Comprehensive guide to the posterior (Moore/Southern) approach to the hip - true internervous plane, piriformis preservation, capsular repair, and dislocation prevention for Orthopaedic exam
Reviewed by OrthoVellum Editorial Team
Orthopaedic clinicians and medical editors • Published by OrthoVellum Medical Education Team
True Internervous Plane | Sciatic Nerve at Risk | Capsular Repair Reduces Dislocation
Only hip approach with true internervous plane: gluteus maximus (inferior gluteal nerve) and gluteus medius (superior gluteal nerve). No muscle denervation occurs with proper technique.
The sciatic nerve runs 1cm medial to the greater trochanter and posterior to the acetabulum. At highest risk during: posterior retractor placement, acetabular exposure, and leg lengthening. Injury causes foot drop and severe disability.
Capsular repair with or without short external repair reduces dislocation from 7% to 2-3%. This is now standard of care. Use non-absorbable suture to reattach capsule and piriformis to femur.
Post-op dislocation occurs with combined flexion, adduction, internal rotation. Highest risk: getting out of low chair, tying shoes, picking up objects. Patient education is critical for first 6-12 weeks.
| Clinical Scenario | Posterior Approach | Alternative | Key Pearl |
|---|---|---|---|
| Primary THA, standard anatomy | EXCELLENT - true internervous plane, familiar | Lateral or anterior also acceptable | 35% market share in Australia per AOANJRR |
| Revision THA, posterior column defect | BEST CHOICE - direct posterior access | No good alternative | Superior posterior column/wall exposure |
| DDH with high hip center, need proximal exposure | EXCELLENT - can extend proximally safely | Lateral approach limits proximal (SGA nerve risk) | No proximal nerve danger in posterior |
| Young patient, high dislocation risk concern | ACCEPTABLE if capsule repaired | Consider anterior approach (lower native dislocation) | Capsule repair mandatory - reduces risk to 2-3% |
Memory Hook:POSTERIOR approach - remember to REPAIR the capsule to prevent dislocation!
Memory Hook:Protect the SCIATIC nerve - it's the most important structure at risk!
Memory Hook:FLAIR position = Flexion + Adduction + Internal Rotation - the danger zone after posterior THA!
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:
Australian Epidemiology (AOANJRR 2023):
Key Advantages:
Key Disadvantages:
The posterior approach exploits the only true internervous plane to the hip joint:
No muscle denervation occurs when the approach is performed correctly, as both muscles remain fully innervated.
The sciatic nerve is the primary structure at risk:
The posterior capsule is the primary restraint to posterior dislocation:
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:
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:
These muscles are divided (Moore) or partially divided (Southern), not split along an internervous plane.
Clinical Significance:
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."
Standard Position: The patient is placed in lateral decubitus with the operative hip uppermost.
Key Positioning Elements:
Pelvis Perpendicular to Floor
Lower Leg Positioning
Operative Leg Free
C-arm Access
Common Positioning Errors:
| Error | Consequence |
|---|---|
| Pelvis tilted forward | Cup appears more anteverted than actual (false security) |
| Pelvis tilted backward | Cup appears more retroverted than actual (may overcorrect) |
| Patient rolling backward | Makes posterior exposure easier but causes anteversion error |
| Inadequate padding | Pressure injuries, nerve compression |
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.
Native Hip Stability Mechanism: The hip joint relies on several stabilizers that must be understood when selecting and performing surgical approaches:
Pathophysiology of Posterior Dislocation: After posterior approach THA, instability occurs through a specific mechanism:
Why Capsular Repair Works: The posterior capsule contributes approximately 60-70% of posterior hip stability. Repair addresses:
Sciatic Nerve Vulnerability: The sciatic nerve is at risk due to its anatomical course:
| Feature | Moore Approach | Southern Approach |
|---|---|---|
| Piriformis | Divided | Preserved |
| Short externals | All divided | Gemelli + obturator only |
| Exposure | Slightly better | Slightly less |
| Theoretical stability | Less (piriformis divided) | More (piriformis intact) |
| Evidence | Extensive | Limited |
| Current practice | Common | Less common |
Current consensus: Capsular repair is more important than piriformis preservation for preventing dislocation. Either approach acceptable if capsule repaired.
History:
Physical Examination:
Pre-operative Nerve Documentation: Critical for posterior approach given sciatic nerve risk:
Pre-op Documentation
Always document pre-operative nerve function before posterior approach THA. If patient has pre-existing foot drop or numbness, this must be noted. Post-operative neuropraxia rate is 0.5-2% even with perfect technique - defending against medicolegal claims requires thorough pre-operative documentation.
Radiographs (mandatory):
CT Scan (selective indications):
| Parameter | Measurement | Goal |
|---|---|---|
| Cup size | Acetabular diameter | 2mm larger than native |
| Cup position | Inclination/anteversion | 40° / 15-20° |
| Stem size | Femoral canal diameter | Fill canal metaphyseally |
| Femoral offset | Contralateral comparison | Restore to native |
| Leg length | Contralateral comparison | Equal or slightly longer (under 10mm) |
| Neck osteotomy | Lesser trochanter reference | Match template level |
Standard pre-operative workup:
Posterior dislocation:
Sciatic nerve palsy:
Heterotopic ossification:
Dislocation Risk Position
The risk position after posterior approach THA is: Flexion beyond 90° + Adduction + Internal Rotation. Common scenarios: getting out of low chair, tying shoes, picking object off floor, sitting on toilet. Patient education to avoid this position for 6-12 weeks is critical.
Day of Surgery:
Mobilization Protocol:
| With Capsular Repair (Modern) | Without Capsular Repair (Historical) |
|---|---|
| No hip precautions | Strict hip precautions |
| Sit on normal chair | Raised toilet seat, cushion |
| Bend to floor if able | Avoid flexion over 90° |
| Cross legs if comfortable | No leg crossing |
| Enhanced recovery | Traditional pathway |
DVT Prophylaxis (Australian Guidelines):
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Dislocation:
Functional Recovery:
Revision Rates:
Causes of Revision:
| Cause | Percentage |
|---|---|
| Loosening | 25% |
| Infection | 20% |
| Dislocation/instability | 18% |
| Fracture | 12% |
| Other | 25% |
Good Prognosis:
Poorer Prognosis:
Practice these scenarios to excel in your viva examination
"Walk me through the posterior approach to the hip for THA. What is the internervous plane and what structures are at risk?"
"A 72-year-old woman undergoes primary THA via posterior approach. In recovery, she has complete foot drop and numbness over the lateral leg and dorsum of foot. How do you assess and manage this?"
"A 65-year-old man has had three posterior dislocations in 6 months following primary THA via posterior approach. Each has been closed reduced successfully. How do you manage this?"
Internervous Plane Question
Q: What is the internervous plane for the posterior approach to the hip? A: Between gluteus maximus (inferior gluteal nerve) and gluteus medius (superior gluteal nerve). This is the only hip approach with a true internervous plane.
Sciatic Nerve Location Question
Q: Where does the sciatic nerve run in relation to the greater trochanter? A: Approximately 1cm medial to the greater trochanter and posterior to the acetabulum. It exits the pelvis below the piriformis muscle (85% of patients).
Capsular Repair Question
Q: What is the effect of capsular repair on dislocation rate after posterior approach THA? A: Capsular repair reduces dislocation rate from approximately 7% to 2-3%. This is now considered standard of care for posterior approach.
Dislocation Position Question
Q: What combination of hip positions causes posterior dislocation after posterior approach THA? A: Flexion (over 90 degrees) + Adduction + Internal Rotation. Common scenarios: getting out of low chair, tying shoes, picking up objects from floor.
Short External Rotators Question
Q: What is the difference between the Moore and Southern modifications of the posterior approach? A: Moore approach: Divides piriformis and all short external rotators. Southern approach: Preserves piriformis, divides only obturator internus and gemelli. Both require capsular repair - more important than piriformis preservation.
AOANJRR Data:
Training implications:
Enhanced Recovery Protocols:
Orthopaedic Exam Preparation
For Orthopaedic clinical and viva exam, you must know: (1) True internervous plane (Gmax vs GM), (2) Sciatic nerve location and protection, (3) Moore vs Southern modifications, (4) Mandatory capsular repair to reduce dislocation, (5) Dislocation risk position (flexion + adduction + IR), (6) Management of sciatic nerve palsy and recurrent dislocation.
High-Yield Exam Summary