Hip arthroplasty in adult DDH
Anatomical defects
Acetabulum
- Shallow & sloping
- Deficient superolaterally & anteriorly
- Decreased in AP dimensions
- posterior wall is usually adequate
Femur
- Hypoplasia
- Narrow canal
- Valgus neck
- Persistent femoral anteversion
- Hypoplastic posteriorly placed greater trochanter. This may cause impingement on external rotation
- Small femoral head
Classification of adult DDH (Hartofilakidis)
| Type | Description | |
|---|---|---|
| Type 1 | Dysplasia | with femoral head remaining in true acetabulum |
| Type 2 | Low dislocation | with the femoral head in a false acetabulum, the inferior lip of which is in contact with or overlaps the true acetabulum |
| Type 3 | High dislocation | in which the false acetabulum has no contact with the true acetabulum |
Principles of management
- four main issues in management of these patients are
- Limb length discrepancy
- Placement & coverage of the cup
- Need for small femoral & acetabular components
- Surgical technique
Limb length discrepancy
- Patients are interested in having their LLD corrected
- LLD is corrected by
- Acetabulum
- restoring the acetabulum back to its correct position
- Femur
- inserting a femoral component that is longer than the length of femoral bone removed.
- Lengthening of up to 4cm or 6% of the length of the limb (whichever is lesser) is acceptable
- Strategies to avoid damaging the nerve include
- When performing a trial reduction keep the knee flexed, & check the tension in the nerve as the knee is extended
- A wakeup test is useful if the nerve is felt to be under tension – the patient is instructed to dorsiflex
- Somatosensory evoked potentials can be useful
- If the nerve is under excessive tension the hip centre must be reconstructed higher up or the femur must be shortened
- femur can be shortened in the subtrochanteric area (which has a higher rate of nonunion) or in the proximal femur (but this removes metaphyseal bone which is needed to maintain stability & ingrowth).
- femoral nerve can also be damaged if there is excessive lengthening
- If there is a femoral nerve palsy the hip should be flexed to 70° to take tension off the nerve
- If the sciatic nerve is also involved the knee should be flexed
Placement & coverage of the cup
- Cup placement depends on two factors
- Bone stock
- Leg length discrepancy
- cup can be positioned
- At the correct anatomical level, with or without a graft
- In a high hip centre position
- In a centralized position
High hip centre
- For:
- Avoid use of autograft
- Easier
- Against:
- Higher rate of component loosening
- Potentially higher rate of dislocation because of impingement against acetabulum
- Smaller cup with less poly can be used
- Bone stock has not been restored so further surgery is more difficult
- Need extra-long femoral neck or calcar replacement prostheses to restore leg length
Anatomical hip centre
- Cotyloplasty (controlled fracture of medial wall & autogenous bone graft) is useful in this situation
- Gross advocates using an anatomical hip centre, with shelf autograft if there is less than 70% coverage. This can be used to form a flying buttress. This is very helpful in providing bone stock for future revision
- Using cement to obtain superior coverage leads to poor results.
Components
- Cups with an outer diameter of as small as 36mm should be available, but the thickest possible polyethylene should be used
- Straight stems with diameters of 5-10mm should be available. A small head is used to maximize polyethylene thickness
Abductors
- Patients may have poor abductor function, & hypoplastic greater trochanters may be a sign of this
- If the abductor muscles have dubious function, & operative exploration shows they are inadequate, it may be necessary to abandon the arthroplasty
Surgical technique
- Need accurate clinical & radiological measurements of leg length prior to surgery. If the patient has a fixed pelvic obliquity the apparent leg length should be measured to determine the amount of leg lengthening required
- Exposure
- Type I hips can be approached through conventional posterior or lateral exposures
- Where grafting procedures, or more than 3cm of lengthening is required, a trochanteric slide (where the GT is transected with gluteus medius & vastus lateralis attached) prevents proximal migration of the GT & is the preferred approach
- Neck cut
- neck cut is usually made at the level of the lesser trochanter to avoid the problem of excessive anteversion
- level of the true acetabulum
- obturator foramen
- intraoperative radiographs
- how far to ream
- drill through the medial cortex & measure the depth available
- Stop reaming .5-1cm from the inner cortex
- A trial cup is then inserted, & if less than 70% of the cup is covered, the femoral head is used as bone graft
- cancellous surface should abut the cup, & the cortical surface face the soft tissues
- Morsellized autogenous graft is then packed between the femoral head structural graft & the ilium
Webpage Last Modified: 24 September, 2011