Influence of reduced tip-apex distance on helical blade fixation-a biomechanical study.

Migration profile of helical blades differs from conventional screw design. Tip-apex distance (TAD) greater than 25 mm is associated with early failure in hip screws. This study investigates the effect of a reduced TAD on helical blade fixation. Six pairs of human femoral heads were used. Bone mineral density (BMD) was determined by pQCT. Local bone quality was evaluated by breakaway torque. Helical blades were implanted with TAD of 20 mm (normal) and 6 mm (reduced). Specimens were cyclically tested under progressively increasing physiologic loading at 2 Hz with starting peak force of 1'000N, increasing by 0.1N/cycle. Implant migration was monitored by radiographs every 250 cycles. Paired Student's t-test and Spearman rank correlation coefficient were used for statistical evaluation (p < 0.05). Mean BMD was 246.7 mgHA/cm3 SD48.7 (normal), 244.3 mgHA/cm3 SD47.6 (reduced); p = 0.93. Mean breakaway torque was 3.59 Nm SD2.11 (normal), 3.72 Nm SD1.83 (reduced); p = 0.91. Number of cycles to failure (1 mm vertical migration) was 16'416 SD7972 (normal), 20'000 SD5232 (reduced); p = 0.38. They correlated significantly with BMD (p = 0.01;R2  = 0.91) and breakaway torque (p < 0.049; R2  = 0.814) (normal). Breakaway torque correlated significantly with BMD (p = 0.02; R2  = 0.898) (normal). In reduced TAD these parameters did not correlate significantly. Normal TAD resulted in failure in varus collapse (n = 6), whereas reduced TAD showed blade perforation (n = 3), rotation (n = 2), varus collapse (n = 1). Fixation stability of helical blades correlates with bone quality when implants are fixed in cancellous bone. Near cortical blade fixation might increase the risk of intraarticular blade perforation.