: May feel more "natural" earlier - emphasize balance exercises, Monitoring: Same radiographic follow-up - watch for alignment drift
Long-Term Surveillance
| Time Point | Clinical Assessment | Radiographic Assessment | Red Flags |
|---|---|---|---|
| 6 weeks | Wound healing, ROM, gait, pain level | Standing AP and lateral knee - component position | Wound dehiscence, excessive pain, ROM under 70° |
| 3 months | ROM, function, return to activities | Standing long-leg radiograph - confirm alignment | Persistent instability, ROM plateau under 90° |
| 1 year | Oxford Knee Score, Forgotten Joint Score, satisfaction | Standing AP/lateral - radiolucent lines, wear | Radiolucent lines greater than 2mm, component migration |
| Annual thereafter | Symptoms, function, any changes | Every 2-5 years or if symptomatic | New pain, swelling, instability, loss of function |
Alignment Monitoring in KA-TKA
Kinematic alignment patients require specific attention to alignment on follow-up long-leg radiographs. Confirm HKA angle is maintained (should match immediate post-op). Any drift toward further varus (in constitutional varus patients) may indicate medial component subsidence or polyethylene wear - requires close monitoring and may warrant earlier revision.
Patient Education and Expectations
Setting Realistic Expectations
Regardless of alignment philosophy, counsel patients that TKA outcomes mature over 12 months. Peak pain relief at 3-6 months, peak function at 6-12 months. Approximately 80-90% patient satisfaction overall (varies by alignment type - KA may be higher). Realistic ROM expectations: 0-115° is acceptable, 0-120° is excellent. Activities to avoid: High-impact running, jumping sports, contact sports. Implant longevity: Greater than 95% survivorship at 15 years with mechanical alignment - kinematic alignment long-term data still emerging.
Postoperative care for TKA is similar across alignment philosophies - the key difference is patient-reported satisfaction and "naturalness" which may be higher with kinematic alignment.
Outcomes and Evidence Base
Patient-Reported Outcomes
| Outcome Measure | Mechanical Alignment | Kinematic Alignment | Clinical Significance |
|---|---|---|---|
| Forgotten Joint Score (FJS) | 78-82 at 2 years | 85-88 at 2 years | Higher is better - KA patients more likely to forget artificial joint |
| Oxford Knee Score (OKS) | 38-42 at 2 years | 40-43 at 2 years | Minimal clinically important difference 5 points - no clear advantage |
| WOMAC Function | 85-90 at 2 years | 88-92 at 2 years | Small improvement with KA - unclear if sustained long-term |
| Patient Satisfaction | 80% satisfied (20% dissatisfied) | 85-90% satisfied (10-15% dissatisfied) | Consistent trend favoring KA in multiple studies |
Forgotten Joint Score (FJS)
The Forgotten Joint Score asks patients how often they are aware of their artificial joint during activities of daily living. Higher scores (range 0-100) indicate better ability to "forget" the joint. KA consistently shows 5-10 point advantage over MA at 2-5 years - this is the strongest patient-reported outcome difference between alignment philosophies.
Radiographic Outcomes and Survivorship
Evidence Timeline by Follow-Up Duration
Multiple RCTs (Dossett 2014, Calliess 2017, Waterson 2016) show non-inferior or superior patient satisfaction with KA vs MA. No difference in complications, revision rates, or radiographic loosening at 2 years. FJS scores favor KA by 5-10 points.
Young 2017 (5-year RCT): No difference in revision rates, radiographic loosening, or WOMAC scores. KA maintained higher FJS (85 vs 80). McEwen 2020 (5-year cohort): Similar survivorship MA vs KA (98% vs 97%). No edge loading failures in KA group (HKA range 0-5° varus).
Howell 2013 (10-year case series): 98% survivorship for KA-TKA. No aseptic loosening. Maintained constitutional alignment (average 3° varus). Limitation: Case series, not RCT. No direct MA comparison.
Australian Registry (AOANJRR): MA malalignment greater than 3° associated with increased revision. KA data limited (small numbers). UK Registry: No clear advantage for KA or MA - both show excellent survivorship if alignment within safe range.
Evidence Quality Assessment
Current evidence is Level 1-2 for short-term (2-5 years) showing non-inferior or superior patient satisfaction with KA. No Level 1 evidence beyond 5 years comparing KA to MA. No Level 1 evidence beyond 10 years for any alignment philosophy - we rely on registry data and case series for long-term survivorship.
Evidence Base and Key Studies
Does Kinematic Alignment Improve Short-Term Functional Outcomes After TKA? (Dossett et al.)
- Retrospective cohort: 88 KA-TKA vs 88 MA-TKA matched pairs
- KA showed superior WOMAC function scores at 6 months (90 vs 85, p less than 0.05)
- KA patients had higher satisfaction (91% vs 80%, p less than 0.05)
- No difference in complications or early revision rates
Kinematic vs Mechanical Alignment in TKA: A Randomized Controlled Trial (Calliess et al.)
- RCT: 80 patients randomized to KA vs MA using patient-specific instrumentation
- KA showed higher Forgotten Joint Score at 2 years (88 vs 78, p less than 0.01)
- No difference in Oxford Knee Score or complications
- KA group had less ligament releases (15% vs 45%, p less than 0.001)
- Both groups showed 0° HKA on average (restricted KA used - limited to 3° varus)
Kinematic Alignment in TKA Better Reproduces Normal Gait Than Mechanical Alignment (Blakeney et al.)
- Prospective cohort: 40 KA-TKA vs 40 MA-TKA, gait analysis at 1 year
- KA showed gait pattern closer to healthy controls (knee adduction moment)
- MA patients showed abnormal gait kinematics despite correct alignment
- No difference in clinical scores between groups at 1 year
No Difference in Clinical Outcomes Between Kinematic and Mechanical Alignment at 5 Years (Young et al.)
- RCT: 99 patients randomized to KA vs MA with 5-year follow-up
- No difference in revision rates (2% vs 1%, p equals 0.6)
- No difference in radiographic loosening or polyethylene wear
- KA maintained higher Forgotten Joint Score (85 vs 80, p less than 0.05)
- Both groups showed excellent survivorship (98% vs 99%)
Restricted Kinematic Alignment Leads to Similar Patient Satisfaction as Kinematic Alignment (MacDessi et al.)
- Prospective cohort: 100 restricted KA (HKA 0° ± 3°) vs 100 unrestricted KA
- No difference in Forgotten Joint Score at 2 years (87 vs 88, p equals 0.7)
- Restricted KA had no outliers greater than 3° HKA (by definition)
- Unrestricted KA had 12% patients with HKA greater than 3° varus
- No edge loading failures in either group at 2 years
Australian Registry Data: TKA Alignment and Revision Risk (AOANJRR Annual Report)
- Over 500,000 TKAs analyzed for alignment and revision risk
- MA-TKA with HKA greater than 3° varus or valgus shows increased revision rate (hazard ratio 1.4)
- Neutral alignment (HKA 0° ± 3°) associated with lowest revision rate
- Limited data on KA-TKA (small numbers, recent technique adoption in Australia)
- No clear signal of early KA failures in registry to date
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Alignment Philosophy Discussion (2-3 min)
"A 65-year-old active male presents with end-stage medial compartment osteoarthritis. Standing long-leg radiograph shows constitutional varus alignment of 4° (HKA 4° varus) bilaterally. He has read about kinematic alignment online and asks if it is appropriate for him. Discuss your approach to TKA alignment in this patient."
Scenario 2: Kinematic Alignment Surgical Technique (3-4 min)
"You have decided to perform kinematic alignment TKA using the caliper technique (Howell method). Walk me through your surgical technique for distal femoral and proximal tibial bone cuts in a patient with medial compartment osteoarthritis."
Scenario 3: Alignment Complication Management (2-3 min)
"You performed a kinematic alignment TKA on a patient with constitutional varus of 5°. You restored the full 5° varus alignment. At 18 months post-operatively, the patient presents with medial knee pain and difficulty with activities. Radiographs show maintained 5° varus alignment and subtle medial tibial component subsidence. Discuss your assessment and management."
MCQ Practice Points
Mechanical Axis Definition Question
Q: What is the mechanical axis of the lower limb and how does it differ from the anatomical axis? A: The mechanical axis is a line drawn from the center of the femoral head to the center of the ankle (talus dome) on a standing long-leg radiograph. It represents the load-bearing axis of the lower limb. The anatomical axis is the midline of the femoral or tibial shaft. The femoral mechanical axis typically differs from the femoral anatomical axis by approximately 5-7° in the coronal plane (anatomical axis is more varus). This 5-7° difference is why the distal femoral cut in mechanical alignment TKA is made 5-7° valgus relative to the anatomical axis (referenced by intramedullary alignment rod) to achieve a cut perpendicular to the mechanical axis.
Hip-Knee-Ankle (HKA) Angle Question
Q: What is the normal HKA angle and what does deviation from neutral signify? A: The HKA angle is measured on a standing long-leg radiograph as the angle between the femoral mechanical axis and the tibial mechanical axis. Normal average is 1.3° varus (range 3° varus to 3° valgus) - representing constitutional alignment. Mechanical alignment TKA targets 0° ± 3° (neutral alignment). Deviations beyond 3° from neutral (i.e., greater than 3° varus or greater than 3° valgus) are associated with increased revision rates in mechanical alignment TKA based on registry data. Kinematic alignment accepts constitutional alignment (typically 3° varus) as the target, while restricted kinematic alignment limits to HKA 0° ± 3° to avoid extreme outliers.
Kinematic Alignment Principle Question
Q: What is the fundamental principle of kinematic alignment and how does it differ from mechanical alignment? A: The fundamental principle of kinematic alignment is to restore the native joint line obliquity and ligament isometry by performing co-level bone resections that account for cartilage wear. This reproduces the patient's pre-arthritic (constitutional) alignment, typically 3° varus HKA. The technique uses caliper measurement of remaining cartilage on medial and lateral sides, then resects bone equal to implant thickness PLUS cartilage wear on each side independently. This contrasts with mechanical alignment which creates perpendicular cuts to the mechanical axis (horizontal joint line) and targets neutral 0° HKA alignment regardless of constitutional alignment. KA preserves native ligament lengths (minimal releases), while MA requires systematic ligament balancing to achieve rectangular gaps.
Restricted Kinematic Alignment Boundaries Question
Q: What are the safe zone boundaries for restricted kinematic alignment and what is the rationale? A: Restricted kinematic alignment (rKA) combines kinematic principles with defined safety boundaries to avoid extreme outliers. The HKA safe zone is 0° ± 3° (same as traditional mechanical alignment acceptable range). Component position limits are: femoral component 5° varus to 10° valgus from mechanical axis, tibial component 5° varus to 5° valgus from mechanical axis, and tibial slope 0-7° posterior. The rationale is based on (1) historical mechanical alignment registry data showing increased failure rates with HKA deviation beyond 3°, (2) concern for edge loading with extreme varus or valgus causing rim contact on polyethylene, and (3) theoretical risk of accelerated wear or aseptic loosening with asymmetric loading patterns. rKA allows patient-specific alignment restoration when constitutional anatomy is within safe boundaries, but adjusts alignment (via bone cuts or ligament releases) if exceeding thresholds.
Forgotten Joint Score (FJS) Question
Q: What is the Forgotten Joint Score and what does the evidence show comparing kinematic vs mechanical alignment? A: The Forgotten Joint Score (FJS) is a patient-reported outcome measure (scale 0-100, higher is better) that assesses how often a patient is aware of their artificial joint during activities of daily living. Questions focus on joint awareness during walking, stairs, sitting, and recreation. KA consistently shows 5-10 point higher FJS compared to MA at 2-5 year follow-up in multiple randomized controlled trials (e.g., Calliess 2017: KA 88 vs MA 78, p less than 0.01). This represents the most consistent patient-reported advantage of KA over MA - patients report a more "natural" feeling knee and better ability to forget they have an implant. However, whether this advantage is sustained beyond 5-10 years is unknown, and FJS is a surrogate outcome - it does not measure implant survivorship or complications.
Evidence Quality for Long-Term Survivorship Question
Q: What is the longest follow-up data available for kinematic alignment TKA and how does it compare to mechanical alignment? A: Mechanical alignment has over 40 years of registry data with multiple studies showing greater than 95% survivorship at 15 years. This represents the gold standard for long-term implant survivorship. Kinematic alignment has maximum 10 years follow-up (Howell 2013 case series showing 98% survivorship), with most randomized controlled trials limited to 2-5 years. No Level 1 evidence exists beyond 5 years comparing KA directly to MA. The longest RCT (Young 2017) showed non-inferior survivorship at 5 years (KA 98% vs MA 99%) with maintained higher Forgotten Joint Scores for KA. The critical knowledge gap is whether KA's constitutional varus alignment and asymmetric loading pattern leads to accelerated polyethylene wear or aseptic loosening at 15-20 years - this remains unknown. Registry data (AOANJRR) has limited KA numbers due to recent technique adoption. Patients must be counseled about this uncertainty when choosing KA.
Australian Context and Medicolegal Considerations
AOANJRR Data on Alignment
Registry tracking: Over 500,000 TKAs analyzed for alignment-revision correlation, MA malalignment finding: HKA greater than 3° varus or valgus shows hazard ratio 1.4 for revision, Neutral alignment optimal: HKA 0° ± 3° associated with lowest revision rate, Limited KA data: Small numbers due to recent adoption - no clear early failure signal, Safe zone concept: Australian registry data supports 0° ± 3° boundary
Australian TKA Guidelines
ACSQHC Hip and Knee Clinical Care Standard: Emphasizes patient-centered care and shared decision-making, Alignment discussion: Surgeons should discuss alignment options with patients when appropriate, Informed consent: Disclose limited long-term data for KA (under 10 years) compared to MA (over 40 years), Public vs private sector: MA remains predominant in public hospitals (cost, familiarity), KA adoption: Growing in private sector, high-volume centers with robotic or PSI technology
Medicolegal Considerations
Informed Consent for Kinematic Alignment
Key documentation requirements when performing kinematic alignment:
- Discuss alignment options with patient and document rationale for choosing KA (e.g., patient desire for natural feeling knee, constitutional varus restoration)
- Counsel on evidence: KA shows superior patient satisfaction short-term (2-5 years) but lacks long-term survivorship data beyond 10 years compared to MA (40+ years proven survivorship)
- Disclose uncertainty: Unknown risk of accelerated polyethylene wear or aseptic loosening with constitutional varus alignment - theoretical concern not yet proven in practice
- Restricted vs unrestricted: If using restricted KA, explain safety boundaries (HKA 0° ± 3°). If using unrestricted KA, explicitly counsel on outlier risk (HKA greater than 5° may increase edge loading risk)
- Alternative offered: Document that mechanical alignment was discussed as the traditional gold standard alternative
- Patient understanding: Confirm patient understands the trade-off (potential satisfaction benefit vs long-term uncertainty) and accepts this
Common litigation issues in TKA alignment:
- Failure to discuss options: Patient alleging they were not offered kinematic alignment when it may have been appropriate
- Early failure of KA: Patient with extreme varus alignment (greater than 5°) developing early loosening - failure to use restricted boundaries
- Patient dissatisfaction with MA: Patient reading about KA post-operatively and questioning why it was not offered - document discussion pre-operatively
Australian Hospital Systems and Practice Patterns
| Setting | Predominant Alignment | Considerations | Future Trends |
|---|---|---|---|
| Public hospitals (teaching) | Mechanical alignment 90%+ | Training programs teach MA as foundation, limited PSI/robotic access | Gradual KA adoption as trainees gain experience |
| Private hospitals (high-volume) | MA 60%, KA 30%, restricted KA 10% | Robotic systems available, patient-specific instrumentation access | Shift toward restricted KA as compromise approach |
| Rural/regional centers | Mechanical alignment greater than 95% | Limited technology access, surgeon familiarity with MA, lower volume | MA likely to remain predominant |
Australian surgeons should be familiar with both mechanical and kinematic alignment principles - the trend is toward individualized alignment based on patient anatomy and expectations.
TKA ALIGNMENT OPTIONS
High-Yield Exam Summary
Alignment Definitions
- •Mechanical axis = Hip center to ankle center (load-bearing axis)
- •HKA angle = Angle between femoral and tibial mechanical axes (normal 1.3° varus)
- •Anatomical axis = Femoral/tibial shaft midline (differs from mechanical by 5-7°)
- •Joint line obliquity = Native distal femur 3° valgus, proximal tibia 3° varus
Three Main Philosophies
- •Mechanical Alignment (MA): Neutral 0° ± 3° HKA, perpendicular cuts, 40+ years data
- •Kinematic Alignment (KA): Restore native joint line, co-level resection, 10 years max data
- •Restricted KA: Kinematic within HKA 0° ± 3° safe zone - compromise approach
- •MA = 95%+ survivorship 15 years | KA = Higher forgotten joint scores 2-5 years
MA-TKA Technique
- •Distal femur: 5-7° valgus from anatomic axis (9mm resection), perpendicular to mechanical
- •Proximal tibia: 0° perpendicular to mechanical axis (8-10mm resection)
- •Femoral rotation: 3° external rotation from posterior condylar axis
- •Gap balancing: Rectangular extension gap, parallel flexion gap via ligament releases
KA-TKA Technique
- •Measure cartilage wear with calipers on medial and lateral sides
- •Co-level resection: Bone cut = Implant thickness + Cartilage wear (each side independent)
- •Femoral rotation: 0° from posterior condylar axis (not 3° external rotation)
- •No ligament balancing - preserve native tension, minimal releases
Restricted KA Boundaries
- •HKA safe zone: 0° ± 3° (cap varus/valgus at 3° boundary)
- •Femoral component: 5° varus to 10° valgus from mechanical axis
- •Tibial component: 5° varus to 5° valgus from mechanical axis
- •Edge loading risk if HKA greater than 5° - restrict to prevent catastrophic outlier