A 5–8 Year Retrospective Follow-Up of the C

The Journal of Arthroplasty xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
The Journal of Arthroplasty
journal homepage: www.arthroplastyjournal.org
A 5–8 Year Retrospective Follow-Up of the C-Stem AMT Femoral
Component: Patient Reported Outcomes and Survivorship Analysis
James R. Berstock, MBChB, MRCS , Michael R. Whitehouse, PhD, MSc (Orth Eng), FRCS (Tr&Orth),
Danielle C. Piper, Stephen J. Eastaugh-Waring, MB, ChB, FRCS (Tr&Orth),
Ashley W. Blom, MD, PhD, FRCS (Tr&Orth)
University of Bristol Musculoskeletal Research Unit, AOC (Lower Level), Southmead Hospital, Westbury-on-Trym, Bristol, UK
a r t i c l e
i n f o
Article history:
Received 17 February 2014
Accepted 27 April 2014
Available online xxxx
Keywords:
hip
arthroplasty
replacement
bone cement
treatment outcome
survival rate
a b s t r a c t
We report midterm functional, radiographic and survivorship data for the cemented, triple taper C-stem AMT
femoral component from a consecutive cohort of 415 hip arthroplasties in 386 patients at a non-developer centre.
Follow-up ranges were from 60 to 99 months, with a mean of 76 months. 32 hips were lost to follow-up. The
median OHS was 40, median SF-12 mental component score (MCS) was 50, and median SF-12 physical
component score (PCS) was 39. At 99 months follow up, stem survivorship is 96.9% (95% conﬁdence interval
(CI) 82.5–99.5), and construct survivorship is 96.0% (95% CI 84.2–99.0). Adverse events such as calcar fracture,
greater trochanter fracture and dislocation were rare at b 1%. There have been no revisions for aseptic loosening.
© 2014 Elsevier Inc. All rights reserved.
The prevalence of osteoarthritis continues to increase and it will
represent the fourth leading cause of disability worldwide by 2020 [1].
The demand for total hip arthroplasty (THA) is predicted to rise by
174% between 2003 and 2030 [2]. In England and Wales, approximately equal numbers of cemented and cementless implants are used.
Registry data from the National Joint Registry for England, Wales and
Northern Ireland indicate higher early to midterm survivorship of
cemented implants compared to cementless implants [3]. For the
most often used combinations in each category, the cumulative
percentage probability of ﬁrst revision at 5 and 8 years were 1.39%
(95% CI 1.26–1.52) and 2.11% (95% CI 1.88–2.37) for cemented
implants (Exeter/Contemporary; Stryker, Newbury, UK) and 3.14%
(95% CI 2.94–3.34) and 5.89% (95% CI 5.11–6.79) respectively for
cementless implants (Corail/Pinnacle; DePuy Int., Leeds, UK). There
are potential confounding factors in survivorship analysis such as age,
activity, bearing size and bearing couple.
Aseptic loosening is the most common cause of failure in THA [1,4].
The taper slip cemented stem design has proven successful in nondeveloper centres [5,6] and it has been suggested that the original
polished, ﬂat-back Charnley femoral component may have acted as a
taper slip [7]. Bone resorption secondary to stress shielding leads to a
reduced survival of prostheses [8], particularly in the case of calcar bone
loss [9]. Calcar bone loss may lead to more complex revision surgery
being required. Periprosthetic bone mineral density (BMD) has been
consistently shown to decrease following cemented THA [10,11] with
the greatest loss (up to 20% of bone stock) in Gruen zone 7 [12].
Double taper designs such as the Exeter stem and the CPT stem
(Zimmer Ltd., Swindon, UK) taper in the craniocaudal and the
anteroposterior directions. In the triple taper design, a third taper is
added from lateral to medial with the intention of loading the
proximal femur, particularly the calcar to reduce resorption [13].
Despite this, reduction in BMD is still seen in triple taper designs but
with improved preservation of medial compared to lateral zones [14].
Comparison of a triple taper (C stem; DePuy Int., Leeds, UK) and a
double taper design (Exeter Universal) has shown no differences in
clinical scores, complications or subsidence [15]. The original C stem
design (DePuy Int., Leeds, UK) was modiﬁed to permit a more
controlled insertion by the addition of a shoulder to the stem, the
addition of a high offset option and a change in the taper to a 12/14
geometry (mini Articul/EZE) to create the C stem AMT, which is the
subject of this report.
The aim of this study was to ascertain the functional, radiological and
survivorship outcomes of the triple tapered cemented stem in a multisurgeon, single centre study at a minimum follow up of ﬁve years.
Materials and Methods
The Conﬂict of Interest statement associated with this article can be found at http://
dx.doi.org/10.1016/j.arth.2014.04.034.
Reprint requests: James R. Berstock, MBChB, MRCS, University of Bristol Musculoskeletal
Research Unit, AOC (Lower Level), Southmead Hospital, Westbury-on-Trym, Bristol,
BS10 5NB, UK.
Between March 2005 and April 2008, a consecutive series of 415
primary C-stem AMT hip arthroplasties in 386 patients were
performed under the care of three surgeons at our institution. The
http://dx.doi.org/10.1016/j.arth.2014.04.034
0883-5403/© 2014 Elsevier Inc. All rights reserved.
Please cite this article as: Berstock JR, et al, A 5–8 Year Retrospective Follow-Up of the C-Stem AMT Femoral Component: Patient Reported
Outcomes and Survivorship Analysis, J Arthroplasty (2014), http://dx.doi.org/10.1016/j.arth.2014.04.034
2
J.R. Berstock et al. / The Journal of Arthroplasty xxx (2014) xxx–xxx
C-stem AMT was implanted with an Ogee cup (DePuy Int., Leeds,
UK) in 52% of cases, and a Pinnacle cup (DePuy Int., Leeds, UK) in
44%. When all the patients had reached the 5-year anniversary of
surgery (range 60–99 months), functional questionnaires were sent
out by mail. We used the Oxford hip score [16] (OHS), Short Form-12
Health Survey [17] (SF-12), and the Self-Administered Patient
Satisfaction Scale [18]. In the event of non-response, reminders
were sent by post before the patients were contacted by telephone.
We also asked directly about subsequent hip surgery to
identify those who may have had revision surgery in other units.
Non-responders were contacted by telephone to complete questionnaires. Where medical problems prevented completion of
questionnaires, collateral history was taken from carers to ensure
no revision surgery had occurred.
The quality of the cement mantle on the immediate postoperative
radiograph was assessed by the method described by Barrack et al
[19]. The most recent follow-up radiograph was assessed for evidence
of implant loosening in the zones described by Gruen et al [12], for
evidence of progressive lucency, calcar resorption according to Engh
[20], and osteolysis.
The ﬂow of patients through this study is provided in Fig. 1.
Statistics
Statistical analysis was performed using SPSS version 19 software
(IBM Corporation, New York, USA). The distribution of data was
tested using the Kolmogrov–Smirnov statistic. Normally distributed
data are reported as a mean and standard deviation and tested with a
two-tailed t-test, whereas non-parametric data are reported as a
median and interquartile range, and tested with the independent
samples Kruskal–Wallis test. Implant survivorship was analysed
using Kaplan–Meier survivorship curves with 95% conﬁdence
intervals. Patients with surviving implants were censored at either
the time of most recent questionnaire, radiograph, or upon death.
Results
Surgical Characteristics
The mean age of the patients at time of surgery was 74 years
(range 39–92), with a 66% female preponderance. The indication for
surgery was osteoarthritis in 97% of cases, with 57% of cases
performed through a posterior approach and an omega approach
used for the remainder. Metal on polyethylene bearings were used in
84%, metal on metal in 9%, ceramic on ceramic in 7%, and ceramic on
polyethylene in b 1%. Intraoperatively there were two calcar cracks
(0.5%) both treated with cables, and two greater trochanter fractures
(0.5%), both reattached with tension band wires.
Follow-Up
As summarised in Fig. 1, 86 patients with 93 hips (22%) had died at
the time of ﬁnal follow-up. Medical problems prevented 17 patients
(4%) from completing forms. Seven patients (2%) declined
participation. In total, of the 298 hip eligible for ongoing inclusion,
266 questionnaires were returned, OHSs for 2 of these were only
partially complete, with 32 (11%) hips lost to follow-up. The
mean questionnaire follow-up period was 76 months, (range 60–
99 months). Recent pelvic radiographs were available for all
32 patients lost to questionnaire follow-up at a mean of 30 months
(range 3–73). There was no statistically signiﬁcant difference in age
at surgery between the group lost to follow-up and the remaining
patients (two-tailed t-test P = 0.17).
Fig. 1. Flow diagram of study follow up.
Please cite this article as: Berstock JR, et al, A 5–8 Year Retrospective Follow-Up of the C-Stem AMT Femoral Component: Patient Reported
Outcomes and Survivorship Analysis, J Arthroplasty (2014), http://dx.doi.org/10.1016/j.arth.2014.04.034
J.R. Berstock et al. / The Journal of Arthroplasty xxx (2014) xxx–xxx
3
Radiographic follow-up occurred in 401 patients (97%) at a mean
of 40 months post-operatively. Follow-up reporting is summarised
in Table 1.
Survivorship
There have been two stem revisions and four acetabular revisions.
Single component C-stem AMT Kaplan–Meier survivorship at
99 months is 96.9% (95% CI 82.5–99.5), see Fig. 2. Construct
survivorship is 96.0% (95% CI 84.2–99.0). The two stem revisions
were both for sepsis, occurring at 24 and 88 months post-operatively.
Both patients underwent successful two-stage revision of both
components. Two other patients underwent single component
acetabular revisions, one to a constrained liner at 38 months
following recurrent dislocations, and one for presumed aseptic
loosening at 88 months. In the case of acetabular loosening, tissue
specimens sent as a precaution at the time of revision were positive
for microorganisms and a course of antibiotics was commenced. The
stem was not revised, and the patient has been clinically free of
infection without antibiotics for 18 months. A total of four patients
(1%) experienced one or more dislocations. There was one immediate
postoperative dislocation, one late dislocation, and one patient
experienced two late dislocations. All were treated by closed
reduction, and have not required revision, except the case revised to
a constrained liner discussed above. Two other patients have had soft
tissue hip procedures, one for trochanteric bursitis and one for
abductor muscle reattachment. There has been one postoperative
greater trochanter fracture, and one Vancouver C periprosthetic
fracture, both treated with open reduction and internal ﬁxation
without stem revision.
Clinical results
At a minimum ﬁve-year follow-up, the median OHS was 40 (on a
scale of 0–48 with 48 representing best function [21]), with an
interquartile range (IQR) of 15. Postoperative OHSs are summarised in
Fig. 3. The median SF-12 MCS score was 50 (IQR 20) and PCS score was
39 (IQR 22). Median satisfaction score was 100 (IQR 19). There was no
signiﬁcant difference in functional outcome between the different
bearing types, independent samples Kruskal–Wallis test P = 0.41.
Radiographic results
Fig. 2. Single component C-stem AMT Kaplan–Meier survivorship curve.
was available for 401 (97%) of patients. Of these, 61% had some
radiographic lucency in one or more Gruen zones. The commonest
zones of lucency were zone 1 (55% of stems) and zone 7 (in 35% of
stems implanted). No polymethylmethacrylate fractures were identiﬁed. One osteolytic lesion has been noted and there are three cases
of progressive lucency, none of which has been revised. Calcar
resorption was evident in 5% of cases, as per Engh grading [18] which
is summarised in Table 2.
Discussion
Cemented total hip arthroplasty is one of the most cost effective
and successful surgical interventions available today. In our retrospective cohort, the C-stem AMT femoral component showed
excellent survivorship of 96.9% (95% CI 82.5–99.5), and construct
survivorship is 96.0% (95% CI 84.2–99.0) at 99 months follow-up.
These results are similar to previous survivorship reports of the
original C-stem femoral component [13]. The only other study of the
C-stem AMT that we are aware of reports encouraging results with no
loosening, subsidence, dislocation or revision at a mean 9 month
follow-up in a cohort of 34 patients [22]. Implant survivorship in this
series compares favourably with data from the National Joint Registry
for England, Wales and Northern Ireland [3], despite 9% of the
constructs having metal-on-metal bearing surfaces. Metal-on-metal
Of the 415 hips, immediate post-operative radiographs were
available for 398 (96%). The majority of those were not available predate the upgrade to the computerised Picture Archiving Communication Service (PACS). Cementation was graded on immediate postoperative radiographs as Barrack A in 60%, B in 38%, and C in 2%. A
follow-up radiograph taken at a mean of 39 months post-operatively
Table 1
Summary of Follow-Up Details.
Original Number of Patients in Study
Number of Hips at Start of Study
Number of Hips Lost Due
to Patients Death
Number of Hips Lost to Follow-Up
Number of Hips Revised for
Any Reason
Number of Hips Surviving at
End of Study
Number of Patients Examined
Number of Patients
Questionnaire/Telephone Review
Mean Follow-Up and Range
Revisions
Follow-Up Period
Kaplan–Meier Survival
386
415
93
32
4 (2 both component revisions and
2 single component acetabular revisions)
294
0
266
76 months (60–99)
4
99 months
96.0% (95% CI 84.2–99.0)
Fig. 3. Chart of postoperative Oxford hip scores.
Please cite this article as: Berstock JR, et al, A 5–8 Year Retrospective Follow-Up of the C-Stem AMT Femoral Component: Patient Reported
Outcomes and Survivorship Analysis, J Arthroplasty (2014), http://dx.doi.org/10.1016/j.arth.2014.04.034
4
J.R. Berstock et al. / The Journal of Arthroplasty xxx (2014) xxx–xxx
Table 2
Engh Grading of Calcar Resorption.
Grade 0
Grade 1
Grade 2
Grade 3
Grade 4
381 (95%)
9 (2%)
10 (2%)
1 (b1%)
0 (0%)
bearings have been shown to have high failure rates [23] and thus it is
somewhat fortuitous that none of the metal-on-metal bearings in this
series has required revision to date. The median OHS for the 39
patients with metal-on-metal bearings was 39, representing slightly
worse scores than the median for the whole cohort. This difference
was not statistically signiﬁcant.
Although our data also compare favourably with that of cementless stems reported in the National Joint Registry for England Wales
and Northern Ireland, we point out the older age at surgery in our
cohort, and that in our unit, this stem was preferentially utilised in
older patients which may impact the survivorship in patients not in
that demographic set with longer follow up.
Compared with the original C-stem, which has now achieved a 10A
Orthopaedic Device Evaluation Panel (ODEP) rating within the UK, the
C-stem AMT has undergone design changes meaning that the C-stem
AMT is a new entity according to ODEP, and is currently 7B rated. This
means that currently, only weak data exist to support a failure rate of
less than 7% at 7 years. To achieve the top 10A rating, strong evidence
of a failure rate of less than 10% at 10 years follow-up must exist.
Like many other collarless polished cemented stems, the C-stem
draws on the taper-slip philosophy, which depends on controlled
subsidence of the tapered stem due to natural cement creep [24,25]. The
importance of this slip philosophy was demonstrated by the relative
failure of a matte coated Exeter stem [26]. Double taper implants e.g. CPT
and Exeter are tapered in the anteroposterior plane and mediolateral
planes. These stems have excellent results out to 10 years at
independent centres [6,27]. Occasionally, excessive subsidence has
been associated with cement mantle fracture, and failure of the femoral
component in these double taper devices [6,28,29].
The C-stem was the ﬁrst taper-slip stem to employ a triple taper
design, however some previous matte ﬁnished stems were also triple
tapered. This additional third taper is from the wide lateral to the
narrow medial plane, making the stem slightly oval in cross-section.
Clinical evidence in support of the triple taper design in the form of
positive bone remodelling at the medial calcar has been observed
radiographically after C-stem implantation [13,30,14], however the
most recent randomised trial has not shown a signiﬁcant clinical
difference between double taper and triple taper designs [31]. A
further clinical trial has been set up to compare radiostereometric
subsidence following C-stem AMT and Exeter stem implantation (trial
identiﬁer: NCT00722982).
In this study, we observed good proximal bone preservation, with
95% of cases showing no calcar resorption (Engh grade 0) [20]. This is
an improvement on the results reported by Hook et al [6], suggesting
only 26% grade 0 calcar resorption with the Exeter stem at 10 year
minimum follow-up. However, our radiographic follow-up occurred
at a mean of only 39 months, so we may observe additional calcar
resorption with longer follow-up.
The rate of periprosthetic joint infection in our series was 3 out of
415 cases (0.7%) which is at the lower end of the range reported by
other authors [32–35]. There have been no revisions for aseptic
loosening at this stage, representing a potential cost saving if these
results are maintained.
In line with good research practise in arthroplasty cohort studies,
we report both clinical and survivorship outcomes [36]. The median
postoperative OHS of 40, is similar to that found in other series of
comparable length follow-up [37,38]. This equates to a good outcome
using the Harris hip score [38]. The ceiling effect of the OHS is evident
in Fig. 3, affecting 9% of patients which is acceptable, but reﬂects the
score’s inability to differentiate between the best performing patients.
Our study is limited by a lack of pre-operative scores and losses to
follow-up. Every effort was made to contact the 32 patients lost to
follow-up. We presume that these patients had either died or moved
to other regions, as we were unable to make contact. All the patients
lost to follow-up had undergone radiographic assessment during the
ﬁve years following surgery, with no evidence of failure or revision,
and for the purpose of Kaplan–Meier survivorship these patients are
censored at the time of radiographic assessment. The fate of all other
implants is known, including in the deceased. Our rate of loss to
follow-up is comparable with other similar sized cohorts beyond ﬁve
years [39–42].
The C-stem AMT demonstrates excellent implant survivorship at
5–8 year follow-up, as well as good midterm functional outcome.
Aseptic femoral component loosening has yet to be observed at
midterm review of this consecutive series of C-stem AMT hip
arthroplasties. Despite continued improvements in implant design
and cementing techniques, aseptic loosening remains the leading
cause of late failure and revision of cemented femoral stems
[3,25,43,44]. Hence there is a need for longer term surveillance of
this series.
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Please cite this article as: Berstock JR, et al, A 5–8 Year Retrospective Follow-Up of the C-Stem AMT Femoral Component: Patient Reported
Outcomes and Survivorship Analysis, J Arthroplasty (2014), http://dx.doi.org/10.1016/j.arth.2014.04.034

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