The painful total knee replacement – the radiographic subtleties Mumith A

The painful total knee replacement – the radiographic subtleties
Mumith A1, Lasker I2, Dawood MT3,Lasker R4, Y Ghani5
1University
Hospital Southampton, 2Northwick Park Hospital
3Homerton University Hospital, 4Barking, Havering and Redbridge University Hospitals, 5High Wycombe Hospital
BACKGROUND
COMPONENT ALIGNMENT
q The Royal College of Radiologists state
we as trainees must develop a fundamental
understanding of the aetiology of painful
joints including complications of hip and
knee arthroplasty.
q In the UK 73,000 knee replacements were
carried out in 2013 (National Joint Registry
Report -2013), approximately 10% of these
underwent revision surgery most commonly
due to aseptic loosening.
q As a musculoskeletal radiologist
assessing TKR radiographs accurately will
enable community referrals to streamline
their patients to local orthopaedic
departments to manage such cases. Those
radiographs taken in emergency
departments for patients with painful total
knee replacements will be reported with
greater accuracy and therefore enable
earlier accurate diagnoses to be made.
q We present subtle radiographic signs that
must be looked for on a TKR radiograph.
q Deformities of the knee are accounted for by surgeon.
q Important to align components to optimise load bearing.
q If deformity still remains it causes increased stresses
on soft tissues and implant components causing pain
early failure of prosthesis
PROSTHESIS TYPES
There are many brands of prosthesis, three
main categories exist;
q Unconstrained: Minimal conformity
between the femoral component and
polyethylene liner. Allows for ‘femoral
rollback’: during flexion the femur rolls back
on the tibia allowing for greater flexion. (See
right) Relies on surrounding soft tissues to
provide stability.
q Semi-constrained: Greater conformity
between liner and femoral component,
improved AP stability at the cost of
decreased flexion.
q Constrained: True or rotating hinge.
Usually used in the revision situation and in
instability.
SS
Flexed femur
FEMORAL COMPONENT
q Should be at 90±3º to the femoral shaft in sagittal plane.
q 90º is neutral, <90º = flexed component, >90º = extended
component.
q Important for range of movement.
q Coronal plane alignment is best assessed on long leg
alignment views. Mechanical axis should pass centrally
through knee.
TIBIAL COMPONENT
q Should lie 90±3º to the femoral
shaft. Varus/valgus malposition
causes pain, ligamentous
instability and undue stresses on
implant.
MIGRATION -M
q Subsidence of prosthesis,
both tibial and femoral
components.
q >1mm a year is significant
Anterior slope
POLYETHYLENE WEAR -PW
q Decreasing thickness of joint
space, more significantly if
asymmetric suggests wear of
the polyethylene liner leading to eventual metal-on-metal
contact. This leads to pain, ligamentous instability and
eventually requires revision.
PW
CEMENT MANTLE -CM
q The tibial component is usually
the first to loosen as it has to withstand tensile and shearing forces.
A minimum of 4mm of cement
penetration into bone is said to
resist this.
Femoral
component
Poly. liner
Tibial component
M
POSTERIOR TIBIAL SLOPE
(PTS)
q Modern prosthesis aim for
PTS of 3-7º.
q Important for anteroposterior
stability, range of motion and
strain on the posterior cruciate
ligament, depending on
prosthesis type.
q Neutral or anterior slope
reduces range of flexion.
CM
SS
Varus
mal-alignment
STRESS SHIELDING -SS
q Bone is anisotropic and responds according to Wolf’s Law;
remodelling in response to forces applied to it. Osteopenic
areas are seen where the forces are offloaded by
components. Commonly the anterior distal femur and under
the tibial component.
q Usually after 2 years this reaches equilibirum and no further
radiographic changes are seen.
q Must be correlated with clinical picture also can be
differential for infection.
SO WHY IS IT PAINFUL?
Minimal conformity,
allowing for high
flexion with femoral
rollback
q Anterior slope of tibial component has led to impingement posteriorly on flexion. Over time this has led to
the femoral component being forced into flexion with loosening behind the trochlea component surface.
There is subsidence of the anterior tibial plate with asymmetrical polyethylene wear. All leading to abnormal
stresses passing through the soft tissue and implant leading to instability and pain.