Hip Posterior Approach (Moore/Southern)

The posterior (Moore/Southern) approach to the hip is a workhorse surgical approach for total hip arthroplasty, hip fracture surgery, and revision procedures. First described by Austin Moore in 1957 and modified by Southern in 1980, it remains one of the most commonly used approaches worldwide.

Historical Context:

• 1957: Austin Moore - original posterior approach with piriformis division
• 1980: Southern modification - piriformis preservation technique
• 1990s-2000s: Recognition of capsular repair importance for dislocation prevention
• 2010s-present: Enhanced recovery protocols with routine capsular repair

Australian Epidemiology (AOANJRR 2023):

• Posterior approach used in 35% of primary THAs in Australia
• Second most common approach (lateral 40%, anterior 20%)
• No difference in long-term revision rates between approaches
• Dislocation rates declining with routine capsular repair protocols

Key Advantages:

• True internervous plane - the only hip approach with no muscle denervation risk
• Excellent acetabular exposure (especially posterior wall)
• Extensile - can extend proximally or distally without neurovascular risk
• Familiar to most orthopaedic surgeons worldwide

Key Disadvantages:

• Higher native dislocation rate (5-7%) if capsule not repaired
• Sciatic nerve at risk (0.5-2% transient injury)
• Posterior soft tissue disruption
• Post-operative position restrictions (flexion + adduction + internal rotation)

The True Internervous Plane

The posterior approach exploits the only true internervous plane to the hip joint:

• Gluteus maximus (supplied by inferior gluteal nerve L5-S2)
• Gluteus medius (supplied by superior gluteal nerve L4-S1)

No muscle denervation occurs when the approach is performed correctly, as both muscles remain fully innervated.

Short External Rotators (Superficial to Deep)

1. Piriformis - Key landmark; sciatic nerve runs below this muscle
2. Superior gemellus
3. Obturator internus tendon
4. Inferior gemellus
5. Quadratus femoris - Most inferior; sciatic nerve lies on its anterior surface

Sciatic Nerve Anatomy

The sciatic nerve is the primary structure at risk:

• Exits pelvis through greater sciatic foramen (below piriformis in 85%)
• Runs 1cm medial to greater trochanter
• Lies posterior to acetabular posterior column
• Contains both common peroneal (lateral) and tibial (medial) divisions
• Most at risk during: posterior retractor placement, acetabular reaming, leg lengthening

Posterior Capsule and Stability

The posterior capsule is the primary restraint to posterior dislocation:

• Thickest at the posterior-superior quadrant
• Iliofemoral ligament provides anterior restraint (not at risk posteriorly)
• Capsular repair restores 60-70% of native posterior stability
• Short external rotator repair adds additional posterior restraint

The True Internervous Plane of the Posterior Approach

The posterior approach to the hip is the only hip approach with a true internervous plane. This fundamental anatomical advantage distinguishes it from all other hip approaches.

Superficial Internervous Plane:

• Laterally: Gluteus maximus (inferior gluteal nerve, L5-S2)
• Medially: Gluteus medius (superior gluteal nerve, L4-S1)

These muscles are separated without denervating either. Gluteus maximus is split in the line of its fibers (not across), preserving its innervation.

Deep Internervous Plane: There is no true deep internervous plane. The short external rotators share innervation from sacral plexus branches:

• Piriformis: S1-S2
• Gemelli and obturator internus: L5-S1
• Quadratus femoris: L4-S1

These muscles are divided (Moore) or partially divided (Southern), not split along an internervous plane.

Clinical Significance:

• No abductor denervation (unlike lateral approaches)
• Gluteus maximus function preserved
• Lower risk of Trendelenburg gait than lateral approaches
• Main limitation: posterior instability from capsular disruption (addressed by repair)

Examination Pearl: When asked "What is the internervous plane of the posterior approach?" the answer is: "Gluteus maximus (inferior gluteal nerve) and gluteus medius (superior gluteal nerve). This is the ONLY true internervous plane to the hip."

Lateral Decubitus Position for Posterior Approach

Standard Position: The patient is placed in lateral decubitus with the operative hip uppermost.

Key Positioning Elements:

1. Pelvis Perpendicular to Floor

   • Most critical positioning factor
   • Use pelvic positioner or sandbags anteriorly and posteriorly
   • Check with C-arm: obturator foramina symmetric, coccyx midline
   • Any pelvic tilt causes equivalent error in cup anteversion

2. Lower Leg Positioning

   • Knee flexed 90 degrees for comfort
   • Padded between legs (axillary roll and knee)
   • Secured to table

3. Operative Leg Free

   • Must move through full range during surgery
   • Needs to achieve flexion, adduction, and internal rotation for dislocation
   • Needs to be positioned for trial reduction and stability testing

4. C-arm Access

   • Position allows true AP and lateral fluoroscopy
   • Verify landmarks before draping

Common Positioning Errors:

Examination Tip: Always mention pelvic position when describing the posterior approach. Examiners expect you to recognize that pelvic tilt is the most common cause of cup malposition.

Biomechanical Rationale for Posterior Approach

Native Hip Stability Mechanism: The hip joint relies on several stabilizers that must be understood when selecting and performing surgical approaches:

1. Bony containment: Deep acetabulum with 170° arc of coverage
2. Labrum: Increases acetabular depth by 21%, adds suction seal
3. Capsule: Posterior capsule is thickest and most critical for stability after posterior approach
4. Ligamentum teres: Minor stabilizer, typically sacrificed in arthroplasty
5. Muscular stabilizers: Short external rotators provide dynamic posterior stability

Pathophysiology of Posterior Dislocation: After posterior approach THA, instability occurs through a specific mechanism:

• Combined motion vector: Flexion + adduction + internal rotation brings femoral head toward posterior-inferior capsular defect
• Critical angle: Risk increases dramatically when hip flexed beyond 90° with adduction and IR
• Capsular deficiency: Untreated posterior capsulotomy creates path of least resistance
• Loss of short external rotators: These muscles provide dynamic posterior restraint when intact

Why Capsular Repair Works: The posterior capsule contributes approximately 60-70% of posterior hip stability. Repair addresses:

• Recreates mechanical barrier to posterior translation
• Restores proprioceptive feedback from capsular receptors
• Allows early mobilization without strict hip precautions
• Reduces dislocation rate from 5-7% to 2-3%

Sciatic Nerve Vulnerability: The sciatic nerve is at risk due to its anatomical course:

• Exits pelvis through greater sciatic foramen below piriformis
• Runs 1cm medial to greater trochanter tip
• Courses posterior to short external rotators and posterior capsule
• Vulnerable to direct injury, retractor compression, and leg lengthening stretch

Pre-operative Assessment

History:

• Primary complaint: Hip pain (groin, lateral, posterior), stiffness, limp
• Duration and progression of symptoms
• Previous hip surgery or injections
• Dislocation risk factors: Age over 70, female, neurological disorder, cognitive impairment, prior hip surgery
• Medical comorbidities affecting anesthesia and rehabilitation

Physical Examination:

• Gait: Antalgic, Trendelenburg (abductor weakness)
• Range of motion: Flexion, extension, abduction, adduction, internal and external rotation
• Limb length: Measure from ASIS to medial malleolus
• Neurovascular status: Document pre-operative nerve function (especially if revision)
• Skin quality: Previous incisions, skin condition

Pre-operative Nerve Documentation: Critical for posterior approach given sciatic nerve risk:

• Ankle dorsiflexion (L4-L5, deep peroneal)
• Great toe extension (L5)
• Ankle plantarflexion (S1-S2, tibial)
• Sensation in sciatic distribution

Pre-operative Imaging

Radiographs (mandatory):

• AP pelvis: Assess acetabular morphology, femoral offset, leg length, osteophytes
• Lateral hip: Femoral version, anterior osteophytes, femoral bow
• Templating: Pre-operative planning for implant size and positioning

CT Scan (selective indications):

• Significant bony deformity (DDH, previous trauma)
• Revision surgery - assess bone stock
• Complex acetabular morphology
• 3D assessment of version and bone loss

Pre-operative Templating

Laboratory Investigations

Standard pre-operative workup:

• FBC, U&E, coagulation studies
• Group and hold (or crossmatch for revision)
• ECG (if indicated by age/comorbidity)
• Infection markers if concern (ESR, CRP, joint aspiration if indicated)

Posterior dislocation:

• Incidence: 2-3% (with repair), 5-7% (without repair)
• Direction: Posterior (flexion + adduction + internal rotation)
• Timing: 80% in first 3 months, risk persists lifelong
• Prevention: Capsular repair, patient education, avoid risk positions
• Treatment: Closed reduction (first time), revision surgery if recurrent (2+ dislocations)

Sciatic nerve palsy:

• Incidence: 0.5-2% (transient), 0.1-0.5% (permanent)
• Presentation: Foot drop (common peroneal division), numbness lateral leg
• Risk factors: Revision surgery, DDH, excessive lengthening (over 4cm), direct trauma
• Recovery: 60% recover by 1 year if neuropraxia, poor if neurotmesis
• Management: Ankle-foot orthosis (AFO), physiotherapy, nerve exploration if progressive

Heterotopic ossification:

• Incidence: 20-40% (radiographic), 5-10% (symptomatic)
• Risk factors: Male gender, DISH, hypertrophic osteoarthritis, previous HO
• Prevention: Indomethacin 25mg TDS × 6 weeks OR single-dose radiation (7 Gy)
• Treatment: Excision if symptomatic after maturation (12+ months)

Immediate Post-operative Period (Day 0-2)

Day of Surgery:

• Weight bearing as tolerated with walking frame
• DVT prophylaxis: Rivaroxaban 10mg daily × 5 weeks OR enoxaparin OR aspirin
• Pain management: Multimodal (paracetamol + NSAID + opioid PRN)
• Ice to surgical site

Mobilization Protocol:

DVT Prophylaxis (Australian Guidelines):

• Chemical: Rivaroxaban 10mg daily × 35 days OR Enoxaparin 40mg daily OR Aspirin 100-150mg daily
• Mechanical: TED stockings, intermittent calf compression intra-operatively
• Early mobilization: Most important factor

Week 1-6

• Progressive gait training: Frame → crutches → single stick → unaided
• Hip strengthening exercises (abductors, flexors, extensors)
• Range of motion exercises (within precautions if applicable)
• Wound review at 2 weeks
• Follow-up radiographs

Week 6-12

• Increase activity level
• Driving typically allowed at 6 weeks
• Return to low-impact activities (swimming, cycling, golf)
• If capsule repaired: No restrictions
• If capsule not repaired: Continue precautions until 12 weeks

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Short-term Outcomes (under 2 years)

Dislocation:

• With capsular repair: 2-3%
• Without capsular repair: 5-7%
• 80% occur in first 3 months
• Most can be managed with closed reduction

Functional Recovery:

• 90% satisfied with pain relief by 3 months
• Full recovery of function: 6-12 months
• Return to driving: 4-6 weeks
• Return to golf/swimming: 3-6 months

Long-term Outcomes (AOANJRR Data)

Revision Rates:

• 5-year revision rate: 5% (similar across approaches)
• 10-year revision rate: 7-8%
• 15-year revision rate: 10-12%
• No difference between posterior, lateral, or anterior approaches

Causes of Revision:

Prognostic Factors

Good Prognosis:

• Primary OA indication
• Age 60-80
• BMI under 30
• No cognitive impairment
• Good soft tissue quality
• Experienced surgeon with capsular repair

Poorer Prognosis:

• Revision surgery
• Neuromuscular disorder
• Cognitive impairment
• Age over 80 or under 50
• BMI over 35
• AVN or DDH

AOANJRR Data:

• Posterior approach: 35% of primary THAs in Australia
• Second most common approach (lateral is 40%, anterior 20%)
• Dislocation rates declining with routine capsular repair protocols
• No difference in long-term revision rates between approaches

Training implications:

• Essential skill for Orthopaedic exam and practice
• Most Australian fellowship programs teach posterior as one of two primary approaches
• Understanding of both posterior and lateral approaches expected

Enhanced Recovery Protocols:

• Australian units increasingly adopting enhanced recovery (ERAS)
• Posterior approach with capsular repair compatible with ERAS
• Reduced or no hip precautions if capsule repaired