Complete separation of the superior from the inferior hemipelvis through the acetabulum — the most demanding acetabular revision scenario
- Definition: complete separation of the superior hemipelvis (ilium) from the inferior hemipelvis (ischium and pubis) through the acetabulum — a fracture or non-union through both anterior and posterior columns
- Risk factors: female sex, rheumatoid arthritis, pelvic irradiation, massive osteolysis, prior acetabular fracture, multiple revisions
- Radiographic triad on AP pelvis: visible fracture line crossing anterior AND posterior columns, break in Kohler (ilioischial) line, asymmetry or rotation of the obturator ring; confirm with Judet views and CT with metal artefact reduction
- Intraoperative confirmation: after implant removal, apply a stress to the inferior hemipelvis (lever on the ischium or push on superior dome) and look for independent motion between superior and inferior segments
- Treatment philosophy splits into COMPRESSION (aim for union: plating plus cup, distraction) versus BRIDGING/BYPASS (accept non-union: cup-cage, custom triflange)
- Structural allograft plus conventional cage is a historic solution with high mechanical failure because the cage has no biologic fixation and fatigues
- “A hemispherical cup alone — however large — cannot reliably treat chronic discontinuity: micromotion at the non-union causes early loosening
- “The acetabular distraction technique deliberately springs the discontinuity open by 2 to 3 mm so that elastic recoil of the pelvis compresses the oversized porous cup
- “In the cup-cage, the cage protects the porous shell from load while ingrowth occurs; the polyethylene liner is CEMENTED into the cage so version can be adjusted independently of cage position
- “The ischial limb of a cup-cage may be slotted ONTO or INTO the ischium; intraosseous placement risks less sciatic nerve irritation from a prominent flange
Discontinuity missed on planning films leads to intraoperative disaster: a jumbo cup impacted into an unrecognised discontinuity displaces the inferior hemipelvis and cannot obtain stability. Scrutinise Kohler line and obturator ring on every revision AP pelvis; obtain CT with metal artefact reduction for any Paprosky IIIA or IIIB defect.
Biology dictates strategy. Acute fracture in healthy bone: compress and heal (plate plus cup). Chronic discontinuity with dead, irradiated or rheumatoid bone: union will not occur — bridge it with a cup-cage or custom triflange. Applying a compression-only strategy to non-healing bone is a classic failure pathway.
The sciatic nerve is at risk during ischial dissection for cage or triflange limbs and during distraction/lengthening of a chronically shortened limb. Intrapelvic structures (external iliac vessels, obturator neurovascular bundle) are at risk from medial screws, retained cement and quadrant violations — remember the Wasielewski quadrant system.
Massive osteolysis causing discontinuity may be septic. Full infection workup — inflammatory markers, aspiration with cell count and culture — before any single-stage reconstruction. Reconstructing a discontinuity onto infected bone guarantees failure.
Definition, Pathoanatomy and Aetiology
Pelvic discontinuity is a complete separation of the superior hemipelvis (ilium and acetabular dome) from the inferior hemipelvis (ischium and pubis), through the acetabulum, involving both the anterior and posterior columns. It represents either an acute fracture, an established non-union, or gradual dissolution of the columns by osteolysis.
Distinguish two entities:
- Acute intraoperative discontinuity — created during component removal or cup impaction, usually in reasonable-quality bone. Behaves like an acute transverse-pattern acetabular fracture: high healing potential if compressed and stabilised.
- Chronic discontinuity — established at presentation, usually with severe associated bone loss (classically Paprosky IIIB, occasionally IIIA). The fracture surfaces are sclerotic, atrophic or osteolytic; union potential is low, particularly in irradiated bone.
Risk factors
- Female sex (thinner columns, osteoporosis) — the large majority of reported cases are women
- Rheumatoid and other inflammatory arthropathies
- Prior pelvic irradiation (osteonecrotic, avascular bed — essentially zero healing potential)
- Massive particle-induced osteolysis
- Prior acetabular fracture, prior multiple revisions, developmental dysplasia with deficient columns
The two hemipelves move independently under load — the inferior segment typically flexes and internally rotates relative to the ilium. Any implant fixed to only one segment, or spanning the gap without rigid fixation to both, experiences cyclical micromotion at the interface, preventing ingrowth and causing early loosening or implant fracture.
Classification and Decision Framework
The Paprosky acetabular classification frames the bone loss; discontinuity is a modifier most commonly attached to type IIIB (superior migration greater than 3 cm, Kohler line broken, severe ischial lysis, unsupportive rim). Berry and colleagues (Mayo) additionally stratified discontinuity by associated bone loss and prior irradiation — the two variables that still drive treatment.
Decision framework — three questions:
- Is it acute or chronic? Acute equals fracture surgery: compress and heal.
- Can it heal? Healing potential depends on bone quality, vascularity (irradiation), inflammatory disease and defect size.
- Can host bone support a porous implant? If contact with viable ilium and ischium is achievable, cup-cage or distraction works; if landmarks are destroyed, custom triflange.
- Best indication
- Acute intraoperative discontinuity; chronic with good bone and healing potential
- Principle
- Compression osteosynthesis of the discontinuity; cup with screws into both segments
- Key weakness
- Fails if bone cannot heal — chronic atrophic non-union
- Best indication
- Chronic discontinuity, workhorse; moderate residual ilium and ischium
- Principle
- Porous tantalum shell for biologic ingrowth, protected by an overlying ilioischial cage while ingrowth occurs; liner cemented into cage
- Key weakness
- Requires enough host bone for shell contact; ischial limb sciatic risk
- Best indication
- Chronic discontinuity with adequate columns to grip an oversized cup
- Principle
- Under-ream, then impact a jumbo porous cup 2 to 3 mm larger, springing the discontinuity open; elastic recoil compresses bone onto the cup
- Key weakness
- Technique-sensitive; risk of iatrogenic column fracture; needs elastic, not rigid, non-union
- Best indication
- Massive bone loss, failed cage, absent landmarks, irradiated pelvis
- Principle
- CT-designed monoblock spanning ilium, ischium and pubis with locked screws into all three
- Key weakness
- Cost, lead time (weeks), no intraoperative adjustability, wound/nerve complications
- Best indication
- Historic; occasionally young patients to restore bone stock
- Principle
- Allograft fills defect; cage bridges and protects it
- Key weakness
- No biologic fixation of the cage — fatigue failure and high mid-term loosening rates; largely superseded
PDC-TCOptions for pelvic discontinuity
Hook:Plate the acute, Distract or Cup-cage the chronic, Triflange the catastrophic, Cage-allograft belongs to history.
Diagnosis: Imaging and Intraoperative Confirmation
Three signs on a well-centred AP pelvis, systematically sought in every acetabular revision:
- Visible fracture line crossing both columns — a lucency traversing the acetabulum from the anterior column (superior pubic ramus region) to the posterior column (ilioischial region)
- Break in Kohler line — the ilioischial line, drawn from the medial border of the ilium down the medial ischium; a step, offset or discontinuity in this line indicates medial displacement of the inferior segment and column disruption
- Obturator ring asymmetry or rotation — the affected obturator foramen appears rotated, smaller or larger than the contralateral side because the inferior hemipelvis has rotated (usually flexed and internally rotated)
Also assess: teardrop destruction (medial wall loss), superior migration of the hip centre (dome loss), and ischial lysis (posterior column and posterior wall loss) — the standard Paprosky radiographic criteria.
Surgical Techniques
chronic pelvic discontinuity with Paprosky IIIA/IIIB bone loss where sufficient host ilium and ischium remain for shell contact. Why over alternatives: combines the biologic fixation of a highly porous shell with the immediate mechanical span of a cage — the cage stress-shields the discontinuity and protects the shell-bone interface from micromotion during ingrowth.
- Position: lateral decubitus, secure pelvic supports (accurate cup version depends on stable pelvic position); radiolucent table.
- Imaging/equipment: image intensifier available; highly porous (tantalum or equivalent) revision shells including jumbo sizes, matched ilioischial cage system, augments, long screws, cement; explant tools; neuromonitoring optional.
- Preparation: full infection workup completed; cell salvage; cross-match — these are high blood-loss cases.
- Approach: posterior approach with extensile capability; identify and protect the sciatic nerve throughout, especially during ischial exposure. Trochanteric slide or extended trochanteric osteotomy if femoral component removal needed.
- Dissection: remove components and all membrane; send tissue for culture and frozen section. Expose the ischium distally and the outer ilium proximally for cage limbs. Confirm the discontinuity by stress testing.
- Reconstruction: ream cautiously for maximal host contact (both segments). Impact the porous shell — augments fill superolateral or posterior defects and are unitised to the shell with cement. Fix the shell with multiple screws into ilium AND ischium/pubis where possible.
- Cage: contour the cage inside the shell. The iliac flange is fixed to the outer ilium with multiple screws; the ischial limb is either slotted into the ischium (intraosseous — lower sciatic irritation) or screwed onto its surface.
- At-risk structures: sciatic nerve (ischial limb), superior gluteal bundle (iliac screws/flange), intrapelvic vessels (anterosuperior/anteroinferior quadrant screws — Wasielewski), obturator bundle.
- Liner: cement a polyethylene liner into the cage in appropriate anteversion and inclination — decoupling liner orientation from cage position is a core advantage. Consider a constrained or dual-mobility bearing given high instability risk from abductor deficiency.
- Closure/aftercare: meticulous closure; protected weight bearing (toe-touch to partial) for 8 to 12 weeks; abduction precautions.
- Pitfalls/salvage: inadequate shell-host contact (add augments, do not rely on cage alone); vertical cemented liner (dislocation); cage limb fracture indicates failed shell ingrowth — salvage is usually custom triflange.
Sciatic nerve: at direct risk during ischial exposure, ischial cage/triflange limb placement and from limb lengthening when correcting a chronically migrated hip centre — limit acute lengthening and check the nerve is lax with the knee flexed. Intrapelvic vessels: screws in the anterosuperior and anteroinferior quadrants threaten the external iliac and obturator vessels; a migrated intrapelvic cup may require preoperative CT angiography and a retroperitoneal-capable surgical plan for extraction.
SPANCup-cage construct logic
Hook:The cage SPANs the gap while the shell grows in.
Outcomes, Failure Modes and Complications
- Acute discontinuity, plate plus cup: the best-prognosis group; most heal when compressed in healthy bone.
- Chronic discontinuity, cup-cage: modern series report survivorship for aseptic failure commonly in the 80 to 90 percent range at 5 to 10 years — the benchmark against which alternatives are judged.
- Distraction: encouraging mid-term results from the originating centres, with radiographic stability in the large majority; longer-term multicentre data still accruing.
- Custom triflange: good fixation survivorship in most series but a high overall complication rate — dislocation is the leading complication (deficient abductors, large exposure), followed by infection and nerve injury.
- Allograft plus cage: historic mid-term mechanical failure and loosening rates were high; the construct is now rarely used as definitive treatment for discontinuity.
Failure modes to recognise:
- Cage or flange fatigue fracture — hallmark of failed biologic fixation of the underlying shell or graft
- Shell migration with recurrent discontinuity motion — inadequate initial contact or infection
- Dislocation — abductor deficiency; mitigate with dual-mobility or constrained liners cemented into the cage
- Periprosthetic joint infection — these are long, high-blood-loss, multiply revised cases; infection rates exceed routine revision
A broken ischial cage limb on follow-up radiographs means the load never transferred to ingrown bone — the shell is loose. Do not simply replace the cage: reassess bone stock with CT, exclude infection, and plan escalation, usually to a custom triflange.
Guidelines, Registries & Global Practice
- Global epidemiology: discontinuity complicates roughly 1 to 5 percent of acetabular revisions in published series; as revision burden rises worldwide with ageing arthroplasty cohorts, absolute case numbers are increasing in every registry region.
- Registry perspective: national registries (NJR, AJRR, AOANJRR, SHAR, Norwegian, NZJR) capture revision for acetabular loosening and bone loss but do not code discontinuity as a discrete entity, so evidence rests on institutional series and multicentre cohorts rather than registry survivorship curves — an important limitation to state in an examination answer.
- Society guidance: AAOS and NICE address hip revision pathways (infection exclusion, centralisation of complex revision) rather than discontinuity-specific technique; the AO principles of column fixation underpin the plate-plus-cup strategy for acute discontinuity. In several health systems (for example UK revision networks and similar European models), complex acetabular revision including discontinuity is increasingly regionalised to high-volume revision centres.
- Resource-setting variation: custom triflanges and porous tantalum systems are costly and require CT-to-implant infrastructure; in resource-limited settings, plate-plus-cup, ring/cage constructs with impaction or structural graft, and staged strategies remain in use, accepting higher mechanical failure risk. Distraction with a standard jumbo porous cup is comparatively resource-efficient where porous shells are available.
Controversies & Areas of Uncertainty
- Cup-cage versus distraction versus custom triflange for chronic discontinuity: no comparative trials exist; selection is driven by bone stock, surgeon experience and implant availability. Distraction advocates cite single-component simplicity; triflange advocates cite fixation into all three residual bones.
- Should the discontinuity be made to heal at all? Bridging constructs succeed without union; whether adding compression plating or grafting to a cup-cage improves durability is unproven.
- Bearing choice: dual-mobility versus constrained liners cemented into cages — constrained liners reduce dislocation but transmit torque to a fragile fixation interface; many revision surgeons now favour dual-mobility.
- 3D-printed custom implants: expanding indications and enthusiastic early series, but cost-effectiveness and long-term survivorship compared with off-the-shelf cup-cage remain undefined.
- Weight-bearing protocols: protected weight bearing for 8 to 12 weeks is conventional but evidence-light; earlier loading may be tolerated by well-fixed distraction or triflange constructs.