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Current thoughts on total hip replacement

Current thoughts on total hip replacement

  • Charnley low-friction arthroplasty - gold standard
    • excellent results > 80% of patients at a mean follow-up of almost 20 years
  • first-generation cementless prostheses - problems
    • anterior thigh pain common
    • early loosening from under-sizing & poor fixation
  • cementless arthroplasty was introduced as a bone stock-preserving option, but
    • bone stock loss
    • cancellisation & rounding off of the calcar
    • aggressive focal granulomatous lesions

Principles of THR - McKee

  • inert (bio-compatible) materials
  • design - adequate mechanical strength
  • correct operative technique
  • fixation of the components to bone

Further criteria

  • high corrosion resistance, good wear resistance of materials
  • low friction

Materials

Metals
Super-alloys

  • improved fatigue resistance - << incidence of mechanical failure of stem
  • modulus of elasticity : chrome-cobalt (210 Gpascal) > titanium (110Gp) > bone (20Gp)
  • may be affected by chemical corrosion or released in form of small particles as a result of wear
    • mostly from fretting corrosion at the head neck coupling
    • cause >> levels of metals in serum & urine
  • hypersensitivity may occur

chrome-cobalt

    • stiffer than titanium & bone
    • may be toxic
    • ? cytotoxic & even carcinogenic
    • anaemia, osteomalacia, dementia

titanium

    • as an articulating surface contraindicated - susceptible to abrasive wear
    • cause more wear of polyethylene than stainless steel & cobalt chrome

favourable characteristics

    • high tensile and fatigue strength
    • relatively low modulus of elasticity (closer to bone)
    • superior bio-compatibility
    • attractive option for cementless hip replacement

unfavourable characteristics

    • poor resistance to wear
    • susceptibility to abrasive wear by particulate cement
    • relative contraindication in cemented implants

High-density polyethylene (HDP)

  • wear at a very low rate
    • X-ray findings previously ascribed to wear were d/t plastic deformation
  • HDP debris - formation of aggressive granulomatous lesions
    • biochemical and enzyme responses - aseptic loosening
  • 3rd generation Ultra-high molecular weight polyethylene (UHMWPE)
    • more durable, wear-resistant plastics

Methylmethacrylate (PMMA)

  • used as a grout, not a glue
  • well tolerated in bulk
  • in particulate form it may cause aggressive granulamatous response so-called 'cement disease'
  • improved cementing techniques - << aseptic loosening of femoral component
    • meticulous preparation of the bone bed by pulse lavage
    • drying with adrenalin-soaked sponges or hydrogen peroxide
    • fixation better w/ use of a plug, a cement gun, w/ or w/out venting & pressurisation mechanical properties of cement
    • brittle and susceptible to crack propagation
    • can be reduced of porosity either thru centrifuging or by vacuum mixing

New bio-active materials

Bioglass ionomer cement

  • excellent bonding, but brittle and stiff
  • some may be toxic

Ceramics

Alumina, zirconia & titanium oxides

    • excellent bio-compatibility and wear
    • hard and durable, but brittle - may break
  • use of modular ceramic heads to articulate w/ HDP - choice for younger, more active patient
  • ion-bombarded cobalt-chrome as scratch resistant & less brittle

Composite materials

  • composites w/ poly-acetal polyethylene, carbon and acrylic
  • reduce stiffness of the femoral stem - match structural characteristics of host bone

Femoral component

Femoral head
Head-neck complex

  • adequate offset to permit a good ROM
    • more easily achieved w/ a 32 mm than a 22 mm head
    • expense of increased friction and torque
  • head size on polyethylene wear
    • least rate of linear wear associated w/ size 28 mm head
    • 22 mm head most
    • greatest volumetric wear with 32 mm components
    • highest rate of acetabular revision w/ 32 mm femoral heads
  • optimal head size at present 28 mm
  • modularity allows fine tuning of offset & length & allows only head change at time of acetabular revision

Femoral stem

Cemented

desirable features

  • rounded-trapezoidal or wedge-shaped
  • broad lateral surface to reduce the lateral cement tensile stresses
  • neck-shaft angle should be anatomical with
    • 10° anteversion
    • rounded 132° calcar region to reduce stress risers in the cement
  • doubtful whether a calcar-bearing collar realises any advantages

small cement collar

  • enhances pressurisation and provides a defined end-point for component insertion

Neck

  • should have a Morse taper to accept modular heads of variable sizes & lengths

stem

  • approximately 14 cm long
  • smooth surface to allow settling in visco-elastic cement mantle
  • anatomical asymmetrical stem incorporating a gentle proximal posterior bow to provide an adequate cement mantle
    • this reduces stem stresses & protect against cement fragmentation & failure

cement centralisation plug

  • enhance pressurisation
  • position the stem centrally in cement

Cementless

  • common in Europe for years
  • popular in North America in early 1980s NB - matching cortical dimensions of the femur w/ the implanted stem
  • shape of the proximal femur - most stem rely on a double taper or wedge to engage proximal cortical bone

lateral profile

  • may have a posterior bow in metaphyseal region to provide an 'anatomical' shape
  • or may have a straight stem to promote three-point fixation
    • may cause significant thigh pain & subsequent bone resorption at contact points

biological fixation (USA)

  • application of multi-layered micro-porous bead and mesh coatings
  • pore size ranging 100 - 500 um

bio-active materials

  • capable of bonding adjacent bone
  • hydroxyapatite (HA) best
    • HA porous coating in the metaphyseal region (prox. 1/3 or less)
    • plasma sprayed on titanium stems ( 50-100 micron thick)
    • osteoconductive properties (as allograft)
    • partially disolves & >> local concentration of Ca & Pho

Indication

    • healthy, active pt.
    • < 60 yrs
    • non-inflammatory arthritis
    • good bone quality

Acetabular component

Cemented

  • higher incidence of aseptic loosening than stem
  • flexibility of HDP results in stresses in subjacent cement
    • specially large 32 mm head
    • HDP is thin

Sockets

  • should consist of pure HDP
  • should be flanged to enhance pressurisation of the acetabular cement

Cementless

  • high incidence of radiological and clinical loosening at 8 years
  • d/t giant-cell foreign body erosions caused by polyethylene debris
  • contact between bone and polyethylene should be avoided
  • acetabular components inserted w/out cement should be metal-backed

three different designs

  • hemispherical cups w/ porous coated surfaces
    • most popular
    • press fit
    • fixation enhanced by screws or pegs
  • chamfered cylindrical components
    • fixation w/ host bone thru an interference fit
    • procedure technically demanding
  • external threads
    • secure fixation at time of surgery (Zwei-Mueller)
    • inferior results, migration
  • recommend use of a hemispherical press-fit prosthesis w/ pegs or spikes & porous coating

Conclusion

  • cementless hip replacement for the very young
  • hybrid replacement (cemented stem, uncemented cup) for middle aged
  • fully cemented replacement in elderly

Complications

Neurological
Central

  • prolonged hypoxia
  • CVA

Fat embolism

  • from fat & marrow elements during reaming & cementing
  • intraop. hypotension, hypoxia or cardiac arrest can occur
  • if foramen ovale patent - embolism avoids lung capillaries - systemic embolism, CVA
  • can cause peri-operative confusion
  • prevention
    • pulse lavage of osseous bed before cementing to minimise fat particles
    • use a vent hole to allow extravasation of fat, blood during placement of the component
    • don't use excessive pressurisation in elderly

Peripheral

Incidence

  • 0.6 -1.5 in primary
  • 7.5 in revision surgery
  • increased prevalence in women
    • << muscle bulk & size, shorter limbs
    • 6% stretch limit on nerves allows limited traction

Site

  • sciatic N 90%
    • half of it peroneal division only
  • femoral N
    • next most common
  • obturator N
  • superior gluteal N
    • w/ Hardinge or transgluteal approach

Etiology

  • Seddon type neuropraxia, axonotmesis or neurotmesis
  • traction, compression or ischaemia
    • from retractors
    • haematoma formation
    • acute limb-lengthening > 2-4 cm
  • direct damage
  • revision surgery
    • more extensive & difficult dissection
    • disturbed anatomy & location of N/V structures

Treatment

  • prevention preferable
  • assess N/V status postop.
  • exploration only if risk of direct injury
    • complete trans-section
    • encirclement w/ cerclage wire

Prognosis

  • related to injury
  • usually neuropraxia → good prognosis
    • retention or fast recovery of motor function
  • better for femoral N than for sciatic N

Vascular complications

  • acute bleeding intraop.
  • postop. haemorrhage
  • DVT or arterial thrombosis
  • formation of A-V fistula or false aneurysm

Prevalence

  • 0.25 %
  • external iliac & femoral arteries most often
  • more common in revision surgery

Etiology

  • direct damage
    • sharp instruments - scalpels, osteotomes
    • acetabular reaming thru' medial wall - common iliac vein
    • screw fixation
    • extraction of implants
  • indirect
    • kinking of the vessels
    • thrombosis later

Site

  • femoral artery & vein
  • obturator artery
  • external iliac artery
  • common iliac artery & vein
  • superior gluteal artery & vein

anterior aspect of acetabular rim

medial to the femoral neck

  • femoral artery & medial & lateral circumflex vessels

Prevention

  • careful operative technique & knowledge of anatomy
  • careful retractor placement especially over anterior acetabular rim

Treatment

  • control bleeding intraop.
    • coagulation or ligation
    • repair major vessel
  • DVT prophylaxis

Peri-prosthetic fracture of the femur

Incidence

  • 0.1-1.1 %
  • increasing
    • >> number of THR
    • younger, more active pts. get THR
    • aseptic loosening still a problem

Classification - Vancouver system

  • Type A
    • trochanteric #s
  • Type B
    • #s about the stem or at tip of stem
    • B1 - stem well fixed
    • B2 - some loosening around stem
    • B3 - marked osseous destruction
  • Type C
    • # distal to tip of stem

Treatment

Prevention

  • during initial arthroplasty avoid creation of cracks, defects or windows
    • fix cracks w/ cerclage wire or bone grafting
  • regular radiographic follow up to detect osteolytic defects
    • may lead to #
    • x-ray at 1, 2, 5, 7 & 10 yrs, then every 2-3 yrs

Treatment

  • options
    • traction
    • cast, braces
    • ORIF
    • Revision
  • Type A # (peri-trochanteric)
    • avulsion type → non-operative
    • severe loosening around stem → revision
  • Type B1 (well fixed stem)
    • internal fixation, BG
  • Type B2-3 (loosening)
    • revision
    • ? custom maid stems
  • Type C (below tip)
    • internal fixation