Unicompartmental Knee Arthroplasty Relieves Pain and Improves Function More Than Total Knee Arthroplasty
This study compared outcomes as assessed by 12-item Short-Form Health Survey (SF-12) and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) between patients who underwent unicompartmental (UKA) and patients who underwent total knee (TKA) arthroplasty. We prospectively collected preoperative demographic and SF-12 and WOMAC data on 128 TKAs and 70 UKAs. Postoperatively, SF-12 and WOMAC outcomes were recorded during annual follow-up visits. At baseline, patients who underwent UKA had a higher Charlson Comorbidity Index than patients who underwent TKA; otherwise, preoperative characteristics were similar. At a mean follow-up of 3.0 years for UKA and 2.9 years for TKA, patients who underwent UKA reported higher SF-12 physical component and mental component scores and WOMAC pain/stiffness/physical function scores (confirmed with multivariate analysis). Furthermore, patients who underwent UKA had significantly larger improvements in both SF-12 outcomes and WOMAC pain and physical function scores from baseline than did patients who underwent TKA.
Unicompartmental Knee Arthroplasty Relieves
Pain and Improves Function More Than Total
Manish S. Noticewala, BA, Jeffrey A. Geller, MD, Jonathan H. Lee, MD,
and William Macaulay, MD
Abstract: This study compared outcomes as assessed by 12-item Short-Form Health Survey (SF-
12) and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) between
patients who underwent unicompartmental (UKA) and patients who underwent total knee
(TKA) arthroplasty. We prospectively collected preoperative demographic and SF-12 and
WOMAC data on 128 TKAs and 70 UKAs. Postoperatively, SF-12 and WOMAC outcomes were
recorded during annual follow-up visits. At baseline, patients who underwent UKA had a higher
Charlson Comorbidity Index than patients who underwent TKA; otherwise, preoperative
characteristics were similar. At a mean follow-up of 3.0 years for UKA and 2.9 years for TKA,
patients who underwent UKA reported higher SF-12 physical component and mental component
scores and WOMAC pain/stiffness/physical function scores (confirmed with multivariate
analysis). Furthermore, patients who underwent UKA had significantly larger improvements in
both SF-12 outcomes and WOMAC pain and physical function scores from baseline than did
patients who underwent TKA. Keywords: unicompartmental knee arthroplasty, pain and
function, total knee arthroplasty.
© 2012 Elsevier Inc. All rights reserved.
The clinical outcome of unicompartmental knee arthro-
plasty (UKA) is a topic of renewed debate. Although
initial studies reported poor outcomes in patients who
underwent UKA, reﬁnements have been made in the
interim [1,2]. As a result, UKA has many potential
advantages over total knee arthroplasty (TKA). The
beneﬁts of UKA include a smaller incision, preservation
of more native tissue, decreased blood loss, better pro-
prioception, less perioperative morbidity, reduced pain,
greater range of motion (ROM), shorter hospitalization
stays, and a faster rehabilitation course [3-8]. Although
registry studies have reported signiﬁcantly higher revi-
sion rates after primary UKA as compared with TKA,
multiple studies including randomized controlled trials
have shown implant survivorship to be comparable
between UKA and TKA [5,8-10].
Studies have separately reported excellent UKA and
TKA functional outcomes over short-term, midterm,
and long-term follow-up periods [5,10-24]. However,
there is scant literature directly comparing UKA and
TKA functional outcomes for similar follow-up time
points. The objective of this study was to compare phy-
sical function, mental well-being, pain, and stiffness
outcomes between patients who underwent UKA and
those who underwent TKA.
All data used in this investigation were collected
through the Center for Hip and Knee Replacement
Joint Registry, a prospective database approved by our
institutional review board. Between the inception of
this registry and March 2009, 410 patients undergoing
447 primary TKAs have provided informed consent to
be included in this database. Likewise, 89 patients un-
dergoing 93 primary UKAs have provided informed
consent over the same period. For the purposes of this
study, patients who ultimately underwent reoperation
were included in the study cohort and classiﬁed ac-
cording to their index procedure. Bilateral simulta-
neous procedures were excluded; however, bilateral
procedures staged with greater than a 3-month interval
From the Center for Hip and Knee Replacement, New York–Presbyterian
Hospital at Columbia University, New York, New York.
Submitted August 14, 2011; accepted March 26, 2012.
The Conflict of Interest statement associated with this article can be
found at doi:10.1016/j.arth.2012.03.044.
Reprint requests: William Macaulay, MD, Department of Ortho-
paedic Surgery, Columbia University Medical Center, PH 11th Floor,
Rm 1146, 622 W 168th St, New York, NY 10032.
© 2012 Elsevier Inc. All rights reserved.
The Journal of Arthroplasty Vol. 00 No. 0 2012
Preoperatively, patient demographic and clinical data
were recorded: age, gender, body mass index (BMI),
comorbidities, procedure, and diagnosis. For analysis in
this study, patients' comorbidities were classiﬁed ac-
cording to the Charlson Comorbidity Index, a validated
method for estimating the risk of mortality from comor-
bid disease . Before their surgery, the patients com-
pleted the 12-item Short-Form Health Survey (SF-12)
and Western Ontario and McMaster Universities Oste-
oarthritis Index (WOMAC) questionnaires. Both ques-
tionnaires were mailed to patients' homes 4 weeks
before surgery dates and were considered valid for
inclusion in this study only if postmark dates on return
envelopes were before surgery dates. The SF-12 form is
a validated health status questionnaire concerning
physical function and mental health . A physical
component score and a mental component score are
derived from this questionnaire. The WOMAC survey is
another validated questionnaire that consists of 3 do-
mains: joint-speciﬁc pain (5 items), stiffness (2 items),
and physical function (17 items) . Each item is
scored using a 5-point Likert scale, and each domain
score is transformed to range from 0 to 100. A higher
score on SF-12 or WOMAC reﬂects a better condition
(0-100, worst to best).
Patients selected to undergo UKA had anteromedial
knee osteoarthritis (OA) consistent with stages I to III of
the Ahlback radiologic classiﬁcation, intact cruciate
ligaments, ﬂexion deformity of 10° or less, and passively
correctable varus deformity of 10° or less . The
diagnosis of anteromedial OA was based on radiologic
and physical examination ﬁndings and conﬁrmed
intraoperatively. Advanced age, obesity, mild patellofe-
moral OA, and chondrocalcinosis were not considered
contraindications; inﬂammatory arthritis was a contra-
indication. All patients who did not fulﬁll the criteria for
UKA underwent TKA. These patients had either
bicompartmental or tricompartmental advanced degen-
erative disease of the knee.
Postoperatively, at each follow-up visit, the patients
completed SF-12 and WOMAC questionnaires. Only
patients with minimum 2-year postoperative question-
naire data were included. A total of 128 TKA and 70
UKA procedures were identiﬁed as having complete
records for analysis. Data collection and maintenance
were performed using Patient Analysis and Tracking
System (PATS 4.0) software (Axis Clinical Software,
A 9-point difference has been suggested by other as
the minimal clinically important difference for the
WOMAC scale . For the purposes of a power analy-
sis, to have an 80% chance of detecting as signiﬁcant (at
the 2-sided 5% level), a 9-point difference in mean
WOMAC subscales, with a standard deviation of 20
units, 60 patients who underwent UKA and 119 pa-
tients who underwent TKA were needed. When com-
paring continuous variables, a 2-tailed ttest or
Wilcoxon rank sum test was performed. When com-
paring categorical variables, a χ
test was performed. A
multivariate linear regression analysis was used to
determine whether an operative procedure (UKA or
TKA) impacted postoperative SF-12 and WOMAC
scores while controlling for preoperative differences.
When evaluating crude and adjusted postoperative
scores, we deﬁned the minimal clinically important dif-
ference for the SF-12 as 3 units and for the WOMAC
index as 9 units, as suggested by others [29,30].
The data from 128 TKAs and 70 UKAs were analyzed.
Table 1 compares baseline parameters between the 2
cohorts. The 2 groups were not signiﬁcantly different
Table 1. Univariate Analysis of Baseline Parameters Between UKA and TKA Cohorts
Variable Patients Who Underwent UKA (n = 70) Patients Who Underwent TKA (n = 128) P
Age (y) 66.84 (46-88, 9.41) 67.27 (40-87, 9.43) .763
Female 46 (65.7) 94 (73.4)
Male 24 (34.3) 34 (36.6)
) 29.84 (21.46-43.46, 5.18) 31.19 (18.81-48.80, 6.23) .139
Charlson Comorbidity Index 2.6 (0-7, 1.59) .86 (0-8, 1.36) b.001
OA 66 (94.3) 125 (97.7)
Osteonecrosis 4 (5.7) 0 (0)
Rheumatoid arthritis 0 (0) 3 (2.3)
SF-12 physical component 30.88 (18.26-54.56, 7.29) 30.24 (16.13-56.65, 8.30) .494
SF-12 mental component 50.32 (26.39-70.99, 10.41) 49.97 (27.45-67.57, 11.32) .993
WOMAC pain 40.43 (0-80, 21.19) 39.84 (0-100, 26.38) .862
WOMAC stiffness 48.21 (0-100, 22.15) 46.68 (0-100, 26.64) .643
WOMAC physical function 49.13 (7-90, 19.37) 45.98 (0-93, 22.35) .483
2The Journal of Arthroplasty Vol. 00 No. 0 Month 2012
with regard to age, gender, BMI, or diagnosis. However,
there was a signiﬁcant difference between the 2 cohorts
with regard to the Charlson Comorbidity Index (Pb
.00001). Patients undergoing UKA had an average
comorbidity index of 2.60 ± 1.59, whereas those
undergoing TKA had an average index of 0.86 ± 1.36.
At baseline, there were no clinically or statistically
signiﬁcant differences in SF-12 physical component and
mental component, WOMAC pain, WOMAC stiffness,
and WOMAC physical function scores between patients
undergoing UKA and patients undergoing TKA.
Patients undergoing TKA were followed up at a mean
of 2.89 years (range, 2-8.5 years), whereas patients
undergoing UKA were followed up at a mean of 3.05
years (range, 2-6.5 years). The upper half of Table 2
compares all crude postoperative SF-12 and WOMAC
scores between the 2 cohorts. Postoperative SF-12
physical component (P= .003289), SF-12 mental
component (P= .04924), WOMAC pain (P= .009208),
WOMAC stiffness (P= .03901), and WOMAC physical
function (Pb.0002) scores did signiﬁcantly differ
between the 2 procedure groups. All scores were
signiﬁcantly greater in favor of UKA. In addition to
being statistically signiﬁcant, the difference in average
SF-12 physical component and SF-12 mental compo-
nent scores between patients who underwent UKA and
patients who underwent TKA also surpassed criteria for
a minimal clinically important difference (ie, 3 units).
Furthermore, the differences in average WOMAC pain
and WOMAC physical function scores between patients
who underwent TKA and those who underwent UKA
also exceeded the criteria for a minimal clinically im-
portant difference (ie, 9 units).
Subsequently, multivariate linear regression analysis
controlling for age, gender, Charlson Comorbidity
Index, diagnosis, and preoperative health status mea-
sures (SF-12 and WOMAC) demonstrated that type of
procedure was signiﬁcantly associated with all post-
operative SF-12 and WOMAC measures. Unicompart-
mental knee arthroplasty was signiﬁcantly associated
with higher postoperative SF-12 physical component
(Pb.0003), SF-12 mental component (P= .0051),
WOMAC pain (P= .0012), WOMAC stiffness (P= .0375),
and WOMAC physical function (Pb.0006) scores than
TKA. The procedure coefﬁcient (β) associated with each
postoperative outcome measure represents the adjusted
increase in score for a UKA procedure relative to a TKA
procedure. All adjusted increases surpassed their re-
spective thresholds for a minimal clinically important
difference. The lower half of Table 2 summarizes the
output of the linear regression analysis.
Lastly, changes in SF-12 and WOMAC scores (post-
operative minus preoperative) were compared between
the UKA and TKA cohorts (Fig.). Changes in WOMAC
stiffness scores were not found to be signiﬁcantly
different between patients who underwent UKA and
those who underwent TKA. However, patients who
underwent UKA did demonstrate a signiﬁcantly larger
improvement in SF-12 physical function (P= .032),
WOMAC physical function (P= .045), SF-12 mental
component, and WOMAC pain. Of note, patients who
underwent UKA reported a larger improvement in all
SF-12 and WOMAC measures compared with patients
who underwent TKA.
In the present study, we used a single-center joint
registry to compare patient-reported outcomes after
UKA and TKA. At baseline, the 2 cohorts did not display
clinically signiﬁcant differences, with the exception of
Table 2. Univariate Analysis of Crude Postoperative SF-12 and WOMAC Scores and Multivariate Analysis of Associations
Between Postoperative SF-12 and WOMAC Scores and Procedure *
Variable Patients Who Underwent UKA (n = 70) Patients Who Underwent TKA (n = 128) P
SF-12 physical component 45.25 (23.15-58.95, 11.53) 40.25 (12.66-58.96, 11.88) b.004
SF-12 mental component 53.94 (34.16-64.24, 7.17) 50.65 (22.48-66.11, 10.13) .049
WOMAC pain 90.86 (45-100, 15.67) 81.33 (0-100, 25.97) b.010
WOMAC stiffness 76.43 (25-100, 24.03) 68.55 (0-100, 26.20) .039
WOMAC physical function 82.70 (32-100, 20.77) 71.77 (0-100, 21.63) b.001
Variable Procedure Coefficient (β)
SF-12 physical component 7.219 b.001
SF-12 mental component 4.206 b.006
WOMAC pain 12.594 b.002
WOMAC stiffness 9.491 .038
WOMAC physical function 13.442 b.001
* In multivariate linear regression analysis, age, gender, Charlson Comorbidity Index, diagnosis, and preoperative SF-12 physical component,
SF-12 mental component, WOMAC pain, WOMAC stiffness, and WOMAC physical function scores were controlled for.
†In multivariate linear regression analysis, the TKA procedure was coded for as the dummy variable and assumed a value of 0. The UKA
procedure assumed a value of 1.
UKA Relieves Pain and Improves Fuction Noticewala et al 3
patients who underwent UKA having a signiﬁcantly
higher Charlson Comorbidity Index compared with
patients who underwent TKA. Univariate analysis
demonstrated a statistically signiﬁcant and clinically
perceptible difference in postoperative SF-12 physical
component, SF-12 mental component, WOMAC pain,
and WOMAC physical function scores in favor of UKA
over TKA. More rigorous multivariate linear regression
analysis, controlling for potential confounders, demon-
strated signiﬁcantly improved physical function, mental
well-being, pain, and stiffness after UKA relative to TKA.
Furthermore, analysis of differences between postoper-
ative and preoperative measures revealed that patients
who underwent UKA experienced signiﬁcantly larger
improvements in physical function, mental well-being,
and pain than did patients who underwent TKA.
In general, there is a dearth of studies comparing the
clinical outcomes of patients after UKA and TKA.
Among the studies available, the consensus conclusion
to be drawn is that patients who underwent UKA have
clinical outcomes similar to patients who underwent
TKA, with the exception of possibly a better postoper-
ative ROM, but TKA remains the preferred surgical
intervention in light of the higher revision rate after
UKA . However, a careful review of these studies
reveals many limitations. In a retrospective study by
Dalury et al  comparing UKA and TKA in the same
patient, UKA knees were capable of more ﬂexion than
TKA knees. Otherwise, UKA and TKA knees performed
similarly in Knee Society pain and function scores.
Besides the small cohort size (n = 23), a confounding
factor that was not controlled for in the ﬁnal analysis by
Dalury et al was timing of procedure: some patients
received simultaneous knee arthroplasties, whereas
others received staged procedures at variable intervals.
Patients undergoing simultaneous bilateral knee surgi-
cal procedures have been shown to have worse post-
operative outcomes than those who undergo staged
procedures . In a study by Weale et al ,2
independent cohorts of patients who underwent UKA
and TKA were compared with respect to the Oxford
Knee score, response to each Oxford Knee survey
question, and Oxford Pain score. Although no signif-
icant differences were reported, responses to certain
survey questions were in favor of UKA over TKA, and
signiﬁcance was approached (Pb.05) but not reached
due to lack of power (TKA: n = 130; UKA: n = 31).
Also, in analyzing Oxford scores, only the postoperative
scores were compared, whereas the preoperative scores
were not provided, thus making comparisons of
improvement impossible. In a more recent study by
Lygre et al  involving the Norwegian Arthroplasty
Register, patients who underwent UKA were found to
demonstrate no clinical differences in Knee Injury and
Osteoarthritis subscales, visual analog scale for pain and
satisfaction, and EuroQoL-5D quality of life index
when compared with patients who underwent TKA
at minimum 2-year follow-up. However, this study
only compared retrospectively collected postoperative
outcome measures and did not prospectively record
preoperative scores that could serve as an internal
control for assessing the magnitude of improvement
after either procedure. Newman et al  prospectively
randomized 102 knees with unicompartmental OA to
be treated with either UKA or TKA. At 2-year follow-
up, there were no differences in the Bristol Knee score
and Bristol Knee score pain component between the 2
cohorts, but the proportion of knees treated with UKA
that had a ROM greater than 120° was signiﬁcantly
greater than that treated with TKA. Although Bristol
Knee score is a widely used outcomes measure, 33% of
a patient's Bristol Knee score is derived from clinical
(Post -Op Minus Pre-Op)
Fig. A signiﬁcantly greater improvement in SF-12 physical component, SF-12 mental component, WOMAC pain, and WOMAC
physical function scores was seen after UKA relative to TKA (***Pb.05).
4The Journal of Arthroplasty Vol. 00 No. 0 Month 2012
examination ﬁndings by the physician . Therefore,
because blinding was not performed in this study, a
strong potential for surgeon bias exists. A more recent
randomized controlled trial performed by Sun and Jia 
compared UKA and TKA treatments for unicompart-
mental OA. The investigators found no differences
between the 2 cohorts with respect to postoperative
(minimum 3-year) Knee Society score, ROM, hip-knee
angle alignment, and visual analog scale pain score. In
this study, 7 of 28 patients who underwent UKA required
revision, whereas 0 of 28 patients who underwent TKA
was revised. The 28 UKA procedures performed by the
authors over 5 years were the initial such procedures
performed by these surgeons. Therefore, surgeon expe-
rience with the more difﬁcult UKA procedure was
limited. This inexperience may have contributed to the
25% short-term revision rate, which far exceeds the
typical revision rate seen in most other studies [11-19].
Reduced UKA prosthetic survivorship has also been
cited to support the superiority of TKA over UKA .
Traditionally, survivorship of UKA implants has been
found to be less than that of TKA implants [1,2,9].
However, more recent studies (published after 2000)
report long-term survivorship rates of UKA as high as
96% [11-19].Table 3 lists the survivorship rates of UKA
prostheses from recent reports and compares them to the
rates of TKA prostheses over similar follow-up periods.
Only O'Donnell and Neil  reported a UKA survivor-
ship rate less than 86%. Their suboptimal outcomes, as
compared with Romanowski and Repicci , who used
a similar implant, may be due to inexperience because
they reported on their ﬁrst 100 patients who underwent
UKA. Overall, the recent survivorship rates of UKA are
comparable with those of TKA.
We do acknowledge limitations in our study. First, a
neutral baseline for comparison of the 2 cohorts was not
possible because patients who underwent UKA had a
signiﬁcantly higher Charlson Comorbidity Index than
did those who underwent TKA. However, we feel that
the difference in comorbidity indices was not a con-
founder in this study because linear regression analysis,
while controlling for Charlson Comorbidity Index,
conﬁrmed uniformly greater postoperative SF-12 and
WOMAC scores after UKA in comparison with TKA.
Second, we have reported on a small of number of
patients who underwent UKA, and only minimum 2-
year follow-up is available. These patients will need to
be followed up annually to assess whether these gains
in physical function endure over the long term. Third,
when comparing functional outcomes between patients
who underwent UKA and those who underwent TKA,
we did not match for radiologic severity of osteoar-
thritic disease. Instead, we chose to match our cohorts
based on the symptomatic severity of disease as dem-
onstrated by similar preoperative SF-12 and WOMAC
scores. Matching cohorts based on both radiologic and
symptomatic severity is extremely difﬁcult because the
radiologic grade of knee OA does not always correlate
with the symptomatic severity of knee OA . Ideally,
the 2 procedures should be compared in patients with
identical radiologic and symptomatic disease. Finally,
although the magnitude of the difference in postoper-
ative scores between the cohorts was large enough to
suggest a clinically perceptible difference, we did not
correlate physical function and pain scores with clinical
examination ﬁndings [29-30]. This eliminates surgeon
bias, but more outstanding differences may not be
highlighted by these clinical indices.
Table 3. Comparison of Long-Term Survivorship Rates Between UKA and TKA
Study Implant Follow-Up (y) Survivorship (%)
Argenson et al  Miller-Galante (n = 160) 10 94
Lidgren  Oxford (n = 749) 10 86
Romanowski and Repicci  Repicci (n = 136) 8 96
Gioe et al  Multiple Designs (n = 473) 10 89
Naudie et al  Miller-Galante (n = 113) 10 86
O'Rourke et al  Marmor (n = 136) 24 86
Price et al  Oxford (n = 439) 15 93
Newman et al  St George Sled (n = 52) 15 90
O'Donnell and Neil  Repicci II (n = 114) 10 78
Ritter et al  AGC (n = 4583) 15 99
Rand et al  Multiple Designs (n = 11,606) 10 91
Dixon et al  PFC Total Knee (n = 136) 15 93
Tarkin et al  Porocoat LCS (n = 70) 17 76
Vessely et al  PFC Total Knee (n = 1008) 15 96
Newman et al  Kinematic (n = 50) 15 79
AGC indicates anatomically graduated components; PFC, press-ﬁt condylar.
UKA Relieves Pain and Improves Fuction Noticewala et al 5
Our results regarding the efﬁcacy of UKA are distinctly
different from the results reported in the studies
described previously. We believe that the primary
reason we were able to uncover superior outcomes for
UKA was through accounting and adjusting for baseline
parameters. Another possibility is improved patient
satisfaction owing to perception of more normal knee
function after UKA [36,37]. Furthermore, biomechan-
ical studies have conﬁrmed that tibial axial rotation and
femoral rollback after UKA more closely recapitulate
normal knee kinematics than that after TKA .
Consequently, UKA leads to a greater increase in
mental well-being, relief of pain, and improvement in
physical function compared with TKA. Although our
ﬁndings are contrary to the outcomes reported by
others, we believe that prior studies may not have
been able to detect the superior outcomes of UKA
because they did not account nor adjust for baseline
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