Total Hip Revision Arthroplasty Techniques - Acetabular revision

Classification systems

AAOS classification

Segmental = complete loss of bone in supporting hemispherical structure of the acetabulum.
In this classification system the rim includes the medial wall.
Cavitary = Localised volumetric loss of bone, without disruption of acetabular rim.

Type 1 – Segmental deficiency.
Peripheral – Superior, Anterior, Posterior.
Central – medial wall absent.

Type II – Cavitatory.
Peripheral – Superior, anterior, posterior.
Central – Medial wall intact.

Type III – Combined.

Type IV – Pelvic discontinuity.

Type V – Arthrodesis.

Paprosky classification

Type I – Undistorted acetabular rim.

Type II – Distorted but intact rim with adequate bone to support hemispherical cementless cup.

Type III – A non-supportive rim.
Host acetabulum is unable to support an acetabular component in the anatomical hip centre.
Will need augmentation to support a non-cemented cup which will be difficult.
Most likely to require a reconstruction ring of some kind..

Aim of revision

  • Restore normal centre of rotation.
  • Rigid construct.
  • Implant must be immediately stable.
  • Major segmental defects more problematic.
  • Superior and posterior deficiencies may require structural allograft.
  • Anterior defects not as problematic – particulate graft may suffice.
  • Increase bone stock.
  • Cavitatory defects should be filled with morsellised cancellous bone.

Crucial questions

  • Will stable fixation in host bone be possible (with or without morsellised graft) at the anatomic hip centre?
  • Are there segmental defects that require structural allograft – or can I use a high hip centre ?
  • Is bone loss so extensive that even with structural grafts, stability and fixation will be questionable?
  • Is there a pelvic discontinuity that will need reconstruction and fixation?
  • What is the optimal exposure to achieve my aims?

My ladder of options

  • The aim is to use a porous coated rim fit cup supplemented by screws. If this can achieve a stable fixation these will give the best results – places in an anatomical hip centre.
  • Fill any contained defects with morsellised graft.
  • If this results in > 50% of the cup in contact with graft it must be supplemented by a reconstruction cage and a cemented cup.
  • Get sufficient stability with what rim remains and the screws – what is sufficient? No consensus.
  • Do not be concerned about the medial wall or small anterior defects – it is rim fit which provides fixation.
  • Walter – “any 3 point peripheral fixation”.
  • Literature criteria of minimum requirements. 2/3 rim intact. 50% contact with native host bone. 70% coverage.
  • May augment segmental defects with structural graft – autograft (iliac crest, fibular), or structural allograft (anatomical graft, or non-anatomical) – however these have a high rate of failure and I would be likely to move to a cage and cemented cup instead.
  • If this is untenable I would move to a reconstruction cage with a sup cemented into it.

Points to consider

  • The best results are obtained with a porous cup placed in an anatomical hip centre which is STABLE. If stability cannot be achieved it will not work.
  • Use criteria for stability as above – if this criteria cannot be satisfied you must move to a ring and cemented cup.
  • Structural allografts don’t have good results in the literature.
  • Problems with revascularization-remodelling and resorption failure. Reflects the fact that they are used in the most difficult cases – inherent bad results in this group.
  • Reconstruction rings/cages with a cemented cup has better results than large structural allografts.
  • Restore bone stock.
  • Graft contained defects.
  • Consider structural grafts even in the setting of reconstruction rings and cemented cups in order to restore bone stock.
  • Hip centre.Anatomical hip centre is the best result.
  • May be a tradeoff between getting a good fixation with a porous cup in a slightly high hip centre (where bone stock is better) versus restoring hip centre but thus necessitating either structural bone graft or use of a ring and cemented cup to achieve it. The best option here is not clear – there are pros and cons to both sides.

Osteolytic defects around stable porous cups

Osteolysis around porous cups tends to be an isolated, expansile lesion, in contrast to the osteolysis seen around cemented cups which tends to be linear.
Removal of the well-fixed cup will likely result in further loss of bone stock, thus treatment is either:

Liner exchange
After removal of the liner the granuloma in the defect can be debrided through any screw holes, and the defect packed with morsellised bone graft.
If locking mechanism is intact simply insert a new liner.

Superior window
This is useful where the granuloma is inaccessible through the cup (either due to its location, or lack of holes in the cup).
Cement a new liner directly into the cup.
If locking mechanism is not intact the poly liner must be moved, debride and graft through the holes, roughen the floor of the cup, and cement in a new cup.

It is contentious whether success depends upon:

  • Debriding the granuloma that forms in the osteolytic defect, OR
  • Removing the poly load that is driving the lysis. This is thought to be the most important step. 
  • Supported by the Maloney study – in cases of just liner exchange (no debridement), no lesions progressed after exchange, and 1/3 completely resolved.