Keywords
unicompartmental knee arthroplasty - polyethylene - loosening - bearing dislocation
- revision - conversion
Introduction
During the last 20 years, the use of unicompartmental knee arthroplasty (UKA) has
considerably increased. Between 1998 and 2005, use of UKA increased at a rate nearly
triple that of total knee arthroplasty (TKA).[1] UKA can be performed through a minimally invasive approach and is associated with
a relatively low incidence of complications.[2]
[3] Several reports have already demonstrated survival rates greater than 90% at 10
years after modern UKA implantation.[1]
[4]
[5]
[6]
[7] Despite this, results of knee replacement registries still show a relatively high
revision and failure rate for UKA, especially when compared with traditional TKA.[8] Failures of UKAs have distinctive characteristics, and consensus on the causes and
the proper treatment methods has been elusive. Additionally, the incidence and type
of complications are different, depending on the type and design of implant, follow-up
period, and the surgeon who performed the procedure.[9] When the revision is required, most UKAs are converted to TKAs; surgical challenges
of UKA revisions, and outcomes of UKA conversion to TKA appear still controversial.
The purpose of this concept review was to analyze the recent literature to elucidate
the main causes and mechanisms of UKA failure, the principal surgical aspects that
should be considered during UKA revision, and the main outcomes of UKA conversion
to TKA.
Causes of Unicompartmental Knee Arthroplasty Failure
Causes of Unicompartmental Knee Arthroplasty Failure
The main causes of UKA failure include bearing dislocation (in mobile designs), aseptic
mechanical loosening, polyethylene wear, progression of osteoarthritis (OA) in unreplaced
compartments, infection, impingement, periprosthetic fracture, retaining of cement
debris into the joint, arthrofibrosis, and unexplained pain.[1]
[9]
[10]
[11] Bearing dislocation continues to be advocated as the predominant mechanism of failure
in mobile UKA,[2]
[9]
[12] whereas polyethylene wear and aseptic loosening remains the main cause of failure
of fixed UKA.[1]
[9] Degeneration of the unreplaced compartments has been reported in both mobile and
fixed designs. Regardless of the implant design, most of the failures occur in medial
UKA when compared with lateral UKA.[9]
Bearing Dislocation
Bearing dislocation is certainly the major complication of mobile UKA. Possible causes
of insert dislocation could be malposition of the components, unbalanced flexion–extension
gap, impingement of the insert with adjacent bone or tibial/femoral component, or
instability due to medial collateral ligament (MCL) injury or secondary to femoral/tibial
component loosening.[9]
[13] Dislocations are particularly common in lateral UKA, as the lateral collateral ligament
is slack in flexion, in contrast to the medial side in which the MCL is tight.[12]
[14]
MCL is the most important structure determining the flexion–extension gap and affecting
stability (with the anterior cruciate ligament) after UKA. Therefore, the surgeon
should take utmost care to protect the MCL during surgery. For the same reasons, in
UKA, soft tissue release should never be performed. The aim of UKA is to restore knee
kinematics by restoring ligament tension to normal; correct ligament balance is restored
by positioning the components accurately and inserting an appropriate thickness of
bearing.[3] On the contrary, ligament release may result in excessive laxity of the periprosthetic
soft tissues, with the consequent trend of the insert (and implant) to dislocate anteriorly.
Additionally, this may cause early polyethylene wear as well as aseptic loosening.[15]
Bearing dislocation can be treated with bearing change, revision UKA, or conversion
to TKA, according to the cause of dislocation.
Aseptic Mechanical Loosening
While aseptic mechanical loosening has been recently reported as the most frequent
cause of failure of the modern TKA,[16] newer instrumentation and prosthetic design have significantly reduced its incidence
in modern UKA. Younger age, overweight, and varus deformity have been advocated as
possible risk factors for mechanical failure of unicompartmental implants. Fixed-bearing
UKA results in greater contact stress on the polyethylene insert due to low conformity,
which may eventually lead to a failure associated with tibial component loosening
or subsidence.[9] Moreover, it is known that tibial component loosening is more frequent in the all-poly
designs than the metal-backed ones.[17] Mechanical loosening following UKA may be potentially determined by component malalignment,
undercorrection of the predegeneration deformity, anterior cruciate ligament deficiency,
excessive tibial slope, and bearing dislocation in mobile designs. Additionally, all
these factors may also produce wear-induced periprosthetic osteolysis (due to an increased
asymmetrical loading), with a further increase of the component subsidence and/or
loosening.[15]
Component loosening should be treated with a revision UKA or a conversion to TKA,
according to the cause of loosening and the residual integrity of bone stock and ligaments.
Progression of Osteoarthritis
Progression of OA in the contralateral compartment and/or in the patellofemoral joint
(PFJ) is one of the major causes of failure following UKA, both mobile and fixed.
Overcorrection of the leg mechanical axis may cause degenerative changes in the contralateral
compartment.[3] Progression of OA may occur in case of patients with systemic inflammatory diseases,
that is, rheumatoid or psoriatic arthritis. Degeneration of PFJ may occur in the presence
of an oversized femoral component with possible impingement with patellar cartilage.[1] Progression of OA in the contralateral and/or patellofemoral compartment can be
characterized by radiographic evidence of joint space narrowing and osteophyte formation
in the initial stage, which eventually leads to development of pain, subchondral sclerosis,
and loss of joint space in unreplaced compartments.[18]
Progression of OA should be treated with replacement of the newly affected compartment
(bi- or tri-UKA) or with conversion to TKA.
Polyethylene Wear
Polyethylene wear is a complication inherent to the design of fixed bearings, secondary
to higher surface deformation and delamination in comparison to mobile bearings.[19] Revision for polyethylene wear usually occur after a minimum of 8 years, but some
early catastrophic failure from wear have been reported.[15] Polyethylene wear increases the debris volume at the bone–implant interface, producing
osteolysis, which plays a role in component loosening and degeneration of unreplaced
compartments. Additionally, wear affects joint alignment and stability, leading to
an increased load at the bone–implant interface that further accelerates loosening.[15]
Factors associated with accelerated polyethylene wear after UKA are component malpositioning,
undercorrection of the predegeneration deformity, polyethylene thickness of less than
6 mm, manufacturing process and sterilization method of polyethylene, and reduced
conformity in the design.[12]
[15] Modern instrumentation helps to avoid component malposition and edge loading, even
with a minimally invasive approach. Furthermore, polyethylene is now thicker than
6 mm for fixed bearings, and the shelf age of polyethylene inserts sterilized with
gamma irradiation in air is well-documented. Finally, recent improvements in manufacturing
processes such as cross-linking may be valuable in some fixed and mobile designs.
If a failure occurs due to polyethylene wear, insert change can be considered as treatment
choice; however, conversion to TKA is the most common option in these cases.
Periprosthetic Infection
Incidence of infection after UKA is significantly lower than that after TKA, varying
from 0.2 to 1%.[20]
[21] The low incidence of infection in UKA is because it can be performed through a minimally
invasive approach, with less damage to the adjacent soft tissue and sparing of bone
and ligamentous structures.
During periprosthetic infection, cellular mechanisms of osteolysis and septic loosening
can be determined by the direct damage of infectious organisms and by the host inflammatory
response, mainly due to bacterial toxins and lipopolysaccharides.[22]
In patients with an acute UKA infection, the strategy of irrigation and debridement
with insert exchange still persists, given the emotional investment in dealing with
this complication by both patient and surgeon. In failed debridement or in chronic
infections, resection of all components is necessary, and a one-stage or two-stage
revision surgery with a primary TKA should be performed.
Periprosthetic Fracture
Periprosthetic fractures are rare but represent a serious complication in UKA. They
are mostly observed around the tibial condyles; this can be attributed to the increased
pressure and load applied on the proximal tibia. A tibial condylar fracture following
UKA can be managed with conservative treatment if slight translation or varus deformity
is present. Rarely, periprosthetic fracture of the femoral condyle may occur; factors
leading to this complication could be the impaction force or direction, or a diminished
load resistance of the distal femur.[23] If the fracture is accompanied by translation or deformity, percutaneous osteosynthesis
with cannulated screws, or open reduction and internal fixation with plates should
be performed.[24] In case of nonunion of the fracture, severe translation, or tibial component loosening,
UKA needs to be revised to TKA.[9]
Arthrofibrosis
The incidence of arthrofibrosis after UKA is much lower than that after TKA because
minimally invasive procedure causes reduced damage to the extensor mechanism and suprapatellar
pouch and lesser scar formation.[25] Additionally, modern designs of UKA have greatly reduced the incidence of prosthetic
impingement and/or impingement of the femoral component with the patella.
Postoperative arthrofibrosis in UKA can be treated with manipulation under anesthesia,
which often requires arthroscopic removal of intra-articular fibrous tissue and scar
tissue.
Unexplained Pain
Revision for unexplained pain has been reported more common after UKA than after TKA.[26] Common causes may include loose bodies, chronic regional pain syndrome, implant
failure, meniscus rupture, and cement extrusion. Additionally, unexplained pain may
be caused by subtle problems that the surgeon is unable to document. Revision for
unexplained pain after UKA may be related to the surgeon's experience.[13] Finally, failure due to unexplained pain has been particularly reported in UKAs
with all-polyethylene tibial components, which may increase load transfer to the proximal
tibia, thus resulting in persistent bone remodeling.[19]
Unicompartmental Knee Arthroplasty Revision: Surgical Aspects
Unicompartmental Knee Arthroplasty Revision: Surgical Aspects
Depending on the type, severity, and consequences of UKA failure, revision surgery
might be indicated, such as simple polyethylene insert exchange, revision UKA, or
conversion to TKA.
Polyethylene insert exchange could be indicated in case of insert dislocation or wear
associated with stable components and no severe osteolysis. Revision UKA can be performed
in the presence of complete ligament integrity and sufficient bone stock; obviously,
there must be no signs of degeneration of unreplaced compartments. Nevertheless, most
of the failed UKAs are generally converted to TKA.[13]
[27]
[28] The reasons for conversions to TKA are repeated dislocation in a mobile UKA, mechanical
loosening with associated bone defect, visible torn anterior cruciate ligament, valgus
overcorrection, and progression of OA in the lateral and/or patellofemoral compartment.[13]
Conversion to TKA may be technically demanding, especially in septic revisions.[20] Preoperative planning is mandatory to anticipate the need for revision components
as well as the availability of constrained implants.[29] Challenges of UKA revision can be joint exposure, component removal, bone defects,
and instability.
Regarding exposure, there is generally no necessity of quadriceps snip, V-Y plasty,
or tibial tuberosity osteotomy, and a standard approach can be considered according
to surgeon's preference.
Although unicompartmental components can be easily removed with an osteotome or a
thin blade saw, the surgeon should be careful to preserve bone stock. In the presence
of an uncemented prosthesis, removal of a stable implant could cause significant bone
loss, whereas removal of a loosed implant generally preserves much more bone stock.
In the presence of a cemented prosthesis, implant removal is usually easier because
a detachment usually occurs at the cement–metal interface.
Bone defects may occur at the site of a failed UKA, so conversion to TKA may require
revision devices such as metal augments, bone grafts, and stems. Causes of bone loss
after the UKA failure could be metal-backed design (when compared with all-polyethylene
tibia), osteolysis from wear, periprosthetic infection, loosening with component subsidence,
removal of components and/or cement, periprosthetic fracture, and the presence and
successive removal of spacer in septic two-stage conversion. In most cases of UKA
failure, residual femoral defects are generally small and contained, thus requiring
only autologous morcellized bone grafts or cement. At revision, correct femoral component
rotation can be established before removing the UKA; otherwise, the epicondylar and
transtrochlear axes can be also established after removing the UKA.[30] More frequently, in a failed UKA, bone loss affects the tibial side, primarily due
to the native tibial bone cut (especially in metal-backed implants). On the preoperative
radiographs, if the line orthogonal to the mechanical axis drawn 10 mm below the joint
line of the unaffected compartment lies below the tibial component, then an augmentation
will not be necessary ([Fig. 1]). On the contrary, if the defect is greater than 10 mm, then the surgeon could necessitate
a metal block/wedge or a bone graft; in this case, a tibial stem may be recommended
([Fig. 2]).[11] Finally, mediolateral stability in the coronal plane is generally preserved in failed
UKA, thus allowing the surgeon to use a primary TKA, with or without posterior ligament
sacrifice. Anyway, when there is deficiency of collateral ligaments, soft tissue instability,
and inability to balance the flexion–extension spaces, a more constrained prosthesis
is necessary.
Fig. 1 When the line orthogonal to the mechanical axis drawn 10 mm below the joint line
of the unaffected compartment lies below the tibial component (A), then tibial augmentation is not generally necessary and a primary tibial component
is sufficient (B).
Fig. 2 When the line orthogonal to the mechanical axis drawn 10 mm below the joint line
of the unaffected compartment lies inside the tibial component (A), then a metal block or wedge can be used during revision to avoid an excessive tibial
resection and therefore the use of an excessively thick polyethylene insert. In this
case, it is also recommended to add a short stem extension to decrease stress at the
bone–implant interface (B).
Literature Review
Complications following UKA are various and depend on the design and type of implant,
follow-up period, and the surgeon who performed the surgery.[9]
Pandit et al[31] reported a 2.9% rate of complications after 1,000 Oxford phase 3 medial UKAs at
a mean follow-up of 5.6 years. In their study, the most common causes for revision
were progression of OA in the lateral compartment (0.9%), bearing dislocation (0.6%),
and unexplained pain (0.6%). Of the 29 reoperations, 19 were conversions to a primary
TKA (2 of those required revision devices as stems and wedges), 6 were open reductions
for dislocation of the bearing, 3 were secondary lateral UKA, and 1 was revision of
a tibial component.
Epinette et al[32] in a multicenter study including 418 failed UKAs reported loosening as the main
reason for failure (45%), followed by OA progression (15%) and wear (12%). Other reasons
were technical problems in 11.5% of cases, unexplained pain in 5.5%, and failure of
the supporting bone in 3.6%. The infection rate was 1.9%. Recently, Kim et al[9] reported a total of 89 (5.6%) failures on 1,576 UKAs. The causes of failure included
bearing dislocation (n = 42), component loosening (n = 23), femoral component loosening (n = 11), tibial component loosening (n = 8), both femoral and tibial component loosening (n = 4), periprosthetic fracture (n = 6), polyethylene wear/destruction (n = 3), progression of arthritis to the other compartment (n = 3), MCL injury (n = 2), impingement (n = 2), infection (n = 5), ankylosis (n = 1), and unexplained pain (n = 2). The most common complication in mobile UKA was bearing dislocation, and in
fixed UKA, it was mechanical loosening. Polyethylene wear and progression of OA were
relatively rare. Complications were treated with conversion to TKA in 58 cases, bearing
change in 21, and revision with another UKA in 1. In all the cases of conversion to
TKA (including infected UKAs), a primary TKA was used; in 74% of the conversions to
TKA, a metal augmentation was used.
In a systematic review of 17 published studies that evaluated the causes of reoperation
after UKA using the Oxford phase 3 implant, Kim et al[13] assessed a total of 3,138 UKAs with a median follow-up period was 5.6 years. UKA
failure occurred in 146 (4.6%) knees. Bearing dislocation was found to be the most
important cause of revision, with a pooled percentage of 1.5% (47/3,138 knees). The
rate of bearing dislocation was significantly higher in Asian population than that
in western population. Loosening of either the tibial or femoral component was the
second most common reason for further surgical intervention (1%). Persistent pain
(0.8%), OA of the lateral compartment (0.5%), infection (0.3%), and medial tibial
plateau fracture (0.2%) were the remaining causes of failure.
In a systematic review and meta-analysis, Peersman et al[28] reported the outcomes of 4,330 fixed-bearing and 5,133 mobile-bearing UKAs. Mean
follow-up was 8.7 years for fixed UKAs and 5.9 years for mobile UKAs. Although mobile
and fixed designs presented comparable revision rates, major complications among fixed
implants were progression of OA (0.29%) and component loosening (0.22%), whereas those
among mobile implants were component loosening (0.44%), bearing dislocation (0.29%),
and progression of arthritis (0.23%).
In a systematic review, Ko et al[27] found that the overall reoperation rate and the overall incidence of complications
were similar between the mobile bearings (1,392) and fixed bearings (1,377); however,
mobile bearings were more susceptible to reoperations due to aseptic loosening, progression
of OA, and implant dislocation. Major incidence of component loosening reported in
mobile UKA was associated with osteolysis due to wear of submicron particles typical
of mobile-bearing designs; these smaller wear-particles are biologically more active
and have high propensity for osteolysis.
Conversion of a failed UKA to TKA could be a demanding procedure. Technical difficulty
may depend on how conservative the initial procedure was and on the primitive mode
of failure and its consequences on bone stock and ligament integrity, often with the
need for metal augmentation and stems.[29]
[30]
Controversies still exist about functional outcomes and final survivorship of TKA
after a failed UKA. Studies on conversion to TKA of failed first-generation UKAs reported
poorer results than those of primary TKA and comparable to revision TKA.[33]
[34]
[35] On the contrary, conversion of second-generation UKA resulted in better outcomes
and therefore comparable to primary TKA.[11]
[36]
Some authors demonstrated how the cause of UKA failure could affect outcomes of the
successive conversion to TKA. Kerens et al[37] found that conversion of UKA to TKA for unexplained pain generally resulted in a
less favorable outcome than revision for a known cause of pain. Furthermore, studies[11]
[38] where the reason of revision was mostly progression of OA, polyethylene wear without
loosening, or polyethylene dislocation showed better outcomes than studies where loosening
was the primary reason of conversion.[39]
[40]
Lunebourg et al[33] reported that the main reason of revision was progression of OA and that the results
of revised UKA with TKA were close to those of revised TKA. The authors compared 48
UKAs converted to TKA, 48 primary TKAs, and 48 revision TKAs. The groups were matched
for age, gender, body mass index, preoperative Knee Society Score, length of follow-up,
and date of the index surgery. Surgical characteristics, clinical outcomes, and complications
were compared at a mean follow-up of 7 years. Regarding the 48 converted UKAs, 43
patients received a primary posterior-stabilized TKA, whereas 5 patients received
a semiconstrained implant. An autologous graft was used in 6 patients, a tibial metal
augment in 14 patients, and a tibial stem in 35 patients. The authors concluded that
even if the conversion of UKA was technically less demanding than a revision TKA,
functional scores, quality of life, complications, and survival rate after revision
UKA were more comparable to revision TKA than to primary TKA.
Järvenpää et al[41] compared the long-term clinical and functional results of 21 UKAs converted to TKA
with those of 28 primary TKAs matched for age, sex, and operation time point. The
mean follow-up period of the patients was 10.5 years. The UKA-converted patients were
less satisfied, as measured by the WOMAC (Western Ontario and McMaster Universities
Arthritis Index) scale. Two patients were revised twice in the UKA conversion group.
There was one revision in the primary TKA group. Improvement in range of motion was
better in the TKA patients compared with the UKA conversion patients. The authors
concluded that UKA conversion to TKA is associated with poorer clinical outcome as
compared with primary TKA.
Similarly, Craik et al[42] in 25 UKAs revised with TKA, found that 10 (34%) patients required augments, stemmed
implants, or bone grafts. Outcomes following UKA converted to TKA were poorer than
those following successful UKA and primary TKA, albeit these results were found to
be a consequence of poor preoperative function of the failed-UKA patients.
Jonas et al[43] concluded that revision of UKA with TKA is not a straightforward procedure comparable
to a standard primary knee replacement. Additionally, despite the cost of components
not being significantly higher than that in primary TKA and despite the clinical outcomes
being similar at 5 years, there were multiple hidden follow-up costs in converted
UKAs.
Finally, Cankaya and Della Valle[44] found that conversion of a failed UKA is similar to primary TKA with respect to
blood loss and transfusion rate.
Conclusion
Although incidence of failure is greatly diminished in the last generation UKA, bearing
dislocation, polyethylene wear, mechanical loosening, and progression of OA continue
to affect the final survivorship of contemporary UKA. Converting a UKA to TKA is more
complicated than performing a primary TKA, and clinical and functional results after
revision TKA for a failed UKA could be inferior to those of a primary TKA. A primary
implant can be used to convert a failed UKA to a TKA; however, surgeons should be
aware that in revision UKAs, stems and augments on the tibial side are often required.
