Subvastus Approach to Knee

Primary Indications

Primary Total Knee Arthroplasty - Selected Cases

• Straightforward primary TKA in thin to normal-weight patients (BMI less than 30)
• Good range of motion preoperatively (at least 90 degrees flexion)
• No significant deformity (varus/valgus less than 15 degrees)
• No significant flexion contracture (less than 10 degrees)
• Patient desires faster recovery

Advantages Over Medial Parapatellar

• Preserves VMO muscle fibers and innervation
• Faster return of quadriceps function
• Earlier straight leg raise ability (2-3 days vs 7-10 days)
• Better patellar tracking (preserved soft tissue balance)
• Lower incidence of patellofemoral complications

Relative Indications

Bilateral Staged TKA

• Subvastus on first knee for faster rehab
• Allows better function before second knee surgery

Young Active Patients

• Higher functional demands
• Benefit from preserved extensor mechanism

Contraindications

Absolute

• Revision TKA (inadequate exposure)
• Previous open knee surgery with scarring
• Need for extensive ligament releases or balancing

Relative

• Obesity (BMI greater than 30) - very difficult exposure
• Limited preoperative ROM (less than 90 degrees flexion)
• Severe deformity (greater than 15 degrees varus/valgus)
• Significant flexion contracture (greater than 10 degrees)
• Muscular or athletic build (thick VMO makes dissection difficult)
• Patella baja (low-lying patella difficult to evert)
• Short stature (limited operative space)

Patient Selection - CRITICAL

Ideal Candidate Profile

• BMI less than 25-30
• Flexion at least 90 degrees
• Minimal deformity (less than 10 degrees)
• No previous open knee surgery
• Primary osteoarthritis (not inflammatory arthritis)

Poor Candidate Profile

• Obese (BMI greater than 30)
• Stiff knee (flexion less than 90 degrees)
• Severe deformity requiring releases
• Rheumatoid arthritis (bone fragile, difficult dissection)
• Muscular build (thick VMO)

Clinical Assessment

History

• Pain pattern and functional limitations
• Previous knee surgeries or trauma
• Activity level and expectations
• Comorbidities (diabetes, peripheral vascular disease)

Physical Examination

• Range of motion - CRITICAL for approach selection
  • Flexion: minimum 90 degrees needed
  • Extension: document flexion contracture
  • Passive vs active ROM
• Alignment (varus, valgus, neutral)
• Ligamentous stability (cruciate, collateral)
• Patellar tracking and mobility
• BMI calculation and thigh circumference

Imaging Protocol

Standard Radiographs

• AP and lateral knee
• Merchant view (patellar alignment)
• Full-length standing alignment films (mechanical axis)

Measurements

• Mechanical axis deviation
• Severity of joint space narrowing
• Patella height (Insall-Salvati ratio)
  • Less than 0.8 = patella baja (relative contraindication)
  • Normal: 0.8-1.2
• Bone quality assessment

CT or MRI - Selected Cases

• Complex deformity
• Extra-articular deformity planning
• Bone loss assessment

Surgical Planning

Implant Selection

• Standard TKA components (CR vs PS based on PCL integrity)
• Standard instrumentation (no special instruments needed)
• Have thicker polyethylene options available

Anesthesia Discussion

• Spinal vs general anesthesia
• Femoral nerve block consideration
  • Pros: Excellent pain control
  • Cons: Delays quadriceps function (defeats purpose of subvastus)
  • Recommendation: Adductor canal block instead (preserves quadriceps)

Standard TKA Equipment

Basic Instruments

• Standard TKA instrumentation system
• Self-retaining retractors (bent Hohmann, narrow)
• Lamina spreaders (helpful for exposure)
• Army-Navy retractors (superficial)
• Electrocautery

TKA Components

• Femoral component (cruciate-retaining or posterior-stabilized)
• Tibial baseplate and polyethylene insert
• Patellar button (standard)

Specialized Retractors

• Narrow retractors (limited exposure)
• Subperiosteal elevators
• Right-angle retractors

Fluoroscopy

Not Routinely Needed

• Standard TKA technique uses instrumentation
• Consider for:
  • Confirming femoral/tibial cut alignment
  • Assessing component position if concerned

Standard Supine Positioning

Positioning Technique

• Supine on standard operating table
• Small bump under ipsilateral hip (optional - aids exposure)
• Leg holder or post at thigh level
• Tourniquet on proximal thigh

Leg Holder Setup

• Side post allows 90-degree flexion
• Important for subvastus: flexion aids VMO visualization
• Pad carefully (peroneal nerve compression risk)

Foot Position

• Foot of bed free (allow knee flexion)
• OR: leg holder device (hands-free positioning)

Tourniquet Use

Advantages

• Bloodless field for dissection
• Better visualization of VMO muscle fibers
• Shorter operative time

Recommendation

• Use tourniquet for subvastus approach
• Deflate before closure to ensure hemostasis
• Limit time to 90-120 minutes if possible

Bony Landmarks

Palpable Surface Anatomy

• Patella - mobile anteriorly
• Medial epicondyle of femur
• Adductor tubercle (superior to medial epicondyle)
• Tibial tubercle - anterior midline reference
• Medial joint line - 1cm distal to medial epicondyle

Neurovascular Anatomy - CRITICAL

Vastus Medialis Obliquus (VMO) Neurovascular Supply

• Nerve to VMO: A distal branch of the femoral nerve that descends within the adductor (subsartorial / Hunter's) canal and enters the muscle on its deep, proximal surface
• Descending genicular artery: Arises from the superficial femoral artery in the adductor canal, contributing the muscular supply to the VMO; runs with the nerve as a proximal neurovascular pedicle
• Key point for subvastus: The pedicle enters the muscle PROXIMALLY from the adductor canal, so distal subperiosteal elevation of the VMO off the medial femur/tibia leaves the supply intact
• Risk: Injury from excessive PROXIMAL dissection into the muscle substance or aggressive proximal extension of the arthrotomy, not from distal elevation
• Protection: Stay in the avascular subperiosteal plane distally; limit proximal dissection to the level of the superior pole of the patella

Saphenous Nerve and Vein

• Run subcutaneously on medial aspect
• Risk with skin incision
• Usually not visualized if incision properly placed

Popliteal Vessels

• Posterior to joint, safe distance
• Not at risk with standard subvastus approach

Muscular Anatomy

Vastus Medialis Obliquus (VMO)

• Origin: Medial lip of linea aspera, adductor magnus tendon
• Insertion: Medial patella, medial retinaculum
• Function: Terminal 15 degrees of knee extension, patellar stabilization
• Innervation: Femoral nerve (L2-L4), via the nerve to VMO running in the adductor canal
• Key anatomic detail: Muscle fibers run obliquely (approximately 50-55 degrees from the long axis of the femur) toward the patella, distinct from the more vertically oriented vastus medialis longus

Subvastus Plane

• Between: VMO muscle belly and medial tibial periosteum
• Contains: Loose areolar tissue (easy dissection plane)
• Advantage: No muscle cut, preserves VMO innervation

Other Quadriceps Components

• Vastus lateralis (lateral)
• Vastus intermedius (deep)
• Rectus femoris (superficial, anterior)

Step 1: Skin Incision

Incision Planning

• Midline or slightly medial incision
• Proximal extent: 5-6cm proximal to superior pole of patella
• Distal extent: Tibial tubercle or 2cm distal
• Length: 10-15cm (similar to medial parapatellar)

Skin Incision

• Sharp incision through skin and subcutaneous tissue
• Achieve hemostasis with electrocautery
• Identify and protect saphenous vein/nerve if encountered

Subcutaneous Dissection

• Develop medial flap to expose medial border of patella
• Expose VMO muscle belly medially
• Identify distal VMO insertion on medial patella

Step 2: Identify VMO and Subvastus Plane

VMO Identification

• Palpate VMO muscle belly on medial thigh
• Identify muscle fibers running obliquely toward patella
• Trace fibers distally to patellar insertion

Find Subvastus Plane

• Key step: Identify interval UNDER (deep to) VMO
• Palpate medial tibial metaphysis through VMO
• Plane is between VMO muscle and tibial periosteum

Initial Dissection

• Make small incision in fascia at distal VMO
• Use blunt dissection to develop plane between VMO and tibia
• Elevator slides easily if correct plane

Step 3: Subperiosteal VMO Elevation

Medial Tibial Periosteal Elevation

• Begin at medial aspect of tibial tubercle
• Elevate periosteum from medial tibial metaphysis
• Work proximally and posteriorly under VMO

VMO Mobilization

• Blunt elevation of VMO off tibial periosteum
• Sweep VMO muscle belly laterally and proximally
• Preserve VMO muscle fibers (no cutting)

Extent of Dissection

• Proximal limit: To level of superior pole of patella
• Posterior limit: To posteromedial capsule (MCL superficial fibers)
• Distal limit: Tibial tubercle

Retractor Placement

• Place narrow retractor deep to VMO
• Retract VMO laterally to expose joint capsule
• VMO "dome" created above joint

Step 4: Capsular Incision and Arthrotomy

Medial Arthrotomy

• Incise medial capsule and retinaculum
• Start at proximal end of exposure
• Extend distally to tibial tubercle
• Stay medial - do not compromise VMO insertion on patella

Patellar Eversion

• Flex knee to 90 degrees
• Gently evert patella laterally
• NOTE: More difficult than medial parapatellar due to preserved VMO
• Avoid excessive force (VMO tearing risk)

If Inadequate Exposure

• Options:
  1. Extend capsular incision more proximally
  2. Use lamina spreaders to widen exposure
  3. Convert to medial parapatellar if truly inadequate
• Do NOT force patellar eversion (VMO avulsion risk)

Step 5: Standard TKA Technique

Joint Exposure Assessment

• Visualize femoral condyles
• Access tibial plateau
• Check for bone spurs (remove to improve exposure)

Tibial Cut

• Insert tibial cutting guide
• Perform proximal tibial resection
• Standard extramedullary or intramedullary guide

Femoral Preparation

• Intramedullary femoral guide (standard)
• Distal femoral cut
• Anterior, posterior, and chamfer cuts
• Sizing and rotation assessment

Balancing and Component Implantation

• Assess extension and flexion gaps
• Soft tissue balancing as needed (limited releases possible)
• Trial components to assess fit and tracking
• Cement and implant final components

Patellar Resurfacing

• Standard technique
• Ensure adequate exposure before cutting
• Check tracking after trial reduction

Step 6: Closure

Arthrotomy Closure

• Re-approximate medial capsule and retinaculum
• Use #1 absorbable suture
• Ensure watertight closure

VMO Positioning

• Allow VMO to fall back into anatomic position
• No sutures through VMO muscle (it was elevated, not cut)
• VMO naturally sits over medial tibia

Deep Fascia Closure

• Close fascia over VMO with 2-0 absorbable suture
• Eliminate dead space

Subcutaneous and Skin

• 2-0 Vicryl in subcutaneous layer
• 3-0 Monocryl or staples for skin

Drain Placement

• Consider intra-articular drain
• Remove when output less than 30mL/shift

Intraoperative Complications

Inadequate Exposure (Most Common)

• Incidence: 10-20% in early learning curve
• Causes:
  • Obesity (thick soft tissues)
  • Muscular build (thick VMO)
  • Limited preoperative ROM
  • Patella baja
• Management:
  • Extend capsular incision
  • Use lamina spreaders
  • Convert to medial parapatellar if needed (don't force it)

VMO Muscle Damage

• Tearing of VMO fibers: From excessive retraction or forced patellar eversion
• VMO avulsion from tibia: Poor subperiosteal technique
• Prevention:
  • Gentle retraction
  • Correct subperiosteal plane dissection
  • Avoid forcing patellar eversion
• Management:
  • If VMO torn: repair with sutures
  • If extensively damaged: consider completing as medial parapatellar

VMO Nerve Injury

• Incidence: Less than 1%
• Cause: Excessive dissection into VMO substance
• Result: Denervated VMO, loss of terminal extension strength
• Prevention: Stay in subperiosteal plane, avoid deep VMO dissection

Early Postoperative Complications

Quadriceps Dysfunction (Paradoxical)

• Despite muscle-sparing technique, some patients have weak quadriceps
• Causes: VMO nerve injury, muscle trauma, pain inhibition
• Prevention: Correct technique, early mobilization
• Management: Aggressive physical therapy

Wound Complications (Similar to Medial Parapatellar)

• Hematoma: 2-5%
• Superficial infection: Less than 1%
• Deep infection: Less than 1%

Patellar Maltracking

• Lower incidence than medial parapatellar (VMO preserved)
• If occurs: May indicate excessive VMO damage or malrotation of components

Late Complications

Extensor Lag

• Incidence: 2-5% (vs 5-10% medial parapatellar)
• Preserved VMO reduces risk
• Management: Physical therapy

Knee Stiffness

• Incidence: 3-5%
• Manipulation under anesthesia at 6-12 weeks if severe

Component Malposition or Loosening

• No difference from medial parapatellar
• Surgeon experience critical (harder to assess landmarks with limited exposure)

Immediate Postoperative Care (0-48 hours)

Positioning and Mobilization

• Leg elevated to reduce swelling
• Knee immobilizer or brace (0-2 days)
• Early mobilization - key advantage of subvastus
  • Sit at bedside Day 0 evening or Day 1
  • Ambulate with walker Day 1

Pain Management

• Multimodal analgesia
• Avoid or limit femoral nerve block (delays quad function)
• Prefer adductor canal block (preserves quads)

Drain Management

• Remove when output less than 30mL/shift
• Typically 24-48 hours

Physical Therapy - Accelerated Protocol

• Straight leg raise (SLR):
  • Attempt Day 1-2 (achievable 2-3 days in most)
  • vs 7-10 days with medial parapatellar
• Quadriceps sets, ankle pumps Day 0
• CPM machine optional (controversial benefit)

Weight-Bearing and Mobilization

Weight-Bearing

• Weight-bearing as tolerated (WBAT) from Day 1
• Walker or crutches for safety first 2-4 weeks
• Progress to cane by 4-6 weeks

Range of Motion Goals

• Passive ROM to 90 degrees by Week 1
• Active ROM to 90 degrees by Week 2
• Goal: 0-110 degrees by 6 weeks

Radiographic Follow-up

2 Weeks

• AP and lateral knee radiographs
• Assess component position and alignment

6 Weeks

• Repeat films
• Assess for component shift or subsidence

6 Months and 1 Year

• Long-term implant assessment
• Check for radiolucencies or loosening

Physical Therapy

Phase 1 (0-2 weeks): Early Mobilization

• Focus on achieving SLR (faster than medial parapatellar)
• Gentle ROM exercises
• Quadriceps strengthening (sets, SLR)
• Gait training with assistive device

Phase 2 (2-6 weeks): Progressive Strengthening

• Increase ROM (goal 0-110 degrees)
• Closed-chain exercises (mini-squats, leg press)
• Stationary bike when ROM adequate
• Wean off assistive device

Phase 3 (6-12 weeks): Functional Restoration

• Return to normal activities of daily living
• Discontinue assistive devices
• Sport-specific training (low-impact)

Return to Activity

Activities of Daily Living: 2-4 weeks Driving: 4-6 weeks (off narcotics, adequate control) Sedentary Work: 4-6 weeks Light Labor: 8-12 weeks Golf/Swimming: 12 weeks Full Activities: 12-16 weeks

Global Practice Pattern

The medial parapatellar arthrotomy remains the default approach for primary TKA worldwide because of its versatility and excellent exposure. The subvastus (and mini-subvastus) approach is used selectively by arthroplasty surgeons internationally as an extensor-mechanism-sparing option within enhanced-recovery (fast-track) pathways, reserved for thin patients with good preoperative range of motion and minimal deformity. Its uptake is limited chiefly by a steep learning curve and restricted exposure in obese, muscular, stiff or severely deformed knees.

Major Joint Registries

National arthroplasty registries do not generally stratify revision risk by surgical arthrotomy (parapatellar vs subvastus vs midvastus), so there is no registry-level signal that the subvastus approach changes implant survivorship.

The key registry-level message for vivas: the choice of arthrotomy is an early-recovery decision, not an implant-survival decision.

Society Guidance and Consensus

• AAOS (US) / NICE & BOA (UK) / AO Foundation / EFORT (Europe): none mandate a specific arthrotomy for primary TKA; approach selection is left to surgeon judgement and patient anatomy. Enhanced-recovery (ERAS) consensus statements endorse quadriceps-sparing approaches and motor-sparing analgesia as components of fast-track care where they suit the patient.
• Antibiotic prophylaxis: a single weight-adjusted dose of a first-generation cephalosporin (e.g. cefazolin 2 g IV, 3 g if over 120 kg) within 60 minutes of incision is the globally consistent standard; glycopeptide where MRSA colonised or beta-lactam allergic. This is independent of the arthrotomy chosen.
• Regional anaesthesia: across all settings, when a subvastus approach is used to accelerate quadriceps recovery, a motor-sparing block (adductor canal block) is preferred over a femoral nerve block, which would blunt the very quadriceps recovery the approach is intended to deliver.

Practical Global Take

The functional advantages of the subvastus approach (earlier straight-leg raise, earlier flexion, less lateral release, lower early pain) are real but concentrated in the first weeks and attenuate by 3 to 6 months. It is therefore best framed globally as a tool for selected fast-track patients rather than a routine replacement for the medial parapatellar approach.