**3. The "squeaking" hip**

Another concern with hard-on-hard bearing couples such as ceramic-on-ceramic and metalon-metal is the incidence of noise or "squeaking" in these joints. Audible sounds such as squeaking, clicking, snapping, cracking, grinding, rustling, crunching and tinkling are all re‐ ferred to in this text as "squeaking". "Squeaking" can be present during different kinds of activity including stair climbing, bending forward, squatting, standing from a chair and walking. The occurrence of this "squeaking" has been reported by many workers to differ‐ ent degrees.

Jarrett *et al* (2009) [52] described a group of 131 patients from which 14 (10.7%) suffered an audible "squeak" during normal activities (however, only 4 of these patients were able to reproduce the "squeak" during the clinical review session). They stated that none of the pa‐ tients had undergone revision surgery specifically because of "squeaking", however, longerterm follow-up was needed to monitor this noise.

Mai *et al* (2010) [70] reported noise or "squeaking" in 17% of 320 ceramic-on-ceramic hips and Keurentjes *et al* (2008) [71] found "squeaking" in 20.9% of cases. From their study they concluded that short neck length of the femoral component was a potential risk factor for "squeaking".

A study performed by Cogan *et al* (2010) [72] reported on the occurrence of "squeaking" in a patient population with at least one ceramic-on-ceramic hip; 10.6% of 265 hips demonstrated "squeaking". Two other published studies report on "squeaking" in 15% and 8.8% of the pa‐ tient population [44, 39]. Other published literature suggests that "squeaking" occurs in 0% - 6% of patients [73-75, 40, 65, 50, 61, 56, 76, 43, 49, 51, 37, 58, 41, 77]. The "squeaking" phenomenon tends to appear at an average of 28.8 months (range of averages 5.7 - 66 months) post-operatively.

After 10 years of follow-up Chevillotte *et al* (2012) [73] discussed the performance of 100, thirdgeneration ceramic-on-ceramic joints. By use of a questionnaire, 5% of these patients reported the occurrence of "squeaking". All of these patients were active, sporty and heavy men. "Squeaking" was not related to any malpositioning, wear or loosening of the joint and none of the 100 patient group suffered from any component fracture. In this paper it was stated that "squeaking" noise seems to be an isolated phenomenon with no consequences for the patient regarding functional results and on the implant longevity at 10 years of follow-up'.

Chang *et al* (2009) [69] reported on the clinical and radiographic outcomes when using thirdgeneration ceramic-on-ceramic joints in revision THR of 42 failed metal-on-UHMWPE hips. This was an interesting study as most published literature discuss the choice of bearing ma‐ terial for primary surgery and, as stated by the authors, '...few studies have focussed on the choice of bearing surface in revision...'. This was a young patient group (mean age: 48.8 years, range: 32 – 59 years) and the mean length of time between primary and revision sur‐ gery was 9.5 years (range: 3.3 – 16.1 years). The mean duration of follow-up after this revi‐ sion surgery was 64 months (range: 38 – 96 months). At the time of publication of this article, no hips needed additional revision surgery and no hips showed radiolucent lines, acetabular cup migration or osteolysis. This study gives very favourable results for the use of ceramic-on-ceramic hip joints in revision surgery, especially for the younger patient as the

Another concern with hard-on-hard bearing couples such as ceramic-on-ceramic and metalon-metal is the incidence of noise or "squeaking" in these joints. Audible sounds such as squeaking, clicking, snapping, cracking, grinding, rustling, crunching and tinkling are all re‐ ferred to in this text as "squeaking". "Squeaking" can be present during different kinds of activity including stair climbing, bending forward, squatting, standing from a chair and walking. The occurrence of this "squeaking" has been reported by many workers to differ‐

Jarrett *et al* (2009) [52] described a group of 131 patients from which 14 (10.7%) suffered an audible "squeak" during normal activities (however, only 4 of these patients were able to reproduce the "squeak" during the clinical review session). They stated that none of the pa‐ tients had undergone revision surgery specifically because of "squeaking", however, longer-

Mai *et al* (2010) [70] reported noise or "squeaking" in 17% of 320 ceramic-on-ceramic hips and Keurentjes *et al* (2008) [71] found "squeaking" in 20.9% of cases. From their study they concluded that short neck length of the femoral component was a potential risk factor for

A study performed by Cogan *et al* (2010) [72] reported on the occurrence of "squeaking" in a patient population with at least one ceramic-on-ceramic hip; 10.6% of 265 hips demonstrated "squeaking". Two other published studies report on "squeaking" in 15% and 8.8% of the pa‐ tient population [44, 39]. Other published literature suggests that "squeaking" occurs in 0% - 6% of patients [73-75, 40, 65, 50, 61, 56, 76, 43, 49, 51, 37, 58, 41, 77]. The "squeaking" phenomenon tends to appear at an average of 28.8 months (range of averages 5.7 - 66

After 10 years of follow-up Chevillotte *et al* (2012) [73] discussed the performance of 100, thirdgeneration ceramic-on-ceramic joints. By use of a questionnaire, 5% of these patients reported

likelihood of the need for further revision is greater.

548 Advances in Biomaterials Science and Biomedical Applications

term follow-up was needed to monitor this noise.

**3. The "squeaking" hip**

ent degrees.

"squeaking".

months) post-operatively.

The largest study to date (Sexton *et al* (2011)) [75] reported on the occurrence of "squeaking" and the role of patient factors and implant positioning in 2406 ceramic-on-ceramic hips at a mean follow-up of 10.6 years. Seventy-four hips (73 patients, 3.1%) made "squeaking" sounds at a mean time post-operatively of 40 months. Taller, heavier and younger patients with higher activity levels were found to be more at risk of developing a "squeaking" hip. However, there was no relationship between BMI and the prevalence of "squeaking". Inter‐ estingly, it was found that at a mean follow-up of 9.5 years, 11 of these hips (15%) had stop‐ ped "squeaking". Therefore, "squeaking" is not necessarily a permanent complication.

There is great debate over the cause of "squeaking" in ceramic-on-ceramic THRs. Several possible causes of "squeaking" are edge-loading [78], component malpositioning [78-80] or component or stem design [74, 81, 70, 77]. Other workers have also related "squeaking" to patient weight, height or age [70, 78, 79]. It is possible that a number of factors need to be present to initiate the "squeaking" phenomenon. The occurrence of "squeaking" has not been found to compromise the results of ceramic-on-ceramic hip joints; however, some pa‐ tients do request revision surgery in order to solve the "squeaking" issue [82].

Recently, laboratory studies have been performed in an attempt to re-create the conditions required to generate this "squeaking". Some authors have observed "squeaking" with ce‐ ramic-on-ceramic hip joints lubricated under dry conditions [83, 84, 73]. However, such an extreme lubrication regime is not expected to occur *in vivo* [85]. Work performed by Currie *et al* (2010) [86] suggested that the rolling/sliding mechanism of the bearing surfaces can in‐ duce vibration, of an audible frequency, resulting in "squeaking". Under lubricated condi‐ tions, Sanders *et al* (2012) [85] were able to reproduce the "squeak" using edge-loading during short-term wear tests. The "squeaking" was found to occur with a high contact force centred above or near the margin of the wear patch on a previously edge-worn femoral head. The authors state 'the results reveal key conditions that yield recurrent squeaking *in vitro* in various scenarios without resorting to implausible dry conditions'. In support of this, Walter *et al* (2011) [87] analysed 12 ceramic-on-ceramic components retrieved from "squeaking" joints and compared these with 33 'silent' ceramic-on-ceramic hip retrievals. All 12 "squeaking" hips showed evidence of edge-loading with up to 45 times greater wear than that reported for then the 33 'silent' hips [54]. The authors suggested that although the causes of "squeaking" are unknown, the high contact pressures experienced during edgeloading may result in a breakdown of the fluid film lubrication leading to some asperity contact and an increase in friction. Also, any surface damage, which may have been caused by the edge-loading, will result in an increased roughness of the bearing surfaces thus lead‐ ing to possible destruction of the fluid film leading to asperity contact.

Although there are a few reports on poor ceramic-on-ceramic hip prosthesis performance, the majority of authors give good and optimistic results. There have been little or no frac‐ tures, dislocation, infection or osteolysis. Also, the few patients suffering from "squeaking" hips have, in the majority of cases, had no need for revision surgery. These are, therefore, excellent results, but Lee *et al* (2010) [44] did suggest that these small risks should be a con‐ cern to surgeons and that patients should also be made aware of these before surgery. In ad‐ dition to this, they mentioned that longer-term follow-up is needed to assess the effects of these small risks on the prosthesis performance.

### **4. The younger patient**

As these artificial hip joints have been found to perform well in the younger patient (45 to 55 years), some surgeons have chosen to replace the diseased joints of even younger patients with this material combination.

A case study was reported by Capello and Feinberg (2009) [88] where ceramic-on-ceramic joints were implanted in a 13 year-old child with bilateral end-stage arthritis of the hip. Seven and eight years post-operatively the patient had no pain, no limp, and was able to walk long distances. The radiographs showed no implant loosening, osteolysis or wear. This is a very en‐ couraging result, however, it was stated that the patient is still very young (20 years of age at the time of report) and, therefore, the need for revision surgery will be more than likely.

Other studies on younger patients have not had as good results as those reported by Capello though. Nizard *et al* (2008) [89] reported on ceramic-on-ceramic hips that had been implant‐ ed in a group of 101 patients (132 hips) younger than 30 years old (mean age: 23.4 years, range: 13 – 30 years). These joints were implanted from 1977 to 2004 and, because of this, different implant designs and modes of fixation were used. Of these 132 joints, 17 were re‐ vised for aseptic loosening leading to a survivorship of 82.1% at 10 years and 72.4% at 15 years. These survivorship rates are quite low and may create cause for concern. It was, how‐ ever, found that the higher rate of failures of joints replaced for treatment of slipped capital epiphysis or trauma influenced the survivorships greatly. Also, these artificial hip joints were implanted over a period of 26.5 years, during which time the ceramic materials have been improved and the fixation methods have changed. It is hoped that ceramic of the new generation with improved prosthesis design and mode of fixation will perform better and provide improved longer-term results.

### **5. Overview**

From the results reported here it is clear that ceramic-on-ceramic hip joints have good tribolog‐ ical results: low friction, good lubrication and very low wear *in vitro* and *in vivo*. In addition to this, ceramic particles are biologically inert. Also, the fracture risk is relatively low. With good implant positioning these joints have the potential to perform incredibly well. These bearings, therefore, deserve to be high on the list for both primary and revision implants, especially for the younger, more active patient. However, for the best results, the choice of bearing combina‐ tion/design should be patient-specific; as one design does not suit all.
