Comparing the volume of vascular intersection of two femoral neck fracture fixation implants using an In silico technique

Femoral neck fracture displacement with subsequent vascular disruption is one of the factors that contribute to trauma-induced avascular necrosis of the femoral head. Iatrogenic damage of the intraosseous arterial system during fixation of femoral neck fracture is another possible cause of avascular necrosis that is less well understood. Recently, Zhao et al (2017) reconstructed 3D structures of intraosseous blood supply and identified the epiphyseal and inferior retinacular arterial system to be important structures for maintaining the femoral head blood supply after femoral neck fracture. The authors therefore recommended placing implants centrally to reduce iatrogenic vascular injuries. Our in vitro study compared the spatial footprint of a traditional dynamic hip screw with an antirotation screw versus a newly developed hip screw with an integrated antirotation screw on intraosseous vasculature.

Methods

Three dimensional (3D) µCT angiograms of 9 cadaveric proximal femora were produced. Three segmented volumes-porous or cancellous bone, filled or cortical bone, and intraosseous vasculature-were converted to surface files. 3D in silico models of the fixation systems were sized and implanted in silico without visibility of the vascular maps. The volume of vasculature that overlapped with the devices was determined. The ratio of the vascular intersection to the comparator device was calculated, and the mean ratio was determined. A paired design, noninferiority test was used to compare the devices.

Results

Results indicate both significant (P < 0.001) superiority and noninferiority of the hip screw with an integrated antirotation screw when compared with a dynamic hip screw and antirotation screw for the volume of vasculature that overlapped with each device in the femoral neck.

Conclusions

Combining established methods of vascular visualization with newer methods enables an implant's impact on vascular intersection to be assessed in silico. This methodology suggests that when used for femoral neck fracture management, the new device intersects fewer blood vessels than the comparator. Comparative clinical studies are needed to investigate whether these findings correlate with the incidence of avascular necrosis and clinical outcomes.

An investigation into PEEK-on-PEEK as a bearing surface candidate for cervical total disc replacement

BACKGROUND CONTEXT

Total disc replacement (TDR) is a relatively new reconstructive non-fusion technology for the spine that aims to overcome some of the limitations of fusion technology. The first generation artificial discs were mainly based on well-known material combinations from total hip replacement.

PURPOSE

To evaluate the feasibility of PEEK-on-PEEK as a bearing surface material for use in cervical TDR.

STUDY DESIGN

In vitro biotribological study including the assessment of different parameters, including the influence of radial clearance, axial load, and angular motion on the wear performance of a PEEK-on-PEEK bearing.

METHODS

PEEK-on-PEEK bearing couples with ball-and-socket articulation were prepared using standard production methods. Two groups of bearing couples, one with large radial clearances and the other with small radial clearances, were manufactured for testing. Wear tests were performed using a servo-hydraulic wear simulator that allowed controlled motions to be applied in three axes corresponding to flexion-extension, lateral bending, and axial rotation. In addition, a dynamic axial compressive load was applied to simulate the weight of the head. All tests were performed at 37°C (±2°C) in bovine calf serum with a 30 g/l protein concentration. In the first test series, the influence of the radial clearance on the wear behavior was evaluated using the load and motion parameters for cervical TDR as defined in the ISO standard 18192-1. Subsequent tests were performed to check if surface degradation was influenced by thermal effects. Finally, in a third series of tests, the effect of load and motion profiles on wear performance was investigated by applying increased loads and corresponding motion parameters for lumbar TDR as defined in the ISO standard 18192-1.

RESULTS

The measured wear rates for a PEEK-on-PEEK bearing under cervical test conditions were considerably larger than those of similar testing using UHMWPE-on-CoCr and seemed to depend on initial radial clearances. The PEEK bearing surfaces exhibited severe surface degradation for large and small clearances, but the group with smaller clearances seemed to have less severe damage. Thermal effects were not found to play a role in the surface degradation, as the temperature near the bearing surface was measured and found to vary only a few degrees during testing. A change of the wear test parameters to simulate lumbar loading resulted in a considerable wear rate reduction as well as in the preservation of the bearing surfaces.

CONCLUSIONS

It was found that a cervical TDR using a PEEK-on-PEEK bearing may be subjected to severe long-term degradation of the bearing surfaces. In this study, the use of PEEK as an articulation material for cervical TDR was found to be sensitive to loading conditions as well as to the initial clearance of the bearing surfaces. Despite the excellent properties of PEEK as an implant material, its use for articulating surfaces for cervical TDR must be critically reviewed.

Influence of the loading frequency on the wear rate of a polyethylene-on-metal lumbar intervertebral disc replacement

Pre-clinical wear testing of intervertebral disc prostheses is commonly carried out according to ISO 18192-1. Ten million multiaxial loading cycles are applied at a frequency of 1 Hz. At this frequency, testing takes about 4 months. Testing at higher frequencies would therefore be desirable. ISO 18192-1 also offers testing at 2 Hz; however, it says the impact on the implant material behaviour as well as on the accuracy of the test machine shall be investigated by the user. Since such data are not available so far, the aim of this study was to carry out comparative wear tests at 1 and 2 Hz. Seven Prodisc-L lumbar disc prostheses were tested. After a pre-soak period, the implants were placed in specimen cups filled with calf serum, mounted to a Spine Wear Simulator and loaded according to ISO 18192-1. Testing was carried out at a temperature of 37 ± 2 °C. Four million loading cycles were applied at 1 Hz and eight million at 2 Hz in an alternating sequence. Each time after 12 days of testing the implants were removed to measure the weight and the height of the polyethylene cores. Then, the test serum was exchanged and the implants were remounted to the testing machine. The mean wear rate was 5.6 ± 2.3 mg per million cycles at 1 Hz and 7.7 ± 1.6 mg per million cycles at 2 Hz during the first six million loading cycles (p < 0.05) and 2.0 ± 0.6 and 4.1 ± 0.7 mg per million cycles during the second six million cycles (p < 0.05). Similarly, the mean heightloss was also smaller at 1 Hz than at 2 Hz (p < 0.05) with -0.02 ± 0.02 mm versus -0.04 ± 0.02 mm per million cycles during the first half of testing and -0.01 ± 0.01 versus -0.02 ± 0.01 mm per million cycles during the second half. The accuracy of the test machine was within the limits described by ISO 18192-1 at both frequencies. The results showed that the wear rate was higher at the beginning than at the end of testing. Also, the results indicated that testing at 2 Hz increases the wear rate compared with 1 Hz in case of a polyethylene-on-metal implant design. In the absence of retrieval studies it is difficult to decide which rate results in a more physiological wear pattern. However, a loading frequency of 1 Hz is probably closer to physiology than 2 Hz since the loading amplitudes prescribed by ISO 18192-1 are high. They rather represent movements like tying shoes or standing up from a chair than walking or sitting. The authors therefore suggest testing at 1 Hz.

A new test for bipolar prosthesis disassociation

Because of several failures by dissociation of a redesigned bipolar prosthesis, a new, dynamic test was developed. This dynamic cam-out test represents a closer simulation of one possible clinical mechanism of bipolar disassociation. The results of dynamic testing are affected by the bipolar design, particularly the locking mechanism that was the problem with the redesigned prosthesis.

Cementation of a polyethylene liner into a metal shell. Factors related to mechanical stability

BACKGROUND

Various clinical situations may make polyethylene liner exchange desirable in the setting of a well-fixed metal shell. Options have included a simple polyethylene liner exchange or revision of the entire acetabular shell. A recently introduced technique involves cementation of a new liner into a well-fixed metal shell. The purposes of this study were to quantify the mechanical integrity of this method of liner fixation, to evaluate the factors that may influence immediate liner fixation, and to compare this construct with a standard locking mechanism.

METHODS

One modular acetabular cup design was evaluated. Variables that may contribute to the mechanical integrity of the construct were evaluated via lever-out and multiaxis dislocation testing methods. The variables included the size of the liner (undersized versus oversized), type of liner (all-polyethylene versus modular design), modification of the modular liner (unmodified versus grooved), and groove configuration (cruciform versus circumferential). Metal shells with and without screw-holes were also tested. The modular locking mechanism was used as the control.

RESULTS

None of the constructs failed at the cement-metal interface. All undersized liners required significantly higher loads to failure than either the controls (p < 0.001) or the oversized liners (p < 0.001). Oversized unmodified liners failed at significantly lower loads than the controls did (p < 0.01). The creation of circumferential grooves in the oversized liners significantly improved the strength of the constructs (p < 0.01), making them comparable with the controls. No significant differences were found among the four undersized groups (p > 0.3). The standard locking construct (control) and the oversized unmodified construct failed at a force of 2000 N in the multiaxis dislocation test. No other construct failed with use of this test mode.

CONCLUSIONS

Cementation of a polyethylene liner into a metal shell can be stronger than a conventional locking mechanism if the liner is undersized. Cementation of an oversized liner into a shell should be performed with caution. The long-term durability of this fixation remains unknown.