Tangential Bicortical Locked Fixation Improves Stability in Vancouver B1 Periprosthetic Femur Fractures: A Biomechanical Study

Abstract

Objectives: The biomechanical difficulty in fixation of a Vancouver B1 periprosthetic fracture is purchase of the proximal femoral segment in the presence of the hip stem. Several newer technologies provide the ability to place bicortical locking screws tangential to the hip stem with much longer lengths of screw purchase compared with unicortical screws. This biomechanical study compares the stability of 2 of these newer constructs to previous methods.

Methods: Thirty composite synthetic femurs were prepared with cemented hip stems. The distal femur segment was osteotomized, and plates were fixed proximally with either (1) cerclage cables, (2) locked unicortical screws, (3) a composite of locked screws and cables, or tangentially directed bicortical locking screws using either (4) a stainless steel locking compression plate system with a Locking Attachment Plate (Synthes) or (5) a titanium alloy Non-Contact Bridging system (Zimmer). Specimens were tested to failure in either axial or torsional quasistatic loading modes (n = 3) after 20 moderate load preconditioning cycles. Stiffness, maximum force, and failure mechanism were determined.

Results: Bicortical constructs resisted higher (by an average of at least 27%) maximum forces than the other 3 constructs in torsional loading (P < 0.05). Cables constructs exhibited lower maximum force than all other constructs, in both axial and torsional loading. The bicortical titanium construct was stiffer than the bicortical stainless steel construct in axial loading.

Conclusions: Proximal fixation stability is likely improved with the use of bicortical locking screws as compared with traditional unicortical screws and cable techniques. In this study with a limited sample size, we found the addition of cerclage cables to unicortical screws may not offer much improvement in biomechanical stability of unstable B1 fractures.

Figure 1

Custom fixture used to hold synthetic femur in a reproducible position, guide an oblique femoral neck cut, and direct initial drilling of the intramedullary canal.

Figure 2

Flouroscopic images of representative constructs from each group.

Figure 3

Schematics of mechanical loading setups for (A) axial and (B) torsional loadings.

Figure 4

Maximum force reached, for both torsional (A) and axial (B) loadings. Bicortical constructs typically did not fail during testing; the above results for those two construct types were limited by force capacity and space limitations in the testing setup. Horizontal bars at the top indicate pairs of constructs having significantly different means (p < 0.05). Error bars indicate standard deviation, and diamonds represent the data from each individual specimen tested.

Figure 5

Stiffness (up to 500 N load) for both torsional and axial loadings. Horizontal bars at the top indicate pairs of constructs having significantly different means (p < 0.05). Error bars indicate standard deviation, and diamonds represent the data from each individual specimen tested.