title: Mechanical torque measurement predicts load to implant cut-out: a biomechanical study investigating DHS anchorage in femoral heads.
abstract: BACKGROUND
  Bone strength plays an important role in implant anchorage. Bone mineral density (BMD) is used as surrogate parameter to quantify bone strength and to predict implant anchorage. BMD can be measured by means of quantitative computer tomography (QCT) or dual energy X-ray absorptiometry (DXA). These noninvasive methods for BMD measurement are not available pre- or intra-operatively. Instead, the surgeon could determine bone strength by direct mechanical measurement. We have evaluated mechanical torque measurement for (A) its capability to quantify local bone strength and (B) its predictive value towards load at implant cut-out.
  
  
  METHODS
  Our experimental study was performed using sixteen paired human cadaver proximal femurs. BMD was determined for all specimens by QCT. The torque to breakaway of the cancellous bone structure (peak torque) was measured by means of a mechanical probe at the exact position of subsequent DHS placement. The fixation strength of the DHS achieved was assessed by cyclic loading in a stepwise protocol beginning with 1,500 N increasing 500 N every 5,000 cycles until 4,000 N.
  
  
  RESULTS
  A highly significant correlation of peak torque with BMD (QCT) was found (r = 0.902, r (2) = 0.814, P < 0.001). Peak torque correlated highly significant with the load at implant cut-out (r = 0.795, P < 0.001). All specimens with a measured peak torque below 6.79 Nm failed at the first load level of 1,500 N. The specimens with a peak torque above 8.63 Nm survived until the last load level of 4,000 N.
  
  
  CONCLUSIONS
  Mechanical peak torque measurement is able to quantify bone strength. In an experimental setup, peak torque identifies those specimens that are likely to fail at low load. In clinical routine, implant migration and cut-out depend on several parameters, which are difficult to control, such as fracture type, fracture reduction achieved, and implant position. The predictive value of peak torque towards cut-out in a clinical set-up therefore has to be carefully validated.
authors.affiliation: AO Development Institute, Clavadelerstrasse 8, 7270 Davos Platz, Switzerland.
authors.forename: Norbert
authors.initials: N
authors.lastname: Suhm
authors.forename: Clemens
authors.initials: C
authors.lastname: Hengg
authors.forename: Ronald
authors.initials: R
authors.lastname: Schwyn
authors.forename: Markus
authors.initials: M
authors.lastname: Windolf
authors.forename: Volker
authors.initials: V
authors.lastname: Quarz
authors.forename: Markus
authors.initials: M
authors.lastname: Hänni
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D015519:Bone Density; D002102:Cadaver; D005270:Femur Head / Q000502:physiology*; D006801:Humans; D008875:Middle Aged; D019736:Prostheses and Implants; D019415:Torque