Primary Total Hip Replacement of the Dysplastic Hip

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Primary Total Hip Replacement of the Dysplastic
Hip

• Q. Ikram, D.O.
• Garden City Hospital

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Introduction

• Developmental abnormalities are the most common
  cause of secondary osteoarthritis of the hip.
• Despite screening programs, a large number of pts
  still have the sequelae of dysplasia of the hip
  in adulthood.
• This lecture hopes to review the assessment of
  the dysplastic hip and summarize the current
  knowledge about THA dysplastic hips.

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Etiology Risk Factors

• Genetic and ethnic factors play a key role
• 25-50/1000 in Lapps and Native Americans
• Low rate among Chinese and African descents.
• Positive family hx in 12-33 of affected pts
• True incidence is 1-1.5/1000 live births

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Diagnosis

• In the newborn, check for Ortolanis and Barlows
  signs.
• For late dx, most reliable physical finding is
  limitation of abduction
• Others signs include apparent femoral shortening,
  asymmetry of the gluteal, thigh, or labial folds,
  and limb length inequality.
• Imaging studies include ultrasound for newborns
  and plain x-rays for infants and adults.

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• The natural hx of complete dislocations depends
  on 2 factors
• Presence or absence of a false acetabulum
• bilaterality
• The severity of hip dysplasia varies widely, from
  a shallow acetabulum to the completely dislocated
  and high-riding hip.
• The anatomic abnormalities that are present
  depend on the severity of the dysplasia.

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Anatomy

• In a mild subluxation, the following occur
• A shallow acetabulum with a wide, oval opening
• Anteromedial aspect of acetabular wall may be
  very thin, but better bone stock posteriorly
• In a high, complete dislocation, following occurs
• Affected side of pelvis is smaller
• Acetabular wall is thin, soft, and often grossly
  anteverted
• Prox femur has a small femoral head with a short
  neck that is markedly anteverted, post
  displacement of GT, and a narrow, straight,
  tapered femoral canal with a tight isthmus.
• Neck-shaft angle is often increased.

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• Secondary anatomic anomalies include
• Hamstring,adductor, quad muscles are shortened
• Prox migration of femoral head leads to a
  relatively horizontal orientation of the abductor
  muscle mass
• Hip capsule may be thickened with a hypertrophic
  psoas tendon
• Sciatic nerve is shortened, may be vulnerable if
  limb lengthening is attempted
• Anatomy of the femoral nerve and profunda femoris
  artery altered due to high-riding femur

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Classification

• Crowe and assoc. classified dysplastic hips
  radiographically into 4 categories on the basis
  of the extent of prox migration of the femoral
  head
• They estimated normal height of femoral head is
  20 of height of pelvis.
• Class I has lt 50 subluxation of femoral head,
  Class II is 50-75 subluxed, Class III is 75-99
  subluxed, and Class IV is 100 subluxed or high
  dislocation.

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Pre-operative Planning

• Tx depends on the severity of the disease, extent
  of arthritic changes, age and fxnal goals of pt,
  and availability of bone stock.
• For most pts, pain in the hip is the primary
  symptom, as well as limb-length discrepancy, a
  severe limp, pain in the back or knee symptoms.
• Pts should be counseled on increased risk of inj
  to fem and sciatic nerves, vascular inj,
  prosthetic failure, and infection.
• Even with successful reconstruction, may also be
  left with a residual limb length discrepancy and
  a noticeable limp

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• Historically, THA after a previous femoral
  osteotomy has been associated with higher rates
  of complications and revision.
• Shinar Harris noted IT osteotomy did not affect
  the expected excellent results
• Boos and assoc compared 74 primary THA vs 74 THA
  after previous osteotomy and showed no
  significant difference in the rate of
  perioperative complications or rate of revision.
• Only diff was greater difficulty with exposure
  and longer OR time in osteotomy group.

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• Pre-op planning is essential to ensure
  appropriate equipment and prostheses.
• Standard AP/lat of hip and pelvis films may be
  supplemented with Judet views
• CT may be helpful in determining available
  acetabular coverage and estimate degree of
  femoral anteversion
• On acetabular side, decision should be made as to
  whether to attempt to restore acetabulum to its
  original location
• Assess bone stock for satisfactory fixation and
  coverage

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• On the femoral side, the size of the femoral
  canal and the need for special or custom
  components should be assessed.
• Particular attention should be paid to the need
  for a 22-mm inside cup diameter and femoral head.
• Decision for shortening with or without
  rotational osteotomy of the femur should be made
  preop.

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The Acetabulum

• The reconstruction of the acetabulum is the most
  important part of the whole procedure!
• Determines the approach that is used, the type of
  bone graft needed, and the type of femoral
  reconstruction.
• The acetabular component is optimally placed at
  the site of the true acetabulum
• A high, but not lateral, position can be
  accepted.
• Obtaining satisfactory acetabular coverage is the
  key step.

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• For most, this necessitates only deeper reaming
  and use of a small diameter acetabular component
  that is porous-coated or inserted with cement.
• Alternatives include use of cement or bone graft
  to augment the acetabulum or the use of
  reinforcement rings.

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Acetabular coverage

• The definition of adequate acetabular support is
  that 70 of cup should be covered with intact
  host bone.
• Remaining 30 may be covered with morcellized
  autogenous graft or allograft.
• Linde Jensen analyzed 123 Charnley THA to
  determine factors leading to loosening
• Main predictor was lack of lateral osseous
  support
• Others included degree of preop dislocation and
  height of acetabular comp relative to true
  acetabulum

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• When there is deficient superolateral coverage,
  the stresses shift to the posterosuperior aspect
  of the acetabulum and to the bone-cement
  interface.

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Small acetabular components

• If there is only a moderate reduction of
  acetabular bone stock, the use of a small
  acetabular component may be satisfactory.
• Care should be taken to avoid overreaming, as
  this reduces the bone stock and provides the
  potential for axial migration of the cup, loss of
  position, and fatigue fx of the acetabulum.
• Sochart Porter reviewed 43 hips using a small
  or extra small acetabular component (38 mm or
  less)
• Survival of acetabular comp was 97 at 10 yrs,
  58 at 25 yrs.

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Cotyloplasty

• Dunn Hess, Hess Umber recommended a
  deliberate fx of the medial wall of the
  acetabulum in order to place the acetabular
  component within the available iliac bone.
• Medial wall can be augmented with mesh or
  reinforced with an autogenous graft, and a small
  cup is inserted with cement.
• Protected wt bearing x 3-4 months.
• 94 had good or excellent results at a mean of 7
  yrs.

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High hip center

• Alterations in the center of hip rotation
  dramatically change hip biomechanics and may
  influence the survival of the hip reconstruction
• Forces thru the hip were lowest when the hip
  center was as far medial as possible and somewhat
  anterior and inferior
• Greatest forces generated when hip center was
  lateral, posterior, and superior
• Delp used 3-D model to study effects of moving
  hip center and showed sup displacement alone
  could easily be compensated for by increasing fem
  neck length

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• However, Yoder reviewed 116 Charnley hips with
  cement and found that location of hip center did
  not influence rate of acetabular loosening but
  considerably increased rate of femoral loosening.
• Pagnano reviewed 145 cemented THA and found that
  if acetabular comp was more than 15 mm superior
  to the normal center of rotation of the hip,
  there were substantially higher rates of
  loosening and revision of both fem and acet
  components.

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Cement Augmentation

• The use of cement to fill the superior acetabular
  defect provided gratifying early results but has
  not been associated with satisfactory long-term
  results
• MacKenzie et al reported on 46 cemented THA that
  were Crowe II-IV with a mean 16 yr f/u
• 14 had revision of acetabular comp and 32 of
  unrevised had evidence of radiographic loosening.

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Acetabular Augmentation

• The bone stock may be augmented by bulk bone
  grafting using the pts own femoral head or an
  allograft.
• The graft should provide post as well as superior
  coverage and should ideally be buttressed by the
  most lat part of the acetabulum
• Structural support with an autogenous fem head in
  a dysplastic acet and cementing of acet comp into
  graft has provided satisfactory short term
  results
• Long term studies are mixed
• Revision of acet comp ranges from 0-46 at
  10-12yrs.

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Reinforcement rings

• Gill et al reported results of use of an
  acetabular reinforcement ring, designed by
  Muller, in 87 pts with class II-IV arthritis.
• At a 9.4 yrs mean f/u, only 2 acetabular
  revisions needed, and both used cement rather
  than bone graft.
• A later publication showed 4 failures in 33
  consecutive hips using an acetabular roof
  reinforcement ring with a hook, designed by Ganz.

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The Femur

• Femoral reconstruction may be complicated by a
  small medullary canal, femoral hypoplasia, severe
  developmental distortion of femoral shape and
  version, and effects of previous intertroch and
  subtroch osteotomies.
• Previous osteotomies may necessitate a repeat
  osteotomy for safe femoral comp placement
• If narrow canal, increased risk of reaming thru
  fem cortex and causing fx
• Can be overcome by splitting prox 8-10 cm of
  shaft both ant post.
• Bone graft and stabilize split with lag screws.

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• For hips with class I-III dysplasia, it may be
  sufficient to use a conventional femoral stem
• For class IV dysplasia, a straight, narrow stem
  with a limited medial curvature should be used,
  as the resection level leaves no calcar and no
  prox-med femoral curve.
• When there is gt 400 of anteversion, a corrective
  rotational osteotomy or a custom implant which
  version of femoral neck can be varied should be
  performed.

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• If the acetabulum is brought down to its true
  level, the femur may have to be shortened in
  order to reduce the risk of injury to the
  sciatic nerve
• Femoral shortening may be carried out at the
  level of the trochanter or in the subtrochanteric
  region.
• This allows for any femoral rotational correction
  and still effectively lengthens the limb even
  though the femur itself is shortened.
• Osteotomies include a transverse, step-cuts,
  double chevron, or oblique osteotomies

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Complications and Pitfalls

• Several series confirm higher rates of
  complications after THA in pts who have dysplasia
  than in pts who have primary osteoarthritis.
• Prevalence of nerve palsy after THA has been
  reported as 0.5-2 however increases to 3-15.
• lengthening up to 4 cm or 6 of length of limb is
  acceptable
• Highest rates of dislocation (5-11) reported in
  pts who had congenital dysplasia or dislocation
  of hip
• 10-29 trochanteric nonunion rate

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• B/c of a narrow medullary canal, there is an
  increased risk of intraoperative fx of the femur.
• Young pts with THA for dysplasia also have
  reported higher rates of infection.
• May be due to duration and complexity of
  operation
• Extensive dissection and stripping of soft
  tissues and use of bone graft may also contribute
  to higher rates of infection

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Summary

• THA relieves pain and improves fxn in those with
  end stage arthritis secondary to DDH
• These pts are often young and will not tolerate
  an arthrodesis
• A mildly dysplastic hip may not require any
  special expertise, but a hip with a complex
  dislocation represents one of the most
  multifaceted challenges facing the reconstructive
  surgeon.

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• Pre op planning is essential to plan the
  appropriate equipment and prostheses to be used.
• Must be aware of anatomic changes that occur with
  DDH, including false acetabulum and the size of
  femoral head and canal
• The reconstruction of the acetabulum is the most
  important part of the whole procedure!!
• Must be aware of potential complications and
  inform pts of such.