Tao has reported in some detail on the phenomenon of rattling after tau total hip arthroplasty. In general, this is a benign complication that patients can tolerate and only rarely requires revision surgery. The etiology of the rattling is not completely understood, but it is clear that it is a multifactorial phenomenon. We investigated the demographic data and the implantation position of the prosthesis and other factors that contribute to hip rattling after hip arthroplasty, and conducted a series of studies on the mechanism of rattling for this group of prostheses to propose a mechanism of rattling generation.
Key words: heterodyne; ceramic; hip; arthroplasty
The phenomenon of hip joint reverberation after arthroplasty is not a recent phenomenon, as it was first reported in the 1990s in Judet hemiarthroplasty. Modern total hip arthroplasty is generally a complication of the hard-to-hard joint interface, with more recent reports of tau-to-tau friction interface.
Despite the high number of reports on the phenomenon, its etiology remains controversial. The rattling usually occurs in abnormal situations, such as direct friction of the metal femoral head against the metal socket cup after polyethylene wear, ceramic lining rupture, or the presence of ceramic abrasive debris, but some normal-performing ceramic-to-ceramic total hip replacements (THR) have been associated with rattling. The incidence of ceramic-on-ceramic friction interface heterodyne in the orthopaedic literature varies widely, from 1% to 20%.
In general, patients tolerate the rattling well, and there is no difference between patient satisfaction and hip scores for hips with rattling and hips without sound. However, in a few cases, the rattling can also be unbearable for the patient, leading to revision surgery.
In order to clarify the etiology of the heteroacusis, we have found in previous studies that the tau to tau hip is associated with patient factors and prosthesis position factors. We also propose a mechanism for the generation of the heteroacusis.
Demographic factors of hip heterodyne
Factors associated with hip heterodyne can be broadly categorized into patient-related factors and prosthesis position-related factors.
We have previously examined the demographic and prosthetic factors that may be associated with an increased incidence of hip rales. This is the largest study to date to include two factors associated with tau-to-tau hip arthroplasty, with a total of 2406 tau-to-tau primary total hip arthroplasties performed by three surgeons at the same center from June 1997 to December 2008. With a mean follow-up of 10.6 years, 74 hips showed heterogeneous ringing at different stages, with a mean of 40 months postoperatively. The frequency of the heterogeneity was highly variable, with four (5%) patients experiencing heterogeneity at every step and some patients only occasionally experiencing it during hip flexion or extension, with pronation being the most frequent action inducing heterogeneity in this sequence. 15% of the heterogeneity disappeared at an average of 9.5 years postoperatively, suggesting that this may be a temporary problem.
In this study, Sexton et al. performed a demographic data analysis and found that younger people with greater height and weight and higher activity levels were prone to develop hip oscillations, and these demographic characteristics fit with the results of another independent study by Murali et al. thus reinforcing the level of evidence for this correlation. These factors by themselves or in combination can increase the mechanical load on the hip joint, making it susceptible to rales. Obesity by itself does not increase the odds of heteronormativity.
It is important to note that the presence of heterodyne was not found in our study. There was a significant difference in satisfaction or Harris scores between patients with and without heterotaxy. The median Harris score was 96 (51-100) for the heterogeneous hip and 95 (29-100) for the hip without heterogeneity (P=0.34,MannCWhitneyUtest).
Factors of prosthesis position for hip heteroacusis
We identified four prosthetic position factors that contributed to the heterodysplasia. The presence of one or more of the following factors may lead to an increased risk of hip heterogeneity.
1. Excessive abduction angle of the acetabular prosthesis.
This condition increases wear at the metal-to-polyethylene interface. In vitro experiments at the tau-to-tau friction interface suggest that poor acetabular abduction angles also increase wear-through rates11.
2., The center of the hip joint is displaced.
This will increase the forces acting on the hip joint.
3, The acetabular prosthesis is too large or too small in anterior tilt.
In this case, the femoral prosthesis and the acetabular prosthesis are prone to impingement and increased load on the acetabular rim.
4.The eccentric distance of the femur is too large.
Height may be a confounding factor at this point, and the femoral eccentric distance will be larger in taller patients (when the mechanical load on the hip joint is greater).
In a series of studies, Walter et al. tried to find the pathogenesis of the eccentricity, analyzing hip radiographs of the eccentric and non-eccentric hips, considering 15° to 35° of anteversion and 35° to 55° of abduction as acceptable ranges, and found that only 6 of 16 cases of eccentric hips had acetabular abduction and anteversion angles within acceptable ranges, and correspondingly, 16 of 17 cases of non-eccentric hips were within acceptable ranges.
An acetabular prosthesis with too much or too little anteversion increases the risk of impingement of the femoral neck against the acetabular rim, thereby increasing rim loading, and may contribute to the occurrence of heterotaxy. In contrast, studies have also found that patients with better postoperative internal or external rotation mobility are more likely to experience heterogeneous ringing.
However, even with a perfectly positioned prosthesis, a hip joint without femoral neck-acetabular rim impingement may still experience heterodyne, suggesting that heterodyne is not an isolated factor, but rather a multifactorial phenomenon.
Noise generation in hip replacements
Walter et al. reported cumulative data based on the analysis of several hip replacement noise mechanism studies. A study by this group performed an in-depth analysis of prostheses that were removed after hip replacement due to noise or pain and noise undergoing revision surgery. The average removal was 23 months postoperatively. The study analyzed a total of 12 third-generation alumina ceramic-to-tau friction interfaces collected from 11 different physicians in 3 different countries. The length, width and depth of wear were measured and the edges of the titanium acetabular cups were observed for impingement. The total amount of wear was calculated based on the measured parameters.
The average annual wear rate was 2.9 mm3 for the femoral head and 3.4 mm3 for the acetabular liner of the odd-rattle hip, which was significantly higher than the wear rate of the previously reported odd-rattle-free hip, which was 0.1 mm3 for the femoral head and 0.04 mm3 for the acetabular liner. all 12 sets of the removed friction subsets exhibited edge loading wear, with seven sets exhibiting femoral neck-acetabular edge impingement. In addition, some of the acetabular liners showed relative activity to the titanium acetabular cup.
Although impingement was present in the majority of the prostheses removed from the rattling hips, it was not a necessary condition for rattling to occur. The common denominator in all of the hip removal prostheses was the presence of edge loading wear. The team used finite element analysis to further investigate edge loading.
CT scans of the entire pelvis were obtained and reconstructed with the implantation of a titanium acetabular cup and alumina ceramic liner. Material assignments were made to the titanium alloy and alumina ceramic based on published literature. Using specialized software, loads were applied at 20 nodes at the outer edge of the liner in a semi-elliptical shape at the moment of toe off the ground during the gait cycle. Tests were performed in two different states: acetabular abduction at 45° and 42° of anteversion or 24° of anteversion.
This study found a stiffness incompatibility between the acetabular cup and the liner. As a result of the imbalance, the cup tended to deform and the cup-liner taper system became mismatched. When a mismatch occurs, the liner tilts away from the cup in the opposite direction of the applied stress (see Figure 1). The greater the anterior tilt of the acetabulum, the greater the separation of the cup from the liner. For example, with a forward tilt of 42°, the separation is 40 μm, while with a forward tilt of 24°, the separation is only 4 μm. The investigators believe that this separation makes the cup an oscillator and produces a heterodyne. The correlation between heteroacusis and increased anterior tilt is consistent with previous studies.
We believe that the heterodyne after hip arthroplasty is the result of forced resonance, consisting of a driving force and a dynamic response. The driving force is the high frictional force that occurs when there is a lack of fluid film lubrication at the hard-to-hard interface. The dynamic response is the resonance of a part of the device (resonator) according to its inherent vibrational frequency.
Liquid film lubrication requires a delicate balance of many factors, including sliding velocity, lubricant flow viscosity, friction interface roughness, clearance and contact pressure. Liquid film lubrication breakdown may result in edge loading (reduced contact area), trisomal particles in the joint (e.g. ceramic abrasive debris), joint surface damage (increased roughness), or friction interface diameter mismatch.
Mismatch in the socket cup lining system appears to be a cause of edge loading, as well as increased relative activity between prostheses, which may contribute to frictional driving forces. The team went on to compare the frequency of occurrence of the heterodyne hip to the intrinsic frequency of each part of the prosthesis in an attempt to discover which one or parts might be resonators.
This involved in-depth in vitro and in vivo acoustic and modal analysis of the frequencies and nature of the sounds emitted by the heterodyne hip, as well as a similar analysis of the individual Tao total hip replacement prostheses. A combination of digital video recorders, external microphones, and acoustic software were used to collect hip heterodyne sounds from named patients who had heterodyne hips after Tao-to-Tao hip replacement. Once the heterodyne was identified, it was easy to measure the predominant frequency of the heterodyne component.
The study found that the fundamental frequency of the in vivo audible ringing ranged from 400 Hz to 7,500 Hz. Each patient had at least one fundamental frequency at each occurrence of the rattling. The reverberations were recorded in three patients on two different occasions, each with a different frequency.
In vitro modal analysis of each part of the prosthesis was also performed. The prosthetic components subjected to analysis included.
1. the femoral stalk containing the femoral head
3. three different cup and liner combinations – fully matched tapered joint; tapered joint with contact only at the mouth of the cup; tapered joint with contact only at the base of the cup
3.Separate cups
4. Separate ceramic liner
The results of the modal analysis suggest that the ceramic prosthesis will resonate only at frequencies above the human audible range, while the metal prosthesis will resonate at frequencies within the human audible range. The analysis also suggested that the ceramic liner and titanium cup combination would also resonate at frequencies outside of the audible range.
The team also performed further acoustic analyses to test this hypothesis. Again, the results suggested that the inherent frequencies of the ceramic components were not in the audible range and therefore unlikely to play a role in the generation of the rattles. However, the much lower intrinsic frequencies of the titanium cup and metal femoral prosthesis are more likely to be responsible for the rattling. This could mean that the intrinsic frequencies of the titanium socket cup and metal femoral prosthesis are associated with the clinically seen rattling, suggesting that the resonance occurs in the metal portion, rather than the ceramic portion.
Conclusion
It is clear that no single factor causes the heterodyne after tau-to-tau hip arthroplasty and that this should be a multifactorial phenomenon. We propose one mechanism for tau-to-tau hip resonance: under ideal conditions, the hard-to-hard friction interface should have liquid film lubrication with very low friction. However, if the liquid film lubrication disintegrates, the frictional force will increase substantially due to sliding contact. If the increased friction force provides more energy than the system can dissipate (e.g., through thermal or subaudible noise), the friction may cause instability in the form of resonance and acoustic radiation. Acoustic and modal analyses show that metal partially amplifies this resonant driving force into audible sound.
Mechanical loading of the hip joint in young people who are taller and heavier and more active will inevitably increase, thus increasing the risk of early wear of the prosthesis. The high incidence of malposition of the acetabular prosthesis in patients with hip rattling suggests that the rattling may be secondary to marginal loading due to disintegration of the fluid membrane lubrication.
Clearly, only a portion of the factors associated with heterogeneity identified to date are within the control of the surgeon, and although poor prosthesis position is associated with heterogeneity, there are undoubtedly still other causes. We recommend that patients undergoing ceramic hip replacements undergo CT scans for heterogeneous responses to rule out ceramic rupture, which may not be evident on radiographs. If this can be ruled out, the patient should be informed that revision surgery should only be considered if the patient is deeply disturbed by the noise (no other factors exist that would suggest revision).
In most cases, the noise is not a serious clinical problem, and no correlation has been observed between the noise and early failure of the friction interface. Given its very low wear rate and incidence of osteolysis, the tau-to-tau friction interface remains the first choice for young active patients. Although it will take time to completely explain the post-operative rattle after tau-to-tau hip arthroplasty, its etiology is becoming increasingly clear. The role of prosthesis design in heterogeneity still needs further investigation.