Demographic data, including age, sex, height, weight, and body mass index, were recorded prospectively for all patients (Table I).
Intraoperative data, including prosthesis type, stem size, head size, and liner size, were also recorded. All patients were managed with either a 28, 32, or 36-mm ceramic head. Head size was based on the maximum allowable size for the implanted cup. In Group 1, thirty-four hips (22.4%) received a 28-mm head, ninety-two hips (60.5%) received a 32-mm head, and twenty-six hips (17.1%) received a 36-mm head. In Group 2, twenty-seven hips (17.8%) received a 28-mm head, seventy-nine hips (52.0%) received a 32-mm head, and forty-six hips (30.3%) received a 36-mm head (Table II).
Postoperative anteroposterior and lateral radiographs were reviewed to assess the position of the acetabular component9. With use of OrthoView software (Meridian Technique, Southampton, Hampshire, United Kingdom), the anteroposterior radiographs were uploaded, the images were calibrated, and the position of the acetabular component was assessed with use of the inclination-anteversion tool from the system. This evaluation was done by an orthopaedic surgeon (C.R.) and an adult reconstruction fellow (Z.D.P.). Cup position was assessed with regard to abduction, version, and height. The stem was evaluated with regard to position and offset.
Outcome measurements, specifically, the Physical Health and Mental Health dimensions of the Short Form-36 (SF-36) Health Survey as well as the Harris hip score, were obtained preoperatively and at the time of the latest follow-up for all patients.
Data regarding the presence of squeaking were obtained prospectively. The 247 patients (282 hips; 92.8%) who were seen for clinical follow-up either expressed the squeaking phenomenon themselves or were asked about it by the physician. The 19 patients (twenty-two hips; 7.2%) who were not seen at a recent clinical follow-up visit were contacted by telephone and were asked specifically about noises that might be associated with the hip replacement. If squeaking was confirmed, the patient was asked to characterize its intensity (slightly, moderately, or highly audible), frequency (continuous, intermittent, or a single event), and the time between the date of the operation and the date of onset. Patients who had only clicking were not included in the study.
Complications, including infection, loosening, and reoperation for any reason, were recorded.
Demographic data as well as prosthetic femoral head size and neck length were analyzed and compared between groups of patients and between squeaking and nonsqueaking hips. All statistical tests were performed with alpha set at 0.05. The t test was performed to evaluate continuous variables (age, height, weight, body mass index, and radiographic findings), and the Fisher exact test was used to evaluate nominal values (sex, head size, and neck length). A baseline logistic regression model adjusting for the covariates of age, sex, and body mass index was performed. We tested confounding by adding several femoral variables to the baseline model individually and then tested whether it resulted in a change of odds ratio by 10%.
Source of Funding
There was no direct external funding for this study, although one of the authors is a consultant for Stryker Orthopaedics.
The two groups did not differ significantly with regard to age, sex, height, weight, or body mass index (Table I).
On the postoperative radiographs, cup abduction, cup anteversion, cup medialization, and femoral offset were not significantly different between the two groups (Table III). Although the values for cup abduction and anteversion were high, cup malpositioning alone has been demonstrated not to correlate with squeaking8.
The only measured parameter that showed a significant difference between the two groups was the prevalence of squeaking. Three patients (four hips; 2.6%) reported squeaking in Group 1. The average time of onset in this group was 16.5 months (range, 1.2 to sixty-nine months) postoperatively. In contrast, twenty-seven patients (twenty-eight hips; 18.4%) reported squeaking in Group 2 (p < 0.0001). The average time of onset in this group was 13.2 months (range, 0.7 to seventy-seven months) postoperatively. The frequency and severity of squeaking did not change through the study period for this specific cohort. Of the thirty-two hips with squeaking, six (all from Group 2) were revised to a polyethylene liner, with complete resolution of the noise in all six cases. After logistic regression, none of the variables—age, sex, body mass index, head size, stem offset—showed a change of 10% in the odds ratio. After adjusting for the covariates, we found an association between the group and squeaking (p = 0.0001) (Table IV).
Compared with preoperative baseline values, both groups exhibited significantly increased values on the SF-36 questionnaire in terms of both the Physical Health score (with increases of 45 points in Group 1 and 47 points in Group 2) and the Mental Health score (with increases of 35 points in Group 1 and 37 points in Group 2), but there was no difference between the two groups (p = 0.956 and p = 0.987, respectively). As expected, the Harris hip score was better after surgery in both groups in comparison with the baseline values (with improvement of 27 points in Group 1 and of 25 points in Group 2), but again there was no significant difference between the two groups (p = 0.893).
All patients, including the ones who experienced squeaking, were satisfied with the result of total hip arthroplasty (except for the squeaking itself). No other orthopaedic and non-orthopaedic complications were found in either group.
A definitive etiology for squeaking in ceramic-on-ceramic-bearing hips has been elusive and controversial. Factors such as cup position and patient age, height, and weight have all been implicated as being associated with audible squeaking10. However, other reports and our own data have shown no association with these same factors8. The current study suggests a new explanation. We found a clear relationship between the prevalence of squeaking and the type of femoral component implanted. Patients in our series who were managed with a thicker femoral component with a C-taper neck and a stem made of titanium-aluminum-vanadium were seven times less likely to report squeaking than those with a thinner stem component with a V-40 taper neck made of a titanium-molybdenum-zirconium-iron alloy.
The current explanation for ceramic-on-ceramic squeaking is based on the phenomenon of stripe wear. Walter et al.11 first reported finding stripe wear in 2004. In a retrieval analysis, those investigators found a heavy wear pattern in a location of the head that suggested wear during activities at the extremes of motion. Specifically, their analysis showed that this wear did not usually occur during normal walking but rather occurred during rising from a seated position or other high-flexion activities. During these activities at the extremes of motion, edge loading between the ceramic head and the posterior rim of the ceramic cup occurred. The investigators concluded that it was these edge-loading activities that led to the visible wear stripe.
In a retrieval analysis, Lusty et al. reported that twelve of twelve squeaking ceramic-on-ceramic bearings had evidence of edge-loading wear, suggesting that a wear stripe is likely involved in the production of squeaking12. As a ceramic-on-ceramic bearing passes over the stripe, it could generate a vibration that produces an audible sound (Fig. 5). This explanation is consistent with the fact that ceramic-on-ceramic squeaking does not occur until an average of fourteen to eighteen months after surgery8,10. During this time, the wear stripe has not fully developed. Once a wear stripe is established, squeaking occurs. In addition, the theory that edge loading creates a wear stripe is consistent with studies that have shown an increased prevalence of squeaking in patients with malpositioned acetabular cups10. A cup in excessive retroversion, for example, would lead to increased edge loading in flexion and, subsequently, a higher prevalence of squeaking.
The basic presumption about squeaking is that the noise is due directly to vibration of the ceramic bearing. However, a study by Walter et al. demonstrated that ceramic heads and ceramic liners generally resonate at a frequency that is above the upper limit of human hearing (20,000 Hz) and well above the in vivo range of squeaking hips (400 to 7500 Hz)13. This discrepancy poses an obvious question: If ceramic heads and liners do not resonate in a range that can be heard, what is causing the audible squeaking? Further evaluation of the data from the study by Walter et al. showed that a titanium femoral stem vibrates between 2000 and 20,000 Hz, easily within the audible human range. These data, combined with our findings, suggest that the difference in the rate of squeaking in our two groups is directly related to the differences in the composition and design of the femoral components. It is our hypothesis that there is a breakdown of fluid film lubrication in the hip, with an increase in friction. This generates increased frictional energy in the form of a vibration, which is then transmitted to a flexible stem that amplifies the vibration by resonating, resulting in an audible sound.
We found no difference between Groups 1 and 2 in terms of patient-related parameters. In addition, we found no difference between the patients with and without squeaking in terms of cup or stem position. These findings are in contrast to those of some published studies10 but are consistent with others7. All procedures in the present study were performed by the same surgeon, who has performed thousands of primary total hip arthroplasties during his career. The only difference noted between groups was the prevalence of squeaking.
The stem used in Group 1 was made of a traditional titanium-aluminum-vanadium alloy. The neck on that stem had a C-taper, and the stem itself was more robust proximally. The stem used in Group 2 was made of a titanium-molybdenum-zirconium-iron alloy. That stem had a V-40 neck geometry and was much more slender proximally. One of the purported benefits of the titanium-molybdenum-zirconium-iron material is that it has a modulus of elasticity that is 30% to 40% less than that of titanium-aluminum-vanadium. Therefore, it is a more flexible material even with the same geometry. Furthermore, the titanium-molybdenum-zirconium-iron stem in our study was thinner from front to back (10 mm compared with 13 mm), thus reducing the bending stiffness in the sagittal plane by an additional 50%. These properties create the potential for less thigh pain and better stress distribution to bone, with less proximal bone atrophy. In addition, they make for a much more flexible stem. The V-40 neck is smaller than the C-taper on the titanium-aluminum-vanadium stem. The decreased cross section of the V-40 neck is another purported advantage of using titanium-molybdenum-zirconium-iron. The smaller neck diameter should lead to less impingement. However, the smaller diameter of the V-40 neck leads to a lower bending stiffness and lower resonant frequency and is more capable of amplifying vibrations generated by the ceramic-on-ceramic articulation to cause an audible squeak. The relatively high prevalence of squeaking in Group 2 is consistent with this mechanism. In contrast, the thicker C-taper neck in Group 1 has a higher bending stiffness and a higher natural frequency and is therefore less able, in theory, to amplify vibrational energy into audible resonance.
Three published studies support our proposed theory. Jarrett and colleagues reported that ten (7%) of 143 patients reported squeaking6. Of note, the stem that was used also had a thin profile with a V-40 neck and was composed of titanium-molybdenum-zirconium-iron alloy. Another study, which demonstrated a 20% prevalence of squeaking, evaluated a different femoral component, but one that was also composed of titanium-molybdenum-zirconium-iron with a V-40 neck14. In contrast, Capello and colleagues reported a 1% prevalence of squeaking in a study that evaluated a component with a C-taper that was composed of titanium-aluminum-vanadium alloy5.
In conclusion, the etiology of squeaking of a ceramic-on-ceramic-bearing hip appears to be complicated and multifactorial. Our data show a direct relationship between the prevalence of squeaking and the type of femoral component used. It is our belief that ceramic-on-ceramic continues to be a good alternative for young patients. Avoiding the use of a V-40 neck and titanium-molybdenum-zirconium-iron stem may reduce substantially the prevalence of squeaking.
NOTE: The authors thank Dr. William Walter for his invaluable input on the revision of this manuscript.
Investigation performed at the Thomas Jefferson University Hospital, Rothman Institute, Philadelphia, Pennsylvania
Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from Stryker Orthopaedics. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity.
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