Hip Arthroplasty in Adult DDH

Western Health Orthopaedic Registrar presentation – Hip Dysplasia by Dr James Churchill

Western Health Orthopaedic Registrar presentation – Hip Pain in the Young Adult by Dr Sam Bewsher

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