Mouth versus cavity defect in protrusio acetabuli
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protrusio acetabuli surgical reconstruction
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protrusio acetabuli "mouth defect" "cavity defect" hip arthroplasty reconstruction
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mouth defect cavity defect protrusio acetabuli classification reconstruction
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protrusio acetabuli "mouth" "cavity" defect classification THA Ranawat Gill
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Now I have sufficient information from both the textbooks and web sources. The "mouth defect" vs "cavity defect" terminology specifically refers to the type of bone defect encountered in protrusio acetabuli during THA, particularly related to how the acetabular bone loss affects the rim versus the medial floor. This comes from the classification framework used in acetabular reconstruction. Let me compile a comprehensive answer.
The terminology "mouth defect" vs "cavity defect" in the context of protrusio acetabuli is a descriptive classification of the morphology and location of acetabular bone loss, which critically guides the reconstruction strategy at THA. Here is a thorough breakdown:
Mouth Defect vs. Cavity Defect in Protrusio Acetabuli
Background
Protrusio acetabuli (PA) is defined radiographically by medial protrusion of the acetabular wall beyond the ilioischial (Köhler's) line by ≥3 mm in men and ≥6 mm in women. The condition creates a medialized center of rotation, deficient medial wall, and variable bone stock loss that complicates THA. To guide reconstruction, surgeons classify the resulting bone deficiency by both location and containment.
The Two Defect Types
1. Cavity Defect (Cavitary / Contained Defect)
| Feature | Detail |
|---|---|
| Definition | Volumetric loss of cancellous bone within the acetabulum without disruption of the peripheral structural rim |
| Structural integrity | Rim is intact; the defect is enclosed ("contained") by surrounding host bone |
| Analogous Paprosky type | Type IIA–IIB (medial wall cavitary loss with intact columns and rim) |
| In protrusio | Seen when the medial wall has thinned or fractured inward but the superolateral rim remains competent — the classic "deep cup" morphology |
| Implication | Containment means morcellized/impacted cancellous graft can be packed into the defect and will be supported on all sides — this is the preferred biological reconstruction |
Reconstruction: Morcellized autograft (femoral head) or allograft impacted into the defect to fill the medial dead space, then a cementless hemispheric cup seated on the reconstructed periphery at the true hip center. The Ranawat–Zahn guidelines apply: graft is required when protrusion exceeds 5 mm with a thin medial wall.
2. Mouth Defect (Segmental / Uncontained Defect)
| Feature | Detail |
|---|---|
| Definition | Loss of a structural segment of the acetabular rim, columns, or medial wall such that the defect opens to the outside — it lacks a complete bony boundary ("uncontained") |
| Structural integrity | Rim continuity is disrupted; the defect has an open "mouth" — no bony wall on at least one side |
| Analogous Paprosky type | Type III (massive segmental loss) |
| In protrusio | Occurs with gross medial wall deficiency (wall absent or perforated), or when anterior/posterior column support is lost; more common in severe RA, revision settings, or after medial wall fracture intraoperatively |
| Implication | An uncontained defect cannot be reconstructed with impaction graft alone — particulate graft will extrude through the open mouth. Structural support is mandatory |
Reconstruction options:
- Structural bone graft (femoral head allograft) to bridge the segmental defect
- Metallic mesh placed medially to convert a mouth defect into a contained defect, enabling impaction grafting
- Reinforcement rings (e.g., Burch–Schneider anti-protrusio cage, Ganz ring) spanning the defect across intact bone superiorly and inferiorly
- Porous metal augments / cup-cage constructs for massive segmental loss
- Custom triflange cups in extreme cases
Practical Surgical Decision Framework
Protrusio acetabuli on preop XR
│
▼
Is the medial wall intact?
├── YES (thinned but present) → Cavity defect
│ → Peripheral reaming (avoid deepening)
│ → Impaction morcellized graft to fill medial dead space
│ → Cementless hemispheric cup on intact rim
│
└── NO (absent/perforated wall or segmental rim loss) → Mouth defect
→ Structural graft OR metallic mesh to convert to contained
→ Reinforcement cage / augments / cup-cage
→ Restore anatomic center of rotation
Key Technical Principles (Both Types)
- Do not ream medially — deepen the socket only peripherally to achieve press-fit on the rim
- Restore the true center of rotation (low and medial, not superior/lateral) to minimize joint reactive forces
- Peripheral reaming preserves medial wall integrity; if the wall is already absent, this becomes moot
- Cementless biologic fixation is preferred over cement (cemented cups in protrusio have high loosening rates from poor containment and weak medial support)
- Screws should engage the posterior column and the inferior hemisphere where bone is best
- The greater trochanter may impinge anteriorly if lateral offset is not restored — critical in protrusio where the femur is medialized
Summary
| Cavity Defect | Mouth Defect | |
|---|---|---|
| Containment | Contained — rim intact | Uncontained — rim/wall deficient |
| Bone loss type | Cancellous / medial volumetric | Structural / segmental |
| Graft type | Morcellized impaction graft | Structural graft ± mesh to convert |
| Cup fixation | Hemispheric cementless cup ± screws | Cage, augments, or cup-cage construct |
| Difficulty | Moderate | High |
The distinction is clinically vital: treating a mouth defect as if it were a cavity defect (packing morcellized graft without structural support) leads to graft extrusion, cup medialization, and early failure. Correctly identifying uncontained defects preoperatively (on CT or MRI showing medial wall thinning/perforation) allows proper implant and graft selection.
Sources:
- Miller's Review of Orthopaedics (9th ed.) — Acetabular defect classification: cavitary vs. segmental deficiency
- Ranawat CS, Zahn MG (1980, 1986) — Original bone grafting guidelines for protrusio at THA
- THA in Protrusio Acetabuli — Systematic Review (PMC10929539)
- Managing Native Hip Protrusio: Simplified Classification (MDPI)
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