Full-Quantitative Analysis of Cementless Stem Hammering Sound Changes During Total Hip Arthroplasty.

Full-quantitative characterization has not been performed to analyze changes in the hammering sound in cementless hip arthroplasty. We analyzed the frequency spectrum of the hammering sound during stem insertion for 20 cases of uncomplicated cementless total hip arthroplasty for osteoarthritis using a proximal-coated stem. The absolute sound pressure (Pa) and normalized sound pressure of each frequency bands in early and late stage of femoral stem insertion were determined by the Fast Fourier Transform analysis. The absolute sound pressures (Pa) of a majority of frequency bands was signicantly higher in the late-stage stem insertion than in the early stage The 1.0–1.5-kHz frequency band showed a signicant change in normalized sound pressure in all cases between the early and late stages of stem insertion (p=0.000). The femoral morphology and canal ll ratio were correlated with late stage normalized sound pressure in specic frequency bands. In the 5.0–5.5 kHz band, the Dorr A femoral morphology was signicantly higher normalized sound pressure than those in the Dorr B (p=0.004). This study revealed the hammering sound frequency with full-quantitative value altered during cementless stem insertion. Frequency bands of 1.0–1.5 kHz, 5.0–5.5 kHz were the key bands for predicting stem xation.


Introduction
Total hip arthroplasty (THA) is one of the most successful surgical treatments and is reported to greatly reduce pain and restore hip function and the quality of life of patients with end-stage hip disease in both short-term and longterm follow-up 1,2 . As the population ages, the demand for primary THA and revision THA has increased rapidly in recent years 3 .
Although the use of cementless xation in THA has increased globally, complications, such as intraoperative femoral fracture and implant subsidence, can occur after inappropriate and inadequate stem implantation 4,5 . These complications can compromise the surgical effect and increase the risks of dislocation, aseptic implant loosening and revision surgery 6,7 .
Several new technologies, such as preoperative three-dimensional (3D) templating and intraoperative navigation, have been used to avoid inadequate stem selection and achieve better stem positioning. Schiffner et al. reported that the accuracy of predicting the right stem size improved from 45.7% to 58.6% using 3D templating compared with 2D surgical planning 8 . Weber et al. found similar accuracy in biomechanical hip reconstruction of the leg length and global and femoral offset in THA between intraoperative navigation and uoroscopy 9 .
Addition to those new imaging tools, because experienced surgeons use the auditory sensation of a hammering sound during stem insertion to determine proper/improper stem sitting, researchers have attempted to analyze the hammering sound. Morohashi et al. reported on acoustic frequency patterns and found that a natural hammering frequency of approximately 7 kHz was the most prominent frequency in patients without complication 10 . Connell et al. reported that a frequency around 1 kHz could better predict an adequately sized stem using spectrographs 11 .
However, no full-quantitative characterization has been performed for the changes in hammering sound frequency during cementless stem insertion, and the relationship between the hammering sound frequency and the femoral morphology and canal ll ratio is unknown. Therefore, we asked two questions. 1) Is the hammering sound frequency with full-quantitative value altered during cementless stem insertion? 2) Is the hammering sound frequency correlated with the femoral morphology and canal ll ratio of the stem? This study was conducted to objectively analyze the changes in hammering sound frequency during cementless stem insertion and the relationship between the hammering sound frequency and femoral morphology and canal ll ratio of the stem.

Patients
All procedures performed in this study involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The study protocol was approved by the Ethics Committee of Juntendo University Hospital, Tokyo, Japan. Informed consent was obtained in a manner approved by the Ethics Committee from all individual participants included in this study. From November 2018 to October 2020, 62 patients (65 hips) undergone Primary THA who agreed to participate to this study were initially included (Fig. 1). Exclusion criteria were (i) patients who underwent THA for osteonecrosis, femoral neck fracture, or rheumatoid arthritis (ii) the surgery used a prosthesis other than the Accolade II stem, (iii) the surgery was performed by a junior surgeon, and (iv)

Surgical procedure
The surgeries were performed by four experienced orthopedic surgeons via the direct anterior approach with a cementless proximal hydroxyapatite-coated stem (Accolade II, Stryker, Tokyo, Japan) using the distal part of the Smith-Petersen approach with the patient in the supine position on a surgical traction table as previously described 13 . The following is the brief description of this surgical procedure 13 . The fascia of the tensor fascia lata muscle was incised at approximately 2 cm lateral to the skin incision to prevent lateral femoral cutaneous nerve injury. The intermuscular space between the tensor fascia lata and sartorius muscles was then bluntly entered. The anterior articular capsule was exposed and incised. Intraoperative X-ray photography was used to con rm that the broach was appropriately aligned, and the porous part of the broach contacted the cortical bone. The stem size was determined using the same size of the last trial insertion of the broach. All patients underwent standardized postoperative rehabilitation with full weight bearing 1-day post-surgery.
Sound data collection during the THA A highly sensitive sound level meter (LA-7500, Onosokki, Kanagawa, Japan) was used to record the hammering sound of the stem insertion. In all cases, the sound level meter was set on a tripod mount at 1 m high and 2 m away from the surgical table in the same operating room (Fig. 2). Recordings were made in the range of 40-110 dB using Z frequency weighting ( at-weighted lter) and fast time weighting at a sampling rate of 64 kHz and a 16-bit sampling depth.

Sound data analysis
Oscope ver 2.1, (Onosokki, Kanagawa, Japan) was used for the sound analysis. Recorded sound data were analyzed using a rectangular weighted window and 50% overlap at a maximum range of 12.5 kHz via fast fourier transform (FFT) analysis (Fig. 3). The rst three and last one hammering sounds during the stem insertion were excluded from the analysis to avoid potential hammering inconsistencies, The fourth to sixth hammering sounds were de ned as early-stage insertion hammering sounds. The second to fourth hammering sounds from the end were de ned as late-stage insertion hammering sounds. If noises were mixed in with these hammering sounds, or an improper hammering was detected on the spectrogram, those hammering sound would be switched to the previous or next one.
The following analysis compared the early-and late-stage insertion hammering sounds. The overall spectrum frequency of the recorded sound was divided into 25 frequency bands in the range of 0.5 kHz from 0-12.5 kHz.
Because frequency bands below 0.5 kHz were mixed with noises ranging from 0.08-0.26 kHz, such as voiced speech from a typical adult, 0-0.5 kHz was thought to inaccurately re ect sound changes during the stem insertion.
Moreover, previous studies detected no changes below 0.5 kHz 10,11,14 . Therefore, the 0-0.5 kHz frequency band was excluded from the comparison. Sound changes between the early and late stages were compared rst by using the absolute sound pressure (Pa) and then by using the normalized sound pressure in each frequency band.
Because the average overall absolute sound pressure (Pa) differed between the early and late stages, the analysis using the normalized sound pressure was used. Normalized sound pressure was calculated as the ratio of the absolute sound pressure (Pa) of each frequency band to the average overall frequency spectrum (0.5-12.5kHz).
Next, correlations were determined between the femoral morphology, canal ll ratio and absolute sound pressure (Pa), followed by normalized sound pressure in the late stage. Finally, sound changes between Dorr A and B were determined for the late stage.
Assessment ofthefemoral morphology and canal ll ratio of the stem.
Radiographs of the femoral morphology and canal ll ratio were assessed using the nal preoperative and immediate postoperative Anterior Posterior hip radiographs. Preoperative radiographs were used to analyze ve morphologic parameters as follows.
(i) Canal-calcar ratio (CCR): ratio of the intracortical diameter of the femoral canal isthmus at 10 cm below the lesser trochanter to the intracortical diameter of the proximal femur at the medial tip of the lesser trochanter 15 .
(ii) Canal-are index (CFI): ratio of the intracortical diameter of the proximal femoral isthmus at 2 cm above the lesser trochanter to the intracortical diameter of the femoral canal isthmus at 10 cm below the lesser trochanter 16 .
(iii) Morphologic cortical index (MCI): ratio of the extracortical diameter of the femur at the medial tip of the lesser trochanter to the intracortical femoral diameter at 7 cm below the lesser trochanter 15,17 .
(iv) Canal-bone ratio (CBR): ratio of the intracortical and extracortical diameters of the femoral canal isthmus at 10 cm below the lesser trochanter 18 .
Postoperative radiography was used to assess the canal ll ratio (CFR) of the stem, de ned as the stem width divided by the canal width at four points at the lesser trochanter, 2 cm above, 2 cm below, and 7 cm below the lesser trochanter. The proximal-distal matching ratio of the CFR at 2 cm above and 7 cm below the lesser trochanter was also considered 19 .
A single observer (S.I.) who was not involved in the sound analysis analyzed the measurements. Radiographs were assessed using the ruler function of the Picture Archiving and Communication System at our institution (Fuji lm Synapse 3.2.1 SR-356; Fuji lm Corp, Tokyo, Japan).

Statistical analysis
Statistical analysis was performed using SPSS software, ver 26.0 (IBM, Armonk, NY, USA). Patient demographics are expressed as the mean ± standard deviation. Two-tailed paired t-tests and Wilcoxon signed-rank tests were used to compare paired data. Spearman rank correlation was used to evaluate relationships between variables.
Differences and correlations were considered statistically signi cant if p < 0.05.

Results
Natural frequencies of the surgical instruments Full-quantitative analysis of the sound changes between the early and late stages The absolute sound pressures (Pa) of a majority of frequency bands except 5.0-6.0, 8.5-10.0 and 12.0-12.5 kHz was signi cantly higher in the late-stage stem insertion than in the early stage (Fig. 5A).

Correlations between femoral morphology, CFR and normalized sound pressure
The CCR and MCI were signi cantly correlated with the normalized sound pressures of 5.0-5.5 kHz (CCR: r=-0.567, p=0.009; MCI: r=0.490, p=0.028) in the late-stage stem insertion (Table.1

Comparisons between Dorr A-type and Dorr B-type femurs
Patients characteristic in each group is shown in Table.2. In the Dorr A group, the normalized sound pressure of 1.0-1.5 kHz in the late stage was signi cantly higher than that in the early stage (p=0.022), (Fig. 6). The normalized sound pressure of 5.5-6.0 and 6.0-6.5 kHz in the late stage were signi cantly lower than those in the early stage (p=0.008, p=0.016, Fig. 6A). In the Dorr B group, 0.5-1.0 and 1.0-1.5 kHz in the late stage were signi cantly higher than those in the early stage (p=0.005, p=0.019). And the normalized sound pressure of 2.0-2.5, 5.0-5.5, 5.5-6.0 and 9.0-9.5 were signi cantly lower than those in the those in the early stage (p=0.045, p=0.004, p=0.002, p=0.033, Fig.   6B). Comparing the normalized sound pressures in the late stage between Dorr A and B, 5.0-5.5 kHz (p=0.006) in the Dorr A group were signi cantly higher than those in the Dorr B group (Fig. 6C).

Discussion
Although cementless THA can relieve pain and restore mobility, the incidence of speci c complications, such as intraoperative fractures or postoperative subsidence, remain problematic. Previous studies have shown the acoustic evaluations based on the hammering sounds during THA can predict stem stability 10,11,20 . However, the sound changes reported in these studies differed and could not easily be distinguished. Moreover, no full-quantitative analysis of the sound frequency has been performed thus far. In the present study, we performed a full-quantitative analysis using normalized sound pressure to quantify the sound quality and found that the characteristic of the sound frequency changed during cementless stem insertion and that the sound frequency was correlated with the femoral morphology and CFR.
We believe that using normalized sound pressure to assess the hammering sound frequency is more reliable and objective. Assessments based on absolute sound pressure make the results unclear because they are affected by the different hammering forces. Although the surgeons who participated in our study were asked to deliver a consistent hammering force during the stem insertion, standardizing the hammering force among the surgeons was di cult. The absolute sound pressure was higher in the late stage than in the early stage, likely because the surgeons are more cautious and more likely to deliver less force at the beginning of a stem insertion. Previous studies suggested that presence of the prominent frequency accentuation of the absolute sound pressure could effectively predict stability of the broach or stem 10,11 . However, some of our FFT analysis results showed that the prominent frequency differed among patients despite the surgery having been performed awlessly without complications (Fig. 3). In some cases, the prominent frequency occurred in the early-stage rather than late-stage stem insertion. Regarding the femoral morphology and CFR, McConnell et al. reported a positive correlation between sound change in the recorded frequency and femoral length, but not with the cortical thickness 11 . Our study yielded that hammering sound in the late stage were signi cantly correlated with the femoral morphology/CFR with the assessment of normalized sound pressure.
Our data suggest that two principals are important for understanding the sound analysis and further study of the sound frequency. First, the hammering sound frequency depends on femoral morphology. Second, the hammering frequency differs in whether the cementless stem reaches the aimed xation contact area of the femur. Ideally, a high CCR indicates that the intracortical diameter of the distal femoral canal isthmus will be relatively large, and the stem is more likely to become xed at the proximal femur. Our results showed that sound pressures of 5.0-5.5 kHz in the late stage were signi cantly lower in the Dorr B group than in the Dorr A group (those with "champagne-ute" morphology of the proximal femur). In addition, the normalized sound pressure of 5.0-5.5 kHz between the early and late stages was decreased in the Dorr B group (Fig. 6B) but not in the Dorr A group (Fig. 6A). We speculate that the sound pressure changes of 1.0-1.5 kHz could be used to predict whether the stem is well-xed and that 5.0-5.5 kHz could distinguish whether the stem is xed proximally or distally. Regarding 1.0-1.5kHz, our data showed that the changes from early to late stem insertion at 1-1.5 kHz of normalized sound pressure were signi cantly higher in both Dorr A and B patients (Fig. 6A, 6B). This nding is similar to that of McConnell et al., who analyzed frequency changes during the femoral broach. McConnell et al. found an additional frequency band around 1 kHz in the nal femoral broach, which they thought indicated that the broach was well tted 11 . Whitwell et al. theorized that the 1-kHz frequency band was the sound wave created by the femoral canal, as a well-tted broach yields better bone contact, leading to more e cient energy transfer and hence a greater bone vibration 21 . Although we agree with the theory of McConnell et al., we postulate that the stem impactor could also generate the sound change around 1 kHz. Although measuring the natural bone-muscle composite frequencies was di cult, our preliminary experimental results con rmed that 1 kHz was a natural frequency of the impactor. When the stem is xed with the bone, both the bone and the stem impactor yield greater vibrations owing to increased reaction forces, leading to the changes around 1 kHz.
At 5.0-5.5 kHz, we believe that the femur-stem system generates the sound change when the stem is xed at the proximal part. When the proximal-coated stem is xed proximally at the femoral canal, the stem is su ciently lled proximally, whereas if the proximally-coated stem is xed distally, the stem moves like a windshield wiper, known as proximal-distal-mismatch 1922 . The bone cannot be well integrated with the stem and thus cannot vibrate as a single system. Jaecques et al. reported a similar result after using an arti cial bone and a custom made stem to simulate the vibration system change between a "loosely inserted" initial stage to a well xed nal stage, but without using a stem impactor. The sound value around 4 kHz was gradually increased and the sound value around 8.5 kHz was gradually decreased as the stem became more xed to the bone 23 .
This study had several limitations. First, the sample size was small. However, despite the small number of patients, the characteristic objective data showed signi cant differences. Second, the hammering style and force were not practically standardized; thus, different vibration modes may have affected the accuracy of the results. Although we used normalized sound pressure to quantify sound changes, this method still required a baseline of the average overall frequency spectrum. Thus, the frequency range should be chosen carefully. Third, noises in the operating environment, such as the electrocardiograph monitoring alarm, could affect the recorded sound quality. Although the hammering sounds with obvious background noise were excluded, they will inevitably have in uenced the results. However, the possible sound effects from the noises in the operating environment were minimal in our analysis compared with the hammering sounds. Moreover, we aimed to determine the possibility of clinically using the sound analysis in the most practical sitting environment with general noise from the operating eld.

Conclusion
This study revealed the full-quantitative sound changes during proximal-coated cementless stem insertion using normalized sound pressure, which was useful for quantifying the sound analysis with no effect of hammering force. The sound changes were correlated with the femoral morphology and CFR. Frequency bands of 1.0-1.5 kHz, 5.0-5.5 kHz were the key bands for predicting stem xation. Further study is needed to determine the relationship between complications and characteristic sound frequencies in the key bands.

Declarations Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.