Metal-on-metal hips. “What’s to see on MRI?”

1. The following presentation was given at the Radiological Society of North America (RSNA) in 2009 Metal-on-metal hips. “What’s to see on MRI?” metalions@ucl.ac.uk S. Sabah, A. Mitchell, J. A. Skinner, J. Henckel, J. Cobb, A. Hart MOM Hips: “What’s to see on MARS MRI?”

2. introduction • Use of metal-on-metal hip resurfacing for primary hip arthroplasty is increasing • ref • Proposed advantages over THR include: • Bone conservation • Reduced incidence of dislocation • ref MOM Hips: “What’s to see on MARS MRI?”

3. Introduction Epidemiology • Metal-on-metal hip resurfacing now accounts for 8% of primary hip replacement in the UK • UK National Joint Registry 5th Annual Report: • MOM-HR have the highest revision rates for primary hip prostheses • Estimated 2.8% at 3-years “Unexplained”43% MOM Hips: “What’s to see on MARS MRI?”

4. Our experience • We are a tertiary referral centre for painful MOM-HR • We run weekly “unexplained painful MOM hip” clinics and have performed MARS MRI in over 100 MOM-HR • We are linked to the London Implant Retreival Centre which has analysed over 200 explanted hip resurfacings from across UK MOM Hips: “What’s to see on MARS MRI?”

5. Introduction MARS MRI • Metal artefact-reduction sequences for MRI can produce diagnostic quality images of the periprosthetic tissues • Toms et al, ClinRadiol [2008], Hart et al, JBJS(Br) [2009] • Recent papers have proposed soft tissue disease (especially so-called “pseudotumours”) may account for high failure rates of MOM-HR • Fang et al, SkelRad [2008], Pandit et al, JBJS(Br) [2008], Hart et al, JBJS(Br) [2009] Axial T1W – Iliopsoas fluid collection MOM Hips: “What’s to see on MARS MRI?”

6. “Pseudotumours” • Aetiology • Unknown • Histopathology • Classification as aseptic lymphocyte-dominated vasculitis association lesions (ALVAL) • Willert, JBJS(Am) [2005] • Subsurface band of macrophages with perivascular lymphocytes • Hart, AAOS Annual Meeting[2009] • Subtypes • Solid, fluid and mixed • Fang [2008], Pandit [2008], Hart [2009] Hip neocapsule specimen. H&E [top]. CD68 immunohistochemistry (macrophage marker) [bottom] MOM Hips: “What’s to see on MARS MRI?”

7. “Pseudotumours” solid masses Axial T1W + contrast MOM Hips: “What’s to see on MARS MRI?”

8. “Pseudotumours” – solid masses imaging features • Intermediate signal intensity on T1W and T2W • No fluid characteristics • Poorly demarcated border • Local regional soft tissue invasion Axial T1W MOM Hips: “What’s to see on MARS MRI?”

9. “Pseudotumours” – solid masses Complications • Lymphatic stasis • Limb swelling • Nodal enlargement • Sciatic nerve involvement Axial T1W + contrast Coronal T1W MOM Hips: “What’s to see on MARS MRI?”

10. “Pseudotumours” – solid masses Clinical • With late presentation: • Severe pain • Reduced range of movement • Very low hip scores MOM Hips: “What’s to see on MARS MRI?”

11. “Pseudotumours” – solid masses Management • Dictated by clinical evaluation • Surgical debridement with maximal preservation of normal tissue • Ensure attention to other soft tissue structures involved within mass • INSERT OPERATION PICTURE MOM Hips: “What’s to see on MARS MRI?”

12. “Pseudotumours” Fluid collections Axial T2W MOM Hips: “What’s to see on MARS MRI?”

13. “Pseudotumours” – fluid collections classification • Radiological • Location • Anterior • Posterior • Lateral • Signal characteristics • Core • Pseudocapsule • Clinical • Aetiology • Infected or sterile • Status • Symptomatic/asymptomatic Axial T2W Coronal STIR MOM Hips: “What’s to see on MARS MRI?”

14. “Pseudotumours” – fluid collections classification • Radiological • Location • Anterior • Posterior • Lateral • Signal characteristics • Core • Pseudocapsule • Clinical • Aetiology • Infected or sterile • Status • Symptomatic/asymptomatic MOM Hips: “What’s to see on MARS MRI?”

15. “Pseudotumours” – fluid collections Signal characteristics • Core • Signal intensity on T1W • Intermediate • High • Mixed Axial T1W NEED BESAGNI T1W FOR ‘MIXED’ MOM Hips: “What’s to see on MARS MRI?”

16. “Pseudotumours” – fluid collections Signal characteristics • Pseudocapsule • Smooth, linear? • Irregularly thickened? • Areas of susceptibility artefact? Coronal STIR Coronal STIR MOM Hips: “What’s to see on MARS MRI?”

17. “Pseudotumours” – fluid collections classification • Radiological • Location • Anterior • Posterior • Lateral • Signal characteristics • Core • Pseudocapsule • Clinical • Aetiology • Infected • Sterile • Status • Symptomatic/asymptomatic Axial T2W. Infected fluid collection in “dumb-bell” about iliopsoas tendon (asterisk) • appear identical MOM Hips: “What’s to see on MARS MRI?”

18. Other pathology • Muscles • Oedema • Atrophy • Tendons • Avulsion • Bones • Fracture • Metastases • Osteolysis Coronal STIR. Extensive muscle oedema posterior to right joint MOM Hips: “What’s to see on MARS MRI?”

19. Other pathology - muscles Muscle atrophy • Individual muscles easily differentiated on MARS MRI • 2 labelled diagrams of normal max, med, min, oe, qf max = Gluteus maximus med = Gluteus medius min = Gluteus minimus oe = Obturatorexternus qf = quadratusfemoris MOM Hips: “What’s to see on MARS MRI?”

20. Other pathology - muscles Muscle atrophy • Assess on T1W images • Compare to contralateral side • Example grading system: • Bal et al, Intr Course Lect [2008] MOM Hips: “What’s to see on MARS MRI?”

21. Other pathology - muscles Muscle atrophy • Significance unknown • surgically induced or indicator of disease? Axial T1W – Atrophy of obturatorexternus and quadratusfemoris on right-side MOM Hips: “What’s to see on MARS MRI?”

22. Other pathology - tendons tendons • Tendon avulsion is best seen on T1W images as discontinuity of the low signal muscle attachment • Toms [2008] • Avulsion may be found with: • Inadequate/absent surgical repair • Local space-occupying lesion (e.g. fluid collection) Axial T1W – Lateral fluid collection (arrow) associated with marked atrophy of gluteus medius muscle and avulsion of its tendon MOM Hips: “What’s to see on MARS MRI?”

23. Other pathology - bones bones • Screen MARS MRI images for fracture, osteolysis and metastatic disease • Always refer to plain radiographs of pelvis and hips for comparison Coronal T1W showing diffuse metastases – NEED TO CHANGE MOM Hips: “What’s to see on MARS MRI?”

24. Other pathology - bones bones • Fracture • Acetabulum • Often difficult to visualise due to artefact • Pubic rami Axial T1W image of a fracture of the inferior pubic ramus on the right (circled) MOM Hips: “What’s to see on MARS MRI?”

25. Other pathology - bones bones AP Radiograph (left) and Axial T1W images of racture of acetabulum and inferior pubic ramus (circled) MOM Hips: “What’s to see on MARS MRI?”

26. Other pathology Incidental pathology • Dermoid MOM Hips: “What’s to see on MARS MRI?”

27. conclusion • MARS MRI can produce diagnostic quality images of the periprosthetic tissues • “Pseudotumours” have characteristic features on MARS MRI and may be important causes for failure of MOM-HR • Musculotendinous and incidental pathologies should not be overlooked MOM Hips: “What’s to see on MARS MRI?”