title: Short- or long-stem prosthesis for intramedullary bypass of proximal humeral fractures with severe metaphyseal bone loss: evaluation of primary stability in a biomechanical model.
abstract: BACKGROUND
  Proximal humeral fractures with substantial metaphyseal comminution are challenging to treat. In the elderly with osteoporotic bone, arthroplasty sometimes remains the only valuable option; however, the minimally required length of stem fixation is not known. The aim of this study was to investigate the primary stability of cemented short- and long-stem prostheses with different intramedullary fracture bypass lengths.
  
  
  METHODS
  Osteoporotic composite bone models of the humerus (Synbone, Malans, Switzerland) with 3 different fracture levels (group A, 6 cm distal to surgical neck; group B, 7 cm distal to surgical neck; and group C, 8 cm distal to surgical neck) were prepared with a cemented standard short (S)- or long (L)-stem prosthesis and were tested for torque to failure. As a reference, we used models with intact bone (group R-O) and a short-stem prosthesis implanted at the surgical neck (group R-P). The radiographic bypass index (BI) was calculated before testing (fracture level to stem tip [in millimeters]/outer cortical diameter at fracture level [in millimeters]).
  
  
  RESULTS
  The resulting BIs of each group were as follows: 1.7 in group A-S, 3.4 in group A-L, 1.4 in group B-S, 3.2 in group B-L, 1.0 in group C-S, and 2.9 in group C-L. Compared with group R-O, the torques to failure of groups B-S and C-S were significantly lower, whereas only group C-S was significantly weaker than group R-P (P < .01). Comparing short- and long-stem bypasses of different fracture heights, we found that only group C-L showed a significantly higher resistance to torque (P < .01).
  
  
  CONCLUSIONS
  A short-stem bypass with a BI of 1.7 was sufficient for primary stability tested by torque to failure in this biomechanical setting. For smaller BIs, a long-stem prosthesis should be considered.
  
  
  METHODS
  Basic science study, biomechanics.
authors.affiliation: Department of Orthopaedics and Traumatology, Kantonsspital St Gallen, St Gallen, Switzerland.
authors.forename: Christian
authors.initials: C
authors.lastname: Spross
authors.forename: Lukas
authors.initials: L
authors.lastname: Ebneter
authors.forename: Emanuel
authors.initials: E
authors.lastname: Benninger
authors.forename: Matthias
authors.initials: M
authors.lastname: Erschbamer
authors.forename: Johannes
authors.initials: J
authors.lastname: Erhardt
authors.forename: Bernhard
authors.initials: B
authors.lastname: Jost
mesh_terms: D000368:Aged; D019643:Arthroplasty, Replacement / Q000379:methods; D001696:Biomechanical Phenomena; D005260:Female; D018460:Fractures, Comminuted / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D006801:Humans; D006811:Humerus / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery; D007595:Joint Prosthesis; D008954:Models, Biological; D010024:Osteoporosis / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D011859:Radiography; D012784:Shoulder Fractures / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery*; D012785:Shoulder Joint / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery; D019415:Torque
keywords: Basic Science Study; Biomechanics; Intramedullary bypass; arthroplasty; comminution; fracture; proximal humerus