Improved primary stability and load transfer of a customized osseointegrated transfemoral prosthesis compared to a commercial one

Abstract

Background: Transfemoral osseointegrated prostheses, like other uncemented prostheses experience the risk of aseptic loosening and post-operative periprosthetic fractures, with an incidence between 3% and 30%. To date, however, osseointegrated off-the-shelf prostheses are manufactured in a limited number of sizes, and some patients do not meet the strict eligibility criteria of commercial devices. A customized osseointegrated stem was developed and a pre-clinical in vitro investigation of the stem was performed, to evaluate its biomechanical performance.

Materials and methods: Six human cadaveric femurs were implanted with commercial stems, while the six contralateral were implanted with customized stems. Three more femurs that did not meet the eligibility criteria for the commercial stems were implanted with the customized stems. Two different loading scenarios (compression-flexion, and torsion) were simulated to measure the primary implant stability and the load transfer. For both loading scenarios, the displacements of the implant with respect to the host bone, and the strains on the bone surface were measured using digital image correlation (DIC). To measure the pull-out force, a tensile force was applied to the prostheses.

Results: The translational inducible micromotions during the compression-flexion test of the OsteoCustom stem were more than 4 times smaller than the commercial one (p < 0.05). The rotational inducible micromotions of the OsteoCustom stem were more than 3 times smaller than the commercial one (p < 0.05). Similar results were found from the torsional test. The full-field strain distribution of the commercial stem showed a slightly higher strain concentration near the stem tip (maximum principal strain = 1928±127 µɛ) than the OsteoCustom (maximum principal strain = 1758±130 µɛ). Similar results were found for the femurs that did not meet the eligibility criteria for the commercial stems and could be implanted with the OsteoCustom. No statistically significant difference was found in the extraction force between the two groups.

Discussion and conclusion: These results support the hypothesis that the OsteoCustom stem can offer better primary stability and load distribution compared to commercial implants. The outcome highlighted the potential benefits of the OsteoCustom prosthesis, which is capable of including a wider range of femoral anatomies than the current standard.

Keywords: In vitro biomechanical test; Implant micromotions; Lower-limb amputation; Primary stability; Stem-bone load transfer; Transfemoral osseointegrated prosthesis.

Fig. 1

LEFT: Replica of the commercial stem OFI-C, RIGHT: Example of the OsteoCustom stem. Also indicated are the main dimensions and the most relevant cross-sections. For the Commercial stem, the only relevant dimension was the diameter (Section A), which is constant along the stem. For the OsteoCustom stem, the parameters customized to each femur were: the diameter of the section at mid-length (Section B, which is circular), and the major (Max) and minor (Min) axes of the most distal section (Section C, which was elliptical), and the radius of curvature of the proximal portion

Fig. 2

LEFT) CT scan of a typical implantation of the commercial stem (sagittal plane, with a zoom of a distal cross-section showing the contact area of the prosthesis into the bone). RIGHT) CT scan of the contralateral implantation of the OsteoCustom stem (sagittal plane, with a zoom on a distal cross-section)

Fig. 3

A) Overview of the experimental setup of the compression-flexion test: a uniaxial-servo-hydraulic testing machine was used to deliver the force to the distal end of the specimens. The four cameras of the DIC framed the implanted specimen from a mediolateral and anterior view. B) Overview of the experimental setup for the torsional cyclic test. The four cameras of the DIC framed the medial, posterior, and lateral sides of the implanted specimen. C) Schematic of the analysis of the DIC-measured displacements to compute the permanent migrations and the inducible micromotions. The frames corresponding to the load peaks and load valleys were extracted. The permanent migrations were computed as the difference between the position of the implant relative to the bone at the last peak and at the first peak (Peak100–Peak1). The inducible micromotions were computed as the difference between the position of the implant relative to the bone at each load peak and at the corresponding valley (Peak N–Valley N). A subsection of (C) is reproduced from [18] under the Creative Commons Attribution License (CC BY)

Fig. 4

Translational and rotational primary stability in the compression-flexion test in terms of permanent migrations and inducible micromotions around the craniocaudal (CC), anteroposterior (AP), and mediolateral (ML) axes for both the commercial and the OsteoCustom stems. For each group, the individual specimens (small circles), the median (horizontal line), the 25–75% percentile (solid box), and the 5th and 95th percentile (whiskers) are indicated. Statistically significant differences (p < 0.05) between the commercial and the customized stem are highlighted with *. For the inducible micromotions, the pink band indicates the critical range of micromotions which is likely to prevent osseointegration [23, 24]. Therefore, the inducible micromotions of a stable implant should be lower than the pink bands. The dotted line represents the intrinsic error affecting the DIC-measured micromotion

Fig. 5

Translational and rotational primary stability through the torsional test in terms of permanent migrations and inducible micromotions around the craniocaudal (CC), anteroposterior (AP), and mediolateral (ML) axes for both the commercial and the OsteoCustom stems. For each group, the individual specimens (small circles), the median (horizontal line), the 25–75% percentile (solid box), and the 5th and 95th percentile (whiskers) are indicated. Statistically significant differences (p < 0.05) between the commercial and the customized stem are highlighted with *. For the inducible micromotions, the pink band indicates the critical range of micromotions which is likely to prevent osseointegration [23, 24]. Therefore, the inducible micromotions of a stable implant should be lower than the pink bands. The dotted line represents the intrinsic error affecting the DIC-measured micromotion

Fig. 6

Full-field distribution of the maximum principal strains (ε1) of one typical specimen with commercial stem (left) and the contralateral with OsteoCustom stem (right) at the peak load (simulatin the heel strike phase of gait)

Fig. 7

Comparison of the strain distribution in ROI 1 (top charts) and ROI2 (bottom). The maximum principal strain (ε1, charts on the left) and minimum principal strain (ε2, on the right) were analyzed for the commercial and the contralateral OsteoCustom implants. For each group, the individual specimens (small circles), the median (horizontal line), the 25–75% percentile (solid box), and the 5th and 95th percentile (whiskers) are indicated. No statistically significant differences were found between the commercial and the customized stem (p > 0.05)

Fig. 8

Comparison of the pull-out force of the commercial stems and OsteoCustom stems. For each group, the median (horizontal line), the 25–75% percentile (solid box), and the 5th and 95th percentile (whiskers) are indicated. No statistically significant differences were found between the commercial and the customized stem (p > 0.05)