The experimental protocol of this study was approved by our institutional review board in priori. The femoral head was retrieved from patients who underwent hip arthroplasty owing to femoral neck fracture between January 2018 and December 2019. The specimens from the patients who agreed to donate the retrieved femoral head for the purpose of this study were selected for analysis. The femoral heads were excluded if acquired from patients 1) who had underlying pathologic conditions that may influence bone quality other than osteoporosis, 2) who had undertaken medication that may potentially influence the quality of the bone, 3) whose BMD was not obtained from the contralateral hip owing to the remaining implants from a previous surgery, and 4) with a history of other osteoporotic fractures.
Twenty-four femoral heads that fulfilled our inclusion criteria were retrieved, which constituted the basis of our study. The demographic data of the donors are listed in Table 1.
Table 1. Demographic data of the donor patients
|
Mean ± standard deviation
|
Minimum/maximum
|
Age (years)
|
77.10±9.92
|
50/94
|
Sex (% of women)
|
12 (60%)
|
|
Time from fracture to retrieval (days)
|
4.74±3.12
|
1/7
|
Body mass index (kg/m2)
|
20.89±2.94
|
14.47/26.23
|
BMD measurement of the non-fractured hip
The BMD of the donor patients was measured from the non-fractured hip using dual-energy X-ray absorptiometry (DXA; Prodigy Advance, GE Healthcare, USA) at the time of admission for surgery. The BMD was measured and collected using the Encore program (GE Lunar Prodigy, USA), with the region of interest (ROI) set at the total proximal femur and femoral neck.
All measurements were performed by two radiotechnologists. Quality control was performed for both technologists and densitometry devices according to the protocol recommended by the International Society for Clinical Densitometry (ISCD). Precision assessment was also performed in priori to measure the least significant difference (LSC), which was 4.2% in the total hip and 5.1% in the femoral neck. The measured LSC in our institution is within the recommendation of the ISCD.[12]
Retrieval of the femoral head
At the time of hip arthroplasty, the femoral head was retrieved with caution to minimize iatrogenic damage. If the ligamentum teres were intact, it was resected with scalpel to prevent avulsion of the femoral head during the dislocation process. The retrieved femoral head was washed with saline and dried at room temperature for 1 h. This was then fresh frozen at -20°C for later experiments.
BMD measurement of the fractured hip
Direct BMD measurement of the retrieved femoral head was performed using the Quantum GX micro-CT imaging system (PerkinElmer, Hopkinton, MA, USA) within 10 days of the retrieval. The fresh frozen femoral head was thawed at room temperature for 24 h before the micro-computed tomography (CT) scan. The X-ray source was set to levels of 90 kV and 80 μA, with a field of view of 72 mm (voxel size, 144 μm; scanning time, 4 min). Three-dimensional imaging was represented by an existing software within the Quantum GX, and images with a resolution of 4.5 μm were obtained for visualization and display. The BMD of the femur was estimated using a hydroxyapatite phantom (QRM-Micro-CT-HA, Quality Assurance in Radiology and Medicine GmbH, Germany), which was scanned using the same parameters.
The spherical cap region of 3 cm from the fovea capitis was selected as an ROI for the BMD measurement of the retrieved femoral head (Figure 1).
Bone mechanical property test
Femoral head preparation
Immediately following the BMD measurement with micro-CT, the femoral head was fixed to the custom-made jig, which was used as a guide for femoral head resection and fixation during the mechanical test. The jig was manufactured with stainless steel and includes a spherical cap engraving at a depth of 30 mm to accommodate the femoral head. It includes two holes to insert 2.8-mm K-wires, so that the femoral head is firmly fixed within the engraving (Figure 2a). An additional hole was developed at the inferior most region of the engraved sphere to enable penetration of the lag screw during the biomechanical test. A multiple jig with the same design was manufactured with a diameter of engraved hemi-sphere in 4-mm increments from 40 mm to 56 mm, so that different sizes of the femoral head could be adopted.
The femoral head was fixed in the jig in the position where the region 5 mm above the fovea capitis is placed at the inferior most part of the sphere cap engraving. The alignment of the femoral head was determined in priori based on the direction of typical lag screw insertion. After placement of the femoral head, the protruded bone out of the jig was resected, so that the resultant femoral head was in the form of a sphere cap of a 30-mm height (Figure 2b).
Lag screw migration test
The mechanical test was performed using a servo-hydraulic universal test machine (MTS Bionix Landmark 370, MTS System Corporation, USA). A helical blade-type lag screw from a commercially available proximal femoral nail system (PFNA-II blade, Depuy Synthes, Switzerland) was utilized to test the mechanical properties of the femoral head. The specifications from the provider indicate that the PFNA-II helical blade has a diameter of 12.2 mm. We used lag screws of 85 mm in length for the experiment. For the setup, lateral locking of the PFNA-II helical blade was released, so that the blade portion of the PFNA-II lag screw can be freely rotated. This was determined to reproduce the failure mechanism of the lag screw where the femoral head is typically rotated along the lag screw.[13, 14]
Initially, the lag screw was advanced axially along the center of the resected surface of the femoral head until the tip of the lag screw was placed 20 mm from the outer surface of the femoral head. This was then advanced at 15 mm/min until the lag screw penetrated the femoral head and advanced for an additional 5 mm. The load–displacement curve was acquired during the 25-mm advancement of the lag screw. The mechanical strength was defined as the 1) maximum compressive load and 2) accumulated compressive load during the 30-mm advancement. The maximum compressive load was defined as the highest load measured in the load–displacement curve, while the accumulated compressive load was defined as the area under the load–displacement curve during the 25-mm advancement of the lag screw.
Statistical methods
The measured results were expressed as means and standard deviations. The normality of the distribution of the data was assessed using the Kolmogorov–Smirnov test. Pearson correlation and linear regression analysis were used to determine the correlation between 1) the BMD of the contralateral hip and that of the fractured femoral head, 2) BMD of the fractured femoral head and mechanical properties of the fractured femoral head, and 3) BMD of the contralateral hip and mechanical properties of the fractured femoral head. Statistical analysis was performed using the SPSS software version 20 (SPSS Inc., IL, USA). All P-values were two-sided, and P-values of <0.05 were considered significant.