Oral Presentation ANZBMS-MEPSA-ANZORS 2022

A six-dimensional envelope to describe failure in an amputated femur with an osseointegrated implant under external loads (#32)

Hans A Gray 1 , Dale L Robinson 1 , Ryan Tiew 1 , David Ackland 1 , Peter Lee 1
  1. The University of Melbourne, Melbourne, VICTORIA, Australia

Osseointegrated implants for femoral amputation are gaining popularity over conventional socket designs. However, these implants are associated with a high rate of femoral fractures [1]. To help understand these femoral fractures better, we aimed to expand the concept of a failure envelope [2] to six dimensions. The aim of this study was to use finite element analysis to quantify all external loading conditions (forces and moments) that when applied externally on the osseointegrated implant were just sufficient to fracture the femur.

A patient-specific finite element (FE) model of one right femur with an osseointegrated implant was created from CT data (details in [3]). A set of 116,444 six-dimensional (6D) unit vectors defined unique linear combinations of three moment and three force components that were applied to the distal end of the implant. The force and moment components were scaled up gradually until femoral fracture occurred. Femoral fracture was predicted when the volume of contiguous failed elements reached 350 mm3 [4]. The points in 6D space that describe the failure loads defined the surface of the failure envelope in 6D space.

The largest direct force the implant-femur construct was able to withstand was in the superior direction (3.4 BW = 2768 N, Fig. 1C) while the largest moment was about the mediolateral direction (1.1 BWK = 138 Nm, Fig. 1A).

A six-dimensional failure envelope for the implanted femur in a transfemoral amputee was created. Published mean force and moment data measured for level walking across 10 patients [5] fell within the envelope throughout the entire activity with a minimum safety factor of 1.2. The methods described in the current study used in conjunction with experimental validation may provide useful data to understand femoral fractures and inform the design of safer osseointegrated implants.  

 

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  1. [1] J. S. Hoellwarth et al., “Periprosthetic osseointegration fractures are infrequent and management is familiar,” Bone Jt. J., vol. 102 B, no. 2, pp. 162–169, 2020, doi: 10.1302/0301-620X.102B2.BJJ-2019-0697.R2.
  2. [2] R. Korabi, K. Shemtov-Yona, A. Dorogoy, and D. Rittel, “The Failure Envelope Concept Applied To The Bone-Dental Implant System,” Sci. Rep., vol. 7, no. 1, p. 2051, 2017, doi: 10.1038/s41598-017-02282-2.
  3. [3] D. L. Robinson et al., “Load response of an osseointegrated implant used in the treatment of unilateral transfemoral amputation: An early implant loosening case study,” Clin. Biomech., vol. 73, no. January, pp. 201–212, 2020, doi: 10.1016/j.clinbiomech.2020.01.017.
  4. [4] W. B. Edwards and K. L. Troy, “Finite element prediction of surface strain and fracture strength at the distal radius,” Med. Eng. Phys., vol. 34, no. 3, pp. 290–298, 2012, doi: 10.1016/j.medengphy.2011.07.016.
  5. [5] L. Frossard, “Loading characteristics data applied on osseointegrated implant by transfemoral bone-anchored prostheses fitted with basic components during daily activities,” Data Br., vol. 26, p. 104492, 2019, doi: https://doi.org/10.1016/j.dib.2019.104492.