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.