Genetic susceptibility to aseptic loosening following total hip arthroplasty: a systematic review

Genetic susceptibility to aseptic loosening

following total hip arthroplasty: a systematic

review

Angelo Del Buono1, Vincenzo Denaro1, and Nicola Maffulli2*

1Department of Orthopaedic and Trauma Surgery, Campus Biomedico University of Rome,Via Alvaro del Portillo, Rome, Italy, and 2Centre for Sports and Exercise Medicine, Barts andThe London School of Medicine and Dentistry, Mile End Hospital, 275 Bancroft Road,London E1 4DG, UK

Introduction: Aseptic loosening is the most common cause of total hip

arthroplasty (THA) failure and revision surgery. Genetic polymorphisms could be

determinant factors for implant loosening.

Source of data: We performed a comprehensive search of Medline, CINAHL,

Googlescholar, Embase and Cochrane databases, using various combinations of

the keyword terms ‘aseptic loosening’, ‘gene’, ‘hip arthoplasty’, ‘genetics’,

‘loosening’. Twelve studies detailing the genetic investigation of patients with

aseptic loosening of a THA were identified.

Areas of agreement: SNPs of GNAS1, TNF-238 A allele, TNF-a promoter

(-308G!A) transition, IL6-174 G allele, interleukin (IL)-6 (-597) and (-572), MMP-

1-promoting gene, C/C genotype for the MMP1, MT1-MMP, MMP-2,

transforming growth factor-beta1 signal sequence (29T!C) transitions, A/A

genotype for the OPG-163, and MBL were overexpressed in patients with aseptic

loosening and periprosthetic osteolysis.

Areas of controversy: Data from single centre studies do not allow one to

compare the results of different studies.

Conclusion: Several gene pathways and genes contribute to the genetic

susceptibility to aseptic loosening following THA. Further studies will enhance

the understanding of prosthesis failure, and may inform and direct

pharmaceutical interventions.

Growing points: Further multi-centre prospective studies are necessary to confirm

the general validity of the findings reported. Single-centre findings should be

replicated in other centres and populations to open new avenues for pre-surgical

genetic testing and to investigate immune response modulation in THA.

Areas timely for developing research: Research in this field could lead to better

understanding of mechanisms behind aseptic osteolysis, and improve the results

of THA.

British Medical Bulletin 2012; 101: 39–55

DOI:10.1093/bmb/ldr011

& The Author 2011. Published by Oxford University Press. All rights reserved.

For permissions, please e-mail: [email protected]

*Correspondence address.

Centre for Sports and

Exercise Medicine, Barts

and The London School

of Medicine and

Dentistry, Mile End

Hospital, 275 Bancroft

Road, London E1 4DG,

UK. E-mail: n.maffulli@

qmul.ac.uk

Published Online June 7, 2011

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Keywords: hip/arthroplasty/loosening/genetics

Accepted: March 9, 2011

Introduction

Almost 1 million of total hip arthoplasties (THA) are implanted world-wide annually. Prosthetic implants improve quality of life and relievesymptoms, but some of these benefits are time limited in many cases.1

A prosthesis which presents loosening required revision surgery, whichhas a high morbidity and mortality rate, especially in elderly patientswith comorbidities.2 While sepsis, fracture and dislocation are rela-tively rare, prosthetic failure secondary to aseptic loosening hasbecome increasingly more common.3 Arising from aseptic inflamma-tory reactions to the prosthetic implants, aseptic loosening results inprogressive bone loss and periprosthetic osteolysis,4 accounting for75.7% of all THA revisions.5,6

To try and identify preventive measures, possible biological, physicaland biophysical factors have been investigated. At present, it is thoughtthat susceptibility to aseptic loosening results from a combination ofenvironmental and genetic factors. Environmental factors such as typeof prosthesis, material, fixation method, implant design, surgical tech-nique and postoperative sport activity level have been widely studied,7,8

and investigations on ‘genetic factors’ may well give a more definiteanswer to this issue. To date, the precise aetiology of aseptic looseningis still unclear.9 Even though inflammatory cytokines trigger a chronicinflammatory status which, if untreated, may determine soft tissue andsupporting bone damage, basic sciences studies are being increasinglyaimed at better understanding the molecular aspects of aseptic loosen-ing, without restricting them to investigate only traditional inflamma-tory patterns.3,4

Genetic

The present review focuses on the genetic aspects of early and lateaseptic loosening and osteolysis after THA implantation. Genetic poly-morphisms are genetic variations that are considered biologicallynormal and can be found in at least 1% of the population. These vari-ations may influence protein transcription, and contribute towardsindividual susceptibilities to certain pathological conditions.10 Therapidly evolving evidence on genetic factors and occurrence of numer-ous common diseases has led to integrate human genomics into the

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practice of medicine and public health.11 Identifying and characterizingassociated risk or protective factors, genetic determinants could behelpful to better understand several aetiologic aspects of disease, anddevelop potential treatment strategies. Thousands of reports havedescribed gene–environment and gene–gene interactions probablyinvolved in the development of pathological status,11,12 but the currentevidence of studies on gene–disease associations is limited by severalmethodological limitations.13,14 Several commentaries have been pro-duced, and different recommendations have been proposed. Giventhese intrinsic methodological limitations, a set of guiding principlesfor reporting results of genetic association studies has been provided ina recently published statement, the STrengthening the REporting ofGenetic Association studies (STREGA).15

Methods

A comprehensive search was performed combining the keywords‘aseptic loosening’, ‘gene’, ‘hip arthoplasty’, ‘genetics’, ‘loosening’ withno limit regarding the year of publication. Medline, CINAHL,Googlescholar, Embase and Cochrane databases were accessed tosearch English, Spanish, French, Portuguese and Italian publications.The first electronic search identified 46 relevant abstracts of articlespublished in peer-reviewed journals. Full-text versions were obtained ifthe abstract did not allow one to include or exclude the study. Thereference lists of all selected articles were reviewed by hand to identifyarticles not included at the electronic search. Studies detailing thegenetic investigation of patients with aseptic loosening of hip prostheticimplants were selected, and their bibliographies thoroughly reviewed toidentify further related articles. Biomechanical reports, studies onanimals, cadavers, in vitro, case reports, literature reviews, technicalnotes, letters to editors and instructional course were excluded. Elevenstudies published from 1998 to 2010 were considered relevant to ourinterest. Genetic data were extracted. Additionally, data concerningloosening imaging assessment, demographic features, body mass index(BMI), type of prosthesis, time from prosthesis to revision surgery andosteolysis-free survival time were extracted (Tables 1 and 2). TheSTREGA statement was used to assess reporting quality (Table 3). Thisstatement provides additions to 12 of the 22 items on the previouslydefined STROBE checklist.15 The additions concern population stratifi-cation, genotyping errors, modelling haplotype variation, Hardy–Weinberg equilibrium, replication, selection of participants, rationalefor choice of genes and variants, treatment effects in studying


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Table 1 Sample features.

Author Type of study Ethnic

group

Sample size Patient age

at surgery

Patient age at

last assessment

BMI (kg/m2) Type of prosthesis Time to

loosening

Time to

revision

Bachman

et al.16

Retrospective

German

Caucasian 57 64.39+11.1 68.44+10.5 27.38+6.0 42 mo

(1–120)

Gallo et al.35 Case–control 205 Mild

osteolysis group

(89); severe

osteolysis group

(116)

48 (27–58) in

mild osteolysis

group; 45

(24–68) in

severe

osteolysis

group

28.1 (20.3–

35.7) in mild

osteolysis

group; 27.2

(16.0–42.6) in

severe

osteolysis

group

Cementless prosthesis Aesculap

Bi-Contact prostheses

9 (2–13)

years in mild

osteolysis

group; 6 (3–

12) in severe

osteolysis

group

(P , 0.001)

Godoy-Santos

et al.42

Case–control 58 Control

group: 31

patients with

successful

uncemented

THA Test group:

27 patients with

early aseptic

loosening

Uncemented Aesculap Bi-Contact

prostheses

Gordon

et al.40

Case–control 612 65+8 in the

control group;

59+9 in the

osteolysis

group

(P , .001)

28+5 in

control

group; 28+5

in the

osteolysis

group

(P . 0.05)

Cemented THA with

polyethylene-on-metal-bearing

couple

12+4 in

control

group;

10+5 in the

osteolysis

group

(P , 0.001)

Kolundzic

et al41

Retrospective Caucasian

Croatian

41 44 (26–58) 25 (20–34) Endler polyethylene uncemented

actabular cup, Zweymueller

uncemented femoral stem

15 years

(5–18)

Malik et al.33 Case–control Caucasian

English

312 150 control

group, 91

aseptic group

and 71 septic

group

Overall mean:

68.6

years+7.8

(29–92)

Cemented Charnley monoblock

femoral stem and cemented

Charnley or Ogee flanged

polyethylene acetabular cup

5.1 years

70.9 in the

control group;

66.2 in the

aseptic group

and 68.7 in the

septic group

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Malik et al.43 Case control Caucasian

English

312 150 control

group, 91

aseptic group

and 71 septic

group

Overall mean:

68.6

years+7.8

(29–92)





5.1 years

70.9 in the

control group;

66.2 in the

aseptic group,

and 68.7 in the

septic group

Malik et al.22 Case–control Caucasian

English

312 150 control

group, 91

aseptic group

and 71 septic

group

Overall mean:

68.6

years+7.8

(29–92)





5.1 years

70.9 in the

control group;

66.2 in the

aseptic group

and 68.7 in the

septic group

Mrazek

et al.25

Retrospective 205

Wedemeyer

et al.26

Retrospective Caucasian 87

Wilkinson

et al.37

Case–control 481 214

osteolysis group

patients and 267

control group

64+8 in the

control group

(follow-up);

59+9 in the

osteolysis

group

(P , 0.001)

28+5 in the

control

group; 28+4

in the

osteolysis

group

(P . 0.05)

10.4 years

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Table 2 Polymorphisms and loosening.

Author SNPs Loosening assessment

Bachman et al.16 GNAS1 T393C

Gallo et al.35 TNF-238 A/A Radiographic Saleh classification

Godoy-Santos

et al.42

MMP-1 G/G

Gordon et al.40 rs1800795 (IL6-174) Radiographic: Harris and McGann criteria and Harris

and Penenberg criteria were used to define femoral

and pelvic loosening

Kolundzic et al41 IL-6 promoter (-597G/A) and

(-572G/C); TGF-b1 T/C

transition in the signal

sequence; TNF-a (308G/A)

promoter

Radiographic: Krugluger and Eyb’s criteria. Level 2

radiological findings (acetabulum or stem) were

considered as failure if combined with clinical

symptoms. Level 3 or 4 findings were considered as

failure, regardless of the clinical symptoms.

Prostheses with aseptic loosening 22: isolated aseptic

loosening of the cup in 15, aseptic loosening of the

cup and the stem in 7

Malik et al.33 OPG-163 A allele promoter;

OPG-245; OPG þ 1181;

RANK þ 575 C/T T allele

and T/T genotype

Femoral stem aseptic loosening was defined either

by surgical findings, and radiographic criteria of

Harris et al., or progressive endosteal cavitation

across zones as described by Gruen et al.

Acetabular loosening was confirmed either at the

time of revision surgery or by using the criteria of

Hodgkinson et al. [10]

Well-fixed control THAs were defined as clinically

asymptomatic for over 10 years with no radiographic

features of aseptic loosening as described by

Pachecho et al.

Malik et al.43 MMP1-1 (RS5854) C/C;

IL6-174; VDR-L and VDR-T







Hodgkinson et al.




Pachecho et al.

Malik et al.22 Mannose-binding lectin

(MBL) 550 C/C; codon 54 G/G







Hodgkinson et al. [10]




Pachecho et al.

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quantitative traits, statistical methods, relatedness, reporting of descrip-tive and outcome data and the volume of data issues.

Proteins, receptors and intracellular mediators

In a study on 57 German Caucasian patients undergoing revisionsurgery for aseptic loosening of a THA, Bachman et al.16 investigatedthe T393C polymorphism of GNAS1, the gene coding for the stimu-latory Gas subunit, an ubiquitously expressed cAMP-dependentG-protein contributing in different pathways to bone physiology andaseptic loosening pathophysiology. Gas and Gas/cAMP pathways areinvolved in parathyroid hormone (PTH)-mediated osteoblastogenesis,mediate bone disorders such as McCune-Albright syndrome andAlbright hereditary osteodystrophy, and functional defects of osteo-blasts,17,18 and regulate interleukin-6 (IL-6) activation in osteoblasts,19

macrophages20 and osteoclasts.21 In female patients, time and mediantime to aseptic loosening were not significantly associated with singleT393C genotypes. However, combined comparison of TC and CC gen-otypes with TT genotype revealed significantly longer time and mediantime to aseptic loosening in female TT carriers (P ¼ 0.022). In malepatients, the CC genotype carriers had longer time and median time toloosening than TC and TT genotype patients (P ¼ 0.018, P ¼ 0.023).Compared with a reference group of T393 homozygous male individ-uals, heterozygous patients had a 6.25-fold lower risk with a hazardratio of 0.160 (95% CI: 0.04–0.71; P ¼ 0.016), while male patientscarrying the CC genotype showed a significant (more than 11-fold)lower risk of loosening, with a hazard ratio of 0.088 (95% CI: 0.02–0.50; P ¼ 0.006).

The association between mannose-binding lectin (MBL) deficiencyassociated with SNPs at codons 52 and 54 and at promoter positions

Table 2 Continued

Author SNPs Loosening assessment

Mrazek et al.25 P2RX7 gene: rs3751143;

rs1653624, rs28360457

(Arg307Gln); rs35933842

(splice site mutation at the

first intron)

Radiographic Saleh classification

Wedemeyer

et al.26

CALCA-1786T . C

(rs3781719) and -1752C . G

(rs1553005)

Wilkinson et al.37 Promoter region TNF-238 A

allele

Radiographic: Harris and McGann criteria and Harris

and Penenberg criteria were used to define femoral

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-550 and -221 and the development of aseptic loosening or deep infec-tion around THAs has been investigated in a case–control study of 312patients.22 MBL is a liver-derived serum protein, which increasesduring the acute-phase response to infection, takes part in the opsoni-zation of bacteria and mediates complement activation. Acting as thecomplement component C1q and activator of the complement cascade,

Table 3 STREGA assessment.

Item Item

number

Extension for genetic association (STREGA)

Introduction

objectives

3 State if the study is the first report of a genetic association, a

replication effort or both

Methods participants 6 Give information on the criteria and methods for selection of

subsets of participants from a larger study, when relevant

Variables 7 (b) Clearly define genetic exposures (genetic variants) using a

widely used nomenclature system. Identify variables likely to be

associated with population stratification (confounding by ethnic

origin)

Data sources/

measurement

8 (b) Describe laboratory methods, including source and storage of

DNA, genotyping methods and platforms (including the allele

calling algorithm used and its version), error rates and call rates.

State the laboratory/centre where genotyping was done. Describe

comparability of laboratory methods, if there is more than one

group. Specify whether genotypes were assigned using all of the

data from the study simultaneously or in smaller batches

Bias 9 (b) For quantitative outcome variables, specify if any investigation

of potential bias resulting from pharmacotherapy was undertaken.

If relevant, describe the nature and magnitude of the potential bias

and explain what approach was used to deal with this

Quantitative

variables

11 If applicable, describe how effects of treatment were dealt with

Statistical methods 12 State software version used and options (or settings) chosen

(f) State whether Hardy–Weinberg equilibrium was considered

and, if so, how

(g) Describe any methods used for inferring genotypes or

haplotypes

(h) Describe any methods used to assess or address population

stratification

(i) Describe any methods used to address multiple comparisons or

to control risk of false-positive findings

( j) Describe any methods used to address and correct for

relatedness among subjects

Results participants 13 Report numbers of individuals in whom genotyping was attempted

and numbers of individuals in whom genotyping was successful

Descriptive data 14 Consider giving information by genotype

Outcome data 15 Report outcomes (phenotypes) for each genotype category over

time

Main results 16 (d) Report results of any adjustments for multiple comparisons

Other analyses 17 (b) If numerous genetic exposures (genetic variants) were

examined, summarize results from all analyses undertaken (c) If

detailed results are available elsewhere, state how they can be

accessed

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MBL removes immune complexes.23,24 The C allele (P ¼ 0.001; OR ¼2.23; 95% CI ¼ 1.42–3.50) and the genotype C/C (P ¼ 0.004) for the550 SNP were highly associated with aseptic failure compared withcontrols. The codon 54 SNP G allele (P ¼ 0.12; OR ¼ 2.17; 95% CI ¼1.18–3.98) and G/G genotype (P ¼ 0.027) were associated withaseptic failure. No statistically significant relationship was foundbetween aseptic loosening and the promoter 221 or codon 52 SNPs.The C allele (P ¼ 0.01; OR ¼ 1.90; 95% CI ¼ 1.14–3.16) and geno-type C/C (P ¼ 0.05) for the 550 SNP were significantly more frequentamong septic than control patients. Regarding the codon 54 SNP, theG/G genotype (P ¼ 0.05; OR ¼ 1.47; 95% CI ¼ 0.79–2.72), but notthe allele frequency, was significantly associated with septic failure ofTHA. No significant relationship was noted between septic failure andthe promoter 221 or codon 52 SNPs.

Four P2RX7 polymorphisms responsible for receptor loss-of-function(LOF) were investigated in 205 patients operated on for severe acetabu-lar osteolysis and THA failure.25 The authors hypothesized that theLOF of P2RX7 may impair the balance between the osteolytic andosteogenetic process at the bone–prosthesis interface, increasing osteo-lysis and bone loss. Although the carriers of any investigated P2RX7LOF variants were moderately over-represented among patients withsevere periprosthetic osteolysis (47% of carriers vs. 53% wild-type gen-otypes) compared with those with mild/no osteolysis (38% of carriersvs. 62% wild-type genotypes), this difference was not statistically sig-nificant. Two rare variants associated with complete loss of P2RX7receptor function were observed only in THA revision patients. TheP2RX7 variant causing complete loss-of-function (P2RX7 568Asn,307Gln and rs35933842 null allele) were over-represented in 15 of 157revised (9.6%) and in one of 48 (2.1%) unrevised prosthesis (P ¼ 0.09).

Wedemeyer et al.26 investigated the possible influence of the func-tional BCL2 (-938C!A) and CALCA (-1786T!C) polymorphisms in87 patients with aseptic loosened hip arthroplasties. In normal tissues,the proteins of the Bcl-2 family regulate proliferation and apoptosis27

by blocking or accelerating it.28 Bcl-2 itself is an inhibitor of apoptoticcell death, and can act as an anti-apoptotic and anti-proliferativefactors.29 The BCL2 gene has two promoters, P1 and P2. The activatedpromoter P2 decreases the activity of the other promoter P1, with nega-tive effect on P1 activity.30 The common single nucleotide polymorph-ism (SNP, -938C!A (rs 2279115)) in the inhibitory P2 promoter ofthe BCL2 gene alters the P2 and P1 promoter activities.31

Alpha-CGRP is a neurotransmitter involved in bone remodelling.32

The CALCA gene encodes both alpha-CGRP and Calcitionin. Thesepolymorphisms are neither associated with the prognosis of THA norinfluence the time to aseptic loosening.



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In a case–control study, Malik et al.33 investigated 312 CaucasianEnglish patients who had undergone a cemented THA. The patientswere divided into three groups: the study groups consisted of 91patients with early aseptic loosening and 71 patients with deep infec-tion, the control group consisted of 150 patients with clinically andradiological well-fixed THAs. OPG-163 A allele promoter, RANK þ575 C/T allele, OPG-245 and OPG þ 1181were isolated and amplified.The RANK/RANKL/OPG system is directly implicated in the develop-ment of osteolysis around THAs.34 The osteoblasts RANKL binds toosteoclasts RANK, and directly stimulates the differentiation of osteo-clast progenitor cells into osteoclasts. OPG acts as a soluble decoyreceptor to RANKL and competes with RANK for RANKL binding,blocking osteoclast differentiation.33 The frequency of the A allele (P ¼0.001, OR ¼ 3.76) and homozygous genotype A/A (P ¼ 0.001) for theOPG-163 SNP were highly associated with aseptic failure comparedwith the controls. The RANK þ 575 C/T SNP T allele (P ¼ 0.004,OR ¼ 1.77) and T/T genotype (P ¼ 0.008) frequencies were associatedwith aseptic failure. OPG-245 and OPG þ 1181 SNPs were not signifi-cantly associated with aseptic loosening. Compared with the patients inthe control group, the patients in the septic group had a significantlyhigher frequency of the A allele (P ¼ 0.001, OR ¼ 3.42, 95% CI ¼1.98–5.91) and homozygous genotype A/A (P ¼ 0.001) for theOPG-163 SNP, with no statistically significant differences for theOPG-245, OPG þ 1181 and RANK þ 575 SNPs.

Cytokines

In a case–control study on 205 Czech Caucasian patients, Galloet al.35 investigated 22 SNPs in 13 genes encoding inflammatory cyto-kines and cytokine receptors (IL-1a, IL-1b, IL-1R, IL-1Ra, IL-4Ra,IL-12, IFN-g, transforming growth factor-b (TGF-b), TNF-a, IL-2,IL-4, IL-6 and IL-10). According to the Saleh radiographic scale,36 89patients presented mild osteolysis and 116 presented severe osteolysis.The TNF-238 A allele, higher in the patients with severe than in thosewith mild osteolysis (OR ¼ 6.59, P ¼ 0.005, PAR ¼ 5.2%), was associ-ated with severe acetabular osteolysis and risk for premature failure, incontrast to the TNF-238 GG genotype. Compared with the GG geno-type patients, the TNF-238 A carriers were six times more prone todevelop severe osteolysis and had increased cumulative hazard of THAfailure (P ¼ 0.024). However, the TNF-238 A allele occurs rarely inpatients who exhibit no evidence of osteolysis. The IL6-174G allele, anIL6 gene promoter SNP, was over-represented among patients withsevere osteolysis compared with those with mild osteolysis (OR ¼ 2.51,

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P ¼ 0.007, PAR ¼ 31.5%). Although univariate and multivariate ana-lyses revealed increased risk of severe osteolysis, the presence of theIL6-174G allele in THA patients did not change the cumulative hazardof THA failure. Instead, IL2-330G allele was under-represented inpatients with severe osteolysis than in those with mild osteolysis (OR ¼0.55, P ¼ 0.043), appearing to protect against severe osteolysis. Thisprotective function was further demonstrated by a lower cumulativehazard of failure of the arthroplasty in carriers compared with non-carriers (P ¼ 0.019).

In a study on 481 patients, Wilkinson et al.37 investigated two SNPsin the promoter region of TNF, namely TNF-238 and TNF-308.Patients were included in the control and osteolysis groups accordingto the radiographic criteria of Harris and McGann38 and Harris andPenenberg.39 The frequency of the TNF-238A allele was higher in theosteolysis group than in the THA controls (P ¼ 0.05) and the localbackground population (P ¼ 0.001), and highest in subjects with bothfemoral and pelvic osteolysis. No difference in allele frequency wasobserved comparing subjects with unilateral and bilateral osteolysis(P . 0.05). The frequency of the TNF 308A allele was similar in THAsubjects (P . 0.05 for all comparisons), with no difference comparedwith the local background population (P . 0.05).

Common polymorphisms in the genes encoding the pro-inflammatoryIL-1 and IL-6 family cytokines were isolated in 612 patients at anaverage follow-up of 17 years after implantation of a THA.40

According to Harris and Mc-Gann38 and Harris and Penenberg39

radiographic criteria, 272 patients were included in the osteolysisgroup, and 340 in the control group of patients who showed no evi-dence of osteolysis on plain antero-posterior and lateral radiographs.The IL1RA þ 2018C (SNP rs419598) allele was underrepresented inthe osteolysis group compared with the control group (P ¼ 0.01 bychi-square test) with OR for osteolysis of 0.66 (95% CI: 0.48–0.91)(P ¼ 0.012). Compared with the TT genotype, the CC genotype wasassociated with increased IL-1Ra mRNA production in response toPBMC stimulation. On the other hand, no significant differences ingenotype or OR were observed between the THA groups for theremaining IL1 or IL6 SNPs (P . 0.05). The haplotype assessmentrevealed a statistically significant association between the rare IL-6haplothype-174G/-572G/-597A and osteolysis (2.4% in the osteolysisgroup vs. 0.8% in the control group; P ¼ 0.02). Kolundzic et al.41

assessed 41 Caucasian Croatian patients at an average follow-up of 15years after an uncemented THA. SNPs in genes encoding forpro-inflammatory transforming growth factor-b1 (TGF-b1), IL-6 andTNF-a were investigated. IL-6 is a pleiotropic cytokine involved in thebone metabolism regulation and immune response, and is upregulated



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in the periprosthetic interface loosening membrane. IL6 stimulatesosteoclast activity and bone resorption by an indirect mechanism, giventhe osteoblastic production of downstream effectors such as receptor/activator of nuclear factor-kb (RANKL). The TGF-b1 signal sequence(29T!C) transitions suggested a higher risk of developing aseptic loos-ening associated with T/T genotype than T/C or C/C genotypes. Theassociation between aseptic loosening and TNF-a promoter(-308G!A) transition observed in two patients did not allow to evalu-ate the potential effects of this SNP on prosthesis survival after THA.Although the IL-6 (-597) and (-572) SNPs did not appear to have a rel-evant effect on prosthesis survival, the 597 G!A and 572 G!C geno-types were associated with a markedly increased risk of asepticloosening.

Enzymes

Godoy-Santos et al.42 studied the frequency of polymorphism of theMMP-1 promoting gene among patients with early aseptic loosening ofTHA (test group) and among patients who underwent a successful unce-mented THA at a long-term follow-up (control group). The allele 2Ghad a frequency of 20.97% in the control group, and 83.33% in theaseptic loosening group. The genotype 2G/2G was observed in 66.66%of the test group patients, whereas the genotype 1G/1G was found in67.74% of the control group subjects. Individuals with the allele 2Gseemed to present greater propensity for loosening of the implant (P ,

0.001). Therefore, the polymorphism of the promoting gene for MMP-1may be a genetic marker for failure of osteointegration.

In another study on the same population of patients, Malik et al.43

investigated the role of four common polymorphisms of MMP1, namelyMMP1-1(RS5854), MMP1-2 (RS554499), MMP1-3(RS2397776) andMMP1-4. Degradation products of collagen and matrix components arechemotactic for monocytes, and may lead to the accumulation of macro-phages in the periprosthetic connective tissues.

The vitamin D system contributes to and directly modulates the osteo-protegerin (RANK/RANKL/OPG triad) system, which acts as theprimary control over osteoclast differentiation and activation44.Therefore, an alteration in the protein product of vitamin D receptor(VDR) may adversely affect the bone response to bacterial challenges inthe presence of prosthetic implants and particulate debris. The frequen-cies of the C allele (P ¼ 0.001; OR ¼ 3.27; 95% CI: 2.21–4.83) and C/C genotype (P ¼ 0.001) for the MMP1 SNP were associated withaseptic failure when compared with controls. The MMP2, MMP4,IL6-174 or VDR-L SNPs were not significantly related to aseptic

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loosening. The T allele (P ¼ 0.007; OR ¼ 1.76; 95% CI: 1.16–2.66)and T/T (P ¼ 0.028) genotype for VDR-T were significantly associatedwith deep infection compared with controls.

STREGA statement

Four16,26,35,40of the 11 studies firstly reported a genetic associationbetween a genetic polymorphism and aseptic loosening. All authorsprovided information on criteria and methods for selection of subsetsof participants. Genetic variants and variables likely associated withpopulation stratification (confounding by ethnic origin) were identifiedin four studies.26,35,40,42 Laboratory methods, including source andstorage of DNA, genotyping methods and platforms, were described inall the studies. No study specified neither described the nature andmagnitude of potential biases, whereas the item regarding the effects oftreatment could not be applied to any study. Statistical details andinformation on method of stratification of the examined populationwere provided in 10 of 11 studies. The item concerning the numbers ofindividuals in whom genotyping had been attempted and had been suc-cessful could not be applied in any study. Information on genotype,numbers of patients and phenotypes for each genotype category werereported in seven studies.22,33,40,42,43 Results were adjusted for multiplecomparisons in four studies.16,37,40,41 Results were summarized in fivestudies.22,26,40,41,42

Discussion

Pro-inflammatory mediators are implicated in aseptic periprostheticosteolysis.7 Individual susceptibility to aseptic loosening after THA isdetermined by patient-related factors other than demographic charac-teristics or co-morbidities. There is a genetic susceptibility componentin this condition, which is likely to include contributions by manypolymorphisms, and genes encoding for cytokines involved in thedevelopment of aseptic loosening.16,35,42 According to the reportedstudies, TNF-238A, IL6-174G allele and IL6 597 G!A and IL6 572G!C,TGF-b1 29T!T, MBL C!C 550 SNP, MBL codon 54 SNPG!G genotype; polymorphism of GNAS1 393T!T; the OPG-163A!A, the RANK þ 575 C!T SNP and T!T genotype, and 2G poly-morphism in the region of the gene promoting MMP-1 supposedly pre-dispose to aseptic loosening (Table 2). On the other hand, although theIL1RA-2018C allele is positively associated with IL-1Ra mRNA



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production and seems to be protective against osteolysis, its role inrelation to the IL1RA intron 2 VNTR is unclear.40

At present, the mechanisms of gene regulation and activation are stillunclear, and key elements are yet to be identified.45,46 Collectively,these data expand the current paradigm of osteolysis as a predomi-nantly inflammatory process. Nevertheless, two regulators of osteoclastdifferentiation and activation, RANKL47,48 and osteoprotegerin(OPG),49 have been identified. As part of the inflammation process,genes of cytokines, chemokines, growth factors, cell adhesion mol-ecules and some acute-phase proteins will be turned on, and frequentlyover-expressed. Part of this genetic regulation is controlled by thenuclear factor kappa B (NFkB), probably involved in the developmentof osteoclastogenesis.50,51 This abnormality occurs as a result of adefect either during osteoclast precursor cell differentiation or duringthe maturation of osteoclasts. It is possible that the osteoclastogenicactivity of several cytokines, such as TNF, IL-1 might be amelioratedby prevention of NF-kB, binding to their respective binding sites of thepromoter sequence. Even though the regulation of NF-kB activationcould represent a therapeutic strategy for reducing inflammatory tissuedamage, a complete and persistent blockage of NF-kB activation couldlead to immune deficiency, up to cell death.50

All these data come from single-centre studies. If these single-centrefindings are replicated in other centres and populations, new avenueswill open for pre-surgical genetic testing and also to investigateimmune response modulation in THA. Therefore, although the findingssupported the role of SNPs genotypes in aseptic loosening, furthermulti-centre prospective studies are necessary to confirm the generalvalidity of the findings reported. Additionally, other genes and geneticpolymorphisms could play a major role in the interaction of biomater-ials and the environment. Therefore, research in this field could lead tobetter understanding of mechanisms of aseptic loosening and improvethe results of THA.

Even though the STREGA assessment could not be fully applied,similar limitations concerning population stratification, description ofthe nature and magnitude of potential biases, and adjustment for mul-tiple comparisons have been found in almost all studies. Genetic associ-ation studies have introduced new data, but the likelihood of very smallindividual effects, including inadequate reporting of results, even fromwell-conducted studies, hampers assessment of a study’s strengths andweaknesses, and hence the integration of evidence.52,53 For the future,studies on associations between genetics and pathological aspectsshould keep to the STREGA statement. These recommendations do notaim to prescribe or dictate how a genetic association study should be

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designed, but to improve the transparency of their reporting, regardlessof choices made during design, conduct or analysis.

Conclusion

Several gene pathways and genes contribute to the genetic susceptibilityto aseptic loosening following THA. Association studies can be veryhelpful to clarify the linkage between complex disease phenotype andspecific genotype. Prospective studies with larger samples are requiredto improve the strength of the studies. Genetic studies in the field ofaseptic loosening will further enhance the understanding of prosthesisfailure, and may inform and direct pharmaceutical interventions. Thegoal would be to ascertain whether and how a given genotype can berelated to the phenotype ‘failure of THA’. This target would influenceclinical practice, allowing better planning and anticipating the need forearly intervention.

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Documents

Genetic susceptibility to aseptic loosening following total hip arthroplasty: a systematic review