title: Stress shielding around radial head prostheses.
abstract: OBJECTIVE
  Stress shielding is known to occur around rigidly fixed implants. We hypothesized that stress shielding around radial head prostheses is common but nonprogressive. In this study, we present a classification scheme to support our radiographic observations.
  
  
  METHODS
  We reviewed charts and radiographs of 86 cases from 79 patients with radial head implants from both primary and revision surgeries between 1999 and 2009. Exclusion criteria included infection, loosening, or follow-up of less than 12 months. We classified stress shielding as: I, cortical thinning; II, partially (IIa) or circumferentially (IIb) exposed stem; and III, impending mechanical failure.
  
  
  RESULTS
  Of 26 well-fixed stems, 17 (63%) demonstrated stress shielding: I = 2, II = 15 (IIa = 12, IIb = 3), and III = 0. We saw stress shielding with all stem types: cemented or noncemented; long or short; and straight, curved, or tapered. The only significant difference was that stems implanted into the radial shaft had less stress shielding than stems implanted into the neck or tuberosity (P = .03). The average follow-up was 33 months (range, 13-70 mo). Stress shielding was detectable by an average of 11 months (range, 1-15 mo). The pattern of bone loss was similar in 16 of 17 cases (94%), starting on the outer periosteal cortex. The 3 cases with circumferential exposure of the stem (stage IIb) averaged 2.6 mm (range, 1-4 mm) of exposed stem. Stress shielding never extended to the bicipital tuberosity, and there were no cases of impending mechanical failure.
  
  
  CONCLUSIONS
  Stress shielding around radial head prostheses is common, regardless of stem design. However, it is typically minor, nonprogressive, and of questionable clinical consequence.
  
  
  METHODS
  Therapeutic IV.
authors.affiliation: Biomechanics Laboratory, Department of Orthopedic Surgery, Mayo Clinic, Rochester MN, USA.
authors.forename: Cholawish
authors.initials: C
authors.lastname: Chanlalit
authors.forename: Dave R
authors.initials: DR
authors.lastname: Shukla
authors.forename: James S
authors.initials: JS
authors.lastname: Fitzsimmons
authors.forename: Kai-Nan
authors.initials: KN
authors.lastname: An
authors.forename: Shawn W
authors.initials: SW
authors.lastname: O'Driscoll
mesh_terms: D057929:Arthroplasty, Replacement, Elbow; D001862:Bone Resorption / Q000145:classification / Q000000981:diagnostic imaging*; D005260:Female; D006801:Humans; D007595:Joint Prosthesis / Q000009:adverse effects*; D008297:Male; D010521:Periosteum / Q000000981:diagnostic imaging; D011474:Prosthesis Design; D011859:Radiography; D011884:Radius / Q000000981:diagnostic imaging* / Q000601:surgery; D011885:Radius Fractures / Q000601:surgery; D012189:Retrospective Studies; D013314:Stress, Mechanical