- Email: [email protected]
Patient-Specific Contact Stress Does Not Predict Polyethylene Wear Rate in a Specific Pressfit Cup
Accepted Manuscript Patient-specific contact stress does not predict polyethylene wear rate in a specific pressfit cup Georg Matziolis, Linda Krakow, Frank Layher, Klaus Sander, Joerg Bossert, Steffen Brodt PII:
S0883-5403(17)30643-5
DOI:
10.1016/j.arth.2017.07.027
Reference:
YARTH 56007
To appear in:
The Journal of Arthroplasty
Received Date: 15 May 2017 Revised Date:
10 July 2017
Accepted Date: 17 July 2017
Please cite this article as: Matziolis G, Krakow L, Layher F, Sander K, Bossert J, Brodt S, Patientspecific contact stress does not predict polyethylene wear rate in a specific pressfit cup, The Journal of Arthroplasty (2017), doi: 10.1016/j.arth.2017.07.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
in a specific pressfit cup
Georg Matziolis, 1Linda Krakow, 1Frank Layher, 1Klaus Sander, 2Joerg Bossert, 1
Steffen Brodt
SC
1
RI PT
Patient-specific contact stress does not predict polyethylene wear rate
Orthopaedic Department, University Hospital Jena, Campus Eisenberg, Germany
2
Otto Schott Institute of Materials Research, Friedrich Schiller University Jena, Germany
Corresponding author Prof. Dr. Georg Matziolis
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1
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Head of the Orthopaedic Department University Hospital Jena
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Campus Eisenberg
Klosterlausnitzer Straße 81
07607 Eisenberg, Germany Phone: +49 36691 8-1010 Fax: +49 36691 8-1807
E-mail: [email protected]
ACCEPTED MANUSCRIPT 1 2 3 4 5
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Patient-specific contact stress does not predict polyethylene wear rate
8
in a specific pressfit cup
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1
ACCEPTED MANUSCRIPT Abstract
14
Background: The most common reason for revision total hip arthroplasty remains polyethylene wear.
15
Development dysplasia of the hip and revision situations require a conscious compromise of implant
16
position. The surgeon should know about the consequence on wear via a possible change in hip
17
contact force. The objective of the present study was to investigate whether annual wear is
18
dependent on hip contact force.
19
Methods: Forty-five inserts (Duraloc,DePuy) that were explanted in our department were included.
20
Three-dimensional gravimetric determination of the wear was performed by fluid displacement.
21
Then, the hip contact force was determined using radiographs according to the Blumentritt model.
22
Results: No correlation was found between patient-specific factors and the annual wear. The hip
23
contact force estimated by the Blumentritt model also showed no correlation between hip contact
24
force and annual wear. Two single model parameters correlated significantly with wear: VRECAB as a
25
ratio of the lever length of the spinocrural and the pelvitrochanteric muscles and the angle Alpha as a
26
measure for the position of the centre of rotation in relation to the greater trochanter. The wear was
27
greater, the greater the ratio spinocrural/pelvitrochanteric lever arm (R=0.408,p=0.005) and the
28
greater the Alpha angle (more valgus the femoral neck) (R=0.377,p=0.011).
29
Conclusions: These results lead to the conclusion that neither patient specific factors nor the
30
estimated hip contact force have a major influence on annual wear in the case of Duraloc cups. Only
31
a coxa valga and a small femoral offset contribute in a limited amount to an increase of wear.
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Key Words
34
wear; total hip arthroplasty; hip contact force
35 36
Introduction
37
Despite the constant optimisation of materials and surfaces, wear remains the main indication for
38
revision total hip arthroplasty [1-4]. In the meantime, it is considered to have been sufficiently well
39
established that the wear after implantation of a total hip replacement is practically linearly
2
ACCEPTED MANUSCRIPT dependent on the number of gait cycles [5-7]. Corresponding to this, as early as the year 2000
41
Schmalzried showed that it is not the implantation time but the use of a total hip replacement that is
42
a predictor of wear [8]. The cause appears to be related to the contact area travelled by the articular
43
partners. Accordingly, small heads produce less wear than large heads, if the contact pressure is
44
below the failure pressure of the PE, which explains the excellent implantation times of the first
45
implants with 28mm heads [9-12].
46
For the first time, The showed a connection between hip contact force and wear 1 year, but no
47
longer 2 and 5 years after surgery in an RSA study [13]. Only one type of implant from one
48
manufacturer was used and the method enabled three-dimensional determination of the volumetric
49
wear. The main limitation of the study was the small case number of 31 patients. In contrast to this,
50
Kosak showed on 80 patients that the hip contact force at the time of an aseptic revision operation
51
correlates with wear [14]. However, in this study, the wear was only determined two-dimensionally
52
on radiographs. A problematic aspect would appear to be that both the determination of wear and
53
of the biomechanical parameters was carried out on the same radiograph, so that correlations might
54
have come about through a common influence of both measured variables (pelvic tilt, pelvic
55
rotation, distortion through position of the central beam, etc.). It appears to be inconclusive that,
56
independently of the implantation time (4 – 17 years), the total wear depended on the hip contact
57
force, although an implant should produce vastly more wear after 17 years than after 4 years. In
58
addition, the implants of 5 different manufacturers were included, although it is evident that wear is
59
largely dependent on the type of PE and its specific processing, here especially sterilisation [15-18].
60
Whereas the best possible reconstruction of the pre-arthritic anatomy makes sense in uncomplicated
61
primary implantation, revision situations and a hip anatomy altered by dysplasia or previous
62
operations require a conscious choice and/or compromise of stem geometry and cup positioning.
63
Therefore, for everyday clinical routine, it is relevant whether changes in hip geometry also affect
64
wear via a possible change in hip contact force. The surgeon should know about consequences on
65
wear if he can choose between different options to reconstruct the hip. As a result of the unclear
66
data situation, this clinically relevant question has not been satisfactorily unanswered to date.
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3
ACCEPTED MANUSCRIPT 67
Therefore, using a three-dimensional method that was independent of the biomechanical model to
68
investigate the polyethylene type of a single manufacturer with a sufficient number of cases, the
69
present study was designed to test the hypothesis that the annual wear of an implant at the time of
70
aseptic revision is dependent on the hip contact force.
RI PT
71 Material and Methods
73
In this retrospective study, all patients were included in whom an aseptic revision of a Duraloc cup
74
insert with a head size of 32mm was performed in our department between 2010 and 2014,
75
regardless of whether the entire cup and/or the stem were replaced at the same time. The study was
76
approved by the local Institutional Review Board.
77
From the patients' files, the age of the patients at the time of primary implantation, the period
78
between implantation and revision of the insert, sex, height and weight were determined. The latter
79
were used to calculate the body mass index.
80
The explanted inserts were placed on a balance (BP 121S Sartorius AG, Göttingen, Germany) in such
81
a way that the equatorial plane was roughly parallel to the table plane. Then a PTFE ball with a
82
diameter of 32mm was placed in the insert and the balance was set to zero. The cavity remaining
83
between the insert and the ball was filled with contrast medium with a density of 1.328 g/ml and the
84
weight was documented. The volume of wear was calculated from this. In a previous study, this
85
method was validated using computed tomography, and air pockets between the PTFE ball and the
86
insert that would falsify the result were ruled out.
87
The hip contact force was determined according to the Blumentritt model on the basis of digital
88
radiographs immediately prior to revision [19-22] (Fig. 1). In a previous study strong correlations
89
between calculated and by gait analysis estimated hip forces could be shown [20].
90
The images were standardised using reference balls and the respective landmarks were read into the
91
MediCAD software (Hectec). Together with the height and weight of the patients, the software
92
determined different geometric and biomechanical parameters of the hip, including the relative and
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ACCEPTED MANUSCRIPT 93
absolute hip contact force. The model has been validated [20,23,24] and is used in routine clinical
94
practice.
95 Results
97
The average follow-up of the 19 men and 26 women was 10.5 ± 4.3 (5 – 20) years. The patients'
98
height was 167 ± 9 (150 – 190) cm, their weight 79.4 ± 15.5 (53 – 130) kg. This resulted in an average
99
BMI at the time of revision of 28.3 ± 4.3 (18.8 – 42) kg/m2.
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The right side was operated on in 22 cases, the left side in 23 cases. The wear of the explanted
101
inserts was on average 0.9 ± 0.4 (0.2 – 1.9) cm3. This was equivalent to a wear of 95 ± 51 (17 – 246)
102
mm3 per year.
103
In 14 cases, a metal head was used, in 31 cases a ceramic head. The wear in the subgroup of patients
104
with a metal head was 117 ± 68 (34 – 247) mm3 per year and did not differ significantly from the
105
wear in the subgroup treated with a ceramic head (85 ± 38 (17 – 203) mm3 per year). Also in the
106
multivariate analysis, the material of the femoral head was not shown to be an independent factor of
107
wear. Therefore, in the following, wear was analysed independently of the head material with regard
108
to a connection to biomechanical parameters.
109
No connection was found between patient-specific factors such as sex, age and time of primary
110
implantation, height, weight or body mass index, and the annual wear.
111
In the Blumentritt model, there was again no connection between the absolute or relative hip
112
contact force and the annual wear. Just two model parameters correlated significantly with the wear:
113
VRECAB as a ratio of the lever length of the spinocrural and the pelvitrochanteric muscles and the
114
angle Alpha as a measure for the position of the centre of rotation in relation to the greater
115
trochanter (coxa valga corresponding to a positive Alpha angle). The wear was greater, the greater
116
VRECAB (the ratio spinocrural / pelvitrochanteric lever arm) (R = 0.408, p = 0.005) and the greater the
117
Alpha angle (more valgus the femoral neck) (R = 0.377, p = 0.011) (Fig. 2). This corresponds to a coxa
118
valga and a small femoral offset but the correlation coefficients were low pointing out a limited
119
influence of hip geometry on wear.
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ACCEPTED MANUSCRIPT 120 Discussion
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The main result of this study is that the hip contact force has no influence on the volumetric wear
123
after total hip arthroplasty with a Duraloc cup with a 32mm PE insert. Just two parameters of the des
124
biomechanical model used showed a statistical correlation with the wear. In view of the method, the
125
relevance of the statistical significance must be questioned, since the main hypothesis was refuted
126
and correlations found by chance must be examined in detail. Also in view of the rather moderate
127
correlation (R = 0.408 and 0.377), no conclusions should be drawn from these findings with regard to
128
practical application.
129
The surprising rejection of the main hypothesis may have different reasons. The biomechanical
130
model according to Blumentritt routinely applied in our department was used in this study [19].
131
Despite its validation, uncertainty remains regarding the correctness of the calculated hip contact
132
force. The correlation shown in other studies may be attributable to the common use of the same
133
source of information for determining the biomechanical data and the wear (plain radiographs).
134
Deviations in the imaging technique and positioning of the pelvis can lead to errors in the
135
determination of both wear and hip contact force, so that correlations are produced that in fact are
136
not present. This could be avoided in the present study by strict separation of the methods. Wear
137
was determined on the explanted inserts, hip contact force on the radiographs. The number of cases
138
was sufficient to show a clinically relevant connection, so that a weak relationship between contact
139
force and wear might have evaded detection. The individual activity level of the patients could not be
140
quantified post-hoc and therefore represents a confounder. However, as a surrogate parameter for
141
the level of activity, the patients' age did not show a correlation to annual wear, either alone or in a
142
multivariate analysis.
143
These present results lead to the conclusion that neither patient specific factors nor the estimated
144
hip contact force have a major influence on annual wear in the case of Duraloc cups. Only a coxa
145
valga and a small femoral offset contribute in a limited amount to an increase of wear.
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146 6
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Legends to Figures
148
Figure 1
Biomechanical model for estimation of hip contact force according to Blumentritt. The parameters correlating with wear are given in detail.
150
VRECAB = ratio between the length of two lever arms. First, the green line
151
representing the lever arm from center of rotation to force vector of
152
spinocrural muscles. Second the blue line representing the lever arm from
153
center of rotation to force vector of pevitrochanteric muscles.
154
Alpha = angle between center of rotations connecting line and line between
155
center of rotation and tip of greater trochanter (positive values = coxa valga).
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Figure 2
The ratio between the lever arm of the pevitrochanteric and the spinocrural
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muscles (VRECAB) correlates with the annual PE wear (A). The angle Alpha
158
between center of rotations connecting line and line between center of
159
rotation and tip of greater trochanter correlates with the annual PE wear
160
pointing out coxa valga producing more wear (B). Both correlations were
161
weak but significant.
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References
164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214
[1]
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[6]
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[5]
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[4]
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[3]
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[2]
Ulrich SD, Seyler TM, Bennett D, Delanois RE, Saleh KJ, Thongtrangan I, et al. Total hip arthroplasties: what are the reasons for revision? Int Orthop 2008;32:597–604. doi:10.1007/s00264-007-0364-3. Malchau H, Herberts P, Eisler T, Garellick G, Söderman P. The Swedish Total Hip Replacement Register. J Bone Joint Surg Am 2002;84-A Suppl 2:2–20. Bozic KJ, Kurtz SM, Lau E, Ong K, Vail TP, Berry DJ. The epidemiology of revision total hip arthroplasty in the United States. J Bone Joint Surg Am 2009;91:128–33. doi:10.2106/JBJS.H.00155. Garellick G, Kärrholm J, Lindahl H, Malchau H, Rogmark C, Rolfson O. The Swedish Hip Arthroplasty Register Annual Report 2014 2015. Penmetsa JR, Laz PJ, Petrella AJ, Rullkoetter PJ. Influence of polyethylene creep behavior on wear in total hip arthroplasty. J Orthop Res 2006;24:422–7. doi:10.1002/jor.20042. Turell M, Wang A, Bellare A. Quantification of the effect of cross-path motion on the wear rate of ultra-high molecular weight polyethylene. Wear 2003. Dirix Y, Becker A, Schmotzer H. UHMW-PE in a hip-simulator: comparison between linear and volumetric wear n.d. Schmalzried TP, Shepherd EF, Dorey FJ. Wear is a function of use, not time. Clinical Orthopaedics … 2000. Lachiewicz PF, Heckman DS, Soileau ES, Mangla J, Martell JM. Femoral head size and wear of highly cross-linked polyethylene at 5 to 8 years. Clin Orthop Relat Res 2009;467:3290–6. doi:10.1007/s11999-009-1038-9. Jasty M, Goetz DD, Bragdon CR, Lee KR, Hanson AE, Elder JR, et al. Wear of polyethylene acetabular components in total hip arthroplasty. An analysis of one hundred and twentyeight components retrieved at autopsy or revision operations. J Bone Joint Surg Am 1997;79:349–58. Livermore J, Ilstrup D, Morrey B. Effect of femoral head size on wear of the polyethylene acetabular component. J Bone Joint Surg Am 1990;72:518–28. Pedersen DR, Callaghan JJ, Johnston TL, Fetzer GB, Johnston RC. Comparison of femoral head penetration rates between cementless acetabular components with 22-mm and 28mm heads. J Arthroplasty 2001;16:111–5. The B, Hosman A, Kootstra J, Kralj-Iglic V, Flivik G, Verdonschot N, et al. Association between contact hip stress and RSA-measured wear rates in total hip arthroplasties of 31 patients. J Biomech 2008;41:100–5. doi:10.1016/j.jbiomech.2007.07.010. Košak R, Kralj-Iglic V, Iglič A, Daniel M. Polyethylene wear is related to patient-specific contact stress in THA. Clin Orthop Relat Res 2011;469:3415–22. doi:10.1007/s11999-0112078-5. Five-year Experience with Crossfire (R) Highly Cross-linked Polyethylene. 2005. Triclot P, Grosjean G, Masri El F, Courpied JP, Hamadouche M. A comparison of the penetration rate of two polyethylene acetabular liners of different levels of cross-linking. A prospective randomised trial. J Bone Joint Surg Br 2007;89:1439–45. doi:10.1302/0301620X.89B11.19543. Rajadhyaksha AD, Brotea C, Cheung Y, Kuhn C, Ramakrishnan R, Zelicof SB. Five-year comparative study of highly cross-linked (crossfire) and traditional polyethylene. J Arthroplasty 2009;24:161–7. doi:10.1016/j.arth.2007.09.015. Kyomoto M, Moro T, Takatori Y, Tanaka S, Ishihara K. Multidirectional wear and impact-towear tests of phospholipid-polymer-grafted and vitamin E-blended crosslinked polyethylene: a pilot study. Clin Orthop Relat Res 2015;473:942–51. doi:10.1007/s11999014-3995-x. Blumentritt S. The Relationship Between the Gait of Humans and the Hip Joint Structure in the Frontal Plane 1990;136:677–93. Layher F, Sander K, Babisch J, Blumentritt S, Schmalz T. The change of the hip joint load by
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[24]
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[21]
implantation of a total hip endoprosthesis of patients with dysplasia coxarthritis – a comparison between gait analysis and biomechanical modeling. Presented at the Orthopaedic Research Society 2007 in San Diego n.d. Asseln M, Eschweiler J, Damm P, Hares Al G, Bergmann G, Tingart M, et al. Evaluation of Biomechanical Models for the Planning of Total Hip Arthroplasty. Biomed Tech (Berl) 2013. doi:10.1515/bmt-2013-4116. Eschweiler J, Fieten L, Dell'Anna J, Kabir K, Gravius S, Tingart M, et al. Application and evaluation of biomechanical models and scores for the planning of total hip arthroplasty. Proc Inst Mech Eng H 2012;226:955–67. doi:10.1177/0954411912445261. Sander K, Layher F, Babisch J, Roth A. [Evaluation of results after total hip replacement using a minimally invasive and a conventional approach. Clinical scores and gait analysis]. Z Orthop Unfall 2011;149:191–9. doi:10.1055/s-0030-1250590. Babisch J, Layher F, Ritter B. Computer-assisted Biomechanically Based Two-dimensional Planning of Hip Surgery. Orthopädische Praxis 2001;37:29–38.
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ACCEPTED MANUSCRIPT Acknowledgements
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The authors wish to thank the “Deutsche Arthrose-Hilfe”, which funded the project with 10.000 €.
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pelvitrochanteric muscle force
spinocrural muscle force
COR Alpha
ACCEPTED MANUSCRIPT
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300
SC M AN U
200
y = 122.7x + 34.7 R = 0.408
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150
EP
100
50
0 0
0.1
0.2
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Wear [cmm per year]
250
0.3
0.4
0.5 VRECAB
0.6
0.7
0.8
0.9
1
ACCEPTED MANUSCRIPT
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300
SC
Wear [cmm per year]
250
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200
y = 1.86x + 80.8 R = 0.377
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100
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150
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50
0
-30
-20
-10
0
10 Alpha
20
30
40
S0883-5403(17)30643-5
DOI:
10.1016/j.arth.2017.07.027
Reference:
YARTH 56007
To appear in:
The Journal of Arthroplasty
Received Date: 15 May 2017 Revised Date:
10 July 2017
Accepted Date: 17 July 2017
Please cite this article as: Matziolis G, Krakow L, Layher F, Sander K, Bossert J, Brodt S, Patientspecific contact stress does not predict polyethylene wear rate in a specific pressfit cup, The Journal of Arthroplasty (2017), doi: 10.1016/j.arth.2017.07.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
in a specific pressfit cup
Georg Matziolis, 1Linda Krakow, 1Frank Layher, 1Klaus Sander, 2Joerg Bossert, 1
Steffen Brodt
SC
1
RI PT
Patient-specific contact stress does not predict polyethylene wear rate
Orthopaedic Department, University Hospital Jena, Campus Eisenberg, Germany
2
Otto Schott Institute of Materials Research, Friedrich Schiller University Jena, Germany
Corresponding author Prof. Dr. Georg Matziolis
TE D
M AN U
1
EP
Head of the Orthopaedic Department University Hospital Jena
AC C
Campus Eisenberg
Klosterlausnitzer Straße 81
07607 Eisenberg, Germany Phone: +49 36691 8-1010 Fax: +49 36691 8-1807
E-mail: [email protected]
ACCEPTED MANUSCRIPT 1 2 3 4 5
RI PT
6 7
Patient-specific contact stress does not predict polyethylene wear rate
8
in a specific pressfit cup
9 10
AC C
EP
TE D
M AN U
SC
11 12
1
ACCEPTED MANUSCRIPT Abstract
14
Background: The most common reason for revision total hip arthroplasty remains polyethylene wear.
15
Development dysplasia of the hip and revision situations require a conscious compromise of implant
16
position. The surgeon should know about the consequence on wear via a possible change in hip
17
contact force. The objective of the present study was to investigate whether annual wear is
18
dependent on hip contact force.
19
Methods: Forty-five inserts (Duraloc,DePuy) that were explanted in our department were included.
20
Three-dimensional gravimetric determination of the wear was performed by fluid displacement.
21
Then, the hip contact force was determined using radiographs according to the Blumentritt model.
22
Results: No correlation was found between patient-specific factors and the annual wear. The hip
23
contact force estimated by the Blumentritt model also showed no correlation between hip contact
24
force and annual wear. Two single model parameters correlated significantly with wear: VRECAB as a
25
ratio of the lever length of the spinocrural and the pelvitrochanteric muscles and the angle Alpha as a
26
measure for the position of the centre of rotation in relation to the greater trochanter. The wear was
27
greater, the greater the ratio spinocrural/pelvitrochanteric lever arm (R=0.408,p=0.005) and the
28
greater the Alpha angle (more valgus the femoral neck) (R=0.377,p=0.011).
29
Conclusions: These results lead to the conclusion that neither patient specific factors nor the
30
estimated hip contact force have a major influence on annual wear in the case of Duraloc cups. Only
31
a coxa valga and a small femoral offset contribute in a limited amount to an increase of wear.
SC
M AN U
TE D
EP
AC C
32
RI PT
13
33
Key Words
34
wear; total hip arthroplasty; hip contact force
35 36
Introduction
37
Despite the constant optimisation of materials and surfaces, wear remains the main indication for
38
revision total hip arthroplasty [1-4]. In the meantime, it is considered to have been sufficiently well
39
established that the wear after implantation of a total hip replacement is practically linearly
2
ACCEPTED MANUSCRIPT dependent on the number of gait cycles [5-7]. Corresponding to this, as early as the year 2000
41
Schmalzried showed that it is not the implantation time but the use of a total hip replacement that is
42
a predictor of wear [8]. The cause appears to be related to the contact area travelled by the articular
43
partners. Accordingly, small heads produce less wear than large heads, if the contact pressure is
44
below the failure pressure of the PE, which explains the excellent implantation times of the first
45
implants with 28mm heads [9-12].
46
For the first time, The showed a connection between hip contact force and wear 1 year, but no
47
longer 2 and 5 years after surgery in an RSA study [13]. Only one type of implant from one
48
manufacturer was used and the method enabled three-dimensional determination of the volumetric
49
wear. The main limitation of the study was the small case number of 31 patients. In contrast to this,
50
Kosak showed on 80 patients that the hip contact force at the time of an aseptic revision operation
51
correlates with wear [14]. However, in this study, the wear was only determined two-dimensionally
52
on radiographs. A problematic aspect would appear to be that both the determination of wear and
53
of the biomechanical parameters was carried out on the same radiograph, so that correlations might
54
have come about through a common influence of both measured variables (pelvic tilt, pelvic
55
rotation, distortion through position of the central beam, etc.). It appears to be inconclusive that,
56
independently of the implantation time (4 – 17 years), the total wear depended on the hip contact
57
force, although an implant should produce vastly more wear after 17 years than after 4 years. In
58
addition, the implants of 5 different manufacturers were included, although it is evident that wear is
59
largely dependent on the type of PE and its specific processing, here especially sterilisation [15-18].
60
Whereas the best possible reconstruction of the pre-arthritic anatomy makes sense in uncomplicated
61
primary implantation, revision situations and a hip anatomy altered by dysplasia or previous
62
operations require a conscious choice and/or compromise of stem geometry and cup positioning.
63
Therefore, for everyday clinical routine, it is relevant whether changes in hip geometry also affect
64
wear via a possible change in hip contact force. The surgeon should know about consequences on
65
wear if he can choose between different options to reconstruct the hip. As a result of the unclear
66
data situation, this clinically relevant question has not been satisfactorily unanswered to date.
AC C
EP
TE D
M AN U
SC
RI PT
40
3
ACCEPTED MANUSCRIPT 67
Therefore, using a three-dimensional method that was independent of the biomechanical model to
68
investigate the polyethylene type of a single manufacturer with a sufficient number of cases, the
69
present study was designed to test the hypothesis that the annual wear of an implant at the time of
70
aseptic revision is dependent on the hip contact force.
RI PT
71 Material and Methods
73
In this retrospective study, all patients were included in whom an aseptic revision of a Duraloc cup
74
insert with a head size of 32mm was performed in our department between 2010 and 2014,
75
regardless of whether the entire cup and/or the stem were replaced at the same time. The study was
76
approved by the local Institutional Review Board.
77
From the patients' files, the age of the patients at the time of primary implantation, the period
78
between implantation and revision of the insert, sex, height and weight were determined. The latter
79
were used to calculate the body mass index.
80
The explanted inserts were placed on a balance (BP 121S Sartorius AG, Göttingen, Germany) in such
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a way that the equatorial plane was roughly parallel to the table plane. Then a PTFE ball with a
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diameter of 32mm was placed in the insert and the balance was set to zero. The cavity remaining
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between the insert and the ball was filled with contrast medium with a density of 1.328 g/ml and the
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weight was documented. The volume of wear was calculated from this. In a previous study, this
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method was validated using computed tomography, and air pockets between the PTFE ball and the
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insert that would falsify the result were ruled out.
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The hip contact force was determined according to the Blumentritt model on the basis of digital
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radiographs immediately prior to revision [19-22] (Fig. 1). In a previous study strong correlations
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between calculated and by gait analysis estimated hip forces could be shown [20].
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The images were standardised using reference balls and the respective landmarks were read into the
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MediCAD software (Hectec). Together with the height and weight of the patients, the software
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determined different geometric and biomechanical parameters of the hip, including the relative and
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absolute hip contact force. The model has been validated [20,23,24] and is used in routine clinical
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practice.
95 Results
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The average follow-up of the 19 men and 26 women was 10.5 ± 4.3 (5 – 20) years. The patients'
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height was 167 ± 9 (150 – 190) cm, their weight 79.4 ± 15.5 (53 – 130) kg. This resulted in an average
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BMI at the time of revision of 28.3 ± 4.3 (18.8 – 42) kg/m2.
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The right side was operated on in 22 cases, the left side in 23 cases. The wear of the explanted
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inserts was on average 0.9 ± 0.4 (0.2 – 1.9) cm3. This was equivalent to a wear of 95 ± 51 (17 – 246)
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mm3 per year.
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In 14 cases, a metal head was used, in 31 cases a ceramic head. The wear in the subgroup of patients
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with a metal head was 117 ± 68 (34 – 247) mm3 per year and did not differ significantly from the
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wear in the subgroup treated with a ceramic head (85 ± 38 (17 – 203) mm3 per year). Also in the
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multivariate analysis, the material of the femoral head was not shown to be an independent factor of
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wear. Therefore, in the following, wear was analysed independently of the head material with regard
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to a connection to biomechanical parameters.
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No connection was found between patient-specific factors such as sex, age and time of primary
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implantation, height, weight or body mass index, and the annual wear.
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In the Blumentritt model, there was again no connection between the absolute or relative hip
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contact force and the annual wear. Just two model parameters correlated significantly with the wear:
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VRECAB as a ratio of the lever length of the spinocrural and the pelvitrochanteric muscles and the
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angle Alpha as a measure for the position of the centre of rotation in relation to the greater
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trochanter (coxa valga corresponding to a positive Alpha angle). The wear was greater, the greater
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VRECAB (the ratio spinocrural / pelvitrochanteric lever arm) (R = 0.408, p = 0.005) and the greater the
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Alpha angle (more valgus the femoral neck) (R = 0.377, p = 0.011) (Fig. 2). This corresponds to a coxa
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valga and a small femoral offset but the correlation coefficients were low pointing out a limited
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influence of hip geometry on wear.
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The main result of this study is that the hip contact force has no influence on the volumetric wear
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after total hip arthroplasty with a Duraloc cup with a 32mm PE insert. Just two parameters of the des
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biomechanical model used showed a statistical correlation with the wear. In view of the method, the
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relevance of the statistical significance must be questioned, since the main hypothesis was refuted
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and correlations found by chance must be examined in detail. Also in view of the rather moderate
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correlation (R = 0.408 and 0.377), no conclusions should be drawn from these findings with regard to
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practical application.
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The surprising rejection of the main hypothesis may have different reasons. The biomechanical
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model according to Blumentritt routinely applied in our department was used in this study [19].
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Despite its validation, uncertainty remains regarding the correctness of the calculated hip contact
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force. The correlation shown in other studies may be attributable to the common use of the same
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source of information for determining the biomechanical data and the wear (plain radiographs).
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Deviations in the imaging technique and positioning of the pelvis can lead to errors in the
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determination of both wear and hip contact force, so that correlations are produced that in fact are
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not present. This could be avoided in the present study by strict separation of the methods. Wear
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was determined on the explanted inserts, hip contact force on the radiographs. The number of cases
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was sufficient to show a clinically relevant connection, so that a weak relationship between contact
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force and wear might have evaded detection. The individual activity level of the patients could not be
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quantified post-hoc and therefore represents a confounder. However, as a surrogate parameter for
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the level of activity, the patients' age did not show a correlation to annual wear, either alone or in a
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multivariate analysis.
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These present results lead to the conclusion that neither patient specific factors nor the estimated
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hip contact force have a major influence on annual wear in the case of Duraloc cups. Only a coxa
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valga and a small femoral offset contribute in a limited amount to an increase of wear.
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Legends to Figures
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Figure 1
Biomechanical model for estimation of hip contact force according to Blumentritt. The parameters correlating with wear are given in detail.
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VRECAB = ratio between the length of two lever arms. First, the green line
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representing the lever arm from center of rotation to force vector of
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spinocrural muscles. Second the blue line representing the lever arm from
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center of rotation to force vector of pevitrochanteric muscles.
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Alpha = angle between center of rotations connecting line and line between
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center of rotation and tip of greater trochanter (positive values = coxa valga).
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Figure 2
The ratio between the lever arm of the pevitrochanteric and the spinocrural
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muscles (VRECAB) correlates with the annual PE wear (A). The angle Alpha
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between center of rotations connecting line and line between center of
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rotation and tip of greater trochanter correlates with the annual PE wear
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pointing out coxa valga producing more wear (B). Both correlations were
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weak but significant.
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References
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The authors wish to thank the “Deutsche Arthrose-Hilfe”, which funded the project with 10.000 €.
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pelvitrochanteric muscle force
spinocrural muscle force
COR Alpha
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y = 122.7x + 34.7 R = 0.408
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