· major complications of THR are
· infection
· loosening
· both cause pain
· difficult to differentiate
· no test is 100% specific or sensitive
· pain may be due to causes
· intrinsic to arthroplasty
· extrinsic to arthroplasty
· infection
· loosening
· micromotion (thigh pain in uncemented)
· heterotopic ossification
· stress fracture
· prosthesis failure
· muscular
· irritation of psoas over acetabulum
· stretching of adductors
· trochanteric bursitis
· nonunion of trochanteric osteotomy
· lumbar pathology
· spinal stenosis
· knee pathology
· degenerative joint disease
· acute fulminating infection
· develops within 1 month of surgery
· wound infection or infected haematoma that has progressed to deep infection
· typically
· febrile patient
· red swollen discharging wound
· more indolent
· develop more than 1 month from surgery
· typically
· well patient
· healed wound
· worsening of pain
· never pain-free interval
· acute onset of clinical symptoms
· can occur several years after surgery
· typically
· well patient
· previously well functioning hip
· history and examination should be able to identify
· early postoperative infection
· causes extrinsic to arthroplasty
· difficulty differentiating
· late infection
· aseptic loosening
· no imaging modality can reliably differentiate them
· should be able to
· identify early postoperative infection
· differentiate extrinsic from intrinsic causes
Pain
· most common symptom
Nature
· absence of pain-free interval suggests
· indolent infection
· pathology elsewhere (hip or spine)
· pain-free interval suggests
· loosening
· infection
· implant failure
· mechanical pain suggests loosening
· rest pain and night pain suggests infection
Location
· buttock and groin pain suggests acetabular pathology
· thigh and knee pain suggests femoral patholog
· pain over greater trochanter suggests
· trochanteric bursitis
· nonunion of trochanteric osteotomy
· pain in other locations suggests other causes
· buttock, thigh and calf in spinal stenosis
· knee and shin in osteoarthritis of knee
· calf in ischaemic claudication
Drainage
· next most common symptom
· strongly suggestive of infection if still present 1 month after operation
Bacteraemia
· history of bacteraemia suggests infection
· pain with hip motion suggests
· loosening
· infection
· implant failure
· wound induration, erythema and drainage suggests infection
· examine spine, knee and vascular system
· next step in investigation
· problems
1. may be normal in face of pathology in arthroplasty
2. cannot always differentiate infection and loosening
· suggestive features
· radiolucent lines
· focal osteolysis with endosteal scalloping
· periosteal new bone formation *
· x-rays may be normal in presence of infection
· easier to identify loosening in femur than acetabulum
· femur - 90% accuracy
· acetabulum - 65% accuracy
· lucent lines do not necessarily represent problem
· often due to remodelling
· may be present in well-fixed prosthesis
Definite signs
Stem
· migration of component
· cement fracture
· fracture of stem
Cup
· migration
· crack in cement mantle
Probable signs
Stem
· continuous radiolucent line at bone-cement interface
Cup
· continuous radiolucent line > 2 mm
· WCC, ESR and CRP obtained
· useful markers but not specific for infection
· uniformly found to be of little value
· raised only in fulminant sepsis
· widely used
Problems
Variable response to surgery
· usually slightly increased before surgery
· variable pattern after surgery
· may remain elevated for 12 months
May be elevated by concurrent conditions
· esp. inflammatory arthropathy
Does not differentiate between infection and loosening
· may be increased with loosening
· not always increased with infection
· less widely used
Advantages
More predictable response to surgery
· normal before surgery
· rapid rise with peak 2 days after surgery
· rapid decrease after 3 days
· normal by 3 weeks
Helps to differentiate between infection and loosening
· rarely increased with loosening
· almost invariably increased with infection
· next step in investigation
Advantages
Sensitive test
· pathology unlikely if bone scan negative
· very unlikely if x-rays and bone scan negative
Problems
Variable response after THR
· increased uptake postoperatively
· usually return to baseline by 6 months
· may persist
· 20% at 6 months
· 10% at 12 months
· but uptake is stable or decreasing
Does not differentiate causes of increased uptake
· poor specificity
· increased with
· infection
· loosening
· heterotopic bone formation
· fracture
· suggested that
· diffuse uptake suggestive of infection
· focal uptake suggestive of loosening
· not useful alone
· used as sequential test
· with technetium scan
· infection suggested if
· uptake incongruent
· Ga uptake > Tc uptake in same area
Advantages
Sensitive
· few false positives
· sensitivity 90%
Disadvantages
Not specific
· numerous false negatives
· sensitivity 50%
· thus has
· strong positive predictive value
· weak negative predictive value
· initially thought to be superior to sequential Tc-Ga scan
· later results less promising
· little place in routine workup
Disadvantages
Ineffective in detecting chronic infections
· response less granulocytic
Expensive and labour-intensive
· white cells must be harvested, labelled and re-injected
· not offered at many centres
Technique
· insert needle into pseudocapsule
· under II control
· aspirate hip joint
· if dry tap, inject saline and aspirate again
· take Trucut biopsy of capsule
Results
· false-negative may be seen with
· unsuccessful aspiration
· concurrent antibiotic administration
· technical laboratory factors
· sensitivity reported at 66%
· false-positive may be seen with contamination of specimen
· specificity reported at 80-90%
Technique
· inject contrast into pseudocapsule
· inject until patient feels discomfort
· image with II
Results
· strongly predictive of loosening with contrast at cement-bone interface
HISTORY AND EXAMINATION->extrinsic->APPROPRIATE Rx
\/
intrinsic
\/
PLAIN X-RAY->not loose->BONE SCAN->not hot->OBSERVE
\/ \/
loose hot
____ \/____________________\/__
\/ \/ \/
ESR and CRP and ASPIRATION
\/ \/ \/
all positive some positive none positive
\/ \/ \/
INFECTED NOT INFECTED
\/ \/ \/
2-stage revision 1-stage revision with biopsy \/
aspiration positive OR aspiration negative
Hx,Ex,XR,ESR,CRP Hx,Ex,XR,ESR,CRP
not suggestive of infection suggestive of infection
\/
REPEAT ASPIRATION
\/ \/
positive negative
\/ \/
INFECTED NOT INFECTED