Keywords
total knee arthroplasty - patella - resurfacing - patelloplasty - patellofemoral joint
Introduction
The patellofemoral (PF) joint (PFJ) is involved in over 45% of cases of knee osteoarthritis
(OA), independently or concurrent with OA in the other compartments.[1] Despite its incidence, few papers in the literature have focused on it. Patellar
disorders are strictly linked to anterior knee pain (AKP) and sometimes it can be
traced back to malalignment of the extensor mechanism.[2] PF malalignments seem to have several predisposing intrinsic factors, such as neural
inflammation, altered proprioception, patellar tendinitis, Osgood–Schlatter disease,
coronal knee malalignment, and joint hypermobility.[3]
[4]
[5]
[6]
[7] There are also some others causes of AKP that are not related to malalignment, such
as inflammatory disease, tumors, infections, stress fractures, fat pad inflammations,
and psychological causes.[8]
[9]
Total knee arthroplasty (TKA) is the most effective treatment for advanced knee OA.
The intervention does not always include a resurfacing of the patella, even if PFJ
is frequently involved. PFJ is a critical aspect of kinetic features of the implant
design in TKAs and is supposed that implant kinematics contributes to PF complications.
A successful TKA generally is able to increase knee flexion angle, thus enhancing
the PF contact and pressing forces.[10]
Long-term changes for nonresurfaced patella have been described. Laughlin et al demonstrated
how the lateral tilt increases over time.[11] Shih et al showed that in patients with long-term follow-up the patellar tracking
and the PFJ remained normal in 60% of the TKAs, but progressive degenerative changes
and patellar maltracking are common radiographic findings. Retention of the native
patella is associated with progressive degeneration of the lateral patellar facet
in 85% of cases.[12]
Patients with preoperative patellar maltracking could be considered at risk of developing
these abnormalities and clinical symptoms. Indeed, leaving the native patella is related
to a high incidence of AKP, up to 47%.[13]
[14] For this reason, some authors recommend routinely resurfacing the patella.[15]
[16]
Nevertheless, some studies showed that patella complications in TKA are more frequent
in the resurfaced group than in nonresurfaced one.[17]
[18]
[19] Some others suggested a selective decision based on factors such as preoperative
AKP, evaluation of patellar thickness, severity of OA in the third compartment, and
the experience of the surgeons.[20]
[21] Therefore, during surgery, surgeons that do not routinely resurface the patella
should consider several issues, including the severity of OA, knee alignment, PF tracking,
implant design, and position of the components.
If the surgeon decides to retain the native patella, several options could be chosen
to reduce the rate of AKP and improve the outcomes. Options that have been described
include patelloplasty, denervation, and circumferential resection of the osteophytes.
Patelloplasty has been defined as the surgical options aiming at the improvement of
the congruency between the patella and the trochlea, conducting a real reshaping of
the patella.[22]
Despite the excellent records about the long-term outcome of TKA, there remains controversy
about the correct management of the PFJ.
Patellar Resurfacing
In 1955, McKeever tried to reduce pain in PFJ OA by patella resurfacing as an alternative
to patellectomy or debridement.[2] Even though TKA has shown good results almost since its creation, in the 1970s many
physicians identified AKP as a frequent complication of TKA. Since then, the implants
included the patellar component. This choice resulted in reduction of incidence of
AKP, but despite this, complications related to resurfacing started to emerge.
Several designs and materials have been used for patellar resurfacing. At the beginning,
all-polyethylene patellar components were used, but they presented with early deformation
and accounted for almost 50% of all TKA revisions.[23] Some years later, a metal backing was incorporated into the design to give load
transfer to the patellar bone and to allow cementless fixations using a porous metal
surface. Unfortunately, new complications arose related to the shearing forces between
the polyethylene surface and the metal backing under eccentric loads.[24] One of the most frequent complications, the polyethylene wear, resulted in exposure
of the metal backing leading to metal-on-metal contacts. Loosening and migration became
frequent, leading to exposition of the femoral metal to the patellar bone. Several
studies conducted onto the metal-backed patella reported 5 to 33% failure rate.[25]
[26] Later, engineers tried to modify the polyethylene dome shapes to normalize the contact
forces, thus improving polyethylene wear patterns.[27] The introduction of a low-contact-stress press-fit metal-backed rotating patella
increased the congruity of the PFJ. These innovative designs led to the reduction
of contact pressure, which resulted in fewer complications.[28]
Actually, modern prostheses incorporate an all-polyethylene patellar component with
pegs (from 1 to 3) and cemented fixation, reducing the high risk of failure with the
cementless metal-backed implants.
The implants are available to the surgeons in the range of 8- to 10-mm thickness.
A caliber should be used to measure the central patellar thickness, with the aim of
maintaining the anatomical thickness. A residual bone thickness of approximately 15 mm
is required.[29] The patella resection is performed with the aim of being flat. The implant is placed
and generally cemented, because, as we have previously said, complications rate may
decrease with cement. Medial placement of the implant improves patellar tracking when
compared with central placement.[30] Only if the patella tracks laterally or lifts off medially, a lateral patellar retinacular
release should be performed. Some authors describe an additional lateral facetectomy,
proper of patelloplasty, and it can be performed when a malaligned patella is addressed.
Open partial lateral patellar facetectomy has been shown to improve function but with
poor correlation with radiological findings.[31]
Several complications can occur following patellar resurfacing. Instability is probably
the first one, requiring revision surgery in about 0.5 to 0.8% cases. Factors that
could contribute to patellar instability include malposition of the components, soft
tissue imbalance, excessive femoral component size, and inadequate patellar resection.[32]
[33] Treatments for instability are component revision, proximal and distal realignment
of the soft tissue, and osteotomy of the tibial tubercle. The most dangerous complication
is patellar fracture (0.05–8.5% of TKA cases).[34]
[35]
[36] Decreased bony thickness combined with osteopenia, avascularity, trauma, fatigue,
and stress can generate this kind of adverse event, whose risk increases when lateral
retinaculum release is simultaneously performed.[37]
Fractures may occur during surgery too: an extremely thin patella is at higher risk
of fracture due to the high mechanical pressures. Patellar fractures are generally
treated with a conservative approach, especially when an implant is not loose and
the extensor mechanism is not disrupted, whereas surgical options include open reduction
with internal fixation, excision of fracture fragments, and patellectomy.[38]
[39] Another complication is wearing of the patellar component (incidence ranging between
5 and 11%).[23] Factors that can be associated to this complication are maltracking and abnormal
contact force. Maltracking is able to induce component loosening in 0.6 to 1.3% of
cases, as well as traumas.[40] Soft-tissue impingement, the so-called patellar clunk syndrome, results from the
formation of a fibrous nodule over the patella proximal pole and is strictly associated
with posterior stabilized design.[38] Generally, it can be treated through an arthroscopic or open resection. The patellar
ligament rupture (0.22–0.55%)[41] can be due to excessive dissection, knee manipulation, and trauma. Either staple
fixation or grafts can be used in treating acute ruptures, but with poor outcomes.[42]
The advent of modern prosthetic designs with better PF congruence, smoother patellar
tracking, and superior patellar fixation, has led toward fewer patellar complications.
Recent studies have reported lower PF complication.[43]
Some authors have proposed algorithms to assess the ideal candidate to patella resurfacing.
Bourne and Burnett[44] formulated five criteria for choosing the resurfacing option: severe PF OA, poor
tracking, inflammatory arthritis, obesity, and age over 60. Several studies have denied
all these beliefs: no differences have been found between obese and nonobese[45]; no correlation has been proved with severity of OA[46]; and studies on alignment have also considered the correlation between malalignment
and necessity for a resurfacing, without a certain result.[47]
Some gross data derived from randomized clinical trials (RCTs) seem to support those
surgeons who are used to resurface the patella.[48] Anyway, some systematic reviews and meta-analysis summarized that the decision is
still difficult. Pakos et al[49] reviewed 10 RCTs that favored patellar resurfacing when compared with the relative
risk of revision and AKP. Parvizi et al[50] conducted a meta-analysis on 14 studies, which proved that there was no difference
between resurfaced and un-resurfaced patellas in reoperation rate, but showed that
patellar resurfacing results in the reduction of AKP and achieve better patient satisfaction.
Nizard et al[51] demonstrated a lower risk of reoperation for resurfaced patella with respect to
un-resurfaced (0.43 vs. 0.71). The Swedish Knee Arthroplasty Register, in 2004, reported
a 10-year follow-up in over 30,000 patients, where secondary resurfacing was found
to be the main cause for reoperation.[52] In 2007, the Australian National Joint Registry reported a relative risk of reoperation
in 4.7% for un-resurfaced patellas, as compared with 3.8% for resurfaced. The same
registry shows no differences regarding the type of fixation of the component.[53] Calvisi et al[54] found that resurfacing might reduce the risk of reoperation and AKP. Patil et al[55] evaluated functional scores such as the Knee Society Score (KSS), 36-Item Short
Form Health Survey, and satisfaction, and found no differences during comparisons.[55] Li et al conducted a large review founding no differences between the two groups.[56] Chen et al[57] published the most recent, important, and valuable meta-analysis about this issue
4 years ago. They reviewed 14 RCTs, with homogenous parameters, with a cumulative
size of over 1,700 patients. Relative risk of reoperation favored the patellar resurfacing
(0.50 vs. 0.76), but no significant differences were found for AKP. Anyway, the authors
revealed a higher KSS in the resurfaced group.[57]
Patelloplasty
Patelloplasty consists of different procedures, such as patellar decompression, lateral
patellectomy, and patella reshaping, which are aimed at creating a good congruence
between the patella and femur. It has the advantage of conforming to the articular
facets of the patella toward a large variability of tracking surfaces. In fact, the
shape of the trochlea could vary and the shape of the femoral prosthetic component
can vary as well according to the implant design.[58]
Reshaping of the patella has the scope of achieving a good matching between the prosthetic
trochlea and the articular surface of the patella. This procedure is conducted with
or without a resection of cartilage layer.[22] Unfortunately, this is not a well-standardized procedure. There are no specific
and worldwide-recognized steps of procedure. There is an excessive variability of
gestures. Some author use to reduce the thickness of the patella to less than 20 mm,
while some other between 22 and 26 mm. Some authors totally remove the cartilage layer,
while some others leave the native one. Therefore, as there is a great variability
in the procedure, a great variability could exist in outcomes as well. All these gestures
are original, without a real validation in the literature.
Rate of complications is controversial due its variability in procedures. Although
this is a technique characterized by rare intraoperative complications,[59] there could be some postoperative ones, such as maltracking for a new patellar shape
and a controversial adverse reduction of the strength of the extensor mechanism; but
this topic has not yet been investigated in the literature.
The literature is still poor of relevant studies about this issue. Sun et al[59] published a retrospective study with 152 patients, divided in two groups: patelloplasty
versus denervation. They found that patelloplasty was better in both relieving AKP
and improving patient's satisfaction and knee function.[59] Župan et al[60] reported better clinical outcomes in patients with patelloplasty with respect to
the simple denervation and removal of osteophytes. They found better outcomes in Knee
injury and Osteoarthritis Outcome Score (KOOS) and in Oxford Knee Score, but revealed
no significant differences in KSS and pain scores. They also correlated the results
to postoperative patella thickness, but they found no significant correlation. The
first systematic review focusing on patelloplasty in primary TKA has been conducted
by Cerciello et al[22] about clinical results, the comparison between patelloplasty and simple denervation
and osteophytes removal revealed lower AKP in patelloplasty (2.9% vs. 4.6%), as well
better clinical outcomes at KOOS, KSS, and Feller patellar score.[22]
[61]
[62]
[63]
Though all these evidences show that patelloplasty is a safe procedure without frequent
intraoperative and postoperative complication, the quality and quantity of studies
are still poor.
Control groups in the studies are often lacking. In some other studies, preoperative
data are missing, so comparisons are extremely difficult.
Other Alternatives
Intraosseous hypertension is another important hypothetical cause of AKP in the third
space after TKA.[64]
[65] In degenerative pathologies of the knee, an increased intraosseous pressure is described.[66] This patellar hypertension, though not specifically studied yet, is reported to
be associated with idiopathic AKP.[67]
In several experimental studies, an impaired venous drainage are linked with hypertension,
especially in patients with patellar chondromalacia and OA.[68] Causes for an altered venous drainage include direct compression of vascularization
from fat pad, osteophytes of the superior patella, a sustained knee flexion position,
chondromalacia, and OA.[67]
[69]
[70]
Patellar drilling has been proposed for improving impaired drainage and it could indirectly
relieve AKP. The first results described after decompression by drilling via the infrapatellar
fat pad reported a direct reduction of intraosseous pressure and pain relief.[71]
Ertürk et al[72] published a trial on 49 TKAs treated with patelloplasty and decompression of the
patella. They conducted the decompression drilling on the edge of the patella with
the aim of not damaging the articular cartilage and drilling at vertical plane to
minimize the risk of fracture. The authors reported excellent clinical results, especially
for range of motion (ROM) and patellar scores. Anyway, the study did not use a control
group.[72]
Lee et al[67] compared patelloplasty to patelloplasty decompression. A 3.5-mm drill has been used
via fat pad under tissue protection in the group with decompression. The authors reported
AKP in 21.3% of the control group and in 18.5% of the study group. Higher KSS was
observed in patients with decompression (78.2 vs. 71.8).
Discussion
The incidence of AKP is high after TKA, reported between 4 and 49%.[73] Daily activities are impaired due to this complication: they include stair climbing,
cycling, and getting up from a chair. Pathogenesis of AKP is still unclear but several
potential factors have been proposed to be linked.
Over time, the most studied options for the management of this complication have been
the treatments of patella during a TKA.
The gross data reviewed in this article show that the patella resurfacing might reduce
pain but is associated to severe complications. Moreover, some systematic reviews
and meta-analysis did not succeed in finding significant differences in terms of clinical
outcomes.
Unfortunately, patelloplasty is still poorly studied, even if it seems the best alternative
solution to resurfacing of the patella. Systematic reviews and meta-analysis are affected
by poor quality of RCTs both in the methodology and in uniformity of samples of patients
and outcomes.
RCTs are generally conducted through clinical and radiographic evaluations, but few
articles are available about biomechanical performances. Smith et al[74] evaluated the gait pattern by a meta-analysis, showing no significant differences
between resurfaced and un-resurfaced patellas. Myles et al[75] used an electrogoniometer to evaluate improvements in ROM, but they found no differences
between the two groups of patients.
Scientific literature lacks RCTs that focus on comparison between patella resurfacing
and patelloplasty. Campbell et al[76] found no significant differences in the International Knee Society and Western Ontario
and McMaster Universities Osteoarthritis Index scores at 10-year follow-up. Similar
uncertain results have been reported by Burnett el al.[47] However, despite the clinical scores, patient's preference was toward resurfaced
patella. Smith et al[77] evaluated 164 patients through KSS, reoperation rate, AKP, and patient's satisfaction.
No significant differences have been revealed, but a lower AKP was reported for TKAs
with resurfaced patella.[77]
Several other elements have gained importance for the assessment of the PFJ. Abnormal
postoperative PF tracking has been described as linked to retropatellar pressure.[78] However, some studies did not find a clear association between patellar tilt or
subluxation and AKP.[79]
Generally, in the past, physicians believed that patellar height was linked to AKP.
Patella baja, although it has a poor incidence in TKA, is surely a predisposing factor
for flexion deficit, but there are no studies that correlated this condition to AKP.
On the contrary, patella alta is generally associated with higher contact forces between
the trochlea and patella. However, studies have not confirmed that it is a certain
predisposing factor for developing AKP.[80] van Houten et al[80] analyzed the effects of patella positioning on AKP in TKA. The authors found that
the amount of patellar displacement or patellar tilt is not related to AKP.
Another critical aspect that is generally discussed is the type of bearing surface
of TKA. Fixed bearing has been always believed to be of higher risk compared with
mobile bearing designs. Type of bearing surface might influence patellar kinematics,
because some studies showed lower PF contact stress in the mobile bearing compared
with fixed bearing.[81] However, few studies demonstrated a potential benefit of mobile bearing in AKP,
some of those suggested that the performances of mobile bearing decline over time.[82]
[83]
[84]
[85]
The tibiofemoral contact point is another potential predisposing factor for AKP. Greater
anterior positioning of the tibiofemoral contact point is the complication of the
lever arm of the extensor mechanism, which leads to PF disadvantage.[86]
The last important element that is able to create a variability in the onset of AKP
is the vascular supply to the patella. In fact, the amount of resection of fat pad
can play an important role.[87]
[88]
Conclusion
The overall incidence of postoperative anterior knee in TKA is still too high. Evidence-based
publications cannot be easily interpreted. There is statistical heterogeneity, perhaps
because of methodological diversity. The general trends seem to present a reduction
of risk of reoperation in TKA with a resurfaced patella, but without significant benefits
to knee function and satisfaction with respect to patelloplasty. So the decision of
whether or not to resurface the patella should be assessed according to the evaluation
of the PF joint, the design of the prosthesis, the experience of the surgeons, and
patient's features.[89]
[90] Anyway, treatment of the patella does not seem to be the only factor that can influence
AKP in TKA. Positioning of the femoral and tibial components, both in rotation and
in AP direction, can influence the contact forces in the third space. The design of
the prosthesis and choice of implant may also play an important role.
Treatment of smooth tissues is still poorly considered as a predisposing factor for
AKP, such as the treatment of patellar fat pad, treatment of retinacula, and balancing
of peripheral compartments could be extremely important both for vascular supply of
the extensor mechanism and for biomechanical tracking of the third space.
