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Effect of Reduced Diameter Neck Stem on Incidence of Radiographic Cup Loosening and Revisions in Charnley Low-Frictional Torque Arthroplasty
The Journal of Arthroplasty Vol. 24 No. 1 2009
Effect of Reduced Diameter Neck Stem on Incidence of Radiographic Cup Loosening and Revisions in Charnley Low-Frictional Torque Arthroplasty B. Michael Wroblewski, FRCS, Paul D. Siney, BA, and Patricia A. Fleming
Abstract: Previous studies of the Charnley low-frictional torque arthroplasty have established an exponential correlation between the depth of cup penetration and the incidence of ultrahigh molecular weight polyethylene cemented cup migration. Impingement of the neck of the stem on the rim of the cup was considered to be the cause. We compared the incidence of radiographic loosening and revision of the cup in 2 groups of patients: those with 12.5-mm-diameter neck stem (972 hips) and those with 10-mm-diameter neck stem (261 hips) over a 20-year period, at comparable depths of cup penetration. The benefit of the 10-mm-diameter neck could be expressed as delaying radiographic loosening and revision of the cup by approximately 2 mm of cup penetration because loosening occurred at 2 mm penetration with a 12.5-mmdiameter neck but not until 4-mm penetration with a 10-mm-diameter neck. When a particular depth of cup penetration is reached will clearly depend on factors affecting wear. Key words: hip arthroplasty, reduced diameter neck, cup loosening and revision, impingement. © 2009 Elsevier Inc. All rights reserved.
The possibility of impingement of the neck of the stem on the rim of the cup and cup loosening was first considered by Charnley and Halley [1], “…..it is thought that more than 5 mm of wear might cause impingement of the neck of the prosthesis against the inner rim of the socket and cause loosening of the bone-cement bond in the acetabulum.” Clinical experience has shown that there was an exponential correlation between the depth of cup penetration
and the incidence of cup migration [2,3] and that impingement of the neck of the stem on the rim of the cup was the likely cause [4]. Theoretical model studies have shown that reducing the diameter of the neck from 12.5 to 10 mm “would allow a probable increase of 18° of the angular movement before impingement; equivalent to 3 mm of plastic before the angular movements were reduced to the 90° possible with the 12.5 mm neck” [4]. In a study of 74 explanted Charnley acetabular cups, acrylic casts and shadowgraph techniques were used to examine direction and depth of penetration, as well as rim impingement. Comparison of the results was made between the cases where 12.5-mm-diameter and 10-mm-diameter neck stem had been used. Impingement was modeled using logistic regression with penetration depth as the only dependent variable. A strong positive association was found between impingement and depth of cup penetration. Inclusion of the neck diameter as a
From the John Charnley Research Institute, Wrightington Hospital, Wigan, Lancashire, UK. Research supported by The Peter Kershaw and The John Charnley Trusts. No benefits or funds were received in support of the study. Submitted July 10, 2006; accepted January 21, 2008. Reprint requests: Paul D. Siney, BA, The John Charnley Research Institute, Wrightington Hospital, Hall Lane, Appley Bridge, Nr. Wigan WN6 9EP, UK. © 2009 Elsevier Inc. All rights reserved. 0883-5403/08/2401-0002$36.00/0 doi:10.1016/j.arth.2008.01.312
10
Effect of a Reduced Diameter Neck Wroblewski et al
variable improved the model specification further. The net effect of the 10-mm-diameter neck stem was to delay the probability of impingement by 2.1 mm of cup penetration. The conclusion was, “If impingement is a problem, then the reduced diameter neck would appear to be a solution in cutting rates of long-term cup loosening…” [5]. A number of factors affecting cup wear have been identified: cement ingress and head damage [6,7], quality of the ultrahigh molecular weight polyethylene (UHMWPE) [8] and patient activity level [9]. We set out to establish what, if any, reduction in the incidence of aseptic cup loosening and revisions would be achieved with the reduced 10-mm-diameter neck stem at comparable depths of cup penetration.
Materials and Methods The Charnley stem has been consistently manufactured in stainless steel—originally in EN58J then in 316L. The introduction of high nitrogen content stainless steel, together with the cold-forming process (1982), resulted in a very strong material ORTRON (DePuy International, Leeds, UK). Fatigue tests of the 10-mm-diameter neck in ORTRON carried out by the manufacturer “in-house” conformed to the then relevant British Standards Institution test DD91. The first stem was implanted on November 17, 1983. The details of the patients included in the study have been published [10]. They were all 50 years or younger at the time of surgery and were part of a long-term follow-up, the operations having been carried out between November 1962 and December 1990. All the operations had been carried out in the CharnleyHoworth clean air enclosure by 20 surgeons. Lateral transtrochanteric approach was used routinely. Cold curing acrylic cement, polymethalmethacrylate, was used for component fixation. During the period under study, a number of modifications had been introduced: long posterior wall cup (1972), flange added to the rim of the cup (1977). Sterilization of the UHMWPE during the period was 1962 to 1968 immersion in formaldehyde; 1969 to 1998, gamma irradiation in air; and gamma irradiation in nitrogen since [11]. Whether any of these factors affect the rate of cup wear is outside the scope of this study. Our interest is in the incidence of cup loosening and revisions once a certain depth of penetration has been reached. We excluded 2 groups: those wherein a threequarter neck length stem had been used because this brings the base of the neck of the stem closer to the
11
rim of the cup and those with an offset bore cup [12] because the cup has no chamfer. Wear measurements were made on serial and prerevision radiographs using the method previously described [13] and verified by establishing “an extremely good correlation between radiographic and real (shadowgraphs of acrylic casts) measurement of wear” [4]. All radiographic assessments and measurements were carried out by an experienced Senior Research Fellow (P.D.S.). Bone-cement interface of the cup was classified according to Hodgkinson et al [14], with loosening defined as demarcation of bonecement interface greater than 1 mm in all 3 zones or change in cup position on serial radiographs. Because with progressive cup penetration and cup migration wear measurements become less accurate [4], we placed all hips with a total penetration greater than 4 mm into a single group. Impingement of the standard 12.5-mm-diameter neck on the rim of the cup was shown likely to occur once the depth of penetration of 0.4 to 0.56 mm has been reached. The restriction of movement, however, will depend not only on the depth of penetration but also on the direction of the wear path. The more central the wear path with respect to the face of the cup, the more likely the restriction of movement and possibility of impingement. Conversely, the more peripheral the wear path the lesser the restriction of movement and, therefore, delayed impingement. These variables have been examined for both the standard 12.5-mm-diameter and the reduced, 10 mm, diameter neck [4]. Radiographic methods of wear measurement are based on the assumption that the wear path is primarily in the coronal plane. However, any digression from that path [4,15] or change in the cup position with loosening or migration will also have an effect on the measurement. Cup orientation and the joint reaction force are other factors that affect the direction of cup penetration. Because of Table 1. Patient's Details Neck Diameter No. of patients No. of hips (LFAs) Males Females Age, mean (range), y Follow-up, mean (range), y Weight, mean (range), kg Penetration rate, mean (range), mm/y Cup penetration, total ( range), mm
12.5 mm
10.0 mm
715 972 272 443 41.3 (15-51) 16.8 (1-36) 65.2 (30-108) 0.1 (0.01-0.71)
218 261 121 97 41.5 (17-50) 12.7 (1-20) 69.8 (32-105) 0.09 (0.01-0.55)
1.62 (0.1-8)
1.10 (0.1-7.5)
LFA indicates low friction torque arthroplasty.
12 The Journal of Arthroplasty Vol. 24 No. 1 January 2009 Table 2. The Correlation Between the Depth of Cup Penetration and Radiographic Cup Loosening (Includes All Cups Showing Full Demarcation Greater Than 1 mm or Migration)
Table 3. The Correlation Between the Depth of Cup Penetration and Revision for Aseptic Cup Loosening Wear (mm) 0
Wear (mm) 0
b1
b2
Standard 12.5-mm-diameter neck No. of cups 34 391 222 No. of cups 0 47 54 loose % of cups loose 0 12 24.3 10 mm reduced diameter neck No. of cups 11 161 30 No. of cups 0 13 3 loose % of cups loose 0 8.1 10
b3
b4
≥4
173 63
85 46
67 38
36.4
54.1
56.7
36 8
15 4
22.2
26.7
8 1
b1
Standard 12.5-mm-diameter neck No. of cups 34 391 No. of cups revised 0 21 % of cups revised 0 5.4 10 mm reduced diameter neck No. of cups 11 161 No. of cups revised 0 3 % of cups revised 0 1.9
b2
b3
b4
≥4
222 30 13.5
173 35 20.2
85 21 24.7
67 20 29.9
30 1 3.3
36 4 11.1
15 2 13.3
8 1 12.5
See Fig. 2.
12.5
See Fig. 1.
the number and complexity of confounding variables and their possible effect on the direction of wear path, we have taken the depth of cup penetration as a single parameter. We compared the incidence of radiographic cup loosening and cup revisions in the 2 groups of patients: with the standard 12.5-mm-diameter neck stem (972 hips), and the reduced 10-mm-diameter neck stem (261 hips) at comparable depths of cup penetration. The study was done over a 20-year period.
and Fig. 2. At zero penetration, none of the cups were radiologically loose, and none had been revised, irrespective of the neck diameter. With increasing depth of cup penetration, there was an increasing difference in the incidence of cup loosening. The incidence of revisions for aseptic cup loosening followed a comparable pattern. The benefit of the 10-mm-diameter neck of the Charnley stem could be expressed as delaying the incidence of both aseptic cup loosening and revision equivalent to 2 mm of cup penetration as compared with the results obtained in cases with 12.5-mm-diameter neck of stem.
Results
Discussion
The details of the 2 groups of patients including cup wear data are shown in Table 1. The correlation between the depth of cup penetration and the incidence of cup loosening is shown in Table 2 and Fig. 1 and between the depth of penetration and revision for aseptic cup loosening in Table 3
We set out to establish the effect of reducing the diameter of the neck of the Charnley stem from 12.5 to 10 mm on the incidence of radiographic cup loosening and revisions for aseptic cup loosening. It has been suggested that impingement of the neck of the stem on the rim of the cup is the likely cause [4,5]. The exponential correlation between the depth of cup penetration and the incidence of cup migration has been documented [2,3]. Reducing the diameter of the neck could not be expected to alter that pattern but place it at a lower level of incidence.
Fig. 1. The correlation between the depth of cup penetration and radiographic cup loosening (includes all cups showing full demarcation greater than 1 mm or migration). The reduction in the incidence of cup loosening and migration at 4 mm penetration or greater is because at the time of the study a number had been revised. They are included in Table 3 and Fig. 2.
Fig. 2. The correlation between the depth of cup penetration and revision for aseptic cup loosening.
Effect of a Reduced Diameter Neck Wroblewski et al
The difference with the 10-mm-diameter neck would be expected to be approximately equivalent to the extra depth of the cup penetration before the range of movements is reduced to, and becomes comparable with, that of the 12.5-mm-diameter neck. This, in turn, would depend on the factors affecting wear, cement ingress [6], damage to the metal head [7], quality of UHMWPE batch to batch variation or degradation in vivo [8], as well as patients' activity level [9]. Although these can be estimated, in general, it may not be possible to use the information as a predictive factor for an individual patient, except as a broad guideline. Our study has shown that in the range of 2- to 4-mm cup penetration, the benefit of the 10-mmdiameter neck is approximately equivalent to 2 mm extra of plastic wear. If the mean rate of cup penetration is accepted as 0.1 mm/y [10], then the “extra life” of the cup could be as high as 20 years. With the penetration rate of 0.67 mm/y [10], it would be no more than 3 years. Statistical analysis of the results was undertaken but did not prove of value. The number of cases in the 10-mm-diameter neck group showing high penetration or revision rate is too small. However, this should not negate the value of the results of the study. The objective was to establish whether reducing the diameter of the neck of from 12.5 to 10 mm was important in reducing the incidence of aseptic cup loosening and revision—which the results have shown to be so—and not whether that reduction was statistically significant. Furthermore the result, 2 mm, closely corresponds to the 2.1 mm suggested from examination of the explanted cups [5]. Whether the causes of loosening of the UHMWPE cups are mechanical or biological or a combination of the two is outside the remit of this study. The exponential correlation between the depth of cup penetration, and thus the volume of UHMWPE wear particles shed into the tissues, and cup loosening [2,3,10] would support the biological theory. However, a 10-fold increase in the cup penetration rate had no effect on the incidence of aseptic stem loosening [15]. The results of this study suggest that the cause is more likely to be mechanical than biological. The reasoning is as follows: Comparable depths of cup penetration shed comparable volumes of UHMWPE wear particles. At comparable volumes of UHMWPE wear particles shed into the tissues, the incidence of aseptic cup loosening and revisions is reduced when impingement is put off with the 10-mm-diameter neck. To reach the comparable incidence of aseptic cup loosening and revisions with the 10-mm-diameter
13
neck, comparable to the results with the 12.5-mmdiameter neck, the volume of the UHMWPE wear particles shed into the tissues is expected to be doubled from the equivalent of 2 to 4 mm cup penetration. Furthermore, calcification and even new bone formation—a healing process—has been documented in the presence of polytetrafluoroethylene wear particles [16]. When a particular depth of cup penetration and, therefore, the incidence of loosening are reached will depend on factors affecting wear. Because clinical results do not reflect the mechanical state of the arthroplasty [17], the regularity of follow-up and timing of revisions should be governed by the surgeon's awareness of the problem.
References 1. Charnley J, Halley D. Rate of wear in total hip replacement. Clin Orthop 1975;112:170. 2. Wroblewski BM. Charnley low-friction arthroplasty in patients under the age of 40 years. In: Sevastik Y, Goldie I, editors. The young patient with degenerative hip disease. Wiksell: Stockholm; 1985. p. 197. 3. Wroblewski BM, Siney PD. Charnley low-friction arthroplasty in the young patient. Clin Orthop 1992; 285:45. 4. Wroblewski BM. Direction and rate of wear in Charnley low-friction arthroplasty. J Bone Joint Surg Br 1985;67:757. 5. Hall RM, Siney PD, Unsworth A, et al. Prevalence of impingement in explanted Charnley acetabular components. J Orthop Sci 1998;3:204. 6. Isaac GH, Atkinson JR, Dowson D, et al. The role of cement in the long term performance and premature failure of Charnley low friction arthroplasties. Eng Med 1986;15:19. 7. Wroblewski BM, McCullagh PJ, Siney PD. Quality of the surface finish of the head of the femoral component and the wear rate of the socket in long-term results of the Charnley low-friction arthroplasty. Proc Inst Mech Eng [H] 1992;206:181. 8. Weightman B, Swanson SA, Isaac GH, et al. Polyethylene wear from retrieved acetabular cups. J Bone Joint Surg Br 1991;73:806. 9. Feller JA, Kay PR, Hodgkinson JP, et al. Activity and socket wear in the Charnley low-friction arthroplasty. J Arthroplasty 1994;9:341. 10. Wroblewski BM, Siney PD, Fleming PA. Charnley low-friction arthroplasty in patients under the age of 51 years. Follow-up to 33 years. J Bone Joint Surg Br 2002;84:540. 11. Issac GH, Dowson D, Wroblewski BM. An investigation into the origins of time dependent variation in penetration rates with Charnley acetabular cups—
14 The Journal of Arthroplasty Vol. 24 No. 1 January 2009 wear, creep or degradation? Proc Inst Mech Eng [H] 1996;210:209. 12. Izquierdo-Avino RJ, Wroblewski BM, Siney PD. Polyethylene wear in the Charnley offset-bore acetabular cup. J Bone Joint Surg Br 1996;78:82. 13. Griffith MJ, Seidenstein MK, Williams D, et al. Socket wear in Charnley low friction arthroplasty of the hip. Clin Orthop 1978;137:37. 14. Hodgkinson JP, Shelley P, Wroblewski BM. The correlation between roentgenographic appearances and operative findings at the bone-cement junction of
the socket in the Charnley low-friction arthroplasties. Clin Orthop 1988;228:105. 15. Wroblewski BM, Siney PD, Fleming PA. Wear of the cup in the Charnley LFA in the young patient. J Bone Joint Surg Br 2004;86:498. 16. Wroblewski BM, Raut VV, Siney PD, et al. An intrapelvic polytetrafluoroethylene granuloma. Orthopaedics International edition Vol 3 1995, No.5: 439. 17. Wroblewski BM, Siney PD, Fleming PA. Charnley low-frictional torque arthroplasty of the hip: 20-30 year results. J Bone Joint Surg Br 1999;81:427.
Effect of Reduced Diameter Neck Stem on Incidence of Radiographic Cup Loosening and Revisions in Charnley Low-Frictional Torque Arthroplasty B. Michael Wroblewski, FRCS, Paul D. Siney, BA, and Patricia A. Fleming
Abstract: Previous studies of the Charnley low-frictional torque arthroplasty have established an exponential correlation between the depth of cup penetration and the incidence of ultrahigh molecular weight polyethylene cemented cup migration. Impingement of the neck of the stem on the rim of the cup was considered to be the cause. We compared the incidence of radiographic loosening and revision of the cup in 2 groups of patients: those with 12.5-mm-diameter neck stem (972 hips) and those with 10-mm-diameter neck stem (261 hips) over a 20-year period, at comparable depths of cup penetration. The benefit of the 10-mm-diameter neck could be expressed as delaying radiographic loosening and revision of the cup by approximately 2 mm of cup penetration because loosening occurred at 2 mm penetration with a 12.5-mmdiameter neck but not until 4-mm penetration with a 10-mm-diameter neck. When a particular depth of cup penetration is reached will clearly depend on factors affecting wear. Key words: hip arthroplasty, reduced diameter neck, cup loosening and revision, impingement. © 2009 Elsevier Inc. All rights reserved.
The possibility of impingement of the neck of the stem on the rim of the cup and cup loosening was first considered by Charnley and Halley [1], “…..it is thought that more than 5 mm of wear might cause impingement of the neck of the prosthesis against the inner rim of the socket and cause loosening of the bone-cement bond in the acetabulum.” Clinical experience has shown that there was an exponential correlation between the depth of cup penetration
and the incidence of cup migration [2,3] and that impingement of the neck of the stem on the rim of the cup was the likely cause [4]. Theoretical model studies have shown that reducing the diameter of the neck from 12.5 to 10 mm “would allow a probable increase of 18° of the angular movement before impingement; equivalent to 3 mm of plastic before the angular movements were reduced to the 90° possible with the 12.5 mm neck” [4]. In a study of 74 explanted Charnley acetabular cups, acrylic casts and shadowgraph techniques were used to examine direction and depth of penetration, as well as rim impingement. Comparison of the results was made between the cases where 12.5-mm-diameter and 10-mm-diameter neck stem had been used. Impingement was modeled using logistic regression with penetration depth as the only dependent variable. A strong positive association was found between impingement and depth of cup penetration. Inclusion of the neck diameter as a
From the John Charnley Research Institute, Wrightington Hospital, Wigan, Lancashire, UK. Research supported by The Peter Kershaw and The John Charnley Trusts. No benefits or funds were received in support of the study. Submitted July 10, 2006; accepted January 21, 2008. Reprint requests: Paul D. Siney, BA, The John Charnley Research Institute, Wrightington Hospital, Hall Lane, Appley Bridge, Nr. Wigan WN6 9EP, UK. © 2009 Elsevier Inc. All rights reserved. 0883-5403/08/2401-0002$36.00/0 doi:10.1016/j.arth.2008.01.312
10
Effect of a Reduced Diameter Neck Wroblewski et al
variable improved the model specification further. The net effect of the 10-mm-diameter neck stem was to delay the probability of impingement by 2.1 mm of cup penetration. The conclusion was, “If impingement is a problem, then the reduced diameter neck would appear to be a solution in cutting rates of long-term cup loosening…” [5]. A number of factors affecting cup wear have been identified: cement ingress and head damage [6,7], quality of the ultrahigh molecular weight polyethylene (UHMWPE) [8] and patient activity level [9]. We set out to establish what, if any, reduction in the incidence of aseptic cup loosening and revisions would be achieved with the reduced 10-mm-diameter neck stem at comparable depths of cup penetration.
Materials and Methods The Charnley stem has been consistently manufactured in stainless steel—originally in EN58J then in 316L. The introduction of high nitrogen content stainless steel, together with the cold-forming process (1982), resulted in a very strong material ORTRON (DePuy International, Leeds, UK). Fatigue tests of the 10-mm-diameter neck in ORTRON carried out by the manufacturer “in-house” conformed to the then relevant British Standards Institution test DD91. The first stem was implanted on November 17, 1983. The details of the patients included in the study have been published [10]. They were all 50 years or younger at the time of surgery and were part of a long-term follow-up, the operations having been carried out between November 1962 and December 1990. All the operations had been carried out in the CharnleyHoworth clean air enclosure by 20 surgeons. Lateral transtrochanteric approach was used routinely. Cold curing acrylic cement, polymethalmethacrylate, was used for component fixation. During the period under study, a number of modifications had been introduced: long posterior wall cup (1972), flange added to the rim of the cup (1977). Sterilization of the UHMWPE during the period was 1962 to 1968 immersion in formaldehyde; 1969 to 1998, gamma irradiation in air; and gamma irradiation in nitrogen since [11]. Whether any of these factors affect the rate of cup wear is outside the scope of this study. Our interest is in the incidence of cup loosening and revisions once a certain depth of penetration has been reached. We excluded 2 groups: those wherein a threequarter neck length stem had been used because this brings the base of the neck of the stem closer to the
11
rim of the cup and those with an offset bore cup [12] because the cup has no chamfer. Wear measurements were made on serial and prerevision radiographs using the method previously described [13] and verified by establishing “an extremely good correlation between radiographic and real (shadowgraphs of acrylic casts) measurement of wear” [4]. All radiographic assessments and measurements were carried out by an experienced Senior Research Fellow (P.D.S.). Bone-cement interface of the cup was classified according to Hodgkinson et al [14], with loosening defined as demarcation of bonecement interface greater than 1 mm in all 3 zones or change in cup position on serial radiographs. Because with progressive cup penetration and cup migration wear measurements become less accurate [4], we placed all hips with a total penetration greater than 4 mm into a single group. Impingement of the standard 12.5-mm-diameter neck on the rim of the cup was shown likely to occur once the depth of penetration of 0.4 to 0.56 mm has been reached. The restriction of movement, however, will depend not only on the depth of penetration but also on the direction of the wear path. The more central the wear path with respect to the face of the cup, the more likely the restriction of movement and possibility of impingement. Conversely, the more peripheral the wear path the lesser the restriction of movement and, therefore, delayed impingement. These variables have been examined for both the standard 12.5-mm-diameter and the reduced, 10 mm, diameter neck [4]. Radiographic methods of wear measurement are based on the assumption that the wear path is primarily in the coronal plane. However, any digression from that path [4,15] or change in the cup position with loosening or migration will also have an effect on the measurement. Cup orientation and the joint reaction force are other factors that affect the direction of cup penetration. Because of Table 1. Patient's Details Neck Diameter No. of patients No. of hips (LFAs) Males Females Age, mean (range), y Follow-up, mean (range), y Weight, mean (range), kg Penetration rate, mean (range), mm/y Cup penetration, total ( range), mm
12.5 mm
10.0 mm
715 972 272 443 41.3 (15-51) 16.8 (1-36) 65.2 (30-108) 0.1 (0.01-0.71)
218 261 121 97 41.5 (17-50) 12.7 (1-20) 69.8 (32-105) 0.09 (0.01-0.55)
1.62 (0.1-8)
1.10 (0.1-7.5)
LFA indicates low friction torque arthroplasty.
12 The Journal of Arthroplasty Vol. 24 No. 1 January 2009 Table 2. The Correlation Between the Depth of Cup Penetration and Radiographic Cup Loosening (Includes All Cups Showing Full Demarcation Greater Than 1 mm or Migration)
Table 3. The Correlation Between the Depth of Cup Penetration and Revision for Aseptic Cup Loosening Wear (mm) 0
Wear (mm) 0
b1
b2
Standard 12.5-mm-diameter neck No. of cups 34 391 222 No. of cups 0 47 54 loose % of cups loose 0 12 24.3 10 mm reduced diameter neck No. of cups 11 161 30 No. of cups 0 13 3 loose % of cups loose 0 8.1 10
b3
b4
≥4
173 63
85 46
67 38
36.4
54.1
56.7
36 8
15 4
22.2
26.7
8 1
b1
Standard 12.5-mm-diameter neck No. of cups 34 391 No. of cups revised 0 21 % of cups revised 0 5.4 10 mm reduced diameter neck No. of cups 11 161 No. of cups revised 0 3 % of cups revised 0 1.9
b2
b3
b4
≥4
222 30 13.5
173 35 20.2
85 21 24.7
67 20 29.9
30 1 3.3
36 4 11.1
15 2 13.3
8 1 12.5
See Fig. 2.
12.5
See Fig. 1.
the number and complexity of confounding variables and their possible effect on the direction of wear path, we have taken the depth of cup penetration as a single parameter. We compared the incidence of radiographic cup loosening and cup revisions in the 2 groups of patients: with the standard 12.5-mm-diameter neck stem (972 hips), and the reduced 10-mm-diameter neck stem (261 hips) at comparable depths of cup penetration. The study was done over a 20-year period.
and Fig. 2. At zero penetration, none of the cups were radiologically loose, and none had been revised, irrespective of the neck diameter. With increasing depth of cup penetration, there was an increasing difference in the incidence of cup loosening. The incidence of revisions for aseptic cup loosening followed a comparable pattern. The benefit of the 10-mm-diameter neck of the Charnley stem could be expressed as delaying the incidence of both aseptic cup loosening and revision equivalent to 2 mm of cup penetration as compared with the results obtained in cases with 12.5-mm-diameter neck of stem.
Results
Discussion
The details of the 2 groups of patients including cup wear data are shown in Table 1. The correlation between the depth of cup penetration and the incidence of cup loosening is shown in Table 2 and Fig. 1 and between the depth of penetration and revision for aseptic cup loosening in Table 3
We set out to establish the effect of reducing the diameter of the neck of the Charnley stem from 12.5 to 10 mm on the incidence of radiographic cup loosening and revisions for aseptic cup loosening. It has been suggested that impingement of the neck of the stem on the rim of the cup is the likely cause [4,5]. The exponential correlation between the depth of cup penetration and the incidence of cup migration has been documented [2,3]. Reducing the diameter of the neck could not be expected to alter that pattern but place it at a lower level of incidence.
Fig. 1. The correlation between the depth of cup penetration and radiographic cup loosening (includes all cups showing full demarcation greater than 1 mm or migration). The reduction in the incidence of cup loosening and migration at 4 mm penetration or greater is because at the time of the study a number had been revised. They are included in Table 3 and Fig. 2.
Fig. 2. The correlation between the depth of cup penetration and revision for aseptic cup loosening.
Effect of a Reduced Diameter Neck Wroblewski et al
The difference with the 10-mm-diameter neck would be expected to be approximately equivalent to the extra depth of the cup penetration before the range of movements is reduced to, and becomes comparable with, that of the 12.5-mm-diameter neck. This, in turn, would depend on the factors affecting wear, cement ingress [6], damage to the metal head [7], quality of UHMWPE batch to batch variation or degradation in vivo [8], as well as patients' activity level [9]. Although these can be estimated, in general, it may not be possible to use the information as a predictive factor for an individual patient, except as a broad guideline. Our study has shown that in the range of 2- to 4-mm cup penetration, the benefit of the 10-mmdiameter neck is approximately equivalent to 2 mm extra of plastic wear. If the mean rate of cup penetration is accepted as 0.1 mm/y [10], then the “extra life” of the cup could be as high as 20 years. With the penetration rate of 0.67 mm/y [10], it would be no more than 3 years. Statistical analysis of the results was undertaken but did not prove of value. The number of cases in the 10-mm-diameter neck group showing high penetration or revision rate is too small. However, this should not negate the value of the results of the study. The objective was to establish whether reducing the diameter of the neck of from 12.5 to 10 mm was important in reducing the incidence of aseptic cup loosening and revision—which the results have shown to be so—and not whether that reduction was statistically significant. Furthermore the result, 2 mm, closely corresponds to the 2.1 mm suggested from examination of the explanted cups [5]. Whether the causes of loosening of the UHMWPE cups are mechanical or biological or a combination of the two is outside the remit of this study. The exponential correlation between the depth of cup penetration, and thus the volume of UHMWPE wear particles shed into the tissues, and cup loosening [2,3,10] would support the biological theory. However, a 10-fold increase in the cup penetration rate had no effect on the incidence of aseptic stem loosening [15]. The results of this study suggest that the cause is more likely to be mechanical than biological. The reasoning is as follows: Comparable depths of cup penetration shed comparable volumes of UHMWPE wear particles. At comparable volumes of UHMWPE wear particles shed into the tissues, the incidence of aseptic cup loosening and revisions is reduced when impingement is put off with the 10-mm-diameter neck. To reach the comparable incidence of aseptic cup loosening and revisions with the 10-mm-diameter
13
neck, comparable to the results with the 12.5-mmdiameter neck, the volume of the UHMWPE wear particles shed into the tissues is expected to be doubled from the equivalent of 2 to 4 mm cup penetration. Furthermore, calcification and even new bone formation—a healing process—has been documented in the presence of polytetrafluoroethylene wear particles [16]. When a particular depth of cup penetration and, therefore, the incidence of loosening are reached will depend on factors affecting wear. Because clinical results do not reflect the mechanical state of the arthroplasty [17], the regularity of follow-up and timing of revisions should be governed by the surgeon's awareness of the problem.
References 1. Charnley J, Halley D. Rate of wear in total hip replacement. Clin Orthop 1975;112:170. 2. Wroblewski BM. Charnley low-friction arthroplasty in patients under the age of 40 years. In: Sevastik Y, Goldie I, editors. The young patient with degenerative hip disease. Wiksell: Stockholm; 1985. p. 197. 3. Wroblewski BM, Siney PD. Charnley low-friction arthroplasty in the young patient. Clin Orthop 1992; 285:45. 4. Wroblewski BM. Direction and rate of wear in Charnley low-friction arthroplasty. J Bone Joint Surg Br 1985;67:757. 5. Hall RM, Siney PD, Unsworth A, et al. Prevalence of impingement in explanted Charnley acetabular components. J Orthop Sci 1998;3:204. 6. Isaac GH, Atkinson JR, Dowson D, et al. The role of cement in the long term performance and premature failure of Charnley low friction arthroplasties. Eng Med 1986;15:19. 7. Wroblewski BM, McCullagh PJ, Siney PD. Quality of the surface finish of the head of the femoral component and the wear rate of the socket in long-term results of the Charnley low-friction arthroplasty. Proc Inst Mech Eng [H] 1992;206:181. 8. Weightman B, Swanson SA, Isaac GH, et al. Polyethylene wear from retrieved acetabular cups. J Bone Joint Surg Br 1991;73:806. 9. Feller JA, Kay PR, Hodgkinson JP, et al. Activity and socket wear in the Charnley low-friction arthroplasty. J Arthroplasty 1994;9:341. 10. Wroblewski BM, Siney PD, Fleming PA. Charnley low-friction arthroplasty in patients under the age of 51 years. Follow-up to 33 years. J Bone Joint Surg Br 2002;84:540. 11. Issac GH, Dowson D, Wroblewski BM. An investigation into the origins of time dependent variation in penetration rates with Charnley acetabular cups—
14 The Journal of Arthroplasty Vol. 24 No. 1 January 2009 wear, creep or degradation? Proc Inst Mech Eng [H] 1996;210:209. 12. Izquierdo-Avino RJ, Wroblewski BM, Siney PD. Polyethylene wear in the Charnley offset-bore acetabular cup. J Bone Joint Surg Br 1996;78:82. 13. Griffith MJ, Seidenstein MK, Williams D, et al. Socket wear in Charnley low friction arthroplasty of the hip. Clin Orthop 1978;137:37. 14. Hodgkinson JP, Shelley P, Wroblewski BM. The correlation between roentgenographic appearances and operative findings at the bone-cement junction of
the socket in the Charnley low-friction arthroplasties. Clin Orthop 1988;228:105. 15. Wroblewski BM, Siney PD, Fleming PA. Wear of the cup in the Charnley LFA in the young patient. J Bone Joint Surg Br 2004;86:498. 16. Wroblewski BM, Raut VV, Siney PD, et al. An intrapelvic polytetrafluoroethylene granuloma. Orthopaedics International edition Vol 3 1995, No.5: 439. 17. Wroblewski BM, Siney PD, Fleming PA. Charnley low-frictional torque arthroplasty of the hip: 20-30 year results. J Bone Joint Surg Br 1999;81:427.