MINI-SYMPOSIUM: SPINAL DEFORMITY
(v) Unicompartmental knee
arthroplasty
ligament was considered essential. Technical factors included the
need to under-correct the coronal plane deformity and to avoid
femoral component impingement on the patella. In addition,
malalignment of the components led to edge loading, high
contact stress, accelerated polyethylene wear and implant loosening. With respect to prosthesis design, increasing conformity of
the femoral-tibial articulation in fixed-bearing designs7 and thin
tibial polyethylene (<6 mm)8 were associated with high failure
rates. Uncemented UKAs9 were less durable. Mobile-bearing
components fared better than fixed.10
With the above in mind, improved understanding of surgical
principles, surgical techniques and designs have led to excellent
long-term results according to numerous published series, with 10year survivorship ranging from 91% to 100%, and 15-year survivorship of 93%.11 This has led to a resurgence in popularity of the
use of UKA over high tibial osteotomy or total knee arthroplasty in
younger patients with unicompartmental degenerative disease.
S Thambapillay
G Chakrabarty
Abstract
Unicompartmental knee arthroplasty (UKA) is a treatment option when
only one compartment of the knee is affected with arthritis. There has
been increasing enthusiasm in unicompartmental knee arthroplasty,
with improved understanding of surgical principles, newer techniques
for improving surgical precision including the use of smaller incisions,
and the introduction of newer designs. Past experiences from several
centres have been paramount in the education of surgeons with regards
to patient selection, technical considerations, and importantly avoiding
common pitfalls can lead to early failure of the components.
Patient selection
The patient should have a diagnosis of osteoarthritis (OA), posttraumatic arthritis or spontaneous osteonecrosis involving only
one compartment of the knee joint. Most importantly, the patient
should have only unicompartmental pain, as can be demonstrated clinically with a positive ‘one finger test’. Usually, the
patient points to the compartment with one finger compared with
a ‘knee grab’, where the patient literally grabs the entire knee due
to inability to localize the pain.
The main clinical indications are:
severe pain arising from one knee compartment,
patients with a relatively sedentary occupation,
any varus deformity should be less than 10 ,
the flexion range should be at least 90 ,
there should be no significant flexion contracture,
the anterior cruciate ligament should be intact and
any instability should be medial only.
With improved reported survivorship in recent studies,
patient selection has been extended to two further categories.
First, UKA should be considered for the middle-aged patient with
OA who desires a reliable initial result with retention of both
cruciates and easy revision to total knee arthroplasty if necessary. The second group are elderly patients with severe medical
co-morbidities. UKA may improve the patient’s lifestyle and
reduce a significant amount of their ambulatory and rest pain. In
addition, these patients are unlikely to survive the lifespan of the
UKA. The patient should be counselled appropriately and be
managed on an individual basis.
Radiologically, the indications for UKA are 50% unicompartmental collapse (Ahlback I)12 or complete collapse
(Ahlback II) on weight-bearing radiographs (Figure 1). Stress
views may occasionally be indicated. Some surgeons perform
initial arthroscopic assessment of the knee to assess the integrity
of the anterior cruciate ligament and both the contralateral and
the patellofemoral compartments.
The contraindications include:
inflammatory arthritis,
decreased range of motion with flexion contracture,
patients with a very active lifestyle (athletes/sports),
obesity,
Keywords arthritis; arthroplasty; knee; unicompartmental
Introduction
The concept of a knee hemiarthroplasty was introduced in the
1950’s, when a spacer was inserted in one half of the tibiofemoral
joint to prevent bone-on-bone apposition. McKeever1 was the
first to introduce his Vitallium (Zimmer Inc., Warsaw, IN) tibial
plateau hemiarthroplasty in 1957, followed by MacIntosh,2 and
the current-day version is the Unispacer (Smith & Nephew, Inc.,
Memphis, TN). Unfortunately, this implant has a very high
failure rate and is no longer recommended.3
The Gunston,4 Marmor and polycentric unicompartmental
knee arthroplasties were introduced in the early 1970’s. The
revision rates were 10% at two-year follow-up.5 Following that,
other reported series did not show encouraging results, with 70%
survivorship at 5e7 years, and 65% survivorship reported at 11
years in the 1970’s and 80’s.6
The results were promising initially but when compared with
total knee arthroplasty, the latter yielded superior outcomes.
Many surgeons held reservations about UKA during the early
stages due to this. Despite that, some held great enthusiasm
towards the development of UKA. Much was learned from the
early experiences. Patient selection was critical. Obese patients
and patients with moderate to severe deformities did worse.
Contralateral compartment degenerative changes were also
considered a poor prognostic factor. An intact anterior cruciate
S Thambapillay MRCS Specialty Trainee Registrar, Orthopaedic
Department, Huddersfield Royal Infirmary, Lindley, Huddersfield,
Yorkshire, UK.
G Chakrabarty D (Orth) MS (Orth) MCh (Orth) FRCS (Ed) FRCS Tr & Orth Consultant
Orthopaedic Surgeon, Orthopaedic Department, Huddersfield Royal
Infirmary, Lindley, Huddersfield, Yorkshire, UK.
ORTHOPAEDICS AND TRAUMA 25:6
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Ó 2011 Elsevier Ltd. All rights reserved.
MINI-SYMPOSIUM: SPINAL DEFORMITY
Figure 1 (a) Pre-operative radiographs of knee. (b) Post-operative radiographs of UKA.
coexistent patellofemoral arthritis,
secondary osteonecrosis, and
knee instability with absence of the anterior cruciate
ligament.
However, some centres have reclassified some of the above
criteria as relative rather than absolute contraindications.
Radiographic contraindications are grade IV Ahlback changes,
with joint space obliteration, or medial-lateral subluxation of
3e4 mm or greater. Intra-operative findings are equally important and contraindications include significant involvement of the
other compartment(s), an absent anterior cruciate ligament, and
greater than 10 of varus deformity. Some surgeons are performing UKAs in patients with patellofemoral OA and/or ACL
deficiency (the procedure being combined with reconstruction of
the ACL), with promising reported results.13
a study comparing patients who had UKA in one knee and TKA
on the contralateral side, there was a greater range of movement
in the UKA side postoperatively.18 31% stated that their UKA
knee was their better knee overall, 15% stated that their TKA
knee was their better knee, and 54% could find no difference.
These findings are probably due to the fact that UKA tends to
preserve more normal knee kinematics better than does TKA.
Surgical techniques
One of the key factors to the overall survival of the implants in
UKA surgery is the level of experience of the surgeon, as this
influences significantly the likely accuracy of component
implantation and limb alignment.
The ability to use a mini-incision technique, i.e. a short incision performed medial to the patellar tendon, with subluxation of
the patella rather than dislocation or eversion of the patella,
using appropriate modern instrumentation, is one of the key
features that has led to a resurgence of UKA as an appropriate
treatment option for unicompartmental arthritis versus TKA.
Studies have shown faster recovery and shorter hospital stay in
the mini-incision group versus TKA or versus UKA through
a larger incision with eversion of the patella.19 Accuracy of UKA
component implantation was the same with the mini-incision as
it was with the standard (TKA-type) approach.
UKA versus high tibial osteotomy and total knee arthroplasty
With improvement in the design and surgical techniques in UKA,
and with promising long-term results, it is vital to compare and
contrast the advantages and disadvantages of the UKA with High
Tibial Osteotomy (HTO) and Total Knee Arthroplasty (TKA).
The outcome following UKA compared with HTO in two
different series revealed superior results with 5e17-year followups. These showed 80% 10-year and 65% 17-years survivorship for HTO, versus 86% 10-year and 88% 17-year survivorship
for UKA, respectively.14,15 More intra- and post-operative
complications were noted in the HTO group. Patients in the
UKA group had a higher satisfaction score, quicker recovery, less
blood loss, lower risk of infection and easier revision to total
knee arthroplasty than patients with HTO.
With respect to TKA, studies have shown comparable or even
superior results with UKA.16,17 UKA allows preservation of bone
stock, improved range of movement, reduced intra-operative
blood loss, reduced inpatient stay and decreased cost. In
ORTHOPAEDICS AND TRAUMA 25:6
Navigation assisted UKA surgery
Several centres have already started to use navigated systems to
assist minimally invasive UKA, and preliminary results have
shown significant improvement in the alignment of the limb, in
both the coronal and sagittal planes.20 Accuracy of implant alignment in arthroplasty surgery is of paramount importance, with
well aligned prostheses giving better function and increased
longevity.21 Navigation assisted surgery will probably be one of
the future core elements of arthroplasty surgery.
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MINI-SYMPOSIUM: SPINAL DEFORMITY
Implant design
implants and accuracy of surgical technique have made the wear
rate in UKA comparable to that of primary TKR. Appropriate
patient selection has also made a significant contribution to the
improved survivorship of the UKA, for example the exclusion of
patients with too high a body mass index.
Cemented versus uncemented
Uncemented implants have fared badly compared to their
cemented counterparts,9 with a higher rate of loosening and
implant failure due to poor bony ingrowth. This is particularly
noted in the metal-backed tibial components. Hence, it has
become common practice to use cemented UKA implants.
Progression of contralateral compartment OA
One of the commonest reasons for revision surgery is progression
of the OA to the contralateral compartment. Initially, overstuffing
of the implants in the medial compartment was thought to be
a potential cause for contralateral disease progression, from
increasing the load on to the lateral compartment (Figure 2). It is
now appreciated that it is important to under-correct the coronal
plane deformity and achieve optimal soft tissue balancing intraoperatively. One should also ensure that there are no significant
degenerative changes in the contralateral compartment at the time
of the UKA procedure. Early or mild patellofemoral osteoarthritis is
a relative contraindication, and medial UKA should be avoided in
patients with actual patellofemoral symptoms.
Fixed bearing versus mobile bearing
The indications for the use of an implant with a mobile versus
a fixed bearing are still not clear. The long-term results of mobile
and fixed bearings are comparable, but there are significant
differences in resulting knee joint kinematics, tribological properties and implant-associated complications.
The mobile bearings in UKA restore the physiological joint
kinematics better than fixed implants. The decoupling of mobile
bearings from the tibial implant allows a high level of congruence
with the femoral implant, resulting in larger contact areas than
with fixed bearings. This fact, in combination with the more
physiological joint kinematics, leads to less wear and a lower
incidence of osteolysis with mobile bearings. Mobile-bearing
articulations also allow a metal-backed component to be used
with a composite thickness as thin as 6 mm. However, the potential
disadvantages of mobile bearings are the higher complication and
early revision rates resulting from bearing dislocation and
impingement syndromes, often secondary to suboptimal implantation technique or instability. This problem has been largely
overcome with improved implant design and instrumentation.
In cases with ligamentous pathology, fixed bearings involve
a lower complication rate. It seems that their use can also be
beneficial in patients with a low level of activity, as problems
related to wear may be of lesser importance for this subgroup.
The decision as to whether to use mobile or fixed-bearing
components can, therefore, be tailored to the individual
patient’s circumstances.
Patellar impingement
Patellar impingement can be a problem in lateral UKA, with some
patients developing patellofemoral pain. However, this is rarely seen
now with improvement in surgical expertise and newer implants.
Malaligned components
The main reason for component realignment is inexperience of
the surgeon, particularly with the earlier instrumentation. This
can lead to early failure of implants, increased polyethylene
wear, impingement and dislocations of mobile-bearing designs,
necessitating revision surgery. Fortunately, this has largely been
overcome with improved modern instrumentation and with
navigation surgery.
Bearing dislocation
Bearing dislocation is a problem that has been reported particularly in mobile-bearing designs, especially in lateral UKA.
Fortunately, however, this is rarely seen nowadays.
Metal-backed versus polyethylene tibial tray
Some surgeons are proponents of the use of metal-backed tibial
components. With the metal-backed tibial tray, there is the
potential for greater resection of the tibial bone stock, to be able
to accommodate an appropriate thickness of the polyethylene
insert (i.e. >6 mm). This diminution in bone stock may potentially make revision surgery technically more demanding.
However, the Oxford UKA utilizes the metal-backed component
with a mobile-bearing polyethylene insert (starting at 3 mm) with
good long-term survival.22 In addition, with improvement in the
design and manufacturing of the polyethylene, wear rates have
been drastically minimized. The reported results for either type
of component have been promising.
Failures of UKA
Apart from the normal modes of failure inherent to any arthroplasty surgery, there are a few specific potential failure mechanisms with UKAs:
Polyethylene wear and associated wear
Polyethylene wear is generally one of the main reasons for
revision surgery. Improved tribological properties of current-day
ORTHOPAEDICS AND TRAUMA 25:6
Figure 2 Degenerative changes in contralateral compartment of the knee
after medial UKA.
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MINI-SYMPOSIUM: SPINAL DEFORMITY
Implant breakage
Implant breakage is most commonly associated with implant
malalignment, early wear and loosening, which can very occasionally lead to this catastrophic consequence.
Bony defects are occasionally noted in knees undergoing
revision arthroplasty. Bone loss is usually classified according to
the nature of the defect at the end of the preparation during
revision surgery. Technical difficulties can arise but nevertheless,
surgeons with a major interest in revision knee arthroplasty have
often reported that bone defects have no major impact on the
technical difficulty of revision surgery due to the improvements
in implant designs.
Many papers have shown comparable outcomes with revision
surgery following UKAs versus total knee arthroplasty.23,24,25 The
Revision surgery
Revision of unicompartmental knee arthroplasties may be
undertaken for the above listed modes of failure. The commonest
reasons for revision are progression of arthritis in the contralateral compartment and implant failure (Figure 3).
Figure 3 (a) X-ray showing loosening around a tibial implant. (b) Post-revision to total knee arthroplasty in same patient.
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MINI-SYMPOSIUM: SPINAL DEFORMITY
majority of the patients had no defects or only contained osseous
defects (78%) at revision surgery and were dealt with by performing routine arthroplasty.23 However, revision of UKA to
another UKA is not advocated in current practice. These centres
have also had good overall survivorship of the primary UKA.
With improved designs of the modern revision implants (e.g.
modularity, stems, metal wedges or steps with stems) that allow
reconstruction, UKA is now a definite viable option for selected
patients with unicompartmental pathology.
Learning points
Indications for unicompartmental knee arthroplasty
unicompartmental pathology from the following:
C
primary osteoarthritis
C
spontaneous osteonecrosis
C
post-traumatic arthritis
are
Patients undergoing UKA should have confirmed clinical and
radiological evidence of symptomatic, isolated unicompartmental
arthritis of the knee.
UKA is contraindicated in patients with:
C
inflammatory arthropathy
C
secondary osteonecrosis
C
associated symptomatic patellofemoral arthritis
C
decreased range of movement with flexion contracture
C
varus deformity greater than 10
Lateral UKA
Lateral UKA is performed less frequently and is technically more
difficult than medial UKA. The ratio of lateral to medial UKA is
1:10. The surgical approach is either through the medial or lateral
approach. Technically, the patella is more vulnerable to
impingement on the leading edge of the femoral component. The
wear pattern is more posterior than the medial compartment, and
achieving mediolateral congruency is technically more
demanding. It is usually indicated for primary osteoarthritis with
relatively good results. However, the results are inferior when
lateral UKA is undertaken for post-traumatic arthritis. Two longterm studies of lateral unicompartmental knee arthroplasty
showed survival rates of 83% at 10 years and 74% at 15 years,26
and 100% at 12.4 years.27
Relative contraindications to UKA are:
C
ACL deficiency (although combined ACL reconstruction can be
undertaken)
C
obesity
C
very young patients
One should beware of patients with medial sided knee pain that
might actually be referred pain from a stiff or arthritic hip.
There is good evidence that UKA gives comparable or even
superior outcomes compared to TKA for patients with
unicompartmental arthritis of the knee, and it has been shown to
be safe, reliable and repeatable.
Many papers have shown comparable results with revision surgery
following UKAs and total knee arthroplasties, and without major
technical difficulties.
Patellofemoral UKA
Several centres are currently performing patellofemoral UKA for
isolated patellofemoral arthritis. The results have been promising, provided clear indicative criteria are met, with good patient
selection.28 This particular topic is not dealt with in this paper as
it merits separate dedicated review and discussion.
The future
Unicompartmental knee arthroplasty has become an established
and viable option over the last two to three decades for the
treatment of unicompartmental OA. The long-term results to-date
have been promising and the indications for the procedure are
broadening.29,30 The future is likely to see refined criteria for
patient selection and should bring better surgical techniques and
prosthetic designs. Improved polyethylene will increase
longevity of the prostheses, and mobile-bearing articulations
may also extend longevity by decreasing wear and allowing
a metal-backed component with relatively thin polyethylene
inserts for a conservative arthroplasty. Ultimately, for UKA as for
any joint arthroplasty, the aim is to perform the right operation
for the right patient using the correct surgical technique and
a proven implant design.
Research directions
Longer term studies are required to:
C
compare fixed versus mobile-bearing implants
C
determine the potential value of computer aided surger
REFERENCES
1 McKeever DC. Tibial plateau prosthesis. Clin Orthop 1960; 18: 86e95.
2 MacIntosh DL, Hunter GA. The use of the hemiarthroplasty prosthesis
for advanced osteoarthritis and rheumatoid arthritis of the knee.
J Bone Joint Surg Br 1972; 54: 244e55.
3 Bailie AG, Lewis PL, Brumby SA, Roy S, Paterson RS, Campbell DG.
The unispacer knee implant: early clinical results. J Bone Joint Surg Br
2008; 90: 446e50.
4 Gunston FH. Polycentric knee arthroplasty: prosthetic simulation of
normal knee movement. J Bone Joint Surg 1971; 53B: 272.
5 Marmor L. Marmor modular knee in unicompartmental disease:
minimum four-year follow-up. J Bone Joint Surg Am 1979; 61:
347e53.
Conclusion
Unicompartmental knee arthroplasty is an excellent operation
when undertaken by the right surgeon for the right patients
meeting the appropriate criteria. It is a minimally invasive
procedure with comparable or better outcomes compared with
total knee replacements. Revision of unicompartmental knee
replacements to total knee replacements is no longer considered a technically challenging concept in many knee
centres.
A
ORTHOPAEDICS AND TRAUMA 25:6
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Ó 2011 Elsevier Ltd. All rights reserved.
MINI-SYMPOSIUM: SPINAL DEFORMITY
6 Marmor L. Unicompartmental arthroplasty of the knee with
a minimum ten-year follow-up period. Clin Orthop 1988; 228: 171e8.
7 Batley RE, Stulberg SD, Robb III WJ, Sweeny HJ. Polyethylene wear in
unicompartmental knee arthroplasty. Clin Orthop Relat Res 1994;
299: 18e24.
8 Knutson K, Jonsson G, Langer Anderson J, Larusdottir H, Lidgren L.
Deformation and loosening of tibial component in knee arthroplasty
with unicompartmental endoprosthesis. Acta Orthop Scand 1981; 52:
667e73.
9 Lindstrand A, Strenstrom A, Egund N. The PCA unicompartmental
knee. a one to four year comparison of fixation with or without
cement. Acta Orthop Scand 1988; 59: 695e700.
10 Argenson JN. Biomechanical study of the Oxford knee prosthesis with
mobile meniscus. Chirurgie 1993e1994; 119: 268e72.
11 Khanna G, Levy BA. Oxford unicompartmental knee replacement:
literature review. Orthopedics 2007; 30(5 suppl): 11e4.
12 Ahlback S. Osteoarthrosis of the knee. A radiographic investigation.
Acta Radiol Diagn (Stockh) 1968; 277(suppl): 7e72.
13 Dervin GF, Conway AF, Thurston P. Combined anterior cruciate ligament reconstruction and unicompartmental knee arthroplasty:
surgical technique. Orthopedics 2007; 30(5 suppl): 39e41.
14 Broughton NS, Newman JH, Baily RA. Unicompartmental replacement
and high tibial osteotomy for osteoarthritis of the knee. A comparative study after 5e10 years’ follow-up. J Bone Joint Surg Br 1986;
68: 447e52.
15 Weale AE, Newman JH. Unicompartmental arthroplasty and high tibial
osteotomy for osteoarthrosis of the knee. A comparative study with
a 12- to 17-year follow-up period. Clin Orthop Relat Res 1994; 302:
134e7.
16 Emerson Jr RH. Unicompartmental mobile-bearing knee arthroplasty.
Instr Course Lect 2005; 54: 221e4.
17 Hopper GP, Leach WJ. Participation in sporting activities following
knee replacement: total versus unicompartmental. Knee Surg Sports
Traumatol Arthrosc 2008; 16: 973e9.
18 Laurencin CT, Zelicof SB, Scott RD, Ewald FC. Unicompartmental
versus total knee arthroplasty in the same patient. A comparative
study. Clin Orthop Relat Res 1991; 273: 151e6.
ORTHOPAEDICS AND TRAUMA 25:6
19 Price AJ, Webb J, Topf H, Goodfellow JW, Murray DW. Rapid recovery
after oxford unicompartmental arthroplasty through a short incision.
J Arthroplasty 2001; 16: 970e6.
20 Jenny JY, Ciaobanu E, Boeri C. The rationale for navigated minimally
invasive unicompartmental knee replacement. Clin Orthop Relat Res
2007; 463: 58e62.
21 Konywes A, Willis-Owen CA, Spriggins AJ. The long-term benefit of
computer-assisted surgical navigation in unicompartmental knee
arthroplasty. J Orthop Surg Res 2010; 5: 94.
22 Simpson DJ, Gray H, D’Lima D, Murray DW, Gill HS. The effect of
bearing congruency, thickness and alignment on the stresses in
unicompartmental knee replacements. Clin Biomech (Bristol, Avon)
2008; 23: 1148e57.
23 Chakrabarty G, Newman JH, Ackroyd CE. Revision of unicompartmental arthroplasty of the knee. Clinical and technical
considerations. J Arthroplasty 1998; 13: 191e6.
24 Dudley TE, Gioe TJ, Sinner P, Mehle S. Registry outcomes of unicompartmental knee arthroplasty revisions. Clin Orthop Relat Res
2008; 466: 1666e70.
25 Johnson S, Jones P, Newman JH. The survivorship and results of total
knee replacements converted from unicompartmental knee replacements. Knee 2007; 14: 154e7.
26 Ashraf T, Newman JH, Evans RL, Ackroyd CE. Lateral unicompartmental knee replacement survivorship and clinical experience over 21 years. J Bone Joint Surg Br 2002; 84: 1126e30.
27 Pennington DW, Swienckowski JJ, Lutes WB, Drake GN. Lateral unicompartmental knee arthroplasty: survivorship and technical
considerations at an average follow-up of 12.4 years. J Arthroplasty
2006; 21: 13e7.
28 Lonner JH. Patellofemoral arthroplasty. Instr Course Lect 2010; 59:
67e84.
29 Berend KR, Lombardi Jr AV, Adams JB. Obesity, young age, patellofemoral
disease, and anterior knee pain: identifying the unicondylar arthroplasty
patient in the United States. Orthopedics 2007; 30(5 suppl): 19e23.
30 Sah AP, Springer BD, Scott RD. Unicompartmental knee Arthroplasty
in octogenarians: survival longer than the patient. Clin Orthop Relat
Res 2006; 451: 107e12.
440
Ó 2011 Elsevier Ltd. All rights reserved.