Bone Deficiency in Primary Total Knee Arthroplasty

1
Bone Deficiency in Primary Total Knee Arthroplasty

• Douglas Dennis, M.D.
• Reviewed by K. Ikram, D.O.
• Jan. 11, 2000

2
Introduction

• numerous etiologies exist for presence of bone
  defects at time of TKA
• axial malalignment
• trauma
• previous osteotomy
• prior TKA
• defects must be localized and classified as
  central or peripheral

3

• varus deformities charac. assoc. with posteromed.
  defects
• valgus assoc. with central lateral defect
• post.lat. defects present following HTO
• bone defects most frequently occur in prox. tibia

4

• Goals of reconstruction of bone defects
• preservation of host bone
• restoration of anatomic jt. line
• axial alignment
• ligamentous stability
• flex-ext. space symmetry and balance
• secure implant fixation

5

• various surgical options available for handling
  such defects including
• thicker osseous resection
• shifting the component away from the defect
• filling the defect with methylmethacrylate (with
  or without screw augmentation)
• modular prosthetic augmentation

6

• use of custom-designed components
• bone graft (auto vs. allograft)
• purpose of this report is to review the various
  treatment options including indications and
  clinical results of each tx. method.

7
Increased Bone Resection

• thicker tibial resection to base of defect should
  be reserved for shall defects
• lt 5 mm medially or lt 10 mm laterally
• numerous studies show reduction in strength with
  more distal resection
• lessens osseous support of tibial comp.
• also may lead use of smaller tibial comp., has
  dec. surface area, leading to inc. unit loading
• should be limited to pts. of advanced age in
  which revision TKA is unlikely.

8
Component Shifting

• shifting tibial component medial or lat. away
  from defect may eliminate or reduce the size of
  the defect
• if shift medially, create a relative
  lateralization of tibial tubercle which may
  result in pat.-fem. instab.
• a smaller tibial comp. often required again
  diminishing surface area therefore inc. unit load

9
Cement and Screws

• filling defects with cement alone is simple and
  efficient but premature radiolucent lines often
  develop
• large mass of cement creates risk of thermal
  necrosis and often difficult to pressurize
• net shrinkage of 2 as cement polymerizes
• both math. and biomech. analyses show inf.
  implant support when using this technique
• Lotke reported success in 58/59 pts at 7 yr f/u
  if bone defect lt 20 mm involved lt 50 of tib.
  condyle

10

• addition of screws to reinforce strength of
  cement in tx. deficiencies of mod. defects of
  5-10 mm thickness.
• has same disadv. of cement alone (difficulty of
  pressurization, shrinkage, etc.)
• Ritter reported on 47 cases and observed no
  prosthetic loosening or failures at 6 yrs f/u.

11
Modular Prosthetic Augmentation

• early designs were angular in shape and limited
  to tibial component
• current designs include angular hemi-plateau and
  full plateaus wedges and rectangulat block shapes
  with variable thicknesses
• also, distal and post. fem. comp. augmentations
  available.
• repairing defects with these prosthetics allow
  immediate support with satisfactory load transfer

12

• no risk of disease transmission, malunion,
  nonunion, or resorption.
• however, size and shape are limited so difficult
  to use with large, irreg. defects.
• due to modular attachment, potential for debris
  generation and osteolysis is created.

13

• controversy regarding choice of ang. wedge vs.
  rectangular block augmentations persists
• angular wedges preserve more host bone
• rect. augmentations are more biomech. stable
  owing to reduced shear loads at bone-aug.
  interface

14

• defect bed is often sclerotic and should be
  predrilled to enhance cement pressurization and
  interdigitization into host bone.
• failure to pre-drill often leads to premature
  radiolucent lines at augmentation-bone interface
• in cases of large defects, defects with osseous
  fragmenation, or marked obesity, use a diaphyseal
  engaging stem to protect and offload the osseous
  defect and reconstruction.

15
Custom Components

• similar advantages to the use of modular
  augmentations
• however, a new modular interface is not created
  so dec. risk of debris-induced osteolysis
• numerous disadvantages
• manufacturing delays
• high costs
• often poorly fits

16

• indications limited to pts who have bone defects
  that cannot be managed effectively with modular
  augmentation and are not good candidates for bone
  grafting.

17
Bone Graft

• offers advantage of bone stock restoration for
  further revisions and ability to contour bone
  graft to fit defect without resection of host
  bone.
• cost effective, especially if autologous
• provides more physiologic load transfer if union
  occurs
• disadvantages include malunion, nonunion, late
  collapse, and disease transmission.

18

• meticulous surgice technique required for optimum
  results
• minimum resection of sclerotic bone to create a
  healthy cancellous interface enhance union
• geometric shaping of host defect and bone graft
  enhance mech. interlock, graft stability, and
  surface contact area available for healing.
• rigid fixation of bone graft is mandatory to
  assure graft union

19

• as with prosthetic augmentation, if underlying
  host bed is weakened, addition of
  diaphyseal-engaging stem is warranted to reduce
  loads on graft during incorporation.
• excellent short term results noted with success
  rates gt 90.

20
Summary

• numerous options exist for management of minor
  bone defects assoc. with primary TKA
• biomech. studies show filling defects with cement
  results in inf. load transfer under eccentric
  loading conditions
• rectangular augmentations may be superior to
  angular wedges owing to reduction in shear
  stresses at fixation interface

21

• bone graft favored for cavitary defects, massive
  bone loss, and in younger pts in whom additional
  revision surgery is likely
• prosthetic augmentations favored in peripheral
  defects of moderate size in more elderly pts.
• preoperative planning is key when dealing with
  any kind of bony defects.

22
(No Transcript)