Clinical and Radiological Outcome of the Newest Generation of Ceramic-on-Ceramic Hip Arthroplasty in Young Patients

Ceramic-on-ceramic articulations have become an attractive option for total hip arthroplasty in young patients. In this study, we retrospectively evaluated the shorttomidterm clinical and radiographic results in 51 consecutive patients (61 hips) using the newest generation of ceramic implants. Results obtained in our study showed positive clinical and radiological outcomes. Both HHS and UCLA activity scores doubled after surgery and tended to increase over time.There was one infection requiring a two-stage revision and a case of squeaking that began 2 years postoperatively after amechanical fall.The overall survival rate of the implants was 98.4% at six years with revision for any reason as the end point. Based on these results, fourth generation ceramics offer a viable option for young and active patients.


Introduction
One of the most challenging problems that orthopaedic surgeons are facing today is the increasing number of young and active patients requiring long lasting and reliable primary total hip arthroplasty (THA) [1].It is well known that bearing surface wear and particle-driven osteolysis remain the major factors threatening the longevity and limiting the performance of the implant [2,3].Ceramic-on-ceramic (CC) hip articulations made form alumina have become an attractive option for young and active patients who require THA.This is partially due to the excellent wear characteristics and outstanding tribological properties of alumina over metalon-polyethylene (MP) bearings [4,5].In addition, the inert nature of alumina gives the CC surfaces a great advantage over metal-on-metal (MM) surfaces [6].
Alumina CC bearings have a long history of use [7].Initial attempts of ceramic-on-ceramic THA had high failure rates that were mainly related to bad design and flaws in the material [8,9].Further advancements in alumina manufacturing technology such as hot isostatic pressing led to the fabrication of a highly purified alumina (BIOLOX forte; CeramTec AG, Plochingen, Germany) with increased material density, decreased grain structure, and less impurities [10,11].All these material improvements yielded a noticeable decrease in the rate of implant components fracture and chipping [12,13].Despite the advancements made in the quality of alumina ceramic, substantial concerns remain regarding CC bearings including stripe wear, limited sizing option, squeaking, and ceramic implant fracture.Limitations of pure alumina characteristics required the development of an advanced ceramic material.
Ceramic composites were the next step in the development of CC bearings.The newest generation of BIOLOX delta ceramic bearings was introduced in 2000 by CeramTec AG (Plochingen, Germany).BIOLOX delta is an aluminamatrix composite (AMC) consisting of 81.6% aluminium oxide, 17% yttria-stabilized tetragonal zirconia particles, and traces (1.4%) of chromium dioxide and strontium crystals engineered to increase material density and reduce grain size (less than 0.8 m compared with the grain size of alumina 1-5 m) [14][15][16].With these improvements, the risk for crack propagation and component fracture was minimized.Laboratory mechanical and hip simulator wear tests have shown that these manufacturing advances have indeed resulted in a high-strength material with increased fracture toughness and lower wear rates over pure alumina [15,17].
These enhanced mechanical properties should decrease the ceramic fracture rate, allowing the manufacturing of thinner acetabular liner inserts and therefore the use of larger femoral head options.This increased head size should improve joint stability and reduce dislocation rates [18,19].One study also suggests that material properties of the AMC may lead to a different wear response and may decrease or eliminate squeaking [17].These favorable material characteristics make AMC a very promising material and particularly desirable for long lasting bearing surfaces.However, AMC has a relatively short clinical history and further monitoring is necessary.
The aim of this study was to retrospectively evaluate the short-to midterm clinical and radiographic outcome after primary total hip replacements using the fourth generation ceramic BIOLOX delta implants.

Material and Methods
Prior to the onset of the study, institutional review board approval was obtained.We conducted a retrospective evaluation of all patients who received a cementless CC BIOLOX delta THA between December 2004 and December 2009.Our exclusion criteria were a previous total hip replacement, previous hemiarthroplasty, or fusion on the ipsilateral side.In all, we evaluated 60 hips in 50 patients.Among these patients, 10 underwent staged bilateral THAs.There were 10 males (11 hips) and 40 females (49 hips) (Table 1).The mean age at the time of the surgery was 41.8 years (range 21-56 years).All patients underwent primary hip arthroplasty for both noninflammatory and inflammatory degenerative joint diseases.Preoperative diagnoses were osteoarthritis (40%), developmental dysplasia of the hip (22%), rheumatoid arthritis (13%), avascular necrosis (7%), juvenile rheumatoid arthritis (5%), and other causes (11%).Three arthroplasty surgeons performed all procedures through a direct lateral (77%) or posterior approach (23%).Bilateral cases were conducted on separate dates with a mean interval of 10.3 months (3-42 months) between the two procedures.
Twenty-five hips (42%) received 28 mm ceramic heads and 35 hips (58%) received 36 mm ceramic heads, depending on patient anatomy and implant stability.Head size was determined by the inner diameter of the ceramic liner for the corresponding cup size.All patients received one of four cementless Pinnacle acetabular cups (Pinnacle 100, Pinnacle 300, Pinnacle Bantam, Pinnacle Multihole II; DePuy Warsaw, IN) and one of five cementless femoral stems (Summit, Prodigy, Trilock, Corail, or S-ROM; DePuy, Warsaw, IN) (Table 1).Acetabular fixation with screws was used based on the patient's bone quality.
All patients were evaluated clinically and radiographically at 6 weeks, 3 months, and one year postoperatively, followed by subsequent annual examinations.Minimum followup for this study was 2 years, and average followup was 4.6 years (range 2-6.8 years).Preoperative and postoperative Harris hip score (HHS) [20] was recorded.HHS score of 90 points or more was defined as an excellent outcome; 80 to 89 points, a good outcome; 70 to 79 points, a fair outcome; and less than 70 points, a poor outcome.Activity score was assessed preoperatively and postoperatively using the University of California, Los Angeles (UCLA) activity level rating scale from 1 to 10 [21].Presence of hip squeaking and all postoperative complications were documented.In each visit, serial anteroposterior (AP) radiographs of the pelvis and AP and lateral radiographs of the operated side were obtained.Radiographic analysis was performed using Einzel-Bild-Roentgen-Analyse (EBRA) by two of the authors (AR and AA).Survival analysis was performed with use of the Kaplan-Meier method [22].The patient demographics were calculated with arithmetic mean and all errors are reported as standard deviation.A nonparametric Mann-Whitney  test was conducted to determine the significance between preoperative and the last postoperative HHS and UCLA activity scores.All other comparisons between different time points of followup groups were performed using a nonparametric Kruskal-Wallis test followed by a Dunn's multiple comparisons.GraphPad Prism software (GraphPad Software, La Jolla, CA, USA) was used for all data analyses.A  value of less than 0.05 was considered to be statistically significant.

Results
All 50 patients had complete clinical and radiographic data at 2 years of followup.
The mean HHS and UCLA scores at last followup significantly improved ( = 0.00087) from their preoperative values.The mean HHS score for all patients at their last followup was 88 points (range, 40-99).The mean UCLA activity score was 6 (range, 2-10).Table 3 shows the number of hips showing excellent (HHS > 90), good (90 > HHS > 80), fair (80 > HHS > 70), or poor (HHS < 70) outcomes based on HHS score for all patients as well as those who received 36 mm and 28 mm femoral heads.In all cases, excellent outcomes were observed in 67% of hips, while poor results were only observed in minority of 5% of hips.There were no outcome differences between the two head diameters.
We also looked at the evolution of HHS and UCLA activity score over time.Both scores significantly increased until 4 years postoperatively reaching a plateau thereafter (Figures 1 and 2).
At the time of the final followup, there was no radiographic evidence of component loosening and no evidence of implant subsidence, liner fracture or dislocation.All components were stable and osseous-integrated.The mean acetabular inclination and anteversion were 46 degrees (range, 28 to 58 degrees) and 7 degrees (range, 2 to 22 degrees), respectively (Table 2).
There was a case of deep infection requiring a twostage revision.One female patient presented with audible repetitive "squeaking" noise that began after a fall 2 years postoperatively.She had a 36 mm head and she was able to    3(a) and 3(b)).The patient was completely pain-free and she refused a revision surgery.The third patient presenting with poor result was a female patient with juvenile rheumatoid arthritis and reduced mobility.
The Kaplan-Meier survival with revision for any reason as the end point was 98.4% at 6 years.In summary, no patients showed osteolysis, one patient had audible squeaking, and only one patient required revision surgery for infection.Both HHS and UCLA activity scores significantly improved after surgery and tended to increase over time.Of all procedures, 72% of 28 mm heads and 63% of 36 mm heads had excellent HHS outcomes.No statistically significant difference was noted between 28 mm and 36 mm heads in both HHS

Discussion
Wear debris-induced osteolysis and subsequent implant loosening are major factors limiting the survivorship and performance of THA.This problem may be overcome with the use of CC bearings known to produce the lowest volumetric wear rate and reduced incidence of periprosthetic osteolysis [23,24].Despite their elevated hardness and toughness, the CC surfaces are not immune to wear or surface damage and concerns arising about ceramic implants are chipping and component fracture, stripe wear, and the poorly understood phenomenon of squeaking [12].Moreover, the lack of large modular options continues to exist [10,13,19].The BIOLOX delta AMC was developed to address some of the drawbacks of currently available alumina designs.Its increased fracture toughness combined with the wider range of head sizes make this implant an attractive selection.In this study our aim was to evaluate the short-and midterm outcome of this new material.
One of the major disadvantages of the alumina ceramic bearing is the risk of fracture.Although sporadic, these component fractures can be catastrophic and continue to be a clinical concern for both surgeon and patient [12].The CC fracture failure is principally related to the crack propagation and is favoured by stress concentration zones caused by microstructural flaws either at the surface or within the material [25].Incremental advances in the manufacturing process, design, and quality control of alumina components have addressed these issues improving significantly the structural characteristics and fracture toughness of pure alumina [26].
Currently, component fractures with modern CC bearings are rare (0.004%) [26,27].Both D' Antonio et al. [23] and Lusty et al. [5] showed no ceramic bearing fractures in 213 and 301 primary alumina-on-alumina THAs, respectively, at an average of five years.The estimated risk of fracture of these implants is from 0.03% to 0.05% for femoral heads and from 0.017% to 0.013% for the alumina ceramic acetabular insert and is more likely to occur within the first three years following implantation [27].Furthermore, fractures of 28 mm heads occur more frequent than 32 mm and 36 mm heads [28].Because of the BIOLOX delta's increased fracture toughness and burst strength, the expectation is that fracture would occur with far less frequency than the estimated fracture rate of alumina ceramic heads (0.02%) [27,29].In our study, there was no incidence of head or liner fracture.Moreover, there was no difference between 28 mm and 32 mm heads in terms of clinical and radiological outcome.
In their prospective randomized study, D' Antonio et al. showed that liners that were not fully seated were prone to peripheral chipping during impaction [30].They observed a peripheral chip in 1.2% of cases; however, none of these resulted in a fracture or in revision surgery.A subsequent design encased the ceramic acetabular component in a titanium sleeve, which eliminates the risk of insertional chips [28,30].
Recognition of squeaking in ceramic articulations as a clinical problem has increased recently [16,17,31] with its incidence ranging from 0.48% to 7% [11,18,32].Squeaking has been shown to be due to ceramic or metal wear, component factors, implant design, impingement, implant positioning [11,18], and patient characteristics [27].Squeaking often occurs in younger, heavier, and taller patients [33].Edge loading appears to be the predominant causative factor for squeaking [33].Ki et al. showed that increased BMI and certain cup designs were more prone to squeaking [34].Swanson et al. reviewed 233 patients comparing 4 implant combinations representing 4 manufacturers and found that Trident acetabular cups paired with Stryker Accolade femoral stems showed a dramatically higher incidence of squeaking.They concluded that prosthetic design plays a key role in squeaking occurrence [35].Another significant cause of squeaking is cup design that results in premature femoral neck-acetabular component rim impingement such as the presence of modular ceramic liner designs that are placed within a titanium encasement with extended rims [29,34].Most studies report that squeaking is rare with no clinical consequences [17], but in some studies, squeaking has led to revision [32,36].Yang et al. also suggested that squeaking can be reduced due to material properties, such as the wear response of BIOLOX delta [17].In our study squeaking was noted in only one patient with increased inclination of the acetabular component.
Our study has certain limitations.The number of patients is relatively small, three surgeons were involved using two different approaches, and four acetabular cups matched with five femoral stems were implanted.We also recognize that our study is of relatively short duration with a minimum followup of two years and a mean followup of just over four years.To the best of our knowledge, this is the first study reporting the survival and functional outcome of solely fourth generation ceramic delta implants.

Conclusion
Alumina-based ceramic composite materials, with its superior mechanical properties and excellent wear behavior, have been successfully implanted and have offered outstanding performance in the last 11 years.This retrospective study demonstrates that BIOLOX delta bearings provided good to excellent overall functioning outcomes with no implant related complications or osteolysis.An increase in head size did not lead to significantly different HHS and UCLA activity score results.However, the duration of followup is relatively short.Therefore, we cannot conclude whether this new ceramic material will outperform other bearing couplings over time.Long-term follow-up studies are needed to determine the stability and reliability of fourth generation ceramic articulation in THA.However, based on the results we observed and those already present in the literature, fourth generation ceramic bearing couplings appear to be very promising and should be considered for young and active patients.

Figure 1 :
Figure 1: Average and standard deviation (SD) values of HHS scores for all patients over time.

Figure 2 :
Figure 2: Average and standard deviation (SD) values of UCLA activity scores for all patients over time.

Figure 3 :
Figure 3: (a) AP radiograph of the patient presenting with audible hip squeaking.(b) Analysis with the EBRA software revealed cup inclination of 58 degrees and anteversion of 19 degrees.

Table 2 :
Clinical and radiographic results at the last followup.

Table 3 :
The number of hips showing the clinical outcome based on HHS score.