Revision THA – Acetabulum Bone Loss

Acetabulum/revision
Title: Acetabular Bone Loss During Revision Total Hip Replacement: Preoperative Investigation and Planning

Reference: Campbell, D.G., Masri, Garbuz, Duncan, AAOS ICL, Chapter 7, Volume 48, 1999

Main Message

Planning, planning, planning. This review goes over the general considerations, the preoperative evaluation of infection, radiographic analysis of bone loss, the numerous classifications of bone loss, a few key examples of how to deal with certain deficiencies, then closes with a brief discussion of approaches.

Points of Interest

When evaluating the failed THA, it is often useful to consider the acetabular and femoral components separately, but keeping in mind the interaction between the two in terms of leg length and stability.

Long term studies on cemented THA have clearly shown a higher failure rates for the cup compared to the femoral component, especially past 10 years.

Much osteolysis can occur silently, before symptomatology arises.

Patient Factors to Consider: Age, medical conditions, sex, mental status (capacity to rehab)
Local Factors to Consider: Infection, stability, leg length, previous incisions, abductor strength, heterotopic bone, status of the femur (will it need to be exposed?)

Rule out sepsis!

Radiographic Analysis: AP pelvis, Judets

Assessment of Bone Deficiency
– Note that xrays often UNDERESTIMATE the amount of bone loss: be aware that the final decision of bone deficiency classification can only be done IN THE OR. Therefore, you should have a back-up plan should the intraoperative findings be much worse than expected.

– Anticipate: cemented cups have big lug holes superiorly, then into the pubis and ischium – this is where the osteolysis will be. Uncemented cups fall into retroversion and dig away more at the anterior than posterior columns.

– Determining where the acetabulum should be: obviously, if present, look at the normal hip. If not present, you can use the medial wall of the teardrop, or the superior/medial border of the obturator foramen (sits within 5mm of the medial wall). Alternatively, you can use the Ranawat method – finds the intersection between Shenton�s line and the ilioischial (�Kocher�) line – the infero-medial acetabulum sits 5 mm medial to this intersection, and the acetabular height is 1/5 the height of the pelvis at this point.

Classification

AAOS
Type I – Segmental: Peripheral, Medial Type IV – Pelvic Discontinuity
Type II – Cavitary: Peripheral, Central Type V – Arthrodesis
Type III – Combined

Also note that there are other classification systems:

Engh & Glassman – 3 types
Chandler and Penenberg – 10 types
Paprosky – 8 types
Gross – 4 types

Preoperative Planning

Remember: the bone loss is often more than predicted based on the xrays!

Questions:

1. Is there pelvic discontinuity? – be prepared to fix the ischium
2. If the defect is primarily cavitary, is it amenable to a standard/large component with morselized allograft?
3. If the defect is segmental, is a structural graft required, or would it be better to just accept a high hip centre?
4. Is the defect so large that not even a structural graft will suffice, and then what type of reconstruction ring is required?
5. What approach should be used?

Examples

Big cavitary defect – morsellized bone graft, then impacted a cemented jumbo cup, accepting a slightly higher hip centre
Medial wall defect – reconstruction cage
Superior dome defect – accept high hip centre, bulk structural allograft, impaction grafting

Approaches

If bioingrown stem – remove femoral head (if modular), retract stem anteriorly, remove stem if cemented and re-cement it back into its mantle
If intrapelvic cup – angiogram! Then consider retroperitoneal exposure
Trochanteric slide is great for providing wide acetabular exposure, particularly when the cup is medialized. If the femur needs addressing as well, then an extended trochanteric osteotomy is ideal.

Thoughts….

Well, it�s a classic review. Nice summary of the classification systems (described also in another ICL).