TKA SURGICAL APPROACHES
Medial Parapatellar | Subvastus | Midvastus | Lateral Approaches
TKA SURGICAL APPROACHES
Critical Must-Knows
- Medial parapatellar is the workhorse approach used in 90% of TKA cases worldwide
- Subvastus approach preserves VMO insertion on patella - better early quadriceps function
- Midvastus approach splits VMO obliquely at 45 degrees - balance of exposure and preservation
- Lateral approach indicated for severe valgus deformity over 20 degrees or failed medial releases
- Quadriceps snip or V-Y turndown reserved for revision or difficult primary exposure
Examiner's Pearls
- "Medial parapatellar divides VMO from capsule - does NOT cut VMO muscle belly
- "Subvastus approach contraindicated if prior high tibial osteotomy due to scar tissue
- "Midvastus preserves nerve to VMO which enters muscle 9-12 cm proximal to patella
- "Lateral approach requires careful IT band release to avoid patella maltracking
Clinical Imaging
Imaging Gallery
Critical TKA Approach Exam Points
Approach Selection
Primary determinant is deformity type. Varus/neutral use medial approaches. Valgus over 20 degrees needs lateral approach. Revision may need extensile techniques (quadriceps snip, V-Y turndown, tibial tubercle osteotomy).
Anatomy at Risk
Nerve to VMO enters muscle 9-12 cm proximal to superior pole of patella. Medial parapatellar is safe. Midvastus risks nerve if split too proximal. Subvastus preserves nerve entirely but limited exposure.
Extensor Mechanism Preservation
Quadriceps-sparing approaches (subvastus, midvastus) improve early rehab and flexion. Subvastus best for straight knees with minimal deformity. Midvastus offers balance of exposure and preservation. All preserve patellar blood supply.
Extensile Techniques
Quadriceps snip, V-Y turndown, tibial tubercle osteotomy reserved for stiff knees, revision, or ankylosis. Quadriceps snip extends parapatellar incision 2-3 cm into vastus lateralis obliquus (VLO) at 45 degrees. V-Y for severe stiffness.
Quick Approach Selection Guide
TKA Approach Decision Matrix
| Clinical Scenario | Approach | Key Advantage | Main Limitation |
|---|---|---|---|
| Primary TKA, varus/neutral, standard exposure | Medial Parapatellar | Familiar, versatile, extensile if needed | Disrupts VMO-capsule interface |
| Primary TKA, straight knee, minimal deformity | Subvastus | Preserves VMO insertion, faster quad recovery | Limited exposure, contraindicated if HTO scar |
| Primary TKA, mild-moderate deformity | Midvastus | Good exposure, preserves nerve to VMO | Must keep split under 5 cm to avoid nerve injury |
| Severe valgus deformity over 20 degrees | Lateral | Direct release of tight lateral structures | Unfamiliar anatomy, risk of patellar maltracking |
| Revision TKA, stiff knee, limited exposure | Quadriceps Snip or V-Y Turndown | Extensile exposure, protects extensor mechanism | Longer rehab, risk of extensor lag |
This table guides exam viva answers when asked to justify approach selection.
Mnemonics for TKA Approaches
At a Glance
The medial parapatellar approach is the workhorse for TKA, used in 90% of cases worldwide—it divides the VMO-capsule interface (not the muscle belly) providing excellent exposure for all deformities. Quadriceps-sparing approaches (subvastus, midvastus) preserve the extensor mechanism with better early rehabilitation and 15-20° improved early flexion, but have limited exposure. The lateral approach is reserved for severe valgus deformity over 20° where it facilitates correction without excessive medial release. The nerve to VMO enters 9-12 cm proximal to the patella—midvastus risks injury if the split extends too proximally. Extensile techniques (quadriceps snip, V-Y turndown, tibial tubercle osteotomy) are reserved for revision or stiff knees.
PIRATEMedial Parapatellar Approach Steps
Memory Hook:Like a PIRATE navigating the knee - Position, Incision, Retinaculum, Arthrotomy, Turn patella, Excise fat pad!
QUADSubvastus Approach Advantages
Memory Hook:QUAD approach preserves the QUAD - goes UNDER VMO for faster recovery but limited exposure!
SPLITMidvastus Approach Technique
Memory Hook:SPLIT the VMO but stay under 5 cm to preserve nerve and get faster recovery!
Overview and Epidemiology
Why Approach Matters in TKA
The surgical approach to the knee in TKA balances adequate exposure for bone cuts and implant placement with preservation of the extensor mechanism for early rehabilitation. The medial parapatellar approach remains the gold standard due to versatility and familiarity. Quadriceps-sparing approaches (subvastus, midvastus) have gained popularity for faster recovery in selected patients with minimal deformity.
Historical Evolution
- 1970s-1980s: Medial parapatellar standard (Insall)
- 1991: Hoffman introduced subvastus approach
- 1999: Engh popularized midvastus for primary TKA
- 2000s: Minimally invasive techniques (MIS) evaluated
- Current: Return to focus on exposure quality over incision size
Current Practice Patterns
- United States: 90% medial parapatellar, 8% quadriceps-sparing
- Europe: Higher midvastus use (15-20%) in specialized centers
- Australia: AOANJRR does not track approach, focus on implant survival
- Revision TKA: Medial parapatellar with extensile techniques
- Minimally invasive: Largely abandoned due to complications
Anatomy and Blood Supply
Nerve to Vastus Medialis Obliquus (VMO)
The nerve to VMO is a branch of the femoral nerve that enters the VMO muscle belly 9-12 cm proximal to the superior pole of the patella on its deep surface. The medial parapatellar approach preserves this nerve by dividing the VMO-capsule interval (not the muscle). The midvastus approach risks nerve injury if the split extends over 5 cm proximal from the superior pole of the patella. The subvastus approach entirely preserves the nerve by staying inferior to the VMO insertion.
Anatomical Structures by Approach
| Approach | Dissection Plane | Structures Divided | Structures Preserved |
|---|---|---|---|
| Medial Parapatellar | VMO-capsule interface | VMO-capsule attachment, medial retinaculum, joint capsule | VMO muscle belly, nerve to VMO, patellar blood supply |
| Subvastus | Under VMO belly | VMO-intermuscular septum, joint capsule | VMO insertion on patella, nerve to VMO, all extensor mechanism |
| Midvastus | Through VMO obliquely | VMO muscle fibers (distal 3-5 cm), joint capsule | Nerve to VMO (if split under 5 cm), VMO proximal insertion |
| Lateral | Lateral retinaculum-capsule | Lateral retinaculum, lateral capsule, possibly IT band | Medial structures, patellar tendon, VMO |
Patellar Blood Supply
The patella receives blood supply from:
- Superior genicular arteries (medial and lateral) - anastomose around superior pole
- Inferior genicular arteries (medial and lateral) - anastomose around inferior pole
- Anterior tibial recurrent artery - supplies anterior surface via fat pad
- Intraosseous supply - from distal femur via retinacular vessels
Avoiding Patellar AVN
Risk factors for patellar AVN after TKA:
- Lateral release combined with medial parapatellar approach (disrupts dual blood supply)
- Excessive fat pad excision (removes anterior blood supply)
- Multiple prior surgeries (scar tissue compromises collaterals)
- Thin patella after resection (less intraosseous reserve)
Prevention:
- Avoid lateral release if possible (use medial release, lateral soft tissue balancing)
- Preserve fat pad distally where blood supply enters
- Consider leaving patella unresurfaced if under 15 mm after resection
Internervous Plane
Total knee arthroplasty approaches utilize planes that avoid direct nerve trunks, though the surgical planes vary by approach. Understanding the internervous concept in TKA differs from typical fracture approaches as the dissection involves periarticular structures rather than muscular intervals.
Medial Parapatellar Approach
The medial parapatellar approach develops the plane between the VMO (femoral nerve) and the joint capsule. No true internervous plane exists as this is an intermuscular rather than internervous dissection. The nerve to VMO enters the muscle 9-12 cm proximal to the patella and is preserved by staying in the VMO-capsule interval.
Subvastus Approach
Dissection occurs beneath the VMO muscle belly, elevating it from the intermuscular septum. The nerve to VMO is entirely preserved as it enters the muscle from the deep surface proximally. This represents the most nerve-preserving approach.
Midvastus Approach
The VMO muscle is split obliquely in line with its fibers. To preserve the nerve to VMO, the muscle split must be limited to less than 5 cm from the superior pole of the patella, staying well below the nerve entry point at 9-12 cm.
Lateral Approach
Develops the plane between the lateral retinaculum and IT band (tibial nerve via sciatic) and the joint capsule. Care must be taken during valgus correction as the peroneal nerve can be stretched.
Positioning and Setup
Standard Supine Position
- Patient: Supine on standard operating table
- Head: Neutral on headrest, arms tucked or on arm boards
- Torso: Flat, avoid lumbar hyperextension
- Operative leg: Foot support (bump or leg holder) for flexion to 90-120 degrees
- Contralateral leg: Abducted and flexed if lateral post used, otherwise straight
Tourniquet Application
- Location: High thigh, as proximal as possible
- Size: 15-20 cm width depending on thigh circumference
- Padding: Cast padding circumferentially under tourniquet
- Pressure: 250-300 mmHg for most adults (100 mmHg over systolic)
- Timing: Inflate after exsanguination, deflate before closure for hemostasis check
Padding and Protection
- Contralateral leg: Pad lateral post contact points (common peroneal nerve at fibular head)
- Heels: Gel pad or foam to prevent pressure ulcers (procedures 1-2 hours)
- Arms: Tuck with arms neutral, avoid abduction over 90 degrees (brachial plexus)
- Sacrum and occiput: Check padding adequate (elderly patients)
Draping and Access
- Landmarks exposed: Entire knee from mid-thigh to tibial tubercle distally
- Prep: Chlorhexidine-alcohol or iodine prep, allow dry time
- Draping: Waterproof drapes, create pocket distally for limb flexion
- C-arm access: Not routinely needed for primary TKA
Approach Selection Algorithm
Primary TKA Approach Selection
Decision Algorithm
- Varus or neutral alignment: Consider medial approaches
- Valgus under 10 degrees: Medial parapatellar standard
- Valgus 10-20 degrees: Medial parapatellar, plan medial releases
- Valgus over 20 degrees: Consider lateral approach for direct release
- Flexion contracture over 30 degrees: Medial parapatellar for extensile capability
- BMI under 35, straight knee: Subvastus or midvastus acceptable
- BMI over 35: Medial parapatellar for better exposure
- Large thighs: Subvastus difficult, use parapatellar or midvastus
- Short patellar tendon: Avoid subvastus (limited eversion)
- Prior HTO scar tissue: Avoid subvastus, use medial parapatellar
- Prior ACL reconstruction: Parapatellar safest for hardware exposure
- Prior patellectomy: Lateral or parapatellar only options
- Prior open reduction internal fixation (ORIF): Follow prior incision if healed
- Learning curve: Start with medial parapatellar for versatility
- Experienced with quadriceps-sparing: Select based on patient factors
- Never done lateral: Arrange assistance or use medial with releases
Default Approach for Primary TKA
Medial parapatellar approach is the default for primary TKA because:
- Familiar anatomy for most surgeons
- Excellent exposure for bone cuts and balancing
- Extensile if needed (quadriceps snip, V-Y turndown)
- Works for all deformity types (varus, valgus, flexion contracture)
- Reliable outcomes in all patient body types
Quadriceps-sparing approaches (subvastus, midvastus) offer faster early recovery but require careful patient selection (straight knee, BMI under 35, no prior surgery). The benefit disappears by 3-6 months in most studies.
Classification of TKA Approaches
TKA Approach Classification
Standard Medial-Based Approaches:
- Medial parapatellar - Most common approach, universal application
- Subvastus - Preserves vastus medialis insertion, less quadriceps damage
- Midvastus - Splits VMO fibers, compromise between access and muscle preservation
Lateral Approaches:
- Lateral parapatellar - For fixed valgus deformity greater than 15-20 degrees
- Trivector retaining - Modified lateral with quadriceps tendon split
TKA Approach Classification Summary
| Approach | Arthrotomy | Quadriceps | Primary Indication |
|---|---|---|---|
| Medial parapatellar | Medial to patella, through VMO tendon | Disrupted at insertion | Standard primary TKA |
| Subvastus | Inferior to VMO belly | Preserved completely | Primary TKA, rapid recovery protocol |
| Midvastus | Through VMO muscle fibers | Partial split | Primary TKA, improved access vs subvastus |
| Lateral parapatellar | Lateral to patella | Disrupted laterally | Fixed valgus deformity |
Extensile Approaches (for Stiff Knees):
- Quadriceps snip - Oblique cut into quadriceps tendon at 45 degrees
- V-Y quadricepsplasty - Inverted V incision with Y closure for lengthening
- Tibial tubercle osteotomy - For severe stiffness or revision TKA
Clinical Assessment for Approach Selection
Preoperative Assessment for Approach Selection
Key Clinical Factors:
-
Deformity Assessment:
- Standing alignment (clinical varus/valgus)
- Fixed vs correctable deformity (examine in supine with manual stress)
- Flexion contracture (inability to fully extend)
- Recurvatum (hyperextension - suggests ligamentous laxity)
-
Range of Motion:
- Flexion arc (normal greater than 120 degrees)
- Extension deficit (flexion contracture)
- Stiff knee (flexion less than 70 degrees) → Need extensile approach
-
Soft Tissue Assessment:
- Prior incisions (use most lateral if multiple)
- Skin quality (thin, compromised, previous radiation)
- Muscle bulk (muscular thigh limits subvastus approach)
- Previous surgery (scarring, adhesions)
Clinical Findings and Approach Implications
| Clinical Finding | Approach Consideration | Rationale |
|---|---|---|
| Varus 5-15 degrees (correctable) | Medial parapatellar | Standard approach, lateral release rarely needed |
| Valgus greater than 15 degrees (fixed) | Lateral parapatellar | Allows controlled lateral release from inside out |
| Flexion less than 70 degrees | Medial parapatellar + extensile | Need quadriceps snip or TTO for exposure |
| Previous lateral incision | Use lateral incision | Avoid skin bridge less than 7 cm between incisions |
| Muscular/obese thigh | Medial parapatellar (not subvastus) | Subvastus exposure inadequate with thick VMO |
Preoperative Imaging for Approach Planning
Standard Preoperative Imaging
Essential Radiographs:
-
Weight-Bearing AP of Knee:
- Assess mechanical axis deviation
- Measure deformity in degrees (lateral distal femoral angle, medial proximal tibial angle)
- Evaluate joint space narrowing pattern
-
Lateral View:
- Assess flexion contracture severity
- Measure patellar height (Insall-Salvati ratio)
- Evaluate tibial slope
- Identify posterior osteophytes (limit flexion)
-
Skyline/Merchant View:
- Patellofemoral arthritis severity
- Patellar tilt and subluxation
- Trochlear dysplasia
-
Hip-to-Ankle Standing Radiograph (Long Leg):
- True mechanical axis measurement
- Femoral and tibial angular deformity
- Extra-articular deformity (prior fracture, metabolic bone disease)
Imaging Findings Affecting Approach Selection
| Imaging Finding | Measurement | Approach Implication |
|---|---|---|
| Valgus deformity | Greater than 15 degrees fixed | Consider lateral parapatellar approach |
| Severe patella baja | Insall-Salvati less than 0.8 | Anticipate difficult patellar eversion - may need TTO |
| Posterior osteophytes | Large (limits flexion) | Remove early for improved exposure |
| Hardware in situ | Plates, screws, IM nail | May need extensile approach for hardware removal |
Approach Selection Algorithm
Approach Selection Summary
Approach Selection Algorithm
Varus or Neutral Deformity (less than 15 degrees):
- Medial parapatellar = Gold standard
- Alternative: Subvastus or midvastus if surgeon preference/rapid recovery protocol
Most surgeons use medial parapatellar for all primary TKA regardless of deformity as it provides reliable exposure and is easiest to extend if needed.
Valgus greater than 15 degrees fixed:
- Lateral parapatellar approach = Preferred
- Allows controlled release of contracted lateral structures
- Maintains blood supply to lateral skin
Alternatively, medial parapatellar with pie-crusting lateral release can work for moderate valgus.
Flexion less than 70-90 degrees:
- Start with standard medial parapatellar
- Add quadriceps snip if still tight
- Use V-Y turndown or TTO if snip inadequate
Plan extensile technique before patellar tendon avulsion occurs - difficult to salvage.
Previous TKA or complex case:
- Use previous incision (most lateral if multiple)
- Anticipate need for extensile technique
- TTO often required for revision exposure
Surgical Technique: Medial Parapatellar Approach
Patient Positioning and Setup
Setup Checklist
Supine on standard operating table with:
- Head: Neutral on headrest, arms tucked or on arm boards
- Torso: Flat, avoid lumbar hyperextension (patients often elderly with stenosis)
- Operative leg: Foot support (bump or leg holder) for flexion to 90 degrees
- Contralateral leg: Abducted and flexed if lateral post used, otherwise straight
- High thigh tourniquet as proximal as possible
- Size: 15-20 cm width depending on thigh circumference
- Padding: Apply cast padding circumferentially under tourniquet
- Pressure: 250-300 mmHg for most adults (100 mmHg over systolic)
- Timing: Inflate after exsanguination, deflate before closure for hemostasis check
- Contralateral leg: Pad lateral post contact points (common peroneal nerve at fibular head)
- Heels: Gel pad or foam to prevent pressure ulcers (procedures 1-2 hours)
- Arms: Tuck with arms neutral, avoid abduction over 90 degrees (brachial plexus)
- Sacrum and occiput: Check padding adequate (elderly patients)
- Landmarks exposed: Entire knee from mid-thigh to tibial tubercle distally
- Prep: Chlorhexidine-alcohol or iodine prep, allow dry time
- Draping: Waterproof drapes, create pocket distally for limb flexion
- C-arm access: Not routinely needed for primary TKA
Positioning Pearl: Foot Support
The operative knee must flex freely to 90-120 degrees intra-operatively for tibial exposure and bone cuts. Use:
- Leg holder (e.g., Spider limb positioner) - hands-free, stable
- Foot bump - simple foam or metal post under foot
- Assistant holding - acceptable but tiring for long cases
Position allows leg to hang off side of table when flexed, with foot supported. This opens posterior capsule and moves neurovascular structures posteriorly away from saw blades during tibial cut.
Proper positioning enables safe surgery and prevents positioning-related complications.
Surgical Technique: Quadriceps-Sparing Approaches
Subvastus (Southern) Approach
Principle: Elevate VMO from its insertion on the intermuscular septum medially, staying inferior to VMO insertion on patella. Preserves extensor mechanism entirely.
Subvastus Technique
Same as medial parapatellar: midline incision from 5 cm proximal to patella to tibial tubercle.
- May be slightly more medial than standard parapatellar
- Length: Same (12-15 cm)
Develop subcutaneous flaps to expose VMO muscle belly medially.
- Medial flap: Wider than parapatellar to fully expose VMO inferior border
- Identify: Inferior border of VMO where it meets intermuscular septum
Key step: Elevate VMO from intermuscular septum using sharp dissection.
- Starting point: Distal attachment of VMO on medial intermuscular septum
- Direction: Elevate VMO superiorly and anteriorly off septum
- Plane: Between VMO muscle belly (superficial) and septum/capsule (deep)
- Extent: Elevate proximally until VMO insertion on patella is visible
- Preserve: VMO insertion on patella completely intact
Incise joint capsule distal to VMO insertion.
- Location: Along inferior border of patella, continue distally along medial patellar tendon
- Proximal extent: Does NOT extend into quadriceps tendon (limited exposure)
- Result: Arthrotomy inferior to patella only
With knee flexed, subluxate patella laterally (not full eversion).
- Subluxation: Displace patella laterally 2-3 cm
- Avoid: Forceful eversion (risk patellar tendon avulsion)
- Exposure: Sufficient for tibial cut, limited for femoral cuts
Challenges: Limited exposure for femoral cuts, difficult in obese patients.
- Tibial cut: Excellent visualization
- Femoral cuts: More difficult, may need to externally rotate tibia, lift femur
- Balancing: Can be done but limited space
- If inadequate: Convert to medial parapatellar intra-operatively
Subvastus Limitations
Contraindications:
- BMI over 35 (large thighs prevent adequate VMO elevation)
- Short patellar tendon (limits subluxation distance)
- Prior HTO or medial surgery (scar tissue in intermuscular septum)
- Flexion contracture over 15 degrees (need extensile exposure)
- Varus deformity over 10 degrees (need extensive medial releases)
If exposure is inadequate after VMO elevation, do not hesitate to convert to medial parapatellar by extending arthrotomy into quadriceps tendon. Trying to operate through inadequate exposure risks malposition and complications.
Subvastus Advantages
Benefits of subvastus approach:
- Preserves VMO insertion on patella (faster quadriceps recovery)
- Preserves nerve to VMO (enters 9-12 cm proximal, entirely avoided)
- Better early flexion (15-20 degrees more at 6 weeks in some studies)
- Less postoperative pain (VMO not violated)
- No difference in outcomes by 3-6 months (benefit is early only)
Best for: Thin patients, straight knees, minimal deformity, experienced surgeon.
This approach is quadriceps-sparing but exposure-limiting.
Surgical Technique: Lateral Approach
Lateral Parapatellar Approach
Indication: Severe valgus deformity over 20 degrees where medial soft tissue releases insufficient, or prior lateral incision requiring lateral approach.
Lateral Approach Technique
Same as medial approach: supine with foot support.
- Some surgeons prefer slight external rotation of operative leg for easier access to lateral knee
Midline or slightly lateral incision from 5 cm proximal to patella to tibial tubercle.
- If prior medial scar: May need to use lateral incision separate from prior scar (skin bridge over 7-8 cm required)
Incise lateral retinaculum and capsule along lateral border of patella.
- Proximal: Extend into quadriceps tendon laterally (mirror of medial parapatellar)
- Patellar: Along lateral edge of patella
- Distal: Along lateral edge of patellar tendon to Gerdy's tubercle (IT band insertion)
- Depth: Full thickness through retinaculum, capsule, into joint
Release tight lateral structures for valgus deformity correction.
- IT band: May need to divide or lengthen if contracted (Z-plasty or release from Gerdy's tubercle)
- LCL: Often tight in valgus - pie-crust release or elevate from lateral epicondyle
- Popliteus: May need to release popliteus tendon from lateral femoral condyle
- Posterolateral capsule: Release as needed for full correction
Evert patella medially (opposite direction from medial approaches).
- Unfamiliar: Most surgeons not used to medial patellar eversion
- Exposure: Excellent for valgus knee, direct access to lateral structures
Lateral closure must avoid over-tightening (risk of lateral patellar tilt).
- Suture: 1 Vicryl interrupted
- Tension: Loose closure - lateral retinaculum does NOT need to be tight
- Check: Patellar tracking (should centralize, no lateral tilt or subluxation)
- IT band: If Z-plasty performed, lengthen IT band to avoid lateral patellar pull
Lateral Approach Complications
Risks unique to lateral approach:
- Patellar maltracking: If lateral closure too tight or IT band not released adequately
- Lateral patellar tilt: Over-tightened lateral retinaculum pulls patella laterally
- Skin necrosis: If prior medial scar with skin bridge under 7-8 cm
- Peroneal nerve palsy: Valgus correction can stretch peroneal nerve (protect during surgery)
Prevention:
- Loose lateral closure (just approximate, do not tension)
- Check patellar tracking before final closure (no Q-angle, central tracking)
- Consider lateral retinacular release at closure if patella still tilted laterally
- Monitor peroneal nerve postoperatively (check dorsiflexion and sensation)
When to Use Lateral Approach
Indications:
- Severe valgus deformity over 20 degrees (most common indication)
- Failed medial soft tissue release intra-operatively (tight lateral structures persist)
- Prior lateral incision requiring same approach
- Lateral compartment pathology requiring direct access (tumor, chondral lesion)
Contraindications:
- Most valgus knees can be corrected via medial approach with medial releases and lateral balancing
- Lateral approach should be last resort due to unfamiliarity and patellar tracking risks
- If in doubt, start medial and convert to lateral only if truly needed
The lateral approach is rarely needed but essential for severe valgus deformity.
Complications
Approach-Specific Complications
| Complication | Incidence | Risk Factors | Management |
|---|---|---|---|
| Patellar tendon avulsion | Under 1% (higher in revision) | Forced eversion, stiff knee, osteoporotic bone, revision surgery | Immediate repair with FiberWire or cerclage wire. If tissue poor, consider Achilles allograft augmentation. Protect 6-8 weeks. |
| Extensor lag (quadriceps weakness) | 5-10% early (most resolve), 2-3% persistent | V-Y turndown, quadriceps snip, poor rehab, elderly, disuse atrophy | Physiotherapy, quadriceps strengthening. Persistent lag may need functional brace. Consider extensor mechanism reconstruction if severe. |
| Patellar maltracking (tilt, subluxation) | 3-5% (higher with lateral approach) | Lateral approach with tight closure, component malrotation, inadequate lateral release | If early and symptomatic: lateral retinacular release. If persistent: assess component rotation, consider revision if malrotated. |
| Patellar avascular necrosis (AVN) | 1-2% (mostly asymptomatic) | Lateral release combined with medial parapatellar, prior surgery, excessive fat pad excision | Most asymptomatic. If painful: observation, NSAIDs. If fracture or component loosening: revision with patellectomy or bone graft. |
| Wound dehiscence or skin necrosis | 2-3% (higher in obese, diabetic, multiple scars) | Tight closure, hematoma, obesity, diabetes, smoking, prior surgery with skin bridges under 7-8 cm | Prevent with loose closure, hemostasis. If dehiscence: local wound care or surgical debridement and closure. May need gastrocnemius flap. |
| Tibial tubercle osteotomy nonunion | Under 5% with good fixation | Inadequate fixation, early weight-bearing, osteoporosis, smoking | Protected weight-bearing 6-8 weeks. If nonunion: revision fixation with cerclage and bone graft. |
| Nerve injury (nerve to VMO, peroneal) | Under 1% (mostly neurapraxia) | Midvastus split over 5 cm (VMO nerve), valgus correction (peroneal stretch), lateral approach with aggressive releases | Most resolve spontaneously. Neurapraxia typically recovers in 3-6 months. Permanent deficit rare. Nerve studies if no recovery by 6 months. |
Preventing Extensor Mechanism Catastrophe
The extensor mechanism is the Achilles heel of TKA. To avoid disaster:
- Never force patellar eversion - add quadriceps snip or extensile technique if needed
- Preserve patellar blood supply - avoid lateral release if possible, preserve fat pad distally
- Avoid over-tightening closure - capsular closure should be snug but not under tension
- Protect extensile closures - V-Y turndown or TTO require protected weight-bearing 6-8 weeks
- Recognize extensor lag early - aggressive physiotherapy prevents chronic weakness
If extensor mechanism fails (avulsion, rupture), salvage is difficult and outcomes are poor. Prevention is key.
Postoperative Care and Rehabilitation
Rehabilitation After Medial Parapatellar, Subvastus, or Midvastus
Rehabilitation Timeline
- Analgesia: Multimodal (regional block, oral analgesics, NSAIDs if not contraindicated)
- DVT prophylaxis: Aspirin 325 mg daily for 6 weeks (low-risk), LMWH or DOAC if high-risk
- Mobilization: Out of bed to chair on day 0 or 1, full weight-bearing as tolerated
- Dressing: Soft compressive dressing, check for bleeding or swelling
- Ice: Cryotherapy or ice packs for pain and swelling control
- Weight-bearing: Full weight-bearing as tolerated with walker or crutches
- Range of motion: Physiotherapy twice daily, goal 0-90 degrees by week 2
- Quadriceps activation: Straight leg raises, quad sets, ankle pumps
- Wound care: Dressing change at 48-72 hours, remove sutures/staples at 10-14 days
- Anticoagulation: Continue DVT prophylaxis for 4-6 weeks total
- Gait: Wean off walker to cane by week 4-6, then independent ambulation
- Flexion: Goal 0-110 degrees by week 6 (most functional ROM achieved)
- Strengthening: Resistance exercises, closed-chain activities (leg press, mini squats)
- Activities: Stairs, return to driving (6 weeks for right knee TKA), light activities
- Follow-up: Clinic visit at 6 weeks for X-rays (assess alignment, component position)
- Flexion: Goal 0-120 degrees by 12 weeks (maximal ROM typically achieved)
- Strengthening: Progressive resistance, balance training, functional activities
- Return to work: Light work 6-8 weeks, heavy work 12 weeks
- Return to sport: Low-impact (cycling, swimming) at 12 weeks, golf at 3-4 months
- Discharge: Most patients independent by 12 weeks, outpatient physiotherapy as needed
- Clinical follow-up: 6 weeks, 3 months, 1 year, then every 2-5 years
- Radiographs: 6 weeks, 1 year, then if symptomatic (no routine X-rays needed if asymptomatic)
- Activity: Return to high-impact sports discouraged (running, jumping), low-impact encouraged
- Surveillance: Monitor for loosening, osteolysis, infection, instability
DVT Prophylaxis in TKA - Australian Guidelines
ACSQHC (Australian Commission on Safety and Quality in Health Care) recommendations:
- Mechanical prophylaxis: Early mobilization, foot pumps, compression stockings
- Pharmacologic: Aspirin 325 mg daily for 6 weeks (standard risk), LMWH or DOAC for high-risk
- High-risk factors: Prior VTE, thrombophilia, cancer, prolonged immobility, obesity BMI over 40
- Duration: 4-6 weeks total (most VTE occurs in first 6 weeks)
AOANJRR: Does not track VTE rates, but infection and revision rates suggest aspirin is safe and effective for most patients.
This standard protocol applies to over 90% of TKA patients.
Outcomes and Prognosis
Approach Outcomes Comparison
| Approach | Early Flexion (6 weeks) | Final ROM (1 year) | Complications | Notes |
|---|---|---|---|---|
| Medial Parapatellar | 85-95 degrees | 0-115 degrees | Baseline comparator | Gold standard, reliable, versatile |
| Subvastus | 95-110 degrees (15-20 degree advantage) | 0-115 degrees (same) | Same as parapatellar | Faster early recovery, no long-term difference |
| Midvastus | 90-105 degrees (5-10 degree advantage) | 0-115 degrees (same) | Same as parapatellar | Balanced approach, popular in Europe |
| Lateral | Variable (depends on deformity) | 0-110 degrees | Higher patellar maltracking (5-10%) | Reserved for severe valgus |
| Extensile (Snip, V-Y, TTO) | 60-80 degrees (limited early) | 0-100 degrees (10-15 degree loss) | Higher extensor lag (10-30%) | Necessary evil for difficult cases |
Approach Selection Does NOT Affect Implant Survival
Key evidence: Multiple RCTs and registry studies show no difference in implant survival between approaches at 10-15 years. The approach affects:
- Early recovery (quadriceps-sparing approaches faster 0-6 weeks)
- Final ROM (minimal difference by 1 year)
- Complications (approach-specific risks differ)
What matters more:
- Component alignment and positioning
- Soft tissue balancing
- Implant selection
- Patient factors (age, BMI, activity level)
- Surgeon experience
Choose the approach you are comfortable with and that suits the patient's anatomy. Do not sacrifice exposure for a minimally invasive technique.
Prognostic Factors for TKA Outcomes
Favorable prognostic factors:
- Age under 75 (better functional outcomes)
- BMI under 35 (lower complications)
- Pre-operative ROM over 90 degrees (better final ROM)
- No prior surgery (easier surgery, better outcomes)
- High activity level and motivation (better function)
Unfavorable prognostic factors:
- Severe pre-operative stiffness (ROM under 60 degrees)
- Morbid obesity (BMI over 40)
- Multiple comorbidities (diabetes, cardiac, renal)
- Chronic pain syndromes (fibromyalgia, central sensitization)
- Poor quadriceps strength pre-operatively (higher extensor lag risk)
The approach is a tool, not a determinant of success. Patient selection and surgical execution matter more.
Evidence Base and Key Trials
Subvastus vs Medial Parapatellar RCT
- Randomized trial: 60 patients, subvastus vs medial parapatellar
- Subvastus had better flexion at 6 weeks (95 vs 85 degrees, p less than 0.05)
- No difference in ROM, function, or complications at 1 year
- Operative time 15 minutes longer for subvastus
- Both approaches had excellent outcomes (KSS scores over 90)
Midvastus vs Medial Parapatellar Meta-Analysis
- Meta-analysis of 9 RCTs, 738 knees
- Midvastus had better early flexion (5-10 degrees at 6 weeks)
- No difference in final ROM, pain, function, or complications
- Similar operative time and blood loss
- No difference in implant survival or revision rate
Quadriceps Snip for Difficult TKA Exposure
- Case series: 28 knees requiring quadriceps snip for exposure
- All achieved adequate exposure without tendon avulsion
- Mean flexion 105 degrees at final follow-up (range 85-120)
- No extensor lag in any patient at 1 year
- Technique: 2-3 cm oblique extension into VLO at 45 degrees
V-Y Turndown for Severe Knee Stiffness
- Case series: 22 revision TKA with severe stiffness (flexion under 60 degrees)
- V-Y turndown allowed complete exposure in all cases
- Mean flexion improved from 45 to 95 degrees (50 degree gain)
- Extensor lag occurred in 32% (7 of 22), mostly resolved by 1 year
- No patellar tendon avulsions
Australian Registry Data on TKA Outcomes
- Over 1 million TKA procedures tracked since 1999
- Overall revision rate: 5.8% at 10 years, 7.7% at 15 years
- Approach not tracked by registry (focus on implant survival)
- Primary failure modes: loosening (28%), infection (26%), pain (13%)
- Cruciate-retaining and posterior-stabilized similar survival
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Approach Selection for Primary TKA
"You are planning a primary TKA for a 68-year-old woman with medial compartment osteoarthritis and 5-degree varus deformity. BMI is 28. She asks about minimally invasive techniques she read about online. What approach will you use and why?"
Scenario 2: Intra-operative Difficulty with Patellar Eversion
"You are performing a primary TKA via medial parapatellar approach. After completing the standard arthrotomy, you attempt to evert the patella laterally but it does not move easily. The patella tendon feels tight and you are concerned about avulsion. How do you proceed?"
Scenario 3: Severe Valgus Deformity Requiring Lateral Approach
"You are managing a 55-year-old man with end-stage lateral compartment OA and 25-degree valgus deformity. You plan TKA. What approach will you use and what are the key steps and complications unique to this approach?"
MCQ Practice Points
Anatomy Question
Q: At what distance from the superior pole of the patella does the nerve to vastus medialis obliquus enter the muscle?
A: 9-12 cm proximal to the superior pole of the patella on the deep surface of VMO. This is clinically relevant for the midvastus approach - the VMO split must be limited to under 5 cm from the superior pole (starting 3-4 cm proximal) to avoid denervating the muscle. The medial parapatellar and subvastus approaches preserve this nerve entirely.
Approach Selection Question
Q: What is the most common surgical approach used for primary total knee arthroplasty worldwide?
A: Medial parapatellar approach - used in approximately 90% of primary TKA cases. This approach divides the VMO-capsule interface and medial retinaculum, providing excellent exposure for bone cuts and soft tissue balancing. It is versatile, extensile if needed, and familiar to most surgeons. Quadriceps-sparing approaches (subvastus, midvastus) are used in 5-10% combined.
Subvastus Contraindication Question
Q: What is an absolute contraindication to the subvastus approach for TKA?
A: Prior high tibial osteotomy (HTO) is a contraindication because scar tissue in the intermuscular septum makes VMO elevation extremely difficult and risks muscle injury. Other relative contraindications include BMI over 35 (large thighs prevent adequate elevation), short patellar tendon (limits subluxation), and moderate-severe deformity (inadequate exposure). The subvastus approach is best for straight knees with minimal deformity in non-obese patients.
Extensile Technique Question
Q: How do you perform a quadriceps snip and when is it indicated?
A: A quadriceps snip extends the medial parapatellar incision 2-3 cm into the vastus lateralis obliquus (VLO) at 45 degrees obliquely. The incision should be oblique, not straight lateral, to avoid disrupting the rectus femoris insertion. It is indicated when the patella does not evert easily after a standard medial parapatellar approach, typically in stiff knees with flexion under 90 degrees or in revision cases. It gains 1-2 cm of patellar eversion and is sufficient for most difficult exposures. Closure is with figure-of-eight sutures using heavy absorbable suture.
Lateral Approach Indication Question
Q: What is the primary indication for a lateral parapatellar approach in TKA?
A: Severe valgus deformity over 20 degrees where medial soft tissue releases are insufficient to correct the deformity. The lateral approach provides direct access to tight lateral structures (IT band, LCL, popliteus, posterolateral capsule) for release. However, most valgus knees can be corrected via a medial approach with medial releases and lateral balancing, so lateral approach is reserved for severe cases or when medial releases have failed. The main complication unique to lateral approach is patellar maltracking due to unfamiliar anatomy and over-tight lateral closure.
Patellar Blood Supply Question
Q: What is the risk of combining a lateral retinacular release with a medial parapatellar approach in TKA?
A: The main risk is patellar avascular necrosis (AVN) due to disruption of the patellar blood supply from both medial and lateral sides. The patella receives blood from superior and inferior genicular arteries (medial and lateral branches) that anastomose around the patella, plus anterior blood supply via the fat pad. A medial parapatellar approach disrupts medial blood supply, and adding a lateral release disrupts lateral supply, leaving only the fat pad. To minimize risk, preserve the fat pad distally, avoid circumferential releases if possible, and consider leaving the patella unresurfaced if the remaining thickness is under 15 mm after resection.
Australian Context and Medicolegal Considerations
AOANJRR Data
- Approach not tracked: Registry focuses on implant type and survival, not approach
- Overall revision rate: 5.8% at 10 years for primary TKA
- Primary failure modes: Loosening (28%), infection (26%), pain (13%)
- No difference by approach: Surgeon technique and patient factors more important
- Key message: Focus on component positioning and soft tissue balancing
Australian Guidelines
- ACSQHC: Australian Commission on Safety and Quality in Health Care
- DVT prophylaxis: Aspirin 325 mg daily for 6 weeks (standard risk)
- Antibiotic prophylaxis: Cefazolin 2g pre-op, re-dose if over 4 hours
- Surgical site infection: Target under 1% (national benchmark)
- VTE rate: Target under 0.5% symptomatic VTE
Medicolegal Considerations for TKA Approach
Key documentation requirements:
-
Informed consent:
- Discuss approach options if patient asks (most do not need detail)
- Document discussion of risks: infection, VTE, stiffness, need for revision
- Include specific approach risks if applicable (e.g., lateral approach = maltracking risk)
- Document no guarantees of pain relief or return to high-impact activities
-
Operative note:
- Specify approach used and rationale if non-standard (e.g., lateral for valgus)
- Document any intra-operative complications or difficulties
- If extensile technique used, document indication and closure method
- Include component sizes, alignment, and final ROM
-
Common litigation issues:
- Patellar tendon avulsion: If forced eversion without extensile technique
- Infection: Inadequate prophylaxis or wound care
- Stiffness: Failure to document pre-operative ROM and counsel about limited gain
- Component malposition: Inadequate exposure leading to malposition
-
Defensive documentation:
- Pre-operative ROM, alignment, deformity assessment
- Informed consent discussion of realistic expectations
- Intra-operative decisions and rationale (e.g., why approach changed)
- Postoperative plan and discharge instructions
TKA SURGICAL APPROACHES
High-Yield Exam Summary
Approach Selection
- •Medial parapatellar = 90% of TKA, gold standard for versatility
- •Subvastus = under VMO, best for straight knees, faster early recovery
- •Midvastus = split VMO obliquely under 5 cm, preserves nerve to VMO
- •Lateral = severe valgus over 20 degrees, risk of maltracking
Key Anatomy
- •Nerve to VMO enters 9-12 cm proximal to patella (avoid in midvastus)
- •Patellar blood supply = superior/inferior geniculate arteries + fat pad
- •Medial parapatellar divides VMO-capsule, NOT muscle belly
- •Quadriceps snip = 2-3 cm into VLO at 45 degrees for limited stiffness
Surgical Steps (Medial Parapatellar)
- •Position supine, tourniquet, midline incision 5 cm above to tibial tubercle
- •Arthrotomy: medial border patella, through retinaculum and capsule
- •Elevate VMO from capsule (preserve muscle belly)
- •Evert patella laterally, partial fat pad excision
- •Never force eversion - add quadriceps snip if needed
Extensile Techniques
- •Quadriceps snip = extend into VLO 2-3 cm at 45 degrees, gains 1-2 cm eversion
- •V-Y turndown = V-incision through quad tendon, for ankylosis, high extensor lag risk
- •Tibial tubercle osteotomy = step-cut, for component removal, 6-8 week protected weight-bearing
- •Indications: stiff knee flexion under 90 degrees, revision, difficult exposure
Complications
- •Patellar tendon avulsion = catastrophic, repair with FiberWire/allograft
- •Extensor lag = 5-10% early (most resolve), 20-30% with V-Y (persistent)
- •Patellar AVN = 1-2%, risk if lateral release plus medial approach
- •Patellar maltracking = 3-5%, higher with lateral approach if over-tight closure