Shifting global demographics, predominantly an increase in an ageing population, has resulted in an increase in the number of patients with hip osteoarthritis worldwide [1
]. Patients with severe hip osteoarthritis are often indicated for hip surgeries, such as total hip arthroplasty (THA). In Japan, approximately 50,000 patients undergo THA each year with an upward trend that is predicted to continue unabated [2
]. Surgical outcomes have improved because of established surgical techniques and advances in implant design and materials [3
]. However, fractures during surgery and loose implants during the early postoperative period have been reported as instances of complications whose prevalence has increased, particularly due to the increase in the number of surgeries among patients with fragile bones, which correspond to an increase in repetitive surgeries [4
]. Therefore, novel approaches to prevent to THA surgery-related complications are warranted.
One such method is to achieve artificial acetabular cup stability. Insufficient acetabular cup stability can result in additional impaction or screw insertion, and these additional procedures can further lead to iatrogenic fracture or vascular injury [7
]. Because there is no quantitative method to evaluate acetabular cup stability during surgery, this stability is currently measured through manual operator sensing. Although several quantitative methods have been established [8
], they are not in clinical use. Others have evaluated acetabular cup stability according to torque [11
] and pull-down force [12
]. However, because the implants should be removed for measurement, their clinical utility is limited. Therefore, a quantitative method for evaluating acetabular cup stability without requiring the removal of the implant during the operation, thereby aiding in accurate surgery, should be established.
In dentistry, dental implant stability is evaluated by resonance frequency analysis (RFA) [15
], based on the theory that the resonance frequency obtained from dental implants vibrated using a magnetic force generated by small magnet (smart peg) and the vibration waveform detected can be analyzed. Because dental implants are exposed in the oral cavity, it is convenient to adapt RFA for dental implants [20
]. Conversely, adaptation of RFA for an acetabular cup is difficult because it is impacted into the pelvis and is located deep within the human body. For measuring acetabular cup stability, hammer tapping [10
], and implant vibration [8
] techniques have been used. Hammer tapping is subjective, is dependent on the surgeons, and is difficult to tap with the same power. The stability of the vibrator, examined by only manual sensing, is indicated as either stable or unstable. However, the unwieldy and complicated nature of these approaches renders them impractical for clinical use as quantitative methods.
To overcome these limitations, we developed a contactless RFA vibrometer based on pulse laser and laser Doppler that does not involve the direct attachment of devices, such as magnets, to the implant. Laser RFA has been used as one of the laser remote-sensing approach for inspecting internal defects within concrete structures, such as tunnels [25
]. The basic principle of laser RFA is irradiation of the sample by pulse laser to induce surface vibration via laser ablation, which is detected using a laser Doppler vibrometer. The scheme of laser RFA is similar to that of hammer inspection which provides rapid, contactless, and quantitative measurements. Further, due to its remote diagnostic scheme, laser RFA can be performed safely. The aim of the present study was to determine whether peak frequency of laser RFA was a predictor of the pull-down force. To achieve this goal, we used finite element method (FEM) analysis as a preliminary approach to determine the appropriate vibration mode and then examined laser RFA.
To the best of our knowledge, this is the first study to demonstrate that laser RFA successfully predicted the pull-down force of the acetabular cup using a simplified bone surrogate model that indicated the possibility of using laser RFA to evaluate acetabular cup stability. There are some differences and improvements compared with its application in other fields. Remoteness for measuring several meters away is unnecessary because it can be operated at hand. Alternatively, the lower irradiation pulse energy is appealing for surgical operation. Our evaluation is achieved by approximately 1/100 of pulse energy compared with its application in concrete inspection system whose irradiation pulse energy is 1000–4000 mJ [25
]. If we can achieve optimization of the spot size by developing a designated device, it will lead to higher fluence. The higher fluence enables us to acquire signals more efficiently as observed in the previous study [26
Acetabular cup stability can be evaluated using a vibrator with RFA in a cadaver model [8
] or a hammer tapping instrument using model bones [10
]. Although both methods can reasonably measure acetabular cup stability, they are limited in utility due to several reasons. Measurement of pull-down force during surgery is destructive because it leads to the detachment of the acetabular cup from the pelvic bone. Although hammer tapping is non-destructive, the tapping force depends on the operators, leading to poor repeatability and reproducibility.
The advantage with RFA is that the vibration induced by a magnetic force is reproducible and non-destructive and not subjected to the operator’s hand movements. Compared with dental implants, the acetabular cup is located deep within the human body, rendering it difficult to attach the ‘smart peg’ for RFA. Debruyne et al. used the Osstell®
apparatus for inducing vibration with magnetic force and used laser for detecting vibration [28
]. To the best of our knowledge, this is the first study to perform RFA using a laser for both vibration excitation and detecting vibration.
Comparisons of conventional methods are presented in Table 2
. Relative to other stability evaluation systems, laser RFA offers several advantages. First, laser RFA is a contactless system that can rapidly determine acetabular cup stability, with only 10 s from laser irradiation to frequency spectrum analysis. By contrast, hammer tapping is more complex and requires 16 impacts to obtain sufficient data for analysis. Second, laser RFA can repeatedly irradiate a narrow area—an essential property to evaluate cup stability—particularly during surgeries, such as THA, that are performed in a limited working space in the body.
As mentioned above, the impact of the hammer instrument varies considerably [9
], whereas laser consistently emits the same energy with each irradiation and results in precise data by averaging. A previous study showed that the frequency spectrum obtained from hammer tapping of the acetabular cup did not reveal the relationship between fixation strength and polar gap or the relationship between polar gap and frequency shift [29
]. These limitations prevent a comprehensive analysis of implant stability and the experimental results in the previous study require further research.
Compared with the vibration pattern of screw-shaped dental implants, the pattern of the acetabular cup is complex because of its hemispherical shape, outer periphery, and friction between the bone and acetabular cup surface. Higher coverage area was associated with a higher peak frequency in our study. Higher bone contact area was previously associated with greater acetabular cup stability [14
], which is consistent with our results.
The mechanism behind acetabular cup fixation is the engagement surrounding the outer periphery [30
]. The edge form and elasticity of bone are important factors that alter laser-induced vibration. Friction between the bone and acetabular cup surface is another mechanism. A conventional FEM study using composite bone and an acetabular cup (Ti-6Al-4V) showed >10 vibration patterns, and acetabular cup fixation was assessed by modal shape curvature [31
]. In that study, one pattern had a torsional characteristic and the remaining nine had a combination of bending and torsional characteristics. In our study, five vibration patterns were detected, of which four were related to acetabular cup stability. Two patterns were assumed to mainly reflect engagement surrounding the outer periphery, whereas the remaining two patterns were assumed to mainly reflect friction between the acetabular cup surface and polyurethane foam. To determine the peak resonance frequency, it was important to identify the pattern that reflects the pull-down force the most. Because several peak frequencies in the frequency spectrum positively correlated to the pull-down force, it was unfeasible to determine the nominal peak resonance frequency for stability analysis.
The limitation of FEM analysis was the lack of typical FEM models for calculating friction. Due to the complexity of incorporating friction in the FEM model, we hypothesized that the contact area between the acetabular cup and bone leads to friction, thereby reflecting the acetabular cup stability, which in turn affects its natural frequency. Therefore, high acetabular cup stability was associated with high peak frequency, which is consistent with the findings of the RFA experiment.
The press-fit model was difficult to express using the FEM model, and under-reaming was not considered. Laser RFA vibrates the acetabular cup rather than the mounting base. Therefore, the boundary surface should be considered. The effect of the coverage area is sufficient to explain the tendency of the vibration patterns because the coverage area reflects pre-stress. Although pre-stress does not induce the transformation of the acetabular cup, in reality, it is essential to set the interface model considering the residual stress of the bone and non-linear response of the soft tissue.
Our preliminary study verified whether the laser RFA principle was applicable without using the non-linear response due to the use of polyurethane foam. We showed that the vibration was type B, whereas type C was the shift in vibration from 3000 Hz to 5000 Hz, which was based on our vibration frequency region from the results of the FEM analysis. These findings were then extrapolated to a clinical study using cadavers.
In the present study, we selected the pull-down force as the stability force, although pull-down force is not a typical method for comparing pull-out and torque forces. Because our study was based on previous findings [14
], we excluded force displacement curve and attempted to establish pull-down force as an ASTM standard method. Polyurethane foams are widely used as standard test materials for mimicking the human cancellous bone [32
]. The mechanical properties of the human bone and polyurethane foam are shown in Appendix A Table A1
. The dimension of the foam and the adopted value of under-reaming were not determined because a previous study [10
] did not show the value of under-reaming. Vibration induced by laser is focal and the amplitude is <1 μm, and the vibration does not lead to the polyurethane foam block. Clinically, there was no mechanical damage to the bone, showing similar correlations that are independent of the mounting base. Re-using the foam block leads to different results because it was assumed that friction reduces with decrease in the pull-down force. We recorded pull-down force for each insertion and the effect of re-use. Re-tapping is occasionally performed during surgeries because of mal-installation, therefore, the re-use of polyurethane foam reflects clinical situations. It is plausible that the results vary with the bone type and future studies will examine the pelvis bone.
The acetabular cup was not subjected to heat because the examination was performed at room temperature reflecting the clinical situation, and the temperature of the acetabular cup was not measured. The nanosecond pulse width of the impact laser may not increase the temperature of acetabular cup because the specific heat of the titanium material Ti-6Al-4V is 610 mJ/g K and the input energy was only 46 mJ, which is a small energy pulse.
The debris generated by vibration associated with laser is a significant concern because there could be an adverse reaction to metal debris [33
]. A possible solution to this is the design of an external device connected to the acetabular cup. The external device will be cannulated and connected to an insertion rod that will preserve repeatability and reproducibility, because the debris will remain inside the external device and not inside the acetabular cup, thereby preventing adverse effects associated with debris during the use of such a device. Another concern is the use of reflection by fixed mirrors in the current laser system, reducing flexibility in the irradiation range. It is important to place the acetabular cup in the right position for irradiation. Therefore, the body position should be changed vertically and horizontally to evaluate acetabular cup stability. Because free body movement is not possible, assessment will be difficult in the operation room. However, the use of an optical fibre system can help overcome this issue owing to its flexibility. An optical fibre system enables the operator to irradiate an arbitrary point, and this will be a future in vivo study, prior to which, using a cadaver, we will establish the use of an optical fibre system and confirm safety.
The incidence of loosening that could be prevented with this device is indeed low, and complications, such as infection and embolism, cannot be prevented with this device. However, this system enables orthopaedic surgeons to prevent iatrogenic pelvic fracture induced by excessive tapping or vascular injury induced by additional screw insertion, which can be fatal. This system has the potential for use in revision surgery, where often the surgeon is required to decide whether to preserve the existing cup or to replace it. We predict that our system might prevent morbidities caused by the unnecessary removal of the existing cup or by retaining a cup with insufficient fixation in its position.
In conclusion, our findings suggest that laser RFA might be a useful approach to measure acetabular cup stability during surgery.