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Patellar Component
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YoavS.Zvi andEliKamara
Contents
36.1 Introduction – 410
36.2 Case Example – 410
36.3 Surgical Technique – 410
36.4 Relevant Literature – 413
References – 416
409
36
© The Author(s), under exclusive license to Springer-Verlag GmbH, DE, part of Springer Nature 2022
E. Hansen, K.-D. Kühn (eds.), Essentials of Cemented Knee Arthroplasty,
https://doi.org/10.1007/978-3-662-63113-3_36
410
Y. S. Zvi and E. Kamara
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36.1 Introduction
In this chapter, we review the patellar component in
cemented knee arthroplasty. The authors’ preferred
surgical technique for patellar resurfacing is the onlay
method using an all-polyethylene cemented implant.
Surgical technique for patellar resurfacing is discussed,
including tips to avoid pitfalls and minimize potential
complications postoperatively. When resurfacing the
patella, specic considerations regarding bony resection and implant positioning should be kept in mind to
optimize xation, patellofemoral tracking, and quadriceps muscle efciency. It is important to restore the
combined thickness of the resected patella and implant
to the thickness of the native patellar bone. Surgeons
should also select patellar implants that will maximize
bony coverage. Furthermore, implants should be positioned such that the patellar component lies along the
anatomic medial ridge.
Patellofemoral complications including loosening,
fracture, and maltracking are a primary reason for revision knee arthroplasty, highlighting the importance of
these concepts.
sharp, grinding pain that is aggravated with weight bearing, specically while climbing stairs. On exam, her pain
localizes to the medial and lateral joint line, as well as the
anterior aspect of the knee. A moderate, partially correctable valgus deformity is noted, with a range of motion of
5–110°. The patient is otherwise neurovascularly intact.
Radiographic imaging of the knee demonstrates signicant valgus angular deformity, osteophytes, subchondral
sclerosis, and joint space narrowing affecting the medial,
lateral, and patellofemoral joint spaces (. Fig.36.1).
A trial of conservative management is started– this
includes physical therapy, activity modications, nonsteroidal anti-inammatory medications (NSAIDs), as
well as multiple corticosteroid injections to the knee.
Her symptoms are alleviated for 2years, at which point
conservative treatment is no longer effective. At that
time, she is indicated for a total knee arthroplasty for
denitive treatment. Postoperatively, the patient reports
complete alleviation of her pain; her valgus deformity
and joint alignment are restored (.
Fig. 36.2). She
remains well at her most recent postoperative visit,
with no evidence of component loosening or functional decits.
36
36.2 Case Example
A 62-year-old female presents to the orthopedic surgery
clinic with complaints of 2years of atraumatic, progressively worsening left knee pain. The patient describes a
36.3 Surgical Technique
The authors’ preferred surgical technique is an onlay
patellar resurfacing. After joint exposure through a
medial parapatellar approach, femoral and tibial bony
abc
. Fig. 36.1 a–c Preoperative left knee radiographs. Anteroposte-
rior a, lateral b, and sunrise c radiographic views demonstrating signicant tricompartmental disease. A valgus angular deformity is
seen with diffuse osteophytes, subchondral sclerosis, and joint- space
narrowing
Patellar Component
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abc
411
36
. Fig. 36.2 a–c Postoperative left knee radiographs. Anteroposterior a, lateral b, and sunrise c radiographic views demonstrating total knee
arthroplasty with restoration of joint space and mechanical alignment
cuts are made. Using a gap-balancing technique, the
joint is balanced in both exion and extension, ensuring an adequate mechanical alignment is maintained
throughout range of motion. Attention is then shifted
to patellar preparation.
> The goals of resurfacing are
5 to restore the combined thickness of the resected
patella and implant to the native patella thickness,
5 to ensure proper patella tracking of the implant
without bony impingement on the prosthesis.
The borders of the patella are carefully dened using
electrocautery. If present, osteophytes surrounding
the rim of the patella are removed with a rongeur. The
native thickness of the patella is measured with a handheld caliper (.
Fig.36.3). A minimum of 10mm thick-
ness is required for this technique to prevent the onlay
implant peg from protruding through the native patella,
but 12–14mm is preferred to decrease the risk of patella
fracture. Using the patellar cutting guide, the patella is
cut with an oscillating saw to a goal of 10–12mm thickness depending on the implant used (. Fig.36.4).
. Fig. 36.3 Measuring native patella bone thickness using hand-
held caliper
under- or over-resecting the native patella, which can
lead to either over- or under-stufng, respectively, when
the nal component is placed. Over-stufng may lead to
postoperative anterior knee pain, loss of exion, patellar fracture, maltracking, and/or early patellar component loosening, while under-stufng may cause patellar
maltracking, patella fracture, osteonecrosis, and loss of
> Care should be taken to ensure that a level surface
with even thickness throughout the patella is maintained– this will help avoid patellar tilt.
extensor mechanism efciency.
The patella is sized with a measuring plate with
consideration for positioning (.
Fig.36.6). In general,
the apex position of the patellar component should be
The cut patella is then re-measured, and if necessary,
additional cuts are made (. Fig. 36.5). This prevents
along the median ridge of the native patella while maximizing the osseous coverage that is provided by the
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Y. S. Zvi and E. Kamara
. Fig. 36.4 Patella cutting guide positioned for bony cut using
oscillating saw
patellar implant. If between sizes, use of a smaller size
with medialization may be appropriate to achieve this.
Following sizing and placement, the corresponding
drill- guide is used to drill one or three holes into the
patella depending on the implant used (. Fig. 36.7).
A trial dome patella is placed, and the knee is brought
through exion and extension to evaluate patellar
tracking (. Fig. 36.8). When necessary, a lateral facetectomy and/or lateral releases are performed in order
to relieve tension forces that may lead to maltracking
postoperatively. Lastly, the thickness of the resected
patella and trial implant is measured using a handheld
caliper, in order to ensure that the native patellar thickness was restored as closely as possible (. Fig.36.9).
36
. Fig. 36.5 Resected patella thickness measured using handheld
caliper
. Fig. 36.7 Corresponding drill guide used to drill 3 holes into
resected patella
. Fig. 36.6 Patella is sized using a measuring plate
. Fig. 36.8 Trial implant placed and tested for adequacy of patel-
lar tracking in exion and extension
Patellar Component
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. Fig. 36.9 Thickness of resected patella and trial implant mea-
sured using handheld caliper
413
patella to resect a pre-measured thickness of bone, while
the inset method uses a reamer to create a circular dome
within the native patella in which the component will be
placed.
> Regardless of technique that is used, careful attention
to component design, size, and positioning is para-
mount to optimize postoperative patellar tracking
and extensor mechanism efciency.
Patellofemoral complications including anterior knee
pain, maltracking, clunk syndrome, implant loosening,
and patella fracture continue to be a primary reason for
revision knee arthroplasty (Schiavone Panni etal. 2014;
Abdel etal. 2014), highlighting the importance of surgical technique. Review of the literature can offer insight
into certain surgical and component principles that may
yield improved outcomes and minimize these potential
complications.
The role of cemented implant xation when resurfacing the patella has been demonstrated in a study conducted by Healy etal. (1995). They reported on implant
design and patient risk factors on postoperative complications in total knee arthroplasty. In their series of
211 patients treated with total knee arthroplasty, 160
patients treated with cemented patella implants were
compared with 37 patients treated with cementless
patella implants. In the total cohort, 5 patients (2.4%)
were found to have patellar fractures; of these patients,
4 were treated with cementless xation. Additionally,
they found the rate of patellar loosening in cementless
implant xation to be signicantly higher than cemented
implant xation (13.5% vs. 1.2%; p<0.01).
36
. Fig. 36.10 Final patella implant after cementing of all compo-
nents
> After nalizing femoral, tibial, and patellar trial com-
ponents and achieving acceptable soft-tissue balancing and patellar tracking, nal components are
cemented (.
components, the order of insertion of implants being
tibia, femur, trial liner, and patella.
Fig. 36.10). Cement is used for all 3
The authors prefer using an all-polyethylene, three- pegged,
dome-shaped patellar component, but anatomic patellar
components may also be used when deemed appropriate.
36.4 Relevant Literature
When resurfacing the patella, for either a total knee or
patellofemoral arthroplasty, two surgical techniques are
generally used. The onlay method involves cutting the
> Patient outcomes were best with a cemented, all-
polyethylene domed patellar component demonstrat-
ing lower rates of overall complications when
compared with cementless implant xation.
> The study concludes that cement contributes signi-
cantly to patella implant stability. While this study
was conducted a number of years ago, most surgeons
today still prefer to use cemented patella implant xa-
tion for this reason.
Onlay and inset patellar xation techniques are both
widely reported in the literature. It is debated which
surgical technique will yield superior outcomes while
minimizing complications, though both are still in
use with reasonable results. Greenwald et al. sought
to compare the xation strength of onlay and inset
implants through a biomechanical study (Rosenstein
et al. 2007). Synthetic solid foam patellae were prepared with identical dimensions of 29 mm diameter
and 12.5mm height; 3-pegged onlay or inset components were then cemented in place. Mounting blocks
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Y. S. Zvi and E. Kamara
36
were then used to load the components onto an Instron
Testing Machine that recreated a chair rise motion by
applying a compressive joint force perpendicular to the
anterior patella. A shearing force was then introduced,
and the maximal load to failure was recorded. Their
study found the inset component group to have signicantly higher maximal shear force than the onlay component group (mean: 715 lbf, SD: 41.8 vs. mean: 571
lbf, SD 52.7; p<0.01).
> The authors conclude that although this was a small
sample-sized biomechanical study, the increased
resistance to shear forces demonstrated by the inset
components may offer improved durability of patellar xation invivo.
The patella experiences signicant anterior forces as the
knee is exed. This has certain implications when deciding not only on surgical technique, but also on prosthesis selection, amount of bone to resect, and composite
thickness of resected patella and implant.
> Increased surface strains across an overly resected
patella can lead to patella fracture, which is why conservative patella cuts should be considered.
Incavo et al. investigated these concepts in a biomechanical cadaver study (Wulff and Incavo 2000). Using
a cadaveric patella with the quadriceps and patellar tendons attached, onlay and inset implants were used to
resurface the patella. Specimens were then mounted to
a loading xture in 50° of exion, previously reported
to show maximum anterior surface strain, and a strain
gauge was placed along the anterior cortical surface
of the patella. Flexion angle, load rate, and maximum
load (1000N) were all kept constant. The study showed
that both implants increased anterior strain on the
patella; however, when the native patellar height was
reproduced the inset implant had signicantly higher
strain than the onlay implant (28% vs. 22%; p<0.05).
They also demonstrated that increasing patella resection at 2mm increments is directly related to increased
patellar strain. The onlay implant was able to tolerate
more resection than the inset prosthesis. Lastly, they
showed that with an over-resected patella, restoring
the native height by using a thicker prosthesis signicantly increased the anterior patellar strain, regardless
of implant type. Increased anterior forces along the
patella suggest that loosening and fracture are more
likely; however, this study did not specically report on
these complications.
Patellar design is another aspect that can affect both
the size and positioning of the component. A study conducted by Laz etal. sought to determine how round and
oval designs of the patella affected size selection and
coverage (Yang et al.
2017). Their study involved 100
patients undergoing primary total knee arthroplasty,
and analyzed 3 parameters:
5 Intraoperative assessment of component size
5 Digitally measured bony coverage
5 Apex positioning of the patellar implants compared
to the medial-lateral ridge of the native patella
They found that a larger oval implant was selected in
82% of patients when compared to a round implant.
Additionally, oval implants were found to have a greater
percent bony coverage of the resected patella when
compared to round implants (mean 82.7%, SD 10.7%
vs. mean 80.9%, SD 9.7%; p=0.02). The authors also
report that the likelihood of upsizing was higher with an
oval component vs. a round component, and was generally preferred by the operating surgeon. Lastly, they
demonstrated improvement in apex positioning with
respect to the native medial ridge with oval components
than round components (mean 0.25mm lateral to anatomic median ridge, SD 2.45mm vs. mean 1.16mm, SD
2.54mm medial to the anatomic median ridge; p<0.01).
> Admittedly, while these ndings are statistically sig-
nicant and theoretically should correspond to better
outcomes based on component principles, it is difcult to determine whether these marginal differences
translate into long-term clinically relevant improvements.
With patellar resurfacing, it is crucial to restore native
patellofemoral biomechanics such that patellar tracking, contact forces, and quadriceps muscle efciency are
optimized. Shelburne etal. conducted an invivo biomechanical study comparing medialized dome vs. medialized anatomic patellar implants in 20 patients (10 per
group), to investigate these parameters (Ali etal. 2018).
Anatomic patellar implants differ from dome patellar
implants in that they possess a medialized peak that
more closely mimics the geometry of native medial and
lateral facets of the patella. In their study, patients were
asked to perform seated knee extension and weightbearing lunge exercises. Using high-speed stereo radiography (HSSR) they collected patient- specic parameters
while performing these activities. Marker-based motion
capture and force plate data were used for musculoskeletal simulations, and data analysis of each patient’s patellofemoral mechanics was done through nite element
modeling (.
Fig.36.11).
The authors note that kinematic differences between
knee extension and lunge activities were not signicant.
When comparing medialized dome and medialized anatomic subjects, noticeable differences were seen during
the lunge. Anatomic patients demonstrated larger patellofemoral exion–extension angles with an average dif-
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415
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. Fig. 36.11 Workow of study. a HSSR data collection. b Musculoskeletal modeling. c Finite element modeling. (Courtesy of John Wiley
& Sons)
ference of 11°± 3°; dome patients were also found to
have an increase in patellar tilt by an average of 6°±5°.
When comparing quadriceps force, little difference was
patella. Patellar implants have a variety of different
designs that play a critical role in optimizing patellar
tracking, quadriceps efciency, and implant stability.
found during knee extension between groups, though
during lunge, dome patients exhibited higher forces at
mid-exion (60°) while anatomic patients exhibited
higher forces at deep exion (90°).
Contact forces were, as expected, shown to decrease
> Surgeons should pay close attention to surgical tech-
nique with regard to bony resection and implant posi-
tioning, and be aware of the added stability with
cemented implant xation when choosing implants.
during knee extension and increase during lunge –
forces were translated superiorly on the patellar component with exion. Contact force ratios, dened as
With consideration of these concepts when performing
patellar resection, patient outcomes can be optimized.
the ratio between total force due to patellar contact
and total quadriceps force, were shown to be larger in
dome patients compared to anatomic patients during
lunge activity. Patellar force ratio, dened as the ratio of
patellar tendon force divided by total quadriceps force,
decreased as knee exion increased. In knee extension,
similar patellar force ratios were measured between
dome and anatomic patients; however, during lunge,
anatomic patients showed larger patellar force ratios in
deep exion (>75°).
> Taken together, the authors conclude that their nd-
ings imply that anatomic patellar implants more
closely replicate native patellofemoral mechanics.
In summary, there are multiple factors that can affect
Take-Home Messages
5 After nalizing femoral, tibial, and patellar trial
components and achieving acceptable soft-tissue
balancing and patellar tracking, nal components
are cemented. The order of insertion of implants
being the tibia, femur, trial liner, and patella.
5 When resurfacing the patella it is important to
restore the native anatomy as best as possible to
optimize patellar tracking and quadriceps muscle
efciency.
5 The combined thickness of the resected patella and
nal implant should be the same as the native
patellar bone to avoid over- or under-stufng the
patellofemoral joint.
postoperative complications when resurfacing the
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Y. S. Zvi and E. Kamara
36
5 Under-stufng (i.e., over-resecting) can decrease
quadriceps muscle efciency, lead to patellar osteonecrosis or fracture, while over-stufng (i.e., underresecting) can lead to anterior knee pain, decreased
ROM, patellar component loosening, or fracture
of the patella.
5 A minimum of 10mm thickness is required for this
technique to prevent the onlay implant peg from
protruding through the native patella, but
12–14 mm is preferred to decrease the risk of
patella fracture.
5 The apex position of the patellar component
should be along the median ridge of the native
patella.
5 Implant size should maximize the osseous cover-
age of the resected patella. If between sizes, the use
of a smaller size with medialization may be appropriate.
5 A lateral facetectomy and/or lateral releases are
performed in order to relieve tension forces that
may lead to maltracking postoperatively.
5 Cement has been shown to minimize patellar frac-
ture and implant loosening.
5 Both onlay and inset techniques demonstrate suc-
cessful clinical outcomes when performed appropriately.
5 Inset implants have demonstrated higher maximal
shear force to failure, suggesting improved durability over onlay implants.
5 Oval implants were shown to enable maximal osse-
ous coverage and medial ridge positioning when
compared to round implants.
5 Medialized anatomic patella implants more closely
replicate native patellofemoral biomechanics when
compared to medialized dome patella implants.
References
Abdel MP, Parratte S, Budhiparama NC (2014) The patella in total
knee arthroplasty: to resurface or not is the question. Curr Rev
Musculoskelet Med 7(2):117–124
Ali AA etal (2018) In vivo comparison of medialized dome and ana-
tomic patellofemoral geometries using subject-specic computa-
tional modeling. J Orthop Res 36(7):1910–1918
Healy WL etal (1995) Patellofemoral complications following total
knee arthroplasty. Correlation with implant design and patient
risk factors. J Arthroplasty 10(2):197–201
Rosenstein AD, Postak PD, Greenwald AS (2007) Fixation strength
comparison of onlay and inset patellar implants. Knee 14(3):
194–197
Schiavone Panni A etal (2014) Patellar resurfacing complications in
total knee arthroplasty. Int Orthop 38(2):313–317
Wulff W, Incavo SJ (2000) The effect of patella preparation for total
knee arthroplasty on patellar strain: a comparison of resurfac-
ing versus inset implants. J Arthroplasty 15(6):778–782
Yang CC etal (2017) Patellar component design inuences size selec-
tion and coverage. Knee 24(2):460–467
Databases Afterwards
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Database
Contents
Chapter 37 International Registries– AComparison
ofOutcomes – 419
Kevin A. Lawson, Spencer J. Montgomery,
and James I. Huddleston
Chapter 38 USA AJRR– Total Knee Arthroplasty:
Lessons Learned – 439
Paul Hoogervorst and Patrick K. Horst
417
VII
Chapter 39 US National Databases– Total Knee Arthroplasty:
Lessons Learned – 447
Christie Bergerson, Derek Holyoak, and Kevin Ong
Chapter 40 Arthroplasty Register Data: Outcomes ofKnee
Arthroplasty inYounger Patients – 463
Nils P. Hailer and Annette W-Dahl
International Registries–
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AComparison ofOutcomes
KevinA.Lawson, SpencerJ.Montgomery, andJamesI.Huddleston
Contents
37.1 Introduction – 420
37.2 Registries – 420
37.2.1 History – 420
37.2.2
Collaboration – 421
Data Collected – 421
37.2.3
37.2.4 Weaknesses – 422
37.3 Registries andTheir Respective Sizes – 422
419
37
37.4 Outcomes Reporting – 423
37.4.1 Outcomes Based onPatient Characteristics – 429
37.4.2 Outcomes Based onMethod ofFixation – 430
37.4.3 Reasons forRevision – 432
37.4.4 Notable Trends – 432
37.4.5 Patient-Reported Outcome Measures – 433
References – 437
© The Author(s), under exclusive license to Springer-Verlag GmbH, DE, part of Springer Nature 2022
E. Hansen, K.-D. Kühn (eds.), Essentials of Cemented Knee Arthroplasty,
https://doi.org/10.1007/978-3-662-63113-3_37
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