Investigation

 

Aetiology

 

Intrinsic

 

Infection

 

Loosening

 

Thigh pain in uncemented

- micro motion at distal end of stem

- modulus mismatch

 

Stress fracture / insufficiency fracture

- pubic rami, sacral

 

Intra-operative fracture

 

Prosthesis failure

 

Subtle instability

 

Extrinsic

 

Psoas tendon

 

Muscular tendonitis

- irritation of Psoas

- stretching of Adductors

- vas lateralis herniation

 

Trochanteric bursitis / tear G medius

 

Non-union of Trochanteric Osteotomy

 

THR GT Nonunion

 

HO

 

Lumbar / Knee / Pelvic / Abdominal pathology

 

History

 

Nature of Pain

 

°Pain-free interval  

- indolent infection

- pathology elsewhere (pain same as pre-op)

- poor implant fixation

- impingement

 

Pain-free interval 

- loosening

- infection

- implant failure

 

Mechanical pain 

- loosening

 

Start up pain

- pain with initial movement

- recedes as implant settles

- loosening symptoms

 

Rest pain / night pain 

- infection

- tumour

 

Location

 

Buttock / groin pain 

- cetabular pathology

 

Thigh / knee pain 

- Femoral pathology

 

Pain over GT suggests

- trochanteric bursitis / tear G medius

- Non-union of trochanteric osteotomy

 

Pain in other locations 

- spinal stenosis 

- knee OA 

 

Radiating below knee

- radiculopathy

 

Infection

 

Drainage postoperative suggests +++ infection if > 1/12 post-op

History of bacteraemia suggests infection

Prolonged in hospital ABx treatment

 

Examination

 

Pain with ROM 

- loosening - extremes of motion

- infection - pain throughout motion

- implant failure

 

Tenderness over GT

 

Wound 

- induration, erythema & drainage

 

Spine, knee & vascular  exam

 

Groin for inguinal hernia

 

Xray

 

Problems

 

1. May be normal in face of pathology

- serial comparison very important

 

2. Difficult to differentiate infection v loosening on XR

 

Infection

 

Infected THR Endosteal ScallopingInfected THR Periosteal New Bone

 

1. Radiolucent lines

2. Focal Osteolysis with Endosteal scalloping

3. Periosteal new bone 

- almost pathognomonic

- usually at junction meta / diaphysis on medial side

- only seen in 1-2%

 

Loosening

 

Easier to identify loosening in femur than acetabulum

- femur 90% accuracy

- acetabulum 65% accuracy

 

Lucent lines don't necessarily represent problem

- may be present in well-fixed prosthesis (retrieval studies)

- due to remodelling

 

WCC

 

Little value

- increased in 15%

- raised only if sepsis +++

 

ESR 

 

> 30 mm = 80% sensitivy & specificity for infection

 

Problems

- takes 6 - 12 / 12 to normalise post OT

- very non specific, increased in RA and remote pathology

- can be raised in aseptic loosening

 

CRP 

 

> 10 mg/l = 90% sensitiviy & specificity

- rarely increased with loosening

 

More predicable response post OT

- peak 2/7 (~400)

- normal after 3 /52

 

In the absence of other causes of elevation

 

If CRP is negative can be confident is no infection

- negative predictive value 99%

 

If CRP is positive is still a 20% chance that is no infection

- positive predictive value 75%

 

IF both ESR > 30 and CRP >10, 84% probability of sepsis

 

Te99 Scan

 

Bone scan may show increased uptake from

- infection

- loosening

- HO

- Paget's

- stress fracture

- large uncemented stem (modulus mismatch)

- tumors

- RSD

 

Advantage

- pathology unlikely if negative

 

Disadvantage

- very sensitive

- poor specificty

- doesn't differentiate cause

 

Lieberman et al JBJS Br  1993

- no benefit of NMBS over x-ray in diagnosis of infection or loosening

 

Residual activity 

 

Cemented 

- majority return to normal by 1 year

- 20% remain hot at portions of stem / GT / LT past 1 year

 

Uncemented 

- can remain hot for 2 years 

- can remain hot at distal stem for many years

 

Infected prosthesis

 

All phases increased & usually diffuse in 3 phases

- highly suggestive of infection

- can get focal uptake similar to loosening but rarer

 

Loose prosthesis

- localised increased uptake on delayed phase only

- motion of prosthesis causes increased bone turnover due to bone resorption 

- increased uptake @ GT & LT alone may be normal post op change

- well advanced loosening can show diffuse uptake as for an infected hip

 

THR Bone Scan NormalTHR Hot Cup Quiscent Femur

 

Stress sites 

- will see localised area of uptake on scan

- corresponds with cortical thickening on plain XRs

 

Insufficiency fracture

- occur in osteopaenic patients

- pubic rami fractures may cause groin pain

- sacral fractures may cause posterior hip pain

 

Indium 111 Labelled WC Scan 

 

Uncertain role 

- expensive, difficult 

- have to harvest WC

 

More specific for infection

- especially when combine with bone scan

- sensitivity 92%

- range specificity 75 - 100%

 

Aspiration

 

THR Aspiration

 

Technique

- no Abx >4 weeks

- II control & with contrast / confirm in joint

- no LA (bacteriostatic)

- aspirate hip joint x 3 specimens

- if only 1 specimen positive then repeat

 

If dry, inject normal saline & aspirate 

- controversial

 

> 65% PMN infection likely

> 1600 white cells microlitre

 

Results

 

Harris & Barrack JBJS 1996

- 2% positive rate if aspirate all hips

- therefore be selective

 

Lachiewicz et al JBJS Am 1996

- hip pain and elevate ESR

- 92% sensitivity & 97% specificity

 

HCLA

 

Crawford et al JBJS 1998

- 95-100% sensitivity

- ff good results from LA expect same from THR

- demonstrates that the pain is from the hip

 

Intra-Operative Frozen Section

 

PMN Cell Count 

- 40x power, count white cells in that field

- average over 10 fields

 

Mirra 1976 > 5phpf

- 84% sens, 96% spec

 

Lonner 1996 > 10phpf

- 84% sens, 99% spec

 

Intra Operative gram stain & m/c/s

 

Gold Standard

- 10% false positive

- Gram stain sensitivity < 20%, but very specific

 

All revisions no antibiotics for 4 weeks prior

 

Surgical Opinion

 

Sensitivity 70%

Specificity 85%

 

Management

 

Algorithm

 

Xray N / Scan N / ESR & CRP N

- not infected

- explore extrinsic causes

 

Xray Loose / ESR & CRP raised 

- infected

- 2 stage revision with intra-operative M/C/S

 

Xray normal / Hot scan / Raised ESR & CRP 

- infected

- 2 stage revision

- intra operative FFS to confirm

 

Xray / Scan / ESR / CRP all equivocal 

 

Aspirate